Magnetization and electron spin resonance of Fe impurities in (TMTSF)2AsF6: magnetic-field-dependent interaction between impurity spins and spin-density waves

A new spin-density-wave (SDW) system with magnetic impurities (TMTSF)₂(AsF₆)_1−x(FeCl₄)_x was prepared and its magnetic properties were studied by means of magnetization and electron-spin-resonance measurements. The anisotropic g-value and comparison of the Fe concentration with the Curie constant indicate that the Fe³⁺ ions are in a low-spin state. We also found that the magnetization curve of the impurity spins in this compound shows an anomalous behavior. This behavior can be explained if one assumes a field-dependent magnetic interaction between the Fe³⁺ spins and the SDW moment. We suppose that the field dependence of the SDW pinning potential is responsible for this phenomenon.     

Electric and magnetic properties of the perovskites Ba2(FeMo)O x (5.88 ≤ x 6.01)

The purpose of this work is to establish the relation between the magnetic, electric, and magnetotransport properties and the oxygen nonstoichiometry of the compounds Ba₂(FeMo)O _x (5.88 ≤ x ≤ 6.01). The investigations established the behavior of the magnetization, resistance, and magnetoresistance of samples in this series. It is shown that the behavior of the magnetization can be described by assuming that the iron ions become divalent (Fe³⁺→ Fe²⁺) as a result of the reduction of the samples and the molybdenum ions become hexavalent (Mo⁵⁺ → Mo₆₊) as a result the oxidation of the samples. It is established that there are two contributions to the magnetoresistance which arise as result of magnetic ordering of the intragranular interlayer and intergranular transfer of spin-polarized charge carriers. It is inferred that electric transport in samples of this series is determined by percolation processes between granules with metallic conductivity separated by a dielectric interlayers.     

Magnetization studies in niobium and YBa2Cu3O7 samples

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H _max to +H _max) and reverse (+H _max to −H _max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH _peak,H _I,H _II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M _rem(H)−M _FC(H)+M _ZFC(H)) being equal to, less than or greater than zero, whereM _rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.     

Anomalous temperature dependence of the 1.7T magnetic hyperfine field of fluorine in nickel

By means of time-differential perturbed angular distribution (TDPAD) techniques, the magnetic hyperfine field ofF atoms implanted in a ferromagnetic Ni lattice has been measured at temperatures from 80 K to 650 K obtaining, for example, a fieldH ₁(295 K)=+1.72(3)T at room temperature. The temperature dependence of the reduced fieldH ₁(T)/H ₁(0) has been found to deviate up to 38% from that of the reduced magnetization of the Ni host. Comparing this result for the halogenF with other nontransitional impurities in nickel, it seems that such deviations become more pronounced with increasing number of excessp-electrons of the impurity. With a simple two-zone model distinguishing between the hyperfine fields originating in the local and in the more distant undisturbed surrounding of theF impurity, the observed temperature dependence can be explained. In this approach the disturbed magnetization of the inner zone is described by localized Ni moments in the molecular field approximation.A satellite magnetic field of + 9.4T was observed at 80 K only.Supported by the Bundesministerium für Forschung und Technologie     

Magnetic properties of weakly coupled superconducting grains

We investigate the thermodynamic properties of weakly coupled superconducting grains in a magnetic field. Starting from the mean field approximation, we treat the induced field and the nonlinearities self-consistently. Close to but slightly belowT _c (H), the order parameter, the magnetization and the variance of the induced-field distribution. Recent experiments on high-T _c materials are consistent with these predictions.     

CoFeB/MgO不连续多层纳米软磁薄膜微波电磁特性

采用直流/射频磁控溅射方法和不连续多层交替沉积工艺,制备了CoFeB/MgO系列纳米不连续多层薄膜,研究了微波频段下的电磁性能.结果表明,通过调整CoFeB合金相和MgO介质相的相对含量可有效调控薄膜的微结构和电磁性能,且在［Co₆₄Fe₂₄B₁₂(0.7nm)/MgO(0.4nm)］₄₀薄膜中获得了优良的微波软磁性能和高电阻率,其饱和磁化强度1.3 T,难轴矫顽力130 A/m,电阻率3.4mΩ·cm,且共振频率高达2.1 GHz,磁导率实部μ′和虚部μ″在1.59 GHz处均高于240,并且在0.9—2 GHz宽频带范围同时大于100.该薄膜可用于微波吸收材料和电磁兼容的设计中. 关键词： 纳米磁性颗粒膜 磁导率频谱 电磁性能 吸波材料     

Magnetic properties of substitutional solid solutions of nickel and iron hexacyanoferrate–hexacyanochromate

In order to evaluate the influence of substitution at the central metal ion position in transition hexacyanometallates in some detail, the magnetic studies were carried out on a series of solid solutions of metal hexacyanometallates of the composition M[Fe^II(CN)₆]_{1? x} [Cr^III(CN)₆] _x , where M?=?Ni^II and Fe^III. The temperature and field dependences of magnetization were studied using a superconducting quantum interference device magnetometer. The field dependence of the samples at 5?K shows a hysteresis behaviour. For M?=?Ni^II, the transition temperature increases with increase in the substitution of low-spin Fe(III) by Cr(III) in the hexacyanometallate unit. The saturation magnetization was found to decrease with increase in the iron concentration. The observed variation in the magnetic properties, such as the value of T _C and the nature of magnetic ordering, is attributed to the variation in the composition of the transition-metal ion in the coordination sphere of carbon. On the other hand, for M?=?Fe^III, the transition temperature and saturation magnetization remained almost unchanged, indicating that substitution at the carbon coordination site did not produce any change in the magnetic interaction among the transition-metal ions through the cyanide ions.     

Kalibrieren von dauermagnet-messgeräten mit nickel

Pure Ni is proposed as a standard for magnetic flux and field intensity measuring instruments. The calibration of a B(H)-hysteresisgraph for permanent magnets is given, using published values of saturation magnetization of Ni, including the influence of impurities and temperature.     

脉冲磁场下水热法制备Cr掺杂ZnO稀磁 半导体晶体

本文以ZnCl₂, CrCl₃. 6H₂O和氨水缓冲溶液为原料, 在4T脉冲磁场下水热法制备了Cr掺杂ZnO稀磁半导体晶体, 通过X射线衍射分析、扫描电子显微镜观察及采用振动样品磁强计进行磁性分析等, 探讨了脉冲磁场对其微观结构及磁性能的影响. 结果表明: Cr掺杂ZnO稀磁半导体晶体仍保持ZnO的六方纤锌矿结构, 脉冲磁场具有促进晶粒生长及取向排列的作用, 4T脉冲磁场条件下合成的Cr掺杂ZnO稀磁半导体具有良好的室温铁磁性, 其饱和磁化强度(M_s)为0.068 emu/g, 而无脉冲磁场情况下制备的样品室温下呈顺磁性, 并且, 脉冲磁场下制备将稀磁半导体的居里温度提高了16 K.     

Influence of magnetic impurities on superconducting property of polycrystalline iron-based NdFeAsO0.88F0.12

Polycrystalline iron-based superconducting NdFeAsO_0.88F_0.12 was prepared via solid-state reaction in ambient pressure. Small amounts of ferromagnetic impurities were detected by Scanning Electron Microscopy (SEM) analysis. To study the influence of magnetism on superconducting properties of the sample, magnetization M(T, H) measurements were performed in fields up to 1.5 T and in the temperature range of 5-60 K. The abnormal behavior was observed in the χ(T) curves, and the magnetization hysteresis loops (MHLs) can be described by a sum of the contributions of superconductive hysteresis, ferromagnetic hysteresis of the impurities and the paramagnetic background of the isolated Nd³⁺ ions. The influence of the ferromagnetic impurities and the paramagnetic background were investigated and discussed. After the corrections of the magnetism contributions, the intrinsic superconductive MHLs as well as the critical current density were obtained.     

Low-temperature behavior of specific heat curves in ruthenocuprate RuSr<Subscript>2</Subscript>Gd<Subscript>1.5</Subscript>Ge<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10 − δ</Subscript>: Superconducting,magnetic and crossing-point effects

Magnetic and superconducting properties of polycrystalline samples of RuSr₂Gd_1.5Ce_0.5Cu₂O_{(10 − δ)}, asprepared (by solid-state reaction) and annealed in pure oxygen at different pressure are presented. Specific heat and magnetization were investigated in the temperature range 1.8–300 K with a magnetic field up to 8 T. Specific heat, C (T), shows a jump at the superconducting transition (with onset at T ≈ 37.5 K) and a Schottky-type anomaly below 20 K. It is found that curves C(T) taken for different values of magnetic field have the same crossing point (at T _* ≈ 2.7 K) for all samples studied. At the same time, C(H) curves taken for different temperatures have a crossing point at a characteristic field H _* ≈ 3.7 T. These effects are manifestations of the crossing-point phenomenon, which is supposed to be inherent for strongly correlated electron systems.     

Magnetization reversal in Co x (PZT)100 − x ferromagnet-ferroelectric nanogranulated composites

The process of magnetization reversal in Co _x (PZT)_{100 − x} composites is studied experimentally. At room temperature, the ferromagnetic state in as-prepared samples is found to arise only if the composite contains more than 60 at % of the metal phase. The concentration dependences of the coercive force and remanent magnetization derived from magnetic hysteresis loops are discussed in terms of the random anisotropy model.     

Magnetization reversal measurements in mesoscopic amorphous magnets by magneto-optical Kerr effect

A magneto-optical setup based on the transverse Kerr effect has been designed to study the magnetization reversal processes by vector magnetometry in arrays of magnetic nanostructures with a reduced total volume. This system allows the measurement of both the parallel and perpendicular to the field components of the magnetization. It has been used to analyze the behavior of amorphous Co _x Si_1-x lines fabricated by electron beam lithography that present a very well defined shape induced uniaxial anisotropy. When the field is applied near to the hard direction, coherent rotation processes are found to occur with a collapse of this reversal mode at fields very close to the hard axis that allows to estimate the very low anisotropy dispersion of these samples. The analysis of the vector hysteresis loops reveals that the magnetization switches via an incoherent process that starts prior to the Stoner-Wohlfarth instability and that can be described in terms of a localized curling-like reversal mode.Received: 16 June 2004, Published online: 24 September 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.60.Jk Magnetization reversal mechanisms - 75.50.Kj Amorphous and quasicrystalline magnetic materials     

Anomalous magnetic properties of ferrites of the CuFe2−x CrxO4 system with high Cr3+ content

The paper reports on a comprehensive study of magnetic properties of CuFe_2−x Cr_xO₄ ferrites (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0). The curves of magnetization versus temperature, σ _s(T), of the ferrites with x=1.0, 1.4, and 1.6 have anomalous shapes: the magnetization begins to fall off at lower temperatures than the Curie point TC. The experimental results and analysis of exchange interactions suggest that in ferrites with high Cr³⁺ content, magnetic transitions to either a frustrated magnetic structure or a clustered spin glass can take place. Zh. éksp. Teor. Fiz. 111, 1732–1737 (May 1997)     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

M(H) shape reconstruction using magnetic spectroscopy

Knowledge about the magnetization behavior M(H) is crucial for the use of magnetic materials in engineering applications. To date many systems exist that are able to measure the magnetization behavior, e.g. VSM, VCM, MOKE. In addition to their huge costs, complex and space-consuming measurement setup, large amount of preparatory work and restricted surface measurements are handicaps which restrict their field of application. Furthermore, the influence of additional physical quantities such as temperature, strain or pressure can only be investigated with great efforts. These influences are, however, of major importance in the development of magnetic sensor systems that are based on the change in magnetic properties.In this paper, a new measurement principle based on a frequency mixing technique is introduced for investigation of the shape of the magnetization curve of soft non-hysteretic magnetic materials. Based on the Taylor expansion of the magnetization curve and the spectral investigation of the inductively detected signal, a mathematical model for the reconstruction of M(H) is proposed. The model is both numerically and experimentally verified. It is shown that the magnetization curve of a nanocrystalline soft magnetic material used in this study can be reconstructed very accurately and the influence of an additional parameter, i.e. strain, can be investigated in detail as well.     

Magnetoresistance effect in A 2(FeMo)Ox double perovskites (A = Sr, Ca; 5.90≤x≤6.05)

The dependences of magnetic, electric, and magnetotransport properties on oxygen non stoichiometry were investigated in compounds of Ca₂(FeMo)O_x and Sr₂(FeMo)O_x (5.90≤x≤6.05). The regular trends in behavior of the magnetization, resistance, and magnetoresistance of samples of these series are determined. It is established that the magnetoresistance is composed of two parts that appear as a result of magnetic ordering in grain-boundary layers and of the intergrain transport of spin-polarized charge carriers. The electronic transport in the samples is assumed to be governed by percolation processes between grains which have a metallic type of conductivity and are separated by insulating spacers.     

Half-metallic behaviour in doped TiO2 (rutile) with double impurities: ab initio calculation

Dilute magnetic oxides are without doubt among the most interesting classes of magnetic materials. However, the nature of their electronic structure and magnetic exchange is far from understood. Here, we apply the ab initio augmented spherical wave (ASW) method, with corrected generalised gradient approximation to study the electronic structure and magnetic properties of doped TiO₂ rutile with double impurities. The study reveals a half-metallic ferromagnetic behaviour for Ti_1?2x Cr _x Mo _x O₂, and the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Mo, which would lead to the ferromagnetic state through the double-exchange mechanism in transition metal oxides.     

Crystal field effects in RERh4B4 compounds (RE = rare earth)

By analysis of a large body of available data, crystal field parameters have been obtained for compounds in the series RERh₄B₄ (RE = rare-earth). These parameters have then been used to calculate properties related to the electronic and magnetic properties of such compounds, and it is shown that a substantial unification of current information is obtained for properties including bulk magnetization (polycrystalline and single crystal), magnetic susceptibility, magnetic anisotropy, Schottky anomalies in the specific heat and suppression of the superconducting transition temperature due to magnetic impurities. In agreement with previous discussions crystal field effects are found to clarify the systematics of magnetic transition temperature, but not to give a full explanation. Other phenomena, such as discrepancies in the magnetic moment of ErRh₄B₄ as measured by neutron diffraction and by Mo¨ssbauer spectroscopy cannot be explained as crystal field related phenomena.