Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} compounds has been investigated by means of magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase with the cubic NaZn_{13}-type structure. Substitution of Co for Fe leads to an increase of Curie temperature of the material. The magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} and LaFe_{11.03}Co_{0.80}Si_{0.94}Al_{0.23} compounds are 21.8J/(kg·K) to 16.9J/(kg·K) under a magnetic field change of 0－5T at Curie temperature, respectively. Giant magnetic entropy changes are attributed to the higher magnetization and the rapid change in magnetization at Curie temperature.     

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.     

Magnetic entropy change and magnetic phase transition of LaFe11.4Al1.6Cx （x=0-0.8） compounds

The unit cell volume and phase transition temperature of LaFe11.4Al1.6Cx compounds have been studied. The magnetic entropy change, refrigerant capacity and the type of magnetic phase transition are investigated in detail for LaFe11.4Al1.6Cx with x=0.1, All the LaFe11.4Al1.6Cx （x=0-0.8） compounds have the cubic NaZn13-type structure. The addition of carbon atoms brings about a considerable increase in the lattice parameter. The bulk expansion results in the change of phase transition temperature （Tc）, Tc increases from 187K to 269 K with x varying from 0.1 to 0.8, Meanwhile an increase in the lattice parameter can also cause a change of the magnetic ground state from antiferromagnetic to ferromagnetic. Large magnetic entropy change IASI is found over a large temperature range around Tc and the refrigerant capacity is about 322J/kg for LaFe11.4Al1.6C0.1. The magnetic phase transition belongs in weakly first-order one for x=0.1.     

Magnetic properties and magnetocaloric effect in NdxLa1-xFe11.5Al1.5 compounds

Effects of Nd-doping on the magnetic properties and magnetocaloric effects （MCEs） of NdxLa1-xFe11.5Al1.5 have been investigated. Substitution of Nd leads to a weakening of the antiferromagnetic （AFM） coupling and an enhancement of the ferromagnetic （FM） coupling. This in turn results in a complex magnetic behaviour for Nd0.2La0.8Fe11.5Al1.5 characterized by the occurrence of two phase transitions at ～188 K （PM AFM） and ～159 K （AFM-FM）. As a result, a table-like MCE （9 J/kg.K） is found in a wide temperature range （160-185 K） for a field change of 0-5T around the transition temperature, as evidenced by both the magnetic and calorimetric measurements. Based on the analysis of low-temperature heat capacity, it is found that the AFM-FM phase transition modifies the electron density significantly, and the major contribution to the entropy change comes from the electronic entropy change.     

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.     

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.     

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.     

The spike in the relation between entropy change and temperature in LaFe11.83Si1.17 compound

LaFe_11.83Si_1.17 compound shows a first-order transition from paramagnetic to ferromagnetic state. When measurement is carried out from low to high temperature in an increasing field, there is a spike of 51 J/kg K at 174.8 K, followed by a plateau of 20 J/kg K in the curve of entropy change versus temperature determined by the Maxwell relation. However, the nature of the spike is fictitious, which is caused by an obvious superheat of the ferromagnetic state when the Maxwell relation or the Clausius–Clapeyron equation is used to evaluate the entropy change.     

Structure and magnetic entropy change of suck-cast Gd60Co26Al6Ge8 alloy

The Gd₆₀Co₂₆Al₆Ge₈ alloy has been prepared by the copper-mold suck-casting and its phase component has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). It is shown that this alloy consists of primary crystalline Gd₅Ge₃ phase and amorphous matrix. The glass transition temperature (T_g) and crystallization temperatures (T_x) occur at 292 and 320 °C, respectively. The maximal magnetic entropy change (ΔS_M) under 0-5 T field is about 7.6 J (kg⁻¹ K⁻¹) at 155 K and the refrigeration capacity (RC) is about 768 J kg⁻¹, which makes Gd₆₀Co₂₆Al₆Ge₈ bulk metallic glass matrix composite a promising candidate for magnetic refrigerant.     

Magnetic properties and magnetic entropy changes of La1-xPrxFe11.5Si1.5 compounds with 0≤x≤ 0.5

Magnetic properties and magnetic entropy changes in LaFe$_{11.5}$Si$_{1.5}$ have been investigated by partially substituting Pr by La. It is found that La$_{1 - x}$Pr$_{x}$Fe$_{11.5}$Si$_{1.5}$ compounds remain cubic NaZn$_{13}$-type structures even when the Pr content is increased to 0.5, i.e. $x = 0.5$. Substitution of Pr for La leads to a reduction in both the crystal constant and the Curie temperature. A stepwise magnetic behaviour in the isothermal magnetization curves is observed, indicating that the characteristic of the itinerant electron metamagnetic (IEM) transition above $T_{\rm C}$ becomes more prominent with the Pr content increasing. As a result, the magnetic entropy change is remarkably enhanced from 23.0 to 29.4\,J/kg$\cdot$K as the field changes from 0 to 5\,T, with the value of $x$ increasing from 0 to 0.5. It is more attractive that the magnetic entropy changes for all samples are shaped into high plateaus in a wide range of temperature, which is highly favourable for Ericsson-type magnetic refrigeration.     

Effects of Fe－Fe bond length change in NaZn13-type intermetallic compounds on magnetic properties and magnetic entropy change

In this paper the effects of Fe－Fe bond length change on magnetic properties and magnetic entropy change have been investigated on LaFe_{12.4-x}Si_xCo_{0.6} and LaFe_{12.3-x}Al_xCo_{0.7} intermetallic compounds. According to the analyses of Fe－Fe bond length change, the variation of Curie temperature and the unusual magnetic phase transition which results in the large magnetic entropy change were explained. The effects of the substitution of Co and Si for Fe on magnetic entropy change and field-induced itinerant-electron metamagnetic transition in LaFe_{12.4-x}Si_xCo_{0.6} compounds were also studied and the considerable magnetic entropy change has been achieved.     

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.     

Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

Double magnetic entropy change peaks and high refrigerant capacity in Gd1-xHoxNi compounds in the melt-spun form

Gd_1-xHo_xNi melt-spun ribbons were fabricated by a single-roller melt spinning method. All the compounds crystallize in an orthorhombic CrB-type structure. The Curie temperature (T_C) was tuned between 46 and 99 K by varying the concentration of Gd and Ho. A spin reorientation (SRO) transition is observed around 13 K. Different from T_C, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔS_M) were found. One at the higher temperature range was originated from the paramagnet-ferromagnet phase transition and the other at the lower temperature range was caused by the SRO transition. The maximum of ΔS_M around T_C is almost same. The other maximum of ΔS_M around SRO transition, however, had significantly positive relationship with x. It reached a maximum about 8.2 J kg⁻¹ K⁻¹ for x = 0.8. Thus double large ΔS_M peaks were obtained in Gd_1-xHo_xNi melt-spun ribbons with the high Ho concentration. And the refrigerant capacity power reached a maximum of 622 J kg⁻¹ for x = 0.6. Gd_1-xHo_xNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.     

Effect of Dy substitution on magnetic properties and magnetocaloric effects of Tb6Co1.67Si3 compounds

The magnetic and magnetocaloric properties of(Tb1-xDyx) 6 Co 1.67 Si 3(0 ≤ x ≤ 0.8) have been experimentally investigated.The compounds exhibit a Ce6Ni2Si3-type hexagonal structure and undergo a second-order magnetic transition.The Curie temperature decreases from ～ 187 K to 142 K as the content of Dy grows from 0 to 0.8.The maximal magnetic entropy change,for a field change of 0-5 T,varies between ～ 6.2 and ～ 7.4 J/kg.K,slightly decreasing when Dy is introduced.The substitution of Dy leads to a remarkable increase in refrigeration capacity(RC).A large RC value of ～ 626 J/kg is achieved for x = 0.4 under a field change of 0-5 T.     

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.     

Ni47Mn32Ga21多晶合金的磁熵变和磁感生应变

对定向凝固方法制备的Ni₄₇Mn₃₂Ga₂₁多晶合金,通过扫描电镜、金相、电子能谱等手段研究其组份和组织形貌,通过对合金磁化强度与温度关系、等温磁化曲线及磁感生应变曲线等的测量分析,研究了合金结构相变和磁相变过程中的磁熵变及不同压力下的磁感生应变. 研究结果表明:合金组份与设计组份基本一致,室温下合金大部分为马氏体相. 升温过程中合金的磁熵变在居里温度(365 K)附近有最大值,并有较大的磁熵变峰值半高宽,747 kA/m的磁场下该磁熵变最大值为-1.45 J/kg ·K,磁熵变峰值的半高宽为21 K. 合金在室温(298 K)下有较好的双向可恢复磁感生应变,480 kA/m磁场下,无压力时合金的磁感生应变值达到-670×10^-6,并趋饱和;而在与磁场方向平行的27.3 MPa外压力作用下合金的磁感生应变值增大到-1300×10^-6,且未饱和. 关键词： Ni-Mn-Ga 铁磁形状记忆合金 磁熵变 磁感生应变     

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