Monday, May 23, 2016

Research blogging: Magnetism in layered materials

Following on from graphene, there has been enormous interest in other layered materials for the last few years, such as transition metal dichalcogenides (TMDs) like MoS2.   Depending on the constituents and particular structure, these materials can be semiconductors, superconductors, charge density wave compounds, etc., and can have properties that vary strongly as the number of layers in the material is reduced toward one.  You can expand the palette further by substitutionally doping different elements into the chalcogenide layers, or you can intercalate other atoms between the layers.  There are a huge number of possible compounds and variations.  (Fun note:  TMDs have been studied intensely before.  See here for a review from almost 50 years ago!  And magnetism in intercalated TMDs was examined by people like Stuart Parkin and Richard Friend almost 40 years ago.   The resurgence now is due to a combination of improved growth and characterization techniques, interest in low-dimensionality materials, and theoretical appreciation for the richness of possible states in these systems.)

Recently, collaborating with my colleague Jun Lou, we had some fun examining a related material, V5S8, which you can also think of as (V0.25)VS2.  There are vanadium disulfide layers, and intercalated between them are additional vanadium atoms in an ordered pattern.  The bulk version of this material was found in the 1970s to be an antiferromagnet - below the Neel temperature TN ~ 32 K, the spins of the unpaired electrons on the intercalated vanadium atoms spontaneously order into the arrangement shown in the upper panel at right.   If an external magnetic field bigger than about 4 T is applied perpendicular to the planes of the material, the spins flop over into the arrangement shown in the bottom panel - this is called a spin flop transition. 

Prof. Lou's group has figured out how to grow V5S8 by chemical vapor deposition, so that we were able to make measurements on single crystals of a variety of thicknesses, down to about 10 nm.  We found a couple of cool things, as reported here.   

First, we found a previously unreported first-order (in the thicker crystals) phase transition as a function of externally applied magnetic field.   The signature of this is hysteresis in the electrical resistance of the material as a function of the magnetic field, H.  Just below TN, the hysteresis appears near zero magnetic field.  As T is lowered, the magnetic field where the hysteresis takes place increases dramatically - in a thick crystal, it can go from basically 0 T to taking place at 9 T when the temperature is lowered by only three Kelvin!  Indeed, that's probably one reason why the transition was missed by previous investigators:  If you take data at only select temperatures, you could easily miss the whole thing.   This kind of a transition is called metamagnetic, and we think that large applied fields kill the antiferromagnetism (AFM), driving the material into a paramagnetic (PM) state.  We suggest a phase diagram shown in the table-of-contents figure shown here.  The transition extrapolates to a finite value of H at zero temperature.  That implies that it ends up as a quantum phase transition.

Second, we found that there are systematic changes in the magnetic properties as a function of the thickness of the crystals.  In thinner crystals, the antiferromagnetism appears to be weaker, with TN falling.  Moreover, the hysteresis in the field-driven transition vanishes in thinner crystals, suggesting that the metamagnetic transition goes from first-order to second order in the thin limit.   

This work was a lot of fun.  As far as I know, it's the first example of a systematic study of magnetic properties in one of these layered materials as a function of material thickness.  I think we've just scratched the surface in terms of what could be possible in terms of magnetism in this layered material platform. 

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