Observation of giant magnetocaloric effect under low magnetic fields in EuTi_(1-x)Co_xO_3

The magnetic properties and magnetocaloric effect(MCE) in EuTi_(1-x)Co_xO_3(x = 0, 0.025, 0.05, 0.075, 0.1) compounds have been investigated. When the Ti~(4+) ions were substituted by Co2+ions, the delicate balance was changed between antiferromagnetic(AFM) and ferromagnetic(FM) phases in the EuTiO_3 compound. In EuTi_(1-x)Co_xO_3 system, a giant reversible MCE and large refrigerant capacity(RC) were observed without hysteresis. The values of -?S_M~(max) were evaluated to be around 10 J·kg~(-1)·K~(-1) for EuTi_(0.95)Co_(0.05)O_3 under a magnetic field change of 10 kOe. The giant reversible MCE and large RC suggests that EuTi_(1-x)Co_xO_3 series could be considered as good candidate materials for low-temperature and low-field magnetic refrigerant.     

Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)₂I₃ crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH²_a + bH²_b + cH²_c)₀^-3/2/H in terms of H = (H_a, H_b, H_c), the zero field resistivity ₀, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.     

Magnetic anisotropy and pressure dependence of the order temperature in in the Gd3 (FeTi)29 compound

In this work we report ac initial magnetic susceptibility under pressure, singular point detection (SPD) and linear thermal expansion measurements in the Gd₃(FeTi)₂₉ intermetallic compound. From these measurements we have determined the anisotropy field, the order temperature and its pressure dependence. The thermal exapansion measurement shows an Invar-like anomaly at the order temperature, T_C = 520 K, which decreases under increasing pressure with a slope of dT_C/dp = 2.7 K/kbar. As the Gd³⁺ ion is isotropic, no contribution to the anisotropy from the 4f sublattice is and low values of the anisotropy fields have been observed.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Large magnetocaloric effect in van der Waals crystal CrBr3

We study the magnetocaloric effect (MCE) in van der Waals (vdW) crystal CrBr₃. Bulk CrBr₃ exhibits a second-order paramagnetic-ferromagnetic phase transition with T_C = 33 K. The maximum magnetic entropy change −ΔS_M near T_C is about 7.2 J·kg⁻¹·K⁻¹ with the maximum adiabatic temperature change ΔT^max_ad = 2.37 K and the relative cooling power RCP= 191.5 J·kg⁻¹ at μ₀H = 5 T, all of which are remarkably larger than those in CrI₃. These results suggest that the vdW crystal CrBr₃ is a promising candidate for the low-dimensional magnetic refrigeration in low temperature region.     

Ca~(2+)掺杂对SmFeO_3的介电、铁磁特性及磁相变温度的影响

采用固相反应法制备Sm_(1-x)Ca_xFeO_3(x=0,0.1,0.2,0.3)样品,研究Ca~(2+)掺杂对SmFeO_3介电性能、铁磁性及磁相变温度的影响.X射线衍射图谱分析表明:所有样品的主衍射峰与SmFe03相符合且具有良好的晶体结构.随着x的增加,SmFeO_3样品的晶粒尺寸由原来的0.5μm逐渐增大到2μm.当f=1 kHz时,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的ε_r分别是SmFe03的5倍、3倍和2.6倍,而tgσ增大一个数量级.在3T磁场作用下,SmFe03样品的M-H呈线性,随着x的增加,M-H逐渐趋向饱和,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的M_r分别是SmFeO_3的20倍、31倍和68倍.X射线光电子能谱分析表明:Fe~(2+)和Fe3+共存于Sm_(1-x)Ca_xFeO_3样品中,Fe~(2+)/Fe~(3+)比例随着x的增加而增大,证明Ca~(2+)掺杂增加了Fe~(2+)的含量,形成Fe~(2+)—O~(2-)—Fe~(3+)超交换作用,增强SmFe03的铁磁特性.测量了Sm_(1-x)Ca_xFeO_3样品在外加磁场为1000 Oe(1 Oe=79.5775 A/m)的M-T变化关系,观测到其自旋重组温度(T_(SR))和尼尔温度(T_N)分别为438 K和687 K,发现SmFe03样品的T_(SR)和T_N均随着x的增加向低温方向移动,当x=0.3时,自旋重组现象消失.这主要是SmFeO_3样品磁结构的稳定性和Fe~(3+)—O~(2-)—Fe~(3+)及Sm~(3+)—O~(2-)—Fe~(3+)超交换三者共同作用的结果.     

多铁材料Bi1-xCaxFeO3的介电、铁磁特性和高温磁相变

采用溶胶凝胶法制备Bi_1-xCa_xFeO₃ (x=0, 0.05, 0.1, 0.15, 0.2)陶瓷样品. X衍射图谱表明所有样品的主衍射峰均与纯相BiFeO₃相符合且具有良好的晶体结构. 随着x的增大, Bi_1-xCa_xFeO₃样品的主衍射峰由双峰(104)与(110) 逐渐重叠为单峰(110), 当x ≥0.15时, 样品呈现正方晶系结构; 扫描电镜形貌分析可知, 晶粒由原来的0.5 μm逐渐增大到2 μm. Bi_1-xCa_xFeO₃样品介电常数和介电损耗随着x 的增加先增大而后减小. 当f=1 kHz, Bi_0.9Ca_0.1FeO₃ 的介电常数达到最大值, 是BiFeO₃的7.5倍, 而Bi_0.8Ca_0.2FeO₃的介电常数达到最小值, 仅仅是BiFeO₃的十分之一. Bi_1-xCa_xFeO₃样品所呈现的介电特性是由偶极子取向极化和空间电荷限制电流两种极化机理共同作用的结果. 随着Ca²⁺ 的引入, BiFeO₃ 样品的铁磁性显著提高. X射线光电子能谱图表明Fe²⁺和Fe³⁺ 共存于Bi_1-xCa_xFeO₃ 样品中, Fe²⁺/Fe³⁺比例随着Ca²⁺ 掺杂量的增加而增大, 证明Ca²⁺掺杂增加了Fe²⁺的含量, 增强BiFeO₃的铁磁特性. 从M-T曲线观察到BiFeO₃样品在878 K附近发生铁磁相变, 示差扫描量热法测试再次证明BiFeO₃ 在878 K发生相变. Ca²⁺掺杂使BiFeO₃样品的T_N略有变化而T_M基本不变, 其主要原因是Fe-O-Fe反铁磁超交换作用的强弱和磁结构相对稳定.     

Magnetic order in M2Mo3O8 single crystals (M = Mn, Fe, Co, Ni)

Magnetic measurements on hexagonal single crystals of M₂Mo₃O₈ (M = Mn, Fe, Co and Ni) are reported. The Mn compound orders ferrimagnetically at T_c = 41.5 K; the Fe and Co compounds antiferrimagnetically at T_N = 59.5 K and 40.8 K, respectively. No magnetic ordering was found in the Ni compound down to 2 K. All the compounds showed strong magnetic anisotropy when ordered, the results indicating that the magnetic spins' preferred orientation is along the hexagonal axis. Mn₂Mo₃O₈ shows a temperature dependence of the spontaneous magnetization in the ferrimagnetic regime which is rarely observed: as T → 0, M_s → 0.     

Incommensurate antiferromagnetic structures in NdIn3

Among the AuCu₃-type RIn₃ series where R is a rare earth, the NdIn₃ compound presents a complex magnetic phase diagram. In this compound, which orders antiferromagnetically at T_N = 5.9 K, three magnetic phases are separated by two sharp first-order transitions. The phase diagrams, determined for the main crystallographic directions, by magnetization measurements in fields up to 7 T show a strong dependence of the transition temperatures. In order to determine the actual magnetic structures in NdIn₃, a neutron diffraction experiment has been performed on a single crystal under magnetic field. It shows that the magnetic structures are collinear (single q) with the magnetic moments aligned along the fourfold axis. They clearly evidence the existence of two incommensurate phases with q = (1/2, 1/2, τ): for T_N T 5.5 K, τ₁ ≈ 0.037 2π/a and the structure is sine-wave modulated, for 5.5 K > T > 4.7 K, τ₂ = 0.017 2π/a and the structure squares up. Below 4.7 K the q = (1/2, 1/2, 0) commensurate structure is stabilized.     

Mn(SiO2)3(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质及磁性

基于密度泛函理论中的广义梯度近似系统研究M_n(SiO₂)₃(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质和磁学性质.结果表明:Fe,Co原子相对于Ni原子更易于在(SiO₂)₃团簇上聚集;通过分析团簇的分裂途径及其产物,发现稳定性较好的氧化硅是一种很好的用于负载过渡金属"岛膜"的载体材料;M_n(SiO₂)₃团簇的能隙恰好位于近红外光谱范围内.通过磁性分析发现,该复合团簇的磁矩主要局域在过渡金属原子周围,而且,Fe₂(SiO₂)₃和Co₃(SiO₂)₃具有相对较大的磁矩,这主要源于过渡金属原子的d轨道间相互耦合.能隙和磁性两方面性质进一步肯定了二氧化硅磁性复合材料在医学界被用作光动力靶向治疗的可观前景.     

Effective Ru moment in RuSr2Eu2−xCexCu2O10 from high-temperature magnetic susceptibility

We have made high-temperature (250 K<T<800 K) DC susceptibility measurements in the compounds RuSr₂Eu_2−xCe_xCu₂O₁₀ for x=0.6,0.8, and 1.0 in order to determine the Ru effective magnetic moment. After carefully subtracting all contributions to the magnetic susceptibility except that of the Ru ions, we have been able to fit the Ru susceptibility with a law χ_Ru=χ₀+C_Ru/(T−Θ_Ru). We have found that the Ru effective moment falls between the values expected for Ru⁵⁺ in spin states and . We have also found a dependence of μ_eff(Ru) and Θ_Ru with the Ce content x.     

Dy,Co共掺杂对BiFeO3陶瓷磁特性和磁相变温度Tc的影响

采用快速液相烧结法制备BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0, 0.05, 0.1, 0.15)陶瓷样品. 实验结果表明: 所有样品的主衍射峰与纯相BiFeO₃相符合且具有良好的晶体结构, 随着Co³⁺掺杂量的增大, Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品的主 衍射峰由双峰(104)与(110)逐渐重叠为单峰(110), 当掺杂量x>0.05时, 样品呈现正方晶系结构; SEM形貌分析可知: Dy³⁺, Co³⁺共掺杂使BiFeO₃晶粒尺度由原来的3—5 μ减小到约1 μ. 室温下, BiFeO₃样品表现出较弱的铁磁性, 随着Dy³⁺和Co³⁺掺杂, BiFeO₃样品的铁磁性显著提高. 在外加磁场为30 kOe的作用下, Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0.05, 0.1, 0.15)的M_r分别为0.43, 0.489, 0.973 emu/g; M_S分别为0.77, 1.65, 3.08 emu/g. BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品磁矩M随着温度T的升高而逐渐减小, Dy掺杂使BiFeO₃样品的T_N由644 K升高到648 K, 而T_C基本没有变化. Dy和Co共掺杂导致BiFeO₃样品磁相变温度T_C由870 K降低到780 K, 其T_C变化主要取决于Fe-O-Fe反铁磁超交换作用的强弱和磁结构的相对稳定性. 关键词： 铁磁电材料 磁滞回线 磁相变温度     

Incommensurate magnetic structure of CeSi as observed by neutron diffraction

The magnetic ordering of the orthorhombic compound CeSi (Pama space group, FeB type of structure, Z = 4) T_n = 5.6 K was investigated by means of powder neutron diffraction in the temperatures 1.5 and 10 K. Our results show that the magnetic ordering is associated with a three dimensionally amplitude-modulated phase which is incommensurate with the crystal lattice with wave vector q = (q_χ,q_y,q_z). The ordered Ce³⁺ moments are confined to the plane (a,b) at an angle of 16.7° with the b direction. It follows from the present data analysis that the dominant wave vector component is q_z, which approximately corresponds to a transversal modulation, however the incommensurateness involves also the q_x and q_y components. The amplitude of the sine wave is μ⁰_j = 1.667μ_B at 1.5 K which corresponds to quite a reduced ordered moment value compared to the Ce³⁺ free ion value gJ = 2.14μ_B.     

Superconductivity in an intermetallic oxide Hf_3Pt_4Ge_2O

Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf_3Pt_ Ge_2O synthesized through a solid-state reaction. The Hf_3Pt_ Ge_2O crystallizes in a cubic structure(space group Fm-3 m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-II superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf_3Pt_ Ge_2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf_3Pt_ Ge_2O1+δ(-0.25 ≤δ≤ 0.5), and found theδ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5 d-electron in these intermetallic compound systems.     

Evolution of structure and physical properties in Al-substituted Ba-hexaferrites

The investigations of the crystal and magnetic structures of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been performed with powder neutron diffractometry. Magnetic properties of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions have been measured by vibration sample magnetometry at different temperatures under different magnetic fields.The atomic coordinates and lattice parameters have been Rietveld refined. The invar effect is observed in low temperature range(from 4.2 K to 150 K). It is explained by the thermal oscillation anharmonicity of atoms. The increase of microstress with decreasing temperature is found from Rietveld refinement. The Curie temperature and the change of total magnetic moment per formula unit are found for all compositions of the Ba Fe12-xAlx O19(x = 0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.     

Effect of hydrogen absorption on the magnetic properties of some Zr-3d laves phase binary and pseudobinary intermetallics

Magnetization measurements have been performed on ZrCr₂, ZrMn₂, Zr(Cr_xFe_1−x)₂(x = 0.2, 0.5, 0.75) and Zr(Cr_x Co_1−x)₂ (x = 0.75, 0.4) before and after hydrogenation. In all cases both the magnetic moments and the magnetic ordering temperatures (whenever present) appeared to be significantly larger in the hydride phases. The increase of the magnetic ordering temperatures is in a qualitative agreement (but not a quantitative one) with the negative pressure effects on T_c in ZrFe₂.     

Dy3Al5O12磁热性质研究

用量子理论计算了Dy₃Al₅O₁₂的晶场能谱、Zeeman劈裂能级和波函数. 在外磁场H_e为0<H_e<9 T, 温度为3<T<42 K 范围内, 计算了该晶体的磁矩、磁熵变, 计算结果与相关实验数据吻合较好. 该计算结果表明, Dy₃Al₅O₁₂内磁性离子间的交换作用非常微弱, 可以忽略. 从理论上给出了绝热退磁过程中温度变化ΔT与T的关系, 并与Gd₃Ga₅O₁₂晶体进行了比较, 发现不同外磁场下, Dy₃Al₅O₁₂和Gd₃Ga₅O₁₂的低温制冷性能在不同温区有差别. 在进行低温(T<10 K)制冷时, 若外磁场较低, 选择Dy₃Al₅O₁₂作为磁制冷材料较好; 若外磁场较高, 选择Gd₃Ga₅O₁₂作为磁制冷材料较好.     

间隙原子H,B,C对LaFe_(11.5)Al_(1.5)化合物磁性和磁热效应的影响

研究了金属化合物LaFe_(11.5)Al_(1.5)H_x(x=0,0.12,0.6,1.3),LaFe_(11.5)Al_(1.5)B_y(y=0.1,0.2,0.3)和LaFe_(11.5)Al_(1.5)C_z(z=0.1,0.2,0.3,0.4,0.5)的磁性、结构和磁热效应.金属化合物样品均形成了良好的NaZn_(13)型单相结构.基于固相-气相反应或者固相-固相反应引入间隙H,B,C原子后,磁性基态从反铁磁态变为铁磁态,饱和磁化强度(M_s)和居里温度(T_c)均呈升高趋势.值得注意的是:随着B和C含量的增加,化合物的相变性质由弱一级相变过渡至二级相变;而随着H含量的增加,相变性质却从二级相变过渡至弱一级相变.同时,化合物LaFe_(11.5)Al_(1.5)H_x,LaFe_(11.5)Al_(1.5)B_y和LaFe_(11.5)Al_(1.5)C_z均呈现出相当大的磁熵变.在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(1.3),LaFe_(11.5)Al_(1.5)B_(0.1)和LaFe_(11.5)Al_(1.5)C_(0.2)的最大磁熵变分别达到12.3,9.6和10.8 J/kg·K.此外,在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(0.6)的制冷能力达到259.2 J/kg,LaFe_(11.5)Al_(1.5)B_(0.1)的制冷能力达到116.4 J/kg,而LaFe_(11.5)Al_(1.5)C_(0.1)的制冷能力达到230.4 J/kg.     

Effect of Sn doping on the magnetic and transport properties of LaMnO3+δ

We report the influence of the Sn doping on the magnetotransport properties of the LaMnO_3+δ perovskite. Two series of samples with nominal LaSn_xMn_1−xO_3+δ (I series) and La_{(1−x)/(1+x)}Sn_xMn_1−xO_3+δ (II series) compositions (x=0, 0.025, 0.05 and 0.10) were prepared at T_s=750°C. The M(T) data under 0.01 and 0.5 T for the I series reveal a depressed magnetization as the Sn content increases suggesting the presence of magnetic clusters with a superparamagnetic behavior. Resistivity measurements indicate an insulator material for all Sn content independently of the applied magnetic field. On the contrary, for the II series the M(T) and M(H) data reveal FM behavior and an improvement of the magnetization as Sn increases. These samples show magnetoresistance. The magnetotransport properties are discussed in terms of the presence of A-site cation vacancies.     

Correlation lengths and E8 mass spectrum of the dilute A3 lattice model

The exact perturbation approach is used to derive the elementary correlation lengths ξ_i and related mass gaps mi of the two-dimensional dilute A_L lattice model in regimes 1 and 2 for L odd from the Bethe ansatz solution. In regime 2 the A₃ model is the E₈ lattice realisation of the two-dimensional Ising model in a magnetic field at T = T_c The calculations for the A₃ model in regime 2 start from the eight thermodynamically significant string types found in previous numerical studies. These string types are seen to be consistent in the ordered high field limit. The eight masses obtained reduce with the approach to criticality to the ES masses predicted by Zamolodchikov, thus providing a further direct lattice determination of the Es mass spectrum.