Magnetic properties of hematite revealed by an ab initio parameterized spin model

Hematite is a canted antiferromagnetic insulator, promising for applications in spintronics. Here, we present ab initio calculations of the tensorial exchange interactions of hematite and use them to understand its magnetic properties by parameterizing a semiclassical Heisenberg spin model. Using atomistic spin dynamics simulations, we calculate the equilibrium properties and phase transitions of hematite, most notably the Morin transition. The computed isotropic and Dzyaloshinskii--Moriya interactions result in a N\'eel temperature and weak ferromagnetic canting angle that are in good agreement with experimental measurements. Our simulations show how dipole-dipole interactions act in a delicate balance with first and higher-order on-site anisotropies to determine the material's magnetic phase. Comparison with spin-Hall magnetoresistance measurements on a hematite single-crystal reveals deviations of the critical behavior at low temperatures. Based on a mean-field model, we argue that these differences result from the quantum nature of the fluctuations that drive the phase transitions.Comment: 11 pages, 10 figure

Limited Stochastic Current for Energy-Optimized Switching of Spin-Transfer-Torque Magnetic Random-Access Memory

The switching of spin-transfer-torque magnetic random-access memory (STT MRAM) in the simple macrospin model is determined by the amplitude and pulse duration of the applied current, and it requires a current that is higher than a critical current, Ic. However, this critical current misses one fundamental physical issue for the commercialization of STT MRAM; the so-called nonswitching probability (PNS) or write soft-error rate (WSER), which is influenced by the stochastic nature of the switching process at finite temperature. Herein, we propose a limited stochastic switching (LSS) current, which is another definition for the critical current with the PNS incorporated. The definition of the LSS current and the analytical expressions are obtained by solving the Fokker-Planck equation with a given specific PNS value. Most importantly, by using the LSS current and optimizing it together with the related pulse-duration time, we find the optimum combination of current amplitude and pulse duration, which may reduce the energy consumption of the STT MRAM by up to 75%. © 2019 American Physical Society.1

Magnetic-stimuli rheological response of soft-magnetic manganese ferrite nanoparticle suspension

The hydrothermal fabrication of crystalline manganese ferrite (MnFe2O4) nanoparticles and their application as a magnetorheological (MR) fluid dispersed in an insulating oil are described herein. The morphology and crystal structure of the MnFe2O4 nanoparticles are revealed by transmission electron microscopy, scanning electron microscopy, and X-ray diffraction. In addition, with a relatively high saturation magnetization of the MnFe2O4 particles, their typical MR behavior is demonstrated by rheometric steady shear and dynamic oscillation tests under an applied magnetic field. Moreover, the flow and yield stress curves for the MnFe2O4 nanoparticle-based MR fluid are shown to conform to the Herschel–Bulkley model with a slope of 1.5. Finally, under the same magnetic field strength, the dynamic yield stress is shown to be higher than the elastic yield stress. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.FALS

Role of the chiral spin configuration in field-free spin-orbit torque-induced magnetization switching by a locally injected spin current

For deterministic magnetization switching by spin-orbit torque (SOT) in a perpendicular magnetic anisotropy system, an additional in-plane direction magnetic field is essential to break the lateral symmetry. Realizing chirality in a magnetic ordering system can be one approach for achieving asymmetry in the lateral direction for field-free magnetization switching. However, systematic analysis of the influence of the chiral spin system on deterministic switching is still scarce. We investigate the field-free SOT-induced magnetization switching by using a chiral spin configuration experimentally and theoretically with micromagnetic simulations. We designed a system in which only part of the ferromagnetic layer overlaps with the heavy metal layer in the Pt/Co/MgO structure. Therefore, a spin current exerts only on a local area of the ferromagnetic layer, which results in a Neel-type chiral spin configuration. The induced chiral spin configuration can be stabilized (or destabilized) depending on the sign of the interfacial Dzyaloshinskii-Moriya interaction and the direction of the current. The stabilized spin configuration plays a crucial role in the deterministic switching in the zero field. We expect our findings to widen the perspective on chirality-based all-electrical SOT device applications. Published under an exclusive license by AIP Publishing.TRU

Suspension rheology of polyaniline coated manganese ferrite particles under electric/magnetic fields

Manganese ferrite (MnFe2O4) nanoparticles initially fabricated using a solvothermal process were coated with conducting polyaniline (PANI) to produce core/shell-structured MnFe2O4/PANI nanoparticles. An electro/magnetorheological (E/MR) fluid was prepared by suspending MnFe2O4/PANI particles in silicone oil, and the rheological properties under either electric or magnetic fields were investigated. Scanning electron microscope and transmission electron microscope provided the particle morphology and size information. X-ray diffraction and Fourier transform infrared spectroscopy were used to analyze the crystal structure and chemical composition of the particles. Chain formation in E/MR fluids under electric or magnetic fields was observed by optical microscopy, and the rheological properties were evaluated using a rheometer. Steady shear and dynamic oscillatory tests were conducted to confirm the effective E/MR characteristics while varying the electric/magnetic field strength. The dielectric properties of the particles measured using an LCR meter were analyzed based on the Cole-Cole model. The E/MR fluids composed of MnFe2O4/PANI showed a reversible and fast electro/magnetic response. © 2022FALS

Improved spin-orbit torque induced magnetization switching efficiency by helium ion irradiation

Increasing the efficiency of spin-orbit torque (SOT) is of great interest in spintronics devices because of its application to the non-volatile magnetic random access memory and in-logic memory devices. Accordingly, there are several studies to alter the magnetic properties and reduce the SOT switching current with helium ion irradiation, but previous researches are focused on its phenomenological changes only. Here, the authors observe the reduction of switching current and analyze its origins. The analyzed major reasons are improved spin Hall angle represented as the changed resistivity of heavy metal layer and the reduction of surface anisotropy energy at interface between heavy metal and ferromagnet. It is confirmed that almost linear relation between changed SHA and Pt resistivity by helium ion irradiation, which is attributed because of the increase in the scattering sources induced by structural distortion during ion penetration. From the calculated power consumption ratio based on the derived parameter, the requiring power decreases according to the degree of ion irradiation. Our results show that helium ion penetration induced layer and interfacial disturbance affects SOT induced magnetization switching current reduction and may provide possibility about helium ion irradiation based superior SOT device engineering.TRU

Roles of magnetic coupling and spin-orbit torque in the electrical manipulation of exchange bias in a Pt/Co/IrMn heterostructure

Heavy-metal/ferromagnet/antiferromagnet heterostructures have attracted significant attention in manipulating antiferromagnetic ordering electrically. We report electric control of the exchange bias field along both the out-of-plane and in-plane directions in Pt/Co/IrMn with assist magnetic fields. Experiments with different directions and amplitudes of the assist magnetic field show the role of magnetic coupling between ferromagnet and antiferromagnet spins. Furthermore, we find that the magnitude and direction of the exchange bias can be manipulated by the magnitude of the current pulse. The observed results can be explained based on a numerical macrospin model; the antiferromagnetic spins are excited and precessed from their ground state via the spin-orbit torque-driven oscillation and are reoriented, being mediated by magnetic coupling with the ferromagnet. Our results provide an approach to achieve electrical omnidirectional exchange bias manipulation and an understanding of the spin dynamics at the antiferromagnet-ferromagnet interface. © 2023 American Physical Society.FALS

Chirality-dependent energy induced by spin-orbit torque-driven artificial spin texture

Recently, vast research has been under investigation on the role of chirality in magnetization dynamics, an area that currently lacks a comprehensive understanding. To gain further insight into the importance of chirality, we explore the effects of varying the degree of chirality. To investigate these effects, we have fabricated samples with perpendicular magnetic anisotropy symmetry breaking through local helium ion irradiation with various azimuthal angles and degrees of irradiation. In this system, the spin-orbit torque can induce artificial spin texture, and our azimuthal angle and degree of chirality dependent results reveal a clear cosine dependence and a nonlinear increase, respectively. It implies the system not only follows the energy contribution of interfacial Dzyaloshinskii-Moriya interaction but also has a nonlinear impact depending on anisotropy asymmetry induced chirality differences. These experimental observations are consistent with our theoretical model and micromagnetic simulations, supporting our experimental results. Overall, our findings provide further insights into the role of chirality in magnetization dynamics and may have important implications for the development of future magnetic devices. © 2023 Vietnam National University, HanoiTRU

Slater-Pauling behavior of interfacial magnetic properties of 3d transition metal alloy/Pt structures

Ferromagnet (FM)/heavy metal (HM) bilayers are core structures for current-induced magnetization switching and chiral magnetic structure generation. Static and dynamic properties of the FM moment depend substantially on interfacial perpendicular magnetic anisotropy (iPMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI). Therefore, it is of crucial importance to control iPMA and iDMI, and to understand their underlying physics. Here we experimentally show that both iPMA and iDMI exhibit similar Slater-Pauling-like dependence on the FM variation as the saturation magnetization (MS) does. We measure MS, iPMA, and iDMI of the FM/HM bilayers with HM fixed to Pt and FM varied from Mn (electron number Z = 25) to Ni (Z = 28), including their alloys for fractional Z. Our result indicates that the density of states structure important for the Slater-Pauling dependence is crucial also for iPMA and iDMI. This provides a useful method to engineer chiral magnetic textures.11Nsciescopu

Formation of helical spin alignment in the AFM/FM/AFM trilayers by spin–orbit torque controlled exchange bias

Non-collinear spin structures can exhibit unusual magnetic properties that cannot be expected in an ordinary collinear ferromagnet (FM) due to the chiral alignment of magnetic moments, offering new opportunities for applications in the field of spintronics. In the present study, we demonstrate that exchange bias pinning can be applied to a single FM layer in two different directions simultaneously, resulting in modified magnetic behaviors due to the formation of non-collinear helical spin structures in the multilayers of Co0.7Ni0.3O (antiferromagnet, AFM)/Co0.7Ni0.3 (FM)/Co0.7Ni0.3O (AFM)/Pt (heavy metal, HM). The pinning of spins at one interface between FM and AFM/HM was controlled by spin Hall current originating from the electrical current through the HM layer at room temperature, while the spins at the other interface between FM and AFM were pinned in a fixed direction, hence allowing for the formation of a helical spin structure along the FM layer thickness with controllable chirality at room temperature. Modified magnetic behaviors of a helical spin structure were confirmed from measurements of magnetic hysteresis and magnetoresistance, as well as direct observation of magnetic domains