
Are Antiferromagnets the Next Step for MRAM? - rbanffy
https://spectrum.ieee.org/nanoclast/semiconductors/memory/antiferromagnets-next-step-mram
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physicsguy
I did my PhD in a related area of magnetics to this.

One of the problems is finding appropriate materials, and that applies for all
magnetic devices.

Crystal structures are hugely complex, and the way that atoms interact
manifests primarily through the 'exchange' interaction. This is caused by a
combination of the Coulomb interaction (charges repelling/attracting), and
through the quantum mechanical Pauli exclusion principle, which basically
stops electrons with the same state from sitting in the same position. The
problem is, while it's the dominant energy term, there are several forms of
exchange interaction - RKKY, Dzyaloshinskii-Moriya (which affects the
boundaries at interfaces and in certain crystals), etc. and so while the
length scale over which the exchange interaction acts is normally on the order
of a single atom, it can often actually act on next-nearest neighbour atoms
and cause odd effects. This means that finding a 'pure' antiferromagnet is
difficult; many things are actually ferrimagentic.

Many studies which suggest wonderful new storage technologies are
computational and have little bearing on the experimental reality. There are
serious difficulties in predicting the magnetic properties of materials with
density functional theory because the results are often oscillatory as you
refine the model, meaning that the computational cost is enormous - the
properties of even simple systems such as the two species alloy FePt can't be
easily predicted. Real crystal structures are hugely more complex than this,
and so it's questionable whether results given by people are any more than
junk. The work in this area is hampered by the fact that DFT research is
overwhelmingly focused on calculating bonding and chemical structures. DFT
scales poorly, and while there are a few codes that scale better, they're
licensed under weird license structures. There's also some odd fallings-out
between various code authors with accusations of stealing...

The 'next level' in the heirarchy of modelling neglects quantum effects
completely and is known as atomistic modelling. These models are still
enormously costly, but it's the only proper way to treat properly
antiferromagnetism, as the next level in the heirarchy, micromagnetism, takes
a continuum limit of this discrete model, and by doing so has assumptions that
antiferromagnets break. Parameterising these models is enormously difficult,
because it is pretty much impossible to measure experimentally the values that
you need, and DFT gives answers which when used bear little resemblance to the
real material.

You have to sort of realise that many people in magnetics have lost funding
because industry has moved on, and academic labs seem to be left behind. So
everyone tries to work on the 'next big thing' \- there are hundreds of
various proposals for devices that haven't worked out, and it's worth being
very, very sceptical of articles like this which promise that a new technology
will revolutionise things. Just as an example, one of the biggest wastes of
time over the last 10 years has been that of Skyrmions, which have been cited
as the possible hallelujah of magnetic storage, and for which there is
basically no evidence of utility after millions of dollars of research funding
worldwide, and thousands of articles. Many scientists jumped onto this topic
in my opinion because a Nobel prize winner wrote a brief, speculative
computational paper suggesting that Skyrmions could be the panacea to the woes
of current magnetic storage, despite there being even a lot of evidence at
that point to the contrary - in particular, work by Stuart Parkin at IBM in
trying to use magnetic domain walls for storage, and a whole host of work on
magnetic bubble memory that even made it to market in the 70s and 80s but
which was replaced because of poorer performance.

