La0.67 Pb0.33 MnO3 的Preisach分析

测量了居里点TC=36 0K的钙钛矿La0 6 7Pb0 33MnO3在 2K 相似文献   

(La0.2Nd0.8)0.67Pb0.33MnO3的Preisach分析

测量了居里点为TC=220K的钙钛矿(La0.2Nd0.8)0.67Pb0.33MnO3 (LNPMO)在外场为ha=3978.87A/m ，2K相似文献   

稀磁半导体Hg0.89Mn0.11Te磁化强度及磁化率的研究

利用MPMS-7(magnetic property measurement system)型超导量子磁强计对垂直布里奇曼法生长的Hg_0.89Mn_0.11Te晶片磁化强度变化规律进行了测量.试验采用了两种不同的外场和冷却条件.首先在5 K恒温下，-5200到5200 kA/m范围内改变磁场强度进行了测定.然后维持800 kA/m恒定磁场，分别在有场冷却和无场冷却条件下，从5到300 K范围内改变温度，研究了变温条件下的磁化特性.并采用分子场近似模型，用类布里渊函数，最小二乘法对磁化强度随磁场强度变化的实验结果进行拟合和分析，结果表明，Mn²⁺离子之间存在反铁磁相互作用.磁化率和温度关系分析表明：在测试范围内Hg_0.89Mn_0.11Te是单一的顺磁相，在高温区磁化率和温度服从居里-万斯定律，呈线性关系，低于40 K时，磁化率和温度的关系偏离居里-万斯定律，表现出顺磁增强现象. 关键词： 0.89Mn_0.11Te')" href="#">Hg_0.89Mn_0.11Te 磁化强度 磁化率 类布里渊函数     

Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3的磁性和输运性质

利用固相反应法制备了Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3(x=0～0.06)的多晶样品,探讨了Ru掺杂对体系结构,输运性质以及磁电阻的影响.多晶X射线衍射证实所有样品均保持简单立方钙钛矿结构.通过零场冷却(ZFC)和加场冷却(FC)下的磁化曲线的测量发现随温度降低样品发生了顺磁(PM)到铁磁(FM)的相变,且样品的居里温度(Tc)随Ru掺杂发生了显著的变化,从x=0.00时的306.7K,下降到x=0.02时的294.3K,紧接着又上升到x=0.04时的302.4K.测得居里温度明显高于La0.7Ca0.2Ba0.1Mn1-xMoxO3体系,而且其磁性也大为增强.由零场和外加磁场H=1T测量得到样品的ρ～T曲线表明随温度降低样品同时发生了从绝缘体到金属的转变,绝缘体-金属转变温度低于相应的居里温度.适量的Ru掺杂降低了样品的电阻率,增强了低温时的磁电阻.     

SrRuO_3样品的制备及磁学性质的研究

利用化学共沉淀法制备了SrRuO_3样品.对样品的结构和磁学性质作了研究.X射线衍射实验表明样品均为单相钙钛矿结构氧化物.用振动样品磁强计(VSM)和Quantum Design PPMS 9系统测量了样品的磁性能.磁化强度随温度的变化关系曲线(M~T)和d M/dT关系曲线表明SrRuO_3样品在161K温度以下具有铁磁性.磁滞回线、FC和ZFC曲线表明SrRuO_3样品具有很强的磁晶各向异性.     

相分离Gd_5Ge_4合金温度依赖的多重磁转变行为研究

系统研究了Gd5Ge4合金的结构和磁特性的温度依赖关系,结果表明,实验样品Gd5Ge4室温下具有典型的pnma空间群正交结构,通过测量该合金场冷(FC)和零场冷(ZFC)下的热磁曲线以及交流磁化率随温度变化曲线,结合不同温度下的磁滞回线(M～H),观察到当外加磁场为1T时,体系在几个特征温度T1=10K、T2=26K、T3=127K和T4=235K附近磁化强度出现了异常变化,反映在这些特征温度下的体系出现了复杂的多重磁转变行为,初步分析了在各特征温度附近发生的磁相变的属性,探讨了相分离中铁磁成分与温度的关联,其结果对扩展包括强关联锰氧化物在内的相分离体系物理机理的理解具有一定的参考价值和借鉴意义。     

Y_(0.125)Ca_(0.875)MnO_3体系的输运性质与铁磁自旋玻璃行为研究

研究了类稀土锰氧化物Y0.125Ca0.875MnO3的输运性质和磁特性。结果表明,外加磁场对体系的电输运性能有较大影响,在低温时样品表现出磁电阻效应。在外磁场的诱导下,随温度的降低,体系发生了明显的绝缘-金属(I-M)相变,并且在25K出现了类近藤现象。热磁测量表明零场冷(ZFC)和场冷(FC)的磁化强度曲线在118K出现分叉现象。测量了不同频率下的升降温过程的交流磁化率,结果显示在108K附近出现自旋玻璃态。     

超顺磁纳米γ-Fe_2O_3/SiO_2复合材料的制备和磁性能研究

本文以正硅酸乙酯(TEOS)、Fe(NO3)3.9H2O、无水乙醇(Eth)、盐酸(HCl)和去离子水作为原料,以十六烷基三甲基溴化铵(CTAB)为模板剂,采用溶胶-凝胶法制备了纳米γ-Fe2O3/SiO2复合材料.主要研究了CTAB、热处理温度以及Fe2O3的浓度对纳米γ-Fe2O3/SiO2复合材料的形成及磁性能的影响.用X射线衍射分析(XRD)对纳米粒子进行表征以及用Quantum Design Model物理性质测量系统(PPMS)测量纳米颗粒的零场冷却(ZFC)和加场冷却(FC)时的磁化强度随温度的变化关系.通过对XRD衍射图和ZFC/FC曲线分析,可知制备纳米γ-Fe2O3/SiO2复合材料最佳热处理温度为700℃左右,Fe2O3最佳浓度为30wt%左右.尤其加CTAB改性后,所得的纳米γ-Fe2O3/SiO2复合材料较纯正和表现出超顺磁性.     

Dy的高掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La07-xDyxSr03MnO3(x=000，030，040，050，060，070)体系的M_T曲线，M_H曲线，ρ_T曲线和MR_T曲线. 实验结果表明：随着Dy掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态、反铁磁及 亚铁磁状态转变. x=030样品的M_T曲线上在TN附近出现磁化强度尖峰， x=040时尖峰消失. 对x=050样品，ZFC的M_T曲线随温度增加出现一个谷，随 后在TN处出现峰；而FC的M_T曲线在低温下呈现负磁矩. 对x=060和070 样品，不论FC还是ZFC的M_T曲线都类似于x=050的ZFC的M_T曲线. 高掺杂时的输 运性质在其磁背景下发生异常. 这些奇异现象用Nel的双格子模型并联合M_H回滞曲线 给予很好地解释. 关键词： 磁结构 输运行为 庞磁电阻效应     

离子注入金红石单晶生成的金属Ni纳米晶的磁学性能研究

研究了能量为64keV、注量1×1017cm-2的Ni离子注入金红石TiO2单晶制备的植入金属纳米晶的微观结构和磁学性能。注入层的结构和磁学性能采用透射电子显微分析（TEM）和超导量子干涉磁强计（SQUID）进行分析。结果表明，金红石单晶中有尺寸为3～18nm的金属Ni纳米晶生成，注入区域基体明显非晶化。10K温度下金属Ni纳米晶的矫顽力约为16.8kA·m-1，比Ni块材的矫顽力大。样品的零场冷却/有场冷却（ZFC/FC）曲线表明，金属Ni纳米晶的截止温度约为85K。     

Exchange bias in bulk layered hydroxylammonium fluorocobaltate (NH?OH)?CoF?

The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.     

Exchange bias in ferrimagnetic-antiferromagnetic nanocomposite produced by mechanical attrition

Nanoparticles comprising of magnetite (Fe₃O₄) core-hematite (α-Fe₂O₃) shell with mean diameter around 9 nm were synthesized by mechanical grinding. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization and hysteresis loop measurements were carried out over the temperature range 5-300 K. Exchange bias varying from 140 to 10 Oe was observed. This is explained as arising due to ferrimagnetic-antiferromagnetic coupling at the nanointerfaces between the two phases. Analysis of data shows a pronounced increase in the effective anisotropy constant as the milling time is increased.     

Magnetization processes in nanocrystalline gadolinium

The thermal decline in magnetization, M(T), at fixed magnetic field (H) under 'zero-field-cooled' (ZFC) and 'field-cooled' (FC) conditions, the time evolution of ZFC magnetization, M(ZFC)(t), at fixed temperature and field, M(H) hysteresis loops/isotherms, and ac susceptibility have been measured on polycrystalline Gd samples with average grain sizes of d = 12 and 18 nm. The irreversibility in magnetization, M(irr), occurring below a characteristic temperature that reduces with increasing H, is completely suppressed above a grain-size-dependent threshold field, H*. At low fields (H ≤ 100 Oe), M(irr)(T), like the coercive field, H(c)(T), exhibits a minimum at ～16 K and a broad peak at ～50 K before going to zero at T ? T(C) (Curie temperature). At fixed temperature (T < T(C)) and field (H ? H*), where M(irr) is finite, M(ZFC) has a logarithmic dependence on time. The magnetic viscosity (S) at H = 1 Oe and T ≤ 290 K is independent of the measurement time above ～2 ms but for t < 2 ms it is strongly time-dependent. S(T) peaks at T ? T(C) for H = 1 Oe. A magnetic field reduces the peak height and shifts the peak in S(T) to lower temperatures. All the above observations are put on a consistent theoretical footing within the framework of a model in which the intra-grain magnetizations overcome the energy barriers (brought about by the intra-grain and grain-boundary/interfacial magnetic anisotropies) by the thermal activation process. These field- and temperature-dependent energy barriers, that separate the high-energy metastable (ZFC) state from the stable minimum-energy (FC) state, are independent of time for t ? 2 ms and have a very broad distribution. We show that the shape anisotropy plays a decisive role in the magnetization reversal process, and that the magnetocrystalline and magnetostatic fluctuations, prevalent in the grain-boundary and interfacial regions, govern the approach-to-saturation of magnetization in nanocrystalline Gd.     

Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.     

Large vertical loop shifts in mechanically synthesized (Mn,Fe)2O3−t nanograins

Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)₂O_3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.     

Anomalous magnetic properties of an iron film system deposited on fracture surfaces of α-Al2O3 ceramics

An iron film percolation system is fabricated by vapour-phase deposition on fracture surfaces of α-Al2O3 ceramics. The zero-field-cooled （ZFC） and field-cooled （FC） magnetization measurement reveals that the magnetic phase of the film samples evolve from a high-temperature ferromagnetic state to a low-temperature spin-glass-like state, which is also demonstrated by the temperature-dependent ac susceptibility of the iron films. The temperature dependence of the exchange bias field He of the iron film exhibits a minimum peak around the temperature T=5 K, which is independent of the magnitude of the cooling field Hcf. However, for T 〉 10K, （1） He is always negative when Hcf=2kOe and （2） for Hcf= 20 kOe （1Oe≈80 A/m）, He changes from negative to positive values as T increases. Our experimental results show that the anomalous hysteresis properties mainly result from the oxide surfaces of the films with spin-glass-like phase.     

Direct observation of chiral susceptibility in the canonical spin glass AuFe

The extraordinary Hall resistivity rho(xy) and the magnetization M of a canonical spin glass AuFe (8 at.% Fe) were measured simultaneously as functions of temperature with the best care to the thermal and the magnetic field hysteresis. The data of rho(xy) show an anomaly at the spin glass transition temperature T(g) and have different zero field cooling (ZFC) and field cooling (FC) measurements below T(g). Moreover, the value of rho(xy)/M, which represents the chiral susceptibility of the system in the present case, also shows the difference between ZFC and FC measurements. The results are consistent with the predictions of the chirality scenario of canonical spin glasses by Kawamura.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Magnetic properties of iron loaded MCM-48 molecular sieves

Mesoporous molecular sieves of MCM-48 type were loaded with iron by the wet impregnation method, using Fe(III) nitrate or Fe(II) sulfate aqueous solutions as Fe sources, to obtain a magnetic porous composite. The iron loaded materials were characterized by XRD, N₂ adsorption and DRUV-vis and compared with the Si-MCM-48 host. Their magnetic properties were studied by measuring the hysteresis loops up to 1.5 T at different temperatures (5-300 K) and by magnetization vs. temperature curves following the conventional zero field cooling (ZFC) and field cooling (FC) protocols. Materials with high structure regularity and surface area are obtained, which exhibit a mixed paramagnetic and superparamagnetic behavior, arising in isolated iron ions inserted in the host framework, and in small iron oxide clusters or nanoparticles forming inside the pores, respectively. Larger hematite particles (8-13 nm) grown on the external surface provide a quite small ferromagnetic contribution to the hysteresis loop.