Anisotropy Energy Density Research Articles

Hyaluronic acid-coated iron oxide nanoparticles for magnetic fluid hyperthermia and methylene blue dye removal: Preparation, physicochemical characterization, and enhancing the heating efficiency

We report here the magnetic fluid hyperthermia properties of water-dispersible hyaluronic acid-coated superparamagnetic iron oxide nanoparticles (HA-MNPs), prepared using the coprecipitation technique followed by ligand exchange. The presence of the hyaluronic acid coating is confirmed using Fourier transform infrared spectroscopy. Magneto-calorimetric studies show good field-induced heating efficiency of the HA-MNPs, where a high specific absorption rate (SAR) of ∼ 315 W/gFe is obtained for ∼ 5 wt% MNP concentration when exposed to an alternating magnetic field amplitude of ∼ 33.1 kA/m at ∼ 126 kHz. The heating efficiency is found to decrease by ∼ 33.8 % after immobilization of the MNPs within an agar matrix, which is attributed to the abrogation of Brownian relaxation. When subjected to an external DC magnetic field, the MNPs form linear chain-like structures, which causes the effective anisotropy energy density to increase. Magneto-calorimetric studies on agar-based oriented samples reveal a ∼ 19.5 % enhancement in SAR, thereby partly compensating for the loss in heating efficiency due to an increase in medium viscosity. The orientational ordering-induced enhancement in SAR is also verified using sweeping rate modified Stoner-Wohlfarth model-based calculations. In vitro cyto-toxicity studies on the HeLa cell lines reveal good cyto-compatibility of the MNPs. The negative charge on the surface of HA-coated MNPs is exploited to remove the cationic methylene blue dye from the aqueous medium, where the dye molecules are found to be physisorbed on the surface of the MNPs. The experimental findings clearly show the high induction heating efficiency, cyto-compatibility, and cationic dye-adsorption efficiency of the prepared HA-MNPs, thereby indicating the suitability of these MNPs for multi-functional applications like magnetic fluid hyperthermia and waste-water purification.

Origin of perpendicular magnetic anisotropy in Fe0.6Al0.4/Cr0.8Al0.2 metallic superlattice

Abstract In this study, we investigated a perpendicular magnetic anisotropy (PMA) in a ferromagnet/antiferromagnetic stacking structure without using heavy metal elements. The Fe0.6Al0.4/Cr0.8Al0.2/Fe0.6Al0.4 stacked films exhibited perpendicular magnetization. We discussed the origin of the PMA based on the Cr0.8Al0.2 thickness, t Cr-Al (= 0.6 – 3.0 nm) dependences of the uniaxial anisotropy energy density K u, elastic strain ε 1 – ε 3, and unit cell volume V of the Fe0.6Al0.4 and Cr0.8Al0.2 layers. The K u value was approximately 25 kJ/m3, independent of t Cr-Al. The positive ε 1 – ε 3, i.e., the tensile strain in the Cr0.8Al0.2 layer can promote the PMA. The possible degradation of PMA due to the lattice relaxation with increasing t Cr-Al could be compensated by recovering the Cr magnetic moment. Our analysis suggests that PMA is caused by interfacial exchange coupling between ferromagnetic Fe0.6Al0.4 and antiferromagnetic Cr0.8Al0.2.

Effects of interfacial oxygen diffusion on the magnetic properties and thermal stability of Pd/CoFeB/Pd/Ta heterostructure

We investigated the effects of annealing temperatures (TA) on a Pd (5 nm)/CoFeB (10 nm)/Pd (3 nm)/Ta (10 nm) multilayer structure. The as-deposited sample showed an amorphous state with in-plane uniaxial magnetic anisotropy (UMA), resulting in low coercivity and moderate Gilbert damping constant (α) values. Increasing TA led to crystallization, forming bcc-CoFe (110) crystals, which increased in-plane coercivity and introduced isotropic magnetic anisotropy, slightly reducing the α. The two-fold UMA persists up to 600 °C, and the thermal stability of the in-plane magnetic anisotropy remains intact even TA = 700 °C. The TA significantly influenced the magnetic properties such as in-plane saturation magnetization (Ms//), in-plane and out-of-plane coercivities (Hc// and Hc⊥), and in-plane effective magnetic anisotropy energy density (Keff). Above 600 °C, Keff decreased, indicating a transition towards uniaxial perpendicular magnetic anisotropy. Interfacial oxidation and diffusion from the Ta capping layer to the Pd/CoFeB/Pd interfaces were observed, influencing chemical bonding states. Annealing at 700 °C, reduced oxygen within TaOx through a redox reaction involving Ta crystallization, forming TaB, PdO, and BOx states. Ferromagnetic resonance spectra analysis indicated variations in resonance field (Hr) due to local chemical environments. The α reduction, reaching a minimum at 300 °C annealing, was attributed to reduced structural disorder from inhomogeneities. Tailoring magnetic anisotropy and spin dynamic properties in Pd/CoFeB/Pd/Ta structures through TA-controlled oxygen diffusion/oxidation highlights their potential for SOT, DMI, and magnetic skyrmion-based spintronic devices.

An enhanced Energy-Based hysteresis model for electrical steel sheets considering mechanical stress

Mechanical stress significantly influences the hysteresis characteristics of silicon steel, which are crucial for the design of electrical equipment cores. This study develops an improved Energy-based hysteresis model that integrates the effects of mechanical stress by modifying the anhysteretic magnetization model to include total average anisotropy energy density and introducing a linear relationship between coercive force and mechanical stress. The model’s performance is demonstrated using 30QG120 oriented silicon steel sheets, where the hysteresis loops under varying stress conditions are successfully simulated and validated against experimental data. The results confirm the model’s accuracy and practical utility in predicting material behavior under mechanical stress, offering a valuable tool for optimizing electrical steel sheet design.

Experimental and computational Insights Into the magnetic anisotropy and magnetic behaviour of layered room-temperature ferromagnet Cr1.38Te2

We investigate the structural, magnetocrystalline anisotropy, critical behaviour, and magnetocaloric effect in the layered room-temperature monoclinic ferromagnet Cr1.38Te2. The critical behavior is studied by employing various techniques such as the modified Arrott plot (MAP), the Kouvel-Fisher method (KF), and the critical isothermal analysis (CI) around the Curie temperature (T C ) of 316 K. The derived critical exponents are self-consistent and obey the rescaling analysis. The Monte-Carlo simulations reproduce the experimentally obtained critical exponents. However, the derived critical exponents do not suggest any single universality class of the magnetic interactions. On the other hand, the renormalization group (RG) theory suggests 3D-Ising type long-range exchange interactions [J(r)], decaying with distance (r) as J(r) = r −(d+σ) = r −4.73. Further, magnetocrystalline anisotropy energy density (K u ) is found to be temperature dependent. The ground state magnetic easy-axis (b-axis) is identified by analyzing the magnetocrystalline anisotropy energy (MAE) using the density functional theory calculations. Maximum entropy change - ΔSmmax ≈2.51 J/kg-K is found near the T C .

Perpendicular magnetic anisotropy of Cr/CoFeB/MgO modulated by MgO thickness

The dependence of perpendicular magnetic anisotropy (PMA) on the MgO thickness in Cr/CoFeB/MgO/Ta films has been experimentally investigated. A clear PMA is observed in the as-deposited samples with 1.8 nm MgO while no as-deposited PMA is shown in those with 4.0 nm MgO. This may be attributed to the moderate oxidation degree of CoFeB and larger interfacial anisotropy energy density K i to overcome the volume magnetic anisotropy and demagnetization field. On the contrary, samples with 4.0 nm MgO demonstrate PMA only after annealing, which might be due to the oxygen and boron diffusion during the annealing process. These results would provide a method to optimize the design of CoFeB/MgO structures on 3d metals for future applications in perpendicular magnetic devices.

Perpendicular magnetic anisotropy of Pd/Co2MnSi/AlN/Pd multilayer films

In this paper, the perpendicular magnetic anisotropy（PMA）of Pd/Co2MnSi (CMS)/AlN/Pd multilayer film is studied, which is designed based on the two-interface interaction, the orbital hybridization effect of Pd5d-Co3d electrons at the Pd/CMS interface and the orbital hybridization effect of Co3d-N2p electrons at the CMS/AlN interface. Compared with the Pd/CMS/Pd multilayer film, the PMA of the Pd/CMS/AlN/Pd structure shows significant enhancement with the insertion of the AlN layer. The Hall resistance (RHall) of the Pd/CMS/AlN/Pd structure reached 0.02 Ω (increased by 300 %) and the coercive force (Hc) is 388 Oe (increased by about 76 %). The magnetic anisotropic energy density (Keff) is 0.61Merg/cm3 (increased by about 135 %). The PMA was optimized with the film factor (layer thickness) of Pd(6 nm)/CMS(5 nm)/AlN(6 nm)/Pd(6 nm). The X-Ray Diffraction（XRD）and High Resolution Transmission Electron Microscope（HRTEM）analysis showed that the PMA is affected by the crystallinity and interface quality of multilayer films.

Obtaining extremely low coercivity of high Bs FeCoBSiCPCu nanocrystalline alloys through modulation of magnetic anisotropy

Longitudinal magnetic field annealing is utilized for modifying the magnetic anisotropy and enhancing the magnetic softness of Fe75Co8(B10Si3C3P1)1-x/17Cux (x = 0.5, 0.75, 1, 1.25) nanocrystalline alloys. All of the magnetic field-annealed nanocrystalline alloys with Cu content more than 0.5 at.% exhibit significantly improved soft-magnetic properties, including high saturation magnetic flux density up to 1.87 T, effective permeability of 13,000–16,000 under the condition of 1 A/m and 1 kHz, coercivity as low as 1.6 A/m, and core loss of 0.11–0.45 W/kg under the condition of 1.0 T and 50 Hz. The application of a magnetic field promotes the nucleation and inhibits the growth of grains, leading to an increase in the number density of nanocrystals and the crystalline volume fraction, and a reduction in the grain size. The magnetic field annealing reduces the effective magneto-crystalline anisotropy energy to 2–4 J/m3, and induces longitudinal magnetic anisotropy with anisotropy energy density of 400–900 J/m3 which shows dependence on the crystalline volume fraction. The field-induced magnetic anisotropy dominates over the random local magnetic anisotropies, and results in the formation of regular magnetic domains aligned longitudinally, pinning-free domain wall displacement, and thus enhanced soft-magnetic properties.

Interfacial Fe segregation and its influence on magnetic properties of CoFeB/MgFeO multilayers

Abstract We investigated effects of Fe segregation from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy and magnetic measurements revealed that segregated Fe reduced to metallic Fe and ferromagnetism was exhibited at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed a more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with that of a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by insertion of an ultrathin MgO layer between the CoFeB and MgFeO layers. Ferromagnetic resonance measurements also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers.

A novel finite element approximation of anisotropic curve shortening flow

We extend the DeTurck trick from the classical isotropic curve shortening flow to the anisotropic setting. Here, the anisotropic energy density is allowed to depend on space, which allows an interpretation in the context of Finsler metrics, giving rise to, for instance, geodesic curvature flow in Riemannian manifolds. Assuming that the density is strictly convex and smooth, we introduce a novel weak formulation for anisotropic curve shortening flow. We then derive an optimal H^1 -error bound for a continuous-in-time semidiscrete finite element approximation that uses piecewise linear elements. In addition, we consider some fully practical fully discrete schemes and prove their unconditional stability. Finally, we present several numerical simulations, including some convergence experiments that confirm the derived error bound, as well as applications to crystalline curvature flow and geodesic curvature flow.

Manipulation of spin–orbit torque and Dzyaloshinskii-Moriya interaction by varying Mn concentration in Pt1-xMnx/Co bilayer

Reducing critical current density (JC) of current-driven magnetization switching via power-efficient spin–orbit torque (SOT) is a key challenge in spin-based memory and logic devices. The Pt1-xMnx alloy shows prospect for enhancing the damping-like SOT efficiency (ξDL) while maintaining relatively low resistivity and power consumption. In this work, we investigated the Mn concentration x dependence of ξDL in Pt1-xMnx/Co bilayer and observed a nonmonotonic variation with the maximum ξDL ≈ 0.25 appearing around x = 0.2. The increase of the Pt1-xMnx layer resistivity is the main contribution to the enhancement of ξDL. Mn atoms doped in Pt not only modulate ξDL originating from the spin Hall effect of the Pt1-xMnx layer, but also affect the interfacial phenomena at the Pt1-xMnx/Co interface. We find that the related interfacial Dzyaloshinskii-Moriya interaction (DMI) is also tunable by x with the maximum appearing around x = 0.15. The significantly reduced JC of Pt1-xMnx/Co bilayer is mainly due to the enhancement of ξDL and the attenuation of effective perpendicular magnetic anisotropy energy density. Meanwhile, Pt1-xMnx/Co bilayer maintains low power consumption and necessary thermal stability. Our work highlights the potential of alloying Pt with Mn as a valuable means to achieve high-performance SOT spintronic devices.

Facile preparation of bonded NdFeB/SmFeN hybrid magnets with flexibility, anisotropy and high energy density

The application of rubber isotropic bonded magnets is limited due to low energy density, although it remainsunbeatableintermsofprice and production rate. In this paper, high-filled, flexible, and anisotropic NdFeB/SmFeN bonded magnets were prepared using mixtures of fine SmFeN powders and coarse NdFeB powders by calendering molding. The optical microscope images show that adding fine SmFeN powder benefits processability, leading to higher loading while retaining flexibility. The magnetic property and microstructure of the magnets display the anisotropic magnetic property of NdFeB/SmFeN bonded magnets derived from the arrangement of the platelet-shaped NdFeB particles induced by mechanical stress during the calendaring process. To further optimize the magnetic properties of NdFeB/SmFeN bonded magnets, the NdFeB was sieved and then mixed with SmFeN to mold into sheets. Finally, the effect of NdFeB particle size on the magnetic and mechanical properties of NdFeB/SmFeN bonded magnets was investigated. The results showed that when the particle size of the NdFeB particle was 75–100 µm, the magnetic properties of magnets reached the maximum value (Br = 7.77 KG, Hcj = 14.70 KOe, (BH)max = 12.70 MGOe) with density of 5.59 g/cm3. In addition, the tensile strength of the NdFeB/SmFeN bonded magnets increases and the sample standard deviation narrows with decreasing NdFeB particle size. This method provides a facile preparation for anisotropic bonded magnets with a new energy density record of rubber bonded rolled magnets prepared without an external field.

Large ordered moment with strong easy-plane anisotropy and vortex-domain pattern in the kagome ferromagnet Fe3Sn

We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetometry and magnetic force microscopy (MFM) on high-quality single crystals. The dependence of magnetization on the orientation of the external magnetic field reveals strong easy-plane magnetocrystalline anisotropy and anisotropy of the saturation magnetization. The leading magnetocrystalline anisotropy constant shows a monotonous increase from K1≈−1.0×106 J/m3 at 300 K to −1.3×106 J/m3 at 2 K. Our ab initio electronic structure calculations yield the value of total magnetic moment of 7.1 μB/f.u. and a magnetocrystalline anisotropy energy density of −0.57 meV/f.u. (−1.62×106J/m3) both being in reasonable agreement with the experimental values. The MFM imaging reveals micrometer-scale magnetic vortices with weakly pinned cores that vanish at the saturation field of ∼3 T applied perpendicular to the kagome plane. The observed vortex-domain structure is well reproduced by the micromagnetic simulations, using the experimentally determined value of the anisotropy and exchange stiffness.

Physicochemical properties and AC magnetic field induced heating properties of solvothermally prepared thiospinel: Fe3S4 (greigite) nanoparticles

The presence of greigite (Fe3S4) nanoparticles in bacterial magnetosomes, and its lower toxicity have emerged as favourable aspects for its potential applications in various bio-medical applications, including magnetic hyperthermia. Despite having a number of intriguing features, systematic research on the heating efficiency of Fe3S4 nanoparticles (MNPs) in an AC magnetic field is scarce, which is primarily due to the difficulties in preparing phase pure greigite MNPs. In this study, greigite MNPs are prepared using a solvothermal approach, utilizing ethylene glycol as a solvent, and surface functionalized with varied concentrations of poly vinyl alcohol (PVA). Studies using powder x-ray diffraction and electron microscopy demonstrate the development of crystalline Fe3S4 MNPs (average crystallite size: 19–23 nm) with flaky or flower-like morphology. X-ray photoelectron spectroscopy indicates that the lattice is composed primarily of iron and sulphur. The existence of bio-compatible PVA polymer on the surface of the coated MNPs is confirmed using Fourier transform infrared spectroscopy. For the uncoated MNPs, the magnetization at 90 kOe and the effective anisotropy energy density values are found to be ∼ 15.2 emu g−1 and ∼ 22.3 kJ m−3, respectively. Due to the improved colloidal stability, magneto-calorimetric experiments reveal higher AC magnetic field induced heating efficiency for the PVA-coated MNPs. The highest specific absorption rate (SAR) is obtained as ∼ 67.8 ± 2.6 W/gFe in the current study, which is several times higher than the previously published values for synthetic Fe3S4 MNPs. Furthermore, for samples with comparable saturation magnetization and crystallite size, SAR is found to increase with initial susceptibility. The in vitro cytotoxicity studies show good bio-compatibility for the prepared greigite MNPs. The experimental findings provide deeper insights into the preparation of Fe3S4 MNPs using a simple solvothermal technique, and its AC magnetic field induced heating efficiency.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Engineering perpendicular magnetic properties of Pt/Co2MnSi multilayered structures capped with dielectric Ta2O5

Pt/Co2MnSi (CMS) multilayered structures capped with high-K dielectric Ta2O5 are sputter-grown and perpendicular magnetic properties are systematically investigated. Strong PMA with the order of 106 erg/cm3 is observed with ultrathin CMS thickness in the range of 0.6∼0.8 nm. An interfacial perpendicular anisotropy energy density of Ks=∼0.52 erg/cm2 is obtained. When the repetition number of Pt/CMS bilayer is greater than two strong PMA can be demonstrated, which may be ascribed to enhanced exchange coupling between CMS layers. In the case of a large repetition number n ≥ 6 switching starting at negative field occurs, indicating nucleation process dominates. Reduced coercivity and saturation magnetization are witnessed in the stacks with increasing thickness of Ta2O5 capping, which can be mainly ascribed to the oxidation effect at the top interface. Our results show perpendicular magnetic properties of Pt/CMS multilayered structures with high-K dielectric Ta2O5 may be engineered for perpendicular spintronic devices.

Micromagnetic and morphological characterization of heteropolymer human ferritin cores.

The physical properties of in vitro iron-reconstituted and genetically engineered human heteropolymer ferritins were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron energy-loss spectroscopy (EELS), and 57Fe Mössbauer spectroscopy were employed to ascertain (1) the microstructural, electronic, and micromagnetic properties of the nanosized iron cores, and (2) the effect of the H and L ferritin subunit ratios on these properties. Mössbauer spectroscopic signatures indicate that all iron within the core is in the high spin ferric state. Variable temperature Mössbauer spectroscopy for H-rich (H21/L3) and L-rich (H2/L22) ferritins reconstituted at 1000 57Fe/protein indicates superparamagnetic behavior with blocking temperatures of 19 K and 28 K, while HAADF-STEM measurements give average core diameters of (3.7 ± 0.6) nm and (5.9 ± 1.0) nm, respectively. Most significantly, H-rich proteins reveal elongated, dumbbell, and crescent-shaped cores, while L-rich proteins present spherical cores, pointing to a correlation between core shape and protein shell composition. Assuming an attempt time for spin reversal of τ 0 = 10-11 s, the Néel-Brown formula for spin-relaxation time predicts effective magnetic anisotropy energy densities of 6.83 × 104 J m-3 and 2.75 × 104 J m-3 for H-rich and L-rich proteins, respectively, due to differences in surface and shape contributions to magnetic anisotropy in the two heteropolymers. The observed differences in shape, size, and effective magnetic anisotropies of the derived biomineral cores are discussed in terms of the iron nucleation sites within the interior surface of the heteropolymer shells for H-rich and L-rich proteins. Overall, our results imply that site-directed nucleation and core growth within the protein cavity play a determinant role in the resulting core morphology. Our findings have relevance to iron biomineralization processes in nature and the growth of designer's magnetic nanoparticles within recombinant apoferritin nano-templates for nanotechnology.

Charge-mediated voltage modulation of magnetism in Hf0.5Zr0.5O2/Co multiferroic heterojunction

We construct the Hall-bar device with the size of several hundred nanometers based on the HZO/Co multiferroic heterojunction. A remarkable voltage-controlled magnetism is observed in the device that possesses both ferroelectric property and perpendicular magnetic anisotropy (PMA). The nucleation field and coercivity can be modulated by voltage pulse while saturation field keeps stable. The non-volatile and reversible voltage-controlled magnetism is ascribable to interfacial charges caused by ferroelectric polarization. Meanwhile, the effective anisotropy energy density (K u) can also be controlled by voltage pulse, a decrease of 83% and increase of 28% in K u are realized under –3-V and 3-V pulses, respectively. Because the energy barrier is directly proportional to K u under a given volume, a decreased or enhanced energy barrier can be controlled by voltage pulse. Thus, it is an effective method to realize low-power and high-stability magneto-resistive random-access memory (MRAM).

Fixed magnetic nanoparticles: Obtaining anisotropy energy density from high field magnetization

A simple method is proposed to obtain the effective anisotropy energy density Keff of an assembly of randomly oriented magnetic nanoparticles, from their hysteresis loops. It involves the fitting of a high field asymptotic expression of the magnetization in inverse powers of the applied field H, up to H−3. This is derived from the partition function formalism and the Stoner–Wohlfarth model for single domain nanoparticles. This method can be applied to ferrogels, frozen ferrofluids or magnetic nanoparticles powder (or any system where the nanoparticles are fixed in random directions, and not allowed to rotate), when dipolar interactions can be neglected. As a proof of concept, it is applied to a suspension of iron oxide nanoparticles in hexane, at different temperatures, obtaining the anisotropy energy density Keff as a function of temperature below the fusion point.

Enhanced magnetoresistance in perpendicular magnetic tunneling junctions with MgAl2O4 barrier

Perpendicular magnetic tunnel junction with MgAl2O4 barrier is investigated. It is found that reactive RF sputtering with O2 is essential to obtain strong perpendicular magnetic anisotropy and large tunneling magnetoresistance in MgAl2O4-based junctions. An interfacial perpendicular magnetic anisotropy energy density of 2.25 mJ/m2 is obtained for the samples annealed at 400 °C. An enhanced magnetoresistance of 60% has also been achieved. The Vhalf, bias voltage at which tunneling magnetoresistance drops to half of the zero-bias value, is found to be about 1 V, which is substantially higher than that of MgO-based junctions.