Influence of chemical and magnetic interface properties of Co-Fe-B / MgO / Co-Fe-B tunnel junctions on the annealing temperature dependence of the magnetoresistance

TL;DR

This study investigates how chemical and magnetic interface properties of Co-Fe-B / MgO / Co-Fe-B tunnel junctions influence the temperature-dependent magnetoresistance, revealing key changes in interface chemistry and crystallinity with annealing.

Contribution

It provides detailed insights into the chemical and magnetic interface changes that correlate with the annealing temperature and magnetoresistance enhancement in Co-Fe-B / MgO tunnel junctions.

Findings

Interfacial Fe-O reduces after annealing at 275°C, increasing TMR.

Recrystallization of Co-Fe-B occurs at 350°C, enhancing coherent tunneling.

Boron diffuses into MgO forming B2O3 at higher annealing temperatures.

Abstract

The knowledge of chemical and magnetic conditions at the Co40Fe40B20 / MgO interface is important to interpret the strong annealing temperature dependence of tunnel magnetoresistance of Co-Fe-B / MgO / Co-Fe-B magnetic tunnel junctions, which increases with annealing temperature from 20% after annealing at 200C up to a maximum value of 112% after annealing at 350C. While the well defined nearest neighbor ordering indicating crystallinity of the MgO barrier does not change by the annealing, a small amount of interfacial Fe-O at the lower Co-Fe-B / MgO interface is found in the as grown samples, which is completely reduced after annealing at 275C. This is accompanied by a simultaneous increase of the Fe magnetic moment and the tunnel magnetoresistance. However, the TMR of the MgO based junctions increases further for higher annealing temperature which can not be caused by Fe-O reduction. The occurrence of an x-ray absorption near-edge structure above the Fe and Co L-edges after annealing at 350C indicates the recrystallization of the Co-Fe-B electrode. This is prerequisite for coherent tunneling and has been suggested to be responsible for the further increase of the TMR above 275C. Simultaneously, the B concentration in the Co-Fe-B decreases with increasing annealing temperature, at least some of the B diffuses towards or into the MgO barrier and forms a B2O3 oxide.