Fe NMR, magnetization and X-ray study of Cd0.1Cu0.9Fe2O4 ferrite

⁵⁷Fe NMR spin-echo spectra, static magnetization and X-ray diffraction measurements for Cd_0.1Cu_0.9Fe₂O₄ ferrite taken in a wide range of temperatures are presented. The X-ray studies show that above 100 K the structure is cubic. The temperature dependence of the effective magnetic field acting on ⁵⁷Fe nuclei at non-equivalent tetrahedral and octahedral sites has been determined. Different thermal behaviour of the magnetic field at both sublattices and of the specific macroscopic magnetization have been found. Magnetization measurements imply that cadmium substitution does not affect the ferric ion distribution between the sublattices of the ferrite.     

Rotational magnetization processes in exchange biased Ni81Fe19/Fe50Mn50 bilayers

The rotational magnetization process of an exchange coupled Ni₈₁Fe₁₉(10 nm)/Fe₅₀Mn₅₀(10 nm) bilayer was studied by Kerr microscopy. The domain processes in rotating magnetic fields near the exchange bias field H_eb are very sensitive to local variations of coupling strength and direction. A characteristic domain splitting was found that shows a remarkably different behavior for weaker and stronger coupled areas. While the magnetization in weaker coupled areas follows the rotating field for H≈H_eb, the stronger coupled areas switch back spring-like. As a result high-angle walls are formed between both areas causing rotational hysteresis.     

Magnetic properties of the hexagonal NdNi5 and NdCu5 compounds

Magnetization and resistivity measurements performed on the hexagonal NdNi₅ and NdCu₅ compounds are presented. For NdNi₅, magnetization was measured on a single crystal along the three main axes of the orthohexagonal cell. This compound orders ferromagnetically at T_c = 7 K, a being the easy magnetization axis. Quantitative analysis of the experimental data led to the determination of the 4f-Nd and 3d-Ni contributions to the magnetization. In particular crystal field parameters were determined which account for inelastic neutron scattering data. Two original features of the compounds magnetic behaviour due to crystal field effects are noted: i) in the basal plane a large anisotropy of the magnetic moment which strongly increases with the magnetic field between 11 and 130 kOe; ii) a large decrease of the Nd moment during the rotation toward the c axis driven by the field. NdCu₅ orders also ferromagnetically at 15 K. The Nd moment lies perpendicular to c, and its reduction by the crystal field seems smaller than in NdNi₅.     

Low temperature magnetotransport properties of La0.55Ho0.15Sr0.3MnO3

Magnetotransport data measured in thin films of La_0.55Ho_0.15Sr_0.3MnO₃ down to very low temperatures (0.25 K) are reported. The samples presented colossal magnetoresistance with a T_C close to 200 K. A minimum in the resistance vs. T curve and a drop in the ZFC magnetization were also observed. It was also found a T-dependent relaxation effect after the magnetic field was either applied or removed. These results can be understood within the framework where electronic scattering occurs across magnetic domain walls in a reentrant spin-glassy-like phase.     

磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中取向行为的影响

实验研究了磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中(Tb,Dy)Fe_2相取向行为及合金磁性能的影响.结果表明,将强磁场作用于Tb_(0.27)Dy_(0.73)Fe_(1.95)合金的凝固过程可以制备出(Tb,Dy)Fe_2相沿111取向的组织,同时显著提高了合金的磁致伸缩性能;通过提高磁感应强度可以在更快的冷却速率下得到111取向的组织;在4-10 T范围内,随着冷却速率的增加,(Tb,Dy)Fe_2相沿111取向所需的磁感应强度增加,而发生(110)取向的磁感应强度减小.随着冷却速率的增加,合金的饱和磁化强度增加,而强磁场的施加对合金饱和磁化强度的变化没有明显影响.(Tb,Dy)Fe_2相的取向行为受*Tb,Dy)Fe_3相取向行为的影响,且由磁晶各向异性能与磁场作用时间共同控制.     

Spin-polarized tunneling in nanostructured La2/3Sr1/3MnO3

We present magnetic and transport properties of nanocrystalline La_2/3Sr_1/3MnO₃ powders prepared by a gel-combustion method using citric acid as the fuel. The coercive magnetic field H_c is significantly different to the field H_c^* for which the magnetoresistance (MR) is maximum. The MR at low fields (LFMR) exhibits a power-law dependence with magnetization, MR∝Mⁿ, with n=2.5–3.3 for temperatures ranging from 5 to 200 K. The results are discussed in terms of a distribution of particle size in our sample.     

Structural and magnetic characterization of Co67Fe4Ni2Si15B12

 Soft magnetic properties of Co-based amorphous alloy of the composition Co₆₇Fe₄Ni₂Si₁₅B₁₂ have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Harmonic generation in TlBa2Ca3Cu4Oy superconductor

We have studied harmonic generation in the magnetic response of bulk samples of the ceramic superconductor TlBa₂Ca₃Cu₄O_y. Dependence of the odd and even harmonics of magnetization on the DC magnetic field, amplitude of the AC magnetic field and temperature was investigate experimentally. We have used a critical state model recently developed by Müller et al. to calculate the full harmonic response. At fixed temperature, the dependence of the odd order harmonic response on the amplitude of the AC magnetic field H_m shows an unambiguous crossover from an H²_m behaviour (Bean-like) at small amplitudes to an H³_m behaviour for larger H_m, in quantitative agreement with the modified critical state model.     

Tb掺杂双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7的磁熵变和电输运性质

研究了Tb掺杂对双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7磁熵变和电输运性质的影响.样品采用传统固相反应法制备,两样品的名义组分可以表示为(La_(1-x)Tb_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.025),磁场为7 T时的最大磁熵变?S_M分别为-4.60 J/(kg·K)和-4.18 J/(kg·K).比较后发现,Tb元素的掺杂使得最大磁熵变值减小,但同时增大了相对制冷温区.电性测量结果表明,x=0.025的样品在高温区的导电机制可以用小极化子模型解释,与母体三维变程跳跃模型不同;当温度降低至三维长程铁磁有序温度(T_c~(3D))附近时,掺杂样品发生金属绝缘相变;掺杂后样品在T_c~(3D)附近,磁电阻取得极大值(约为56%),表明是本征磁电阻效应.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.     

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

The heat release has been measured in high temperature superconductors during and after a change of the external magnetic field μ₀H_a ≤ 0.5 T applied parallel to the c-axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured ∝ H_a²dH_a/dt. This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t⁻¹ dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.     

Magnetic phase transition in (Fe,Mn)65Ni35 alloys

Magnetization measurements on the Fe₆₀Mn₅Ni₃₅ and Fe₅₀Mn₁₅Ni₃₅ alloy samples were carried out in the temperature range 80T300 K and in magnetic fields up to 8 kOe. The Fe₆₀Mn₅Ni₃₅ was found to order ferromagnetically with a Curie temperature, T_c, above 300 K. From the temperature dependence of the spontaneous magnetization, M_s, it was concluded that the magnetic behavior of Fe₆₀Mn₅Ni₃₅ follows Wohlfarth theory of weak itinerant ferromagnet. The Fe₅₀Mn₁₅Ni₃₅ sample exhibits a magnetic phase transition from ferromagnetism to paramagnetism at T_c=242 K. The critical amplitudes and critical exponents (β, γ and δ) have been determined by using Arrott plots, Kouvel–Fisher method and scaling plots of the reduced magnetization and reduced magnetic field. The values of β, γ and δ are discussed and compared with the results obtained for various theoretical models and also with the experimentally determined values for related systems obtained by others.     

Low-field negative magnetization and coercive-field magnetization reversal in transition metal chalcogenides; Cr2FeSe4 magnetic structure from neutron diffraction

Low-field negative magnetization, of the order of −10⁻¹ emu/g-Oe, from 4.2 K up to room temperature and higher (350 K), and coercive-field magnetization reversal are both present in Cr_(3−x)Fe_xX₄ for X=S, Se, Te and x=0 to 3, and for Cr₅Te₈ and Cr₇Te₈. For Cr₂FeSe₄ the zero-field-cooled (ZFC) magnetization is negative for 5 Oe and below. To obtain a more detailed knowledge of the magnetic phases involved in the observed magnetization versus temperature M(T) curves, we obtained and studied neutron diffraction (n.d.) scans on the compound Cr₂FeSe₄, taken at 14 temperatures from 4.2 to 300 K. For this same n.d. sample, the temperature for magnetization reversal of value −3×10⁻⁴ emu/g-Oe is 80 K in 40 Oe applied field, then the reversal disappears for 65 Oe applied field. The complex magnetic interactions responsible for this reversal are revealed in the hysteresis curves.     

Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f=10 to 10,000 Hz. A prominent peak appears in both χ′ and χ″ as a function of T. The peak temperature T₂ of χ″ depends on f following the Vogel–Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T_B^m94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K=6×10⁶ erg/cm³. Mössbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mössbauer spectrum was fitted with two magnetic subspectra with internal fields H_int of 490, 470 and 515 kOe, corresponding to Fe³⁺ ions in A and B sites.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

Magnetic anisotropic properties in Fe3O4 and CoFe2O4 ferrite epitaxy thin films

Epitaxial thin films of Fe₃O₄ and CoFe₂O₄ on MgO (0 0 1) substrates were grown by molecular beam epitaxy at low temperature growth process. Magnetization and hysteresis loop of both films were measured to investigate magnetic anisotropic properties at various temperatures. Anomalous magnetic properties are found to be correlated with crystalline, shape, and stress anisotropies. The Fe₃O₄ film below Verwey structural transition has a change in crystal structure, thus causing many anomalous magnetic properties. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. The saturation magnetization of Co–ferrite film becomes much lower than that of Fe₃O₄ film, being very different from the bulks. It indicates that the low temperature growth process could not provide enough energy to have the lowest energy state.     

Sm_2Co_(17)型永磁合金的辐照效应研究

Sm_2Co_(17)型永磁合金大量使用在上海同步辐射光源储存环的永磁型波荡器上,在受到长期辐照后会发生磁性能损失的现象,进而影响同步辐射光的品质.为了探索其潜在的微观机理,本文首先对Sm_2Co_(17)型永磁合金所处混合辐射场的粒子及相关物理量进行了计算分析,确定引发磁性能损失的主要粒子是中子.然后采用Ar离子模拟中子辐照损伤的方法对其进行辐照,采用透射电镜对其辐照前后的微观形貌及微观结构进行了研究探讨,采用振动样品磁强计对永磁合金辐照前后的饱和磁化强度进行了分析对比,并讨论了微观结构演化与宏观磁性能变化的联系.结果表明,Ar离子辐照后Sm_2Co_(17)型永磁合金饱和磁化强度的不可逆损失与微观结构变化有直接的关系,其2:17相从单晶结构转变为非晶结构是造成其磁性能损失可能的微观机制.     

Irreversibility line and anisotropy of magnetic melt textured Y1Ba2Cu3O7−δ studied by AC susceptibility

Magnetically melt textured YBa₂Cu₃O_7−δ samples have been studied by AC initial susceptibility measurements in the low field limit. The critical dynamic fields h^*_c and the relative position of the irreversibility line and of the critical line show that magnetic melt texturing leads to the creation of strong pinning sites. The anisotropy factor A is in agreement with the value obtained from magnetization under high DC fields. The A values are lower than in single cystals, but in agreement with the important orientation effects introduced by magnetic field texturing. These values also are typical of fabrication techniques involving a melting at some stage.