Thursday, March 07, 2019

APS March Meeting wrapup

I spent the lion's share of today talking w/ my collaborators.  This was great scientifically, but meant that I only went to a couple of talks. 

  • This one was pretty slick.  If you look at the conduction properties of a Josephson junction as a function of magnetic field through it, you see a Frauenhofer pattern as a function of the enclosed flux (see this pdf, fig 2).  In principle, taking the inverse Fourier transform of this should reveal the real-space current distribution as a function of the distance along the width of the junction.  This group made Josephson junctions using oriented thin pieces of WTe2.  When the current flowed along one direction, they found that the Josephson current was mostly flowing near the edges of the strip of material.  When current flowed along a different direction in the plane, the current distribution was much more uniform.  
  • Similarly evocative, this talk presented work using magnetic focusing plus scanning gate microscopy plus collimating contacts to look at the real-space paths of electrons in a graphene-hBN bilayer w/ a Moire superlattice.  They could then infer the shape of the Fermi surface in momentum-space, confirming that the Moire superlattice results in a roughly triangular (miniband) Fermi surface.  Cooler than my jargon-heavy description sounds.
  • I greatly regret that I was unable to attend the invited session in honor of Millie Dresselhaus.  If one of my readers who did make it could please describe it in a comment, I'd appreciate it.
  • One other random note:  I did actually speak to the APS person who was in charge of the trade show, and I asked what the heck was up with the two weird "pain relief" booths, which seemed borderline late-night-infomercial/much more like something you'd see at a cheesy shopping mall.  This was apparently an experiment in allowing local vendors in, and it sounds very unlikely that it'll ever happen again.  
If I missed a big story from the meeting, please let me know in the comments.

Wednesday, March 06, 2019

APS March Meeting Day 3

A handful of semi-random highlights (broken up by my conversations w/ colleagues and catching up on work-related issues):

  • Laura Heydermann from ETH spoke about "artificial" magnetic systems, where mesoscopic, lithographically patterned arrays of magnetic islands can yield rich response.  A couple of representative papers are here and here, and recently they've been moving into 3D fabrication and magnetically sensitive imaging.  Very neat stuff.  
  • Christian Glatti from Saclay showed a very interesting result, analogous to the ac Josephson effect, but in fractional quantum Hall edge-state tunneling.  The relevant paper, just out in Science, is here.  This idea is, measure electronic shot noise as a function of bias voltage.  Ordinarily this has a minimum at zero bias, and the noise sits at the Johnson-Nyquist level there.  Now shine microwaves of frequency f on the device.  With photon-assisted tunneling, the net result is a change in the noise that has kinks at voltages of +/- hf/e*, where h is Planck's constant, and e* is the effective charge of the low-energy excitations.  Do this in the fractional quantum Hall regime, and you see fractional charge.  
  • On a related topic, Michael Pepper from Cambridge showed a very recent result.  In quantum point contacts at very low charge carrier densities, they see quantized conductance at some very surprising rational fractions of the usual conductance quantum 2e2/h.  I still need to digest this.  
  • I spent much of the afternoon at the big Kavli Symposium, on topics spanning from unit cell all the way to biological cells.  All excellent speakers.  I won't try to summarize this - rather, when the talks become available streaming, I will put the link here.  (Claudia Felser did bring donuts for the audience to talk about topology, always a crowd-pleaser.)

APS March Meeting, Day 2

A random selection from Day 2:

  • Thomas Silva at NIST gave a fun talk about some experiments using the linac coherent light source.  Using pump/probe time-resolved x-ray diffraction, they discovered some surprising acoustic modes in thin, polycrystalline metal films, with systematics suggesting that they might be seeing localization of phonons due to scattering off grain boundaries.
  • Along those lines, Gang Chen of MIT spoke about seeing reductions in thermal conductivity due to phonon localization.  His group was working with semiconductor superlattices, with little ErAs nanodots embedded in a disordered way at the superlattice interfaces.  They see systematics in the thermal conductivity that suggest that they are seeing Anderson localization of the heat-carrying phonons.  
  • I stopped by the session on conveying physics to a popular audience, and caught most of the talk by Allison Eck chock full of advice for would-be science writers, and a skyped-in talk by Sean Carroll about podcasting.  The depressing truth: If I really want to expand my audience, I should probably join twitter.  (The problem is, that's a conversational medium and I don't see how I could do it well given everything else.)
  • Abe Nitzan gave a prize talk that was a nice overview of the last decade's work on understanding electrons, photons, and phonons in molecular junctions.
  • I spent much of the afternoon at this session about the copper oxide superconductors.  Dan Dessau's talk primarily about this paper showed the capabilities of a new technique in analyzing angle-resolved photoemission data, to figure out the actual spatial shape of Cooper pairs in these systems. My collaborator Ivan Bozovic spoke (similar to this), showing the power of his tremendous MBE growth approach, able to create epitaxially perfect materials smoothly and systematically spanning the whole doping range.  The other talks in the session were also very interesting.

Monday, March 04, 2019

APS March Meeting, Day 1

A few things I saw at the APS Meeting today, besides 10 inches of fresh, wet snow on the ground this morning (disclaimer:  for various reasons I was session-hopping quite a bit, so this is rather disjointed):
  • Ignacio Franco at Rochester spoke about some experiments (here) that I'd not remembered, where carefully controlled, intense femtosecond light pulses were used to turn on a transient current in SiO2, normally one of the best insulators out there.  The theory is interesting, and made me start thinking about possible opportunities in this area.
  • A focus topic session on 2D magnetic materials was extremely crowded - so much so that I literally couldn't get in the room for the first talk.  Interesting talks, including Yujun Deng from Fudan presenting this workMasaki Nakano from the University of Tokyo spoke about growing epitaxial films of V5Se8, a cousin of a material with which we've worked; and Boyi Zhou at Washington University in St. Louis presented this work, which seems to show nontrivial electronic conduction in (ordinarily Mott insulating) monolayer RuCl3 layered on graphene.  Lots of interesting activity going on here, many fun ideas.
  • Naomi Ginsberg at Berkeley talked about some impressive imaging techniques used to follow energy flow in complex materials.  Combining super-resolution methods, interferometry, and time-resolved techniques is a heck of an enabling technique!
  • Peter Abbamonte at Illinois presented some remarkable measurements using an angle-resolved electron energy loss technique (M-EELS) to look at the strange metal state of a cuprate superconductor.  The main result is that this material seems to support a very broad plasmon mode with a lot of properties that are inconsistent with what you'd expect in a Fermi liquid, and may make connection with more exotic pictures of strange metals.  
  • Wojciech Zurek's talk about the foundations of quantum mechanics (based on this article) was very engaging (and apparently in a superposition of all possible fonts), though again the room was so full that people were sitting on the floor in the aisles and lining three walls.  The session also was running about 10 minutes ahead of schedule, which definitely was not great for people who ended up missing the beginning of Zurek's talk or Rovelli's before it.
The unwieldy size of the meeting is increasingly clear, with lines in the restrooms, and local fastfood places unable to handle the lunch crowd.  

Sunday, March 03, 2019

APS March Meeting, Day 0

A brief summary of topics/reading material/things I learned today during DCMP and joint DCMP/DMP executive committee meetings:
  • As usual, this will be the biggest March Meeting ever, with 11500 registrants ahead of time.  This is still increasingly problematic in terms of organization and availability of sites.
  • New APS Strategic Plan
  • New APS report on the Impact of Industrial Physics on the US Economy
  • DOE Basic Energy Sciences report (pdf) on the impact of the BES at its 40th anniversary
  • The upcoming privatization of the US Strategic Helium Reserve looks depressingly unavoidable.  Sounds like changing this is a non-starter in Congress.

Michelle Simmons and Si-based quantum computing

A last tidbit before the March Meeting.

Earlier this week, Prof. Michelle Simmons came to Rice for our Chapman Lecture series and gave a great talk about her team's project to develop quantum computing in a silicon platform, with individual phosphorus donor atoms as the qubits.   This idea goes back more than twenty years to this proposal by Bruce Kane.   Actually implementing this approach requires overcoming many technical challenges, including positioning individual phosphorus atoms inside single-crystal Si with nearly atomic precision, and similarly fabricating control and read-out electrodes in registry with those.  

Prof. Simmons' group has made truly remarkable progress in this direction.  The key enabling technique is using a scanning tunneling microscope (STM) as a lithography tool.  Single-crystal Si surfaces are prepared in ultrahigh vacuum and terminated with a hydrogen.  The STM tip is then used to strip off the hydrogen atoms with atomic precision.  (This is a serial technique, and so scaling up to the production numbers of the present-day Si industry would require something different, but for now it's fine.)  Phosphine gas decomposes in a particular way when exposed to the dangling bonds left behind by stripping the hydrogen, placing P atoms in particular locations.  This approach can also be used to make highly conductive wires and gates by doping, enabling transport measurements through single dopant atoms.   Growing more single-crystal Si on top of the dopants without having the dopants move around is another success story, making possible 3D fabrication schemes.  With isotopically pure Si, encapsulating the donors can give long coherence times.

There are many competing platforms for possible quantum computer implementations, and this approach is undoubtedly difficult.  In terms of technical achievement, though, this effort has shown the power of sustained support - progress has been truly impressive.

Thursday, February 28, 2019

APS March Meeting 2019

Once more, it is that time of year, when (mostly) condensed matter physicists gather in ever-increasing numbers to give and watch talks, network, try to get cool swag at the tradeshow, and generally grouse about the poor quality and high price of convention center coffee.  The March Meeting this year is in Boston for the first time since I've been going 2012.  (Strangely, I was unable to find a list of all the March Meeting sites online.  Perhaps a reader knows the last time the meeting was in Boston.)  It's my third and last year as a DCMP member-at-large, so it will be interesting to hear what comes up at the business meetings this time.   As I have done in past years, I'll do my best to write up some of what I see and give my impressions of the conference, though I may be more concise compared to previous years.

Tuesday, February 19, 2019

Why twisting materials is interesting

Twisted bilayer graphene is a hot topic, with 32 preprints on the arxiv using those keywords just since the beginning of the year.  It's worth explaining for non-experts, why this system, comprising two atomic layers of graphene twisted relative to each other by some angle, is so interesting. 

Let's start w/ the basics.  In the (non-relativistic) quantum world, we talk about the wavefunction \(\psi(\mathbf{r},t)\) of a system.  The Schroedinger equation describes how the wavefunction evolves with time, and by solving it we can find the particular energy levels ("stationary states") for a given problem.  The magnitude-squared of the spatial wavefunction, \(|\psi(\mathbf{r},t)|^2\) gives the probability of finding the particle in a particular place at a particular time.  

The wavefunction a free particle with a well-defined momentum \(\mathbf{p}\) can be treated as a wave with a wavevector \(\mathbf{p}/\hbar \equiv \mathbf{k}\), and therefore a wavelength \(2 \pi \hbar/|\mathbf{p}|\).   Higher momentum = shorter wavelength = the wavefunction has more closely spaced wiggles.  The kinetic energy goes like \(p^{2}/2m\), as in classical nonrelativistic mechanics.   (Note that the magnitude-squared of such a wave is constant as a function of spatial position.  That is consistent with the uncertainty principle:  Knowing the momentum precisely means that the position could be anything.)  

Take a particle and stick it in an environment where the local potential energy varies periodically in space - ideally in a system so large that we can neglect boundary effects for now.  The classic example of this is an electron in a crystalline solid.   I've talked about this kind of spatial periodicity before.  There are a couple of ways to think about this situation.  We have replaced "continuous translational symmetry" (the environment of the particle is unchanged if we consider moving the particle anywhere) with "discrete translational symmetry" (now we have to move an integer number of lattice spacings to get back to the same environment for the particle).  Mathematically, the single-particle stationary states can still be labeled by a parameter \(\mathbf{k}\), but they're Bloch waves rather than plane waves, and the energy \(E(\mathbf{k})\) is no longer necessarily proportional to \((\hbar k)^{2}\) all the time.   Physically, when the naive spatial periodicity of the single-particle state matches up with the spatial periodicity (or some harmonic) of the lattice, it makes sense that there should be deviations from what we'd see with a free particle.  The result is "band structure", ranges of energy densely filled with allowed single-particle states, separated by "band gaps", ranges of energy in which there is no way to make a single-particle state and still satisfy the Schroedinger equation with the spatially periodic potential energy.

The particular spatial periodicity of the lattice determines the form of \(E(\mathbf{k})\).  For a hexagonal lattice like single-layer graphene, it turns out that there are two "Dirac points", and that near those special values of \(\mathbf{k}\), the form of \(E(k)\) looks like what is obeyed by photons in free space (!), with energy linearly dependent on \(k\).

The key point here:  if we can tune the spatial periodicity of the potential arbitrarily, we can create interesting forms of \(E(\mathbf{k})\).  That's really a neat idea.  Carefully growing stacked multilayers of semiconductors along one direction has been used to create "minibands" for optoelectronic devices.  Starting from a 2D surface state in copper, people have been able to put down patterns of CO molecules to create spatial periodicities in 2D, creating structures that look and act like graphene, or very recently even fractals.  People have also tried doing this by patterning semiconductor structures, but it's very hard to get sufficient uniformity so that disorder isn't a problem.

Stacking graphene layers with some relative twist angle is a great way to create a 2D modulation with excellent uniformity over large areas (many many lattice spacings).  This 2D modulation shows up because of the Moire pattern, which gives a spatially periodic coupling between the bands in each of the layers.  By tweaking the relative angle, the spatial pattern can be tuned.  By squishing on the bilayer, in principle the strength of the coupling can be tuned.  This kind of 2D modulation should be possible in principle in twisted bilayers of all kinds of stackable materials.

The situation is even more interesting once we start thinking about electron-electron interactions.

Another way to think of bands:  Start from the atomic energy levels of the individual, isolated constituent atoms.  The electronic levels of each atom are sharply defined.  All of the 4s orbitals, say, have the same energy.  If you think about possible electronic states, the "band" made out of isolated (localized to individual atoms) 4s orbitals is very narrow in energy.  If you built up some linear combination of those 4s orbitals that had a parameter \(\mathbf{k}\), the energy \(E(\mathbf{k})\) of that state would basically be independent of \(\mathbf{k}\).  That is, the band would be "flat".   Turn on hopping between atoms, and band broadens out in energy. 

If we throw in a bunch of electrons and ask what is the lowest energy state of the many-electron system, we can often get away with mostly neglecting electron-electron interactions.  Because of the Pauli Principle, we fill up the bands from the bottom up, and very often the (single-particle kinetic + lattice interactions) energy grows very rapidly, so much so that any electron-electron interactions are not very important.   (That's what happens in the periodic table - as you go to atoms containing more and more electrons, the kinetic energy grows fast enough that e-e interactions don't really disrupt the basic hydrogen-like s-p-d-f orbital structure of energy levels.)

In the twisted bilayers, it is possible to end up with some bands that are very flat - so flat that the typical electron-electron interaction energy is comparable or large compared to the bandwidth.  In these flat band situations, electron-electron interactions can end up being very important in determining the collective many-body state of the electrons.  That appears to be what people are seeing in the experiments mentioned previously.

The bottom line:  Twisted stacking is a great, robust way to create a lateral spatially modulated potential, and therefore (within particular geometric limits) a "designer" band structure.  The resulting bands can be very flat, so that electron-electron interaction effects (apparently) can lead to remarkable many-body responses, like the onset of superconductivity or magnetism. 

Sunday, February 10, 2019

More brief items

Some additional interesting links:

  • Another example of emergent universal behavior, as it is demonstrated that runners at the start of a marathon seem to collectively obey hydrodynamics, like a fluid.
  • The Voices of the Manhattan Project oral histories effort has a large number of interviews online.  It’s important for posterity that these were recorded before everyone involved is gone.
  • Maybe massive open online courses were not, in fact, the end of the traditional model of higher education.  Who could have foreseen this?
  • There are people arguing that the preprint arxiv model is a good path toward opens access.  This is definitely something I like, especially more than models involving authors paying thousands of dollars to for-profit publishers for open access journals.

Wednesday, February 06, 2019

Brief items

This is the absolute most dense time of the year in terms of administrative obligations, so posting is going to be a bit sparse.  In the meantime, here is a bit of interesting reading:

Scientific American has an interesting article about the fact that two independent means of assessing the Hubble constant (analysis of the cosmic microwave background on one hand; analysis of "standard candles" on the other) disagree well outside the estimated systematic uncertainties.

Kip Thorne posted a biographical reminiscence about John Wheeler on the arxiv.  I haven't read it yet, but it's in my queue.

Quanta Magazine had put up a very well done article about turbulence.  Good stuff.  I liked the animation.

Tuesday, January 29, 2019

Three brief book reviews

In the spirit of Peter Woit's latest post, I also wanted to offer up three miniature book reviews.

The New Science of Strong Materials: Or Why You Don't Fall Through the Floor by J. E. Gordon.  This book is a fine, accessible (minimal math) introduction to materials science by one of the people who created the field as a distinct discipline.  The first edition came out in 1968, so it is a bit of an historical journey.  For example, the author describes how just recently people were able to achieve the first transmission electron microscopy images that directly showed dislocations.  The only way they could do it was to image a material that was actually a crystal of Pt-containing organic complexes - the Pt has high electron density for imaging, and the organic ligands keep the Pt spatially separated by a large enough distance (a few nm) to resolve in the equipment of the day.  Quite a difference from the present state of the art.  Gordon wrote in an engaging style with a dry UK wit, and clearly had a genuine fondness for wood as an amazing, versatile composite material.  Should be required reading for undergrad mechanical and civil engineers who need to get a real physical picture for stress, strain, and ways to mitigate crack propagation.  A fun read.

How to Invent Everything: A Survival Guide for the Stranded Time Traveler by Ryan North.
I won't spoil the amusing conceit that's used as a frame for this remarkable, fun collection of bite-sized bits of knowledge.   Suffice it to say that, in the event of a global collapse of civilization, this will be a handy tome to have on hand, should you need to recreate, say, agriculture or printing or distillation or the steam engine.  The recurring theme is, there are many societal and technological advancements that the human race seemed to be curiously slow to figure out (like, many tens of thousands of years slower than could have been done).  Just the kind of fun you would expect from the person who brought us Dinosaur Comics.  It does have a bit of a Randall Munroe What If vibe, but it's distinctive.

Math with Bad Drawings: Illuminating the Ideas That Shape Our Reality by Ben Orlin.
This was also very enjoyable.  Parts of it made me think that "Condensed Matter with Bad Drawings" would be a great approach, except that now it would seem hopelessly derivative.  The book takes a free-wheeling path through math in our lives, with large, healthy doses (perhaps a bit lengthy) of statistics (lies and damn lies - what different statistical quantities are telling and not telling you) and economics.  It's well done, and I particularly liked the beginning sections that explain what math really looks and feels like to a mathematician; that really resonated, and I wish I could convey even half as well that aspect of how physicists look at and think about the world around us.

Friday, January 25, 2019

"Seeing" atoms

The power of modern transmission electron microscopy (TEM) is very impressive.  Often in TEM images at high magnification, you can see what looks like the atomic lattice, but that can be a bit illusory.  Because the scattering effects of individual atoms, especially light ones like carbon, can be very slight, often those images are looking at the result from scattering off columns of atoms, with the crystalline structure of the material helping greatly to produce a clean image.  With state of the art instrumentation and processing, however, it is possible to resolve single atoms, even in atomically thin, light materials like graphene.  This image, from a new ACS Nano paper by Lee et al. from Oxford University, is a great example of what is now possible, showing the reconfiguration of carbon bonds as a nanoscale graphene constriction is modified.  Pretty eye-popping.

Sunday, January 20, 2019

Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!

Tuesday, January 15, 2019

This week in the arxiv

Twisted bilayer graphene (TBLG): Is there anything it can't do?  Two recent papers have appeared on the arxiv that show that TBLG looks like a versatile platform for exploring the physics of electrons in comparatively flat bands.  Remember, flat bands as a function of (crystal) momentum = high effective mass = tendency toward localization = interaction effects have an easier time dominating the kinetic energy.  There was a big splash at the APS meeting last year when superconductivity was found in this system that had some resemblance to the phenomena seen in the high-Tc cuprates.

arxiv:1901.03520 - Sharpe et al., Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
In this new work the Goldhaber-Gordon group at Stanford shows that, in TBLG, at the right gate voltage (that is, the right filling of the rather flat bands), the system seems to develop spontaneous ferromagnetism, seen both through the appearance of hysteresis in the electrical resistance as a function of magnetic field, and through the onset of an anomalous Hall signature.  Through non-local transport effects (e.g., drive a current over here, measure a voltage over there) they see indications of edge currents, suggesting that topology is important here. 

arxiv:1901.03710 - Cao et al., Strange metal in magic-angle graphene with near Planckian dissipation
Another feature of strongly correlated electronic materials like the cuprate superconductors is "strange metallicity", when the temperature dependence of the electrical resistance is linear with T over a large range, in contrast with simple expectations of Fermi liquid theory.  There have been arguments that in the limit of very strong electron-electron scattering, a kind of hydrodynamics kicks in for the electrons, with universal bounds on charge diffusion (and hence the resistance).  These arguments are sometimes based on fairly exotic ideas.   Not everyone agrees with the proposed universality.  In this new paper, the MIT group shows that the TBLG system does show resistance similar in form and magnitude to this strange metallicity.

The broad idea of tuning band flatness by stacking and rotating 2d materials continues to show promise as a playground for looking at the competition between different possible ordered states.

Tuesday, January 08, 2019

Magnetic data storage - heat-assisted v microwave-assisted

(Ironically, given my recent missive about the importance of condensed matter beyond applications to information technology, here is a post about condensed matter in information technology.)

It may seem like solid-state drives - basically flash memory - have taken over data storage, particularly in phones, tablets, and laptops.  However, magnetic storage media, particularly in the form of hard drives, are still the main tools of choice for the cloud.  Magnetic storage is very robust and doesn't rely on charge staying put on tiny floating gates to hold your information.  Rather, in a hard drive the zeros and ones of your data are encoded as magnetic domains of particular orientation in a specially engineered thin film of material on a disk platter. 

The amount of research and engineering development that has gone into hard drives is amazing.  The read/write heads "fly" at nanometer separation over incredibly smooth magnetic surfaces.  The smaller you can make the magnetic domains and still manipulate and read them in a controlled manner, the higher the density of the information storage.  The timing and positioning stability required to store and retrieve terabytes per square inch is amazing.  Reading the data requires detecting the magnetic fields produced by the domains.  These days that's done using magnetoresistance, some component of the read head with an electrical resistance that changes depending on the local magnetic environment.  Giant magnetoresistance went from the laboratory to hard drive read heads to Stockholm for the 2007 Nobel Prize in Physics.  Its successor in read heads, tunneling magnetoresistance, now rules the roost.

On the physics side, there are many challenges for continued scaling.  Magnetic domains interact with each other.  To be reliable for storage, it's important that the magnetization \(\mathbf{M}\) of a domain remain fixed once it is set.  The energy scales associated with pinning a domain tend to scale with domain size, meaning that, all other things being equal, tinier domains are easier to flip (for writing, yay, for long term stability, boo).  In the extreme limit, thermal fluctuations can provide enough energy to reorient the magnetization, leading to superparamagnetism.  A number of years ago, IBM and others came up with ways to increase the coercive fields (and anisotropy energies) of the bits in disk media.  Still, as bits get smaller, it's more important to pin them down, but somehow still allow deliberate reorientation of \(\mathbf{M}\) for write operations.

This article spurred me to write a little about this, in part because of what it gets wrong.  Hard drive manufacturers have decided that the best plan is to pin down the bits firmly, and then during the write process, locally kick those bits hard enough to allow reorientation of \(\mathbf{M}\). 

One method that's been under consideration for a while, and is the favorite of Seagate, is HAMR - heat-assisted magnetic recording.  This requires some means of locally heating the disk media right under the write-head so that thermal energy is available to help the bit reorient.  The Seagate approach uses an embedded laser source and a plasmonic antenna to drive the local optical heating (this has nothing to do with an electrical discharge, despite the linked article at the start of this paragraph).

Western Digital is pursuing an alternative approach microwave-assisted magnetic recording (MAMR).  The idea there (video) is to use a local oscillator (one based on spin torque) to generate microwaves locally at the write head, with the frequency of those microwaves tuned to drive ferromagnetic resonance of the bit to flip it.

Tons of physics in all of this, and an enormous amount of engineering cleverness.  Magnetic data storage will be with us for a while yet.