Chapter 20: Magnetic Properties

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Ever notice how that magnetic charger just sort of snaps perfectly onto your phone?

Or you know, the silent whir of a computer hard drive, storing just incredible amounts of data in a space, well, no bigger than your hand.

Yeah, it's easy to take for granted.

Right.

Magnetism isn't some abstract force.

It's really fundamental to our modern world.

It's literally powering our devices and storing our memories.

And while humans have known about its pull for millennia, understanding how it all works scientifically, that's relatively recent.

It really is.

It's a phenomenon that's both, you know, complex and incredibly foundational to so much of our technology.

Welcome to the deep dive.

Today, we're taking a deep dive into magnetic properties, drawing heavily from Callister and Rethwish's Materials, Science and Engineering, 10th edition.

A classic text?

Definitely.

Our mission is to unpack this fascinating world of how materials interact with magnetic fields.

We're tailoring this specifically to help, say, college students grasp these concepts without needing visuals right in front of them.

Right.

We'll try to paint a picture with words.

Exactly.

We'll journey from the tiny dance of electrons all the way to the large scale applications that make our technology possible.

We'll explore why different materials behave the way they do magnetically and how we can even, you know, engineer them for specific uses.

Think of it as a guide to understanding the magnetic, well, personalities of different materials and how we put those personalities to work.

We'll cover the core concepts, explore the five main types of magnetic behavior, see how temperature changes everything.

Oh, yeah.

Temperature is a big one.

Huge.

And then we'll dive into cutting edge applications like digital storage and the truly mind -bending world of superconductivity.

Get ready to connect some dots because we're going to make sure you walk away with a solid grasp of these ideas.

So let's start with a foundational truth.

It's easy to think magnetism is just for, I don't know, iron and a few special things.

Right.

Refrigerator magnets and compasses.

Yeah.

But the reality is all substances are influenced by magnetic fields to some extent.

We see magnetic materials in action, constantly electric motors, power transformers, and of course, the computers we use every day.

And what's truly fascinating fundamentally is that all magnetic forces originate from moving electric charges.

At the atomic level, we're talking about electrons.

Their movement creates what we call magnetic dipoles.

Think of them as essentially tiny bar magnets, just like a regular magnet.

They have a north and south pole.

You've seen this with a simple compass needle, right?

It's a magnetic dipole, and it aligns itself with Earth's magnetic field pointing north.

It's a classic example of an external magnetic field creating a twist or torque to orient these tiny internal magnets.

Exactly.

So to really talk about magnetism properly, we need a precise language.

We use several key terms, key vectors, really, to describe these fields.

First, there's magnetic field strength, usually denoted as H.

OK.

Think of H as the external magnetic push or intensity you're applying.

To a material.

How do you generate an H field, like practically?

Good question.

A common way is using a cylindrical coil of wire, like a spring or a solenoid.

If you run an electric current through that wire, a magnetic field H is generated right along its axis.

And the strength of this H field depends on, well, how much current you run through it, how many turns of wire there are per unit length and the coils overall length.

We measure H in ampere turns per meter or just amperes per meter.

Got it.

So H is the applied field.

What about inside the material?

Right.

That's where magnetic induction or flux density, B, comes in.

While H is what you apply externally, B is the total magnetic field inside the substance after you've applied H.

It's the result.

And it's measured in Tesla's T.

OK.

So H goes in, B comes out.

Well, B is measured inside.

What connects them?

The connection is a material property called permeability, symbolized by the Greek letter mu.

So the basic relationship is B equals mu.

B equals mu H.

Think of permeability as a measure of how easily a magnetic field, the B field, can form and move through a particular material.

It tells you if material is like welcoming to a magnetic field or if it resists it.

Like an open door versus a brick wall for the field.

Exactly.

Now, for a vacuum, there's a baseline relationship, B or O -H -H.

Here, U -naught is the permeability of free space, just the universal constant.

OK.

A fundamental constant.

Right.

And we often compare a material's permeability to this baseline using the relative permeability, A.

It's simply oreo -ye.

It's unitless and tells you how much better or worse a material is at conducting a magnetic field compared to just empty space.

So a high O -H -H means the material really concentrates the field lines.

Precisely.

But here's a really important distinction.

Material doesn't just passively let the field pass through.

It can also actively contribute to the internal magnetic field itself.

OK, so it's not just a conduit, not at all.

That's where magnetization M comes in.

M quantifies the material's own contribution, its response, to the internal field.

So the total internal field B is actually a combination of the external field H, adjusted by ore, and the material's self -magnetization M.

The equation is B equals H plus M.

OK, so B equals H plus M.

The total field is the applied field plus the material's own magnetic response.

You got it.

Essentially, those tiny magnetic dipoles within the material can align with the applied H field, reinforcing it.

And we measure how easily this alignment happens using another property.

Magnetic susceptibility, Chi -M.

Susceptibility, like how susceptible it is to being magnetized.

Exactly.

It's a unitless number that tells us the proportionality between the magnetization M and the applied field H.

So M -M -H.

A high susceptibility means the material gets easily magnetized.

It's actually related to relative permeability by M -R -R -1.

Interesting.

It sounds a bit like how dielectric materials respond to electric fields.

It's a very good analogy, yes.

Permeability and susceptibility in magnetism are analogous to permittivity and dielectric constant in electricity.

OK, that gives us the language, the parameters H, B, M, O -M.

But where do these magnetic moments, the M part, actually come from?

You mentioned electrons.

Right.

It all boils down to the electrons within the atoms.

The macroscopic magnetic properties we observe are fundamentally a consequence of the tiny magnetic moments associated with individual electrons.

So each electron is like a tiny magnet.

In effect, yes.

And it acts like a magnet in two primary ways.

First, there's its orbital motion.

Picture an electron orbiting the nucleus like a tiny planet.

Because it's a moving electric charge, it creates a small current loop.

Like the coil we talked about earlier, but microscopic.

Exactly.

And that loop generates a tiny magnetic field and a corresponding magnetic moment along its rotation axis.

OK, that's one source.

What's the other?

The other is an intrinsic property of the electron called spin.

Each electron effectively spins around its own axis, kind of like a spinning top.

This spin also contributes another magnetic moment.

It can be in one of two directions, often visualized as spin up or spin down.

So orbital motion and spin both create magnetic moments.

Correct.

And this fundamental unit of magnetic moment, the smallest possible chunk associated with electron spin, is so important we have a special name for it.

The Bohr Magneton B.

It's a tiny but fundamental value.

So atoms have all these electrons orbiting and spinning.

Does the atom itself end up being a magnet?

It depends.

Often within an atom, electrons exist in pairs.

When they pair up, their orbital and spin moments frequently point in opposite directions and, well, they cancel each other out.

Cancellation.

Yes.

If all the electron shells in an atom are completely filled, there's total cancellation.

The atom ends up having no net magnetic moment.

And materials made of such atoms like the inert gases, helium, neon, argon, they can't be permanently magnetized.

So the unpaired electrons are key.

Unpaired electrons are absolutely key for permanent magnetism.

This atomic level interplay of electron moments canceling or not canceling is the foundation for all the different magnetic behaviors we see in materials.

Which brings us to the different types of magnetism.

You mentioned five.

Yes.

And they really span a huge range from incredibly weak effects to remarkably strong ones.

Left start with the absolute weakest.

Diamagnetism.

Diamagnetism.

OK.

This is a very subtle, non -permanent form of magnetism that's actually induced in all materials when you put them in a magnetic field.

Even things you think aren't magnetic at all.

So how does it work?

What happens is that the applied external field, H, ever so slightly changes the orbital motion of the electrons.

This change induces a tiny magnetic moment.

But here's the twist.

It points in the direction opposite to the applied field.

Opposite.

So it repels the field.

Exactly.

A very weak repulsion.

Diamagnetic materials have a relative permeability, R, just slightly less than one.

And their magnetic susceptibility is negative, usually tiny, around minus 10 year olds.

So the B field inside is slightly weaker than it would be in a vacuum.

That's right.

Visually, you could imagine atomic dipoles that only appear when an external field is present and they point against it.

But it's such a weak effect, honestly, that it's usually completely massed by other types of magnetism if they're present.

It really has few practical applications on its own.

OK, so that's diamagnetism, weak repulsion, present and everything.

What's next?

Next up is paramagnetism.

This is a weak attraction.

These materials do have permanent atomic dipole moments.

This happens because some electron orbital or spin moments don't fully cancel out within the atom.

So they have tiny inherent magnets.

They do.

But here's the catch.

In the absence of an external magnetic field, these tiny atomic magnets are all pointing in random directions.

Thermal energy keeps them scrambled.

So there's no net magnetism overall for the bulk material.

OK, random compass needles again.

What happens when you apply the H field?

When you apply an external field, these individual permanent dipoles feel a torque and tend to rotate and align with the field.

This alignment enhances the internal field slightly, causing a weak attraction.

So they line up with the field, reinforcing it a bit.

Exactly.

But they act individually without strongly influencing their neighbors.

Think of that crowd of tiny compasses, again randomly oriented initially,

bring a large magnet nearby and they all sort of pivot to point generally in the magnet's direction.

That's a great visual.

So paramagnetic materials are weakly attracted.

Right.

Their relative permeability R is just slightly greater than one.

And they have a small positive magnetic susceptibility, typically between 10 or and 10 hours.

And like magnetism, this effect disappears when you remove the external field.

Absolutely.

The thermal energy just randomizes the dipoles again.

That's why both diamagnetic and paramagnetic materials are generally considered non -magnetic in everyday terms.

Their magnetization is relatively small and requires an external field to even exist.

OK, weak repulsion, weak attraction.

Now for the strong stuff.

Now for the strong stuff, ferromagnetism.

This is what most people think of when they hear magnetism.

These materials like iron, cobalt and nickel, and some rare earth elements display a permanent magnetic moment.

They can have very large permanent magnetizations even without an external field applied.

Wow.

So their susceptibility must be huge.

Enormous.

Can be as high as 10 euro, a million or more.

This means their own internal magnetization M is vastly larger than the applied field term H.

So the total field B is basically dominated by the material's own contribution.

What makes them so different?

Why the permanent alignment?

It's because of strong quantum mechanical coupling interactions between the electron spins of adjacent atoms.

It's not just a weak tendency to align like in paramagnetism.

Here the spins strongly force their neighbors to align parallel with them.

It's a cooperative inherent alignment.

Like a chain reaction to alignment.

Sort of.

Yeah.

This strong mutual alignment creates large regions within the material called magnetic domains.

Within each domain, which can contain billions of atoms, all the tiny magnetic moments are perfectly aligned, parallel to each other.

The domain itself is essentially magnetically saturated.

Okay.

So a chunk of iron isn't one giant magnet, but lots of these tiny, perfectly magnetized domains.

Initially, yes.

We'll get back to that.

But within a domain, you have this maximum possible alignment.

We call the magnetization value in the state, the saturation magnetization, missers.

It's the absolute maxima magnetic moment per unit volume a material can achieve.

And you can calculate this?

You can quite precisely.

If you know the crystal structure of the material, like BCC for iron, how many atoms are in a unit cell, the atomic weight and density to find atoms per volume, and crucially, how many uncancelled bore magnetons each atom contributes, you can calculate, miss.

Fascinating.

Okay.

Ferromagnetism strong permanent due to spin coupling and domains.

Are there other strong types?

There are two more main types closely related, but with key differences.

First, there's antiferromagnetism antiferro.

Sounds like the opposite.

It basically is at the atomic level in antiferromagnetic materials.

The coupling interactions cause the magnetic moments of neighboring atoms or ions to align in exactly opposite directions.

This is called anti -parallel coupling.

So perfectly alternating up, down, up, down spins.

Exactly.

And the result, even though there are strong individual magnetic moments on each atom, they perfectly cancel each other out macroscopically.

So the material as a whole possesses no net magnetic moment.

So it's strongly magnetic atom by atom, but overall appears non -magnetic.

Correct.

Manganese oxide MNO is a classic example.

Picture those equal strength magnets side by side, but pointing exactly opposite to each other, they cancel out any external effect.

Weird.

Okay.

What's the last type?

The last major category is ferromagnetism.

Notice the iferromagnetism.

These materials also show permanent magnetization.

Similar macroscopically to ferromagnets, but the underlying mechanism is different and they are typically found in ceramic compounds, often oxides.

Okay.

So permanent magnets, but ceramic, how does it work?

It also involves anti -parallel spin coupling between different ions, like in antiferromagnetism.

But here the cancellation of magnetic moments is incomplete.

The opposing moments are unequal in magnitude, or there's an unequal number of them pointing opposite ways.

So they try to cancel, but don't quite manage it.

Exactly.

There's a net magnetic moment left over.

A classic example is magnetite ferro, which is a type of ceramic called a cubic ferrite.

It contains iron ions in two different oxidation states, FeO and ferro, along with oxygen ions, which are magnetically neutral.

How does the cancellation work there?

It's related to the crystal structure called the inverse spinal structure.

Basically the fat ions occupy two different types of sites in the crystal lattice and their magnetic moments, which are coupled anti -parallel between these site types, completely cancel each other out.

Okay.

So the fair moments are gone.

What's left?

What's left are all the FeO ions.

They typically sit in one type of site, octahedral, and all their magnetic moments are aligned parallel in the same direction.

This uncancelled moment from the FeO ions gives the material its overall net magnetization.

So it's the FeO ions that make magnetite magnetic.

In essence, yes.

Their moments survive the cancellation.

And again, knowing the number of Bohr magnetons for FeO and the crystal structure allows us to calculate the saturation magnetization for magnetite.

That's really clever.

It sounds like you could tweak the magnetism by changing the ions.

You absolutely can.

This is a huge area of materials design.

For instance, you can increase the saturation magnetization of a ferrite like magnetite by substituting some of the FeO ions with a different divalent ion, like manganese, which happens to have a higher magnetic moment.

Five Bohr magnetons compared to four for FeO.

So you can engineer the magnetic properties by playing with the chemistry.

Precisely.

And a major advantage of these ferromagnetic ceramics or ferrites is that they are typically electrical insulators.

This is crucial for high frequency applications, like in transformers or inductors working at radio frequencies, because it prevents energy loss from unwanted electrical currents, eddy currents that magnetic fields can induce in conductors.

Right.

Insulators don't allow those currents easily.

OK, that covers the main types.

What about temperature?

You said it was a big factor.

It's a huge factor, especially for the strongly magnetic materials, ferro and ferromagnets.

Think about heat as atomic vibration.

Increasing the temperature increases these vibrations.

Makes sense.

Atoms jiggle more when hot.

And these vibrations tend to disrupt the orderly alignment of those magnetic moments.

They knock them out of alignment, essentially fighting against the coupling forces that want to keep them parallel.

So higher temperature means less perfect alignment.

Exactly.

For ferro and ferrimagnets, the saturation magnetization, EMS, is at its maximum value, only at absolute zero zero Kelvin.

As the temperature increases, EMS gradually decreases because the thermal vibrations cause more and more misalignment.

OK, gradual decrease.

But then something dramatic happens.

At a specific critical temperature called the Curie temperature,

the thermal energy becomes string enough to completely overcome the mutual spin coupling forces that hold those domains aligned.

Completely overcome them.

Completely.

The long range magnetic order is destroyed.

The saturation magnetization abruptly drops to zero.

Wow, it just switches off.

It effectively switches off.

Imagine a gratch showing EMS versus temperature.

It starts high at low T, dips gradually as T increases, and then suddenly plummets to zero right at TC.

Above the Curie temperature, the material loses its permanent magnetism and behaves just like a paramagnetic material.

The individual moments are still there, but they're randomized by thermal energy unless you apply an external field.

What are typical Curie temperatures?

They vary a lot.

For iron, it's 768 degrees C.

For cobalt, it's even higher, 1120 degrees C.

Nickel is lower, 355 degrees C.

For the ferret magnetite, it's 585 degrees C.

Antifaramagnets also have a similar critical temperature called the Neil temperature, above which they become paramagnetic.

OK, that's a really critical concept, the Curie temperature.

Now, you mentioned domains earlier for ferromagnets.

Let's dive into that.

Right.

Domains and hysteresis are absolutely fundamental for understanding how these strong magnets actually behave in bulk.

So below the Curie temperature, ferro and ferromagnetic materials aren't typically one single giant magnet.

They are composed of these small regions we call domains.

And within each domain.

Within each domain, all the magnetic dipole moments are perfectly aligned, mutually parallel.

The domain itself is at saturation magnetization.

But the whole piece of material might not be magnetized.

Exactly.

Because a macroscopic piece of, say, iron that hasn't been magnetized yet contains many of these domains.

And the direction of magnetization from one domain to the next is different, often random.

So their individual magnetizations average out and the overall material has no net magnetic moment.

It appears un -magnetized.

How do they separate?

Are there sharp boundaries?

They're separated by relatively thin boundaries called domain walls or domain boundaries.

Across the width of this wall, the direction of magnetization gradually changes or rotates from the orientation in one domain to the orientation in the neighboring domain.

OK, so domains are like little magnetic kingdoms and the walls are the transition zones.

What happens when you apply an external H field to this un -magnetized piece?

This is where it gets dynamic.

If you start with an un -magnetized material, random domains, and slowly increase the external H field, the internal B field starts to increase.

Initially, it increases slowly, then it increases rapidly, and finally, it levels off.

Levels off at saturation.

Levels off at the saturation flux density, Bs, and saturation magnetization, Ms.

What's happening microscopically during this process is fascinating.

Tell me.

First, as you apply a small H field, the domains that happen to be already oriented favorably with the field, meaning their magnetization is roughly aligned with H start to grow by moving the domain walls.

They consume the less favorably oriented domains.

So the well -aligned domains get bigger.

Yes, the domain walls shift.

As you increase H further, this growth continues, and eventually favorably oriented domains might take over most of the material.

Then, for the final push to saturation, the magnetization direction within those remaining domains actually rotates away from its original, easy crystallographic direction to become perfectly aligned with the external H field.

So wall movement first, then rotation.

Generally, yes, wall movement is usually easier at lower fields.

Rotation happens at higher fields to reach full saturation.

At saturation, the entire sample has essentially become one single large domain perfectly aligned with H.

OK, now what happens if you take the H field away?

Does it go back to random domains?

Ah, this is the crucial part.

It doesn't simply retrace its path.

If you start from saturation and begin reducing the applied H field, the B field inside the material decreases, but it doesn't follow the original curve back down.

It lags behind the applied H field.

This phenomenon is called hysteresis.

Hysteresis, meaning it lags.

Yes, from the Greek word for lagging.

OK, even when you reduce the applied field H all the way back to zero, there's still a significant magnetic field B remaining inside the material.

This residual B field is called the remanence BR.

And that's why it could be a permanent magnet, because it remembers the magnetization.

Exactly.

The remanence is the memory of the alignment.

To force the internal magnetic field B back down to zero, you actually need to apply an H field in the opposite direction.

You have to actively push it back to zero.

You do.

The magnitude of this opposing H shield required to bring B back to zero is called the coercivity.

Remanence BR is the leftover magnetism.

Coercivity HC is the resistance to demagnetization.

Perfect summary.

If you continue applying the H field in the negative direction, you'll eventually reach saturation in the opposite direction, B, S, Eps.

And then if you bring H back through zero towards positive saturation again, you trace out the other half of the loop.

This closed loop, traced out by cycling H, is the hysteresis loop.

It graphically shows the material's magnetic memory and energy loss.

Energy loss, how?

The area enclosed within the hysteresis loop actually represents magnetic energy that is lost, converted into heat during each cycle of magnetization and demagnetization.

Interesting.

So if you want to demagnetize a permanent magnet, why do you do it?

Just apply the coercive field.

Applying HC only gets B to zero while HC is applied.

To truly demagnetize it, you need to essentially scramble the domains again.

You do this by cycling the material repeatedly through an H field that alternates direction, positive to negative and back, while gradually decreasing the magnitude of the alternating field down to zero.

This shakes up the domains and leaves them in a more random state.

OK, that makes sense.

And you mentioned earlier how different ferroferromagnets are from paradiamagnets.

The hysteresis loop really highlights that, doesn't it?

Absolutely.

If you were to plot B versus H for a diamagnetic or paramagnetic material, you'd just get a straight line through the origin with a very, very shallow slope.

Compare that to the fat hysteresis loop of a ferromagnet.

The difference in the magnitude of B for the same applied H is staggering orders of magnitude greater.

It visually confirms why we call the former non -magnetic in practice.

Right.

The response is just tiny in comparison.

Now, you also touched on crystal direction mattering, anisotropy.

Yes, magnetic anisotropy.

This is another important factor, especially in single crystals or materials with aligned grains.

It means the ease of magnetization depends on the crystallographic direction along which you apply the magnetic field H.

So it's easier to magnetize along certain crystal axes.

Exactly.

For every ferromagnetic material, there's at least one crystallographic direction called the direction of easy magnetization.

This is the direction where you reach saturation magnetization at the lowest applied H field.

Can you give examples?

Sure.

For nickel, which is FCC, the easy directions are the body diagonals, like the 111 direction.

For iron, BCC, the easy directions are the cube edges, the 100 type directions.

For cobalt, HCP, it's along the hexagonal axis.

The 0001 direction.

And they're also hard directions.

Yes, the hard directions are those where it takes the highest H field to achieve saturation.

So the BH curve and the hysteresis loop will look different depending on whether you measure along an easy or a hard direction.

Is this anisotropy just a curiosity or is it useful?

Oh, it's hugely important practically.

For example, in transformer cores, which are often made of iron silicon alloys, manufacturers go to great lengths to process the material so that the easy magnetization direction, 100 for iron, aligns with the direction the magnetic field will operate in the transformer.

This minimizes the energy needed to magnetize the core during each AC cycle, reducing hysteresis losses.

That's clever material processing, aligning the easy access with the field.

Very clever.

It often involves carefully controlled rolling procedures to create a specific grain texture where most crystals line up the desired way.

This seems like a good point to talk about classifying materials based on these properties,

specifically soft and hard magnetic materials.

You mentioned the hysteresis loop area is energy loss.

Exactly.

That energy loss, which manifests as heat, is often undesirable, especially in devices operating with alternating magnetic fields.

This leads to the first category, soft magnetic materials.

Soft, meaning easy to change.

Precisely soft magnetic materials are characterized by having a narrow, skinny hysteresis loops.

This means they have low energy losses per cycle.

They also typically have high initial permeability, meaning they respond strongly to small fields and crucially, low coercivity.

Low coercivity means it's easy to demagnetize them, right?

Yes, they are easily magnetized and easily demagnetized.

This makes them ideal for applications where the magnetic field is constantly changing or reversing, like in transformer cores, electric motors, generators and switching circuits.

How do you make a material magnetically soft?

Well, the saturation magnetization is mostly set by the material's composition.

But the properties we care about for softness, susceptibility and coercivity are very sensitive to the material's microstructure.

Low coercivity means the domain walls can move easily when the field changes.

So you want smooth sailing for the domain walls.

You got it.

Things like impurities, structural defects, internal stresses or inclusions of non magnetic particles can act like roadblocks pitting the domain walls and hindering their motion.

This increases coercivity.

So soft magnetic materials need to be manufactured to be as free of such defects as possible.

High purity, large grains, minimal stress.

Like that iron silicon alloy for transformers you mentioned.

Exactly.

That's a prime example.

They use specific compositions around 3 % silicon and iron and processing like that grain alignment rolling to achieve low hysteresis losses and high permeability.

What about those 80 currents you mentioned earlier?

The unwanted electrical currents.

Good point.

For applications involving alternating fields, especially at higher frequencies, you also want the soft magnetic material to have high electrical resistivity.

This limits the flow of those induced eddy currents, which also cause energy loss and heating.

How do you get high resistivity?

Alloying helps adding silicon or nickel to iron increases its resistivity.

But for really high frequency work, the best solution is often to use those ceramic ferrites we discussed under ferromagnetism Because they are electrical insulators by nature, their resistivity is extremely high, effectively eliminating eddy current losses.

OK, so that's soft materials.

Easy come, easy go magnetism, low loss used in AC applications.

What about the opposite?

Hard materials.

Hard magnetic materials are the opposite.

They are designed to be permanent magnets.

They need a high resistance to demagnetization.

So they need wide hysteresis loops.

Exactly.

They are characterized by large, fat hysteresis loops.

This means they have high remnants, BR.

They retain a strong magnetic field after being magnetized and high coercivity.

It takes a strong opposing field to demagnetize them.

They also usually have a high saturation flux density, these.

Since their loops are wide, they have high hysteresis losses.

But that doesn't matter as much because they aren't typically cycled repeatedly in applications.

Their job is just to be a magnet permanently.

Precisely.

The key performance metric for a hard magnet is often the maximum energy product denoted as BH max energy product.

Yes.

If you look at the second quadrant of the hysteresis loop, where B is positive, but H is negative, representing the demagnetizing region, BH Max is the area of the largest rectangle you can draw under that curve.

A larger BH Max value indicates a stronger or harder permanent magnet.

It stores more magnetic energy and is more resistant to demagnetization.

How do you achieve this hardness?

If softness requires easy domain wall motion, hardness must require.

Exactly the opposite.

Hard magnetic materials are designed and processed specifically to impede domain wall motion, making them much harder to demagnetize once they've been magnetized.

This is often done by creating microstructures with lots of defects, fine particles or specific phases that pin the domain walls very effectively.

What are some examples of hard magnetic materials?

There's a range.

Conventional hard magnets include things like tungsten or chromium magnet steels, where tiny carbide precipitates obstruct domain walls.

Alloys like cunefe and the well -known alnico alloys, aluminum, nickel, cobalt, iron, which often rely on tiny, strongly magnetic particles embedded in a nonmagnetic matrix.

And also hard hexagonal ferrites like barium ferrites, which are ceramic magnets often used in inexpensive applications.

Their BH Max values range from maybe two up to 80 kilojoule minimum.

OK, those are the conventionals.

Are there better ones now?

Oh, yes.

The development of high energy hard magnets has been revolutionary, especially those based on rare earth elements.

These are typically intermetallic compounds.

Two main families dominate, which are first Cimmerian cobalt magnets like Semco.

These were developed in the 60s and 70s and have very high energy products.

One hundred and twenty two hundred forty kilojoules and large coercivities.

They are usually made using powder metallurgy, grinding the alloy into fine powder, aligning the powder particles in a magnetic field, pressing them together and then sintering at high temperature.

And the second family, I feel like I've heard of these.

You probably have neodymium iron boron magnets, often just called neomagnets and fair developed in the early 80s.

These have become the material of choice for high performance applications because they offer performance rivaling or exceeding Semco.

But neodymium is less expensive and more abundant than Cimmerian.

How are they made?

They can also be made by powder metallurgy, similar to Semco.

But another common method is rapid solidification, sometimes called melt spinning.

The molten alloy is quenched extremely rapidly onto a spinning wheel, forming a fine grained ribbon.

This microstructure is inherently good for hard magnetic properties.

So why use permanent magnets instead of electromagnets, which you can turn on and off?

Great question.

Permanent magnets offer continuous magnetic fields without needing any electrical power input.

This means they generate no heat during operation, unlike electromagnets, which have resistive losses.

They can also lead to much smaller, lighter and often more efficient designs, especially in motors.

Where do we see these hard magnets used?

They are absolutely everywhere now, especially the high energy ones.

Think about motors and cordless drills, power tools, hybrid and electric cars, traction motors, power steering, window lifts, actuators, sensors, loudspeakers, lightweight high performance earphones, hearing aids, computer peripherals like hard drive actuators.

The list is huge.

Wow.

OK,

that dependence on magnets leads perfectly into magnetic storage.

You mentioned hard drives.

Yes, magnetic recording is basically the universal technology for storing vast amounts of digital information, whether it's computer data, sound or video.

The information is ultimately stored by changing the magnetic state of tiny segments on a magnetic medium, typically a tape or a desk.

So writing is magnetizing and reading is sensing the magnetization.

That's the core idea.

Let's look at hard disk drives, HDDs first.

These use rigid circular discs or platters coated with a magnetic material spinning at very high speeds, typically 5400 to 7200 revolutions per minute or even faster.

And how is the data stored on there?

The dominant technology today is called Perpendicular Magnetic Recording, or PMR.

In PMR, the tiny magnetic regions, the bits representing ones and zeros, are oriented with their magnetization pointing either up or down perpendicular to the surface of the spinning disk.

Perpendicular.

OK, how does the writing happen?

There's a tiny right head that flies incredibly close to the disk surface.

It's essentially an electromagnet, an electric current flowing in a coil within the head generates a time varying magnetic flux concentrated at the tip of a main pole.

This creates a very intense localized magnetic field right underneath it, which magnetizes a small region of the storage layer on the desk either up or down.

And when the head moves away, that spot stays magnetized.

Exactly.

The material used for the storage layer has enough coercivity to retain that magnetization that's the stored bit.

How is it read back?

That uses a separate, highly sensitive read head, often based on a phenomenon called magnetor resistance.

This head senses the faint magnetic fields emanating from the written bits on the disk.

These fields cause a measurable change in the electrical resistance of the red head sensor.

By detecting these resistance changes, the original pattern of ones and zeros can be reproduced.

What's the storage layer itself made of?

It's typically a very thin film, maybe only 15 to 20 nanometers thick.

And it's not continuous.

It's composed of incredibly tiny, physically isolated magnetic grains, perhaps only around 10 nanometers in diameter.

These grains are usually a cobalt chromium alloy with platinum, often having an HCP crystal structure.

And each grain acts like a tiny magnet?

Yes.

Ideally, each grain is a single magnetic domain.

They are processed so that their crystallographic easy axis of magnetization, the zero zero zero one direction for HCP coalloys, is oriented perpendicular to the disk surface, which is perfect for PMR.

Are there limits to how small you can make these bits?

Definitely.

For reliable storage, you need a collection of grains, maybe around 100 or so, to represent a single bit to average out noise.

Also, if the grains become too small, thermal energy can become strong enough to randomly flip their magnetization over time, causing data loss.

This is called the superparamagnetic limit.

And it's a major challenge in pushing storage densities higher.

What densities are we talking about now?

Current HDDs achieve densities well over 100 gigabits per square inch.

And the industry is pushing towards one terabit per square inch and beyond using new technologies like heat assisted magnetic recording, HAMR, or microwave assisted magnetic recording, MAR, to overcome the superparamagnetic limit.

Incredible density.

What about the other common magnetic storage magnetic tapes?

Right.

Tapes are still widely used, especially for data backup and archiving, primarily because they offer very low cost per gigabyte, even if their storage densities and access keys are lower than HDDs, density might be about 100 times lower.

How do they work?

Is it similar?

The principle is similar, magnetizing small regions.

But the medium is obviously different.

You have a flexible tape wound between reels passing across a read write head system.

What's the magnetic material on the tape?

Instead of a thin film like in HDDs, tapes typically use tiny magnetic particulates embedded in a binder.

These particles are maybe tens of nanometers in size.

Traditionally, they were needle shaped ferromagnetic metal particles, often pure iron or iron cobalt alloys.

More recently, plate shaped ferromagnetic barium ferrite particles are common.

So tiny magnetic particles glued onto a plastic strip.

Essentially, yes.

These particles are uniformly dispersed within an organic polymer binder, which is then coated as a thin magnetic layer onto a flexible nonmagnetic substrate like a sheet of pit or pen plastic film.

Do the particles have a preferred direction?

They do.

These particles are specifically designed to be magnetically anisotropic, meaning they have an easy direction of magnetization along their length for needles or within their plane for plates.

During the tape manufacturing process, these particles are physically aligned, usually by a magnetic field.

So their easy access is parallel to the direction the tape moves.

So the right head can easily magnetize them either forward or backward along the tape direction to represent one's.

Precisely modern tape technologies like LTO linear tape open can achieve really impressive capacities on a single cartridge, for instance.

LTO eight offers 12 terabytes uncompressed.

So tape is definitely still relevant for large scale data archiving.

OK, that covers storage.

Now for the final really mindbending topic,

superconductivity.

Ah, yes, superconductivity.

It's a truly remarkable electrical phenomenon, put simply, as certain materials are cooled down to very low temperatures, their electrical resistivity, which normally just decreases gradually, suddenly and abruptly plunges from a finite value to virtually zero resistance, like absolutely none for all practical purposes.

Yes.

Below a certain critical temperature, T .C., the resistance becomes immeasurably small.

It's like flipping a switch from normal conductor to perfect conductor.

How low are these critical temperatures for traditional superconductors, which are often simple metals or alloys?

T .C.

is usually very low, typically below 20 Kelvin and actually 253 degrees Celsius.

So you need liquid helium to cool them down, which is expensive and difficult to handle.

Is it just temperature dependent or are there other factors?

It's not just temperature.

The superconducting state is actually quite fragile.

It can be destroyed and the material reverts back to its normal resistive state if you exceed not only the critical temperature, T .C., but also a critical magnetic field, H .C., or a critical current density.

There's a three way limit.

Temperature, field and current density.

Exactly.

You can visualize it as a 3D phase space.

As long as the material stays within the boundary defined by T .C., H .C.

and J .C., it's superconducting across any of those boundaries and it becomes normal.

What's the underlying physics?

How can resistance just vanish?

The detailed theory, BCS theory, is quite complex, involving quantum mechanics.

But the basic idea is that below T .C., the conducting electrons form pairs called Cooper pairs.

These pairs interact with the vibrations of the crystal lattice in a special way that allows them to move through the material without scattering off imperfections or thermal vibrations, which is what normally causes resistance.

They move coherently like a superfluid.

Wow.

OK.

Are all superconductors the same?

No, they are broadly classified into two types based on their behavior in a magnetic field.

Type I superconductors are usually pure metals like aluminum, lead, tin, mercury.

Below their critical temperature and critical field, they exhibit perfect diamagnetism.

Perfect diamagnetism.

We talked about weak diamagnetism earlier.

This is much stronger.

Type I superconductors in the superconducting state completely exclude all applied magnetic fields from their interior.

If you try to apply an external field, they generate surface currents that create an opposing magnetic field, perfectly canceling the applied field inside.

This is known as the Meissner effect.

Is that why magnets can levitate above superconductors?

Precisely.

The levitating magnet is actually being repelled by the magnetic field the superconductor is actively pushing out due to the Meissner effect.

This perfect field exclusion persists until the applied field reaches the critical value, Hc, at which point the superconductivity is abruptly destroyed.

The field penetrates and the material becomes normal again.

OK, that's type one.

What about type two?

Type two superconductors are generally alloys or compounds like niobium titanium or niobium tin and BN.

They also exhibit the Meissner effect, perfect diamagnetism, at very low applied fields.

However, their transition back to the normal state as you increase the magnetic field is gradual.

Gradual.

How so?

They have two critical fields, a lower critical field, Hc1, and a much higher upper critical field, Hc2.

Below Hc1, they exclude all flux, Meissner effect.

Above Hc2, they are fully normal.

But between Hc1 and Hc2, they exist in a mixed state.

Big state.

Yes.

In this mixed state, the magnetic field is allowed to penetrate the material, but only in specific quantized tubes or filaments called fluxons or vortices.

The regions inside these flux tubes are normal, but the bulk material surrounding them remains superconducting.

So it's like a mix of superconducting and normal regions.

Exactly.

And because Hc2 for type two superconductors can be much higher than Hc for type I materials, type two are generally preferred for applications involving high magnetic fields, like building superconducting magnets.

That makes sense.

You mentioned needing liquid helium.

Yeah.

Has there been progress on higher temperature superconductors?

This is where things got really exciting starting in the 1980s.

Researchers discovered complex oxide ceramics, often containing copper oxide layers, that exhibit superconductivity at much higher critical temperatures initially above the boiling point of liquid nitrogen, 77 K, and now even up to around 135 K under normal pressure and higher under extreme pressure.

Above 77 K.

That's a huge deal.

Liquid nitrogen is way cheaper and easier than liquid helium.

It's a game changer for potential applications.

A famous example is utrium barium copper oxide, Y by Euroriojo, often called YBCO, which has a Tc of about 92 K.

These are often called high Tc superconductors.

What's the catch?

Why aren't they everywhere yet?

The main challenge is that these high Tc ceramic materials are inherently brittle.

It's very difficult to fabricate them into useful forms like long, flexible wires or tapes that are needed for magnets or power cables, while still maintaining their excellent superconducting properties.

There's been progress, but it's still a major hurdle.

Riddleness.

OK.

But the potential applications must be enormous.

Absolutely.

Superconductivity is already crucial for high field magnets used in scientific research like particle accelerators and perhaps most visibly in magnetic resonance imaging MRI machines for medical diagnostics.

The strong, stable fields needed for MRI are generated by superconducting magnets, usually meant from type two alloys like niobium, titanium, cooled with liquid helium.

What about future possibilities?

The possibilities are vast, especially if high Tc materials become more practical.

Think about extremely efficient, lossless electrical power transmission over long distances, powerful compact magnets for fusion energy reactors,

faster computer components based on superconducting switches like Josephson junctions and maybe even widespread high speed magnetically levitated maglev trains.

Wow.

Lossless power grids and maglev trains.

The potential is definitely there, but the main deterrent remains the difficulty and cost associated with achieving and maintaining the necessary low temperatures, even with the high Tc materials requiring only liquid nitrogen progress is being made.

But we're not quite there for widespread use yet.

OK, that was a fantastic journey through magnetism and superconductivity.

Let's try to quickly recap the main takeaways.

Sounds good.

So first, magnetic properties fundamentally arise from electron orbital and spin magnetic moments within atoms.

Right.

And we saw a whole spectrum of behaviors based on how these moments interact.

Diamagnetism, that weak universal repulsion.

Paramagnetism, weak attraction due to random permanent moments aligning.

Ferromagnetism, strong permanent magnetism from cooperative spin alignment in domains.

Anti -ferromagnetism, where neighboring moments cancel out completely and ferromagnetism, incomplete cancellation, often in ceramics, leading to net permanent magnetism.

We learned how crucial temperature is, especially the Curie temperature where ferro and ferrimagnets abruptly lose their permanent magnetism and become paramagnetic.

And the concept of hysteresis, the lagging response of B to H, which gives us the hysteresis loop, defining a material's magnetic memory through its remnants, Br, and its resistance to demagnetization, the coercivity HD.

That loops area also tells us about energy loss, leading to the classification of materials as magnetically soft, easy to magnetize, demagnetize, low loss, narrow loop used in AC applications or hard permanent magnets, high resistance to demagnetization, high Br and Hc wide loop.

We saw how magnetic storage technology in both hard disk drives, HDDs using PMR on thin films and magnetic tapes using aligned particles relies on precisely creating and reading tiny magnetized regions to store digital data.

And finally, the amazing phenomenon of superconductivity, zero electrical resistance below a critical temperature, critical magnetic field and critical current density.

We distinguished type I, Meissner effect, low Hc and type two mixed state high Hc two and touched on the promise and challenges of high T .C.

ceramic superconductors.

I think that covers the highlights.

It's a rich field with deep physics and incredibly important applications.

Absolutely.

This has been a deep dive into the magnetic properties of materials.

Thank you so much for joining us on this exploration.

My pleasure.

So as we look to the future, here's something to think about.

What if we could truly engineer those high temperature ceramic superconductors to be not just high T .C.

but also ductile and easily formable like regular metal wires?

How would that fundamentally change everything from our global power grids becoming near lossless to the speed of computation, maybe even enabling everyday objects to defy gravity using magnetic levitation?

The material science challenges are immense, but the potential is truly electrifying.

On behalf of the deep dive team, we hope this deep dive helps you master these concepts.