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.     

Magnetic entropy change in GdCo13−xSix intermetallic compounds

The magnetic entropy change in GdCo_13−xSi_x (x=3.8, 4, 4.1, and 4.2) intermetallic compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. It was found that the magnetic ordering temperatures decrease from 60 K at x=3.8 to 28 K for x=4.2. The magnetic entropy change is calculated from isothermal magnetization versus magnetic field at various temperatures using the Maxwell relation. As a result, the maximum magnetic entropy changes of the investigated compounds, at their Curie temperatures, decrease from 11.5 J/kg K for x=4.2 to 6.86 J/kg K for x=3.8 in a field change of 0-3 T, whereas it decreases from 5.13 J/kg K for x=4.2 to 2.60 J/kg K for x=3.8 in a field change of 0-1 T. Moreover, the maximum value of the magnetic entropy change obtained at a higher field for GdCo_13−xSi_x with x=4 (23.75 J/kg K at 5 T) is comparable to that of various types of compounds with a cubic NaZn₁₃-type structure. Finally, the maximum of the magnetic entropy change is found to decrease with increasing Si content.     

Magnetic and magnetocaloric properties of Gd1−xScxNi2 solid solutions

Magnetic and heat capacity measurements have been carried out on the polycrystalline Gd_1−xSc_xNi₂ solid solutions (0≤x≤1), which crystallize in the cubic C15 Laves phases superstructure (space group F4?3m). These solid solutions are ferromagnetic with a Curie temperature below 76 K. Their Curie temperature decreases from 75.4 K for GdNi₂ to 13.6 K for Gd_0.2Sc_0.8Ni₂. At high temperatures, all solid solutions, except ScNi₂, are Curie-Weiss paramagnets. The Debye temperature as well as phonon, conduction electron and magnetic contributions to the heat capacity have been determined from heat capacity measurements. The magnetocaloric effect has been estimated both in terms of isothermal magnetic entropy change and adiabatic temperature change for selected solid solutions in magnetic fields up to 3 T.     

Structure and magnetic properties of LaFe11.5−xCoxSi1.5C0.2 compounds for magnetic refrigeration near room temperature

The influence of cobalt on the microstructural, magnetic and magnetocaloric properties of LaFe_11.5−xCo_xSi_1.5C_0.2 (x=0.50–0.85) compounds was investigated. The ingots were prepared by using a vacuum induction melting furnace. Before annealing, a large amount of 1:13 phase was distinctly observed. Nearly single 1:13 phase was obtained after annealing at 1353 K for only 3 days. The easy formation of 1:13 phase in the annealing process could be attributed to carbon doping. The Curie temperature (T_C) increases linearly with increasing the cobalt content. Although the maximum magnetic entropy changes of the compounds decrease rapidly when T_C rises from 275 to 298 K, and it decreases mildly when T_C continues to rise. Two composite refrigerants based on the compounds are proposed. Their entropy changes remains approximately constant over the temperature range from 266 to 292 K and 289 to 309 K.     

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.     

Magnetic properties of YFe12−xMox compounds and magnetocaloric effect of YFe9.5Mo2.5

The characterization and magnetic properties of YFe_12−xMo_x (x=2.0, 2.5 and 3.0) with the ThMn₁₂-type structure, and the magnetocaloric effect of YFe_9.5Mo_2.5 were investigated. A directional growth was observed in YFe₁₀Mo₂ alloy. A broad peak in the zero-field-cooling (ZFC) magnetization curve of the YFe_12−xMo_x compounds is ascribed to the existence of ferromagnetic clusters with different site moments and scattered orientations of the moments. The broad range of the peak is reduced with increasing Mo content. A weak peak is observed near 190 K in the ZFC curve of YFe₉Mo₃, which is associated with the 8i sites being mostly occupied by Mo atoms. YFe_9.5Mo_2.5 has a magnetic entropy change of −1.09 J/kg K for a field change of 5 T at 277 K.     

The analysis of the magnetic properties in the intermetallic YxGd1−xNi3 compounds

In the paper an influence of Gd/Y substitution on the magnetic properties and exchange interactions of the Y_xGd_1−xNi₃ (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) polycrystalline compounds have been studied. The partial replacement of Gd by Y atoms is reflected in decreasing of the Curie temperature (T_C) as well as decreasing of effective the magnetic moment (μ_eff). It has been shown that such a behaviour strongly depends on the magnetic interactions. Exchange coupling parameters of R–R (A_RR), T–T (A_TT) and R–T (A_RT) have been evaluated from M(T) magnetization curves (2–300 K, 2 T) based on the mean field theory (MFT) calculation. The magnetocaloric effect (MCE) has been estimated from the family of magnetic isotherms. The magnetic entropy indicates relatively small change with the Gd/Y substitution. The value of ΔS_m(T,H) is higher for Gd-rich compounds and, respectively, decreases with Gd/Y substitution.     

Structure, magneto-structural transitions and magnetocaloric properties in Ni50−xMn37+xIn13 melt spun ribbons

Melt spun Ni_50−xMn_37+xIn₁₃ (2≤x≤5) ribbons were investigated for the structure, microstructure, magneto-structural transitions and inverse magnetocaloric effect (IMCE) associated with the first-order martensitic phase transition. The influence of excess Mn in Ni site (or Ni/Mn content) on the martensite transition and the associated magnetic and magnetocaloric properties are discussed. It was found that with the increase in Mn content, the martensitic transition shifted from 325 to 240 K as x is varied from 2 to 4, and the austenite phase was stabilized at room temperature. The x=5 ribbon did not show the martensitic transition. For the x=3 ribbon, the structural and magnetic transitions are close together unlike in the x=4 ribbon in which they are far (∼60 K) apart. The zero field cooled and field cooled curves support the presence of exchange bias blocking temperature due to antiferromagnetic interactions in the ribbons. A large change in the magnetization between the martensite and austenite phases was observed for a small variation in the Ni/Mn content, which resulted in large IMCE. A large positive magnetic entropy change (ΔS_M) of 32 J/kg K at room temperature (∼ 300 K) for a field change of 5 T with a net refrigeration capacity of 64 J/kg was obtained in the Ni₄₇Mn₄₀In₁₃ ribbon.     

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.     

Metamagnetic transition and magnetocaloric effect in charge-ordered Pr0.68Ca0.32−xSrxMnO3 (x=0, 0.1, 0.18, 0.26 and 0.32) compounds

In this study, magnetic and magnetocaloric properties of Pr_0.68Ca_0.32−xSr_xMnO₃ (x=0, 0.1, 0.18, 0.26 and 0.32) compounds were investigated. X-ray results indicated that all the samples have a single phase of orthorhombic symmetry. The orthorhombic unit cell parameters increase with the increase in Sr content. Large negative magnetic entropy changes (−26.2 J/kg K at 38 K and 5 T for x=0 and −6.5 J/kg K at 83 K and 6 T for x=0.1) were attributed to ultrasharp metamagnetic transitions. The peak value of ΔS_m decreased from −4.1 J/kg K for x=0.18 sample to −2.4 J/kg K for x=0.32 at 1 T magnetic field.     

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.     

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.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Near room temperature magnetocaloric properties of melt-spun Gd100−xBx (x=0, 5, 10, 15, and 20 at%) alloys

The magnetocaloric properties of melt-spun Gd-B alloys were examined with the aim to explore their potential application as magnetic refrigerants near room temperature. A series of Gd_100−xB_x (x=0, 5, 10, 15, and 20 at%) alloys were prepared by melt spinning. With the decrease in Gd/B ratio, Curie temperature (T_C) remains constant at ∼293 K, and saturation magnetization, at 275 K, decreases from ∼100 to ∼78 emu/g. Negligible magnetic hysteresis was observed in these alloys. The peak value of magnetic entropy change, (−ΔS_M)_max, decreased from ∼9.9 J/kg K (0-5 T) and ∼5.5 J/kg K (0-2 T) for melt-spun Gd to ∼7.7 J/kg K (0-5 T) and ∼4.0 J/kg K (0-2 T), respectively for melt-spun Gd₈₅B₁₅ and Gd₈₀B₂₀ alloys. Similarly, the refrigeration capacity (q) decreased monotonously from ∼430 J/kg (0-5 T) for melt-spun Gd to ∼330 J/kg (0-5 T) for melt-spun Gd₈₀B₂₀ alloy. The near room temperature magnetocaloric properties of melt-spun Gd_100−xB_x (0≤x≤20) alloys were found to be comparable to few first-order transition based magnetic refrigerants.     

Magnetic properties and room-temperature magnetocaloric effect in the doped antipervoskite compounds Ga1−xAlxCMn3 (0≤x≤0.15)

We report the effects of Al doping on the structure, magnetic properties, and magnetocaloric effect of antiperovskite compounds Ga_1−xAl_xCMn₃ (0≤x≤0.15). Partial substitutions of Al for Ga enhance the Curie temperature (from 250 K for x=0.0 to 312 K for x=0.15) and the saturation magnetization. On increasing the doping level x, the maximum values of the magnetic entropy change (−ΔS_M) decreases while the temperature span of −ΔS_M vs. T plot broadens. Furthermore, the relative cooling power (RCP) is also studied. For 20 kOe, the RCP value tends to saturate at a high doping level (for x=0.12, 119 J/kg at 296 K). However, at 45 kOe, the RCP value increases quickly with increasing x (for x=0.15, 293 J/kg at 312 K). Considering the relatively large RCP and inexpensive raw materials, Ga_1−xAl_xCMn₃ may be alternative candidates for room-temperature magnetic refrigeration.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Magnetic/structural diagram, chemical composition-dependent magnetocaloric effect in self-doped antipervoskite compounds Sn1−xCMn3+x (0≤x≤0.40)

The effects of Mn substitutions on the crystal structure, magnetic properties, and magnetocaloric effect (MCE) of antiperovskite Sn_1−xCMn_3+x (0≤x≤0.40) have been investigated detailedly. Both the Curie temperature (T_C) and the magnetizations at 40 kOe decrease with increasing x firstly for x≤0.10, and then increase with increasing x further. The type of magnetic transition changes from first-order to second-order around x=0.10 with increasing x. Chemical composition-dependent MCE is also studied around T_C. With increasing x, the maximal magnetic entropy changes decrease and the magnetic phase transitions broaden. Accordingly, the relative cooling power (RCP) increases with increasing x, reaching the largest values of ∼0.56 J/cm³ (∼75 J/kg) and ∼1.66 J/cm³ (∼221 J/kg) with the magnetic changes of 20 kOe and 48 kOe, respectively. Considering the large RCP, inexpensive, and innoxious raw materials, these serial samples Sn_1−xCMn_3+x are suggested to be potential room-temperature magnetic refrigerant materials.     

Theoretical investigation of the conduction and valence band offsets of GaAs1−x Nx/GaAs1−yNy heterointerfaces

Theoretical investigations of the conduction band offset (CBO) and valence band offset (VBO) of the relaxed and pseudo-morphically strained GaAs_1−xN_x/GaAs_1−yN_y heterointerfaces at various nitrogen concentrations (x and y) within the range 0-0.05 and along the [0 0 1] direction are performed by means of the model-solid theory combined with the empirical pseudopotential method under the virtual crystal approximation that takes into account the effects of the compositional disorder. It has been found that for y < x, the CBO and VBO have negative and positive signs, respectively, whereas the reverse is seen when y > x. The band gap of the GaAs_1−xN_x over layer falls completely inside the band gap of the substrate GaAs_1−yN_y and thus the alignment is of type I (straddling) for y < x. When y > x, the alignment remains of type I but in this case it is the band gap of the substrate GaAs_1−yN_y which is fully inside the band gap of the GaAs_1−xN_x over layer. Besides the CBO, the VBO and the relaxed/strained band gap of two particular cases: GaAs_1−xN_x/GaAs and GaAs_1−xN_x/GaAs_0.98N_0.02 heterointerfaces have been determined.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

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.