Magnetic disorder increases the ordering temperature in quantum materials

Abstract:

An umbrella term “quantum materials” brings together inorganic materials with unprecedented conductive, magnetic and optical properties originating from robust quantum effects stable up to ambient conditions. These materials hold promises for advances in electronics, spintronics, photonics, catalysis and quantum technologies.

I apply the toolbox of inorganic materials synthesis to obtain and optimize members of the recently discovered quantum materials’ class – magnetic topological insulators (MTIs) [1]. They are a hotbed for exotic quantum phenomena such as the quantum anomalous Hall effect (QAHE) and the topological magneto-electric effect which enable conceptually new electronic applications. The prerequisites for MTI performance are: a particular electronic band structure (certain topology of the collective wave function), a net magnetic polarization (ferro- or ferrimagnetic bulk order) and a high magnetic ordering temperature.

In reply to these guidelines, I perform materials optimization in a promising family of the layered MTIs (MnX2Te4)(X2Te3)n, X = Sb or Bi, n = 0–4, which I will introduce in my talk. The progenitor MnBi2Te4 is an A-type antiferromagnet with TN = 25 K [2, 3] which was the first MTI to exhibit the QAHE up to the record 6.5 K [4]. The task of fabricating structurally similar ferri- or ferromagnets with an increased TC (Curie temperature) is very pertinent. I show two mechanisms that tailor the magnetic ground state in the desired direction: intralayer coupling and Mn/X site intermixing. The latter is a surprisingly powerful, new tool that I am looking at closely. Disorder is commonly perceived as detrimental to a material’s properties, whereas in the present case it positively affects the magnetic order. For instance, site mixing is capable of pushing the TC from 27 K in Mn0.85Sb2.15Te4 to 58 K in Mn2Sb1.2Te2 thanks to the Mn re-distribution in the same crystalline lattice and the formation of new magnetic exchange pathways [5-8]. Another example is that Mn/Bi site intermixing transforms the magnetic ground state from an antiferro- to a ferromagnet in MnBi6Te10 [9].

REFERENCES
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