Anomalous Crystallinity and Magnetism in Chemically Disordered Coherent Heterostructures

Abstract

ABSTRACT High‐entropy oxide (HEO) thin films uniquely superimpose exceptional chemical disorder with exceptional crystalline quality and coherence—an intersection we term anomalous crystallinity that arises from coupled structural, chemical, and valence degrees of freedom unique to the high‐entropy and entropy‐stabilized conditions. Here, we demonstrate unexpected and predictive control of this state using formulation, epitaxial constraints, and kinetic arrest of metastable macrostates. Specifically, aliovalent cation substitutions, tightly controlled substrate temperatures, and conditions favoring significant adatom kinetic energy, can program the out‐of‐plane lattice parameter of coherent rock salt HEOs while preserving in‐plane epitaxial pinning to MgO. We highlight the exemplar (ScMgCoNiCuZn)O/(CrMgCoNiCuZn)O (JSc/JCr) system where Sc and Cr substitution into the rock salt structure produces pseudomorphic heterostructures between individual antiferromagnets, sustaining an exceptional 5.5% out‐of‐plane lattice parameter difference and enabling abrupt interfaces across which the Co valence switches from mostly 2 + to an even 2 + /3 + mixture. The JSc/JCr valence interface heterostructure is accompanied by a 2 × exchange bias boost compared to single‐layer constituents, that could be attributed to enhanced uncompensated spins in the layers themselves or around the buried JSc/JCr interface. These results establish pseudomorphic valence interfaces with anomalous crystallinity as a source of new magnetic macrostates that host emergent magnetic and spintronic functionality.