Magnetism, magnetocaloric effect and positive magnetoresistance in Fe-doped antipervoskite compounds SnCMn3−xFex (x=0.05-0.20)

The effects of Fe substitution on the structure, magnetic properties, magnetocaloric effect and positive magnetoresistance (MR) effect in antipervoskite compounds SnCMn_3−xFe_x (x=0.05-0.20) have been investigated systematically. Partial substitution of Fe for Mn leads to the monotonic reduction in both the Curie temperature T_C and saturated magnetization (M_S). It can be attributed to the reduction of electronic density of state at the Fermi energy by Fe-doping. The maximum values of magnetic entropy change (−ΔS_M) and positive MR gradually decrease as x increases, due to the broadening of magnetic phase transition. The refrigerant capacity increases initially with x≤0.05, then decreases gradually as x increases further, which is suggested to originate from the competition between the decreasing −ΔS_M and broadening temperature span. Our result indicates that the chemical doping on Mn site is an effective method for manipulating the properties of antiperovskite compounds AXMn₃.     

On the magnetocaloric effect in Gd(Zn1−xCdx)

In this work, we calculate the magnetocaloric effect in the compounds Gd(Zn_1−xCd_x). We use a model Hamiltonian of interacting spins in which the indirect exchange interaction parameter between localized spins was calculated as a function of Cd concentration. The calculated isothermal entropy changes and the adiabatic temperature changes upon magnetic field variations are in good agreement with the available experimental data.     

The magnetic entropy changes in Gd1−xBx alloys

A series of Gd_(1−x)B_x alloys have been prepared by arc melting method. After introducing small quantity of B atom in Gd, the Curie temperature of these alloys increase while the magnetic entropy changes are almost same as that of Gd. The refrigerant capacities of these alloys are also greater than that of Gd. These results suggest that Gd_(1−x)B_x alloys may be utilized as refrigerant in household magnetic refrigeration.     

Investigation on the magnetocaloric effect in DyNi2, DyAl2 and Tb1−nGdnAl2 (n=0, 0.4, 0.6) compounds

The magnetocaloric effect (MCE) in the DyNi₂, DyAl₂ and Tb_1−nGd_nAl₂ (n=0, 0.4, 0.6) was theoretically investigated in this work. The DyNi₂ and DyAl₂ compounds are described considering a model Hamiltonian which includes the crystalline electrical field anisotropy. The anisotropic MCE was calculated changing the magnetic field direction from 〈1 1 1〉 to 〈0 0 1〉 in DyNi₂ and from 〈1 0 0〉 to 〈0 1 1〉 in DyAl₂. The influence of the second- and first-order spin-reorientation phase transitions on the MCE that occurs in these systems is discussed. For the calculations of the MCE thermodynamic quantities in the Tb_1−nGd_nAl₂ systems we take into account a two sites magnetic model, and good agreement with the available experimental data was obtained.     

Low-field AC susceptibility in Pr1−xHoxMn2Ge2

We report the structure and magnetic properties of Pr_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Pr leads to a linear decrease in the lattice constants and the unit cell volume. The samples with x=0 and x=0.8 have spin reorientation temperature. The results are collected in a phase diagram.     

Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

Kondo lattice behaviour in CePt2(Si1−xSnx)2 alloys

Measurements of electrical resistivity are presented for polycrystalline alloys in the CePt₂(Si_1−xSn_x)₂ system. Results of X-ray diffraction indicate that the tetragonal region of the CePt₂(Si_1−xSn_x)₂ alloy system that is amenable for study only extends up to x=0.3. The resistivity maximum characteristic of a Kondo lattice is observed at a temperature T_max=63 K for the parent compound CePt₂Si₂ and shifts to lower temperatures with increase in Sn content. The compressible Kondo lattice model is applied to describe the results of T_max in terms of the on-site Kondo exchange interaction J and the electron density of states at the Fermi level N(E_F). A value of |JN(E_F)|=0.060±0.009 for the parent compound is obtained from the experimental results.     

The magnetic behavior of La1−xTbxMn2Si2 (0≤x≤0.3) in weak magnetic fields

The structure and magnetic properties of La_1−xTb_xMn₂Si₂ (0≤x≤0.3) were studied by X-ray powder diffraction and DC magnetization measurements. All the compounds crystallize in ThCr₂Si₂-type structure. Substitution of Tb for La led to a linear decrease in the lattice constants and the unit-cell volume. A ferromagnetic phase for x≤0.15, and an antiferromagnetic phase for x=0.3 have been observed at about room temperature, whereas the compounds with x=0.2 and 0.25 exhibit a magnetic phase transition from ferromagnetism to antiferromagnetism.     

Influence of Ho substitution for Nd on magnetic properties of the Nd1−xHoxMn2Ge2 germanides

The structure and magnetic properties of Nd_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Nd leads to a linear decrease in the lattice constants and the unit cell volume, and the magnetic interactions in the Mn sublattice cross over from a ferromagnetic character to an antiferromagnetic one. A typical SmMn₂Ge₂-like behavior is observed for x=0.6 and 0.8. The results are collected in a phase diagram.     

The large low-field magnetic entropy changes in Ni43Mn46Sn11−xSbx alloys

A series of Ni₄₃Mn₄₆Sn_11−xSb_x (x=0, 1, and 3) alloys were prepared by an arc melting method. The martensitic transition shifts to higher temperature with the increasing Sb content. The isothermal magnetization curves and Arrott plots around martensitic transition temperatures show a typical metamagnetic behavior. Under a low applied magnetic field of 10 kOe, large magnetic entropy changes around the martensitic transition temperature are 10.4, 8.9, and 7.3 J/kg K, for x=0, 1, and 3, respectively. The origin of the large magnetic entropy changes and potential application for Ni₄₃Mn₄₆Sn_11−xSb_x alloys as working substances in magnetic refrigeration are discussed.     

Magnetocaloric effect of Gd5SixSn4−x Alloys

The crystal structure and magnetocaloric effect of Gd₅Si_xSn_4−x   (with x=2.4$x=2.4$, 2.6 and 2.8) alloys were studied by means of X-ray power diffraction (XRD) and magnetic measurements. From the XRD results, these alloys adopt a Gd₅Si₄-type structure for x=2.8$x=2.8$, Gd₅Si₄-type and Gd₅Si₂Ge₂-type mixed structures for x=2.4$x=2.4$ and 2.6, while some minor phases can also be found. The Curie temperatures of the Gd₅Si_xSn_4−x increases gradually when x increases from 276 K for x=2.4$x=2.4$, to 301.5 K for x=2.8$x=2.8$. Magnetic entropy changes of these alloys at a magnetic field change of 0–1.8 T are 1.88, 2.26 and 1.69 J/kg K for x=2.4$x=2.4$, 2.6 and 2.8, respectively. The temperature-dependent XRD analysis shows that there is no crystallographic transition for these alloys, which can explain their low magnetic entropy changes.     

Formation mechanism of ferromagnetism in Si1−xMnx diluted magnetic semiconductors

Si_1−xMn_x diluted magnetic semiconductor (DMS) bulks were formed by using an implantation and annealing method. Energy dispersive X-ray fluorescence, transmission electron microscopy (TEM), and double-crystal rocking X-ray diffraction (DCRXD) measurements showed that the grown materials were Si_1−xMn_x crystalline bulks. Hall effect measurements showed that annealed Si_1−xMn_x bulks were p-type semiconductors. The magnetization curve as a function of the magnetic field clearly showed that the ferromagnetism in the annealed Si_1−xMn_x bulks originated from the interaction between interstitial and substitutional Mn⁺ ions, which was confirmed by the DCRXD measurements. The magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature was approximately 75 K. The present results can help to improve understanding of the formation mechanism of ferromagnetism in Si_1−xMn_x DMS bulks.     

Magnetic and magnetocaloric properties of Y bMn6Sn6−xInx

We have synthesized three Y bMn₆Sn_6−xIn_x representatives (x=0.45, 0.80, 1.20), the first pseudo-ternary RMn₆Sn_6−xX′_x compounds involving a divalent R metal. The crystal structure is found to evolve with the In concentration without modification of the Yb valency: Y bMn₆Sn_5.55In_0.45 is isotypic with HoFe₆Sn₆ (Immm) while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 crystallize in the TbFe₆Sn₆-type (Cmcm). The In content is also determining as regards the magnetic and magnetocaloric properties: Y bMn₆Sn_5.55In_0.45 () almost behaves like a simple ferromagnet while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 also order ferromagnetically but at significantly lower temperatures ( and 129 K, respectively) and are further characterized by the interference of low temperature antiferromagnetic interactions. The results are discussed and compared to previously published data.     

ESR investigation of the spin dynamics in (Gd1−xYx)2PdSi3

We report electron-spin resonance (ESR) measurements in polycrystalline samples of (Gd_1−xY_x)₂PdSi₃. We observe the onset of a broadening of the linewidth and of a decrease of the resonance field at approximately twice the Néel temperature in the paramagnetic state. This characteristic temperature coincides with the electrical resistivity minimum. The high-temperature behavior of the linewidth is governed by a strong bottleneck effect.     

Optical properties and deep levels in annealed Si1−xMnx bulk materials

The optical properties and the deep levels in bulk Si_1−xMn_x formed by using an implantation and annealing method were investigated. Transmission electron microscopy, X-ray diffraction, and Hall-effect measurements showed that the annealed bulk Si_1−xMn_x samples were p-type crystalline semiconductors. The photoluminescence spectra for the annealed bulk Si_1−xMn_x material showed luminescence peaks corresponding to excitons bound to neutral acceptors and related to dislocations due to the existence of Mn impurities. Deep-level transient spectroscopy results for the annealed bulk Si_1−xMn_x showed deep levels related to the interstitial and substitutial sites of the Mn⁺ ions. These results can help improve understanding of the optical properties and the deep levels in annealed bulk Si_1−xMn_x material.     

Magnetocaloric effect in the intermetallic compound DyNi

Magnetic and heat capacity measurements have been carried out on the polycrystalline sample of DyNi, which crystallizes in the orthorhombic FeB structure (space group Pnma). This compound is ferromagnetic with a Curie temperature of 59 K. Magnetization-field isotherms at low temperatures show a multi-step behavior characteristic of metamagnetic transitions. The magnetocaloric effect has been measured both in terms of isothermal magnetic entropy change and adiabatic temperature change for various applied magnetic fields. The maximum values of the entropy change and the temperature change are found to be 19 J kg⁻¹ K⁻¹ and 4.5 K, respectively, for a field of 60 kOe. The large magnetocaloric effect is attributed to the field-induced spin-flop metamagnetism occurring in this compound, which has a noncollinear magnetic structure at low fields.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Spin-glass-like phase behaviour in CeNi2−xCuxSn2

We report the results of our investigation in CeNi_2−xCu_xSn₂ (x=0, 0.4, 1.0, 1.6 and 2.0), a new pseudoternary series with CaBe₂Ge₂-type tetragonal structure. Substitution of Cu for Ni leads to a linear increase in the constants a, c and the unit cell volume v. As probed by the low temperature dependence of ac susceptibility χ_ac(T), the T_f temperature, which corresponds to the freezing temperature of the spin-glass clusters, is annihilated above 2.0 K significantly for the samples with x≥1.6. This observation proves conclusively that the Ni-rich samples in the series CeNi_2−xCu_xSn₂ have the advantage of forming the spin-glass-like state.