Technology | Highlight
TMR Device on Flexible Substrate Paves Way for Soft Spin Device
T

he tunnel magnetoresistive (TMR) device fabricated on a hard semiconductor silicon (Si) substrate has been widely used as a read head for a hard disc or a solid magnetic memory*1. The Institute of Scientific and Industrial Research (ISIR), Osaka University, in collaboration with the Graduate School of University of Tokyo and Murata Manufacturing Co., Ltd., demonstrated that the tunnel magnetoresistive device*2 (Fig. 1) fabricated on a soft plastic substrate (flexible substrate) has performance comparable to the device on a hard semiconductor substrate. Furthermore, the device has been confirmed to have high durability as it does not break even when the substrate is repeatedly stretched and to have the high heat resistance even for temperature as high as 500°C. These results indicate that the device has performance close to the practical level. In the future, it is expected to open up new industrial applications of spin devices*3. Possible practical applications are magnetic memory embedded in wearable devices, a flexible high-sensitivity magnetic-field sensor, a mechanical sensor that uses the phenomenon that the reversible change of magnetic properties due to stretching of the substrate causes a large resistance change.

New Applications for Spin Devices
A research group at ISIR is working with Murata Manufacturing to develop new industrial applications using spin devices on soft flexible substrates. Spin devices have functions of memory and magnetic field sensing. The softness will add a new function of detecting mechanical movement (detection of motion and strain on the surface of a living body or a structure) that is extremely important in the internet of things (IoT) industry. This opens the door to new industrial applications that are different from the advancement of magnetic recording technology aimed at by conventional spintronics.

Photograph (lower left and upper right) and schematic diagram (center) of a tunnel magnetoresistive device fabricated on a soft plastic substrate (flexible substrate)
Fig. 1: Photograph (lower left and upper right) and schematic diagram (center) of a tunnel magnetoresistive device fabricated on a soft plastic substrate (flexible substrate)
(a) Dependence of the resistance of tunnel magnetoresistive devices fabricated on soft plastic substrate (flexible substrate) on an external magnetic field and (b) dependence of the maximum resistance change on the annealing temperature(a) Dependence of the resistance of tunnel magnetoresistive devices fabricated on soft plastic substrate (flexible substrate) on an external magnetic field and (b) dependence of the maximum resistance change on the annealing temperature
Fig. 2: (a) Dependence of the resistance of tunnel magnetoresistive devices fabricated on soft plastic substrate (flexible substrate) on an external magnetic field and (b) dependence of the maximum resistance change on the annealing temperature
Performance at Par with Hard Substrate
The group has demonstrated that a tunnel magnetoresistive device fabricated on a soft plastic substrate (flexible substrate) has performance comparable to a device on a hard semiconductor substrate. The tunnel magnetoresistive device used in this research has a layered structure consisting of CoFeB/MgO/CoFeB (Fig. 1) widely used in hard disc read heads and solid magnetic memories. Here, CoFeB is a magnetic material and MgO constitutes a tunnel barrier. Fig. 2 (a) shows the dependence of the device resistance on a magnetic field. In the magnetic-field region where the resistance is high, the magnetization directions of the two CoFeB magnetic layers in the magnetic tunnel junction are antiparallel to each other, and in the region where the resistance is low, they are parallel. It can be seen that the resistance change is greatly increased by high temperature treatment (annealing) of the device. The group has succeeded in the fabrication of a device that can withstand high-temperature annealing up to 500°C by using a polyimide substrate. Fig. 2 (b) shows the maximum resistance change of Fig. 2 (a) as a function of the annealing temperature. The resistance change becomes close to 200 percent (equivalent to about three times in the resistance itself) at around 450°C, showing that the device on the flexible substrate shows performance better than or similar to the device fabricated on the hard Si substrate.

It is known that a device that has been annealed once at a high temperature has heat resistance capable of maintaining its performance up to temperatures lower than that of the annealing. Detailed investigation using a transmission electron microscope confirmed that high-temperature annealing promotes crystallization of the MgO layer and increases the resistance change (Fig. 3). Even when the device was stretched and put back into its original state 1,000 times, the device maintained the same resistance and the resistance change of about 200 percent, demonstrating high durability (Fig. 4).

Multitude of Practical Applications
This research showed for the first time that a soft spin device has a satisfactory level of performance for practical use. The spin devices can be easily integrated and have high sensitivity as a sensor. In the future, it can be expected to open up new industrial applications of spin devices. Possible practical applications are magnetic memory embedded in wearable devices, a flexible high-sensitivity magnetic-field sensor, a mechanical sensor and a wearable motion biosensor, both of which use the phenomenon that the reversible change of magnetic properties due to substrate stretching causes a large resistance change.

Transmission electron microscope image of a tunnel magnetoresistive device fabricated on a soft plastic substrate (flexible substrate)
Fig. 3: Transmission electron microscope image of a tunnel magnetoresistive device fabricated on a soft plastic substrate (flexible substrate)
Graph of The resistance and the maximum resistance change as a function of the number of cycles for stretching the substrate and putting back to its original state
Fig. 4: The resistance and the maximumresistance change as a function of the number of cycles for stretching the substrate and putting back to its original state
Glossary:
*1 Solid-state magnetic memory: Non-volatile memory using magnetic material (information recording device that can retain memory with no power turned on). The resistance of the tunnel magnetoresistive device, to be explained in *2, corresponds to the information of 0 or 1.

*2 Tunnel magnetoresistive device: A device with a structure in which a thin insulating layer is sandwiched between two layers of magnetic material. Each layer has thickness of about several nanometers. The tunneling current through the insulating layer becomes larger when the magnetization directions of the two magnetic layers are parallel to each other, and smaller when the magnetization directions are antiparallel. That is, the device resistance is low when the magnetization is parallel and high when the magnetization is antiparallel. By associating these with information 0 and 1, the device resistance is used as the bit of the magnetic memory described in *1. On the other hand, the device resistance changes continuously depending on the relative angle of the magnetization. For this reason, when the relative angle of the magnetization takes an intermediate value depending on an external magnetic field, the device also functions as a magnetic-field sensor. This is used as a hard disc read head.

*3 Spin device: A functional electronic device that uses a thin film or a layered structure of magnetic materials with thickness of about nanometers.