Highlights

3-Dimensional Nanoscale Structural Characterization of Magnetic Tunnel Junction

Scientific Achievement

A magnetic tunnel junction (MTJ) consists in its simplest form of two ferromagnetic layers separated by a thin insulating layer such as MgO. Despite being used commercially as hard disk drive read heads and solid state memory devices, MgO-based MTJs are not very well understood.  Indeed, current state-of-the-art MTJs achieve only a fraction of their theoretical performance capabilities.  To better understand the basic physical structure that leads to these shortcomings, three-dimensional (3D) electron tomography and analytical transmission electron microscopy (TEM) were applied to a simple MTJ structure of Ru/CoFeB/MgO/CoFeB/CoFe Cap to map the morphology and chemistry of both the individual layers and their interfaces at high spatial resolution

The elemental distribution of this MTJ, especially near the interfaces was illustrated by the line scan method in scanning transmission electron microscopy (STEM).  The interfaces of the top and bottom MgO are not so sharp and some Ru diffused into CoFeB.  The good crystallinity in MgO barrier and CoFeB layers after annealing was shown clearly by high resolution TEM. In classic TEM research, the third dimensional information was integrated in the two-dimensional images.  Here we extract the 3D structure of this MTJ by tomography in STEM mode, using image contrast to map the materials with different mean atom numbers.  The layered structure and grains distribution in CoFe cap layer was clearly reconstructed in three dimensions with the STEM tomography.  In future studies, more detailed analyses will be made to evaluate the physical roughness of the MTJ interfaces, which could play a significant role in the resulting magnetotransport of the MTJ and affect the ultimate device performance. 

All the TEM specimen preparation and structure characterizations were carried out on the Zeiss 1540XB FIB system and FEI Tecnai F20 TEM in the EMC.

Significance

This research has significant contribution to the structural characterization of the MTJ.  Most structural characterizations of MTJs have been performed with either 1D (SIMS) or 2D (TEM) data.  Here, we extend this physical characterization to 3D with electron tomography.  We will continue study multilayered structures such as MTJs and other oxide heterostructure by using electron tomography combined with analytical TEM to explain the role of interfacial chemistry and microstructure on transport properties.

Performers

Y. Liu, A. K Petford-Long, D. K Schreiber (Argonne-MSD)