Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures

TL;DR

This study reveals that ferromagnetism in LaAlO3/SrTiO3 heterostructures appears only above a critical layer thickness and is likely induced by local disorder or strain, independent of carrier mobility.

Contribution

It demonstrates the critical thickness for ferromagnetism and shows that magnetism can exist without itinerant carriers, highlighting the role of interface reconstruction and local effects.

Findings

Ferromagnetic patches appear only above a critical LAO thickness.

Ferromagnetism is unaffected by gate voltage.

Magnetism observed even in non-conducting p-type samples.

Abstract

In heterostructures of LaAlO3 (LAO) and SrTiO3 (STO), two nonmagnetic insulators, various forms of magnetism have been observed [1-7], which may [8, 9] or may not [10] arise from interface charge carriers that migrate from the LAO to the interface in an electronic reconstruction [11]. We image the magnetic landscape [5] in a series of n-type samples of varying LAO thickness. We find ferromagnetic patches that appear only above a critical thickness, similar to that for conductivity [12]. Consequently we conclude that an interface reconstruction is necessary for the formation of magnetism. We observe no change in ferromagnetism with gate voltage, and detect ferromagnetism in a non-conducting p-type sample, indicating that the carriers at the interface do not need to be itinerant to generate magnetism. The fact that the ferromagnetism appears in isolated patches whose density varies greatly between samples strongly suggests that disorder or local strain induce magnetism in a population of the interface carriers.