Magnetic properties and magnetocaloric effects of Mn5Ge3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1,0.2,0.3,0.4,0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically.The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9).The average Mn magnetic moments increases with increasing Ga content,reaching a maximum value at x=0.6.The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation.The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3.One is that the magnitude of the magnetic entropy change decreases,the other is that the MCE peak becomes broadened.     

Magnetocaloric effects in RT X intermetallic compounds(R = Gd–Tm,T = Fe–Cu and Pd,X = Al and Si)

The magnetocaloric effect(MCE) of RT Si and RT Al systems with R = Gd–Tm, T = Fe–Cu and Pd, which have been widely investigated in recent years, is reviewed. It is found that these RT X compounds exhibit various crystal structures and magnetic properties, which then result in different MCE. Large MCE has been observed not only in the typical ferromagnetic materials but also in the antiferromagnetic materials. The magnetic properties have been studied in detail to discuss the physical mechanism of large MCE in RT X compounds. Particularly, some RT X compounds such as Er Fe Si,Ho Cu Si, Ho Cu Al exhibit large reversible MCE under low magnetic field change, which suggests that these compounds could be promising materials for magnetic refrigeration in a low temperature range.     

MnFe(PGe) compounds: Preparation,structural evolution,and magnetocaloric effects

The interdependences of preparation conditions,magnetic and crystal structures,and magnetocaloric effects(MCE)of the Mn Fe PGe-based compounds are reviewed.Based upon those findings,a new method for the evaluation of the MCE in these compounds,based on differential scanning calorimetry(DSC),is proposed.The Mn Fe PGe-based compounds are a group of magnetic refrigerants with giant magnetocaloric effect(GMCE),and as such,have drawn tremendous attention,especially due to their many advantages for practical applications.Structural evolution and phase transformation in the compounds as functions of temperature,pressure,and magnetic field are reported.Influences of preparation conditions upon the homogeneity of the compounds’chemical composition and microstructure,both of which play a key role in the MCE and thermal hysteresis of the compounds,are introduced.Lastly,the origin of the"virgin effect"in the Mn Fe PGebased compounds is discussed.     

Crystal structures, phase relationships, and magnetic phase transitions of RsM4 compounds （R = rare earths, M = Si, Ge）

Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 （R = rare earths; M = Si, Ge） are reviewed briefly. First, crystal structures, phase relationships, and magnetic prop- erties of several 5：4 compounds, including Nd5 Si4-xGex, Pr5 Si4_xGex, Gds-xLaxGe4, La5 Si4, and Gd5 Sn4, are presented. In particular, the canted spin structures as well as the magnetic phase transitions in PrsSi2Ge2 and PrsGe4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gds（Si,Ge）4 are summarized. The focus is on the parent compound GdsGe4, which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pro- nounced effects of impurities, and high-field induced magnetic transitions.     

Magnetic properties and magnetocaloric effects in NaZn13-type La（Fe, Al）13-based compounds

In this article,our recent progress concerning the effects of atomic substitution,magnetic field,and temperature on the magnetic and magnetocaloric properties of the LaFe13-xAlx compounds are reviewed.With an increase of the aluminum content,the compounds exhibit successively an antiferromagnetic(AFM) state,a ferromagnetic(FM) state,and a mictomagnetic state.Furthermore,the AFM coupling of LaFe 13-xAlx can be converted to an FM one by substituting Si for Al,Co for Fe,and magnetic rare-earth R for La,or introducing interstitial C or H atoms.However,low doping levels lead to FM clusters embedded in an AFM matrix,and the resultant compounds can undergo,under appropriate applied fields,first an AFM-FM and then an FM-AFM phase transition while heated,with significant magnetic relaxation in the vicinity of the transition temperature.The Curie temperature of LaFe13-xAlx can be shifted to room temperature by choosing appropriate contents of Co,C,or H,and a strong magnetocaloric effect can be obtained around the transition temperature.For example,for the LaFe 11.5Al1.5C0.2H1.0 compound,the maximal entropy change reaches 13.8 J·kg-1 ·K-1 for a field change of 0-5 T,occurring around room temperature.It is 42% higher than that of Gd,and therefore,this compound is a promising room-temperature magnetic refrigerant.     

Giant magnetocaloric effect in Tb5Ge2-xSi2-xMn2x compounds

The crystal structure,magnetic and magnetocaloric characteristics of the pseduo ternary compounds of Tb5Ge2 xSi2 xMn2x(0 ≤ 2x ≤ 0.1) were investigated by x-ray powder diffraction and magnetization measurements.The x-ray powder diffraction results show that all compounds preserve the monoclinic phase as the majority phase and all the synthesized compounds were observed to be ferromagnetic from magnetization measurements.Magnetic phase transitions were interpreted in terms of Landau theory.Maximum isothermal magnetic entropy change value(20.84 J.kg-1.K-1) was found for Tb5Ge1.95Si1.95Mn0.1 at around 123 K in the magnetic field change of 5 T.     

Magnetocaloric effects in Mn1.35Feo.65P1-xSix compounds

The structural and magnetocaloric properties of Mn1.35Fe0.65P1-xSix compounds are investigated.The Sisubstituted compounds,Mn1.35Fe0.65P1-xSix with x = 0.52,0.54,0.55,0.56,and 0.57,are prepared by high-energy ball milling and the solid-state reaction.The X-ray diffraction shows that the compounds crystallize into the Fe 2 P-type hexagonal structure with space group P■2m.The magnetic measurements show that the Curie temperature of the compound increases from 253 K for x = 0.52 to 296 K for x = 0.56.The isothermal magnetic-entropy change of the Mn1.35Fe0.65P1-xSix compound decreases with the Si content increasing.The maximal value of the magnetic-entropy change is about 7.0 J/kg·K in the Mn1.35Fe0.65P0.48Si0.52 compound with a field change of 1.5 T.The compound quenched in water possesses a larger magnetic entropy change and a smaller thermal hysteresis than the non-quenched samples.The thermal hysteresis of the compound is less than 3.5 K.The maximum adiabatic temperature change is about 1.4 K in the Mn1.35Fe0.65P0.45Si0.55 compound with a field change of 1.48 T.     

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.     

Influences of P doping on magnetic phase transition and structure in MnCoSi ribbon

The structure and magnetic properties of Mn Co Si1-xPx(x = 0.05–0.50) are systematically investigated.With P content increasing,the lattice parameter a increases monotonically while both b and c decrease.At the same time,the temperature of metamagnetic transition from a low-temperature non-collinear ferromagnetic state to a high-temperature ferromagnetic state decreases and a new magnetic transition from a higher-magnetization ferromagnetic state to a lowermagnetization ferromagnetic state is observed in each of these compounds for the first time.This is explained by the changes of crystal structure and distance between Mn and Si atoms with the increase of temperature according to the hightemperature XRD result.The metamagnetic transition is found to be a second-order magnetic transition accompanied by a low inversed magnetocaloric effect(1.0 J·kg-1·K-1at 5 T) with a large temperature span(190 K at 5 T) compared with the scenario of Mn Co Si.The changes in the order of metamagnetic transition and structure make P-doped Mo Co Si compounds good candidates for the study of magnetoelastic coupling and the modulation of magnetic phase transition.     

Electric and Magnetic Properties of a New Ferrite—Ceramic Composite Material

We have investigated a new ferrite-ceramic composite material with inductive and capacitive properties fabricated by a solid-state reaction method.We analyse the effects of the composite mechanism and microstructure on the magnetic and electric properties.The results show that the new materials can be used not only as inductor materials,but also as capacitor materials in the wide frequency range of 1kHz-1.8GHz.The real part of permeability of the composite material is between 10 and 5.6,the imaginary part of permeability is between 1.2 and 0.5,and the dielectric constant is about ten times larger than that of ordinary ferrite materials.It is suggested that the new composite materials will be widely used in anti-electromagnetic interference fields and radio frequency communication fields.     

Electronic structure and magnetism of MnFeP1−xSix alloys from first-principles calculations

First-principles calculations of electronic structure and magnetic properties based on density-functional theory were performed for MnFeP_1−xSi_x (0.44?x?0.60) alloys which are considered as promising magnetocaloric refrigerants. We used the full-potential APW+lo method and treated the random order of P(Si) atoms in the ZrNiAl-type structure in a virtual-crystal approximation. A non-monotonic behavior of the alloy magnetization as a function of x was obtained, in qualitative agreement with experiment, and explained in terms of the spin-polarized densities of states.     

Magnetocaloric effects in Mn1.35Fe0.65P1-xSix compounds

The structural and magnetocaloric properties of Mn_1.35Fe_0.65P_1-xSi_x compounds are investigated. The Si-substituted compounds, Mn_1.35Fe_0.65P_1-xSi_x with x = 0.52, 0.54, 0.55, 0.56, and 0.57, are prepared by high-energy ball milling and the solid-state reaction. The X-ray diffraction shows that the compounds crystallize into the Fe₂P-type hexagonal structure with space group P62m. The magnetic measurements show that the Curie temperature of the compound increases from 253 K for x = 0.52 to 296 K for x = 0.56. The isothermal magnetic-entropy change of the Mn_1.35Fe_0.65P_1-xSi_x compound decreases with the Si content increasing. The maximal value of the magnetic-entropy change is about 7.0 J/kg稫 in the Mn_1.35Fe_0.65P_0.48Si_0.52 compound with a field change of 1.5 T. The compound quenched in water possesses a larger magnetic entropy change and a smaller thermal hysteresis than the non-quenched samples. The thermal hysteresis of the compound is less than 3.5 K. The maximum adiabatic temperature change is about 1.4 K in the Mn_1.35Fe_0.65P_0.45Si_0.55 compound with a field change of 1.48 T.     

铁基Heusler合金Fe2Co1-xCrxSi的结构、磁性和输运性质的研究

基于多种实验手段和能带计算的方法, 对四元合金Fe₂Co_1-xCr_xSi的晶体结构、 磁性、输运性质及能带结构进行了研究. 研究发现, 随着Cr的增加, 合金Fe₂Co_1-xCr_xSi保持了高度有序结构, 逐渐从Hg₂CuTi结构的Fe₂CoSi 过渡到L2₁结构的Fe₂CrSi; 由于次晶格网络的破坏, 居里温度逐渐下降; 系列合金的分子磁矩呈现线性下降, 符合半金属特性; 剩余电阻比率与原子占位有序程度密切相关, 呈现两端大、 中间小的特点. 在Cr替代Co的过程中, 材料半金属能隙逐渐打开, 表现半金属特征. 同时费米能级从Fe₂CoSi半金属能隙的价带顶上移至Fe₂CrSi能隙的导带底. 最大的能隙宽度出现在x= 0.75处, 这表明四元合金有可能成为具有更高自旋极化率和更强抗干扰能力的自旋电子学材料.     

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Synthesis and magnetic properties of Fe100-xMox alloy nanowire arrays

The Fe100-xMox(13≤x≤25) alloy nanowire arrays are synthesized by electrodeposition of Fe 2+ and Mo 2+ with different ionic ratios into the anodic aluminum oxide templates.The crystals of Fe100-xMox alloy nanowires gradually change from polycrystalline phase to amorphous phase with the increase of the Mo content and the nanowires are of amorphous structure when the Mo content reaches 25 at%,which are revealed by the X-ray diffraction and the selected area electron diffraction patterns.As the Mo content increases,the magnetic hysteresis loops of Fe100-xMox alloy nanowires in parallel to the nanowire axis are not rectangular and the slopes of magnetic hysteresis loops increase.Those results indicate that the magnetostatic interactions between nanowires and the magnetocrystalline anisotropy both have significant influences on the magnetization reversal process of the nanowire arrays.     

A density functional theory study of the electronic structures and magnetic properties of Fe（1-x）Cox alloy nanowires encapsulated in （10,0） carbon nanotubes

Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe_(1-x)Co_x alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using firstprinciple density functional theory calculations. For the fully relaxed Fe_(1-x)Co_x/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe_(1-x)Co_x/CNT systems are exothermic, and therefore the Fe_(1-x)Co_xalloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe_(1-x)Co_xnanowires to the more electronegative CNTs, and the Fe-C/Co-C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe_(1-x)Co_x/CNT system are smaller than those of the corresponding freestanding Fe_(1-x)Co_xnanowire, and the magnetic moment of the Fe_(1-x)Co_x/CNT system decreases monotonously with increasing Co concentration, but the Fe_(1-x)Co_x/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.     

Magnetic phase transitions and magnetocaloric effect in the Fe-doped MnNiGe alloys

The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNi_{1 - x}Fe_xGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNi_{1 - x}Fe_xGe alloys at temperature below the structural transition temperature.     

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd₅Si_xGe_4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd₅(Si,Ge)₄-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound.     

脉冲激光沉积法制备的Ni1-xFexO稀磁半导体的结构和磁性研究

利用脉冲激光沉积方法制备出了具有室温铁磁性的Ni_1-xFe_xO(x=0.02,0.05)稀磁半导体.X射线衍射(XRD)结果表明Ni_1-xFe_xO的晶体结构为NaCl结构,并且在Fe含量较高的Ni_0.95Fe_0.05O中出现了少量的α-Fe₂O₃物相.X射线吸收近 关键词： 1-xFe_xO')" href="#">Ni_1-xFe_xO XANES 脉冲激光沉积方法     

Limits of solubility,magnetic properties and electron concentration in Fe3-xTxSi system

The role of the outer electrons on the limits of solubility and bulk magnetic properties of the system Fe_3-xT_xSi (T - Ti, V, Cr, Mn, Co, Ni) is investigated. A correlation with the magnetic and structural behavior of Fe_1-cSi_c and Fe_1-cAl_c alloys is demonstrated.