Soft magnetization of a semi-hard/soft magnetic bilayer produced by oblique-incidence evaporation

The possibility of achieving soft magnetization in semi-hard magnetic films such as Fe, Fe_93.5Si_6.5, Fe₅₀Co₅₀ and Fe₇₀Co₃₀ is investigated by depositing films on an Fe₂₀Ni₈₀ underlayer by oblique-incidence evaporation. The magnetic anisotropy of the underlayer is strengthened to a depth of several lattice parameters by vapor deposition of the film at an oblique angle to the substrate surface. This method also allows magnetic anisotropy to be induced in strongly isotropic semi-hard magnetic overlayers to a thickness of a few thousands Angstroms. The coercive force of bilayer films measured along the hard-axis is reduced remarkably by this process, and the strength of the anisotropy field is demonstrated to be readily controllable. When magnetic anisotropy exists in both magnetic layers, a significant change is observed in the magnetization processes of the semi-hard magnetic layer and the coercive forces in the hard magnetization direction is dramatically reduced. Soft magnetization of the semi-hard magnetic layer cannot be achieved when magnetic anisotropy exists in only one of the magnetic layers.     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.

     

Magnetic behavior of nanocrystalline CoFe2O4

Magnetic nanoparticles of CoFe₂O₄ have been synthesized under an applied magnetic field through a co-precipitation method followed by thermal treatments at different temperatures, producing nanoparticles of varying size. The magnetic behavior of these nanoparticles was investigated. As-grown nanoparticles demonstrate superparamagnetism above the blocking temperature, which is dependent on the particle size. One of the nanoparticles demonstrated a constricted magnetic hysteresis loop with no or small coercivity and remanence at low magnetic field. However, the loop opens up at high magnetic field. This magnetic behavior is attributed to the preferred Co ions and vacancies arrangements when the CoFe₂O₄ nanoparticles were synthesized under an applied magnetic field. Furthermore, this magnetic property is strongly dependent on the high temperature heat treatments that produce Co ions and vacancies disorder.     

Mössbauer study of thermal magnetic transitions in Cu0.5Zn0.5Fe2O4

The dependence of the Mössbauer spectra of the ferrite Cu_0.5Zn_0.5Fe₂O₄ on the temperature is investigated. The profiles of the hyperfine magnetic fields distributionP(H) are obtained from the spectra by a decomposition method. The results are interpreted by assuming a variation of the magnetic correlation radius in the region of the thermal magnetic transition.     

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.     

确定磁性体在绝热磁化过程中达到最高温度的方法

在磁性体磁化过程中, 决定其能够达到的最高温度, 对磁热材料的优化选取是重要的. 本文以钆镓石榴石(Gd₃Ga₅O₁₂) 为例, 根据高磁场下趋近饱和定律的思想, 给出了低温、超强磁场下, Gd₃Ga₅O₁₂晶体等效磁化率的定量形式. 在外磁场从0---40 T范围内, 计算了该晶体的磁熵变、声子熵变以及磁性体温度随外磁场的变化, 结果均与实验值符合较好. 利用声子熵变与饱和磁熵变曲线交点的唯一性, 给出了在磁性体磁化过程中, 确定其温度达到最大值的方法, 预言了Gd₃Ga₅O₁₂晶体在绝热磁化过程中达到的最高温度为64.7K. 该方法还可以对所加外磁场大小进行预言或估计.     

Investigations of the magnetic structure of the surface and volume of aluminum-substituted Sr-M type hexaferrites

The magnetic structure of the surface layer of single crystals of hexagonal ferrites of the type Sr-M (SrFe₁₂O₁₉) in which some iron ions are replaced by diamagnetic Al ions is investigated, in direct comparison with the magnetic structure in the bulk of the sample, by the method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy. It is found that under conditions of diamagnetic dilution of the magnetic lattice of hexagonal ferrites of the type Sr-M by Al ions, a layer ~200 nm thick in which the orientation of the magnetic moments is not collinear with the direction of the moments in the bulk of the sample is observed on the surface of SrFe_10.2Al_1.8O₁₉ crystals. Thus a “transitional” surface layer has been observed on macroscopic ferromagnetic crystals.     

Magneto-optic studies of magnetic oxides

A brief review of the use of magneto-optic methods to study magnetic oxides is given. A simple method to obtain the magnetic circular dichroism (MCD) of a thin film on a transparent substrate is described. The method takes full account of multiple reflections in the film and substrate. Examples of the magneto-optic spectra of Co-doped ZnO, Fe₃O₄, and GdMnO₃ are given. The Maxwell–Garnett method is used to describe the effects of metallic cobalt inclusions in Co:ZnO samples, and the change of the MCD spectra of Fe₃O₄ at the Verwey temperature is discussed. Data showing different MCD signals at different energies is presented for GdMnO₃.     

Energies of the H+2 ion in strong magnetic fields

Variational calculations of the energy values and the equilibrium internuclear separations of the H⁺₂ ion in the 1σ_g and 1π_g states are carried out to confirm our previous results based on the Monte Carlo method; in the absence of an external magnetic field the antibonding 1π_g state changes to a bonding state with increasing magnetic field. As a byproduct, a simple variational form is proposed for the wave function of the H atom in strong magnetic fields.     

Effect of an external magnetic field and a trapped magnetic flux on the current-voltage characteristics of a granular high-temperature superconductor YBa2Cu3O7−δ

A comparative study of the current-voltage characteristics of the high-temperature ceramic superconductor YBa₂Cu₃O_～6.95 at T = 77.3 K is performed over wide ranges of external magnetic fields H _ext and “treatment” fields H _treat. It is found that the field dependences of the parameters a and j _c involved in the exponential equation E = a(j ? j _c)^v describing the current-voltage characteristics depend substantially on the method used for applying the magnetic field, whereas the exponent v ～ 2 depends on neither the method of application nor on the magnetic field strength. The field dependence of the trapped magnetic field H _trap is determined.     

Fabrication and magnetic properties of hexagonal BaFe12O19 ferrite obtained by magnetic-field-assisted hydrothermal process

High magnetic field effects on the microstructure and magnetic properties of BaFe₁₂O₁₉ hexaferrites synthesized hydrothermal method have been investigated. The obtained results indicate that the lattice constant decreases gradually as the magnetic field strength increases, which may be attributed to the lattice distortion resulted from the high magnetic field. Polycrystalline BaFe₁₂O₁₉ samples prepared under magnetic field strength at zero and 5 T are single phase. It is found that application of external magnetic field during synthesis can induce orientated growth of the hexaferrite crystals along the easy magnetic axis. The magnetic properties can be effectively regulated by an application of high magnetic fields. It is observed that the BaFe₁₂O₁₉ prepared under a 5 T magnetic field exhibits a higher room-temperature saturation magnetization (66.3 emu/g) than that of the sample (43.6 emu/g) obtained without magnetic field. The results can be explained as the enhanced crystalline, improvement of Fe³⁺ ions occupancy and the oriented growth induced by the external magnetic field. The growing orientation of particles gives rise to increased coercivity due to the enhancement in shape anisotropy. It is expected that an application of magnetic field during the formation of magnetic nanoparticles could be a promising technique to modify magnetic properties with excellent performance.     

Effect of electron correlations on the electronic structure and magnetic properties of the perovskite-like high-pressure phase ErCu3V4O12

The electronic structure and magnetic properties of double perovskite ErCu₃V₄O₁₂ have been investigated in the framework of the projector augmented-wave method. It has been shown that the electron correlations play an important role: the calculations in the one-electron approximation predict that the ground state of ErCu₃V₄O₁₂ is a magnetic metallic state, while with the inclusion of electron-electron correlations, it is a ferrimagnetic semimetallic state. It has been found that the magnetic Cu-Cu and V-V couplings are ferromagnetic and that the Cu-V magnetic coupling is antiferromagnetic.     

Synthesis protocol influence on aqueous magnetic fluid properties

Magnetic colloids containing superparamagnetic Fe₃O₄ nanoparticles have been prepared by co-precipitation method. Three samples of citric acid coated magnetic colloids containing magnetic nanoparticles (ultra-fine particles of Fe₃O₄) have been obtained following three different preparation protocols. Physical tests have been performed on these samples of the magnetic colloids prepared by us (consisting mainly of Fe₃O₄ ultra-fine particles stabilized with citric acid (C₆H₈O₇) and immersed in water), in order to reveal their microstructural and rheological features. Transmission electron microscopy (TEM) and magnetic measurements were the investigation methods used for the assessing of the magnetic nanoparticles size. The dimensional distribution of the ferrophase physical diameter was comparatively presented using the box-plot statistical method. Infrared absorption spectra have been recorded aiming to get some information on the magnetic fluid composition.     

SQUID法和Campbell法测量超导材料的研究

本文对比研究了超导材料磁测量中的SQUID法和Campbell法; 并用高压PIT法制备的超导材料MgB₂作为测量样品,用两种方法,测量了超导样品的临界电流密度, 分别得到了样品的J_c-B关系曲线; SQUID法测量样品的外磁场可以达到6 T, 此时材料已经处于失超状态,此方法测得的结果是样品各个小区域结果的平均值, SQUID还可以用来进一步标度材料的钉扎力行为,研究材料磁特性. Campbell法测量只能测量到外磁场强度为0.4 T,外磁场的交流部分的频率可以达到800 Hz, 用这种测量方法得到的是整块样品的电流,由于测量计及材料内部微观结构缺陷等影响电流传输因素, 所测结果小于直流磁化法,但更切近材料实际电流,能用来深入研究材料内部结构差别对材料电性能的影响.     

The synthesis and characterization of Al/Co3O4 magnetic composite pigments with low infrared emissivity and low lightness

The main challenge of low infrared emissivity coatings based on aluminum flake lies in finding an efficient method to synthesize the composite pigment with low infrared emissivity and low lightness simultaneously. In this work, we overcome this constraint to some extent, synthesizing a novel Al/Co₃O₄ magnetic composite pigments with low infrared emissivity and low lightness by thermal cracking and hot flowing method. The results show that the covering area of Co₃O₄ on the aluminum flake can be tuned by the amount of CoCO₃ adding in precursor and the reaction temperature of hot flowing, both of which pay a key factor on the VIS and IR spectral reflectance and magnetic properties. The magnetic Al/Co₃O₄ composite pigments with low lightness and low infrared emissivity can be obtained at 130 °C for 24 h in hot flowing liquid. The lightness L¹ can be decreased to 69.2, however the infrared emissivity (8–14 μm) is also low to 0.45. Compared with the single Al flakes, Al/Co₃O₄ magnetic composite pigments present stronger magnetic properties. Therefore, the Al/Co₃O₄ magnetic composite pigments have offered new choice for the pigments of low infrared emissivity coatings.     

Exsolution of Nickel Nanoparticles from Mixed-Valence Metal Oxides: A Quantitative Evaluation by Magnetic Measurements

A fast and accurate experimental method is demonstrated to assess the fraction of exsolved metallic nanoparticles using magnetic measurements. As a benchmark, nanometric metallic nickel exsolved from (La_1−xSr_x)(Cr_1−yNi_y)O_3−δ is used for its high relevance as a solid oxide fuel cell component. The method is based on the difference in the magnetic response of the exsolved metallic nickel (ferromagnetic) and Sr-doped lanthanum chromite ceramic matrix (paramagnetic). The exsolved nickel results in coherent nanoparticles pinned on the surface of the Sr-doped lanthanum chromite ceramic matrix, as evidenced by electron microscopy analyses. The results obtained indicate the procedure as a fast and sensitive method to study the exsolution of ferromagnetic nanoparticles.     

Effects of oblique-incidence evaporation on magnetic properties of Fe20Ni80/Fe70Co30 bilayer film

Evaporative deposition at oblique incidence is shown to enhance the magnetic anisotropy of an Fe₂₀Ni₈₀ magnetic film and induce magnetic anisotropy in an overlying, strongly isotropic Fe₇₀Co₃₀ film. This deposition method for the formation of an underlayer of several lattice parameters in thickness and semi-hard overlayer of a few thousands Angstroms in thickness achieves a significant change in the magnetization process and strong suppression of the coercive forces of Fe₇₀Co₃₀ in the hard magnetization direction. Soft magnetization of the Fe₇₀Co₃₀ overlayer is not achieved when one of the layers is deposited at oblique incidence. It is anticipated that shape magnetic anisotropy is responsible in part for the magnetic anisotropy induced in both in Fe₂₀Ni₈₀ under- and Fe₇₀Co₃₀ overlayer by oblique incidence evaporation.     

Neutron diffraction investigation of the magnetic structure of the Mn2Sb pnictide at high pressures

This paper reports on the results of investigations into the influence of high pressures (up to 5.3 GPa) at T=300 K on the magnetic structure of the Mn₂Sb pnictide. The crystal and magnetic structures of the Mn₂Sb compound are studied using a direct neutron diffraction method. It is demonstrated that the magnetic ferrimagnet-antiferromagnet phase transition, which is observed in a number of Mn₂Sb-based substitutional solid solutions upon chemical compression of the Mn₂Sb crystal lattice, does not occur in the Mn₂Sb compound under high pressures in the aforementioned range due to an anisotropic pressure-stimulated strain in the Mn₂Sb lattice. At pressures P≥2.8 GPa, the Mn₂Sb compound is characterized by a spin reorientation with respect to the tetragonal axis and the basal plane of the crystal lattice.     

Phase error elimination in the M x magnetometer and resonance line shape control in an unstable field using the technique of invariant mapping of a spin precession signal

A method of two-dimensional mapping of a magnetic resonance signal is proposed that allows one to exclude the effects of field variations on the processes of monitoring of the magnetic resonance spectra and phase correction in the feedback loop of the M _x magnetometer.