Magnetic structures of Zr6CoAs2-type Ho6−xErxMnBi2 solid solutions

Investigations were made by neutron diffraction on Zr₆CoAs₂-type (space group no. 189) Ho_6−xEr_xMnBi₂ solid solutions. The ferromagnetic ordering temperature decreases from Ho₆MnBi₂ (T_C = 200(6) K) to Er₆MnBi₂ (T_C = 100(4) K), whereas temperatures of ferrimagnetic (or antiferrimagnetic) ordering (T_Ferri and T_AFerri) are found to have non-monotonic dependences on the content of Er: T_Ferri = 58(4) K for Ho₆MnBi₂, T_Ferri = 162(4) K for Ho_4.5Er_1.5MnBi₂, T_Ferri = 150(4) K for Ho₃Er₃MnBi₂, T_AFerri = 78(4) K for Ho_1.5Er_4.5MnBi₂ and T_AFerri = 52(4) K for Er₆MnBi₂.

In these compounds, no local moment was detected on the manganese ion site, except for Ho_1.5Er_4.5MnBi₂ and Er₆MnBi₂ compounds. The manganese magnetic moments (μ_Mn) is 1.5μ_B and these are antiferromagnetically coupled with that of rare earth moments.     

Magnetic entropy change in polycrystalline La1−xKxMnO3 perovskites

Polycrystalline samples of potassium doped lanthanum manganites having nanometric crystallite size have been synthesized by pyrophoric method. The Curie temperature (T_C) of the prepared samples is found to be strongly dependent on K content and spans between 260 and 309 K. Close to T_C, large change in magnetic entropy has been observed in all the samples. The maximum magnetic entropy change observed for samples with different concentration of K, exhibits a linear dependence with the applied magnetic field. Adiabatic temperature change at T_C at 1 T also increases with K doping and attains a maximum of 2.1 K for La_0.85K_0.15MnO₃. Estimated relative cooling power of La_1−xK_xMnO₃ compounds is nearly one-third of pure Gd. In addition to the tuneability of T_C between 260 and 310 K, higher chemical stability, lower eddy current heating and inexpensive preparation technique; the magnetic entropy change in La_0.85K_0.15MnO₃ compound at 1 T magnetic field is found to be 3.00 J/kg K and is 89% to that known for the prototype magnetic refrigerant (pure Gd). Our result on magnetocaloric properties suggests that La_1−xK_xMnO₃ compounds are attractive as a possible refrigerant for near room temperature magnetic refrigeration.     

Electrical, magnetic and elastic properties of La1.2(Sr1−xCax)1.8Mn2O7 (0.0 ≤ x ≤ 0.4)

Polycrystalline bulk samples of double layered manganite system La_1.2(Sr_1−xCa_x)_1.8Mn₂O₇ (0.0 ≤ x ≤ 0.4) were prepared by sol–gel method. After characterizing the samples using XRD and SEM, their electrical, magnetic and elastic properties were investigated. The lattice parameters and cell volume show a monotonous decrease with increase of Ca content, whereas the grain size is found to increase with increasing Ca content. The value of T_IM is found to decrease with Ca content up to x = 0.3 and then a slight increase of T_IM is observed. The low temperature upturn of resistivity is attributed to the spin-glass-like behavior, which is also evidenced by the irreversibility observed between ZFC and FC magnetizations. The conduction mechanism above T_IM can be explained by Mott VRH model. The present magnetization and ultrasonic studies indicate that the system shows a secondary transition at T^*, which decreases with increasing Ca content. Further, the T^* seems to be intrinsic to the present double layered manganite system.     

Crystal and magnetic structures of Laves phase compound NdCo2 in the temperature range between 9 and 300 K

The crystal and magnetic structures of the Laves phase compound NdCo₂ in the temperature range from 9 to 300 K are determined by Rietveld refinement technique, using high-resolution neutron powder diffraction data. The compound crystallizes in space group above the magnetic ordering temperature T_C (≈100 K), in space group I4₁/amd below T_C and in space group Imma below the tetragonal–orthorhombic structural/magnetic transitions at T_M ≈ 42 K. The assignment of the space groups to the crystal structures of NdCo₂ in different temperature ranges complies with the reported Mössbauer studies. Detailed information of the crystal and magnetic structures of NdCo₂ at different temperatures are reported.     

Electronic structure and magnetism of PrNixPt1−x compounds

We have studied influence of the Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi_xPt_1−x compounds. Polycrystalline samples with x = 1, 0.9, 0.75, 0 were prepared and characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The analysis of XRPD data confirmed that the orthorhombic CrB-type structure owned by the parent binary compounds remains conserved through the entire series. The samples were subsequently investigated by specific heat (C_p), magnetization (M) and ac susceptibility measurements in the temperature range 2–350 K and in magnetic fields up to 9 T. All compounds were found to order ferromagnetically. The T_C values monotonously increase with increasing Ni content. To inspect the crystal-field (CEF) effects and magnetocaloric properties specific-heat data were analyzed in detail and the magnetic contribution to the specific heat together with the magnetic entropy have been determined. The results of first principles electronic structure calculations of the PrNi and PrPt confirmed that besides the stable Pr magnetic moments due to localized 4f-electrons only a very small magnetic moments of at most 0.2μ_B is induced at the Ni (Pt) site due to the polarized 3d-electron states (5d-electron states) hybridizing with the Pr 5d-electron states, i.e. the Ni (Pt) moment plays only minor role in the total balance of the magnetic moments in these compounds.     

Temperature dependent neutron powder diffraction study of the Laves phase compound TbCo2

The structure, magnetization and magnetostriction of Laves phase compound TbCo₂ are investigated by temperature dependent high resolution neutron powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure above its Curie temperature T_C and exhibits a rhombohedral distortion (space group ) below T_C. By an appropriate extrapolation of the temperature factor of Co atom above T_C, the Rietveld refinement of the neutron powder diffraction data of the rhombohedral structure converges satisfactorily and reveals that the moments of Co1(3b) and Co2(9e) are almost equal. Tb moment follows well the Brillouin function. The total magnetic moment of TbCo₂ is about 5.8μ_B/f.u., the anisotropic magnetostriction constant λ₁₁₁ is about 4.6 × 10⁻³ and the volume magnetostriction ω_s is about 8.7 × 10⁻³ at 14 K.     

Synthesis characterization of magnetic properties in La0.7−x Lnx Pb0.3 MnO3 (, Nd, Gd, Dy, Sm and Y) perovskite compounds

The influence of La substitution by rare-earth elements on the magnetization behaviors of perovskite oxides, La_0.7−x Ln_x Pb_0.3 MnO₃ (Ln=Pr, Nd, Gd, Dy, Sm and Y), is investigated. The replacement of La ion by Pr, Nd, Gd, Dy, Sm or Y results in a considerable decrease in the ferromagnetic ordering temperature T_C and clearly irreversible behavior in the FC–ZFC curves showing a short-range spin order phase. The fact is in agreement with the smaller ionic radii of Pr, Nd, Gd, Dy, Sm and Y ions in contrast to La ion and the corresponding larger distortion of perovskite structures. The saturation magnetization M_S increases as Pr or Nd content increases while M_S decreases as Sm or Y content increases. Moreover, the saturation magnetization M_S increases and then decreases as Gd or Dy content increases. These results can be explained in terms of the competition between the increase of ferromagnetically interacting spins due to the introduction of magnetic ions with f-shell electrons and suppression of ferromagnetism due to structure tuning induced by the small ionic radius of the interpolated cation into the La-site.     

Effect of Pr and Co substitution on magnetic properties and magnetic entropy changes in LaFe13−xSix compounds

Effect of Pr and Co substitution on magnetic properties and magnetic entropy changes in the cubic NaZn₁₃-type compound LaFe_11.2Si_1.8 has been experimentally investigated. Replacing 30 at.% La with Pr leads to a decrease of Curie temperature from 216 to 203 K, and drives the magnetic transition from second-order to first-order. As a result, magnetic entropy change, under a field change of 0–5 T, increases from 13.7 to 19.4 J/kg K. Substitution of Co for Fe in La₀₇Pr_0.3Fe_11.2Si_1.8 can adjust T_C to around room temperature. A magnetic entropy change of 9.3 J/kg K at T_C = 290 K for a field change from 0 to 5 T is obtained in La₀₇Pr_0.3Fe_10.4Co_0.8Si_1.8. A reversible variation of magnetization with temperature and magnetic field is observed in the present compound, which is highly desired by the magnetic refrigeration application.     

Structural and thermodynamic properties of the pseudo-binary TiCr2−xVx compounds with 0.0≤x≤1.2

The TiCr_2−xV_x compounds with 0.0≤x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature (P–C–T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P–C–T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.     

Evolution of magnetic hardness in the HfFe6Ge6-type RMn6Sn6−xGax and RMn6Sn6−xInx compounds (R=Tb, Dy; 0.2≤x≤1.4)

The HfFe₆Ge₆-type RMn₆Sn_6−xX_x′ solid solutions (R=Tb, Dy, X′=Ga, In; x≤1.4) have been studied by powder magnetization measurements. All the series are characterized by ferrimagnetic ordering and by a decrease in Curie temperatures with the substitution (ΔT_c/Δx≈−39 K for X′=Ga and ΔT_c/Δx≈−75 K for X′=In). The RMn₆Sn_6−xGa_x systems are characterized by a strong decrease in the spin reorientation temperature with substitution (ΔT_t/Δx≈−191 K and −78 K for R=Tb and Dy, respectively) while this transition almost does not change in systems containing indium. The coercive fields drastically decrease with the substitution in the TbMn₆Sn_6−xGa_x system while the substitution of In for Sn has a weaker effect. The coercive fields of the Dy compounds do not vary greatly with the substitution in both series. The behaviour of the TbMn₆Sn_6−xGa_x is compared with the evolutions observed in the TmMn₆Sn_6−xGa_x series. This comparison strongly suggests that the replacement of Sn by Ga changes the sign of the A₀² crystal field parameter.     

Structural and magnetic properties of Sm2Fe17−xCrxC2 nanocrystalline carbides with 0≤x≤2

The crystallographic and the Curie temperature of the Sm₂Fe_17−xCr_xC₂ (x=0.5, 1, 1.5 and 2) carbides have been extensively studied. X-ray diffraction studies have shown that all these alloys are approximately single phases corresponding to the Th₂Zn₁₇ type rhombohedral structure with a small amount of -Fe. The amount of this residual -Fe phase decreases with increasing the Cr atomic content. It decreases from 1 wt% for x=0.5 to 0.4 wt.% for x=2. The lattice parameter c increases as a function of the Cr atomic content x from x=0 to x=1.5 and then decreases. This is due to the Cr atoms which prefer to substitute the Fe atoms in the 6c sites located along the c-axis. The lattice parameter a and the unit-cell volume decrease in all substitution ranges. The insertion of the C atoms leads essentially to an increase of the distances between the 9d and 18h sites and the 9d–18f sites. The Curie temperature reaches a maximum value of 583 K for x=1.5 and then decreases to 551 K for x=2. The enhancement of the T_c for lower Cr contents is due to a lowering of the hybridization of the iron atoms with their neighbors, the magnetovolume effect and the reduction of antiferromagnetic interactions. However, the decrease in T_c for higher Cr content is due to the reduction in the number of Fe–Fe pairs due to the magnetic dilution effect. For given interatomic distances, the exchange coupling of the Cr–Cr atoms is not of antiferromagnetic type and the exchange integral of the Cr–Cr pair is higher than that of the Fe–Fe pair.     

HfFe6Ge6-type YbMn6Ge6−xGax compounds (0.07≤x<0.72) with large magnetocrystalline anisotropy

The HfFe₆Ge₆-type YbMn₆Ge_6−xGa_x solid solution (0.07≤x≤0.72) has been studied by X-ray diffraction, microprobe analysis and powder magnetization measurements. All the compounds order antiferromagnetically between T_N=481 K for x=0.07 and T_N=349 K for x=0.72 and display more or less pronounced spontaneous magnetization at lower temperature. The corresponding Curie points increase from 40 K for x=0.07 to 319 K for x=0.72. The maximum magnetization values of the Ga-rich compounds (M≈5 μ_B/f.u. at 6 K) is compatible with a ferrimagnetic order of the Mn and Yb sublattices whereas the smaller values measured in the Ga-poor compounds suggest the stabilization of non-colinear magnetic structures. All the studied compounds are characterized by rather large coercive fields at low temperature (4.0≤H_c≤8.2 kOe).     

Structural, magnetic and magnetocaloric effect in double perovskite Ba2CrMo1−xWxO6

Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. In double perovskite Ba₂CrMo_1−xW_xO₆ (x = 0, 0.2 and 0.5) prepared by the sol–gel method, the experimental results show the observation of a large magnetocaloric effect (MCE) near the Curie temperature T_C which decreases with the increasing of the substitution of Mo by W. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature T_C, and a large magnetic entropy change (|ΔS_M| ≈ 1.6 J kg⁻¹ K⁻¹) is obtained at 285 K in the sample Ba₂CrMo_0.5W_0.5O₆ under an applied magnetic field of 10 kOe.     

Structural, electrical and magnetic properties of xNiFe2O4 + (1 − x)Ba0.8Sr0.2TiO3 ME composites

Magnetoelectric composites, namely xNiFe₂O₄ + (1 − x)Ba_0.8Sr_0.2TiO₃ were prepared by standard double sintering ceramic method. The X-ray diffraction measurements were carried out to check the phase purity and to calculate the lattice constants. scanning electron microscopy (SEM) micrographs were taken to understand the microstructure of the samples. Dielectric property such as dielectric constant (ε^′) was also studied as a function of frequency in the range 100 Hz–1 MHz. The ac conductivity as a function of frequency and dc resistivity as a function of temperature were studied for different compositions. The hysteresis measurements were done to determine saturation magnetization (M_s) and coersivity (H_c) and magnetic moment was calculated. Effect of resistivity on ME voltage coefficient is studied.     

The effect of substitution of La for Tb on some physical properties of Tb1−xLaxNi2 solid solutions

The standard Gibbs energy of formation, , of MgLa in the temperature range from near absolute 0 to 525 K were determined by calorimetry. The heat capacities, C_p, from 2 K to 525 K were measured by the relaxation method and DSC. Also, a thermal anomaly at 5.9 K, which appeared to be a superconductive phase transition, was found in the obtained C_p values. The values were determined by combining the C_p data with the standard enthalpy of formation at 298 K which was measured by the Calvet-type calorimeter using hydrochloric acid solution. From 2 to 300 K, the increases gradually, and it can be evaluated as a linear function of temperature above 300 K as follows:

This result is expected to be useful as basic thermodynamic data of Mg-based alloys.     

Effect of Mn concentration on the spin glass transition temperature of Fe0.70−xMnxAl0.30 alloys

Magnetization measurements on zero field cooled (ZFC) and field cooled (FC) samples as a function of temperature where made on Fe_0.70−xMn_xAl_0.30 alloys with 0≤x≤0.10. Our data show the existence of two regimes of the spin glass temperature T_f when increasing the concentration x of the Mn atoms. We found that T_f decreases slowly with increasing x up to x=0.06 and then shows a rapid linear decrease above this concentration. The existence of two regimes of T_f behavior with x may be attributed to the disappearance of ferromagnetic clusters which exist for low values of x (x<0.06). Compared to the Mn-free alloy, the relative changes in T_f are bigger and exceed 40% at x=0.10. The second regime (x>0.06) is associated with more influence of x on T_f.     

Structural, magnetic and magnetostrictive studies of Tb0.27Dy0.73(Fe1 − xAlx)2

Measurements of magnetic properties, X-ray diffraction and magnetostriction were made on Tb_0.27Dy_0.73(Fe_{1 − x}Al_x)₂ (x = 0.1, 0.2, …, 0.7) compounds. It was found that the system has the cubic MgCu₂ structure over almost the whole (Fe,Al) concentration range investigated, except for a narrow intermediate range (x = 0.4–0.6) where the hexagonal MgZn₂ structure appears. With increasing Al content x, the lattice constant a increases linearly with x. The first replacement of Fe results in a marked decrease in the Curie temperature, which is followed by a slight decrease in T_C with x. A linear decrease in magnetostriction of |λ_| − λ| at room temperature with x was also observed from 1530 × 10⁻⁶ for x=0 to 36×10⁻⁶ for x=0.3. The saturation magnetization σ_s exhibits a complex concentration dependence in the Tb_0.27Dy_0.73(Fe)_{1 − x}Al_x)₂ system: in the range x < 0.5, σ_s increases linearly with x and, for x = 0.5–0.6, σ_s decreases and then increases again. An enhancement of the magnetic ‘hardness’ in this system was also observed at low temperature.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

Influence of carbon introduced by arc melting on the structural and magnetic properties of RFe12−xVx (R = Y and Dy)

Carbon was introduced by arc melting into RFe_12−xV_x compounds, where R = Y or Dy and x = 2, 3 or 4, and its effects on the crystallographic and magnetic properties of these intermetallics were investigated by X-ray, magnetostatic and Mössbauer effect measurements. Experimental results show that the carbon atoms prefer the 4d sites to the 2b sites, or occupy the sites reserved for Fe atoms in the ThMn₁₂-type crystal structure. Carbon solution increases with increasing vanadium concentration, giving rise to an effect opposite to that of vanadium. An increase in vanadium content causes an increase in the cell volume and a linear decrease in Curie temperature and in the Fe magnetic moment, whereas the addition of carbon leads to a reduction in unit volume while maintaining a high T_c and enhancing the Fe magnetic moment despite decreasing iron content in the samples.     

Lattice effect on the magnetic and magneto-transport properties of (La1/3Sm2/3)0.67Ba0.33−xSrxMnO3 (x = 0.0, 0.1, 0.2 and 0.33) compounds

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of (La_1/3Sm_2/3)_0.67Ba_0.33MnO₃ system, has been investigated. The samples, (La_1/3Sm_2/3)_0.67Ba_0.33−xSr_xMnO₃ (x = 0.0, 0.1, 0.2 and 0.33), synthesized by citrate gel route, crystallize in an orthorhombic structure (space group Pnma, no. 62). The unit cell volume decreases while the metal-insulator transition temperature (T_MI) increases with increasing Sr content. The localization of charge carriers occurs at low temperatures and becomes more pronounced with decreasing Sr content which leads to an enhancement of resistivity. This could be understood by the variation of MnOMn bond-distance and angle. Reappearance of semiconducting behavior (dρ/dT < 0) is observed only in samples with x = 0 and x = 0.1 below certain temperature (T < T_MI). These samples exhibit thermal irreversibility behavior for a field-cooled (FC) and zero-field-cooled (ZFC) magnetization data in a magnetic field of 100 Oe. This is ascribed to the competition between the superexchange and double exchange interactions. The change in physical properties has been correlated to chemical parameters such as ionic radii, tolerance factor, electronegativity and variation in MnOMn angle.