Spin Hall Magnetoresistance as a Probe for Surface Magnetization in Pt=CoFe2O4 Bilayers

Abstract

We study the spin Hall magnetoresistance (SMR) in Pt grown in situ on CoFe2O4 (CFO) ferrimagnetic insulating films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO, but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, the surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are responsible for such intriguing features. In addition, a contribution from the ordinary magnetoresistance of Pt is identified, which is distinguishable only due to the low resistivity of the in situ grown Pt. ; This work is supported by the European Union under the NMP project (263104-HINTS) and the European Research Council (257654-SPINTROS), by the Spanish MINECO (MAT2012-37638, MAT2015-65159-R, MAT2014-56063- C2-1R, and SEV-2015-0496), by the Basque Government (PC2015-1-01), and by the Catalan Government (2014 SGR 734). M. I. and E. S. acknowledge the Basque Government and the Spanish MECD, respectively, for a Ph.D. fellowship (BFI-2011-106 and FPU14/03102). J. F. acknowledges stimulating discussions with Xavier Martí. ; Peer reviewed