FeRh is a metallic alloy that has been well investigated over the past 70 years or so, yet a thorough understanding of some of its key properties remains elusive. In recent years FeRh thin films have seen somewhat of a renaissance in spintronics research. Papers from around 3-5 years ago focussed on experimental study of its magnetic, structural, and transport features, especially focused around its magnetic phase transition (see below paragraph). More recently, several papers have identified FeRh as a material which can be exploited for spintronic application, especially for the purpose of a memory device. This week’s Pick of the Week, “Physical Properties of FeRh alloys: The antiferromagnetic to ferromagnetic transition” in PRB by Kudrnovsky and Drchal, attempts to theoretically fill in some of the gaps of the 3-5 year old material characterisation papers while acknowledging the relevance of their results to the potential of FeRh as a spintronic material.
Going through a phase
The key feature of FeRh is that at temperatures between zero and a “transition temperature”, the alloy is antiferromagnetic. As temperature increases, FeRh transforms from an antiferromagnet to a ferromagnet over a finite temperature range. The FeRh then remains in this ferromagnetic phase until the temperature is increased to its Curie temperature, at which point it becomes paramagnetic. Along with a change in measurable magnetic moment, the antiferromagnetic to magnetic phase transition can also be experimentally detected by a well-defined accompanying decrease in sample resistance. The origin of the magnetic phase change is not fully understood and still remains debated.
FeRh for spintronics?
From a spintronics standpoint FeRh is especially interesting from an antiferromagnetic standpoint. As has previously been pointed out in these articles, there are many benefits to antiferromagnetic spintronics devices, but one of the major obstacles that currently limit them is the lack of ability to control the orientation of the antiferromagnetic moments (the antiferromagnetic spin axis). Exchange pinning to a ferromagnet, and a relativistic spin-orbit coupling-induced torque have been suggested as potential mechanisms. Another route to manipulating the antiferromagnetic spin axis was demonstrated last year in FeRh, whereby cooling the material from its ferromagnetic phase through its transition temperature into the antiferromagnetic phase in the presence of an external magnetic field could set the antiferromagnetic spin axis along different orientations. This is especially interesting as FeRh in its antiferromgantic phase was simultaneously shown to have an antiferromagnetic anisotropic magnetoresistance (AFM-AMR). Therefore, FeRh could be used as a memory device, with the writing done in a similar spirit to heat assisted magnetic recording of ferromagnetic memory devices.
The work of Kudrnovsky and Drchal is a fairly rigorous theoretical look at some of the magnetic and transport properties of FeRh. It is quite a difficult read for an experimental physicist such as myself, so be warned. The majority of the paper focuses on the magnetic properties which are investigated using a unified electron structure model within the local density approximation. Some of the key findings include the observation that the value of the calculated moments show very little change when the effects of spin-orbit coupling are included, and that the size of the moments associated with the Rh atoms are sensitive to their orientation with respect to the Fe moments. The former point bears some thinking about when considering FeRh from an AMR standpoint as AMR generally increases with both the size of the magnetic moment and spin-orbit coupling. The latter finding may have significance when considering growth conditions for a spintronic FeRh layer (growth technique, temperature, substrate etc).
The transport study of FeRh that Kudrnovsky and Drchal conduct may be more relevant to its use a spintronic material. The drop in resistance as FeRh moves from its antiferromagnetic to ferromagnetic phase is shown to be due to a decrease in spin disorder; in the antiferromagnetic phase local moments are orientated opposite to one another so are inherently more disordered than the ferromagnetic phase where they are parallel to one another. This can be considered in a similar vein to the giant magnetoreistance effect. It is also worth noting that the size of this non-relativistic change in resistance that accompanies the magnetic phase change is around 50%, which is significantly larger than any AMR effect, and therefore may have potential in itself to be harnessed in a spintronic device. Kudrnovsky and Drchal also investigate both the ferromagnetic and antiferromagnetic AMRs as a function of the relevant Fe to Rh stoichiometry. As would be expected, the strength of the AMR increases as Fe content increases (Rh is a paramagnetic element). For the AFM-AMR, it is shown that a stoichiometry of around 1:1 between Fe and Rh actually brings about the largest effect, and this is an important point to note when considering growing FeRh for spintronic applications.
FeRh research has taken an interesting turn now that it has been realised that it can possibly be used in spintronic devices. The work of Kudrnovsky and Drchal will help experimental papers continue to move away from material development and towards material functionality. Some practical questions still remain that may hinder the latter point, one being that that the transition temperature needs to be tailored to a value that can be reached by heating within a device (by a laser or current most likely), but that is still high enough to the FeRh is antiferromagnetic at room temperature. Doping with various other transition metal impurities is likely to be crucial to achieve this.
Author’s note: Please note that Pick of the Week will be going on a hiatus over the next month or so as I concentrate my efforts on finishing my thesis and wrapping up some exciting experimental work of my own. Hopefully I will be able to bring it back by around the start of March. Please keep checking www.spinandtonic.co.uk as content from other contributing writers will continue to be uploaded during February.
Photo: Jay, some rights reserved
Kudrnovský, J., Drchal, V., & Turek, I. (2015). Physical properties of FeRh alloys: The antiferromagnetic to ferromagnetic transition Physical Review B, 91 (1) DOI: 10.1103/PhysRevB.91.014435