Spintronic Technology Paves Way for Newly Designed Magnetic Tunnel Junctions

For the past two decades, magnetic tunnel junctions (MTJs) have been a vital component in spintronic devices such as read heads of hard disk drives and nonvolatile magnetoresistive random access memories (MRAMs). Researchers have constantly been trying to enhance their performance.

One of the most noticeable accomplishments that quickened the technology's practical applications was the realization of giant tunnel magnetoresistance (TMR) ratios by using rock-salt type MgO crystalline barrier.

A Japanese team of researchers have managed to apply MgGa2O4 to a tunnel barrier, the core part of an MTJ, as a substitute material to more conventional insulators such as MgAl2O4 and MgO. Their research article has been published in this week's issue of Applied Physics Letters, from AIP Publishing,

An MTJ has a laminated structure comprising of a nanoscale insulating layer, called a tunnel barrier, placed between two magnetic layers. One of the most crucial performance indexes of an MTJ is the tunnel magnetoresistance ratio (TMR ratio), the magnitude of resistance change. Magnesium oxide (MgO) is typically used as a tunnel barrier as a large TMR ratio can be easily acquired.

In order to widen the application range of MTJs further, we wanted to greatly tune the MTJ properties by replacing the tunnel barrier material. Particularly, for many MTJ applications, we need to have a large TMR ratio and low device resistance and for that we chose a tunnel barrier material with a low band gap.

Hiroaki Sukegawa, Scientist, National Institute for Materials Science

The team chose semiconducting MgGa2O4, as it has a comparatively lower band gap than the conventional MgO insulator, and used current technology to make an ultrathin MgAl2O4 layer to attain the parameters they were seeking.

The greatest challenge was acquiring a superior quality MgGa2O4 layer with defect-free interfaces since that is crucial to attaining a large TMR ratio.

We first attempted an oxidation method using an Mg-Ga alloy layer for the MgGa2O4 layer preparation however, this process also caused significant oxidation on the surface of the magnetic layer under the Mg-Ga, and the resulting fabricated structure did not function as an MTJ device.

Hiroaki Sukegawa, Scientist, National Institute for Materials Science

Inspired by their latest work on a superior quality MgAl2O4 fabrication, the team then used a direct sputtering technique; the MgGa2O4 layer was created by radio frequency sputtering from a high-density MgGa2O4 sintered target to decrease the interfacial over-oxidation.

This new technique was highly effective in creating a superior quality MgGa2O4 tunnel barrier with very sharp and defect-free interfaces. It was a pleasant and unpredicted surprise.

We did not expect that we could construct an MTJ showing a large TMR ratio using MgGa2O4 in such a short time since there were few tunnel barrier materials capable of providing the large TMR ratio at room temperature that we were looking for.

Hiroaki Sukegawa, Scientist, National Institute for Materials Science

Contrary to earlier understanding, this research shows that MTJ tunnel barriers can be "designed." It was thought that tuning the tunnel barrier’s physical parameters while maintaining large TMR ratios was virtually impossible. These results strongly indicate that a variety of physical properties of the tunnel barrier can be designed by choosing the composition of spinel based barrier materials as essential while realizing efficient spin-dependent transport (i.e. large TMR ratio).

While there is a lot more work to be performed to realize larger TMR ratios, these results pave the way for using "tunnel barrier design" with a variety of spinel oxides to develop new spintronic applications.

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