Emergent Magnetic State in (111)-Oriented Quasi-Two-Dimensional Spinel Oxides.

Liu, Xiaoran; Singh, Sobhit; Kirby, Brian J; Zhong, Zhicheng; Cao, Yanwei; Pal, Banabir; Kareev, Mikhail; Middey, Srimanta et al. · Nano Lett · 2019

basic_science · Level V

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Abstract

We report on the emergent magnetic state of (111)-oriented CoCr<sub>2</sub>O<sub>4</sub> ultrathin films sandwiched between Al<sub>2</sub>O<sub>3</sub> spacer layers in a quantum confined geometry. At the two-dimensional crossover, polarized neutron reflectometry reveals an anomalous enhancement of the total magnetization compared to the bulk value. Synchrotron X-ray magnetic circular dichroism measurements demonstrate the appearance of a long-range ferromagnetic ordering of spins on both Co and Cr sublattices. Brillouin function analyses and ab-initio density functional theory calculations further corroborate that the observed phenomena are due to the strongly altered magnetic frustration invoked by quantum confinement effects, manifested by the onset of a Yafet-Kittel-type ordering as the magnetic ground state in the ultrathin limit, which is unattainable in the bulk.