Tb0.3Dy0.7Fe2合金磁畴偏转研究

研究了Tb_0.3Dy_0.7Fe₂合金在压磁和磁 弹性效应中的磁畴偏转和磁导率特性. 基于Stoner-Wolhfarth 模型能量极小原理, 绘制了自由能与磁畴偏转角度的关系曲线, 研究了压应力和磁场载荷作用下磁畴角度的偏转特性, 计算分析了不同载荷作用下磁畴偏转的磁导率特性, 并与实验数据进行比较论证. 研究表明,应力和磁场的作用都将使磁畴方向[111]和[111]发生角度跃迁, 直观有效地解释了材料巨磁致伸缩效应的机理; 应力和磁场作用下磁畴的偏转将使材料磁导率呈减小趋势, 其中磁场能对磁导率的影响大于应力能, 这一现象在小载荷作用下尤为明显. 实验结果表明, 磁导率的计算数据与实验数据符合得较好, 验证了计算方法的正确性. 理论分析对Terfenol-D磁畴偏转模型的完善 和磁化过程中磁滞回线的绘制非常有意义.     

压应力对Fe0.81 Ga0.19单晶磁化和磁致伸缩的影响

研究了Fe_0.81Ga_0.19合金单晶沿[100]方向的磁机械效应和磁致伸缩效应.基于Stoner-Wohlfarth模型,通过数值计算获得了在压应力和外磁场联合作用下磁化强度的方向余弦.研究表明,随着压应力的增加,退磁态下合金中的磁各向异性会由三轴各向异性向双轴各向异性转变.这使得合金中 90° 畴的体积分数增加,导致磁致伸缩效应增大. 关键词： FeGa合金 磁机械效应 巨磁致伸缩效应     

Tb0.3Dy0.7Fe2合金的本构参数辨识方法研究

建立了一种简便的、适用于磁畴模型应用的Tb_0.3Dy_0.7Fe₂ 合金本构参数辨识方法. 针对Tb_0.3Dy_0.7Fe₂合金磁畴模型中本构参数不明确且直接实验测试困难的问题, 提出了一种数值计算与实验测试相结合的参数辨识方法. 采用坐标变换与绘制自由能等势曲线相结合的方法, 简化了载荷作用下Tb_0.3Dy_0.7Fe₂ 合金内磁畴角度偏转的数值计算, 研究了合金磁畴角度偏转模型的参数依赖性. 在此基础上, 结合简单的实验测试, 建立了Tb_0.3Dy_0.7Fe₂合金各向异性常数K₁ 和K₂、能量分布因子ω、晶轴取向分布的辨识及修正方法. 该方法能够简单、快速地完成Tb_0.3Dy_0.7Fe₂ 合金磁畴模型中本构参数的辨识, 对完善磁致伸缩材料磁畴偏转的数值计算模型非常有意义. 理论分析可为类磁致伸缩材料磁机耦合模型的建立、完善, 以及材料本构参数的辨识、获取提供参考.     

Tb0.3Dy0.7Fe2合金的本构参数辨识方法研究

建立了一种简便的、适用于磁畴模型应用的Tb0.3Dy0.7Fe2合金本构参数辨识方法.针对Tb0.3Dy0.7Fe2合金磁畴模型中本构参数不明确且直接实验测试困难的问题,提出了一种数值计算与实验测试相结合的参数辨识方法.采用坐标变换与绘制自由能等势曲线相结合的方法,简化了载荷作用下Tb0.3Dy0.7Fe2合金内磁畴角度偏转的数值计算,研究了合金磁畴角度偏转模型的参数依赖性.在此基础上,结合简单的实验测试,建立了Tb0.3Dy0.7Fe2合金各向异性常数K1和K2、能量分布因子ω、晶轴取向分布的辨识及修正方法.该方法能够简单、快速地完成Tb0.3Dy0.7Fe2合金磁畴模型中本构参数的辨识,对完善磁致伸缩材料磁畴偏转的数值计算模型非常有意义.理论分析可为类磁致伸缩材料磁机耦合模型的建立、完善,以及材料本构参数的辨识、获取提供参考.     

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.     

基于力磁耦合效应的铁磁材料修正磁化模型

Jiles-Atherton (J-A)模型和Zheng Xiao-Jing-Liu Xing-En (Z-L)模型在分析应力对铁磁材料磁化的影响方面应用十分广泛.目前, J-A模型中的磁致伸缩应变与应力和磁化强度的关系式采用Jiles给出的经典拟合公式,该拟合公式中磁化强度的二次项和四次项系数与应力均为线性关系,不能准确描述铁磁材料磁致伸缩系数随应力、磁化强度的非线性变化规律; Z-L模型中磁致伸缩应变与应力和磁化强度的关系式采用了双曲正切函数tanh(x),更好地描述了铁磁材料磁致伸缩应变和磁化强度随应力的非线性变化规律,但Z-L模型却没有考虑Weiss分子场、钉扎效应的作用,且由于采用了基于弹性能的接近定理,只能描述弹性应力对磁化过程的影响.针对上述问题,本文结合Z-L模型中的非线性磁致伸缩应变关系式以及J-A模型中的磁滞理论,考虑弹性应力、塑性变形对模型参数的影响,建立了能够反映弹-塑性阶段应力与塑性变形对铁磁材料磁化曲线影响的修正磁化模型,分析了弹性拉、压应力及塑性拉、压变形对磁化曲线、矫顽力和剩余磁化强度的影响规律.通过与试验结果及原有模型的计算结果进行对比,发现修正模型能够更好地反映单次磁化、循环磁化过程中应力、塑性变形对磁化曲线的影响规律,理论预测结果与试验结果之间的相关系数均在0.98以上,可为分析力磁耦合效应对铁磁材料磁化影响规律提供更准确的理论模型.     

CoFe2O4和MnFe2O4纳米复合介质的制备及其磁性研究

采用热分解法制备了分散程度高且平均晶粒尺寸为20 nm的CoFe₂O₄和MnFe₂O₄复合介质.低温磁化曲线测量显示,制备的复合介质具有软-硬磁交换弹性耦合效应,且合成温度以及软磁和硬磁相的成分比例对磁交换弹性耦合的强度有很大的影响.变温磁测量显示,温度为20K时,复合纳米介质的表面自旋冻结效应导致饱和磁化强度显著增加.Henkel测量显示,对分散的CoFe₂O₄和MnFe₂O₄复合介质,磁偶极相互作用占主导作用.     

纳米复合永磁Pr9Fe74Co12B5Snx(x=0, 0.5)的磁化行为与磁黏滞性

用熔体快淬法制备了纳米复合永磁样品Pr₉Fe₇₄Co₁₂B₅ 与Pr₉Fe₇₄Co₁₂B₅Sn_0.5，分析了样品的起始磁化、反磁化过程，测得样品的总磁化率、可逆磁化率以及样品的磁黏滞性.结果表明，两样品在室温下均表现为单一硬磁相磁化行为，在低温下表现为双相行为，且由于添加Sn后使晶粒均匀化从而导致样品低温下的双相行为更加明显.添加Sn后引起样品中软磁相含量和软磁相晶粒尺寸的增加，使磁化反转中可逆磁化部分增多，且使反磁化形核场降低.磁黏滞性研究表明，热激活体积与软磁相晶粒的大小有关. 关键词： 纳米复合永磁 磁化反转 磁粘滞     

Ni47Mn32Ga21多晶合金的磁熵变和磁感生应变

对定向凝固方法制备的Ni₄₇Mn₃₂Ga₂₁多晶合金,通过扫描电镜、金相、电子能谱等手段研究其组份和组织形貌,通过对合金磁化强度与温度关系、等温磁化曲线及磁感生应变曲线等的测量分析,研究了合金结构相变和磁相变过程中的磁熵变及不同压力下的磁感生应变. 研究结果表明:合金组份与设计组份基本一致,室温下合金大部分为马氏体相. 升温过程中合金的磁熵变在居里温度(365 K)附近有最大值,并有较大的磁熵变峰值半高宽,747 kA/m的磁场下该磁熵变最大值为-1.45 J/kg ·K,磁熵变峰值的半高宽为21 K. 合金在室温(298 K)下有较好的双向可恢复磁感生应变,480 kA/m磁场下,无压力时合金的磁感生应变值达到-670×10^-6,并趋饱和;而在与磁场方向平行的27.3 MPa外压力作用下合金的磁感生应变值增大到-1300×10^-6,且未饱和. 关键词： Ni-Mn-Ga 铁磁形状记忆合金 磁熵变 磁感生应变     

Anisotropic magnetomechanical effect in Tb0.3Dy0.7Fe2 alloy

In this paper, domain rotations in Tb_0.3Dy_0.7Fe₂ alloy under a compressive stress applied along various crystallographic axes alone have been investigated on the basis of 3D Stoner-Wohlfarth model by following the conventional free energy minimization procedure. The dependence of both the domain rotations and the strains caused by the compressive stress on the stress directions has been revealed. And it has been found that the anisotropic magnetomechanical effect arises from the dependence of the stress induced anisotropy on the stress direction. This study is very helpful for the better understanding of magnetic behavior of magnetostrictive materials under both stress and field.     

Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 grown on (001) oriented SrTiO3 is studied with in-plane and out-of-plane orientations of the cooling magnetic field, with respect to the substrate plane. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the SrRuO3 layer in the vicinity of interfaces by the antiferromagnetic SrMnO3 layer.     

Magnetization curve and magnetic correlations in a nanochain of ferromagnetic grains with random anisotropy

The magnetization curve and magnetization correlation function are calculated for a ferromagnetic chain of single-domain nanoparticles with a randomly oriented anisotropy axis for different ratios between the exchange correlation and anisotropy energies. It is shown that the coercive force decreases as the exchange correlations increase. For strong exchange correlations, the magnetization curve is described by the following three successive magnetization processes as the applied field is increased: (i) nonuniform rotation of the magnetization of stochastic domains, (ii) collapse of the magnetic solitons, and (iii) nonuniform rotation of exchange-correlated magnetization vectors of the nanoparticles. For high fields, the calculated correlation function of the transverse magnetization components coincides with that predicted from linear theory. At low and zero fields, the main parameters of the correlation function (the variance and correlation radius) tend to certain finite values rather than diverge (as is the case in linear theory). The irreversible variation in the magnetization at low fields (the hysteresis loop) and the hysteresis of the main parameters of the correlation function are calculated.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Magnetization reversal mechanisms in hybrid resin-bonded Nd–Fe–B magnets

The magnetic properties of isotropic epoxy resin-bonded magnets prepared by mixing a hard magnetic powder made from melt quenched Nd–Fe–Co–B ribbons and a soft magnetic iron powder have been examined. The magnetization reversal processes and the magnetic parameters have been studied by the measurement of the virgin magnetization curves, the major and minor hysteresis loops and sets of recoil curves. From these recoil curves the field dependence of the reversible and irreversible magnetization components during the magnetization and demagnetization processes has been derived. The remanence relationship was used to study the nature of magnetic interaction between the grains. A study of interaction domains was conducted using optical microscopy. Groups of domains, each over several grains, were observed. It was found that the reversal process in the samples investigated involves the rotation of magnetization vectors in the iron powder grains and pinning of domain walls at the MQP-B grain boundaries.     

Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.