Direct demonstration of the emergent magnetism resulting from the multivalence Mn in a LaMnO3 epitaxial thin film system

TL;DR

This study demonstrates that multivalence Mn states in LaMnO3 thin films induce emergent ferromagnetism through a double-exchange mechanism, revealing new insights into oxide heterostructures and spintronic applications.

Contribution

It uncovers the role of Mn multivalence states in ferromagnetism of LaMnO3 thin films, distinct from interfacial effects, using advanced spectroscopic techniques.

Findings

Ferromagnetism up to 1.5 μB/Mn observed in LaMnO3 heterostructures

Mn multivalence states (Mn3+, Mn4+, Mn2+) are key to magnetic properties

Magnetic dead layer associated with Mn2+ accumulation near interface

Abstract

Atomically engineered oxide heterostructures provide a fertile ground for creating novel states. For example, a two-dimensional electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining spatially resolved electron energy-loss spectroscopy, X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques, we determine unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+ double-exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all-oxide spintronic devices.