Tb0.3Dy0.7Fe2单晶中巨磁致伸缩的逆效应

研究了Terfenol-D材料中巨磁致伸缩的逆效应,即磁机械效应.基于Stoner-Wohlfarth(SW)模型,考虑磁晶各向异性和应力各向异性能,依据自由能极小原理,获得了退磁态下Terfenol-D单晶中磁化强度方向和压应力的关系.采用数值方法求解了平衡条件下的非线性方程组.理论结果表明,Terfenol-D巨磁致伸缩单晶中的磁各向异性取决于磁晶各向异性和应力各向异性之间的竞争.在压应力的作用下,Terfenol-D单晶中的磁各向异性由立方向单轴转变.理论和实验结果的比较表明,存在一个临界压应力,使磁致伸缩效应达到极大值.该理论结果还解释了压应力使得Terfenol-D单晶材料难于磁化和磁致伸缩效应出现极大值的实验事实.理论计算不仅为研究这类问题提供了一个更准确的方法,而且其结果也有助于理解类似材料中的磁化过程.     

A structural, magnetostrictive and M(o)ssbauer study of Tb0.3Dy0.7(Fe0.9T0.1)1.95 alloys

The effect of ⅢA metal and transition metal T substitution for Fe on crystal structure, magnetostriction and spontaneous magnetostriction, anisotropy and spin reorientation of a series of polycrystalline Tb0.3Dy0.7(Fe0.9T0.1)1.95 (T = Mn, Fe, Co, B, Al, Ga) alloys at room temperature were investigated systematically. It was found that the primary phase of the Tb0.3Dy0.7(Fe0.9T0.1)1.95 alloys is the MgCu2-type cubic Laves phase structure for different substitution. The magnetostriction λs decrases greatly for the substitution of IIIA metal B, Al and Ga, but is saturated more easily for Al and Ga substitution, showing that the Al and Ga substitution is beneficial to a decrease in the magnetocrystalline anisotropy of Tb0.3Dy0.7(Fe0.9T0.1)1.95 alloys. However, the substitution of transition metal Mn and Co decreases slightly the magnetostriction λs. It was also found that the effect of different substitutions on the spontaneous magnetostriction λ111 is distinct. The analysis of the Mossbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry for Al and Ga substitution, namely spin reorientation, but it does not change evidently for B, Mn and Co substitution.     

A structural, magnetostrictive and M?ssbauer study of Tb0.3Dy0.7(Fe0.9 T 0.1)1.95 alloys

The effect of IIIA metal and transition metalT substitution for Fe on crystal structure, magnetostriction and spontaneous magnetostriction, anisotropy and spin reorientation of a series of polycrystalline Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 (T=Mn, Fe, Co, B, Al, Ga) alloys at room temperature were investigated systematically. It was found that the primary phase of the Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 alloys is the MgCu₂-type cubic Laves phase structure for different substitution. The magnetostriction λ{ins} decrases greatly for the substitution of IIIA metal, B, Al and Ga, but is saturated more easily for Al and Ga substitution, showing that the Al and Ga substitution is beneficial to a decrease in the magnetocrystalline anisotropy of Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 alloys. However, the substitution of transition metal Mn and Co decreases slightly the magnetostriction λ{ins}. It was also found that the effect of different substitutions on the spontaneous magnetostriction λ{in111} is distinct. The analysis of the M?ssbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry for Al and Ga substitution, namely spin reorientation, but it does not change evidently for B, Mn and Co substitution.     

Thermal expansion of ZrFe2 and some rare-earth iron compounds

The thermal expansion coefficient of the compounds ZrFe₂, TiFe₂, YFe₂, ErFe₂, ErFe₃, Er₆Fe₂₃ and Er₂Fe₁₇ has been determined from room temperature up to 700°C. The anomalous thermal expansion of some of these compounds observed at temperatures below their Curie temperature (T_c) is shown to be related to a strong pressure dependence of T_c. (dT_c/dP is large and negative).     

Anomalous magnetostrictive effects in （La1—xTbx）2／3Sr1／3 MnO3

A series of (La_{1-x}Tb_x)_{2/3}Sr_{1/3}MnO_3 polycrystalline samples has been studied by means of x-ray diffraction, magnetostriction, and thermal expansion measurements. It has been found that this series undergoes a phase transition from a rhombohedral to an orthorhombic form at the doping level x≈0.20 at room temperature accompanied by an anisotropic magnetostriction up to －50×10^{-6} under a magnetic field of 1T. The linear and volume magnetostrictions vary with chemical composition, even change sign. At T=80K, the magnetostrictions for the samples of x=0.20 and 0.40 exhibit different behaviours. The sample of x=0.20 has positive volume and linear magnetostrictions and a negative anisotropic magnetostriction, while the sample of x=0.40 has an opposite behaviour. The magnitude of volume magnetostriction for both the samples is essential (～10^{-4}) at T=80K under a magnetic field of 4T. We conclude that these anomalous effects are due to the charge delocalization and the structural phase transition between orthorhombic and rhombohedral forms induced by the applied magnetic field.     

Magnetic hyperfine field at s-p impurities on Laves phase compounds

Recent experimental results for the magnetic hyperfine field B_hf at the nuclei of s-p impurities such as ¹¹⁹Sn in intermetallic Laves phases RM₂ (R=Gd, Tb, Dy, Ho, Er; M=Fe, Co) and ¹¹¹Cd in R Co₂, the impurity occupying a R site indicate that the ratio B_hf/μ_3d exhibits different behavior when one goes from RFe₂ to RCo₂. In this work, we calculate these local moments and the magnetic hyperfine fields. In our model, B_hf has two contributions: one arising from the R ions, and the other arising from magnetic 3d-elements; these separate contributions allow the identification of the origin of different behavior of the ratio mentioned above. For ¹¹¹Cd in RCo₂ we present also the contributions for B_hf in the light rare earth Pr, Nd, Pm, Sm compounds. For the sake of comparison we apply also the model to ¹¹¹Cd diluted in R Ni₂. Our self-consistent magnetic hyperfine field results are in good agreement with those recent experimental data.     

Magnetic transitions and magnetostrictive properties of Laves compounds Sm0.88Nd0.12(Fe1−xCox)1.93

The structure, magnetic and magnetostrictive properties of Sm_0.88Nd_0.12(Fe_1−xCo_x)_1.93 (0≤x≤1.0) alloys have been investigated. The alloys have the cubic MgCu₂ structure over the whole composition range and the lattice parameter a decreases with increasing x. For 0≤x≤0.2, substitution of Co for Fe slightly increases the saturation magnetization M_s and Curie temperature T_c, while further substitution causes a decrease in both M_s and T_c. The spin reorientation is observed, and a phase diagram for the spin configurations of the Sm_0.88Nd_0.12(Fe_1−xCo_x)_1.93 system is determined. The spontaneous magnetostriction λ₁₁₁ increases as x is increased, while a monotonic decrease of the saturation magnetostriction λ_s with x originates from the increase of λ₁₀₀ with opposite sign to that of λ₁₁₁, which may be caused by the filling of the d band due to Co substitution.     

Magnetic and electrical properties of RCo2Mn (R=Ho, Er) compounds

The RCo₂Mn (R=Ho and Er) alloys, crystallizing in the cubic MgCu₂-type structure, are isostructural to RCo₂ compounds. The excess Mn occupies both the R and the Co atomic positions. Magnetic, electrical and heat capacity measurements have been done in these compounds. The Curie temperature is found to be 248 and 222 K for HoCo₂Mn and ErCo₂Mn, respectively, which is considerably higher than that of the corresponding RCo₂ compounds. Saturation magnetization values in these samples are less compared to that of the respective RCo₂ compounds.     

Substituting Al for Fe in Pr(Al_xFe_(1-x))_(1.9) alloys:Effects on magnetic and magnetostrictive properties

The magnetostrictive effects of substituting Al for Fe in Pr(AlxFe1-x)1.9(x = 0.0, 0.02, 0.05, 0.10) alloys between5 K and 300 K were investigated. The substitution decreases the Curie temperature and the value of λ111. Fortunately, the substitution slightly increases the magnetostriction in a low magnetic field, which imbues these materials with potential advantages for applications. Rotation of the easy magnetization direction(EMD) from [111] to [100] in the Pr(Al0.02Fe0.98)1.9alloy as temperature decreases was detected by step scanned XRD reflections.     

Anomalous thermal expansion and spontaneous magnetostriction of Gd2Fe16Cr compound

The structural and the magnetic properties of Gd 2 Fe 16 Cr compound are investigated by x-ray diffraction and magnetization measurements.The Gd 2 Fe 16 Cr compound has a rhombohedral Th 2 Zn 17-type structure.There exist an anisotropic strong spontaneous magnetostriction and a negative thermal expansion in the magnetic state of Gd 2 Fe 16 Cr compound.The average thermal expansion coefficient ā=-7.03 × 10-6 /K in a temperature range of 294-454 K and ā=-1.31 × 10-5 /K in 454-572 K are obtained.The spontaneous magnetostrictive deformation and the Curie temperature are discussed.     

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMnsGe2（R=Sm,Gd）

The temperature dependence of lattice constants a and c of intermetallic compounds RMn₂Ge₂ (R=Sm, Gd) is measured in the temperature range 10－800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism－ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10^-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn－Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn－Mn exchange interaction.     

Induced magnetic phase transitions in rare-earth intermetallic compounds RMn2Ge2 in ultrastrong magnetic fields

The differential magnetic susceptibility of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb, Dy, Ho, Y) with a layered tetragonal structure is measured in pulsed magnetic fields up to 130 T. It is found that all these compounds undergo a first-order magnetic phase transition in strong magnetic fields. The nature of this transition is discussed, and it is found that a change in the magnetic state of the manganese sublattice is responsible for the transition.     

Electronic structure and bonding properties for Laves-phase RV2 (R=Ti, Nb, Hf, and Ta) compounds

The electronic structure and bonding properties of Laves-phase compounds RV₂ (R=Ti, Nb, Hf, and Ta) with C15 structure have been investigated using the full-potential linearized augmented plane-wave method. The results show that the chemical bonding is metallic–ionic–covalent in nature in these compounds, and the covalent bonding between V and V atoms strengthens with the atomic number, increasing among the RV₂ (R=Ti, Nb, Hf, and Ta) compounds. The density of states (DOS), equilibrium volume, and elastic properties are discussed, which is important for understanding the physical properties of RV₂ (R=Ti, Nb, Hf, and Ta) and may inspire future experimental research.     

Rearrangement of crystallographic domains driven by magnetic field in antiferromagnetic CoO

The rearrangement was investigated of crystallographic domains in the antiferromagnetic pseudo-tetragonal phase in CoO (Néel temperature: 293 K) when the domains were driven by a magnetic field. A rearrangement is generally observed in ferromagnetic shape-memory alloys. The rearrangement was found to occur at temperatures between 170 K and 293 K, but not at temperatures below 170 K. In order to determine the reason for such a difference, the shear stress driven by a magnetic field, τ _mag, was calculated and compared with the shear stress required for twinning plane movement, τ _req. It was found that τ _mag is equal to or larger than τ _req whenever the rearrangement of crystallographic domains occurs due to the application of a magnetic field, and vice versa. This observation is similar to past observations in the case of many ferromagnetic shape-memory alloys.     

Orientational optical unsteady effects in rare-earth garnet crystals containing chromium or vanadium ions with the electronic 3d 2 configuration

The method of picosecond unsteady spectroscopy was used to study the optical anisotropy manifested in rare-erath garnet crystals doped by chromium and vanadium. In YAG:V³⁺, GSAG:V³⁺ and YAG;Cr⁴⁺ crystals the anisotropy of unsteady absorption and the effect of rotation of the plane of polarization of a probing pulse pulve were revealed. Deceased. Research Institute of Applied Physics at the Irkutsk State University, 20, Gagarin Ave., Irkutsk, 664003, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 597–600, July–August, 1998.     

Structure comparison between Th2Zn17-type and TbCu7-type Sm–Fe intermetallic compounds and their nitrides by means of 57Fe-Mössbauer spectroscopy

Samarium–iron intermetallic compounds were prepared by a melt spinning method with low and high wheel speeds, which resulted in a Th₂Zn₁₇-type and a TbCu₇-type structure, respectively. Structure comparison between these types was investigated for Sm–Fe intermetallic compounds and their nitrides by ⁵⁷Fe-Mössbauer spectroscopy.