Effective Magnetic Anisotropy Research Articles

Control of Anisotropy and Magnetic Hyperthermia Effect by Addition of Cobalt on Magnetite Nanoparticles.

Magnetic hyperthermia (MH) has emerged as a promising technology with diverse applications in medical and technological fields, leveraging the remote induction of temperature elevation through an alternating magnetic field. While Fe3O4 nanoparticles with an average size around 12-25 nm are commonly employed in MH systems, this study introduces a strategy to produce smaller particles (less than or equal to 10 nm) with enhanced heating efficiency, as measured by specific power absorption (SPA). We conducted an exhaustive and detailed investigation into the morphological and magnetic properties of CoxFe3-xO4 nanoparticles, aiming to optimize their MH response. By varying the Co content, we successfully tuned the effective magnetic anisotropy while maintaining saturation magnetization nearly constant. The MH analysis indicates that these nanoparticles predominantly heat through the Néel mechanism, demonstrating robust reproducibility across different concentrations, viscosity mediums, and ac field conditions. Notably, we identified an optimal anisotropy or Co concentration that maximizes SPA, crucial for developing magnetic systems requiring particles with specific sizes. This work contributes to advancing the understanding and application of MH, particularly in tailoring nanoparticle properties for targeted and efficient heat generation in various contexts.

Magnetostrictive strain-sensitivity synergy for laser-beam powder bed fusion processed Fe81Ga19 alloys by magnetic field annealing

Magnetostrictive Fe–Ga alloys have been demonstrated potentialities for numerous applications, whereas, suffering a tradeoff between large magnetostrictive strain and high sensitivity. Herein, bulk polycrystalline Fe81Ga19 alloys were prepared by laser-beam powder bed fusion (LPBF) and then annealed in magnetic field for manipulating the comprehensive magnetostrictive properties. Results indicate that <001> oriented grains are developed in the LPBF-prepared Fe81Ga19 alloys due to high temperature gradient. After magnetic field annealing (MFA), the magnetic domains within the alloys gradually transformed into well-arranged stripe domains, especially, flat and smooth 90° domains were established in the alloys annealed at 2600 Oe. As a result, the induced <001> orientation grains and 90° domains contributed to an improved effective magnetic anisotropy constant (57.053 kJ/m3), leading to an enhanced magnetostrictive strain of 92 ppm. Moreover, the MFA-treated alloys also displayed enhanced magnetostrictive sensitivity (0.097 ppm/Oe) owing to the smooth domain structures and low dislocation densities, demonstrating a fruitful strain-sensitivity synergy. In addition, good magnetostrictive dynamic response and enhanced compressive yield strength were also observed for the prepared alloys. This work demonstrates that LPBF and MFA might be an attractive strategy to resolve the tradeoff between strain and sensitivity, providing a basis for the preparation of high-performance magnetostrictive materials.

Exploring the impact of non-magnetic ion Mg2+ into M- type Ba0.5Ca0.5Fe12-xO19 (x = 0.0, 0.1 and 0.2) hexaferrites on structural, magnetic and magnetocaloric effect properties

The Mg substituted Ba-Ca Hexaferrites Ba0.5Ca0.5Fe12-xMgxO19 (BCFMO) (x = 0.0, 0.1 and 0.2) were prepared through the conventional ceramic technology. The microstructure, morphology, magnetic and magnetocaloric Effect (MCE) of all samples were investigated by XRD, SEM, EDX and BS1 and BS2 magnetometer. XRD analysis revealed the formation of Magnetoplumbite structure with P63/mmc space group and minor secondary phase R-3 m and having crystallites in the range of 121–108 nm. The XRD refinement indicates a preference for Mg2+ ions to occupy the octahedral 12k site. SEM analysis showed the presence of the hexagonal shape for all products. The saturation magnetization Ms decreases from 69.77 to 65.55 emu/g and the coercive field Hc increases from 1.29 to 3.54 kOe with the increase in Mg concentration. The increase of Hc with increasing Mg concentration is attributed to the reduction in crystallite sizes. Additionally, the magnetocrystalline anisotropy parameters (obtained by the "Law of Approach to Saturation method") such as the anisotropy field (Ha) and the effective magnetic anisotropy constant (Keff) were found to be 16.59–24.18 kOe and 5.15–6.94 105 emu/cm for x = 0.0–0.2, respectively.The MFC-MZFC curves for all compounds show a paramagnetic (PM) to ferromagnetic (FM) phase transition at the Curie Temperature TC. Doping with Mg2+ in BCFO lowers the Curie temperature TC down to T ∼700 K. The maximum entropy (-ΔSmmax) (x = 0.1) and the corresponding RCPmax (x = 0.2) at 5 T are found to be 1.80 J.kg-1.K-1 and 114.29 J.kg-1 respectively. The above results demontrate that the samples have great potential for permanent magnets and guide for the Magnetocaloric Effect (MCE) applications.

Stabilization of various structural phases and magnetic properties of chemically synthesized Co-Ni-Ga nanoparticles

Polycrystalline Co-Ni-Ga ferromagnetic shape memory alloy nanoparticles with phase single austenite (A), martensite (M) and gamma (γ) structures have been obtained for the first time through a facile template-less chemical method. Average crystallite size of A, M and γ phase nanoparticles are 25 nm, 22 nm and 21 nm, respectively. All the synthesized nanoparticles exhibit soft ferromagnetic behavior with saturation magnetization varying from 21.22 to 77.65 emu/g and effective magnetic anisotropy of ∼105 J/m3 at 5 K. Ab initio studies have been performed to evaluate the magnetic moments of single A, M and γ phase. All the single phase nanoparticles have been ascertained to be in single-domain regime by estimating the single-domain size limit. This work showcases a simple methodology to prepare pure crystal phases in these nanoparticles. It also provides insight on the role of elemental precursors and processing conditions in designing Co-Ni-Ga nanoparticles for ferromagnetic shape memory applications.

Spin-orbit coupled Hubbard skyrmions

When the material phases exhibit topological quantum numbers, they host defects protected by the nontrivial topology. Magnetic skyrmions are such “quantized” objects, and although many of them are metals, they had been most likely treated in theories as purely classical spin states. Here, we show that the electrons described by the Hubbard model with strong spin-orbit couplings can exhibit various nano- or flake-size skyrmions in their ground state. Importantly, the conducting electrons forming Fermi pockets themselves carry textured magnetic moments in both real and momentum space and on top of that possess Chern numbers in their energy bands. This quantum and conducting skyrmion is related to small skyrmions observed in atomic-layered compounds. We clarify how the effective magnetic interactions and magnetic anisotropies are tied to the spin-orbit coupling, and how they influence the stability of skyrmions beyond the phenomenology. Published by the American Physical Society 2024

Imaging the Magnetic Anisotropy in Ultrathin Fe4GeTe2 with a Nitrogen-Vacancy Magnetometer.

Two-dimensional (2D) FenGeTe2, with n = 3, 4, and 5, has been realized in experiments, showing strong magnetic anisotropy with enhanced critical temperature (Tc). The understanding of its magnetic anisotropy is crucial for the exploration of more stable 2D magnets and its spintronic applications. Here, we report a quantitative reconstruction of the magnetization magnitude and its direction in ultrathin Fe4GeTe2 using nitrogen vacancy centers. Through imaging stray magnetic fields, we identified the spin-flop transition at approximately 80 K, resulting in a change of the easy axis from the out-of-plane direction to the in-plane direction. Moreover, by analyzing the thermally activated escape behavior of the magnetization near Tc in terms of the Ginzburg-Landau model, we observed the in-plane magnetic anisotropy effect and the formation capability of magnetic domains at ∼0.4 μm2 μT-1. Our findings contribute to the quantitative understanding of the magnetic anisotropy effect in a vast range of 2D van der Waals magnets.

Study of Angular-Dependent Magnetic Anisotropy and Spin Pumping-Induced Inverse Spin Hall Effect (ISHE) in Py and Py/Pt Bilayer: Realization of Quantum Metrology

Study of Angular-Dependent Magnetic Anisotropy and Spin Pumping-Induced Inverse Spin Hall Effect (ISHE) in Py and Py/Pt Bilayer: Realization of Quantum Metrology

Effect of moringa oleifera with pure and Zn-doped Mn and Ni nanoferrites for hyperthermia applications

Nanoferrites like spinel manganese ferrite (MnFe2O4) and inverse spinel nickel ferrite (NiFe2O4) have an efficient heating deportment to the destruction of neoplasm. The incorporation of Moringa oleifera (MO) leaf extracts with magnetic nanoparticles as heat mediators is an effective approach for treating magnetic hyperthermia. This study investigated the potential heating behavior of bare, Zn-doped MnFe2O4 and NiFe2O4 nanoparticles (NPs) with Moringa oleifera leaf extracts as magnetic hyperthermia agents. High-resolution transmission electron microscopy showed the spherical-like morphology of MnFe2O4 NPs with no impurities in the EDX spectra. X-ray diffraction spectra confirmed the cubic spinel and inverse spinel structure for MnFe2O4 and NiFe2O4 NPs, respectively. Magnetic properties of the prepared nanoferrites exhibit low coercivity values (in the range of 13–18 Oe representing superparamagnetic behavior. Further, the binding energies of the prepared nanoferrites were revealed by X-ray photoelectron spectroscopy. The effective magnetic anisotropy constant (Keff) was calculated to be around 3.9–5.9 kJ/m3 by electron spin resonance. An increased specific surface area (120.69 m2/g) and wider porosity (17.053 nm) were observed for MnFe2O4/MO NPs through Brunauer-Emmett-Teller (BET) analysis. Thermal properties were investigated by measuring the effective specific absorption rate in an alternating magnetic field ranging from 23 to 31 * 10−9 W/g Oe2 Hz. Results indicated that the green synthesized MnFe2O4/MO NPs are promising agents for magnetic hyperthermia applications in cancer therapy.

Electrical control of the switching layer in perpendicular magnetic tunnel junctions with atomically thin Ir dusting

The use of magnetic tunnel junction (MTJ)-based devices constitutes an important basis of modern spintronics. However, the switching layer of an MTJ is widely believed to be an unmodifiable setup, instead of a user-defined option, posing a restriction to the function of spintronic devices. In this study, we realized a reliable electrical control of the switching layer in perpendicular MTJs with 0.1 nm Ir dusting. Specifically, a voltage pulse with a higher amplitude drives the magnetization switching of the MTJ's bottom electrode, while a lower voltage amplitude switches its top electrode. We discussed the origin of this controllability and excluded the possibility of back-hopping. Given the established studies on enhancing the voltage-controlled magnetic anisotropy effect by adopting Ir, we attribute this switching behavior to the significant diffusion of Ir atoms into the top electrode, which is supported by scanning transmission electron microscopy with atomic resolution.

Tau-borides (Mnx{Ru,Os,Ir}1-x)23B6: X-ray single crystal and TEM data, physical properties

Investigation (electron microprobe and X-ray powder and single crystal diffraction analyses) of the phase relations in the Mn-rich corner (> 45 at% Mn) of the systems Mn-{Ru,Os,Ir}-B, prompted in all three systems a ternary compound with formula (Mnx{Ru,Os,Ir}1-x)23B6. For the systems Mn-Ru-B, Mn-Ir-B phase equilibria have been determined at 950°C (Ru), 900°C (Ir) revealing in both cases a small homogeneity region at constant B-content (Mn1-xRux)23B6 (0.29 < x < 0.37); (Mn1-xOsx)23B6 (0.33 < x < 0.37), as well as (Mn1-xIrx)23B6 (0.29 < x < 0.34). The crystal structure of the three compounds was determined from single crystal and X-ray powder intensity data analyses to be isotypic with the Cr23B6-type (so-called tau-phase, space group Fm3¯m, No. 225). In all cases Mn atoms fully occupy the 4a site (0,0,0) at the origin of the unit cell. For the 8c site (¼,¼,¼) we observed a random distribution of Mn1-x(PM)x with decreasing PM content (PM stands for a platinum group metal atom) going from Ru to Os and Ir, where Mn atoms fully occupy 8c. Whereas the remaining sites (48h, 32f) show various ratios of the two metal species, boron atoms fully occupy the centers (24e site) of the Archimedean metal atom antiprisms. A transmission electron microscopic study confirms the absence of superstructures related to these metal atom disorder, thus Mn and PM atoms randomly share their sites. The latter is well reflected in the behavior of the electrical resistivity of these compounds, which is dominated by disorder scattering. For (Mn0.6{Ru,Os}0.4)23B6, temperature dependent dc magnetization studies reveal distinct antiferromagnetic-like anomalies at TN ≅ 72 K and 114 K, respectively. Low field dc and ac magnetic susceptibility data of (Mn0.7Ir0.3)23B6 display a distinct ferromagnetic-like transition at TC = 280 K, consistent with a pronounced specific heat anomaly and a rather continuous temperature dependent evolution of magnetic anisotropy effects. Mechanical properties (hardness) characterize the tau phases among rather hard and brittle intermetallics (about 5–6 GPa).

Spintronic virtual neural network by a voltage controlled ferromagnet for associative memory

Recently, an associative memory operation by a virtual oscillator network, consisting of a single spintronic oscillator, was examined to solve issues in conventional, real oscillators-based neural networks such as inhomogeneities between the oscillators. However, the spintronic oscillator still carries issues dissipating large amount of energy because it is driven by electric current. Here, we propose to use a single ferromagnet manipulated by voltage-controlled magnetic anisotropy (VCMA) effect as a fundamental element in a virtual neural network, which will contribute to significantly reducing the Joule heating caused by electric current. Instead of the oscillation in oscillator networks, magnetization relaxation dynamics were used for the associative memory operation. The associative memory operation for alphabet patterns is successfully demonstrated by giving correspondences between the colors in a pattern recognition task and the sign of a perpendicular magnetic anisotropy coefficient, which could be either positive or negative via the VCMA effect.

Thermally generated magnonic spin currents in a polycrystalline gadolinium iron garnet thin film with perpendicular magnetic anisotropy

Rare-earth iron garnets (REIGs) are the benchmark systems for magnonics, including the longitudinal spin Seebeck effect (LSSE). While most research has focused on single-crystalline REIGs on complimentary garnet substrates, moving to more, cost-effective complementary metal-oxide semiconductor (CMOS)-compatible substrates is important to integrate REIG thin films with existing technology. In this regard, we grow a 130 nm-thick polycrystalline gadolinium iron garnet (GdIG) film on the Si/SiO2 substrate and investigate the temperature-dependent LSSE. Interestingly, the polycrystalline GdIG film exhibits perpendicular magnetic anisotropy (PMA) at room temperature which is induced by tensile in-plane (IP)-strain originating from the thermal-expansion mismatch between the GdIG film and the substrate during rapid thermal annealing. Further, a spin-reorientation transition from the out-of-plane IP direction below TS = 180 K is observed. Additionally, the film reveals a magnetic compensation temperature, TComp, of ≈240 K. The LSSE voltage not only demonstrates a sign-inversion around TComp, but also shows noticeable changes around TS. As compared to a single-crystalline GdIG film, the lower LSSE voltage for the polycrystalline GdIG is attributed to the higher effective magnetic anisotropy and enhanced magnon scattering at the grain boundaries. Our study not only paves the way for the cost-effective growth of CMOS-compatible REIG-based systems with PMA for magnonic memory and information processing applications, but also highlights the fact that the spincaloritronic and spin-insulatronic properties of the polycrystalline REIGs follow those of their single-crystalline counterparts with reduced spin-to-charge conversion efficiency through LSSE which can be tuned further by controlling the average gran size and interface engineering.

Effect of metal insertion on the magnetic properties and anomalous Hall effect in MgO/CoFeB/Ta/MgO films

CoFeB-based nano-magnetic multilayers can be applied in magnetic sensors, magnetic random access memory and other logic devices, which have attracted wide attention. The magnetic properties and interface structure of the magnetic multilayers still need further research for future applications. The effects of Pt insertion on the perpendicular magnetic anisotropy (PMA) and the anomalous Hall effect (AHE) of MgO/CoFeB/Pt/Ta/MgO multilayers were studied. It is found that the sample with 0.2 nm Pt insertion at the CoFeB/Ta interface and annealed at 250 °C can transform the films from in-plane magnetic anisotropy to PMA. The effective magnetic anisotropy K eff can reach 1.82 × 106 erg cm−3. The Pt insertion and annealing heat treatment can promote the migration of oxygen from MgO at the top layer to CoFeB/Ta interface and combine with Fe to generate iron oxide, and improve the orbital hybridization of Fe 3d and O 2p, thus successfully inducing the PMA of the film samples. In addition, Pt insertion and annealing treatment can improve the side-jump mechanism and the skew scattering mechanism which contribute to the AHE. This work provides guidance for the application of PMA materials in magnetic sensor and memory devices.

The Effect of Magnetic Anisotropy on the Computed Specific Total Loss in Electrical Steel

Grain-oriented (GO) electrical steel (ES) laminates are still very important in industrial applications due to their remarkable crystallographic properties. Cores of large electrical machines and transformers are built from ES. The performances of these devices are significantly influenced by the properties of ES. The improvement of ES properties has been the subject of considerable research for many years. The phenomenon of magnetic anisotropy is highly non-linear, and it should be taken into account by the designers of magnetic circuits. The article proposes a modified model for calculating the angular properties of specific total loss of ES. The modeling takes into account the isotropic component (from classic eddy currents) and the anisotropic component, which is the sum of hysteresis and excess losses. For the directional loss modeling, the Boltzmann function was used. An analysis of the dependency of model coefficients on the frequency is presented.

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

Temperature Evolution of Magnon Propagation Length in Tm3Fe5O12 Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping.

The magnon propagation length, ⟨ξ⟩, of a ferro-/ferrimagnet (FM) is one of the key factors that controls the generation and propagation of thermally driven magnonic spin current in FM/heavy metal (HM) bilayer based spincaloritronic devices. For the development of a complete physical picture of thermally driven magnon transport in FM/HM bilayers over a wide temperature range, it is of utmost importance to understand the respective roles of temperature-dependent Gilbert damping (α) and effective magnetic anisotropy (Keff) in controlling the temperature evolution of ⟨ξ⟩. Here, we report a comprehensive investigation of the temperature-dependent longitudinal spin Seebeck effect (LSSE), radio frequency transverse susceptibility, and broad-band ferromagnetic resonance measurements on Tm3Fe5O12 (TmIG)/Pt bilayers grown on different substrates. We observe a significant drop in the LSSE voltage below 200 K independent of TmIG film thickness and substrate choice. This is attributed to the noticeable increases in effective magnetic anisotropy field, HKeff (∝Keff) and α that occur within the same temperature range. From the TmIG thickness dependence of the LSSE voltage, we determined the temperature dependence of ⟨ξ⟩ and highlighted its correlation with the temperature-dependent HKeff and α in TmIG/Pt bilayers, which will be beneficial for the development of rare-earth iron garnet based efficient spincaloritronic nanodevices.

Bifurcation to complex dynamics in largely modulated voltage-controlled parametric oscillator

An experimental demonstration of a parametric oscillation of a magnetization in a ferromagnet was performed recently by applying a microwave voltage, indicating the potential to be applied in a switching method in non-volatile memories. In the previous works, the modulation of a perpendicular magnetic anisotropy field produced by the microwave voltage was small compared with an external magnetic field pointing in an in-plane direction. A recent trend is, however, opposite, where an efficiency of the voltage controlled magnetic anisotropy (VCMA) effect is increased significantly by material research and thus, the modulated magnetic anisotropy field can be larger than the external magnetic field. Here, we solved the Landau–Lifshitz–Gilbert equation numerically and investigated the magnetization dynamics driven under a wide range of the microwave VCMA effect. We evaluated bifurcation diagrams, which summarize local maxima of the magnetization dynamics. For low modulation amplitudes, the local maximum is a single point because the dynamics is the periodic parametric oscillation. The bifurcation diagrams show distributions of the local maxima when the microwave magnetic anisotropy field becomes larger than the external magnetic field. The appearance of this broadened distribution indicates complex dynamics such as chaotic and transient-chaotic behaviors, which were confirmed from an analysis of temporal dynamics.

Voltage-controlled magnetic anisotropy effect through a high-k MgO/ZrO2/MgO hybrid tunneling barrier

We investigated the voltage-controlled magnetic anisotropy (VCMA) effect in epitaxial magnetic tunnel junctions (MTJs) with a hybrid MgO/ZrO2/MgO tunnel barrier. A metastable cubic ZrO2(001) thin film was successfully grown on a MgO(001) layer, leading to the high dielectric constant of 26.5. Using the hybrid tunneling barrier, we achieved the large VCMA coefficient of −350 fJ V−1 m−1, which is 70% larger than that observed in the magnetic tunnel junction with the single MgO barrier. Introduction of crystalline high-k dielectric tunneling barrier can open up new pathways to improving the VCMA properties in MTJs for voltage-driven spintronic devices.

Influence of Fe3+ substitution by Al3+ on the structural, microstructural, and magnetic properties of Co0.2Ni0.3Zn0.5Fe2O4 nanoparticles

A series of aluminum-substituted ferrite nanoparticles, Co0.2Ni0.3Zn0.5Fe2-xAlxO4 (0 ≤ x ≤ 1) was synthesized using the polyol method. The structural analysis confirmed the formation of pure cubic ferrispinels. The unit cell parameter decreased monotonically with increasing Al3+ content obeying Vegard's law. Besides, it was found that Al3+ ions prefer to enter octahedral sites. The produced particles consisted of roughly spherical ultrasmall nanocrystals with an average particle size of ∼3-6 nm. The nanoparticles are superparamagnetic with magnetic features (blocking temperature, saturation magnetization and Curie temperature) showing a decrease with the increase in Al3+ content. The magnetic tendency correlates with the weakening of the superexchange interactions in the spinel sublattices caused by substituting the non-magnetic Al3+ ions for Fe3+ ones. While, the coercivity and effective magnetic anisotropy increased with the increase of Al3+ concentration, suggesting the development of high surface anisotropy that largely compensates for the decrease of intrinsic magnetic anisotropy caused by the magnetic dilution.

Efficient Optical Control of Magnon Dynamics in van der Waals Ferromagnets

Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe 3.6 Co 1.4 GeTe 2 . The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.