Symmetry broken spin reorientation transition in epitaxial MgO/Fe/MgO layers with competing anisotropies

Isidoro Martínez,Farkhad G Aliev,Mairbek Chshiev,Michel Hehn,Coriolan Tiusan

doi:10.1038/s41598-018-27720-7

Abstract

The observation of perpendicular magnetic anisotropy (PMA) at MgO/Fe interfaces boosted the development of spintronic devices based on ultrathin ferromagnetic layers. Yet, magnetization reversal in the standard magnetic tunnel junctions (MTJs) with competing PMA and in-plane anisotropies remains unclear. Here we report on the field induced nonvolatile broken symmetry magnetization reorientation transition from the in-plane to the perpendicular (out of plane) state at temperatures below 50 K. The samples were 10 nm thick Fe in MgO/Fe(100)/MgO as stacking components of V/MgO/Fe/MgO/Fe/Co double barrier MTJs with an area of 20 × 20 μm2. Micromagnetic simulations with PMA and different second order anisotropies at the opposite Fe/MgO interfaces qualitatively reproduce the observed broken symmetry spin reorientation transition. Our findings open the possibilities to develop multistate epitaxial spintronics based on competing magnetic anisotropies.

Highlights

Magnetic films with the magnetization oriented perpendicular to the film plane are currently the best candidates for magnetic storage devices with respect to the challenge to decrease the bit size
Seminal magnetic tunnel junctions (MTJs) grown by Molecular Beam Epitaxy (MBE) have in-plane anisotropy and typically incorporate about 10 nm thick Fe or FeCo soft ferromagnetic layers (FM) separated by MgO barriers from the magnetically hard layers[19,20]
Both in-plane and out of plane tunneling magnetoresistance (TMR) values are somewhat lower than those typically reported for single barrier MTJs

Summary

Introduction

Magnetic films with the magnetization oriented perpendicular to the film plane are currently the best candidates for magnetic storage devices with respect to the challenge to decrease the bit size. The bottleneck to have PMA is the control of the magnetic anisotropy which is characterized by an effective anisotropy constant (Keff) that has a volume contribution Kv and two surface or interface contributions Ks2–4 As a result, it can be described as Keff = Kv + Ks(I)/t + Ks(II)/t where Ks(I) and Ks(II) are the surface anisotropies at the lower (I) and upper (II) interfaces and t is the ferromagnetic layer thickness. The recent trends in spintronics using MTJs take advantage of PMA to provide large tunneling magnetoresistance (TMR), enhanced thermal stability[6], low spin torque switching currents[7,8] and record small lateral sizes[9] Those features are critical for the progress towards spin transfer torque based magnetic random access memories.

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