Annealing-Induced Solid-Phase Epitaxy at Interfaces between CoFeB Layers and MgO Barrier-Layer in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions and Their Magnetic Properties

Abstract

The present paper describes influences of annealing conditions and chemical composition on grain-to-grain epitaxy, which is induced by annealing after sputter-deposition, in MgO based magnetic tunnel junctions (MTJs) with CoFeB ferromagnetic layers. Annealing-induced crystallization in the amorphous CoFeB layer (at as-deposited state) at the interface with the MgO layer in CoFeB/MgO/CoFeB MTJs is investigated by electron microscopy. Annealing conditions such as temperature and time as well as the chemical composition in the CoFeB layers strongly influence magnetoresistance (MR) ratio. Excess annealing (temperature and time) causes the break-down of synthetic antiferromagnet (SAF) structure in MTJs for practical applications, resulting in the decrease of MR ratio. Fe-rich CoFeB is more suitable to obtain better grain-to-grain epitaxy and higher MR ratio in view of lattice mismatch with the MgO layer. The annealing conditions and chemical composition are optimized to exhibit the highest MR ratio with keeping SAF structure.