Temperature effect on the magnetic properties of the coprecipitation derived ferrimagnetic glass-ceramics

Ferrimagnetic glass-ceramic materials are promising candidates for destruction of cancerous cells by hysteresis loss, as a consequence of the increase of the local temperature when an alternating magnetic field is applied. The glass-ceramics prepared in this work contain magnetite and hematite crystals in a glassy matrix. They can be prepared by melting the coprecipitation-derived precursors at temperatures higher than 1400°C. The influence of the melting temperature on the magnetic properties of these glass-ceramics has been analysed. Room temperature hysteresis cycles in quasi-static conditions were performed using two different magnetic field, one of 12 kOe, and the other one of 500 Oe. The heat generation was estimated from calorimetric measurements, using a magnetic induction furnace. The magnetic hysteresis parameters are strongly affected by the glass-ceramics microstructure, which is influenced by the melting temperature.     

Ultrasound effects on metallic (Fe and Cr); iron sesquioxides (alpha-, gamma-Fe2O3); calcite; copper, lead and manganese oxides as powders

Different kind of materials, as powders, were submitted to ultrasound after mixing in water, dodecane or dilute acetic acid (5%). After treatment, firstly a decrease of particle size, and secondly the formation of unexpected and unknown compounds was observed on a mesoscopic scale (CaO from CaCO3 for example). For iron and iron oxides, it was found that the magnetic properties (susceptibility and effective magnetic moment) were changed slightly. A shift in the Morin temperature transition for alpha-Fe2O3 was attributed to the formation of impurities. For inorganic oxides (Cu2O, PbO, Pb3O4 and Mn3O4) sonicated in dilute acetic acid, the formation of metallic acetates is easily achieved and it is rapid (10 s for PbO).     

Characterization,electrical and magnetic properties of PVA films filled with FeCl3–MnCl2 mixed fillers

Polyvinyl alcohol (PVA) films filled with different filling levels (FLs) of XFeCL₃(15−X)MnCl₂ were studied. The DSC thermograms exhibited an increase in the melting temperature with filling, indicating better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was depicted using XRD scans. Vibrational studies displayed significant structural deformations. The FL dependence of certain IR absorption peaks was discussed. The dc electrical conduction mechanism was interpreted on the basis of the modified interpolaron hopping model. The present results of the dc magnetic susceptibility (χ) suggested the temperature dependence of Curie–Weiss behavior characterized by localized magnetic moments. The effective paramagnetic moment (μ_eff) was estimated; its dependence on the FL exhibited a non-linear character. The electron spin resonance (ESR) study revealed unresolved broad distorted signals characterized by the hyperfine structure. The ESR parameters were evaluated. A correlation between the above-mentioned studies was discussed to relate the structural, electrical and magnetic properties of the filled PVA polymer.     

Effect of thermal fluctuations on magnetic phase separation and on the parameters of spin-spiral waves

Effects of temperature on magnetic phase separation and on the parameters of spin-spiral waves are studied. The study is performed using the two-dimensional single-band Hubbard model and the Hubbard-Stratonovich transformation. Both commensurate (antiferromagnetic, ferromagnetic) and incommensurate (spiral) magnetic phases are considered. The problem is solved using the static approximation with allowance for transverse fluctuations of the magnetic moment. It is shown that the temperature significantly affects the collinear and spiral magnetic phases. With an increase in the temperature, the phase-separation region near the half-filling is sufficiently reduced and substituted by the antiferromagnetic phase. The results are used for the interpretation of the magnetic properties of cuprates.     

Size effects in parameters of both interatomic exchange interactions in Fe–Pt alloy nanoparticles and their superparamagnetism

The objective of this study is to calculate the parameters of strong exchange interactions within magnetic nanoparticles and weak (dipolar + anisotropic) interactions for the patterns of nanoparticles injected into non-magnetic substrate. The Fe–Pt magnetic system was chosen as the best applicable for this purpose. However, computations done in this work may be extended to the other f.c.c. magnetic systems. In this paper, we estimated the parameters of exchange interactions within the magnetic Fe–Pt nanoparticles close to equiatomic composition with 4–10 nm in diameter. Size effect for exchange interaction parameters was found. Temperature dependences of spontaneous magnetizations for Fe and Pt subsystems of nanoparticles with different sizes at fixed equiatomic composition were obtained. Total magnetic energies of weak interactions between Fe and Pt nanoparticles injected different matrixes were also estimated. Magnetic moment ordering temperature was evaluated within the simple model for ordered and disordered Fe–Pt nanoparticles of various sizes (4–10 nm) separated by different interparticle distances (30–50 nm).     

超薄Fe(4(A))膜磁特性极化中子反射研究

利用分子束外延薄膜生长技术,制备了200 (A)V/4 (A) Fe/900 V/MgO(100)薄膜样品,通过X射线反射和极化中子反射两种测量手段获得了薄膜的表面、界面及各层膜厚的相关结构信息.中子反射结果表明,Fe原子磁矩在室温下约为1.0±0.1μB,随着温度的降低,Fe原子磁矩增加,在10 K时达到1.5±0.1μB.利用指数定律拟合磁矩随温度的变化情况,外推得出4(A)铁薄膜样品的居里温度约为310±30 K.

Abstract：

Uhrathin Fe film 200 (A)V/4 (A)Fe/900 (A)V/MgO(100) has been prepared by molecular beam epitaxy (MBE). The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity measurement. The magnetic properties of the thin Fe layer were investigated by polarized neutron reflectometry at different temperature. The result shows that the magnetic moment of an Fe atom is about 1.0 ± 0.1 μB at room temperature and increases to 1.5 ±0.1 μBat 10 K. The Curie temperature of the thin Fe film is estimated to be 310 ± 30 K.     

Spectrum of spin waves in cold polarized gases

The spin dynamics of cold polarized gases are investigated using the Boltzmann equation. The dispersion relation for spin waves (transverse component of the magnetic moment) and the spin diffusion coefficient of the longitudinal component of the magnetic moment are calculated without using fitting parameters. The spin wave frequency and the diffusion coefficient for rubidium atoms are estimated numerically.     

Influence of the inhomogeneity of local magnetic parameters on the curves of magnetization in an ensemble of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

Methods have been proposed and tested for analyzing local magnetic parameters in a system of single-domain ferromagnetic nanoparticles using their magnetization curves. The magnetic inhomogeneity in ensembles of Fe₃C nanoparticles encapsulated in carbon nanotubes has been investigated. It has been established that the Fe₃C nanoparticles encapsulated in carbon nanotubes are characterized by two-modal distribution functions of the local magnetic anisotropy energy. The particle distribution over the blocking temperature is reconstructed from the experimental temperature dependence of the coercive force. The allowance made for the inhomogeneity of the local magnetic parameters of the Fe₃C nanoparticles, which were studied by the proposed methods, explains the discrepancy between the magnetic anisotropy energy determined by the method of the magnetization approaching saturation and the magnetic anisotropy energy estimated from the coercive force of single-domain nanoparticles.     

超薄Fe(4?)膜磁特性极化中子反射研究

Ultrathin Fe film 200  V/4  Fe/900  V/MgO（100） has been prepared by molecular beam epitaxy （MBE）. The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity mea 关键词： 超薄Fe膜 磁特性 极化中子反射 分子束外延     

Fe-Cr-Co-Si永磁合金的磁性和穆斯堡尔效应研究

本文研究了成分为(重量百分比)65Fe-22Cr-12Co-1Si的可加工永磁合金。对高温淬火样品和经过磁场热处理及时效的样品,研究了饱和磁矩与温度的关系,以及室温和约80K的穆斯堡尔谱,计算了谱参数。实验表明:两种样品的磁性都随测量中热循环而改变。时效样品在室温除出现铁磁性相外,还出现顺磁性相;顺磁性相在低温下转变为铁磁性相。在时效样品中发现了磁矩的不完全的取向效应。最后讨论了这一材料的反磁化机制和提高矫顽力的途径。 关键词：     

Structural and magnetic properties of Co2CrAl Heusler alloys prepared by mechanical alloying

Mechanical alloying has been used to produce nanocrystalline samples of Co₂CrAl Heusler alloys. The samples were characterized by using different methods. The results indicate that, it is possible to produce L2₁-Co₂CrAl powders after 15 h of ball-milling. The grain size of 15 h ball milled L2₁-Co₂CrAl Heusler phase, calculated by analyzing the XRD peak broadening using Williamson and Hall approach was 14 nm. The estimated magnetic moment per formula unit is ∼2 μ_B. The obtained magnetic moment is significantly smaller than the theoretical value of 2.96 μ_B for L2₁ structure. It seems that an atomic disorder from the crystalline L2₁-type ordered state and two-phase separation depresses the ferromagnetic ordering in alloy. Also, the effect of annealing on the structural and magnetic properties of ball milled powders was investigated. Two structures were identified for annealed sample, namely L2₁ and B2. The obtained value for magnetic moment of annealed sample is smaller than the as-milled sample due to the presence of disordered B2 phase and improvement of phase separation.     

Heating in the MRI environment due to superparamagnetic fluid suspensions in a rotating magnetic field

In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4 and 7 degree Centigrade above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B₀. Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B₀. Results are presented for the expected temperature increase in small tumors (approximately 1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002-0.01 solid volume fraction) and nanoparticle radii (1-10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful selection of the rotating or sinusoidal field parameters (field frequency and amplitude). The work indicates that it may be feasible to combine low-field MRI with a magnetic hyperthermia system using superparamagnetic iron oxide nanoparticles.     

Effect of magnetic annealing on magnetization of BiFeO3 ceramics

The effect of magnetic annealing treatment on the magnetization of multiferroic BiFeO₃ was studied systematically. A series of pelletized nano-sized BiFeO₃ powders were annealed at high temperature under different magnetic fields. Typical ferromagnetic hysteresis loops were obtained at room temperature of the ceramics which were derived from ferromagnetic BiFeO₃ precursors. On the other hand, antiferromagnetic behaviors were observed in other samples synthesized from nonmagnetic precursors. The enhanced magnetic properties were ascribed to the magnetic anisotropy which was induced by the strong magnetic fields. This work indicates that the strong magnetic annealing method is an alternative approach to tuning the magnetic properties of high performance multiferroic materials with canted antiferromagnetic ordering.     

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.     

Mössbauer spectroscopy of goethite of small particle size

A series of synthetic acicular and natural goethite samples was examined by Mössbauer spectroscopy in the temperature range 5 to 290 K. From the temperature dependence of magnetic moment fluctuations the anisotropy constant has been estimated and the results have been compared with results of the magnetic measurements.     

On the interpretation of magnetization data for antiferromagnetic nanoparticles

We have investigated the influence of anisotropy on the magnetization curves of antiferromagnetic nanoparticles. We show that if such curves are analyzed in a conventional way, i.e. using a Langevin function in combination with a linear term, this usually results in good quality fits, but with an apparent temperature dependence of parameters such as the magnetic moment per particle and the antiferromagnetic susceptibility. In order to avoid the problems associated with anisotropy as well as volume/moment distributions we propose that the initial susceptibility is used when analyzing the temperature dependence of the magnetic moment.     

Magnetic nanoscale aggregates of cobalt and nickel in MgO single crystals

The magnetic properties of Co and Ni nanosized aggregates formed after implantation of nickel and cobalt ions in magnesium oxide single crystals were investigated. The influence of the implantation energy and annealing treatments was characterized. The particle size distribution was determined from the combined analysis of the magnetic moment dependence on both magnetic field and temperature, and used to determine the magnetic anisotropy constant of the aggregates. The results for nickel aggregates indicate the presence of an antiferromagnetic layer after the annealing treatments.     

Ni2MnGa单晶马氏体相变过程摩擦耗能的热动力学计算

根据相界面摩擦原理 ,在推导出计算Ni2 MnGa系统热动力学参量的一般表示式的基础上 ,结合马氏体相变温度分别在室温以下、室温附近、室温以上三种非正配分比Ni2 MnGa单晶自发相变应变和交流磁化率随温度变化的测量结果 ,计算了三种样品马氏体相变过程中界面摩擦所消耗的能量 .结果进一步表明正是相变过程中的界面摩擦导致了相变的热滞后 ,而三种样品马氏体相变过程的摩擦耗能和相变热滞后存在较大差别的原因在于三种样品马氏体相变生成物具有不同的结构     

The lithium- and sodium-enhanced transformation of Ba-exchanged zeolite LTA into celsian phase

The Li⁺- and Na⁺-doped hexacelsians (HC) synthesized from Ba-LTA synthetic zeolite as precursors were used for preparation of monoclinic celsians (MC). The doped pure MC and mixture HC/MC species were obtained by thermally induced polymorphous transformation at 1200 °C. Synthesized-doped MC have been characterized by X-ray powder diffraction and spectroscopic (infrared, Raman and ²⁹Si magic angle spinning nuclear magnetic resonance) methods. The obtained results suggest that in all investigated samples Na⁺ or Li⁺ dopants were incorporated in MC crystal structures during thermal transformation of a zeolite. It has been shown that HC→MC transformation depends on molar fractions of alkali cations. These findings combined with used temperature/time conditions could help in optimization of MC synthesis route.     

Measurement of the magnetic moment in a cold worked 304 stainless steel using HTS SQUID

The magnetic properties of stainless steel have been investigated using a radio frequency (RF) high-temperature superconductivity (HTS) SQUID (Superconducting QUantum Interference Device)-based susceptometer. The nuclear grade 304 stainless steel is nonmagnetic at a normal condition but it changes to a partially ferromagnetic state associated with martensitic transformation under a plastic deformation. The magnetic moment of the 304 stainless steels was increased with an increasing cold work rate, and decreased with an increasing annealing temperature. The change of mechanical properties such as yield strength and ultimate tensile strength (UTS) are also analyzed in terms of deformation-induced martensitic transformation.