Anisotropic Magnetic Materials Research Articles

Сверхбыстрое лазерно-индуцированное управление магнитной анизотропией наноструктур

Employing short laser pulses with a duration below 100 fs for changing magnetic state of magnetically-ordered media has developed into a distinct branch of magnetism —femtomagnetism which aims at controlling magnetization at ultimately short timescales. Among plethora of femtomagnetic phenomena, there is a class related to impact of femtosecond pulses on magnetic anisotropy of materials and nanostructures which defines orientation of magnetization, magnetic resonance frequencies and spin waves propagation. We present a review of main experimental results obtained in this field. We consider basic mechanisms responsible for a laser-induced change of various anisotropy types: magnetocrystalline, magnetoelastic, interfacial, shape anisotropy, and discuss specifics of these processes in magnetic metals and dielectrics. We consider several examples and describe features of magnetic anisotropy changes resulting from ultrafast laser-induced heating, impact of laser-induced dynamic and quasistatic strains and resonant excitation of electronic states. We also discuss perspectives of employing various mechanisms of laser-induced magnetic anisotropy change for enabling processes prospective for developing devices. We consider precessional magnetization switching for opto-magnetic information recording, generation of high-frequency strongly localized magnetic excitations and fields for magnetic nanotomography and hybrid magnonics, as well as controlling spin waves propagation for optically-reconfigurable magnonics. We further discuss opportunities which open up in studies of ultrafast magnetic anisotropy changes because of using short laser pulses in infrared and terahertz ranges.

Temperature-dependent magnetic transitions in CoCrPt-Ru-CoCrPt synthetic ferrimagnets

The magnetic orientations and switching fields of a CoCrPt-Ru-CoCrPt synthetic ferrimagnet with perpendicular magnetic anisotropy have been studied in the temperature range from 2 K to 300 K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet across this temperature range. The first set exhibits three magnetic transitions in the 50 K–370 K range, whereas the second involves only two transitions in the 2 K and 50 K range. The observed magnetic hysteresis curves of the synthetic ferrimagnet are explained using the energy diagram technique framework pioneered by Koplak et al. [1] which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. In this work we expand the framework to include synthetic ferrimagnets (SFMs) comprising higher perpendicular magnetic anisotropy materials and large (4X) interlayer exchange coupling energies which are promising for the development of ultrafast (ps) magnetic switching free layers in MTJ structures. Furthermore, we apply the analysis to predict SFM magnetic hysteresis curves in a temperature regime that includes temperature extrema that a synthetic ferrimagnet would be expected to reliably operate at, were it to be utilized as a free layer in a memory or sensor spintronic device.

Low Gilbert damping and high thermal stability of Ru-seeded L10-phase FePd perpendicular magnetic thin films at elevated temperatures.

Bulk perpendicular magnetic anisotropy materials are proposed to be a promising candidate for next-generation ultrahigh density and ultralow energy-consumption spintronic devices. In this work, we experimentally investigate the structure, thermal stability, and magnetic properties of FePd thin films seeded by a Ru layer. An fcc-phase Ru layer induces the highly-ordered L10-phase FePd thin films with perpendicular magnetic anisotropy (K u ~ 10.1 Merg/cm3). The thermal stability of FePd samples is then studied through the annealing process. It is found that a K u ~ 6.8 Merg/cm3 can be obtained with the annealing temperature of 500 °C. In addition, the damping constant α, an important parameter for switching current density, is determined as a function of the testing temperature. We observe that α increases from 0.006 to 0.009 for as-deposited FePd sample and from 0.006 to 0.012 for 400 °C-annealed FePd sample as the testing temperature changes from 25 °C to 150 °C. These results suggest that Ru-seeded FePd provides great potential in scaling perpendicular magnetic tunnel junctions below 10 nm for applications in ultralow energy-consumption spintronic devices.

Investigation of Spin Transport Properties in Perpendicularly Magnetized MoS2/Pt/[Co/Ni]n Multilayers with Effective Spin Injection into Two-Dimensional MoS2

Injecting spin current from ferromagnetic metals into two-dimensional (2D) transition-metal dichalcogenide semiconductors is expected to trigger a revolution in the next generation of energy-efficient spintronic and valleytronic devices. By investigating the spin transport properties in perpendicularly magnetized ${\mathrm{Mo}\mathrm{S}}_{2}/\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_{2}$ and ${\mathrm{Mo}\mathrm{S}}_{2}/\mathrm{Pt}/\mathrm{Ni}/[\mathrm{Co}/\mathrm{Ni}{]}_{2}$ heterostructures, here we demonstrate a strong and controllable spin injection into the 2D ${\mathrm{Mo}\mathrm{S}}_{2}$ layer through a nonmagnetic $\mathrm{Pt}$ layer. Based on optical dynamic measurements, noticeable changes in the magnetic damping constant show that the spin injection intensity can be controlled by the $\mathrm{Pt}$ layer thickness. The improved spin mixing conductance of the interface is much higher than those reported in $\mathrm{Pt}/\mathrm{Ni}$/permalloy (Py) systems with in-plane magnetic anisotropy. Our results shed lights on the development of next-generation low-power spintronic devices employing both perpendicular magnetic anisotropy and 2D material.

Electrical switching of perpendicular magnetization in a single ferromagnetic layer

Electrical manipulation of magnetization is essential for integration of magnetic functionalities such as magnetic memories and magnetic logic devices into electronic circuits. The current induced spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) bilayers via the spin Hall effect in the HM and/or the Rashba effect at the interfaces provides an efficient way to switch the magnetization. In the meantime, current induced SOT has also been used to switch the in-plane magnetization in single layers such as ferromagnetic semiconductor (Ga,Mn)As and antiferromagnetic metal CuMnAs with globally or locally broken inversion symmetry. Here we demonstrate the current induced perpendicular magnetization switching in L10 FePt single layer. The current induced spin-orbit effective fields in L10 FePt increase with the chemical ordering parameter (S). In 20 nm FePt films with high S, we observe a large charge-to-spin conversion efficiency and a switching current density as low as 7.0E6 A/cm2. We anticipate our findings may stimulate the exploration of the spin-orbit torques in bulk perpendicular magnetic anisotropic materials and the application of high-efficient perpendicular magnetization switching in single FM layer.

THE RECIPROCITY PRINCIPLE FOR A NONLINEAR ANISOTROPIC MEDIUM WITHOUT HYSTERESIS: THEORY AND PRACTICE OF APPLICATION

The construction of the correct vector material equations for nonlinear anisotropic soft magnetic materials remains one of the main reserves for increasing the accuracy of mathematical models in solving magnetostatic problems in the field formulation. The aim of the work is to establish asymptotic expressions for the reciprocity principle, which is a fundamental property of reversible magnetization processes of nonlinear anisotropic media, and to use the obtained results to optimize the computational process when constructing the vector magnetization characteristic and differential permeability tensor. The potentiality property of the magnetic flux density vector B in H -space is used. The main result of the paper is an illustration, using concrete examples, of an alternative method for calculating vector magnetization characteristics for one of the orthogonal families. In order to eliminate the instrumental error and ensure maximum accuracy and reliability of the obtained results, the exact characteristics for the components of the vector magnetization characteristic obtained by differentiating a special analytical expression for the potential were used as initial ones. The principle of reciprocity, by virtue of its universal nature, makes a significant contribution to the theory of nonlinear anisotropic media in the hysteresis-free approximation. Asymptotic expressions for the reciprocity principle are obtained for the first time. The performed computational experiments on the construction of vector characteristics based on the known magnetization characteristics in one of the directions confirm almost complete coincidence with the exact values obtained analytically. The use of asymptotic expressions for the reciprocity principle not only greatly simplifies computational processes for determining the orthogonal magnetization characteristics, but also implements the calculation of differential permeability tensors for arbitrary field values. The proposed method can be implemented in applications for calculating the magnetic field in devices with nonlinear anisotropic magnetically soft materials, primarily with cold rolled sheet electrical steels, which are most used in electrical engineering.

Nano-oscillator based on radial vortex by overcoming the switching of core

The spin torque nano-oscillators (STNOs) have garnered significant interest as a potential nanosized microwave signal generator. However, the core of magnetic radial vortex may be annihilated at the edge with the large spin polarized current density, which leads to unstable STNOs. In this work, we propose new STNOs based on a magnetic radial vortex by overcoming the annihilation of core at the edge, which was realized by adding high magnetic crystal anisotropy materials on the boundary of the nanodisk. It is found that the oscillation frequency reaches up to 2.8 GHz through a circular motion of core of magnetic radial vortex, and the radial vortex-based STNOs have advantages of small size, lower driving current. Our result has important significance to design new STNOs and provide a possible candidate for designing of spintronic devices based on magnetic radial vortex.

Shape anisotropic Fe3O4 nanotubes for efficient microwave absorption

Although Fe3O4 particles have exhibited excellent microwave absorbing capacity and widely used in practical application due to the synergistic effect of magnetic loss and dielectric loss, their applications are still limited for the required high mass fraction in absorbers. To overcome this problem, the development of Fe3O4 materials with low dimensional structures is necessary. In this study, the shape anisotropic Fe3O4 nanotubes (NTs) with low mass ratios were applied to realize efficient microwave absorption. The NTs with different aspect ratios were prepared through facile electrospinning followed by two-step thermal treatments and mechanical shearing. The cross-linked nanotubular structure enabled the absorbers to have much higher electrical conductivity, multiple scattering, polarization relaxation and better anti-reflection surface, while the shape anisotropic NTs maintained significant multiple resonances with stronger coercivity. These all were beneficial to microwave absorption with enhanced dielectric loss, magnetic loss and sterling impedance matching. Results showed that the absorber with 33.3 wt.% of short Fe3O4 NTs had minimum reflection loss of -58.36 dB at 17.32 GHz with a thickness of 1.27 mm, and had the maximum effective absorbing bandwidth (EAB) of 5.27 GHz when the thickness was 1.53 mm. The absorber with 14.3 wt.% of long Fe3O4 NTs presented the widest EAB in certain radar band with attenuated 80.75% X band and 85% Ku band energy bellow -10 dB at the thickness of 2.65 and 1.53 mm, respectively. This study provided an approach for the development of shape anisotropic magnetic absorbing materials, and broadened their practical applications as magnetic absorbers.

Analysis, Design and Optimization of Hysteresis Clutches

Electromagnetic clutches are widely used devices in industrial applications when a torque must be transmitted between two components without mechanical contact avoiding friction and so reducing wear and maintenance time. Hysteresis clutches are a particular variant of electromagnetic couplers used when a constant torque from zero to the synchronous speed needs to be transmitted. This paper deals with the analysis, design and optimization of hysteresis clutches. After a brief introduction on the operating principle, a new method of analysis is then introduced. The latter allows a fast prediction of the performance of hysteresis couplers without sacrificing the accuracy of the performance evaluation even when adopting anisotropic magnetic materials. The introduced method, based on a static finite element simulation followed by a post-processing of the evaluated magnetic field, is then used within an automatic design procedure in order to identify the inevitable trade-offs encountered when optimizing the geometry of any hysteresis clutches with a given outer envelope. The obtained results are commented in-depth and allow to draw general design guidelines with special regards to the selection of the number of poles and the magnetic materials. The effect of the high level of anisotropy of the most common magnetic materials showing the highest energy density is carefully taken into account allowing to deduce the optimal preferred direction of magnetization.

Spin–Orbit Torque Switching in Low-Damping Magnetic Insulators: A Micromagnetic Study

Magnetic insulators, in particular, rare-earth iron garnets, have low damping compared to metallic ferromagnetic materials due to lack of conduction electrons. In analogy to spin-transfer-torque devices, the low-damping nature is presumed to be an advantage for spintronic applications. We report that perpendicular magnetic anisotropy material with low damping actually does not favor reliable spin–orbit torque (SOT) switching. Increasing damping, introducing interfacial Dzyaloshinskii–Moriya interactions, or field-like torques may help SOT switching in some cases. Having notches in a nanometer-scale element, which is a more realistic size for practical applications, can also improve switching stability.

The effects of annealing temperature and heating rate on Ta/TbFeCo bilayers

Among various perpendicularly anisotropic magnetic materials, amorphous rare earth-transition metal RE-TM alloys have attracted much interest due to the advantages of tunable magnetic properties and suitable perpendicular magnetic anisotropy (PMA) strength. In this study, the magnetic properties of the Ta/TbFeCo/MgO structure with various Tb-contents are investigated under different annealing conditions like heating rate and annealing temperature. The samples were annealed at heating rate from 10 °C/min to 50 °C/min and holding time was varied from 10 min to 30 min with annealing temperatures ranging from 100 to 150 °C. We found that fast heating rate and low holding temperature help to improve PMA and significantly increase the saturation magnetization and squareness of the TbFeCo films.

Complex Performance Analysis and Comparative Study of Very High-Speed Switched Reluctance Motors

This paper presents the comparative analysis of the characteristic features of two 4/2 switched reluctance motors (SRMs) designed for a very high-speed drive. The use of two-phase construction of SRM with irregular air-step gap makes it possible to achieve the required parameters of the drive. The analysis presents the comparison of two distinct structures designed for the same very high-speed drive. In this paper, a novel mathematical model that takes into account the nonlinearity of the magnetic circuit is presented. The results of numerical calculations and laboratory tests of the influence of the possible use of an anisotropic magnetic material in the rotor magnetic circuit are presented. The influence of control parameters on the motor properties is specified. Both the structures were compared with the respect to employed electric parameters. It is important for high-speed structures that a maximal speed of the motor should not affect permissible parameters of the rotor in a negative way. The high-speed structures should be properly designed to ensure a safety margin. The selected laboratory test results that show electric parameters are presented following the specification of the first critical speed based on the Campbell diagram, the maximum speed limited by allowable stresses. It was shown that both the structures analyzed fulfill the condition of a safety margin. A number of thermal problems of the structures analyzed are discussed. The design assumptions related to the possibility of using a higher current density in the motor windings were verified under laboratory conditions.

Rotational magnetocaloric effect of anisotropic giant-spin molecular magnets

The magnetocaloric effect, that consists of adiabatic temperature changes in a varying external magnetic field, appears not only when the amplitude is changed, but in cases of anisotropic magnetic materials also when the direction is varied. In this article we investigate the magnetocaloric effect theoretically for the archetypical single molecule magnets Fe8 and Mn12 that are rotated with respect to a magnetic field. We complement our calculations for equilibrium situations with investigations of the influence of non-equilibrium thermodynamic cycles.

Spin-orbit-torque-driven multilevel switching in Ta/CoFeB/MgO structures without initialization

Spin-orbit torque (SOT) has been proposed as an alternative writing mechanism for the next-generation magnetic random access memory (MRAM), due to its energy efficiency and high endurance in perpendicular magnetic anisotropic materials. However, the three-terminal structure of SOT-MRAM increases the cell size and consequently limits the feasibility of implementing high density memory. Multilevel storage is a key factor in the competitiveness of SOT-MRAM technology in the nonvolatile memory market. This paper presents an experimental characterization of a multilevel SOT-MRAM cell based on a perpendicularly magnetized Ta/CoFeB/MgO heterostructure and addresses the initialization-free issue of multilevel storage schemes. Magneto-optical Kerr effect microscopy and micromagnetic simulation studies confirm that the multilevel magnetization states are created by changing a longitudinal domain wall pinning site in the magnet. The realization of robust intermediate switching levels in the commonly used perpendicularly magnetized Ta/CoFeB/MgO heterostructure provides an efficient way to switch magnets for low-power, high-endurance, and high-density memory applications.

Borderline Magnetism: How Adding Mg to Paramagnetic CeCo3 Makes a 450-K Ferromagnet with Large Magnetic Anisotropy

A recent experimental study (Phys. Rev. Appl. 9, 024023, 2018) on paramagnetic CeCo$_3$ finds that Magnesium alloying induces a ferromagnetic transition with intrinsic properties large enough for permanent magnet applications. Here we explain these surprising results \textit{via} a first principles study of the electronic structure and magnetism of Magnesium-alloyed CeCo$_3$. We find the origin of this Magnesium-induced ferromagnetic transition to be Stoner physics - the substantial increase in the Fermi-level density-of-states $N(E_F)$ with Mg alloying. Our calculations suggest that both Ce and Co atoms are important for generating large magnetic anisotropy suggesting the viability of Co-3$d$, and Ce-4$f$ interaction for the generation of magnetic anisotropy in magnetic materials. These results offer a new route to the discovery of ferromagnetic materials and provide fundamental insight into the magnetic properties of these alloys

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L1-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.

Robust emergence of a topological Hall effect in MnGa/heavy metal bilayers

We have investigated the topological Hall effect (THE) in MnGa/Pt and MnGa/Ta bilayers induced by interfacial Dzyaloshinskii-Moriya interaction (DMI). The most evident THE signals have been found based on the MnGa films with small critical DMI energy constant Dc. The large topological portion of the Hall signal from the total Hall signal has been extracted in the whole temperature range from 5 to 300 K. These results open up the exploration of the DMI induced magnetic behavior based on the bulk perpendicular magnetic anisotropy materials for fundamental physics and magnetic storage technologies.

Magnetocrystalline anisotropy in the Co/Fe codoped Aurivillius oxide with different perovskite layer number

AbstractPerpendicular magnetic anisotropic materials are of great interest for their huge potential to realize high‐density nonvolatile memory and logic chips. Designing such materials with lower cost and better magnetoelectric coupling still remains major challenges. In this work, aurivillius oxide ceramics with highly [00l]‐oriented grains were prepared by a facile pressureless sintering method, and the perovskite layer number was modified by varying the cobalt content. Afterwards, the unique perpendicular magnetic anisotropy and the intriguing anisotropic ferroelectric properties have been observed. The magnetic properties of the Aurivillius oxides with different layer numbers have been carefully investigated by field cooling, zero field cooling, and the magnetization with a varying field. The magnetic anisotropy in the oriented ceramics is demonstrated to be caused by the magnetocrystalline anisotropy with the easy magnetization direction along the c‐axis, which possibly arises from the unquenched 3d orbitals combined with the special layered crystal structures. The magnetocrystalline anisotropy becomes weaker in the ceramics with lower numbered perovskite layers, whereas the orientation degree of the ceramics and the ferroelectric anisotropy show quite opposite trends. Furthermore, the weak magnetoelectric coupling is also observed in the ceramics. This special anisotropic multiferroic properties may open up a new window for the Aurivillius materials.

Hybrid magnetic anisotropy [Co/Ni]15/Cu/[Co/Pt]4 spin-valves

Thin film multilayer heterostructures consisting of two magnetically coupled stacks, one with in-plane and the second one with out-of-plane magnetic anisotropies, are very promising hybrid magnetic anisotropy materials for spintronics. Here we present results on magnetic and magnetoresistance properties of [Co/Ni]15 (in-plane)/Cu/[Co/Pt]4 (out-of-plane) spin-valves with hybrid magnetic anisotropy. We demonstrate that the saturation field and magnetoresistance depend on the thickness of the copper interlayer (tCu) and they have peak values at tCu = 2.5 nm, where the antiferromagnetic coupling is maximum. We reveal that an indirect exchange coupling between [Co/Ni]n and [Co/Pt]m stacks decreases the magnetization switching fields making these systems suitable for low-field sensor, spin torque oscillator and bit patterned media applications.

A highly efficient 3D level-set grain growth algorithm tailored for ccNUMA architecture

A highly efficient simulation model for 2D and 3D grain growth was developed based on the level-set method. The model introduces modern computational concepts to achieve excellent performance on parallel computer architectures. Strong scalability was measured on cache-coherent non-uniform memory access (ccNUMA) architectures. To achieve this, the proposed approach considers the application of local level-set functions at the grain level. Ideal and non-ideal grain growth was simulated in 3D with the objective to study the evolution of statistical representative volume elements in polycrystals. In addition, microstructure evolution in an anisotropic magnetic material affected by an external magnetic field was simulated.