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New Magneto resistive materials enable ultrasensitive magnetic field sensors, brain-like computing chips and 3D magnetic memory

Magneto resistor is a type of resistor whose resistance changes when an external magnetic field is applied. In other words, the flow of electric current through the magneto resistor changes when an external magnetic field is applied to it.


Magnetic field is the region present around a magnetic object within which other objects experience an attractive or repulsive force.


The magneto resistors that are placed in the magnetic field will experience a change in resistance. When the strength of the magnetic field is increased, the resistance of magneto resistors also increases. On the other hand, when the strength of magnetic field is reduced, the resistance of magneto resistors decreases. This change in resistance is caused by the magneto resistive effect.


In the absence of magnetic field, the charges carriers in the material move in a straight path. Therefore, electric current flows in a straight path. When the magnetic field is applied to the material, the magnetic forces cause the mobile charge carriers (free electrons) to change their direction from direct path to indirect path. This increases the length of electric current path.


Magneto resistive effect is the property of some materials, which causes them to change their resistance under the presence of magnetic field. This magneto resistive effect occurs in materials such as semiconductors, non-magnetic metals, and magnetic metals.


For the last several years, the search for materials with large magnetoresistances (MRs) and studies on related phenomena have been in the forefront of worldwide research activity owing to their widespread application in the field of magnetic sensors, magnetic memory devices, magnetic switches, etc. The highest value of MR in principle can be achieved if the resistivity of the material can be transformed from an extreme insulating material (such as mica) to a very good metallic material (such as copper) by applying a magnetic field.

Colossal magnetoresistance, in which the resistivity changes by several orders of magnitude (~ 104%) in an external magnetic field, occurs mainly in phase-separated oxide materials, namely, manganites, owing to the phase competition between the ferromagnetic metallic and antiferromagnetic insulating regions.

Magnetoresistance ratio enhancement in Heusler-based alloy opens the door to highly sensitive magnetic field sensors

Magnetic field sensors can enhance applications that require efficient electric energy management. Improving magnetic field sensors below the picoTesla range could enable a technique to measure brain activity at room temperature with millisecond resolution — called magnetic encephalography — without superconducting quantum interference device (SQUID) technology, which requires cryogenic temperatures to work.


A group of researchers from Japan’s National Institute of Materials Science at the University of Tsukuba and LG Japan Lab Inc. explored enhancing the magnetoresistance ratio in a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device by using a half-metallic Heusler CoFeAl0.5Si0.5 (CFAS) alloy. The alloy has 100 percent spin-polarized conduction electrons, which enables very high spin-asymmetry of electron scattering and results in a large magnetoresistance ratio.


They report their findings in the Journal of Applied Physics (“High magnetic field sensitivity in anti-ferromagnetically coupled 001-epitaxial [Co2Fe(Al0.5Si0.5)/Ag]N multilayers”).


Magnetoresistance — a variation of electrical resistance in response to an externally applied magnetic field — is important for all magnetic field sensor applications. To increase the sensitivity of magnetic field sensors, their magnetoresistance ratio (a value defined as electrical resistance change against magnetic field or magnetization) must first be increased.


“We were able to demonstrate further enhancement of the magnetoresistance ratio by making multilayer stacks of CFAS and silver (Ag),” said Yuya Sakuraba, leader of the Magnet Materials Group at NIMS. “By precisely controlling the interfacial roughness of the multilayers, we obtained antiparallel interlayer exchange coupling between each of the CFAS layers, up to six, and achieved not only a high magnetoresistance ratio but also high linearity of resistance change against the magnetic field.”


Previous studies demonstrated that half-metallic Heusler alloys are well suited to enhance the magnetoresistance ratio in CPP-GMR devices. “Heusler-based alloys are expected to be the next-generation read head for hard disk drives with high areal recording density over 2 terabits per square inch,” Sakuraba said.


“And our work has demonstrated that further enhancement of the magnetoresistance ratio is possible by creating a multilayer structure, which now really opens up the potential of Heusler-based CPP-GMR for highly sensitive magnetic field sensor applications,” Sakuraba went on to explain.


The researchers fabricated a fully expitaxial device on a single crystalline magnesium oxide (MgO) substrate. If a similar property can be obtained in a polycrystalline device, it may become a candidate for a new magnetic field sensor with a greater sensitivity than a conventional Hall sensor or tunnel magnetoresistance sensor.



A colossal breakthrough for topological spintronics

Scientists have developed the world’s best-performing pure spin current source made of bismuth–antimony (BiSb) alloys, which they report as the best candidate for the first industrial application of topological insulators. The achievement represents a big step forward in the development of spin-orbit torque magnetoresistive random-access memory (SOT-MRAM) devices with the potential to replace existing memory technologies.


A research team led by Pham Nam Hai at the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology (Tokyo Tech), has developed thin films of BiSb for a topological insulator that simultaneously achieves a colossal spin Hall effect and high electrical conductivity.


Their study, published in Nature Materials, could accelerate the development of high-density, ultra-low power, and ultra-fast non-volatile memories for Internet of Things (IoT) and other applications now becoming increasingly in demand for industrial and home use.


The BiSb thin films achieve a colossal spin Hall angle of approximately 52, conductivity of 2.5 × 105 and spin Hall conductivity of 1.3×107 at room temperature. (See Table 1 for a performance summary, including all units.) Notably, the spin Hall conductivity is two orders of magnitude greater than that of bismuth selenide (Bi2Se3), reported in Nature in 2014.


Until now, the search for suitable spin Hall materials for next-generation SOT-MRAM devices has been faced with a dilemma: First, heavy metals such as platinum, tantalum and tungsten have high electrical conductivity but a small spin Hall effect. Second, topological insulators investigated to date have a large spin Hall effect but low electrical conductivity.


The BiSb thin films satisfy both requirements at room temperature. This raises the real possibility that BiSb-based SOT-MRAM could outperform the existing spin-transfer torque (STT) MRAM technology.


“As SOT-MRAM can be switched one order of magnitude faster than STT-MRAM, the switching energy can be reduced by at least two orders of magnitude,” says Pham. “Also, the writing speed could be increased 20 times and the bit density increased by a factor of ten.”


If scaled up successfully, BiSb-based SOT-MRAM could drastically improve upon its heavy metal-based counterparts and even become competitive with dynamic random access memory (DRAM), the dominant technology of today.


Researchers demonstrate the existence of a new kind of magnetoresistance involving topological insulators

A new discovery, led by researchers at the University of Minnesota, demonstrates the existence of a new kind of magnetoresistance involving topological insulators that could result in improvements in future computing and computer storage. The details of their research are published in the most recent issue of the scientific journal Nature Communications.


“Our discovery is one missing piece of the puzzle to improve the future of low-power computing and memory for the semiconductor industry, including brain-like computing and chips for robots and 3D magnetic memory,” said University of Minnesota Robert F. Hartmann Professor of Electrical and Computer Engineering Jian-Ping Wang, director of the Center for Spintronic Materials, Interfaces, and Novel Structures (C-SPIN) based at the University of Minnesota and co-author of the study.


Emerging technology using topological insulators

While magnetic recording still dominates data storage applications, the magnetoresistive random access memory is gradually finding its place in the field of computing memory. From the outside, they are unlike the hard disk drives which have mechanically spinning disks and swinging heads—they are more like any other type of memory. They are chips (solid state) which you’d find being soldered on circuit boards in a computer or mobile device.


Recently, a group of materials called topological insulators has been found to further improve the writing energy efficiency of magnetoresistive random access memory cells in electronics. However, the new device geometry demands a new magnetoresistance phenomenon to accomplish the read function of the memory cell in 3D system and network.


Following the recent discovery of the unidirectional spin Hall magnetoresistance in a conventional metal bilayer material systems, researchers at the University of Minnesota collaborated with colleagues at Pennsylvania State University and demonstrated for the first time the existence of such magnetoresistance in the topological insulator-ferromagnet bilayers.


The study confirms the existence of such unidirectional magnetoresistance and reveals that the adoption of topological insulators, compared to heavy metals, doubles the magnetoresistance performance at 150 Kelvin (-123.15 Celsius). From an application perspective, this work provides the missing piece of the puzzle to create a proposed 3D and cross-bar type computing and memory device involving topological insulators by adding the previously missing or very inconvenient read functionality.



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