Uniaxial Magnetocrystalline Anisotropy Research Articles

Thermodynamics of SmCo5 compound doped with Fe and Ni: An ab initio study

SmCo5 permanent magnets exhibit enormous uniaxial magnetocrystalline anisotropy energy and have a high Curie temperature. However, their low energy product is a significant deficiency. To increase the energy product in SmCo5, we propose substituting cobalt with iron, that has a much larger magnetic moment, in a SmCoNiFe3 magnet where nickel is used as a thermodynamic stabilizer.

Magnetism of new metastable cobalt-nitride compounds.

The search for new magnetic materials with high magnetization and magnetocrystalline anisotropy is important for a wide range of applications including information and energy processing. There is only a limited number of naturally occurring magnetic compounds that are suitable. This situation stimulates an exploration of new phases that occur far from thermal-equilibrium conditions, but their stabilization is generally inhibited due to high positive formation energies. Here a nanocluster-deposition method has enabled the discovery of a set of new non-equilibrium Co-N intermetallic compounds. The experimental search was assisted by computational methods including adaptive-genetic-algorithm and electronic-structure calculations. Conventional wisdom is that the interstitial or substitutional solubility of N in Co is much lower than that in Fe and that N in Co in equilibrium alloys does not produce materials with significant magnetization and anisotropy. By contrast, our experiments identify new Co-N compounds with favorable magnetic properties including hexagonal Co3N nanoparticles with a high saturation magnetic polarization (Js = 1.28 T or 12.8 kG) and an appreciable uniaxial magnetocrystalline anisotropy (K1 = 1.01 MJ m-3 or 10.1 Mergs per cm3). This research provides a pathway for uncovering new magnetic compounds with computational efficiency beyond the existing materials database, which is significant for future technologies.

Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate

Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.

Reduction of intergranular exchange coupling and grain size for high Ku CoPt-based granular media: Metal-oxide buffer layer and multiple oxide boundary materials

Investigation of magnetic properties and microstructure of granular media with various multiple oxides as the grain boundary material is reported. Saturation magnetization (Ms), uniaxial magnetocrystalline anisotropy (Ku), and magnetic grain diameter (GD) of the granular media show linear correlation with volume weighted average for melting point (Tm) of each oxides (Tmave). Ku of magnetic grains (Kugrain) shows a trade-off relation with GD that it is a big challenge to satisfy both high Kugrain and small GD by only controlling Tmave. To obtain a granular medium with appropriate Kugrain, GD, and low degree of intergranular exchange coupling, the combination of Tmave control of grain boundary material by mixing oxides and employment of a buffer layer are required. Here the degree of intergranular exchange coupling is estimated from the slope of M-H loop at around coercivity (α). By applying this technique, a typical granular medium with Kugrain of 1.0×107 erg/cm3, GD of 5.1 nm, and α of 1.2 is realized.

Screening of rare-earth-lean intermetallic 1-11 and 1-11-X compounds of YNi9In2-type for hard-magnetic applications

We report on theoretical investigations of ferromagnetic rare-earth-transition-metal phases with the structure of YNi9In2 and with additional atoms X at two interstitial positions. By a high-throughput-screening (HTS) approach based on density functional theory the intrinsic key properties of hard magnets, namely the magnetization M, energy product (BH)max and uniaxial magnetocrystalline anisotropy constant K1 are estimated. The HTS identifies several promising phases NdFe10AX with A=Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Si, P, and X=B, C, N. These phases partially outperform Nd2Fe14B in terms of (BH)max and K1 values, and they contain about 35% less rare-earth atoms.

Exchange-coupled hard-soft ferrites; A new microwave material

During the past decade, hexaferrite has become an important candidate for a variety of microwave and millimeter-wave devices due to their large uniaxial magneto-crystalline anisotropy and high saturation magnetization. The goal of this investigation is to synthesize BaFe12O19/Y3Fe5O12 (Barium hexaferrite/Yttrium Iron Garnet - BaM/YIG) hard-soft ferrite nanocomposites (NCs) as an advanced microwave material. A detailed microstructure and magnetic properties of these nano-composites were investigated using various techniques; viz. X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).With recent advances in nanostructure synthesis techniques, we have realized a thin nano-composite layer on top of coplanar waveguide (CPW) planar transmission lines to design microwave notch-filters. Using a vector network analyzer we have checked the device operation from X-to Ka-band through ferromagnetic resonance (FMR) spectroscopy. The FMR linewidth (device bandwidth) is observed to be tunable with (BaM)1-x (YIG)x nanocomposite from 2 kOe (900 MHz) for pure BaM (x = 0) to 0.2 kOe (90 MHz) for pure YIG (x = 1). Various mathematical models were employed to fit the experimentally observed FMR linewidth (ΔH) data to yield intrinsic and extrinsic damping parameters as a function of ‘x’ value in (BaM)1-x (YIG)x nanocomposite.

Effective uniaxial anisotropy in easy-plane materials through nanostructuring

Permanent magnet materials require a high uniaxial magneto-crystalline anisotropy. Exchange coupling between small crystallites with easy-plane anisotropy induces an effective uniaxial anisotropy if arranged accordingly. Nanostructuring of materials with easy-plane anisotropy is an alternative way to create hard-magnetic materials. The coercivity increases with decreasing feature size. The resulting coercive field is about 12 percent of the anisotropy field for a crystal size of 3.4 times the Bloch parameter.

Hexagonal Closed Packed CoPtRu Alloy as a Buffer Layer Material for Promoting Heteroepitaxial Growth for Perpendicular Magnetic Recording Media

Crystal structure and magnetic properties for Co–Pt–Ru alloy films and Co–Pt–Ru–oxide granular films were investigated for material to reduce the lattice mismatch between the magnetic layer and the nuclei control layer of the perpendicular magnetic recording media. The laminate stacking structure of the sample was sub/NiFeCr (20 nm)/Ru (20 nm)/Co–Pt–Ru (20 nm). The amount of stacking faults in the films was evaluated as a probability of occurrence of fcc stacking, Pfcc measured by an in-plane X-ray diffractometry profile. As a result, it was found that: 1) hcp atomic stacking structures of the Co100– x Ru x alloy films were stable compared with that of Co100– x Pt x alloy films for almost all of the additional content x; 2) lattice constant a and c for Co–Pt–Ru ternary alloy film could be controlled between 2.51 to 2.59 A, and 4.07 to 4.41 A, as changing the Pt and Ru content with a total amount of 0 to 50 at.%, respectively. And the lattice constant ratio c/a decreased from 1.62 to 1.59 with increasing Ru content more than 20 at.%; 3) Pfcc in the Co–Pt–Ru ternary alloy films was less than 0.5% for the samples with Ru content were more than 20 at.%; 4) uniaxial magnetocrystalline anisotropy K u could be controlled from 12.6 to 0 106 erg/cm3 by adding Pt and Ru with a total amount of 30 at.% to a Co75Pt25 film; and 5) Co–Pt–Ru–(SiO2) film with the columnar structure was realized for Ar gas pressure of 4 Pa during sputtering.

Effect of Ball-Milling on Magnetic Properties of Uniaxial Nanocrystalline $$\hbox{SmNi}_2 \hbox{Fe}$$ SmNi 2 Fe Compound

We report an interesting structural and nanostructural correlation to the Curie temperature and coercivity, respectively. The effect of Ni substitution and magnetocaloric effect (MCE) response for the uniaxial nanocrystalline $$\hbox{SmNi}_2\hbox{Fe}$$ compound was demonstrated. The nanocrystalline SmNi2Fe( $$\hbox{SmNi}_2\hbox{Fe}$$ ) powder has been synthesized by mechanical milling for the first time. The nanocrystalline $$\hbox{SmNi}_2\hbox{Fe}$$ powder has been synthesized by mechanical milling. The structural properties have been studied by x-ray diffraction and have been refined with the Rietveld method. This analysis reveals that the samples crystallize in the rhombohedral PuNi $$_3$$ -type structure ( $$R\bar{3}m$$ -space group). The Curie temperature of $$\hbox{SmNi}_2\hbox{Fe}$$ material is about 450 K. It is a direct measure of the exchange interaction, which is the origin of ferromagnetic phase transition. This interaction depends markedly on the interatomic distance. The magnetic entropy change $$\Delta S_{\mathrm{M}}$$ , as well as the relative cooling power around the second-order magnetic transition and the Arrott plots for the compound have been reported. It was estimated from isothermal magnetization curves, and it decreases with the increase of Fe content from 2.3 J/kg K ( $$x = 0$$ ) to 0.6 J/kg K ( $$x = 0.8$$ ) at 50 kOe. The compound presents high coercivity exceeding 12 kOe at room temperature, obtained based on the optimization of $$\hbox{SmNi}_2\hbox{Fe}$$ nanostructure. The objective of this optimization is to obtain a homogeneous nanostructure developed by the mechanical milling process, subsequent annealing and to the high uniaxial magnetocrystalline anisotropy.

Fabrication and magnetization reversal of L10 FeMnPt dots surrounded by paramagnetic A1 phase formed by ion irradiation

A nanopatterning method involving a ferromagnetic (FM)–paramagnetic (PM) phase transformation caused by ion irradiation has been developed. A (Fe0.56Mn0.44)50Pt50 film with the ordered L10 structure and uniaxial magnetocrystalline anisotropy constant (Ku) of 2.1×106Jm−3 was fabricated. Mn ion irradiation produced a smooth FM dot array with dot diameters of 100nm surrounded by the PM phase, which was obtained from the induced FM–PM phase transition caused by the structural transformation from the L10 structure to the disordered A1 structure and a slight increase in the Mn content. The magnetization reversal process was studied by investigating the angle dependence of the dot reversal field.

Nature of magnetocrystalline anisotropy in the basal plane of iron borate

Basal (hexagonal) magnetocrystalline anisotropy of iron borate FeBO3, including crystal field and dipole-dipole contributions, is discussed in detail. Previously, the latter contribution had been ruled out on the basis of symmetry; indeed, considering the magnetic dipole-dipole interaction in the approximation of point dipoles, only uniaxial magnetocrystalline anisotropy is accounted for. In the present work we consider the dipole-dipole interaction energy for extended dipoles and calculate the dipole-dipole contribution to basal magnetocrystalline anisotropy for two different models of an extended dipole: a pair of fictitious magnetic charges and a circular current loop. A comparison between theoretical expressions developed and experimental results obtained by antiferromagnetic resonance allows estimating the effective size of magnetic dipoles and calculating the dipole-dipole contribution to basal magnetocrystalline anisotropy constants of iron borate.

Field tuning of domain-wall type and chirality in SrRuO3

${\mathrm{SrRuO}}_{3}$ films have large uniaxial magnetocrystalline anisotropy tilted out of the film plane. When cooled in zero field from above the Curie temperature ($\ensuremath{\sim}150$ K), a magnetic domain structure emerges in the form of 200-nm-wide stripes oriented along the in-plane projection of the magnetic easy axis. We measure the interface resistance of the magnetic domain walls by applying a magnetic field perpendicular to the easy axis in the domain-wall plane and perpendicular to this plane and observe hysteretic behavior. Micromagnetic simulations of the results indicate that, depending on the direction in which the field is applied, the domain-wall type (Bloch or N\'eel) and chirality can be tuned.

Structural and magnetic properties of new uniaxial nanocrystalline Pr5Co19 compound

Highly-coercive nanocrystalline Pr5Co19 powders have been synthesized by mechanical milling for the first time. The structural properties are studied by X-ray diffraction and refined with Rietveld method. This analysis revealed that whatever annealing temperature, samples crystallize in the rhombohedral (3R) of Ce5Co19-type structure (space group R3¯m). The magnetization curve as a function of temperature shows a magnetic transition state at the Curie temperature TC=690K. The optimum hard magnetic properties have been obtained for Pr5Co19 milled for 5h and annealed at 1048K for 30min. These alloys exhibit a coercivity of 15kOe at room temperature. This high coercivity is attributed to the high uniaxial magnetocrystalline anisotropy, nanoscale grain size, and to the homogeneous nanostructure developed by mechanical milling process and subsequent annealing.

Preparation of FePt Nanoparticles by Pulsed Plasma in Liquid Method

FePt alloys are an important class of materials in permanent magnetic applications because of their large uniaxial magnetocrystalline anisotropy and good chemical stability. We have applied the pulsed plasma in liquid method to synthesis nanoparticles. Short duration of pulse and quenching effects of the surrounding liquid limit the size of crystal. That enable synthesis of small size and metastable nanoparticles. In this study, ferromagnetic FePt nanoparticles were successfully synthesized. Face-centered-cubic (FCC) A1-type phase was synthesized from FePt (Fe:PT=50:50, 55:45 in atomic %) alloy rod electrodes using the pulsed plasma in ethanol. The ordered face-centered-tetragonal (FCT) L10-type phase FePt was obtained by annealing the A1-type phase at 400oC for 1 h. The average diameter of L10-type FePt nanoparticles was less than 10 nm. Vibrating Sample Magnetometer (VSM) analysis showed that the coercivity of L10-type nanoparticles was much larger than that of the A1-type phase nanoparticles.

Strain engineering for controlled growth of thin-film FeNi L10

FeNi thin films in the L10 phase were successfully grown by magnetron sputtering on HF-etched Si substrates on Cu/Cu100−xNix buffers. The strain of the FeNi layer, , was varied in a controlled manner by changing the Ni content of the Cu100−xNix buffer layer from to , which influenced the common in-plane lattice parameter of the CuNi and FeNi layers. The presence of the L10 phase was confirmed by resonant x-ray diffraction measurements at various positions in reciprocal space. The uniaxial magnetocrystalline anisotropy energy is observed to be smaller (around 0.35 MJ m−3) than predicted for a perfect FeNi L10 sample, but it is larger than for previously studied films. No notable variation in with strain state is observed in the range achieved (), which is in agreement with theoretical predictions.

Magnetic anisotropy properties of Co1.2Fe1.8–xMnxO4 (0.0 ⩽ x ⩽ 0.3) nanopowders: theory and experiment

We perform a theoretical and experimental investigation of the magnetic properties of a series of Mn-substituted non-stoichiometric cobalt ferrite nanopowders. We propose a theoretical approach based on the Stoner–Wohlfarth model of coherent rotation of single-domains, considering mean-field interactions between the nanograins. In particular, we provide a detailed description of the magnetic anisotropy behavior, considering tetragonal distortions of the cubic spinel structure of the system. Our model shows that the effects of the tetragonal distortion of the cubic structure may be modeled by a superposition of uniaxial and cubic magnetocrystalline anisotropies. We obtain the uniaxial and cubic anisotropy constants and the mean-field parameters by fitting numerically the experimental magnetic hysteresis curves. Thus, the agreement between theory and experiment provides support to confirm the validity of our theoretical approach, in addition our results corroborate experimental works found in the literature reporting that cobalt ferrites present tetragonal distortion of the well-known cubic spinel structure.

BaM/YIG nano-composites: A microwave material for C to U band application

Hexaferrites have become important candidates for a variety of microwave and millimeter wave devices due to their large uniaxial magneto-crystalline anisotropy and high saturation magnetization. The goal of the present investigation is to synthesize Barium hexaferrite/Yttrium Iron Garnet (BaFe12O19/Y3Fe5O12): (BaM/YIG) Nano-Composites (NCs) to be used in broad band microwave frequency range applications, especially as microwave absorber. X-ray diffractometry, Vibrating Sample Magnetometer (VSM), and ferromagnetic resonance (FMR) techniques were used to characterize these NCs. Using a Cu coplanar wave guide and a Vector Network Analyzer, broadband (C to U) microwave absorption were investigated by placing the bulk sample in flip chip mode. Various mathematical models were employed to fit the experimental data to yield intrinsic and extrinsic damping parameters.

Coercivity enhancement mechanism in Dy-substituted Nd–Fe–B nanoparticles synthesized by sol–gel base method followed by a reduction-diffusion process

In current work, Nd15−xDyxFe77.5B7.5 (at%) nanoparticles with different Dy-content (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by sol–gel method followed by a reduction-diffusion process. The effects of Dy on the magnetic properties and the relations between the microstructure and the coercivity of Dy-substituted Nd–Fe–B nanoparticles have been studied. The coercivity of Nd–Fe–B nanoparticles with the addition of Dy first increase, reaches a maximum, and then starts to decrease. The coercivity of Dy-substituted Nd–Fe–B nanoparticle synthesized by sol–gel method increased from 938.9 to 1663.9kA/m while the remanence decreased slightly from 1.16 to 1.06T. The results show that with an increase in Dy content the variation of maximum energy product ((BH)max), lowest-order uniaxial magnetocrystalline anisotropy constant (Ku1), and Curie temperature (Tc) had a trend as same as the coercivity. The Henkel plot showed that the existence of exchange coupling interaction between grains, and the exchange coupling interactions increased with increasing x from 0.0 to 2.0 and then decrease with further increasing x≥2.5. The optimum magnetic properties of Nd–Fe–B nanoparticles with (BH)max =40.38 MGOe, Hc=1663.9kA/m, Br=1.08T were obtained by substituted 2.0at% Dy. The effects of increasing temperature on magnetic properties of Dy-substituted Nd–Fe–B nanoparticle magnets with 2.0at% Dy was investigated. The reduced spin-reorientation temperature was obtained for Dy-substituted Nd–Fe–B nanoparticles with 2.0at% Dy. Below 100K a spin-reorientation transition was takes place. The temperature coefficient of coercivity (β) was −0.36, −0.46, −0.41, −0.34, −0.29, −0.24, −0.25%/°C at different temperature 50, 100, 150, 200, 250, 300, 350°C, respectively. Mössbauer spectroscopy was applied to study the composition and properties of Dy-substituted Nd–Fe–B magnet. Microstructure analysis showed a homogeneous distribution of Dy in produced samples. The possible reason for observed magnetic behavior is improving the intrinsic material parameter and optimizing the microstructure by a uniform enhancement of magnetocrystalline anisotropy by formation the nanocrystalline compound (Nd,Dy)2Fe14B.

Temperature-dependent shape anisotropy in patterned ferromagnetic (Ga,Mn)As films with low Mn concentration

We discuss magnetic properties of lithographically patterned nanoscale stripes of dilute ferromagnetic semiconductor (Ga,Mn)As aligned with [110] crystallographic direction, i.e., perpendicular to the easy axis of uniaxial magnetocrystalline anisotropy of this material. We find that at temperatures where uniaxial anisotropy becomes dominant, shape-induced effects strongly compete with this intrinsic property, tending to align the total magnetization along the stripe direction. The present study revealed, however, that low-temperature annealing of the stripes markedly reduces such shape-induced effects. This effect of annealing is not presently understood, and will require further detailed study.

Current-induced nonlinear magnetoelectric effects in strontium hexaferrite

We report on the observation of nonlinear magnetoelectric effects at room temperature due to a dc current in the ferrimagnetic $M$-type strontium hexaferrite platelets. Utilizing microwave measurement techniques and data on the shift in magnetic mode frequencies, it was found that a dc current along the hexagonal $c$ axis resulted in a significant decrease in the saturation magnetization and an increase in the uniaxial magnetocrystalline anisotropy field. These changes in the magnetic order parameters were directly proportional to the square of applied electric field and were found to be much higher than variations due to Joule heating. A phenomenological theory that takes into account the current-induced magnetobielectric (MBE) effects is proposed. Expressions for coupling coefficients for MBE effects have been obtained and have been calculated from the variations in magnetic order parameters. The electric field $E$ (or current) tuning of the magnetic modes in $\mathrm{Sr}M$ reported here is orders of magnitude stronger than strain mediated $E$ tuning of magnetic resonance in hexaferrite-ferroelectric composites. The nonlinear magnetoelectric effects in hexaferrite, therefore, open up an avenue for the realization of $E$-tunable broadband microwave and millimeter wave ferrite signal processing devices such as resonators and filters.