LaFe11.5Si1.5中的磁不稳定性

通过对LaFe11.5Si1.5化合物进行自旋极化和固定磁矩(FSM)的能带计算,发现LaFe11.5Si1.5化合物具有磁不稳定性的特征,无磁态和铁磁态的能量差别很小,FSM计算表明LaFe11.5Si1.5化合物具有低自旋态和高自旋态的双磁性态特征,在一定条件下能够在两磁性态之间发生变磁转变.通过计算的结果,定性地分析了实验上所观察的一些现象.     

LaFe$lt;sub$gt;11.5$lt;/sub$gt;Si$lt;sub$gt;1.5$lt;/sub$gt;化合物氢化特性及稳定性的研究

系统研究了温度、时间、压力等因素对LaFe_11.5Si_1.5化合物吸氢过程的影响以及效果. 结果表明：吸氢处理会增大化合物的晶格常数,但不会改变其晶体结构. 在温度为423 K,氢气压力为0.0987 MPa时,可以制备出氢分布均匀的LaFe_11.5Si_1.5H_1.6间隙化合物. 氢化处理可以明显提高化合物的居里温度,降低热滞后,并且使磁熵变保持在较高值. LaFe_11.5Si_1.5H_1.6样品随着在空气中暴露时间的延长,居里温度和磁熵值的变化非常小,氢化合金的磁热性能具有良好的时间稳定性. 关键词： P-C-T关系曲线 居里温度 磁熵变 11.5Si_1.5氢化物')" href="#">LaFe_11.5Si_1.5氢化物     

NaZn13型Fe基化合物的结构和热力学性质研究

利用Chen-Mbius晶格反演获得的原子间相互作用势,对NaZn₁₃型Fe基金属间化合物进行原子级模拟研究.计算结果表明,Si原子和Co原子均优先占据96i晶位,Si原子和Co原子替代Fe原子后晶体平均结合能降低.随着Co含量的增加,LaFe_13-x-yCo_ySi_x和NdFe_13-x-yCo_ySi_x的晶格参数逐渐降低.声子态密度中,稀土原子主要激发低频模,Si原子主要激发高频模.LaFe_11.5-yCo_ySi_1.5化合物的德拜温度随Co含量的增加而增高. 关键词： 晶格反演 原子间相互作用势 热力学性质 磁致冷材料     

NaZn13型结构LaFe13－xAlxCy化合物的磁熵变与磁相变的研究

研究了NaZn₁₃型结构LaFe_13-xAl_xC_0.1(x=1.6，1.8)间隙化合物的磁制冷能力和磁相变.利用麦克斯韦关系式计算得到，高Al含量LaFe_13-xAl_x碳化物的最大磁熵变值|ΔS|_m低于低Al含量碳化物的最大磁熵变值.随Al含量的增加，化合物的磁熵变峰展宽，但由于磁熵变大幅降低，衡量磁制冷能力的q值随之降低.基于朗道相变原理，考虑到自旋涨落的影响，磁自由能可以展开到磁化强度的6次方项，材料的相变类型由磁化强度的4次方项系数a₃(T)的符号来进行判断.随着Al含量的增加，研究的碳化物相变由弱的一级相变转为二级相变. 关键词： 13-xAl_x碳化物')" href="#">LaFe_13-xAl_x碳化物 磁制冷能力 磁相变     

(Nd1－xGdx)3Fe27.31Ti1.69化合物的结构和磁性

通过X射线衍射和磁性测量等手段研究了(Nd_1-xGd_x)₃Fe_27.31Ti_1.69(0≤x≤0.6)化合物的结构和磁性.X射线衍射测量结果表明Gd替代后并未改变Nd₃(Fe,Ti)₂₉化合物的晶体结构，但引起了晶胞体积收缩.随着Gd含量的增加，化合物的居里温度T_C和室温磁晶各向异性场B_a单调增加，而自旋重取向 关键词： 1-xGd_x)₃Fe_27.31Ti_1.69化合物')" href="#">(Nd_1-xGd_x)₃Fe_27.31Ti_1.69化合物 磁晶各向异性 自旋重取向 磁相图     

一级磁结构相变材料Mn0.6Fe0.4NiSi0.5Ge0.5和Ni50Mn34Co2Sn14的磁热效应与磁场的线性相关性

磁熵变（△S_M）与磁场（μ₀H）的相关性已在很多二级相变材料中被研究并报道,但一级相变材料的磁热效应与磁场相关性还少有报道.本文在具有一级磁结构相变的Mn_0.6Fe_0.4NiSi_0.5Ge_0.5材料中研究发现△S_M与μ₀H存在线性相关性,并通过麦克斯韦关系式的数值分析详细讨论了这一线性相关性的来源.同时,进一步发现在低磁场时,△S_M近似正比于μ₀H的平方.该线性相关性同样在一级磁结构相变Ni₅₀Mn₃₄Co₂Sn₁₄材料中得到了印证.但由于一级磁弹相变LaFe_11.7Si_1.3材料相变温度具有更强的磁场依赖性,不具有△S_M的线性相关性,因此,本研究表明,当磁结构相变材料的相变温度具有弱磁场依赖性时,△S_M与μ₀H具有线性相关性.进而,在磁场未达到相变饱和磁场以下,利用△S_M与μ₀H的线性相关性可以有效推测更高磁场下的△S_M.     

在永磁体强磁场中Mn1.2Fe0.8P1-xSix系列化合物热磁发电研究

研究了处于永磁体强磁场中Mn_1.2Fe_0.8P_1-xSi_x 系列化合物的热磁发电性能, 采用高性能球磨和固相烧结合成方法制备了Mn_1.2Fe_0.8P_1-xSi_x 系列化合物, 并对该系列化合物的物相结构、磁性和热磁发电性能进行了测量. 结果表明: Mn_1.2Fe_0.8P_0.37Si_0.63和Mn_1.2Fe_0.8P_0.35Si_0.65化合物是具有Fe₂P型六角结构的一级相变软磁性材料, 两者居里温度分别为334 K和348 K, 处于工业余热温区. 根据一级相变磁性材料在居里温度磁化强度发生突变这一特性, 研制热磁发电演示装置, 测量了Mn_1.2Fe_0.8P_0.37Si_0.63和Mn_1.2Fe_0.8P_0.35Si_0.65这两种材料铁磁相变产生感应电流大小与线圈匝数、热磁发电材料质量、表面积、表面上温度梯度的关系. 研究结果表明, Mn_1.2Fe_0.8P_1-xSi_x系列化合物具有很好的热磁发电性能, 有望成为热磁发电候选材料.     

Mn5Ge2.7M0.3 (M=Ga，Al，Sn) 化合物的磁性和磁熵变

研究了Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物的磁性和磁熵变. x射线衍射实验表明，研究的化合物均呈六角Mn₅Si₃型结构. 三种原子对Ge原子的替代，使得平均Mn原子磁矩下降，但居里温度没有明显的变化. 由于磁矩的降低，导致磁熵变值的下降，在磁场变化为4.0×10⁶A·m^-1时，对应于M=Ga，Al和Sn的样品，最大磁熵变值ΔS^max_Ｍ分别为6.1，6.3和5.3J·kg^-1K^-1，但磁熵变峰值的半高宽ΔＴ_ＦＷＨＭ有所增加. 另外，Mn₅Ge_2.7M_0.3(M=Ga，Al，Sn)化合物在高于居里温度的Arrott曲线上出现了一个不连续点，即样品在一定温度下的顺磁磁化率在某一临界磁场下发生了突变，临界磁场与温度几乎呈正比关系.这可能是由于样品在加一定磁场时3d带的费米能级发生了变化，使得有效电子数的减少所致. 关键词： 居里温度 平均Mn原子磁矩 磁熵变 Arrott图     

Er2AlFe16－xMnx化合物的磁性及反常热膨胀

通过X射线衍射及磁测量手段研究了Er₂AlFe_16-xMn_x(x=1，2，3，4，6，8)化合物的结构和磁性. 研究结果表明Er₂AlFe_16-xMn_x化合物具有六角相的Th₂Ni₁₇型结构. 采用X射线热膨胀测定法在103—654K的温度范围内测量了Er₂AlFe_16-xMn_x(x=1，2，3，4)化合物的热膨胀性质，发现这些化合物在低温下存在热膨胀反常现象，在居里点附近具有负膨胀性质. 对自发磁致伸缩的研究结果表明Er₂AlFe_16-xMn_x化合物中存在着较强的各向异性的自发磁致伸缩，低温下自旋重取向的出现使得化合物的自发体磁致伸缩有所增强. 磁测量结果表明Mn的替代导致Er₂AlFe_16-xMn_x化合物的居里温度及自发磁化强度急剧下降，并且使得化合物的磁晶各向异性发生显著改变. 关键词： 2AlFe_16-xMn_x化合物')" href="#">Er₂AlFe_16-xMn_x化合物 反常热膨胀 自发磁致伸缩     

化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

Hydrogen absorption of LaFe11.5Si1.5 compound under low hydrogen gas pressure

Hydrogen absorptions of LaFe11.5Si1.5 compound in 1-atm hydrogen gas at different temperatures are investigated. The hydrogen content in the hydrogenated sample does not increase with the increase of temperature of hydrogen absorption but changes complicatedly. The characteristic of first-order transition in LaFe11.5Si1.5 compound is weakened after hydrogen absorption. It leads the peaks of magnetic entropy to become wider and the hysteresis loss to reduce significantly, but relative cooling power (RCP) is not changed considerably.     

一级相变材料Mn1.2Fe0.8P0.4Si0.6的热磁发电性能

本文报道把热能直接转换电能的热磁发电技术所用一级相变新材料Mn_1.2Fe_0.8P_0.4Si_0.6的磁性和热磁发电性能.用高能球磨机械合金化技术和固相烧结合成方法制备了Mn_1.2Fe_0.8P_0.4Si_0.6化合物.磁性测量结果表明,该化合物呈现从铁磁状态变为顺磁状态的一级相变,居里温度为337K,并伴随巨大的磁化强度的变化.根据该材料的这一特性,设计制作了热磁发电演示装置,测定了热流引起材料的相变而产生的电流,并研究了固定磁场中热致磁转变产生的电流随热流温度和样品质量的变化.研究结果表明Mn_1.2Fe_0.8P_0.4Si_0.6化合物具有很好的热磁发电性能,可作为热磁发电材料.     

Large magnetic entropy change near room temperature in the LaFe11.5Si1.5H1.3 interstitial compound

The LaFe_11.5Si_1.5H_1.3 interstitial compound has been prepared. Its Curie temperature T_C (288 K) has been adjusted to around room temperature, and the maximal magnetic entropy change (|ΔS|～17.0 J·kg^-1·K^-1 at T_C) is larger than that of Gd (|ΔS|～9.8 J·kg^-1·K^-1 at T_C=293 K) by ～73.5% under a magnetic change from 0 to 5 T. The origin of the large magnetic entropy change is attributed to the first-order field-induced itinerant-electron metamagnetic transition. Moreover, the magnetic hysteresis of LaFe_11.5Si_1.5H_1.3 under the increase and decrease of the field is very small, which is favourable to magnetic refrigeration application. The present study suggests that the LaFe_11.5Si_1.5H_1.3 compound is a promising candidate as a room-temperature magnetic refrigerant.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Giant magnetocaloric effect in the Gd5Ge2.025Si1.925In0.05 compound

The magnetocaloric properties of the Gd 5 Ge 2.025 Si 1.925 In 0.05 compound have been studied by x-ray diffraction,magnetic and heat capacity measurements.Powder x-ray diffraction measurement shows that the compound has a dominant phase of monoclinic Gd5Ge2Si2-type structure and a small quantity of Gd 5(Ge,Si) 3-type phase at room temperature.At about 270 K,this compound shows a first order phase transition.The isothermal magnetic entropy change(△SM) is calculated from the temperature and magnetic field dependences of the magnetization and the temperature dependence of MCE in terms of adiabatic temperature change(△Tad) is calculated from the isothermal magnetic entropy change and the temperature variation in zero-field heat-capacity data.The maximum S M is 13.6 J·kg-1·K-1 and maximum △Tad is 13 K for the magnetic field change of 0-5 T.The Debye temperature(θD) of this compound is 149 K and the value of DOS at the Fermi level is 1.6 states/eV·atom from the low temperature zero-field heat-capacity data.A considerable isothermal magnetic entropy change and adiabatic temperature change under a field change of 0-5 T jointly make the Gd5Ge2.025Si1.925 In 0.05 compound an attractive candidate for a magnetic refrigerant.     

Magnetic order of Mn5Si3-type Tb5Sb3 and Tb5Si1.5Sb1.5

Neutron diffraction study has been performed on the Tb₅Sb₃ and Tb₅Si_1.5Sb_1.5 compounds (hexagonal Mn₅Si₃-type, hP16, P6₃/mcm) to understand their magnetic structures. The temperature dependence of neutron diffraction results proves that these intermetallics show a complex magnetic ordering. The Tb₅Sb₃ presents five subsequent changes in magnetic structure at ∼150, 119, 85, 70 and 54 K on cooling: paramagnet→antiferromagnetic flat spiral→ferromagnetic cone→antiferromagnetically canted ferromagnetic cone→canted AF→sine modulated AF. The Tb₅Si_1.5Sb_1.5 shows two subsequent changes in magnetic structure at 123 and 66 K: paramagnet→sine modulated antiferromagnet I→sine modulated antiferromagnet II. The Tb₅Si₃, Tb₅Sb₃ and Tb₅Si_1.5Sb_1.5 have the different magnetic structure in the full temperature range.     

Structural properties, magnetic order and electronic transport in single crystalline UPt2Si2

We present a detailed comparison of the physical properties of as-cast and annealed single crystalline UPt₂Si₂, a compound whose properties we have shown to be governed by strain disorder on the Pt/Si ligand sites. Contrary to common knowledge, and to our surprise, from our data we do not observe a significant improvement of the physical properties of UPt₂Si₂ upon annealing at 900 °C for one week. We attribute this to the specific way the strain disorder is produced in UPt₂Si₂ by presenting evidence that it results from a first order phase transition at ambient temperatures. We discuss the implications of such phase transitions occurring at comparatively low temperatures for the ground state properties of heavy fermion systems and related correlated electron materials.     

Superconductivity in Mo5SiB2

In the Mo-Si binary system, Mo₅Si₃ crystallizes in the W₅Si₃ (T₁ phase) structure type. However, when boron replaces silicon in this compound, a structural transition occurs from the W₅Si₃ prototype structure to the Cr₅B₃ prototype structure (T₂ phase) at the composition Mo₅SiB₂. Mo₅SiB₂ has received much attention in the literature as a candidate for structural application in high-temperature turbines, but its electronic and magnetic behavior has not been explored. In this work, we show that Mo₅SiB₂ is a bulk superconducting material with critical temperature close to 5.8 K. The specific-heat, resistivity and magnetization measurements reveal that this material is a conventional type II BCS superconductor.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.