包钴对α-Fe2O3粉末Morin相变的影响

用穆斯堡尔谱,磁性测量,中子衍射和X射线光电子能谱(XPS)等方法对纯的和包钴α-Fe₂O₃粉末Morin相变的影响以及有关性质进行了研究,发现包钴会使α-Fe₂O₃的Morin温度降低,相变温度的区域扩大,矫顽力明显增大,3d能带变化,假设包钴可使α-Fe₂O₃的单离子各向异性常数K_FS下降来解释上述实验结果。 关键词：     

γ-Fe2O3中的氢含量

利用热中子透射法测定γ-Fe₂O₃的氢含量。利用差热分析、磁分析以及穆斯堡尔效应研究γ-Fe₂O₃的相变,实验结果表明在γ-Fe₂O₃结构中确实含有一定量的氢,当γ-Fe₂O₃结构中的阳离子空位被H¹⁺,Co²⁺,Si⁴⁺,P⁵⁺等离子占据时,将 关键词：     

R2Fe14B(R=Ce,Pr,Gd)磁晶各向异性常数K1,K2随温度的变化

本文在1.5—300K温度范围内测量了R₂Fe₁₄B(R=Ce,Pr,Gd)各向异性常数K₁,K₂和各向异性场H_A随温度的变化。同时用单离子模型计算了Pr³⁺离子对Pr₂Fe₁₄B磁晶各向异性的贡献,得到与实验值半定量符合的结果。 关键词：     

包钴铁氧体型γ-Fe2O3磁粉穆斯堡尔效应的研究

包钴铁氧体型,γ-Fe₂O₃磁粉(简记为CoFe-γ-Fe₂O₃)是针状γ-Fe₂O₃磁粉与Co⁺⁺和Fe⁺⁺溶液起反应,在每个针状颗粒上包覆了一层钴铁氧体固溶体。经此种方法处理后的γ-Fe₂O₃磁粉的矫顽力及其它磁特性有较大的提高。如矫顽力由原来415Oe增加到715Oe;剩磁和矫顽力随时间及温度变化小等。我们利用穆斯堡尔效应并配合其它研究手段进行了研究,认为:CoFe-γ-Fe₂O₃磁粉矫顽力的提高,主要是由于γ-Fe₂O₃磁粉表面包覆了一层钴铁氧体固溶体,γ-Fe₂O₃与钴铁氧体固溶体之间发生磁耦合作用之故。 关键词：     

溅射氧化铁薄膜矫顽力的研究

本文报道了有关溅射氧化铁薄膜磁性能的系统研究,特别着重于矫顽力的温度依赖关系和矫顽力随不同氧化铁相的变化。从理论分析与实验测量结果的对比中,给出了形状各向异性、磁晶各向异性及应力各向异性各自对Fe₃O₄薄膜、γ-Fe₂O₃薄膜及二者混合相薄膜的贡献,并且得到了Fe₃O₄薄膜和γ-Fe₂O₃薄膜的磁晶各向异性常数K₁的温度依赖关系曲线。 关键词：     

包钴型r-Fe2O3磁粉各向异性研究

包钴型γ-Fe₂O₃磁粉的矫顽力可比原γ-Fe₂O₃磁粉提高8000—32000A/m。木文研究探讨了两种包钴型γ-Fe₂O₃磁粉(包钴γ-Fe₂O₃和包钴包亚铁γ-Fe₂O₃)的单轴各向异性的起源和矫顽力增大的机制。包钴γ-Fe₂O₃磁粉矫顽力 关键词：     

化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

AgI(α-Fe2O3)复合离子导体相转变温度相互影响的研究

通过测量AgI(α-Fe₂O₃)复合离子导体的直流和交流电导率,发现α-Fe₂O₃的Morin转变温度随着AgI含量的增加而降低,电导率测量的结果与磁化率测量和M?ssbauer谱测量结果定性的符合。α-Fe₂O₃也使AgI的α→β相转变温度显著下降。结果表明复合离子导体二相材料各自存在的相转变温度彼此产生影响。 关键词：     

Ce1.66Mg1.34Co3和α″-Fe16N2多层梯度膜磁反转性质的微磁学模拟

为了改善永磁薄膜的磁性能,基于微磁学理论,使用编程软件OOMMF针对Ce_1.66Mg_1.34Co₃和α″-Fe₁₆N₂交换耦合多层梯度膜的磁化过程进行模拟,系统研究磁晶各向异性梯度对多层膜性能的影响,分析体系的剩余磁化强度、矫顽力、磁滞回线和磁化反转过程中的能量变化。研究表明：减小磁晶各向异性梯度或增加界面处磁晶各向异性的差值,可以有效提高薄膜的矫顽力和剩余磁化强度,从而改善磁性能。通过计算磁矩分布发现一种磁涡旋态,这种磁涡旋态的产生过程伴随系统能量的增加。     

大面积α-Fe2O3纳米线及纳米带阵列的制备研究

以铁箔为原材料和基片,通过控制热氧化过程中的宏观实验条件(载气流量及其组分、压强、温度分布和反应时间等),实现了α-Fe₂O₃一维纳米结构的可控生长,获得了大面积(10mm×10mm)、单分散性好、沿［110］方向生长的α-Fe₂O₃纳米带或纳米线阵列. 对不同宏观实验条件下所制备的样品进行形貌和晶格结构表征和分析,认为热氧化过程中α-Fe₂O₃一维纳米结构的生长遵循类似气- 关键词： 2O₃')" href="#">α-Fe₂O₃ 一维纳米结构 热氧化法     

Origin of the growth induced anisotropy in (YBi)3Fe5O12 films

The single-ion anisotropy model together with the superposition model of the zero field splitting are used to analyse the growth-induced anisotropy of YIG films containing Bi³⁺ ions. Three different mechanisms are discussed. It is shown that the most probable source of anisotropy is the modification of the electron structure of Fe³⁺ ions by interactions in Fe³⁺O²;Bi³⁺ triads.     

Magnetic anisotropy in geometrically frustrated kagome staircase lattices

This paper reviews experimental results concerning magnetic anisotropy in geometrically frustrated kagome staircase lattices. Following problems are discussed: high-temperature susceptibility measurements of kagome single crystals; inelastic neutron scattering measurements on Co₃V₂O₈ single crystals; EPR of Co²⁺ ions in kagome staircase Mg₃V₂O₈ single crystals. The single-ion anisotropy Hamiltonian is used to analyze experimental results. It is suggested that the magnetic anisotropy in kagome staircase M₃V₂O₈ (M=Co, Ni, Mn) oxides has mainly single-ion origin.     

Shape- and field-dependent Morin transitions in structured α-Fe2O3

We report shape- and field-dependent magnetic properties of ellipsoid-, spindle-, flattened- and rhombohedra-shaped α-Fe₂O₃ samples prepared by solvothermal technique. We observed that a magnetic spin-flip mechanism, mostly known as Morin transition (T_M), depends on the shape of α-Fe₂O₃ as well as on the applied magnetic field. In each of these structures the obtained value of T_M was less than its bulk value of 263 K. We observed that T_M shifted from highest 251.4 K for ellipsoid to lowest 220.8 K for rhombohedra structure, with intermediate values of T_M for the other two structures. However, for rhombohedra structure T_M shifted from 220.8 to 177.5 K under the external magnetic field of 100 Oe-30 kOe, respectively. The observed lowering of T_M in the structured sample was analyzed in terms of elementary size, shape of the nanocrystallites, lattice parameters and occupancy of Fe⁺³ ions as well. These parameters were determined from the Rietveld refinement process using MAUD software.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

Anisotropy of hexagonal ferrites with M,W and Y structures containing Fe3+ and Fe2+ as magnetic ions

The ferrimagnetic saturation moment and hexagonal anisotropy constant K₁ have been measured at 4K on a Zn₂Y single crystal and on polycrystalline BaFe²⁺₂W and SrFe²⁺₂W samples. The moment of Fe₂W is in agreement with a collinear spin coupling and with the known site occupation for the Fe²⁺ ions. The moment of Zn₂Y is 9% lower than the value for a collinear configuration.The uniaxial anisotropy of Fe²⁺ in hexagonal ferrites is discussed and compared with that of Co²⁺. No noticeable Fe²⁺ anisotropy is found in Fe₂W in contrast to LaM = LaFe²⁺Fe³⁺₁₁O₁₉, in which the Fe²⁺ anisotropy is strong. The difference is attributed to the symmetry difference of the sites occupied by the Fe²⁺ ions in both compounds. The current theory does not satisfactorily explain the anisotropy and quadrupole splitting of Fe²⁺ in LaM. From this it is concluded that admixing of ⁵E states and (or) the influence of a dynamical Jahn-Teller effect cannot be neglected.The dipole-dipole anisotropy is computed for the M, W and Y structure and some deviation from the literature data is found. Using these results, a mean anisotropy of 1.3 to 2.3 cm^?1 per Fe³⁺ ion is found for the three structures.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

An NMR Analysis of Transitions in FexNbxV2-2xO4

The first order semiconductor - metall phase transition with the lattice symmetry change from monoclinic to tetragonal and the conductivity jump of several orders of magnitude is observed in pure vanadium dioxide and the dioxide doped by varios impurities [I]. Transition temperature T_t for pure VO₂ is 68°C and depends on the impurity. For example, the impurities which enter the insulating phase as pentavalent ions Nb⁵⁺ [2] give rise to a decrease in metal-semiconductor transition temperature at low concentrations and the trivalent ions Cr³⁺, Fe³⁺ [3,4] lead to an increase in T_t. In the latter case three different monoclinic phases are stabilized at a temperature below T_t. It has been already shown [5] that for double doping of vanadium dioxide by pentavalent Nb⁵⁺ and trivalent Fe³⁺ or Cr³⁺ ions three semiconductor phases exist (M_I, M₂, M₃) but the transition temperature decreases as the impurity concentration increases.     

Magnetic anisotropy of antiferromagnet (CH3)4NMnCl3

The parameters of the electron paramagnetic resonance (EPR) spectra of S ion pairs in diamagnetic crystals are analyzed. A relation between the spin Hamiltonian constants is established for solitary ions and pairs for (CH₃)₄NCdCl₃: Mn²⁺ crystals. In contrast to solitary ions, an additional contribution (which is a linear function of the exchange field) to the “single-ion” spin Hamiltonian constants appears in the case of pairs. It is shown that anisotropic exchange mechanisms do not play a significant part in the formation of the axial constant of the spin Hamiltonian for this crystal. Some aspects of the method of studying “single-ion” anisotropy predicted by the two-ion model are developed with the help of an isostructural diamagnetic analog with impurity concentration of the paramagnetic ions of a magnetically concentrated substance sufficiently high for observing the EPR spectrum of the pairs. It is found that the microscopic quantities determined partially from the EPR spectra for pairs and solitary Mn²⁺ ions in (CH₃)₄NCdCl₃ are in accord with the experimental value of the effective field for the (CH₃)₄NMnCl₃ crystal anisotropy which can be described primarily by the dipole and “single-ion” mechanisms of the exchange origin.     

XRD,impedance, and Mössbauer spectroscopy study of the Li<Subscript>3</Subscript>Fe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> composite for Li ion batteries

The synthesis procedure of the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite is presented. The monoclinic (A type) and hematite phases were detected by X-ray diffraction after the synthesis of the composite. The structural α–β (at a temperature of 460 K) and β–γ (at a temperature of 523 K) phase transitions in the composite were indicated by the anomalies of the electrical conductivity, dielectric permittivity, and changes of activation energies of conductivity. Two phase transitions have been detected in the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite by ⁵⁷Fe Mössbauer spectroscopy: the phase transition in Li₃Fe₂(PO₄)₃ from the paramagnetic to antiferromagnetic phase at temperature T _N?=?29.5 K and the Morin phase transition in Fe₂O₃ at temperature T _M?=?235 K.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.