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. 关键词： Terfenol-D 磁机械效应 巨磁致伸缩效应 磁各向异性     

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.     

Influence of Tb on easy magnetization direction and magnetostriction of ferromagnetic Laves phase GdFe_2 compounds

The crystal structure,magnetization,and spontaneous magnetostriction of ferromagnetic Laves phase Gd Fe2 compound have been investigated.High resolution synchrotron x-ray diffraction(XRD) analysis shows that Gd Fe2 has a lower cubic symmetry with easy magnetization direction(EMD) along [100] below Curie temperature TC.The replacement of Gd with a small amount of Tb changes the EMD to [111].The Curie temperature decreases while the field dependence of the saturation magnetization(Ms) measured in temperature range 5–300 K varies with increasing Tb concentration.Coercivity Hc increases with increasing Tb concentration and decays exponentially as temperature increases.The anisotropy in Gd Fe2 is so weak that some of the rare-earth substitution plays an important role in determining the easy direction of magnetization in GdFe_2.The calculated magnetostrictive constant λ100 shows a small value of 37×10~(-6).This value agrees well with experimental data 30×10~(-6).Under a relatively small magnetic field,GdFe_2 exhibits a V-shaped positive magnetostriction curve.When the field is further increased,the crystal exhibits a negative magnetostriction curve.This phenomenon has been discussed in term of magnetic domain switching.Furthermore,magnetostriction increases with increasing Tb concentration.Our work leads to a simple and unified mesoscopic explanation for magnetostriction in ferromagnets.It may also provide insight for developing novel functional materials.     

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.     

Superconducting properties of the C15-type Laves phase ZrIr2 with an Ir-based kagome lattice

We report systematic studies on superconducting properties of the Laves phase superconductor ZrIr$_2$. It crystallizes in a C15-type (cubic MgCu$_2$-type, space group $Fd\overline{3}m$) structure in which the Ir atoms form a kagome lattice, with cell parameters $a=b=c=7.3596(1)$ Å. Resistivity and magnetic susceptibility measurements indicate that ZrIr$_2$ is a type-II superconductor with a transition temperature of 4.0 K. The estimated lower and upper critical fields are 12.8 mT and 4.78 T, respectively. Heat capacity measurements confirm the bulk superconductivity in ZrIr$_2$. ZrIr$_2$ is found to possibly host strong-coupled s-wave superconductivity with the normalized specific heat change $\Delta C_{\rm e}/\gamma T_{\rm c} \sim 1.86$ and the coupling strength $\Delta_0/k_{\rm B}T_{\rm c} \sim 1.92$. First-principles calculations suggest that ZrIr$_2$ has three-dimensional Fermi surfaces with simple topologies, and the states at Fermi level mainly originate from the Ir-5d and Zr-4d orbitals. Similar to SrIr$_2$ and ThIr$_2$, spin--orbit coupling has dramatic influences on the band structure in ZrIr$_2$.     

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.     

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

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.     

Crystal fields at rare-earth sites in R2Fe14B compounds

Central-field effects are expected to be important in R₂Fe₁₄B compounds. Within a model-independent approach, it is proved that four distinct rare-earth sites exist with respect to the crystalline electric fields, namely, R(4f;z = 0), R(4f; z = 0.5c), R(4g; z = 0) and R(4g; z = 0.5c), and relationships are established between the corresoonding crystal-field coefficients. Further, generalized Stevens parametrizations of the crystal field coefficients are derived at three levels of approximation for the interatomic forces inside the crystal. A crystal lattice dressing effect upon the radial electronic integral is found to occur, the magnitude of which depends on the deviation of the interatomic forces from Coulombian. Finally, computation of crystal-field coefficients in Nd₂Fe₁₄B leads to results which raise questions about the validity of the simple Coulumb point-charge model.     

Low temperature magnetic and magnetostrictive properties in Pr(Fe_(1-x)Co_x)_(1.9) cubic Laves alloys

The structures,spin reorientations,magnetic,and magnetostrictive properties of the polycrystalline Pr(Fe_(1-x)Co_x)_(1.9)(x=0–1.0)cubic laves phase alloys between 5 K and 300 K are investigated.Large low-field magnetostrictions are observed at 5 K in the alloys with x=0.2 and 0.4 due to the low magnetic anisotropies of these two alloys.A large negative magnetostriction of about-1130 ppm is found in PrCo_(1.9) alloy at 5 K.The magnetizations of the alloys with 0≤x≤0.6decrease abnormally at the spin reorientation temperature T_(sr),and an abnormity is detected in the alloy with x=1.0 at its Curie temperature T_c(45 K).The substitution of Fe by Co increases the value of T_(sr) in the alloy with x value increasing from 0.0 to 0.4,and then reduces the value of Tsr with x value further increasing to 0.6.