TMR Overview

In 1975, Julliere observed the TMR effect in the Magnetic Tunnel Junction (MTJ) sandwich structure (Co / Ge / Fe). The MTJ basic structure is (ferromagnetic / nonmagnetic insulating layer / ferromagnetic) or (FM / I / FM). However, this finding did not attract a lot of attention. Progress was gradual on the TMR effect over the next decade or so.  The Spin-Dependent Tunneling (SDT) effect is the physical mechanism of the resistance change that one measures as TMR. 

In 1988, the Brazilian scholar Baibich discovered the giant magnetoresistance (GMR) effect in Fe / Cr multilayers.  He was working in the Department of Physics at the University of Paris, France, in the research group led by Professor Fert. Discoveries of the GMR and TMR effects led to a new branch in condensed matter physics – spintronics.   Over the past 20 years, the GMR Effect has developed very rapidly, and both basic research and applied research are pursued.  The GMR Effect is a basic research result that was rapidly transformed into commercial applications. 

Along with further research into the GMR effect, the TMR effect has begun to attract attention. Although the metal multilayer film can produce a high GMR value, the strong antiferromagnetic coupling effect leads to a high saturation field, so the magnetic field sensitivity is small (compared to TMR), which limits the practical applications the GMR effect. 

In MTJs, the two ferromagnetic layers have very little interlayer coupling. So, only a small external magnetic field is needed to reverse the magnetization direction of a ferromagnetic layer in an MTJ.  The tunneling resistance changes a great deal in the

MTJ, in a smaller field, so the MTJ has a much higher magnetic field sensitivity than a typical GMR film.

Meanwhile, the MTJ structure itself can have very high, or very low resistivity; low energy consumption, and stable magnetic performance. Therefore, MTJ elements used as read heads, sensors, or in Magnetic Random Access Memory (MRAM), have unique advantages. Their future technical prospects are also very good, so TMR has attracted the attention of research teams around the world. 

TMR Market Applications

The TMR effect, due to having a large magnetoresistance ratio, can have relatively high sensitivity to magnetic fields. It also has some other unique advantages, and thus presents very attractive prospects. Generally, TMR material is mainly used in computer hard disk read heads, MRAM, and a great variety of magnetic sensors.

Currently, higher density, higher capacity, and miniaturization have become inevitable trends in computer data storage. Starting in the early 1990s, magnetoresistive read heads have been used in hard disk drive applications, contributing greatly to the hard disk drive performance improvement, so that the surface areal recording density reached the Gb / in² level. Through the 1990s, magnetoresistive read heads evolved from the original Anisotropic Magnetoresistance, to GMR, and then to TMR. 

The main advantages of TMR read head materials are higher magnetic field sensitivity due to larger magnetoresistance ratio than GMR, and the geometry is current-perpendicular to the film plane (CPP) type, which is especially well suited for ultra-thin gap spacing. 

The impact of TMR on production MRAM has been a highly integrated nonvolatile memory that has: a high read and write speed, highly reliable reads and write cycles, strong radiation tolerance, low power consumption, and a long lifetime. It can be integrated with the computer RAM, or be used as an external memory.  As an internal memory, in comparison with the common semiconductor integrated memory, MRAM has the advantages of: non-volatility, high access speed, and strong radiation tolerance.  As an external memory, compared to flash, MRAM has 1000 times faster access speed, low power consumption, and long life.  When compared with a hard disk drive, MRAM has the advantage of being solid state, with no moving parts, it can be convenient like flash.

TMR materials can also be used to make a variety of high-sensitivity magnetic field sensors for quantifying magnetic fields, and detecting low level magnetic fields. Because these sensors are small and highly reliable, they have applications in wide range of applications like automation technology, household appliances, trademark recognition, satellite positioning, navigation systems, and precision measurement technology.  

High sensitivity – As the magnitude of the magnetic field being detected gets very small, one needs greatly improved magnetic sensor sensitivity. Applications include current sensors, angle sensors, gear sensors, space, and environmental measurements.

Temperature Stability - More application areas require sensors with working conditions that are increasingly harsh.  This means the magnetic sensors must have increasingly good temperature stability for industrial applications such as automotive and electronics.

Frequency Characteristics - As the fields of applications grow, requiring sensors with increasing frequency range in applications including water meters, automotive electronics, and the data gathering and storage industry.

Low Power Consumption – There are many applications that demand the sensor itself consume low power to extend the useful life of sensor.

Immunity - many areas where the sensor's environment without any shield, it requires that the sensor itself has a good anti-interference.  Applications include electronic compass, finance heads, etc..

Miniaturization, Integration, Intelligence - to achieve these technical goals requires the integration at all levels: chip-level, module-level, and product-level.

Above excerpt from "physical" Volume 38 (2009) 6, Author: Li Yanbo, Wei Fulin, Yang; Author: Magnetism and Magnetic

Materials, Lanzhou University, Ministry f Education Key Laboratory of Magnetic Materials Institute. Thanks!