Temperature Dependence Of Coercivity Research Articles

Reduction in anti-ferromagnetic interactions in ion-beam deposited Fe3O4 thin films

Phase pure Fe3O4 thin films of thickness ∼42 nm have been prepared on the Si(100) substrate by reactive ion beam sputtering in the growth temperature range of 150–250 °C. A high degree of phase purity in the 175 °C sample has been confirmed by the XRD, Raman shift, and R-T measurements. The polycrystalline films show a sharp Verway transition as supported by temperature dependent resistivity, AC susceptibility, and coercivity behavior. The significant feature of these films is the early saturation of their room temperature magnetization at ∼400 mT, indicating the presence of low anti-ferromagnetic competitions in sharp contrast to most of the previous reports. The noticeable reduction of anti-phase boundaries and its dependence on growth temperature has been correlated with the energetic ion-beam deposition process, and explained in terms of the of ionic vacancy migration approach of Eerenstein et al. [Phys. Rev. B 68, 014428 (2003)]. The electronic conduction of these films is governed by near-neighbor hopping above 240 K and Shklovskii-Efros variable range hopping below this transition temperature.

Superparamagnetism and spin-glass like state for the MnFe2O4 nano-particles synthesized by the thermal decomposition method

MnFe2O4 nano-particles with an average size of about 7nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (MS) and coercivity (HC) are determined. It is shown that above 20K the temperature-dependence of the MS and HC indicates the magnetic behaviors in the single-domain nano-particles, while below 20K, the change of the MS and HC indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization–temperature (M–T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M–T curves peaks can be observed below 160K, superparamagnetism does not appear until the temperature goes above 300K, which is related with the strong inter-particle interaction.

Magnetic characteristics of a new cubic defect spinel Li0.5Mg0.5MnO3

Magnetic properties of Li0.5Mg0.5MnO3−δ nanoparticles (size ≃ 20 nm) synthesized by the Pechini method are investigated using temperature dependence of its magnetization (M) and electron magnetic resonance (EMR) spectra at 9.286 GHz. Analysis of the x-ray diffraction spectra yields its structure to be a cubic defect spinel with the formula 4(Li0.5Mg0.5MnO2.75) = 3{[Li2/3Mg1/3][Mn4/3Mg1/3□1/3]O11/3]} so that Mn occupies the octahedral B-sites only. The data of M versus T yields a blocking temperature TB ≃ 9 K above which the Curie–Weiss law variation with θ = 13 K and μ = 3.96μB characteristic of Mn4+ ions is established. For T < 9 K, temperature dependent coercivity and remanence are observed. The observed temperature dependence of the EMR parameters (linewidth ΔH, resonance field Hr, and intensity Io) for T < 30 K is interpreted in terms of TB (EMR) ≃ 30 K. Formation of ferromagnetic Mn4+ clusters, resulting from the co-presence of non-magnetic Mg2+ and vacancies on the B-sites, is inferred.

Magnetization Reversal and Anisotropy with Chains of Superparamagnetic Ni Nanoparticles

Chains of superparamagnetic (SPM) Ni nanoparticles of 20 and 33 nm in diameter are synthesized by a facile, template-free, wet chemical method in magnetic field. The blocking temperature for the sample of 20 nm appears at T(B) approximately 255 K, above which the sample exhibits SPM behaviors by a M(H) measurement. At T < T(B), the temperature dependent coercivity, Hc(T), is beyond the description by the simple two-well model proposed by Neel and Brown in the early day. In contrast, for the Ni nanochains of diameter larger than the coherence length, Lcoh approximately 25 nm, the magnetization reversal properties are well described by this model. The magnetization reversal behavior of the SPM Ni nanochains with the diameter much smaller than the coherence length is one of the interesting points to study. Possible transition from the localized magnetization reversal mode in the high temperature end to the coherently rotational mode at low temperature is discussed.

Microstructure optimization to achieve high coercivity in anisotropic Nd–Fe–B thin films

A high coercivity of 2.35MAm−1 (29.5kOe) is demonstrated using perpendicular anisotropy Nd–Fe–B films. Single layer columnar grown Nd–Fe–B thin films with a coercivity comparable with that of sintered magnets (1.11MAm−1, 14.0kOe) were used as the starting film. The coercivity was doubled by annealing films capped with Nd, Nd/Cu, and Nd/Ag layers. Transmission electron microscopy indicated that the Nd2Fe14B columnar grains were isolated by a Nd-rich grain boundary phase formed by diffusion of the capped layers into the grain boundaries of the polycrystalline Nd–Fe–B films. The temperature dependence of coercivity was lower than that of commercial sintered magnets. The coercivity of the Nd/Cu diffusion processed sample at 473K was 0.979MAm−1 (12.3kOe), which is higher than that of Nd10Dy4Fe80B6 sintered magnets.

Synthesis and magnetic properties of nickel nanoparticles deposited on the silicon nanowires

Nickel (Ni) nanoparticles with sizes of ∼35 nm were deposited on the surface of silicon nanowires (SiNWs) by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking temperature ( T B ) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC) plots. The M–H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value was ∼4.5 emu/g and the coercivity was ∼375.3 Oe for the loop at 5 K, respectively. While for the loop at 400 K, these values were of ∼2.6 emu/g and ∼33.3 Oe, respectively. The temperature dependence of coercivity followed by the relation H C ( T) = H C0 [1 − ( T/ T B ) 1/2], indicating a superparamagnetic behavior. The magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin function at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic defects on the surface of SiNWs.

Preparation and characterization of Ba2Co2Fe12O22 ferrite via glucose sol–gel method

Ba2Co2Fe12O22 (Co2Y) was synthesized by sol–gel method using glucose as chelating agent. X-Ray diffraction studies indicate that sintering temperature as low as 950 °C is sufficient to produce Co2Y ferrites. Co2Y ferrites calcined at 1,000 °C exhibit good magnetic prosperities in high frequency, with the resonance frequency up to 11 GHz and intrinsic permeability about 5 even at 6 GHz. The heat-treated temperature dependence of coercivity, initial permeability and resonance frequency is close related to the particle shape and size.

TEMPERATURE DEPENDENT COERCIVITY AND RELAXATION PHENOMENA IN AMORPHOUSFe–(Mn)–Zr–BNANOPARTICLES

We report the temperature dependent coercivity behavior and the improvement of room temperature soft magnetic properties in interacting amorphous Fe–Mn–Zr–B powder nanoparticles prepared by mechanical alloying of elemental powders. With increasing Mn concentration from 5 to 20 at.% in Fe–Zr–B alloys, the coercivity (HC) reduces drastically from 170 to 25 Oe. Temperature dependent HCbehavior shows exponential variation of HCwith temperature. Interparticle interactions dominate the low temperature behavior and surface effect on the nanoparticles is more pronounced at low temperatures. Magnetic relaxation behavior shows slow relaxation in the interacting amorphous nanoparticles and the relaxation process is observed to be thermally activated in the present studies.

Temperature dependent coercivity crossover in pseudo-spin-valve magnetic tunnel junctions with perpendicular anisotropy

We report the temperature dependent collapse of tunnel magnetoresistance (TMR) in perpendicular anisotropy magnetic tunnel junctions (pMTJs) with AlOx barriers and (Co/Pt)3 multilayer electrodes, due to the coercivity crossover of the top and bottom (Co/Pt)3 stacks. The different temperature dependence of two (Co/Pt)3 stacks in pMTJs is mainly caused by the additional perpendicular anisotropy created at interface between the ferromagnetic electrode and the AlOx barrier.

High temperature magnetic properties of magnesium ferrite nanoparticles

Magnetic properties such as Curie temperature (TC), saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) of nanoparticles of magnesium ferrites (MgFe2O4) were studied in a broad range of temperatures varying from room temperature to 800 K. The magnetization decreases with increasing temperature, approaching 0 at ∼ 750 K. The Curie temperature, determined by means of the inverse susceptibility versus temperature, was ∼738 K. The saturation magnetization, coercivity, and remanence decreased with increasing temperature, being close to 0 at temperatures near TC. However, for temperatures 100 K above room temperature, these magnetic properties were still the same as those at room temperature. The coercivity temperature dependence could be expressed in terms of T3/4, indicating that MgFe2O4 nanoparticles may form a system of random and noninteracting identical particles. The results are discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution, and other imperfections that exert fields on Mg2+ ions that could increase with temperature.

The effect of nano sized SrFe12O19 additions on the magnetic properties of chromium-doped strontium-hexaferrite ceramics

Strontium hexaferrite (SrFe12O19) which crystallizes in the hexagonal system and has a uniaxial magnetoplumbite structure, displays distinctive magnetic characteristics, good chemical stability, good tribological properties and a weak temperature dependent coercivity at about room temperature. In the present work the synthesis conditions for the solid-state preparation of the chromium-doped hexaferrite SrCr0.3Fe11.7O19 were optimised and the effect on the magnetic properties of this compound of additions of nanosized SrFe12O19 was studied. The nanosized SrFe12O19 additive, synthesized by the citrate–nitrate reaction, was substituted in varying amounts for a commercial calcium silicate borate sintering additive mixture. A combination of 0.75 wt% of nano-sized SrFe12O19 with 0.75 wt% of the commercial additive increases the intrinsic coercivity, remanence magnetization and rectangularity ratio and results in superior magnetic properties than obtained with 1.5 wt% of nanosized SrFe12O19 or the commercial sintering additive alone.

The Spin-Reorientation Transition in a Nanocrystalline Nd-Ho-Fe-Co-B Alloy and its Influence on the Hysteresis Loops

The temperature dependence of coercivity (Hc) of a nanocrystalline (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2 alloy in a wide temperature range, including the spin reorientation (SR) transition interval, has been investigated. A considerable decrease in the Hc for the SR in the range 150–4.2 K was detected and interpreted as being an effect caused by the anisotropy change of the 2-14-1 phase nanograins, i.e., from the uniaxial to the easy-cone type.

Magnetoelastic Anisotropy Induced Effects on Field and Temperature Dependent Magnetization Reversal of Ni Nanowires and Nanotubes

Vertically aligned Ni nanowires and nanotubes have been electrodeposited in alumina templates at room temperature. The detailed study of angular dependent coercivity and squareness demonstrates that the magnetic easy axis of Ni nanowires is perpendicular to that of Ni nanotubes axis. The mechanisms of magnetization reversal in Ni nanowires and Ni nanotubes are found to occur through the nucleation mode with the propagation of transverse domain wall and curling mode, respectively. Field dependant magnetization results at different temperatures have depicted that the magnetocrystalline anisotropy might cause a crossover of easy axis at room temperature to that of low temperature in both Ni nanowires and nanotubes. Furthermore, the variation in temperature dependent coercivity illustrates that the magnetoelastic anisotropy induced by the alumina matrix plays a dominant role in the magnetization reversal of the nanowires and nanotubes at low temperature.

Magnetization reversal of two-dimensional superlattices of Mn3O4 nanocubes and their collective dipolar interaction effects

Magnetic properties of two-dimensional (2D) paddy-field like superlattices of Mn3O4 cubic nanoparticles have been investigated by magnetization measurements. The 2D ordered structure extends over several microns in size. Each nanocube is of single-crystal about 6 nm in size. The magnetic properties are investigated with the powders dispersed in nonmagnetic n-eicosane to “dilute” the dipolar interaction. By accounting for the temperature variation effect of the magnetocrystalline anisotropy, Kmag(T), the temperature dependent coercivity, HC(T), can be well described by the equation, HC(T)=H0kmag(T)/mS(T){1−[kBT ln(t/t0)/E0kmag(T)]3/4}, in which kmag(T)=Kmag(T)/Kmag(0) is the reduced temperature dependent magnetocrystalline anisotropy and mS(T)=MS(T)/MS(0) is the reduced saturation magnetization. The effects of collective dipolar interaction on the magnetic properties are also studied with the as-prepared powder sample. The apparent magnetic anisotropy is seriously reduced with the presence of dipolar interaction. The switching volume is determined by the analysis on the magnetic measurements both with and without the dipolar interaction effect. There is a discrepancy in the value of switching volume determined by the two different analysis methods. Possible reasons are discussed.

Perpendicular Magnetic Anisotropy in CoFeB/Pd Bilayers

Perpendicular magnetic anisotropy is observed in ultrathin (~ 0.6 nm) amorphous Co40Fe40B20 when sputtered on an MgO (001) buffer layer and capped with Pd. The layers are superparamagnetic with a blocking temperature of ~ 230 K, below which they show an exponential temperature dependence of coercivity. Perpendicular magnetic anisotropy is observed in the as-deposited state and the mechanism is different from that of CoFeB/Pt, which requires postannealing. These ultrathin layers could be a model system for studies of electric field effects on magnetic anisotropy.

Magnetization Process in Dilute Magnetic Oxides

Thin films of dilute magnetic oxides often exhibit high-temperature ferromagnetism with magnetization curves of the form M ¿ M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> tanh (H/H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ); the curves are practically anhysteretic and temperature-independent below room temperature. The absence of temperature-dependent coercivity indicates that the magnetization process is dominated by magnetic dipole interactions, not by magnetocrystalline anisotropy. In a model of ferromagnetic grain boundaries, H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ¿ 0.14M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , where M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the magnetization of the ferromagnetic regions. Quantitative analysis of more than 200 films of different oxides reveals that only a few percent of the volume of these films is magnetically ordered.

The role of dipolar interactions for the magnetic properties of ferromagnetic nanoring

We investigate numerically the effects of the dipolar interactions on magnetic properties in small ferromagnetic nanorings using a Monte Carlo technique. Our simulated results show that the strength of dipolar interaction in the magnetic nanoring has an important influence on the magnetization reversal processes and further the coercivity and the remanence. As the dipolar interaction increases, the transition of magnetization reversal processes from the onion-rotation state to the vortex state can occur, which results in an increase in coercivity and a decrease in remanence. On the other hand, it is found that the coercivity and the remanence depend more strongly on the strength of dipolar coupling for the relatively small size nanoring than for the large size nanoring in width. This can be attributed to the stable vortex state without core in smaller width nanoring in contrast to the metastable vortex state with core in larger width nanoring, induced by strong dipolar interactions. Additionally, the temperature dependence of coercivity and remanence in magnetic nanoring is also studied at a fixed dipolar interaction.

The magnetization behavior and magnetic viscosity of Sm(Co,Fe,Cu,Zr)Z ribbons with different temperature dependence of coercivity

Two types of Sm(Co,Fe,Cu,Zr)Z ribbons with different temperature dependence of coercivity are investigated in comparison at different temperatures. It is found that their magnetization behaviors and magnetic viscosity are distinctly different. The magnetization of sample A (with abnormal temperature dependence of coercivity) behaves as a single phase permanent magnet at room temperature, and then becomes similar to a nanocomposite magnet with the increase of temperature. However, sample B (with negative temperature coefficient of coercivity) is similar to a nanocomposite magnet at the whole temperature range. The magnetic viscosity is mainly determined by the irreversible magnetization for both ribbons, while there emerges an extra small peak of magnetic viscosity coefficient S(H) at low field and high temperature for sample B. The different content and distribution of Cu in the cell boundary phase are proposed to be responsible for the differences of temperature dependence of coercivity, magnetization, and magnetic viscosity behaviors of these two types of Sm(Co,Fe,Cu,Zr)Z ribbons.

Role of anisotropy and interactions in magnetic nanoparticle systems

Magnetic nanoparticle systems are characterized by several competing effects like anisotropy, an inherent disorder, the long range dipolar and the short range exchange interactions due to clustering effects. The sensitivity of the observed static and dynamical properties of these systems like the blocking temperature, the hysteresis and the susceptibility, to the methods of preparation, annealing and the resulting morphology is a manifestation of this. However, given the complexity of the system, it is often difficult to isolate the effects which might be dominant in a particular sample, which has a direct bearing on the desired applications. In this paper we report the effects of anisotropy, interactions and particle concentration on the temperature dependent remanence and coercivity through a numerical simulation on an array of single domain magnetic particles which incorporates all the above mentioned factors. Our results show that it is possible to distinguish between purely anisotropic systems and interacting systems with these measurements. In confirmation of our simulation results we also present the experimental results on the remanence and coercivity of nanomagnetic nickel ferrite composites.

Temperature dependent coercivity and magnetization of nickel ferrite nanoparticles

Magnetic nanoparticles of nickel ferrite (size: 24±4 nm) have been synthesized by chemical coprecipitation method using stable ferric and nickel salts. Coercivity of nanoparticles has been found to increase with decrease in temperature of the samples. It has been observed that the coercivity follows a simple model of thermal activation of particle’s moment over the anisotropy barrier in the temperature range (10–300 K), in accordance with Kneller’s law for ferromagnetic materials. Saturation magnetization follows the modified Bloch’s law in the temperature range from 300 to 50 K. However, below 50 K, an abrupt increase in magnetization of nanoparticles was observed. This increase in magnetization at lower temperatures was explained with reference to the presence of freezed surface-spins and some paramagnetic impurities at the shell of nanoparticles that are activated at lower temperatures in core-shell nickel ferrite nanoparticles.