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New theory clarifies why tunnel magnetoresistance oscillates with barrier thickness

Researchers have developed a new theory that explains why tunnel magnetoresistance (TMR)—used in magnetic memory and other technologies—oscillates with changes in the thickness of the insulating barrier within a magnetic tunnel junction (MTJ). This oscillation was clearly observed when NIMS recently recorded the world's highest TMR ratio. Understanding the mechanisms behind this phenomenon is expected to significantly aid in further increasing TMR ratios.

This research is as a letter article in Â鶹ÒùÔºical Review B.

The TMR effect is a phenomenon observed in thin-film structures called magnetic tunnel junctions (MTJs). It refers to changes in electrical resistance depending on the relative alignment of magnetizations in two magnetic layers (i.e., parallel or antiparallel alignment) separated by an insulating barrier. It is desirable to develop MTJs with larger TMR effects—reflected in higher TMR ratios—in order to expand their potential applications, including improvement of magnetic sensor sensitivity and expansion of magnetic memory capacity.

A NIMS research team recently achieved the world's highest TMR ratio, and also found that the TMR ratio oscillates by changing the thickness of the insulating barrier, referred to as the TMR oscillation. This finding indicates that understanding the physical origin of the TMR oscillation is vital to achieving even higher TMR ratios. However, the mechanism responsible for the TMR oscillation had remained unknown despite extensive research conducted on the subject for more than two decades.

The present research team developed a new theory for the TMR oscillation by considering a mechanism that had been overlooked in previous theoretical studies. Interfaces between magnetic layers and the insulating barrier in MTJs have been believed to play an important role in the TMR effect. The team took into account a superposition of wave functions between majority- and minority-spin states occurring at such an interface—the most important and novel contribution made by this study. TMR ratios calculated using this theory were consistent with TMR ratios obtained experimentally, supporting the validity of the present theory.

Previous experiments for the TMR oscillation were conducted using MTJs with limited types of magnetic materials (e.g., iron). Future experimental studies using a broader range of magnetic materials may further advance the understanding of the TMR oscillation by comparing the results with the present theory. In addition, the present theory is expected to contribute to the development of guidelines for the control of TMR oscillation and the design of MTJs with even higher TMR ratios.