The spin-Hall effect (SHE) converts an electric current into a pure spin flow with the help of spin-orbit coupling, which can be further used to drive magnetic moment reversal or precession, known as the spin-orbit moment (SOT) effect. It became the physical basis of the third generation of spin-orbit moment-type magnetic random access memory (SOT-MRAM) in the industrial world. In 2009, the Institute of Physics of the Chinese Academy of Sciences and the Beijing National Research Center for Condensed Matter Physics took the lead in applying for and obtaining the first original patent in the field of SOT-MRAM [**Yang, Han Xiufeng, etc., invention patent number: CN200910076048.].x], in which two core structures were invented, such as the spin flow generation layer, the magnetic metal layer and the spin flow generation layer, the magnetic tunnel junction. These two structures have become the core unit structures for subsequent SOT effect research and SOT-MRAM device development. On this basis, in the past ten years, people have tried to further optimize the spin flow generation layer to make it have higher current-spin current conversion efficiency, spin Hall angle, the ability of ultrafast pulse current to drive magnetic torque flip under zero magnetic field, higher conductivity, etc., and finally make the data nonvolatile SOT-MRAM have lower energy consumption (PJ FJ), faster speed (< 1ns) and longer cycle life (>10 10) and other excellent comprehensive performance.
The spin Hall effect originates from the intrinsic spin-orbit coupling of atoms in materials, and it can be generated by three microscopic mechanisms, including the intrinsic mechanism that is dependent on the band structure, and the scattering-related side jump mechanism and the skew scattering mechanism, which broaden the design space of more elements, their alloys and compounds in addition to the preferred heavy metal elements for enhancing the spin Hall effect. In the process, many methods have been tried to enhance the spin Hall effect, including the alloying of light and heavy metal elements, ultra-thin metal stacking structures, and so on. The physical basis for these designs is the introduction of more spin-orbit coupling-related impurity scattering centers in the composites. However, the scattering associated with spin-orbit coupling includes not only the scattering of impurities between electrons and doped atoms, but also the interaction between electrons and magnetic structures, such as magneton scattering or spin fluctuation scattering. The latter provides a physical possibility for establishing a correlation between the spin Hall effect and the magnetically ordered structure and its phase transitions.
Figure 1Schematic diagram of the principle of spin fluctuation enhancing spin Hall effect in the magnetically ordered structure of antiferromagnetic materials. As the temperature approaches to the magnetically ordered phase transition temperature, the spin fluctuation intensifies. This results in two results, one, an increase in the concentration of the local spin (yellow arrow) that is the center of scattering;Second, the length of the association between local spins increases. These two reasons lead to spin fluctuations, which can increase the probability of oblique scattering and edge jump scattering, thereby enhancing the spin Hall effect.
Although researchers have realized that the correlation between the spin Hall effect and the magnetic structure is expected to provide a new design idea and technical approach to enhance the former, and have experimentally tried to reveal the dependence between the spin Hall effect and the magnetic phase transition of the magnetically ordered structure in magnetically ordered systems, such as ferromagnets and antiferromagnets, it is still very difficult to clearly and unambiguously prove the correlation between the two in physics. Because in the classical SOT effect study material system, the spin-orbit moment effect is not only related to the bulk current-spin flow conversion efficiency of the spin flow-generating layer, but also to the spin transport efficiency of the bilayer interfaceBoth are related to the magnetic structure of the spin flow generation layer, which requires the decoupling of bulk phase effects from interface effects, which are physically hindered by their symbiotic relationship with the magnetic structure.
It is the abundant physical correlation between the spin Hall effect and the magnetic ordered system and the great challenge of such studies that led to this study, namely the use of spin flow-generating layers, tunnel junctions, magnetic metal three-layer film structures, and spin Hall tunneling spectroscopy techniques to study the "strong association" between magnetic ordering and spin Hall effects in the antiferromagnetic material chromium as a spin flow-generating layer. This tunnel junction structure avoids the direct contact between the spin flow generation layer and the magnetic metal layer, so it also effectively avoids the intervention of the above-mentioned interface effect, so that the bulk phase effect can be highlighted. Spin Hall tunneling spectroscopy can measure both positive spin Hall effect (DSHE) and reverse spin Hall effect (ISHE), which is a complementary measurement method that can significantly improve the confidence of experimental data. The selected metal CR material is not only a typical antiferromagnetic material, but also can be easily embedded in the CR Mgo Fe all-single crystal magnetic tunneling junction through molecular beam epitaxy technology, and the single crystal system can significantly reduce the influence of impurity scattering on SHE, which is more conducive to the purification of magnetic ordering and highlights its dominant role.
Figure 2(a) Schematic diagram of the spectral measurement layout of the spin Hall tunnel. (b) Arrangement of inverse spin Hall effect measurements. (c) Arrangement of positive spin Hall effect measurements. (d) Dependence of forward and reverse spin Hall resistance with temperature.
As shown in Figure 2, a current is applied between electrodes 1 and 3, and the spin polarization current is then injected into the CRThe spin flow part is due to the existence of the reverse spin Hall effect, which leads to the generation of transverse currents between the 2-4 electrodes. And because of the open-circuit environment between the 2-4 electrodes, we can finally detect the voltage value due to ISHE between the 2-4 electrodes. The polarity of the voltage value can be changed by inverting the direction of the FE magnetization, which can be used as a basis for identifying the ISHE signal. Conversely, when we introduce a current between the 2-4 electrodes, due to the presence of the positive spin Hall effect in the CR, a non-equilibrium spin accumulation will be formed at the CR MGO interface, resulting in a spin chemical potential at this interface. These spin chemistries can be read out by the tunneling junction of the MGO Fe and the ferromagnetic electrode, which in turn generates a voltage between electrodes 1 and 3. Similarly, the polarity of the detection voltage is related to the direction of magnetization of FE as the detection electrode, which is also the basis for screening DSHE signals. With this measurement method, we obtain the dependence of the spin Hall angle-current-spin flow conversion effect of CR with temperature, and find the maximum value of the spin Hall angle near the Nair temperature-antiferromagnetic-paramagnetic phase transition point of CR.
Figure 3The relationship between the resistivity of different material systems and the efficiency of spin-orbit torque-driven magnetic moment flipping is better at the data point in the upper left region of the figure. (a) Comparison is made in terms of energy efficiency proportional to p. (b) Figures are compared according to p proportional to . where is the resistivity of the material, is the spin diffusion length, and is the spin Hall angle. (b) The figure considers the effect of interfacial spin backflow. Because CR has a long spin diffusion length, its performance is lower to the right in (b). However, in the actual SOT-MRAM device, the spin reflow effect can be suppressed, so the actual device energy consumption is compared according to figure (A), and the CR material can show its advantages over other spin-flow generating layer materials.
The experimental results clearly demonstrate the strong correlation between the spin Hall effect and the magnetically ordered structure in the bulk phase of the antiferromagnetic material Cr, and confirm the feasibility of enhancing the spin Hall effect by the magnetic ordered structure and its spin fluctuation phenomenon near the phase transition temperatureIn addition, the CR material itself has higher electrical conductivity and longer spin diffusion length than traditional heavy metals, and the discovery of CR materials with magnetically ordered structure spin fluctuation to enhance the spin Hall effect provides a new class of material options for the development of low-energy and low-cost SOT-MRAM devices. The work has been published in Nano Express. Researcher Han Xiufeng, associate researcher Wan Caihua of the Institute of Physics of the Chinese Academy of Sciences, and Professor Lu Yuan of the University of Lorraine in France conceived and directed the study as co-corresponding authorsDr. Fang Chi (Ph.D. graduate of the Institute of Physics, Chinese Academy of Sciences, now a postdoctoral fellow at the Max Planck Institute for Microstructural Physics, Germany) is the first authorS. S., Director of the Max Planck Institute for Microstructural Physics s. p.Prof. Parkin provided guidance on data analysis;Professor Tang Ning of Peking University and researcher Wen Zhenchao of the National Institute of Materials Research of Japan provided support for the preparation of thin filmsDr. Satoshi Okamoto of the Oak Ridge National Laboratory in the United States and Professor Naoto Nagaosa of the RIKEN Center in Japan provided theoretical guidance for the study. Other collaborators were involved in data analysis, thin film deposition, and writing. The work is thanks to the Chinese Academy of Sciences, the Ministry of Science and Technology of the People's Republic of China for the key research and development projects and the key projects of the ** Committee and other projects.
Editor: Nilo.