Magnetic Anisotropy Research Articles

Micromagnetic simulations of magnon-magnon coupling in synthetic antiferromagnets with tilted magnetic anisotropy.

Hybrid magnonics has attracted extensive attention for its potential applications in quantum information processing, especially following the discovery of strong coupling in magnon-magnon hybrid systems. In this paper, we studied the coupling phenomena between the left-handed (LH) and right-handed (RH) magnon modes in synthetic antiferromagnets (SAFs) with a tilted perpendicular magnetic anisotropy (PMA). By tilting the PMA at a certain angle from the film normal, we achieved strong magnon-magnon coupling without the need for an external magnetic field. The resulting hybrid eigenmodes exhibit characteristics of a linear combination of pure LH and RH modes. In addition, micromagnetic simulations revealed in detail the gradual change of hybrid resonance modes from linear to elliptic, and eventually to circular polarization as the coupling strength gradually decreases. We also examined the effects of an applied magnetic field on the coupling strength and mechanism between the two eigenmodes. These findings provide valuable insights into magnetic dynamics within hybrid magnonic systems for spintronic applications involving magnon polarization.&#xD.

Leaky-integrate-fire and reconfigurable neuron spiking in a field free spin Hall nano-oscillator with a conically magnetized free layer

Spintronic oscillators being highly nonlinear have gained immense attention to mimic the neuron spiking behavior in spiking neural networks used for building neuromorphic computing hardware. However, the need for an external magnetic field to realize spintronic oscillators imposes significant limitations on their scalability, tunability, and fabrication complexity. So, in this study, we have realized a bias-field-free spin Hall nano-oscillator (SHNO) using a heavy metal/magnetic tunnel junction (MTJ) heterostructure. The field-free operation is achieved by biasing the free layer of the MTJ into an easy cone regime. This regime arises when the first-order magnetic anisotropy field and the demagnetization field are balanced, and the influence of the second-order magnetic anisotropy becomes predominant. We have explored the oscillation properties of this field-free SHNO, focusing on frequency tunability with current and the output power spectral density through the macrospin and micromagnetic simulations. We have theoretically derived the critical current necessary for the onset of oscillations. Using this field-free SHNO, we demonstrated neuron-like spiking behavior analogous to the Hodgkin–Huxley model by applying a constant DC current. The tunability of spiking frequency in response to input current was also examined. Moreover, we showcased leaky-integrate-and-fire neuron spiking behavior by applying a pulsed current and the reconfigurable nature of neuron spiking under a time-varying ramp current. These spiking behaviors underline the potential applications of this device in the practical realization of brain-inspired neuromorphic computing with the spiking neural network.

Room-temperature topological spin textures and magnetic-field-induced skyrmion-bimeron switching in FeSnN3 monolayer

Two-dimensional (2D) polar magnets have received considerable attention due to their intrinsic ability to host Dzyaloshinskii–Moriya interaction (DMI), which is crucial for generating topological spin textures such as skyrmions and bimerons. The ability to switch between skyrmions and bimerons is considered to be important for developing future computing architectures based on multiple different topological bits. Here, using first-principles calculations and Monte Carlo simulations, we predict that the FeSnN3 monolayer with a polar structure is a 2D ferromagnetic half-metal, exhibiting an out-of-plane magnetic anisotropy energy of 0.181 meV, a high Curie temperature TC of 510 K, and a substantial DMI of 2.96 meV. Micromagnetic simulations demonstrate that the DMI-induced skyrmions in the FeSnN3 monolayer can persist above room temperature under feasible magnetic fields. Notably, skyrmion-bimeron switching can be achieved by altering the direction of the external magnetic field. Our findings not only suggest that the FeSnN3 monolayer is a promising candidate for developing spintronic devices based on topological spin textures but also provide alternative insights into skyrmion-bimeron switching through magnetic field.

Optically Readable Multilevel Magnetic Memory States in Perpendicularly Exchange-Biased Ferromagnetic Multilayers.

The construction of multilevel magnetic states using materials with perpendicular magnetic anisotropy (PMA) offers a novel approach to enhancing the storage density and read/write efficiency of nonvolatile magnetic memory devices. In this study, optically readable multilevel magnetic domain states are achieved by inducing asymmetric interlayer interactions and decoupling the magnetic reversal behavior of individual ferromagnetic (FM) layers in exchange-biased FM multilayers with PMA. Hepta-level magnetic domain states are formed in [Co/Pt]n FM multilayers grown on an antiferromagnetic Fe2O3 layer within a relatively low magnetic field range of ∼±400Oe. Raising and lowering operations between states are demonstrated to be achievable, enabling the writing of new information without the need for initialization in multilevel magnetic memory applications. This design concept, leveraging multilevel magnetic domain states and facilitating noncontact optical reading of stored information, demonstrates the potential to enhance the storage density of nonvolatile magnetic memory devices as it eliminates the need for electrical circuits typically required in other resistive memory technologies.

Write error ballooning due to anisotropy variation within the free layer in perpendicular magnetic tunnel junctions

Abstract A finite-temperature micromagnetic investigation is conducted to analyze magnetization switching and write error rates in perpendicular spin-transfer torque random access memory (STT-RAM), including the influence of domain wall (DW) pinning arising from variations within the free layer (FL) magnet and stray field from reference layer assembly in the magnetic tunnel junction. The study reveals that uneven perpendicular magnetic anisotropy within the FL could lead to the formation of pinning sites that give rise to metastable states during switching. Such metastable states could result in a significant increase in the write error rates as compared to the ideal situation. Unlike the case with stray fields arising from reference layer assembly, where the DW propagation is hindered only for one switching direction, inhomogeneity within the FL causes the DW pinning to occur in both switching directions. Combined effects of FL inhomogeneity and stray field from the reference layer result in write error ballooning similar to those reported in recent experiments. The impact of free layer inhomogeneity could be further exacerbated by the stray field for one of the switching directions. Surprisingly, the impact of inhomogeneity is also observed to persist at smaller FL dimensions where quasi-coherent switching is expected. These findings provide deeper insights into the potential factors contributing to anomalous write errors in perpendicular STT-RAM.

Interfacial Strain-Driven Large Topological Hall Effects in Supermalloy Thin Films with Noncoplanar Spin Textures.

Materials exhibiting topological transport properties, such as a large topological Hall resistivity, are crucial for next-generation spintronic devices. Here, we report large topological Hall resistivities in epitaxial supermalloy (NiFeMo) thin films with [100] and [111] orientations grown on single-crystal MgO (100) and Al2O3 (0001) substrates, respectively. While X-ray reciprocal maps confirmed the epitaxial growth of the films, X-ray stress analyses revealed large residual strains in the films, inducing tetragonal distortions of the cubic NiFeMo unit cells. Magnetic force microscopy showed the presence of skyrmion-like features, which may arise from strain-induced spin texturing in the films. Magneto-optic Kerr effect confirmed a 4-fold magnetic anisotropy in the NiFeMo/MgO (100) film which exhibited a more pronounced topological Hall effect compared to that in the NiFeMo/Al2O3 (0001) film. We envisage that the strain-induced disruption of the centrosymmetry in the NiFeMo films possibly leads to Dzyaloshinskii-Moriya (DM) interactions within the spins. This gives rise to the skyrmion-like spin textures and notable topological Hall effects near room temperature hitherto unobserved in NiFeMo thin films. Theoretical simulations based on DM interactions strongly correlate the experimental results and corroborate with the presence of skyrmion-like spin textures in the films. The modulation of topology and magnetism in NiFeMo thin films through strain engineering may provide useful direction for the pursuit of quantum Hall effect based spintronic devices.

First-principles prediction of intrinsic ferrovalley properties in Janus rare-earth PrXY (X ≠ Y = Cl, Br, I) monolayers.

In this work, using first-principles calculations, we predict a promising class of two-dimensional ferromagnetic semiconductors, namely Janus PrXY (X ≠ Y = Cl, Br, I) monolayers. Through first-principles calculations, we found that PrXY monolayers have excellent dynamic and thermal stability, and their band structures, influenced by magnetic exchange and spin-orbital coupling, exhibit significant valley polarization. Between K and -K valleys, the Berry curvature values are opposite to each other, resulting in the anomalous valley Hall effect. In addition, applying moderate biaxial strain can further enhance their magnetic anisotropy energy and valley polarization. These findings do not only emphasize the importance of strain in regulating spin and valley characteristics, but also provide new possibilities for the application of ferromagnetic semiconducting materials in spintronic and valleytronic devices.

Effect of Hubbard U-corrections on the electronic and magnetic properties of 2D materials: a high-throughput study

We conduct a systematic investigation of the role of Hubbard U corrections in electronic structure calculations of two-dimensional (2D) materials containing 3d transition metals. Specifically, we use density functional theory (DFT) with the PBE and PBE+U approximations to calculate the crystal structure, band gaps, and magnetic parameters of 638 monolayers. Based on a comprehensive comparison to experiments we first establish that the inclusion of the U correction worsens the accuracy for the lattice constants. Consequently, PBE structures are used for subsequent property evaluations. The band gaps show a significant dependence on U. In particular, for 134 (21%) of the materials the U parameter induces a metal-to-insulator transition. For the magnetic materials we calculate the magnetic moment, magnetic exchange coupling, and magnetic anisotropy parameters. In contrast to the band gaps, the size of the magnetic moments shows only weak dependence on U. Both the exchange energies and magnetic anisotropy parameters are systematically reduced by the U correction. On this basis we conclude that the Hubbard U correction will lead to lower predicted Curie temperatures in 2D materials. All the calculated properties are available in the Computational 2D Materials Database (C2DB).

Abnormal chirality in antiferromagnetic resonance modes of van der Waals 2D magnets

Two-dimensional van der Waals (2D vdW) materials have attracted widespread research interest due to their unique physical properties and potential application prospects. In this study, an atomistic-level dynamical simulation method is employed to investigate the chirality of antiferromagnetic resonance modes in CrI3 bilayer. Beyond the typical observations of a linear increase in high-frequency resonance mode and a linear decrease in low-frequency resonance mode, we have identified a distinct magnetization precession chirality in the CrI3 bilayer at low magnetic fields: Spins in different layers exhibit opposite precession chirality. This unusual chirality phenomenon is attributed to the weak interlayer coupling inherent in vdW materials, which can be adjusted by tuning their interlayer coupling and perpendicular magnetic anisotropy. These findings provide valuable insights into the intrinsic antiferromagnetic resonance characteristics of atomically thin vdW materials and their potential implications for the development of spintronic devices.

A Triangular Frustrated Eu(II)-Organic Framework for Sub-Kelvin Magnetic Refrigeration.

Attaining sub-Kelvin temperatures remains technologically challenging and often relies on the scarce resource 3He, unless employing adiabatic demagnetization refrigeration. Herein, the active coolant typically consists of weakly coupled paramagnetic ions, whose magnetic interaction strengths are comparable in energy to the relevant temperature regime of cooling. Such interactions depend strongly on inter-ion distances, fundamentally hindering the realization of dense coolants for sub-Kelvin refrigeration. We present a magnetically concentrated triangular coordination network, Eu0.9Ba0.1I2(pyrazine)3, featuring the large s = 7/2 moment of Eu(II). Electron paramagnetic resonance, magnetization, and heat capacity measurements reflect antiferromagnetic correlations between the Eu(II) ions and a dominant easy-plane magnetic anisotropy. The ensuing geometric frustration prevents entropy-annihilating magnetic order down to at least 0.17 K. This remarkably low operational temperature for a low-dimensional, molecule-based magnetic refrigerant opens possibilities for on-chip cryogenic refrigeration, especially in scenarios where established inorganic refrigerants cannot be utilized.

Switching of a magnetic tunnel junction by voltage-controlled magnetic anisotropy

Switching of a magnetic tunnel junction by voltage-controlled magnetic anisotropy

A Compendium of Magnetic Nanoparticle Essentials: A Comprehensive Guide for Beginners and Experts.

Magnetic nanoparticles (MNPs) are advanced materials that combine the unique properties of magnetic materials and nanoscale dimensions, enabling a wide range of applications in biomedicine, environmental science, and information technology. This review provides a comprehensive yet accessible introduction to the fundamental principles, characterization techniques, and diverse applications of MNPs, with a focus on their nanoscale magnetic properties, such as superparamagnetism, single-domain behavior, and surface effects. It also delves into their classification and the critical role of parameters like magnetic anisotropy and blocking temperature. Emphasis is placed on routine characterization methods, including X-ray diffraction, electron microscopy, and magnetometry, as well as advanced concepts like magnetic hyperthermia and self-regulated heating. Designed for newcomers and experts alike, this review serves as both an educational guide and a quick-reference resource, ensuring clarity while maintaining scientific rigor.

Stiffness Tunable Magnetic Actuators Based on Shape Memory Polymer/NdFeB Composite for Segmental Controllable Motion.

Soft magnetic robots have attracted extensive research interest recently due to their fast-transforming ability and programmability. Although the inherent softness of the matrix materials enables dexterity and safe interactions, the contradiction between the easy shape transformation of the soft matrices and load carrying capacity, as well as the difficulty of independently controllable motion of individual segments, severely limits its design space and application potentials. Herein, we have proposed a strategy to adjust the modulus of shape memory polymer composite embedded with hard magnetic particles by in situ Joule heating of printed circuit, which can reversibly change the stiffness from 4.1 GPa at 25°C to 10.9 MPa at 70°C. The stiffness tunable magnetic robots realize the compatibility of fast reversible shape transformation and high load carrying capacity. Furthermore, multiple separated Joule circuits are designed for the independently controllable motion of individual segments. The simulation of Joule heating and magnetic actuation is used to guide the design of devices. The concept of simultaneously programming magnetic anisotropy and stiffness proposed in this work greatly expands the design space and new applications of magnetic actuators, including soft grippers for heavy loads and bionic hand with independent motion of fingers.

SMM Behavior in Distorted Trigonal Bipyramidal and Tetrahedral Cobalt(II) Complexes Based on Tripodal Tetradentate Phenolic Amines

Four CoII complexes [Co(L1)(MeOH)]‧MeOH (1‧MeOH), [Co(L2)(MeOH)]‧MeOH (2‧MeOH), [Co(L3)(H2O)]‧MeOH (3‧MeOH) and [Co(L4)] (4), derived from tripodal tetradentate phenolic amine arms, H2L1‐4, were synthesized and structurally characterized. The complexes 1·MeOH, 2·MeOH and 3·MeOH displayed distorted trigonal bipyramidal geometry, whereas 4 exhibited distorted tetrahedral geometry, depending on the substituents at the phenolate rings and amine arm. The variation of the coordination geometries and interatomic parameters around the CoII center has an impact on the magnetic behavior of the compounds. The complexes show magnetic anisotropy (ZFS) of the MS = ± ½ and ± 3/2 sub‐levels, with D = 29.1(2), 22.7(1), 28.8(2) and 30.9(5) cm‐1 for 1‧MeOH, 2‧MeOH, 3‧MeOH and 4, respectively. The results obtained from ab initio CASSF calculations match well with the experimental data, revealing the origin of magnetic anisotropy. The dynamic ac magnetic investigation of the magnetic susceptibility revealed a slow magnetic relaxation behavior for 2·MeOH, 3·MeOH and 4. The field‐induced slow relaxation of the magnetization occurred through combination of Raman and Direct processes, depending on the variation in the coordination geometries imposed by the coordinated ligand and/or the interatomic parameters around the CoII center, which in turn have definite impact on the magnetic features of the compounds.

First-principles predictions of two-dimensional Ce-based ferromagnetic semiconductors: CeF2 and CeFCl monolayers.

Two-dimensional (2D) ferromagnetic (FM) semiconductors hold great promise for the next generation spintronics devices. By performing density functional theory first-principles calculations, both CeF2 and CeFCl monolayers are studied, our calculation results show that CeF2 is a FM semiconductor with sizable magneto-crystalline anisotropy energy (MAE) and high Curie temperature (290 K), but a smaller band gap and thermal instability indicate that it is not applicable at higher temperature. Its isoelectronic analogue, the CeFCl monolayer, is a bipolar FM semiconductor, its dynamics, elastic, and thermal stability are confirmed, our results demonstrate promising applications of the CeFCl monolayer for next-generation spintronic devices owing to its high Curie temperature (200 K), stable semiconducting features, and stability. Under biaxial strain from -5% to 5%, the CeFCl monolayer is a semiconductor with sizable MAE, its Curie temperature can increase to 240 K, the easy magnetization axes for CeFCl monolayer are still along the out-of-plane directions because the couplings between Cef y(3x 2-y 2) and f x(x 2-3y 2) orbitals in the different spin channels contribute most to the MAE according to second-order perturbation theory.

Tilted chiral spin textures in confined nanostructures with in-plane magnetic anisotropy

Tilted chiral spin textures in confined nanostructures with in-plane magnetic anisotropy

Temperature crossovers in the specific heat of amorphous magnets

It is argued that the specific heat of amorphous solids at low temperatures can be understood to arise from a single branch of collective modes. The idea is illustrated in a model of a correlated spin glass for which magnetic anisotropies are present but they are completely frustrated by disorder. The low-energy spectrum is dominated by soft modes corresponding to propagating Halperin–Saslow spin waves at short wavelengths, evolving into a relaxation and a diffusion mode at the long wavelengths. The latter gives rise to an anomalous temperature behavior of the specific heat at T≪T∗: C∼T in d=3 solids, C∼Tln⁡(1/T) in d=2. The temperature scale T∗ has a non-trivial dependence on the damping coefficient describing magnetic friction, which can be related to fluctuations of the spin-torque operator via a Green–Kubo formula. Halperin–Saslow modes can also become diffusive due to disclination motion (plastic flow). Extensions of these ideas to other systems (including disordered phases of correlated electrons in cuprate superconductors and of moiré superlattices) are discussed.

High-Coercivity Ferrimagnet Co₂FeO₂BO₃: XMCD Insights into Charge-Ordering and Cation Distribution

The multi-sublattice ferrimagnet Co2FeO2BO3, a prominent example of lanthanide-free magnets, was the subject of element-selective studies using X-ray magnetic circular dichroism (XMCD) observations at the L- and K- X-ray absorption edges. Research findings indicate that the distinct magnetic characteristics of Co2FeO2BO3, namely its remarkable high coercivity (which surpasses 7 Tesla at low temperatures), originate from an atypical arrangement of magnetic ions in the crystal structure (sp.gr. Pbam). The antiferromagnetic nature of the Co2+-O-Fe3+ exchange interaction was confirmed by identifying the spin and orbital contributions to the total magnetization from Co (mL = 0.27 ± 0.1 μB/ion and meffS = 0.53 ± 0.1 μB/ion) and Fe (mL = 0.05 ± 0.1 μB/ion and meffS = 0.80 ± 0.1 μB/ion) ions through element-selective XMCD analysis. Additionally, the research explicitly revealed that the strong magnetic anisotropy is a result of the significant unquenched orbital magnetic moment of Co, a feature that is also present in the related compound Co3O2BO3. A complex magnetic structure in Co2FeO2BO3, with infinite Co²⁺O6 layers in the bc-plane and strong antiferromagnetic coupling through Fe3⁺ ions, is suggested by element-selective hysteresis data, which revealed that Co²⁺ ions contribute both antiferromagnetic and ferromagnetic components to the total magnetization. The findings underline the suitability of Co2FeO2BO3 for applications in extreme environments, such as low temperatures and high magnetic fields, where its unique magnetic topology and anisotropy can be harnessed for advanced technologies, including materials for space exploration and quantum devices. This XMCD study opens the door to the production of novel high-coercivity, lanthanide-free magnetic materials by showing that targeted substitution at specific crystallographic sites can significantly enhance the magnetic properties of such materials.

Guanine-based spin valve with spin rectification effect for an artificial memory element.

Non-volatile electronic memory elements are very attractive for applications, not only for information storage but also in logic circuits, sensing devices and neuromorphic computing. Here, a ferroelectric film of guanine nucleobase is used in a resistive memory junction sandwiched between two different ferromagnetic films of Co and CoCr alloys. The magnetic films have an in-plane easy axis of magnetization and different coercive fields whereas the guanine film ensures a very long spin transport length, at 100K. The non-volatile resistance states of the multiferroic spintronic junction with two-terminals are manipulated by a combined action of small external magnetic and electric fields. Thus, the magnetic field controls the relative orientation of the magnetization of the metallic ferromagnetic electrodes, that leads to different magnetoresistance states. The orientation and the magnitude of the electric field controls the orientation of the polarization of the guanine ferroelectric barrier, that leads to different electroresistance states, respectively. Moreover, we have observed a strong interfacial coupling of the two parameters. Consequently, positive and negative magnetoresistance hysteresis loops corresponding to spin rectification effects and non-hysteretic (erased) resistive states are manipulated with the electric field by switching the orientation of the electrical polarization of the organic ferroelectric.

Quantifying Magnetic Anisotropy of Series of Five-Coordinate CoII Ions: Experimental and Theoretical Insights.

Stabilizing large easy-axis type magnetic anisotropy in molecular complexes is a challenging task, yet it is crucial for the development of information storage devices and applications in molecular spintronics. Achieving this requires a deep understanding of electronic structure and the relationships between structure and properties to develop magneto-structural correlations that are currently unexplored in the literature. Herein, a series of five-coordinate distorted square pyramidal CoII complexes [Co(L)(X2)].CHCl3 (where X = Cl (1), Br (2), or I (3)) is reported, all exhibiting easy-axis magnetic anicotropy. The size of the zero field splitting axial parameter (D) is quantitatively determined (1 = -72; 2 = -67 and 3 = -25 cm-1) using a cantilever torque magnetometry which is further firmly supported by magnetic susceptibility, and EPR measurements. The study of the magnetization relaxation dynamics reveals field-induced slow relaxation of magnetization due to the predominant Raman relaxation process. Theoretical calculations on 1-3 and optimized model complexes of 1 reveal insights into the electronic structure and highlight the impact of steric and electronic effects on modulating the D values. Overall, the studies reported pave the way for designing a new generation of CoII complexes with enhanced axiality and a lower rhombicity.