Effective Magnetic Anisotropy Constant Research Articles

Changes in the Magnetic Structure of Multiferroic BiFe0.80Cr0.20O3 with Temperature

The results of a Mossbauer study of the magnetic structure of multiferroic BiFe0.80Cr0.20O3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe0.80Cr0.20O3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Neel temperature, TN = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe0.80Cr0.20O3 multiferroic.

Characterisation of high-anisotropy nanocrystalline alloys based on magnetic susceptibilities in the remanent state

The macroscopic magnetic properties of nanocrystalline alloys are strongly dependent on microstructural and micromagnetic inhomogeneities along with intergranular interactions. In particular, magnetic susceptibility is sensitive to these factors and frequently used for alloy characterisation. In this work reversible magnetic susceptibility in the remanent state of the nanocrystalline alloy with randomly oriented easy magnetisation axes is considered. Due to intergranular interactions the susceptibilities measured parallel and perpendicular to the remanence can be distinctly different from each other. When the intergrain exchange is the dominant interaction this susceptibility feature has potential for its quantitative evaluation. Here this opportunity is studied by means of two approaches: analytical solutions based on the mean-field approximation and simulations using the kinetic Monte Carlo model. The dependence of the magnetic susceptibility on the exchange coupling strength are investigated taking into account the effects of grain size distribution and magnetostatic interactions. These results lead us to propose an experimental susceptibility-based method for the estimation of both intergrain exchange interaction strength and effective magnetic anisotropy constant. This method can be used for the characterisation of high-anisotropy nanocrystalline alloys with randomly oriented easy magnetisation axes that is a widespread case. The method is applied to rapidly quenched Nd2(Fe0.8Co0.2)14B nanocrystalline alloy giving strong exchange coupling and a reasonable value for the anisotropy constant.

The annealing effect on memory state stability and interlayer coupling in perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Abstract Perpendicular magnetic tunnel junctions (p-MTJs) attract great interest because of their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). The annealing process is critical to achieve the required structural and magnetic properties, therefore obtaining high perpendicular tunneling magnetoresistance (p-TMR) and lower switching current. In this work, the CoFeB/MgO/CoFeB-based p-TMR stack was prepared and a high p-TMR value of 155.9% was obtained from CIPT. The p-MTJs were patterned with pillar sizes ranging from 50 nm to 1000 nm and were annealed upon different annealing temperatures (TA) of 300 and 340 °C. The resistance at parallel (RP) and antiparallel (RAP) state in p-MTJs was obtained upon the application of a ±30 mT perpendicular magnetic field, where the angle of magnetization between storage and reference layer at memory state, θP and θAP, were derived from the P and AP resistance ratios with and without magnetic field. The condition θP = θAP of about 63.5° was found in as-deposited samples, indicating a ferromagnetic interlayer coupling between storage and reference layers, where θP = 0° and θAP = 180° at TA = 300 °C and 340 °C demonstrate a stable memory state. The stability of the resistance state was modeled by considering the evolution of effective perpendicular magnetic anisotropy (Keff) and interlayer coupling constant (J), showing a method to evaluate the ratio of Keff/J via the memory state resistance characterization, where the Keff/J of about −1.26 was obtained in our as-deposited samples. Our study assesses the effect of TA on the θP and θAP and supplies the method to enhance the stability through the optimization of perpendicular magnetic anisotropy and interlayer coupling, so to better control the performance of p-MTJ-based STT-MRAMs.

Температурные изменения магнитной структуры мультиферроика BiFe-=SUB=-0.80-=/SUB=-Cr-=SUB=-0.20-=/SUB=-O-=SUB=-3-=/SUB=-

AbstractThe results of a Mössbauer study of the magnetic structure of multiferroic BiFe_0.80Cr_0.20O_3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe_0.80Cr_0.20O_3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Néel temperature, T _N = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe_0.80Cr_0.20O_3 multiferroic.

Mössbauer Spectroscopy Study of the Superparamagnetism of Ultrasmall ϵ-Fe2O3 Nanoparticles

The superparamagnetism of an ensemble of ϵ-Fe2O3 nanoparticles with a mean size of 3.9 nm dispersed in a xerogel SiO2 matrix is studied by the Mossbauer spectroscopy method. It is shown that most nanoparticles at room temperature are in the superparamagnetic (unblocked) state. As the temperature decreases, the progressive blocking of the magnetic moments of the particles occurs, which is manifested in the Mossbauer spectra as the transformation of the quadrupole doublet into a Zeeman sextet. The analysis of the relative intensity of the superparamagnetic (quadrupole doublet) and magnetically split (sextets) spectral components in the range of 4–300 K provides the particle size distribution, which is in agreement with the transmission electron microscopy data. The values of the effective magnetic anisotropy constants (Keff) are determined, and the contribution of surface anisotropy (KS) is estimated for particles of various sizes. It is shown that the quantity Keff is inversely proportional to the particle size, which indicates the significant contribution of the surface to the magnetic state of the ϵ-Fe2O3 nanoparticles with the size of several nanometers.

Synthesis, crystal structure and magnetic characterization of Pr3+ and Zn2+ ions co-doped hexagonal ferrites via the ceramic process

M-type hexaferrites Ca0.2Sr0.8-xPrxFe12-yZnyO19 (0.00 ≤ x ≤ 0.40, 0.00 ≤ y ≤ 0.30) were synthesized by the ceramic process. The X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and a vibrating sample magnetometer (VSM) were used to investigate microstructure and magnetic properties of the M-type hexaferrites. The single-phase with hexagonal structure was obtained in all Pr–Zn substituted M-type hexaferrites, and with increasing Pr–Zn content, the 2θ values of (107) and (114) peaks shifted towards higher angles. With increasing Pr–Zn content, the lattice constant a basically kept unchanged, while the lattice constant c decreased. FESEM images of the hexaferrites showed that the hexagonal platelets had formed in the hexaferrites and the average grain size increased with increasing Pr–Zn content. The saturation magnetization (Ms), remanent magnetization (Mr) and Mr/Ms ratio first increased with increasing Pr–Zn content (0.00 ≤ x ≤ 0.24, 0.00 ≤ y ≤ 0.18), and then decreased with further increasing Pr–Zn content. The coercivity (Hc), magnetic anisotropy field (Ha) and effective magnetic anisotropy constant (Keff) increased with increasing Pr–Zn content (0.00 ≤ x ≤ 0.16, 0.00 ≤ y ≤ 0.12), and then decreased with further increasing Pr–Zn content.

Temperature Mössbauer study of the spatial spin-modulated structure in the multiferroic BiFeO3

57Fe Mössbauer detailed study of the spatial spin-modulated structure of the multiferroic BiFeO3 was carried out in a wide temperature range including the temperature of magnetic phase transition. The Mossbauer spectra have been analysed by fitting in terms of the anharmonic spin cycloid mode. It is established that at temperatures below ~330 K a magnetic anisotropy of the "easy axis" type is realized and above is the magnetic anisotropy of the "easy plane" type. An explanation for the change in the type of magnetic anisotropy is proposed, based on taking into account the different temperature dependences of the two contributions to the effective uniaxial magnetic anisotropy constant: a crystal anisotropy of net antiferromagnet and a weak ferromagnetism.

Investigation of Magnetic Properties of Multilayer Thin Films with Spin-Wave Resonance

A theory has been developed to simulate spin-wave resonance (SWR) modes in the multilayer systems consisting of alternate magnetic and nonmagnetic layers. An equation of motion of magnetization with Gilbert-type damping parameter for simulating SWR modes was used. It has been realized that the theory developed for the magnetic multilayer films is suitable to study the spin dynamics and extract various magnetic parameters. It has been shown that SWR modes strongly depend on an effective magnetic anisotropy constant (K eff), interlayer exchange coupling constant (A 12) and effective magnetization (M eff). The nature of the effective magnetic anisotropy and interlayer exchange coupling constants has been investigated by using the developed SWR theory in detail. The separation between optic and acoustic modes strongly depends on the magnitude of the interlayer exchange coupling constant, whereas the relative position of the acoustic and optic modes depends on the sign of $A_{12}$ . With increasing the interlayer exchange coupling constant, the resonance field of the optic mode decreases (increases) for ferromagnetic (antiferromagnetic) coupling. When the effective magnetic anisotropy constant increases, the resonance field of the acoustic and optic modes increases for both the ferromagnetic and antiferromagnetic couplings. The increasing of the effective magnetization results in decreasing of the resonance field of SWR mode at parallel geometry, whereas that of SWR mode increases at the perpendicular geometry. The results are compatible to the other theories and experimental results.

Effect of Zn substitution on temperature dependent magnetic properties of BaFe12O19 hexaferrites

Zinc doped barium hexaferrite powders with chemical composition BaFe12-xZnxO19 (0.0 ≤ x ≤ 0.3) were synthesized by sol-gel auto combustion method. The effect of zinc ion concentration on the temperature-dependent magnetic properties of BaFe12-xZnxO19 hexaferrites has been comprehensively investigated in the temperature range of 10–300 K and magnetic field of ±50 kOe. XRD powder patterns showed a pure single phase structure. The Rietveld refinement of the XRD patterns revealed the existence of hexagonal structure with P63/mmc space group. The average crystallite size of the powders was found to be in the range of 46–51 nm. The ratio of lattice constants (c/a) lower than 3.98 proves that BaFe12-xZnxO19 hexaferrite powders have a M-type hexagonal ferrite structure. The nonmagnetic zinc ions strongly affect the magnetic properties of the powders. The magnetic hysteresis loops were fitted by using a ‘law of approach to saturation’. From the linear fitting, the magnetic parameters were calculated such as saturation magnetization, effective magnetic anisotropy and anisotropy field. The saturation magnetization values close to bulk values were determined as 66.67 and 95.47 emu/g for 300 and 10 K, respectively, which makes the samples a promising candidate for electronic applications. The saturation magnetization slowly decreases and then increases, whereas the coercive field continuously decreases with the increasing of Zn2+ concentration (x). The squareness ratio (Mr/Ms) of about 0.5 confirmed that the hexaferrite powders have a uniaxial anisotropy. The effective magnetic anisotropy constant (Keff) was observed to decrease with the increase in Zn2+concentration. When the temperature increases, the saturation magnetization continuously decreases because of the weakening of superexchange interaction and the coercive field increases due to the increasing of anisotropy field. FMR measurements confirmed the ferromagnetic behaviour of the substituted barium hexaferrite powders. The high resonance field values of the powders (a maximum resonance field of 16 kOe) originate from the hard magnetic property of the hexaferrites used in this study. The absorption curves have a broad FMR linewidth due to an anisotropy axis randomly distributed and the competing between anisotropy and exchange fields.

Effect of composition and temperature on the magnetic properties of BaBi x La x Fe(12-2x)O19 (0.0 ≤ x ≤ 0.2) hexaferrites

Bi–La-substituted M-type barium hexaferrites, BaBi x La x Fe(12-2x)O19 (0.0 ≤ x ≤ 0.2), were prepared by the standard solid-state reaction method. The effect of temperature and concentration on the structural and magnetic properties of BaBi x La x Fe(12-2x)O19 hexaferrite powders has been comprehensively investigated. XRD analysis confirms the formation of M-type hexagonal crystal structure with P63/mmc space group. The average crystallite size of the powders was determined using Scherrer’s formula (in the range of 66–77 nm.). Lattice parameters were found to increase with the increase in Bi–La substitution. Magnetic properties were enhanced by substitution. Magnetic hysteresis loops were analysed using a ‘Law of Approach to Saturation’ method. It was obtained the saturation magnetization value close to the bulk value. Bi–La substitution gives rise to increase of saturation magnetization, coercive field, and effective magnetic anisotropy constant for both 10 and 300 K. It is observed from FMR measurements that resonance field increases with the concentration. The broad FMR linewidth of almost 2.5 kOe indicates the existence of the particles with randomly distributed anisotropy axis. The enhanced magnetic properties make these powders a good candidate for potential applications in permanent magnets at low cost and microwave absorption devices.

Mn0.5Zn0.5Fe2O4 nanoparticles with high intrinsic loss power for hyperthermia therapy

Nanosized mixed ferrite Mn0.5Zn0.5Fe2O4 with crystalline size ∼15nm has been prepared by hydrothermal route. XRD patterns confirm that the crystallites have single phase cubic spinel structure. The dynamic scaling analysis on the frequency dependence of spin glass-like transition temperature well explains the model of a transition at finite temperature. The analysis gives critical exponent and parameters as: zν=10.48, T0=190K, f0=5.38×1010 and this confirms the occurrence of spin glass-like transition in Mn0.5Zn0.5Fe2O4 particles. The saturation magnetization and the coercivity change with temperature. The effective magnetic anisotropy constant of sample was calculated using the law of approach to saturation. The coercivity at different temperatures was deduced using the mixed coercivity model. The calculated coercivity results are in a good agreement with the experimental ones. The magnetic heating ability of Mn0.5Zn0.5Fe2O4 magnetic fluid was studied with an induction heating system. The calculated intrinsic loss power (ILP) was 3.75gnHm2/kg. This study indicates that the resulting Mn0.5Zn0.5Fe2O4 nanoparticles are promising materials in magnetic hyperthermia.

Buffer layer annealing effects on the magnetization reversal process in Pd/Co/Pd systems

We have investigated the effect of annealing the buffer layer on the magnetization reversal behavior in Pd/Co/Pd thin films using magneto-optical Kerr microscopy. It was found that annealing the buffer layer at 150°C for 1h decreases the coercivity and increases the saturation magnetization and the effective magnetic anisotropy constant. This study also shows that the annealing induces a change of the magnetization reversal from a mixed nucleation and domain wall propagation process to one dominated by domain wall propagation. This result demonstrates that the main effect of annealing the buffer layer is to decrease the domain wall pinning in the Co layer, favoring the domain wall propagation mode.

Modelling the Influence of Size Distribution and Effective Magnetic Anisotropy Constants on the Magnetic Hyperthermia Process

Background: This paper presents a series of theoretical studies about the influence of the size distribution and the effective magnetic anisotropy constants on the Néel relaxation time, the effective relaxation time and the Specific Loss Power of a magnetic nanoparticle system. Methods: We modelled the system by considering the dipolar magnetic interactions among the nanoparticles. Therefore, the Néel relaxation time has been modelled by considering the local magnetic field (i.e., the external magnetic field plus the local magnetic dipolar field) as oblique to each nanoparticle's anisotropy axis. The contribution of our studies have practical applications. In fact, our goal is to propose a robust and reliable model able to optimise the heating process and, consequently, the Specific Loss Power in magnetic hyperthermia applications. Results: In our work, we checked how the dynamic behaviour of monodomain nanoparticles is ifluenced not only by energy barriers but also by a number of other parameters that depend on the effective magnetic anisotropy constants, of the local magnetic field and thus implicitly of the nanoparticle sizes. Conclusion: Our studies contribute to a deeper understanding of the magnetic hyperthermia process and finding ways to control the system parameters for optimising the heating process and, consequently, the Specific Loss Power. Keywords: Hyperthermia process, magnetic modelling, magnetic nanoparticle, specific loss power.

Dynamic Magnetic Properties of La0.8Ag0.2MnO3 Nanoparticles

In thisresearch,nanoparticlesof La0.8Ag0.2MnO3 with mean particle sizes d of 16 and 22 nm have been prepared by sol-gel method. Dynamic magnetic properties of the samples have been carried out by AC magnetic susceptibility and electron spin resonance techniques. The structural properties of the samples have been characterized by X-ray diffraction using Rietveld refinement and transmission electron microscopy. The analysis of the AC magnetic susceptibility by phenomenological models reveals the interacting superparamagnetic behaviors in La0.8Ag0.2MnO3 nanoparticles. From the Vogel-Fulcher model, the values of 1.3 × 104 and 2.8 × 104 erg/cm3 are obtained for the effective magnetic anisotropy constant for d= 16 and 21 nm, respectively. Electron spin resonance signals of the samples are well described by double Lorentzian line shapes which suggest the presence of ferromagnetic clusters in the paramagnetic phase and possible phase separation at room temperature.

The Néel Magnetic Relaxation Dynamics in Nanoparticle Systems - Phenomenological Approach versus Stochastic Approach

In nanomagnetism, the studies of magnetic nanoparticle systems are of particular interest from both experimental and theoretical points of view. Experimentally, the measurements made on such a system are hard to interpret. It is very difficult to distinguish the effect of the magnetic dipole interactions from the effects of size distribution or effective magnetic anisotropy constants. In this respect, the simulation models can help. This paper presents a study comparing the two conventional approaches, using simulation models for the magnetic relaxation dynamics of nanoparticle systems, i.e. a phenomenological Ising-type approach, on two levels, and a stochastic approach. The paper also shows a way of using these approaches in creating a model to simulate the Neel magnetic relaxation time for aligned magnetic nanoparticle systems.

Structural and Magnetic Properties of Single Domain PEG-Coated FexNi80−xCo20 (x = 20, 40) Ternary Alloys Synthesized by Chemical Method

In the present study, we have synthesized nanocrystalline FexNi80−xCo20 (x = 20 and 40) alloys via modified sodium borohydride reduction route in aqueous medium. The phase purity and microstructural analysis of the materials were done using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fe–Ni–Co alloys crystallize in face centered cubic (fcc) structure having the values of lattice parameters, a = 3.5460 and 3.5634 A, for Fe20Ni60Co20 and Fe40Ni40Co20, respectively. The average crystallite sizes were found to be 30 and 26 nm for Fe20Ni60Co20 and Fe40Ni40Co20, respectively. Fe20Ni60Co20 and Fe40Ni40Co20 alloys crystallize in nearly spherical shape morphologies having TEM particle sizes of 24 and 36 nm, respectively. Selected area electron diffraction (SAED) patterns confirm face centered cubic crystal structure and polycrystalline nature of the materials. Low temperature studies indicate that Fe–Ni–Co alloys show ferromagnetism at least up to 300 K. The effective magnetic anisotropy constant (K) for Fe20Ni60Co20 and Fe40Ni40Co20 alloys are found to be 14 and 4.2 kJm−3, respectively. These results have been explained on the basis of size, shape and surface effects in the nano-materials. The contribution corresponding to spin relaxation and dipolar interactions between nano-particles on magnetic characteristics of Fe20Ni60Co20 and Fe40Ni40Co20 alloys have been explained qualitatively during the magnetic analysis.

Synthesis, characterization and Monte Carlo simulation of CoFe2O4/Polyvinylpyrrolidone nanocomposites: The coercivity investigation

To study the influence of polymer matrix on the effective magnetic anisotropy constant and coercivity of magnetic nanoparticles, we have synthesized the Cobalt ferrite/Polyvinylpyrrolidone (PVP) nanocomposites by co-precipitation method in four different processes. In addition the Monte Carlo simulation and law of approach to the saturation magnetization have been applied to achieve the anisotropy constants. The obtained experimental and theoretical results showed a decrease in anisotropy constant relative to the bulk cobalt ferrite. We have showed that the PVP matrix can interact with metal cations and made them approximately immobilized to participate in spinel structure. Hence different anisotropy constants or coercivity were obtained for synthesized nanocomposites. In addition, PVP matrix can attach to the surface of magnetic particles and make them approximately non-interacting. The synthesized samples have been characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). Magnetic measurements were made at room temperature using a vibrating sample magnetometer (VSM).

Temperature dependence of magnetic anisotropy constant in CoFe2O4 nanoparticles examined by Mössbauer spectroscopy

The temperature dependence of the effective magnetic anisotropy constant of CoFe2O4 nanoparticles is determined based on the measurements of SQUID magnetometry and Mossbauer spectroscopy. Under an intuitive assumption that the superparamagnetic fraction of the cumulative area in the particle size distribution at a temperature is equal to the doublet fraction in the Mossbauer spectra at that temperature, we are able to get a relation between r and T B, from which the temperature dependence of the effective magnetic anisotropy constant is determined. The resultant magnetic anisotropy constant increases markedly with decreasing temperature from 2.0 ×105 J/m 3 at 300 K to 8.3 ×106 J/m 3 at 125 K.

Metallic transport and large anomalous Hall effect at room temperature in ferrimagnetic Mn4N epitaxial thin film

We report the electrical transport properties of ferrimagnetic Mn4N (001) epitaxial thin films grown by pulsed laser deposition on MgO (001) substrates. The Mn4N thin films were tetragonally distorted with a ratio of out-of-plane to in-plane lattice constants of 0.987 and showed perpendicular magnetic anisotropy with an effective magnetic anisotropy constant of 0.16 MJ/m3, which is comparable with that of a recently reported molecular-beam-epitaxy-grown film. The thin films exhibited metallic transport with a room temperature resistivity of 125 μΩ cm in addition to a large anomalous Hall effect with a Hall angle tangent of 0.023.

Broadband electron spin resonance in a nanosized La0.25Ca0.75MnO3 manganite

The microwave response of a nanogranular La0.25Ca0.75MnO3 manganite has been studied by means of broadband electron spin resonance experiments performed in a commercial magnetic properties measurement system magnetometer using two purpose-built probes. The results concur with the hydrodynamic model for spin-glass systems and allow to determine the gyromagnetic ratio and the effective uniaxial magnetic anisotropy constant in a wide range of temperatures. The thermal behavior of both magnitudes provides information about structural transitions and magnetic interactions within the nanosized grains that make the sample. The experiments enable to corroborate the validity of the applied model in this kind of magnetic systems.