Evidence for stable room-temperature skyrmions found

 

Thu, 07/02/2015 - 10:17am RIKEN In research published in Nature Communications, researchers from the RIKEN Center for Emergent Matter Science in Japan, along with collaborators in Europe and Japan, have identified a class of materials that displays clear evidence for stable skyrmions at room temperature and above, paving the way for the development of useful spintronics devices. Magnetic skyrmions are tiny, nanometer-sized magnetic-spin vortices that emerge in magnetic materials. Because they are so small, they could potentially be used as extremely dense memory devices, with the presence or absence of skyrmions being used to denote bits in computer calculations. Achieving this could lead to the advent of a new class of low-power-consumption devices dubbed "spintronics", which include high-density magnetic memory. However, skyrmions are not always easy to use. Though they occur ubiquitously in magnets, in a diverse range of circumstances, they are not always easy to control. The ultimate dream of researchers has been to create stable skyrmions—which do not decay—in a set chirality, so that their presence or absence can be manipulated to represent data for storage devices. Skyrmions emerging from a process called the Dzyaloshinskii-Moriya interaction have been considered to be particularly promising, as they are small (under 150 nanometers) and have a fixed direction of spin. As a result, they could host a large number of stable skyrmions which could be manipulated by electrical currents, and hence could be used to create high-density storage. Such stable skyrmions have been found in certain crystal structures, but in the materials studied so far, such as MnSi and Cu2OSeO3, the skyrmions emerge most strongly in low temperatures, requiring low-temperature manipulation. For the present study, the scientists decided to look at a type of magnetic material made up of cobalt, zinc, and manganese, whose structure seemed likely to host stable skyrmions. Using a variety of techniques, they were able to show indeed that when a magnetic field was applied, the material showed the clear presence of skyrmion crystals in both a bulk shape and when shaped into a thin plate. These skyrmions were both stable and chiral—meaning that they spun in a set direction—so that they could be manipulated to encode information. According to Yusuke Tokunaga of CEMS, who is the first author of the work, "We are quite excited about these results, as they may answer the long-held expectation that we can find skyrmion hosting systems in a variety of new materials. In addition, the fact that we have shown that skyrmions can be stabilized at room temperature and above opens the road to looking for ways to integrate skyrmions into spintronics devices without complicated cooling systems." The research was done by CEMS scientists in collaboration with scientists from the Paul Scherrer Institut and Ecole Polytechnique Federale de Lausanne in Switzerland and the University of Tokyo in Japan. SOURCE: RIKEN

Ultrafast heat conduction manipulates nanoscale magnets

 

Tue, 06/09/2015 - 12:27pm Rick Kubetz, University of Illinois at Urbana-Champaign Schematic, cutaway view of the geometry used to generate currents of spin from currents of heat. Pulses of laser light heat the left side of the sample and create an intense current of heat passing through the [Co,Ni] ferromagnet. This current of heat creates a separation of electron spins that then diffuse through the Cu heat sink and affect the magnetization of a second ferromagnetic layer, CoFeB, causing the magnetization to tilt and then precess. The total thickness of the sample is approximately 100 nm. Alex Jerez, Imaging Technology Group, Beckman Institute.Researchers at the University of Illinois at Urbana-Champaign have uncovered physical mechanisms allowing the manipulation of magnetic information with heat. These new phenomena rely on the transport of thermal energy, in contrast to the conventional application of magnetic fields, providing a new, and highly desirable way to manipulate magnetization at the nanoscale. “In our study, we make use of the fact that a heat current passing through a magnetic material creates a separation of electron spins. This process creates a current of magnetic dipoles that we use to manipulate the orientation of a second magnetic layer,” said David Cahill, a Donald B. Willett Professor of Engineering and head of the Department of Materials Science and Engineering at Illinois. “The physics of separating spins with heat currents is related to the operation of thermocouples and the thermoelectric generators that power deep space probes. In those thermoelectric devices, a heat current causes a separation of electrical charges. That separation of electrical charge can then be used to measure a temperature or provide electrical power.” “We use the spin current created by ultrafast heat conduction to generate spin transfer torque. Spin transfer torque is the transfer of the spin angular momentum from conduction electrons to the magnetization of a ferromagnet and enables the manipulation of nanomagnets with spin currents rather than magnetic fields,” explained Gyung-Min Choi, who recently completed his Ph.D. in materials science and engineering at Illinois. “Spin transfer torque has often been realized by passing electrical currents through magnetic layers. In our paper, we showed how spin transfer torque can be generated by an intense current of heat.” Choi is lead author of the paper, “Thermal spin transfer torque driven by the spin-dependent Seebeck effect in metallic spin-valves,” published in Nature Physics. (The term “Seebeck effect” refers to a thermoelectric phenomenon by which temperature differences between two dissimilar materials in a circuit generate a voltage. The spin-dependent Seebeck effect refers to the analogous phenomenon involving the spin of electrons in a ferromagnet.) “We quantify thermal spin transfer torque in metallic spin valve structures using an intense and ultrafast heat current created by picosecond—one trillionth of a second—pulses of laser light,” Cahill added. “This heat current has the impressively large magnitude of 100 GW per square meter and persists for approximately 50 trillionths of a second. The sign and magnitude of the heat-driven spin current can be controlled by the composition of a ferromagnetic layer and thickness of a heat sink layer.” Cahill’s research group at Illinois studies the physical mechanisms governing the interplay of spin and heat at the nanoscale, addressing the fundamental limits of ultrafast spintronic devices for data storage and information processing. In addition to Choi and Cahill—whose work was supported by the Army Research Office MURI program—co-authors of the paper include Byoung-Chul Min, Center for Spintronics Research, Korea Institute of Science and Technology, Seoul; and Kyung-Jin Lee, Department of Materials Science and Engineering and KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul. SOURCE: University of Illinois at Urbana-Champaign