Magnetoelasticity and elasticity of Fe85Ga15 single crystals under coupled magnetomechanical loading

The effect of coupled magnetomechanical loading on magnetostriction and compressive strain of Fe-Ga alloys has been investigated. A shift from negative to positive magnetostriction was observed with increase in compressive stress on a Fe₈₅Ga₁₅ single crystal. Non-linear behavior of the compressive strain in different magnetic fields was observed during the compressive loading and unloading process. These phenomena can clearly be explained by a model based on the magnetic-domain-switching process. The Young's modulus can also be obtained from the measured stress-strain curves.     

铁单质薄膜磁致伸缩行为与磁矩演化研究

磁致伸缩材料在传感器、位移器件等领域应用前景广阔,对此类材料的制备与实验已成为研究热点,但使用分子动力学方法模拟其磁致伸缩过程中内部磁矩的演化仍缺乏相关研究.本研究以磁致伸缩材料铁单质为研究对象,采用分子动力学方法建立单畴铁单质磁致伸缩模型.分析了铁单质薄膜磁致伸缩行为随初始磁矩的变化,以及在磁场作用下微观原子磁矩的变化与宏观磁致伸缩之间的关系.结果表明:模型磁化构型的演化与磁致伸缩行为有着密切联系,随着外加磁场强度增大,原子磁矩与外加磁场方向相同的区域面积逐渐增大,宏观表现为模型的磁致伸缩随磁场强度增大而伸长,并最终达到饱和,而边界处的原子磁矩是模型是否达到饱和磁致伸缩的关键.     

Magnetoelectric properties of Gd and Nd-doped nickel ferrite

Ferromagnetic and ferroelectric characteristics of Gd and Nd-substituted nickel ferrite were investigated. The materials formed in the cubic inverse spinel phase with small amounts of GdFeO₃ and NdFeO₃ as the additional phases in the respective materials. Substitution of Gd and Nd for Fe caused decrease in the saturation magnetization and Curie temperature of the nickel ferrite. However, the saturation magnetostriction is seen not to change significantly by the substitution of Gd and Nd. The existence of the ferroelectricity was confirmed from the ferroelectric loops and magnetocapacitance of −2% and −3% were observed. The large frequency dependence of the (high) dielectric constant reveals a wide dispersion of relaxation times. The ferroelectric transition temperature values of NiO.Fe_1.95Gd_0.05O₃ and NiO.Fe_1.95Nd_0.05O₃ were found to be 498 and 544 K, respectively.     

Structure and magnetostriction of Tb0.2Pr0.8(Fe0.4Co0.6)1.93−xCx intermetallic compounds

The structure and magnetostriction of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93−xC_x intermetallic compounds were studied by X-ray diffraction and magnetic measurements. Almost a single cubic Laves phase forms in the alloys for x ≤0.20, and a small amount of C can inhibit the formation of the 1:3 phase. The lattice parameter increases when 0≤x≤0.15, while the T_c and the spontaneous magnetization decreases with increasing x. The lattice parameter decreases slowly when 0.15≤x≤0.30, while the T_c decreases evidently with increasing x. The magnetostriction λ_a (=λ_∥-λ_⊥) is improved at low magnetic fields at room temperature for the compounds with 0.05≤x≤0.10, indicating that these C-containing compounds are promising magnetostrictive materials.     

Effect of heat treatment on structure, magnetization and magnetostriction of Fe81Ga19 melt-spun ribbons

Structure, magnetization and magnetostriction of melt-spun Fe₈₁Ga₁₉ ribbons were investigated both before and after heat treatment. The matrix of melt-spun Fe₈₁Ga₁₉ ribbons kept a body-centered-cubic (bcc) structure (A2) at room temperature. [1 0 0] preferred orientation was formed during melt-spinning process and became stronger with the increase of the ribbon thickness. For the ribbons with a thickness of 110 μm, maximum saturation magnetostrictive strain of −189 ppm along ribbon length was obtained in the samples heat treated at 800 °C for 3 h and then quenched into water. This value was about 16% larger than that of melt-spun ones, which could be contributed to the single disordered A2 structure and the enhancement of [1 0 0]-oriented texture. However, when the ribbon samples were cooled at 2 and 0.5 °C/min after heat treatment at 800 °C for 3 h, a minor quantity of ordered D0₃ and L1₂ phase was found to precipitate in the A2 matrix, respectively, which resulted in the reduction of both magnetization and magnetostrictive strain.     

The physics of magnetoelectric composites

This paper gives an overview about the basic ideas of magnetoelectric materials. Up to now single-phase materials show the magnetoelectric effect only below room temperature. Mixing a magnetostrictive with a piezoelectric component is a way to overcome this limitation. This delivers a composite which can exhibit a magnetoelectric effect even at room temperature and higher. Possible candidates for these composites (piezoelectric as well as magnetostrictive) are shown, examples from literature and own results are given. The most important coupling mechanism (magnetization, magnetostriction, local stress, charge) between the magnetostrictive and the piezoelectric phase are discussed. Hints for a direct coupling between the electric polarization and the magnetization are also presented. Different measurement methods for determining the magnetoelectric coefficient are discussed. Representative results as obtained on a technical useful composite between 50% Co-Ferrite+50% BaTiO₃ are given. The behavior of a simple “mixed” structure with that of a “core-shell” structure is compared. The later gives a 20-times larger magnetoelectric coefficient.     

Structure and magnetic properties of melt-spun Tb0.27Dy0.73Fex ribbons

The structure and magnetic properties of the melt-spun ribbons of Tb_0.27Dy_0.73Fe_x alloy are investigated as a function of various wheel speeds during melt-quenching using a single-roll technique. It is found that Tb_0.27Dy_0.73Fe_x alloy is difficult to be fabricated as amorphous state by using the melt-quenching method. X-ray diffractions show that all these ribbons for x=1.7−2.0 are the MgCu₂-type phase at the wheel speed of 45 m s⁻¹. For Tb_0.27Dy_0.73Fe_x alloy, the high wheel speed is beneficial to eliminate the RFe₃ phase and form the perfect MgCu₂-type phase. Compared with the bulk of Tb_0.27Dy_0.73Fe_1.95, these ribbons exhibit higher intrinsic coercivity value and their saturation magnetizations increase as well. The magnetostriction of Tb_0.27Dy_0.73Fe_1.95 composite with 4% epoxy resin is 640×10⁻⁶ at 900 kA m⁻¹.     

原位磁场下粉末样品的X射线衍射图谱

To obtain more crystal and magnetic structural information of powder crystals,magnetic field is introduced into X'Pert-MPD XRD apparatus with the strength of 0. 42 T and two different directions:one direction of magnetic field is vertical to the sample holder and another is parallel. XRD patterns in situ magnetic field are obtained for six samples which are representative of paramagnetic,diamagnetic,ferromagnetic,ferrimagnetic,antiferromagnetic substances and reduction product of FeCl2 by NaBH4 respectively. Compared with XRD patterns obtained in the zero magnetic field,there are some diffraction peaks disappear,some occur,or some diffraction angles and counts change. In addition,the patterns are different under the different direction of magnetic field. The cause is that magnetic dipoles in crystals are oriented along with the direction of magnetic field. The oriented magnetic dipoles produce stress in crystals and make crystal lattice changed（such as magnetostriction）or even turn particles aligned along with the direction of magnetic field to form preferred orientation of particles.     

Spin–orbit induced magnetic phenomena in bulk metals and their surfaces and interfaces

First-principles electronic structure studies based on local spin density functional theory and performed on extremely complex simulations of ever increasingly realistic systems, play a very important role in explaining and predicting surface and interface magnetism. This review deals with what is a major issue for first-principles theory, namely the theoretical/computational treatment of the weak spin–orbit coupling in magnetic transition metals and their alloys and its important physical consequences: magneto-crystalline anisotropy, magnetostriction, magneto-optical Kerr effects and X-ray magnetic circular dichroism. As is demonstrated, extensive first-principles calculations and model analyses now provide simple physical insights and guidelines to search for new magnetic recording and sensor materials.     

Magnetomechanical coupling enhancement via high-density nanoprecipitation in Co70Fe30 alloy

Magnetostrictive materials with high magnetomechanical coupling performance are required for transducer design and tunable resonator applications. The magnetostriction of Fe-Ga and Co-Fe alloy can be remarkably increased by high-density and semicoherent nanoparticles. In this work, we found that the magnetomechanical coupling of Co₇₀Fe₃₀ alloy can be obviously enhanced by high-density nanoprecipitation. This outstanding enhancement originates from the semicoherent face-centered tetragonal (fct) nanoparticles embedded in a body-centered cubic (bcc) matrix, which may promote magnetic domain rotations under applications of stress. The result indicates that high-density nanoprecipitation can be used to guide the design of magnetostrictive materials with unusual magnetomechanical coupling performance.