Tunnel Magnetoresistance in the Magnetic Tunnel Junctions with an Amorphous Boron Nitride Barrier Formed via Nitrogen Diffusion

Sputtering is the method widely used to fabricate thin films including nitrides in spintronic research fields. One of the issues to fabricate the nitride thin films via sputtering is the control of N deficiency. In this work, it was demonstrated that the use of atomic diffusion improved the insulating property of amorphous BN (a-BN): BN deposition caused the nitridation of Co underlayer, and the diffusion of N from Co-N into BN reduced the N deficiency of BN. The a-BN formed via this sequence showed flat interfaces and no pinholes and could be used as a tunneling barrier layer. This provides an insight into synthesizing stoichiometric BN thin films. A negative tunnel magnetoresistance (TMR) ratio of -0.2% was observed at room temperature. The TMR ratio increased with a decrease in temperature to -0.7% at 10 K. The effective thickness and height of the barrier were estimated to be 1.78 nm and 0.52 eV, respectively, for 2 nm thick BN-MTJs based on the dependences of transport properties on the bias voltage and temperature.