Perpendicular magnetic tunnel junction with a strained Mn-based nanolayer.

K Z Suzuki,A Sugihara,R Ranjbar,H Tsuchiura,S Mizukami,J Okabayashi,Y Miura

doi:10.1038/srep30249

Abstract

A magnetic tunnel junction with a perpendicular magnetic easy-axis (p-MTJ) is a key device for spintronic non-volatile magnetoresistive random access memory (MRAM). Co-Fe-B alloy-based p-MTJs are being developed, although they have a large magnetisation and medium perpendicular magnetic anisotropy (PMA), which make it difficult to apply them to a future dense MRAM. Here, we demonstrate a p-MTJ with an epitaxially strained MnGa nanolayer grown on a unique CoGa buffer material, which exhibits a large PMA of more than 5 Merg/cm3 and magnetisation below 500 emu/cm3; these properties are sufficient for application to advanced MRAM. Although the experimental tunnel magnetoresistance (TMR) ratio is still low, first principles calculations confirm that the strain-induced crystal lattice distortion modifies the band dispersion along the tetragonal c-axis into the fully spin-polarised state; thus, a huge TMR effect can be generated in this p-MTJ.

Highlights

Magnetic tunnel junctions (MTJs) composed of two magnetic layers separated by a thin insulating barrier, such as Al-O or MgO, exhibit tunnel magnetoresistance (TMR), depending on the relative orientation of magnetisation[1,2,3,4]
Even though the experimental TMR ratio is still low, the first principles calculations confirm that the strained MnGa has fully spin-polarised band dispersion along the tetragonal c-axis; this is distinct from bulk Mn-Ga and similar to Co-Fe(-B)[3,4,30,31] and Mn3Ge32–34, which implies that a huge TMR is possible
The p-MTJ was not annealed after microfabrication to avoid atomic diffusion

Summary

Introduction

Magnetic tunnel junctions (MTJs) composed of two magnetic layers separated by a thin insulating barrier, such as Al-O or MgO, exhibit tunnel magnetoresistance (TMR), depending on the relative orientation of magnetisation[1,2,3,4]. A high bulk PMA and low Gilbert damping constant originate from the special property of Mn, i.e., it has nearly half-filled 3d electron orbital states in a crystal field with tetragonal symmetry[19] Those tetragonal Mn-based alloy films exhibit high PMA fields of 60–200 kOe owing to the low magnetisation, which enables long-lifetime magnetisation precession at a terahertz (THz) frequency[20,21], and they can be applied to STT-oscillators and diodes in the THz frequency range[22,23]. One technological challenge is to realise p-MTJs with an ultrathin Mn-based alloy layer with a large PMA and a typical thickness of 1–3 nm This is crucial for devices driven by the STT effect. Even though the experimental TMR ratio is still low, the first principles calculations confirm that the strained MnGa has fully spin-polarised band dispersion along the tetragonal c-axis; this is distinct from bulk Mn-Ga and similar to Co-Fe(-B)[3,4,30,31] and Mn3Ge32–34, which implies that a huge TMR is possible

Methods

Results

Conclusion