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.     

LaFe11.2Co0.7Si1.1合金在室温区的巨大磁熵变

在具有立方NaZn13型结构的稀土铁基化合物LaFe11.2Co0.7Si1.1中发现室温的巨大磁熵变，磁熵变的峰值位于居里温度Tc=274K处，5T外磁场下达到－20.3J/kg K约为相同的温区下金属Gd的2倍，LaFe11.2Co0.7Si1.1合金中强烈的磁弹性耦合，导致晶格在居里温度处出现巨大负膨胀，这是其巨大磁熵变的来源。     

LaFe11.6Si1.4C0.5间隙化合物的磁性、磁相变和磁熵变

使用了约80％的廉价铁原料，制备了LaFe11．6Sil．4C0．5间隙化合物。研究表明：该间隙化合物仍保持了LaFe11．6Sil．4的NaZnl3立方晶体结构，间隙C原子的引入导致晶格膨胀了约0．35％，居里点提高了约50K；同时，仍然保持大的磁熵变，在0-2T磁场变化下，磁熵变达到Gd的2倍以上，大磁熵变源于磁场诱发的巡游电子变磁相变。该间隙化合物具有大磁熵变。     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

Magnetism and magnetic entropy change in LaFe11Al2Cx compounds around room temperature

Magnetism and magnetic entropy changes in LaFe11A12Cx(x=0.0, 0.2 and 0.5) compounds have been investigated.The Curie temperature TC is conveniently controlled from 200K to room temperature by varying the carbon concentration.Large magnetic entropy change is obtained over a wide temperature range due to the high magnetization and the drastic decrease in the magnetization around TC.The large magnetic entropy change in wide temperature range,low cost and the convenience of controlling TC suggest that the LaFe11Al2Cx compounds are promising candidates for magnetic refrigerants in the corresponding temperature range.     

Change in the magnetic ground state of LaFe11.4Al1.6 compound by the substitution of Mn for Fe

The structure and magnetic properties of La(Fe_{1-x}Mn_x)_{11.4}Al_{1.6} (0≤x≤0.25)compounds have been studied. The NaZn_{13}-type structure is preserved and the lattice parameter increases linearly with increasing the Mn concentration. The magnetic ground state changes from the antiferromagnetic to the spin-glass or the cluster-glass state by the substitution of Mn for Fe. Furthermore, a field-induced transition from cluster glass to ferromagnet is found for the samples with x=0.05 and 0.10.     

Large magnetic entropy change and magnetic properties in La （Fel－xMnx）ll.TSil.3Hy compounds

Magnetic properties and magnetic entropy change in La(Fe_{1-x}Mn_x)_{11.7}Si_{1.3}H_y compounds have been investigated. A significant increase of the Curie temperature T_C and a small increase of the saturation magnetizations μ_S have been observed after the introduction of interstitial H, which caused a slight volume expansion. The first-order field-induced itinerant-electron metamagnetic (IEM) transition remains and brings about a large magnetic entropy change around room temperatures for the compounds. The maximal magnetic entropy change is about 23.4, 17.7 and 15.9J/kg·K under a magnetic field change from 0 to 5T for x=0.01, 0.02 and 0.03, respectively. Therefore, the compounds appear to be potential candidates for magnetic refrigerants around room temperatures.     

直线中的对称问题

对称问题是解析几何中的基本问题,主要有中心对称和轴对称问题两类,本文就直线中的对称问题归纳如下．     

Ce2Fe16Al化合物在居里温度附近的磁性和磁熵变

研究了具有菱方Th2Zn17型结构的Ce2Fe16Al化合物在居里温度TC附近的磁性和磁熵变.实验结果表明，在居里温度附近样品的磁特性符合二级相变规律，样品的居里温度为2758K.通过磁化强度与磁场和温度关系的测量，计算出临界指数β=044±001，γ=130±001，δ＝383±001，临界指数β，γ，δ基本满足标度率方程γ＝β（δ-1)，但偏离三维Heisenberg模型的理论值.Ce2Fe16Al化合物的磁熵变在居里温度处达到峰值，2T外磁场下的最大磁熵变为195J/kg K. 关键词： Ce2Fe16Al化合物 临界指数 磁熵变     

LaFe11.5Si1.5中的磁不稳定性

通过对LaFe_11.5Si_1.5化合物进行自旋极化和固定磁矩(FSM)的能带 计算，发现LaFe_11.5Si_1.5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LaFe_11.5Si_1.5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. 关键词： 磁不稳定性 变磁转变