Magnetic properties of R3Co29Si4B10 (R=La, Ce, Pr, Nd, Sm, Gd and Dy) borides

The crystal structure and magnetic properties of quaternary rare-earth intermetallic borides R₃Co₂₉Si₄B₁₀ with R=La, Ce, Pr, Nd, Sm, Gd and Dy have been studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in a tetragonal crystal structure with the space group P4/nmm. Compounds with R=La, Ce, Pr, Nd and Sm are ferromagnets, while ferrimagnetic behavior is observed for R=Gd and Dy. The Curie temperatures vary between 149 K and 210 K. The Curie temperatures in R₃Co₂₉Si₄B₁₀ (R=Ce, Pr, Nd, Sm, Gd, Dy) compounds are roughly proportional to the de Gennes factors.     

Enhancement of magnetocaloric effects in La0.8R0.2(Fe0.919Co0.081)11.7 Al1.3 (R=Pr, Nd) compounds

The magnetic properties and magnetocaloric effects (MCEs) in La_0.8R_0.2(Fe_0.919Co_0.081)_11.7Al_1.3 (R=Pr, Nd) compounds have been investigated. When Pr and Nd substitute for La, the Curie temperature of compounds decreases. The values of the MCEs are enhanced significantly by a partial substitution of Nd for La. Under the field change of 2 and 5 T, the maximum magnetic entropy changes for La_0.8Nd_0.2(Fe_0.919Co_0.081)_11.7Al_1.3 compound are −4.56 and −9.46 J/Kg K, respectively. It can be exploited for room temperature magnetic refrigeration material.     

Magnetic, magnetocaloric, and magnetotransport properties of RCo1.8Mn0.2 (R=Er, Ho, Dy, and Tb) compounds

The magnetic, magnetocaloric, and magnetotransport properties of RCo_1.8Mn_0.2 (R=Er, Ho, Dy, and Tb) were studied by room temperature X-ray diffraction, magnetization, and resistivity measurements at a temperature interval of 5-400 K and magnetic fields up to 5 T. The Curie temperature of RCo₂ was found to increase significantly when 10% Mn was substituted for Co. The effective paramagnetic moments were found to be in reasonable agreement with their theoretical values. A large magnetoresistance (MR) of Δρ/ρ_o≈−13.5% for R=Ho at T≈153 K for ΔH=5 T has been observed. The maximum relative cooling capacities vary from 467 J/kg at low temperature for R=Er to 202 J/kg at the near room temperature for R=Tb.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Structural and magnetic properties in Bi1−xRxFeO3 (x=0-1, R=La, Nd, Sm, Eu and Tb) polycrystalline ceramics

A series of rare-earth doped BiFeO₃ samples, Bi_1−xR_xFeO₃ (x=0-1, R=La, Nd, Sm, Eu and Tb), were prepared in this work. X-ray diffraction analysis showed that the structure of rare-earth doped BiFeO₃ was transformed from rhombohedral lattice to orthorhombic one by increasing x. The lattice constants and unit-cell volume decreased with the increasing of the doping content, while both the Néel temperature and magnetization were enhanced. A magnetic phase transition was observed at about 35 K for BiFeO₃. The variation of the magnetization with temperature depended on applied field strength and magnetizing history, which was explained according to the antiferromagnetic exchange interaction between Fe and R sites in Bi_1−xR_xFeO₃(x>0). The magnetocrystalline anisotropy contributed by Fe sublattice gave rise to a large coercivity in Bi_xNd_1−xFeO₃ with an orthorhombic structure.     

Magnetic properties of the intermetallic compound Ce5Ge4

Magnetic and magnetocaloric properties of the compound Ce₅Ge₄ have been studied. This compound has orthorhombic Sm₅Ge₄-type structure (space group Pnma, no. 62) and orders ferromagnetically at ~12 K (T_C). The paramagnetic Curie temperature is ~−20 K suggesting the presence of competing ferromagnetic and antiferromagnetic interactions in this compound. The magnetization does not seem to saturate even in fields of 90 kOe at 3 K consistent with the presence of competing interactions. Saturation magnetization value (extrapolated to 1/H→0) of only 0.8μ_B/Ce³⁺ is obtained compared to the free ion value of 2.14μ_B/Ce³⁺. This moment reduction in the ordered state of Ce₅Ge₄ could be due to partial antiferromagnetic/paramagnetic ordering of the Ce moments and may also be due to crystalline electric field effects. Magnetic entropy change near T_C, calculated from the magnetization vs. field data, is found to be moderate with a maximum value of ~9 J/kg/K at ~11 K for a field change of 90 kOe.     

Magnetic properties of ternary rare-earth compounds of the type R2Fe14B

Ternary tetragonal compounds of the composition R₂Fe₁₄B were observed for R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The lattice constants and the X-ray density of these compounds were determined. Also determined were the magnetic properties, comprising the temperature dependence of the magnetization in the range 4.2–700 K and the field dependence of the magnetization at 4.2 K in fields up to 20 T. These latter measurements were made in two mutually perpendicular directions, making it possible to determine the anisotropy fields. The magnetocrystalline anisotropy was found to consist of contributions due to the Fe and rare-earth sublattice, respectively.     

Critical bond lengths and their role in spontaneous magnetostriction of R2Fe17CX (R=Y,Nd, Gd,Tb, Er)

High-energy high-flux synchrotron X-rays have been used to study the spontaneous magnetostriction of R₂Fe₁₇ (R=Y, Nd, Gd, Tb, Er) and their carbides in the temperature range 10–1100 K. Addition of interstitial carbon greatly increases both the Curie temperatures (T_C) and the spontaneous magnetostrain of the compounds, while reduces the anisotropy of the magnetostrain by expanding the distances between rare-earth and neighboring Fe sites. The increase of T_C with carbon is due to the increased spatial separation of the Fe hexagon layers. On the basal plane, the Fe hexagons are squeezed and the contribution of Fe sublattice to spontaneous magnetostriction is attenuated, while that of rare-earth sublattice is enhanced. The average bond magnetostrain around Fe sites are in linear relation with their hyperfine field intensities.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Magnetocaloric effect in DyCu2

The magnetocaloric effect of the rare-earth intermetallic compound DyCu₂ is explored through magnetization measurements. DyCu₂ is paramagnetic at the room temperature but becomes antiferromagnetic below 27 K (Neel temperature). Strong temperature and field dependence of magnetization in DyCu₂ at and around the Neel temperature lead to a large magnetocaloric effect. An appreciable magnetocaloric effect persists well above the Neel temperature probably because of the presence of short-range ferromagnetic correlations in the paramagnetic state of DyCu₂. This along with the absence of magnetic hysteresis lead to a large effective refrigerant capacity of 194 J/kg below 44 K, which makes the material important as a potential magnetic refrigerant for the cryogenic liquefier cycles.     

Magnetic properties of RE0.7Ca0.3Mn0.95Fe0.05O3 (RE=Sm and Gd) manganites at low temperature

The magnetic properties of RE_0.7Ca_0.3Mn_0.95Fe_0.05O₃ perovskite with rare-earth cations (RE=Sm and Gd) were investigated by means of X-ray diffraction, Mössbauer spectroscopy, and low temperature (4.2-266 K) magnetization measurements. Structural characterization of these compounds shows that they both have orthorhombic (Pbnm) structure. The Mössbauer spectra show clear evidence of local structural distortion of the Mn(Fe)O₆ octahedron, which is based on the non-zero nuclear quadrupole interactions for high-spin Fe³⁺ ions. It was found that the local structural distortion increases significantly when Sm³⁺ is replaced by Gd³⁺. This distortion is attributed to the Jahn-Teller coupling strength as estimated from the Mössbauer effect results. The magnetic results indicate that the Curie temperature decreases as a result of replacing Sm by Gd. This is due to the decrease of the average A-site cationic radius 〈r_A〉. The rapid increase of magnetization at low temperature indicates the magnetic ordering of rare earth ions at the A-site.     

Magnetic properties of R2WO6 (where R=Nd, Sm, Eu, Gd, Dy and Ho)

The magnetic and electrical measurements carried out on the R₂WO₆ tungstates showed a paramagnetic behaviour for samples with R=Nd, Gd, Dy and Ho and more complex one for samples with R=Sm and Eu in the temperature range 4.2-280 K and an insulating state at room temperature. With increasing atomic number of the R element the Curie-Weiss temperature increases from −43.5 K for Nd₂WO₆ to −2.7 K for Ho₂WO₆, excluding Sm₂WO₆ and Eu₂WO₆ compounds for that the Curie-Weiss region is not observed and the imaginary part of susceptibility is close to zero. The effective magnetic moment is close to the theoretical one for the free R ion and the magnetic moment measured in magnetic field of 14 T and at temperature of 4.2 K, generally, does not reach the saturation state. The temperature independent residual susceptibility is negative for Nd₂WO₆ and positive for the remaining compounds suggesting different proportions of the Landau, Pauli and van Vleck contributions to the total susceptibility. An increase of the orbital magnetic contribution to the total magnetic moment is suggested from the fitting of the Landé factor in the compounds under study.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

A comparative study of the magnetoelastic properties of the YFe10V2 and NdFe10V2 compounds

The magnetoelastic properties of iron-rich REFe₁₀V₂ (RE=Nd, Y) compounds were studied via magnetostriction and thermal expansion measurements in the 5–300 K range of temperature in up to 6 T external fields. Results of thermal expansion analysis show that the spontaneous magnetostriction of the compounds mostly originates from itinerant magnetization. Besides, the small volume striction appearing in the thermal expansion of the Nd compound close to 50 K suggests the existence of a basal to conical spin re-orientation transition. The volume magnetostriction isotherms of both compounds take minimum values for external field corresponding to the anisotropy field. In addition, the anisotropic and the volume magnetostriction traces of the NdFe₁₀V₂ take marked maxima under low field, with a relatively large initial magnetostrictivity, again more pronounced at the conical–axial spin re-orientation transition (T_SR=130 K). Analysis of the anisotropic magnetostriction of the Nd compound leads to the conclusion that the contribution of Nd–Fe interactions is negligible. The temperature dependence of volume magnetostriction is in good agreement with prediction of a phenomenological model based upon a fluctuating local band theory. This analysis shows that the difference between the forced volume strictions of Y and Nd compounds below and above T_SR originates from the Nd sublattice magnetization.     

Magnetic order and crystal field in Dy2Ru2O7 and Yb2Ru2O7

The magnetic properties of R₂Ru₂O₇ pyrochlore compounds (R=Yb, Dy) were studied using specific heat down to 0.4 K and bulk magnetic measurements. These two rare-earth elements were chosen to demonstrate the effect of Ru-R exchange interaction on R magnetic sublattice, in two cases of anisotropy: axial in Dy and planar in Yb. Dy₂Ru₂O₇ undergoes a second order transition to a fully ordered state at 1.85 K with no signs of the spin-ice state. In Yb₂Ru₂O₇ the Yb sublattice orders gradually around 8 K due to the Ru molecular field and no further transition is observed down to 0.4 K. Including the Ru molecular field at the R site in calculations based on crystal field parameters known from titanates R₂Ti₂O₇, allowed us to interpret experimental data.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

A magnetic study of the ThCr2Si2-type RPd2Ge2 (R=Pr, Nd) compounds. Magnetic structure of PrPd2Ge2 from powder neutron diffraction

The magnetic properties of the PrPd₂Ge₂ and NdPd₂Ge₂ compounds have been investigated by magnetic measurements, specific heat measurements and neutron diffraction experiments. The PrPd₂Ge₂ compound orders antiferromagnetically below T_N=5.0(2) with an original modulated magnetic structure characterized by a magnetic cell three times larger than the chemical one by tripling of the c parameter. The palladium atom is non magnetic and the Pr moments are parallel to the c-axis with a value of ≈2.0 μ_B at 2 K. The specific heat measurements clearly detect a low temperature transition for the NdPd₂Ge₂ compound, interpreted as a Nd sublattice antiferromagnetic ordering below 1.3(2) K.     

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.     

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 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.