Sm(CobalFe0.1Cu0.1Zr0.033)6.9高温永磁合金的矫顽力

对Sm(Co_balFe_0.1Cu_0.1Zr_0.033)_6.9合金, 经810℃等温时效后以0.5℃/min逐渐冷却, 在600℃-400℃温度区间淬火, 研究了不同淬火温度下的磁滞回线、磁畴和矫顽力温度系数β. 发现时效600℃淬火后磁滞回线出现台阶状, 说明畴壁中应存在两处钉扎. 随淬火温度的降低, 合金的室温矫顽力显著增加, 磁滞回线的台阶消失. 通过磁畴形貌发现时效600℃淬火后的磁畴接近条形畴, 1:5相中Cu分布相对均匀, 形成的畴壁钉扎较弱, 从而使磁滞回线出现台阶, 决定矫顽力的畴壁钉扎位于两相界面处; 随时效淬火温度的降低, 磁畴逐渐细化, 畴壁1:5相中的畴壁能降低, 形成了较强的内禀钉扎, 并决定材料的矫顽力, 两相界面处的畴壁钉扎被掩盖. 对不同温度淬火合金的高温矫顽力研究表明, 最强的畴壁钉扎位于两相界面处时, 矫顽力随温度升高逐渐增加, 矫顽力出现温度反常现象; 最强的畴壁钉扎位于1:5相中心时, 矫顽力随温度升高逐渐衰减. 当测试温度达到500℃后不同淬火温度样品的矫顽力几乎相同, 此时最强畴壁钉扎均在两相界面处.     

低温时效处理对铁磁形状记忆合金Mn2NiGa的结构、相变和磁性能的影响

对在较低温度范围的时效处理铁磁形状记忆合金Mn₂NiGa的结构、相变和磁性进行了研究.研究发现,母相基体析出了细小的析出相,引起了晶格扭曲和畸变,导致了系统内产生了很大的内应力.在其浓度超过晶格的容忍度之后,提升了体系的马氏体相变温度,使母相在时效温度下转变成马氏体相,并在其中测量到高达900 Oe的矫顽力.由于这种马氏体相的逆相变温度大幅提高,外推获得其居里温度在530 K附近.细小析出相的粗化使内应力消失,样品又回到母相状态.观察到细小析出相粗化的两个阈值温度,分别为423 K和 关键词： 铁磁形状记忆合金 2NiGa')" href="#">Mn₂NiGa 时效处理 内应力     

分子束外延制备的垂直易磁化MnAl薄膜结构和磁性

系统地研究了利用分子束外延方法在GaAs(001) 衬底上外延生长的MnAl_x薄膜的结构和垂直易磁化特性随组分及生长温度的依赖关系. 磁性测试表明, 可在较大组分范围内 (0.4≤x≤1.2) 获得大矫顽力的垂直易磁化MnAl_x薄膜, 然而同步辐射X射线衍射和磁性测试发现当x≤0.6时MnAl薄膜出现较多的软磁相, 当x >0.9时, MnAl薄膜晶体质量和化学有序度逐渐降低, 组分为MnAl_0.9时制备的薄膜有最好的[001]取向. 随着生长温度的增加, MnAl_0.9薄膜的有序度、垂直磁各向异性常数、矫顽力和剩磁比均增加, 350℃时制备的MnAl_0.9薄膜化学有序度高达0.9, 其磁化强度、剩磁比、矫顽力和垂直磁各向异性常数分别为265emu/cm³、93.3%、8.3kOe (1 Oe=79.5775A/m)和7.74Merg/cm³ (1 erg=10^-7J). 不含贵金属及稀土元素、良好的垂直易磁化性质、 与半导体材料结构良好的兼容性以及磁性能随不同生长条件的可调控 性使得MnAl薄膜有潜力应用于多种自旋电子学器件. 关键词： 分子束外延 大矫顽力材料 磁各向异性     

时效时间对FeNiAlTa形状记忆合金组织结构和性能的影响

本文以 Fe_59.5Ni₂₈Al_11.5Ta₁ 形状记忆合金为研究对象, 采用金相显微镜、X 射线衍射仪、扫描电镜、能谱仪和压力试验机等研究了轧制后不同时 效时间处理对该合金组织结构和性能的影响. 结果表明, 随着时效的进行, γ’ 相和 β’ 相的相继析出, 强化了奥氏体基体. 综合伪弹性曲线看出, 随着时效时间的增加, 600 ℃时效态合金的应力诱发马氏体临界应力先减小后增大, 合金的抗压强度、可恢复的应变和硬度都先增大后减小, 合金的残余应变则先减小后增大, 时效时间为 60 h 时, 合金的抗压强度最大, 到达1306 MPa, 此时合金的可恢复形变最大, 达到14.9%, 合金的硬度也最大, 合金的残余应变相对最小. 但随着时效时间的延长, 合金的最大应变逐渐减小, 合金塑性逐渐减小. Fe_59.5Ni₂₈Al_11.5Ta₁ 形状记忆合金的性能与沉淀相的颗粒大小、分布、体积分数等因素有关. 关键词： 59.5Ni₂₈Al_11.5Ta₁')" href="#">Fe_59.5Ni₂₈Al_11.5Ta₁ 时效处理 伪弹性 硬度     

Pr2Fe14(C,B)/α-(Fe,Co)型纳米晶复合磁体的结构与磁性

研究了Pr_xFe_82-x-yTi_yCo₁₀B₄C₄ (x=9—10.5;y=0, 2)纳米晶薄带的结构与磁性. 结果表明,所有薄带皆主要由2∶14∶1, 2∶17和α-(Fe, Co)三相组成. 对于y=0的合金,其内禀矫顽力随Pr含量x的增加而增加,剩磁随Pr含量x的增加而减小. 以Ti置换部分Fe (y=2),合金的磁性能得到显著提高,表现为:添加Ti后,合金的剩磁B_r基本不降低,x=10.5时合金的B_r值甚至有较明显的提高;同时添加Ti后,合金的内禀矫顽力及退磁曲线的方形度都明显改善. 当x=10.5,y=2时,合金薄带的磁性能达到最佳值为: B_r=9.6 kGs(1 Gs=10^-4 T),_iH_c =10.2 kOe(1 Oe=79.5775 A/m)和(BH)_max＝17.4 MGOe. 随着Pr含量的提高,合金中的硬磁相2 ∶14 ∶1的含量相对增加,内禀矫顽力提高;而Ti置换Fe抑制了软磁相α-(Fe, Co)在快淬和热处理过程中的优先长大,使合金中软磁相和硬磁相的晶粒尺寸及比例趋向最佳组合,交换耦合作用明显增强. 关键词： 纳米晶永磁材料 2Fe₁₄(C')" href="#">Pr₂Fe₁₄(C B) Ti添加 交换耦合     

非平衡Fe-Pd合金的矫顽力与磁致伸缩

通过测量非平衡Fe_1-xPd_x合金的矫顽力H_c与饱和磁致伸缩系数λ_s,随合金成分的变化,发现H_c随x增加而减小,而λ_s随x增加而增大。认为晶粒尺寸减小引起的平均各向异性的降低可能是H_c减小的原因。退火温度T_a>600K时,由于具有高磁晶各向异性的有序FePd相的分离析出,矫顽力H_c随T_a升 关键词：     

Sm(Co,Cu,Fe,Zr)z反磁化过程的微磁学分析

通过微磁学有限元方法研究了微结构对各向异性的Sm(Co,Cu,Fe,Zr)z磁性能的影响， 并 对不同温度下的退磁曲线进行了计算.计算结果表明，矫顽力随着2∶17相晶粒尺寸的增大 而增大，随1∶5晶界相厚度的增大而减小；通过减小晶界相厚度或增大晶粒尺寸可以有效提 高 磁能积.反磁化的物理机制主要为形核机制，主要表现为首先在晶界相形成反磁化核，随 着 磁场的增大反磁化核不断长大，最后导致整个磁体的磁化反转；而当温度升高时，晶界相逐 渐变成非磁性相，使得反磁化核难以形成，因此出现了反常的矫顽力温度依赖关系. 关键词： 微磁学 有限元 微结构 磁性能     

取向烧结Nd-Fe-B永磁合金反磁化过程及矫顽力机理研究

本文对取向烧结Nd-Fe-B合金沿取向易轴饱和磁化后的反磁化过程分四个阶段进行了理论研究。结果表明,主相晶粒表面软磁性区成核及从表面向晶粒内部不可逆畴壁位移对Nd-Fe-B合金的矫顽力起决定性作用。矫顽力随温度升高而急速下降主要是由于热运动破坏了主相四方结构的完整性,从而使软磁性过渡区变厚所致。 关键词：     

利用[Fe/Pt]n多层膜降低L10-FePt有序化温度

采用直流磁控溅射方法制备了Fe/Pt多层膜和FePt单层薄膜,再经不同温度真空热处理得到了有序相Ll0-FePt薄膜.通过x射线衍射谱和磁性研究表明,FePt单层薄膜需要在500℃以上热处理,才能开始有序化转变,而Fe/Pt多层膜可以降低FePt薄膜有序化温度.[Fe(1.5 nm)/Pt(1.5 nn)]13薄膜在350℃热处理后,有序度已经增加到0.6,相应矫顽力达到了501 kA/m.多层膜化促进有序化在较低的温度下进行,这是由于热处理过程中多层膜界面的消失提供了有序化过程额外的驱动力.     

Cr/Ru/PtCo/Ru/PtCo/Ru介质中底层Ru厚度对磁记录特性影响的研究

讨论了Cr/Ru（1）/PtCo（稳定层）/Ru（2）/PtCo（记录层）/Ru（3） 结构的矫顽力Hc_c与层间反铁磁耦合交换场Hex_ex随Ru（1）与Ru（2）厚度变 化的规律.研究发现 ，样品的矫顽力及交换场随Ru（1）厚度增加而增大， 这可能是由Ru（1）hcp结构引起的. 矫顽力及交换场在Ru（2）厚度为08nm处有峰值. 关键词： 磁记录 反铁磁耦合     

Fabrication and magnetic properties of highly ordered Co16Ag84 alloy nanowire array

Highly ordered Co-Ag alloy nanowire arrays embedded in the nanochannels of anodic alumina membranes (AAMs) were successfully fabricated using electrodeposition. Scanning electron microscopy and transmission electron microscopy observations revealed that the ordered Co-Ag alloy nanowires were uniformly assembled into the hexagonally ordered nanochannels of the AAMs. Magnetic measurements showed that the perpendicular coercivity (H_c⊥) of the ordered nanowire arrays increased dramatically as the annealing temperature (T_a) rose from 300 °C, reached its maximum (183 Oe) at 400 °C and then decreased sharply as T_a further increased beyond 400 °C. However, there was little change in the parallel coercivity (H_c∥) of the nanowire arrays during the annealing process. The mechanism of this phenomenon was attributed to the special structure of the AAMs and nanowires. Received: 27 November 2000 / Accepted: 3 May 2001 / Published online: 25 July 2001     

Effect of short-range atomic order on macroscopic magnetic properties of equiatomic FePd alloy

X-ray diffraction, magnetic measurements, and Mössbauer spectroscopy were employed to comparatively analyse the relation of the basic characteristics of highly anisotropic materials– coercive force H_c and Curie temperature T_C to the peculiarities of short-range atomic order that forms upon annealing of disordered samples of equaiatomic FePd alloys produced by different techniques (casting, melt-spinning, severe deformation). It is shown that for all samples, independently of methods of their preparation, the ordered states with the maximal values of coercivity are inhomogeneous in the composition of short-ordered regions, type of atomic ordering, and degree of tetragonality. The Curie temperature depends on the temperature and duration of annealing for ordering similarly to the conventional course of coercivity, which is peculiar to these alloys. The behaviour of these macroscopic characteristics (H_c and T_C) in the course of annealing is shown to correlate with changes in the local atomic configurations revealed in the Mössbauer spectroscopy experiments.     

Magnetic properties of hematite with large coercivity

The magnetic properties of hematite powders produced by a solid state nucleation-and-growth process are studied as a function of temperature T and applied field H. Independently of the temperature, there exists a soft magnetic contribution that is assigned to the canting of spins at the superficial shell of each particle and is not affected by the Morin transition. At 220<T<T _M a magnetic contribution with high coercivity is observed, due to spin–flop in the anti ferromagnetic state and above T _M =248 K the weakly ferromagnetic state has a coercivity that ranges from 6 kOe to 4 kOe when raising T up to room temperature. Different sub-grain structures were obtained by means of isochronal and isothermal annealing. Changes in the susceptibility are directly related to the sub-particle size. It is concluded that sub-boundaries are the defects responsible for the high coercivities observed in the weakly ferromagnetic state.     

Magnetic field and external pressure effects on the spiral order of polycrystalline MnCr2O4

In this paper, the magnetic and specific heat properties of polycrystalline MnCr₂O₄ were investigated, mainly by focusing on the spiral order transition around T_s. With increasing magnetic field, the magnetic anomaly around T_s in the M–T curve is gradually suppressed. However, an external magnetic field up to 5 T has no evident influence on the spiral magnetic transition as revealed by specific heat measurements. Upon cooling below T_s, AC susceptibility displays no frequency dependence, but the coercivity increases abruptly. Applying external pressure increases the coercivity at 5 K, implying the strengthening of the spiral order. It is suggested that with decreasing temperature across T_s, the spiral component develops in the direction perpendicular to the easy axis of the parent collinear ferri-magnetic phase and does not contribute to the saturation magnetization.     

X-ray studies of magnetic phase transitions in the intermetallic compounds RMn2Ge2 (R=La, Sm, Gd, Nd, Tb, and Y)

The lattice parameters a and c of the tetragonal intermetallic compounds RMn₂Ge₂ (R=La, Sm, Gd, Nd, Tb, and Y) have been measured by x-ray diffraction in the temperature interval 10–800 K. Anomalies are observed in the temperature dependence of a and c due to phase transitions from the paramagnetic to the magnetically ordered state in the Mn subsystem, transitions between various magnetically ordered phases due to a change in the magnitude and sign of the Mn-Mn exchange interaction, and magnetic transitions caused by ordering of the rare-earth subsystem leading to a rearrangement of the magnetic structure of the Mn subsystem. It is concluded that, along with the lattice parameter a, the lattice parameter c also has an influence on the Mn-Mn exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 41, 2053–2058 (November 1999)     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

Ferromagnetic phases in spin-Fermion systems

Spin-Fermion systems which obtain their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons are considered. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 Fermi operators of itinerant electrons. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferromagnetic phases in the system: low temperature phase 0 < T < T*, where all electrons contribute the ordered ferromagnetic moment, and high temperature phase T* < T < T _C , where only localized spins form magnetic moment. The magnetization as a function of temperature is calculated. The theoretical predictions are utilize to interpret the experimentally measured magnetization-temperature curves of UGe₂.     

First XANES evidence of a disorder–order transition in a spinel ferrite compound: nanocrystalline ZnFe2O4

In situ Zn K‐edge XANES experiments were performed to investigate the thermal evolution of the non‐equilibrium state in nano‐sized ZnFe₂O₄. The initially disordered ferrite was annealed under oxygen atmosphere and kept at temperatures of 673, 773 and 873 K. Modifications of the XANES features allowed the direct detection of the Zn local surrounding changes from O_h to T_d symmetry. Quantitative analyses of these results were performed by using the principal‐component analysis approach. The ferrite inversion does not change until the activation barrier is overcome at T_a = 585 K. Above T_a, the Zn ions continuously change their environment to their normal equilibrium state. Isothermal treatments confirm that the Zn transference follows a first‐order kinetic process. In addition, the thermal treatment produces a partial recrystallization that increases the average grain size from 13 to 50 nm and reduces the microstrain. The room‐temperature magnetic state changes from ferrimagnetic to paramagnetic, while the blocking temperature increases after the treatment.     

The double-phase transition of ErFe4Ge2. An XRPD study

High-quality powder XRD data of the compound ErFe₄Ge₂ collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (T_N=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P4₂/mnm (T_c, T_N=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>T_N for the tetragonal P4₂/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<T_N, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (≈31%) around 30 K and (c) the LT (low temperature) range, 1.5–20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er–Er, Er–Fe and the Fe–Fe exchange interactions and their coupling with the lattice strains.