Electrical transport in UNi0.5Sb2 single crystals

Single crystals of UNi₀.₅Sb₂ were investigated by means of Seebeck coefficient and Hall effect measurements in the temperature range 5–300 K. The results corroborated the occurrence of two magnetic phase transitions: from para- to antiferromagnetic state at T_N = 161.5 K and a spin-reorientation near T_t = 64 K. The first-order character of the latter feature was proved by studying in detail the electrical resistivity and the magnetic susceptibility of single-crystalline UNi₀.₅Sb₂ in the vicinity of T_t.     

Magnetic ordering in Th3P4-type Tb4Sb3 compound

The nature of the magnetic ordering of Tb₄Sb₃ compound (Th₃P₄-type, cubic; cI28, space group , No. 220, a = 0.91518(7) nm) has been investigated by using the techniques of magnetization and neutron diffraction. AC and DC magnetisation measurements indicate antiferromagnetic ordering at 108 K in zero magnetic field that is accompanied by a field-induced metamagnetic transition to a ferromagnetic state, in fields above 0.3 T. Neutron diffraction experiment in zero applied magnetic field shows that below T_N = 112(4) K Tb₄Sb₃ exhibits an antiferromagnetic flat spiral-type ordering with propagation vector K₁ = [±1/8, ±1/8, ±1/8]. The magnetic moment of Tb atoms is found to be M_Tb = 6.7(3) μ_B at 80 K. The magnetic moment of Tb atoms lie in the (1 1 1) plane of Tb₄Sb₃ unit cell (the cone axis arranges along [1 1 1] direction with cone angle β = 90°). Below T_N2  50 K, Tb₄Sb₃ shows second antiferromagnetic transition with K₂ = [1/2, 1/2, 1/2] with possible re-orientation of Tb magnetic moments.     

Structure and magnetic properties of pseudoternary ThMn12-type uranium compounds

Magnetic properties of UFe₉MnSi₂, UNiMnSi₂ and UNi_9.5Co_0.5Si₂ were investigated at temperature range 4.2–1000 K and in magnetic field up to 14 T at 4.2 K. The first compound is ferromagnetic below T_C=445 K with saturation magnetic moment about 7 μ_B/f.u. The derivatives of paramagnetic UNi₁₀Si₂ are both nonmagnetic. Additionally, ⁵⁷Fe Mössbauer effect was examined in UFe₉MnSi₂ in the temperature range between 20 K and about 500 K. This last experiment suggests that the magnetic order of the iron sublattice is not simple ferromagnetic.     

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.     

Glass formation ability, structure and magnetocaloric effect of a heavy rare-earth bulk metallic glassy Gd55Co20Fe5Al20 alloy

The structure and magnetic entropy changes in melt-spun and annealed LaFe_11.8−xCo_xSi_1.2 (x = 0, 0.4 and 0.8) ribbons have been investigated. It is found that the value of T_c can be increased continuously up to 290 K for x = 0.8 and the phase transition, at T_c from paramagnetic to ferromagnetic, is changed from first- to second-order due to Co substitution. Large values of the magnetic entropy change are 31 and 13.5 J/kg K in the magnetic field change from 0 to 5 T at 201 K for the LaFe_11.8Si_1.2 and at 290 K for the LaFe₁₁Co_0.8Si_1.2 ribbons, respectively. The magnetic entropy change in the LaFe₁₁Co_0.8Si_1.2 ribbons is higher than that reported in the bulk counterpart and that of conventional MCE materials, such as pure Gd. The enhanced magnetic entropy change of ribbons compared to bulk counterpart is attributed to a more uniform microstructure and element distribution resulting from the high cooling rate by melt-spinning.     

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.     

Magnetocaloric effect in Pr6Co1.67Si3 compound

Magnetic properties and magnetocaloric effects of Pr₆Co_1.67Si₃ compound have been investigated by magnetization measurements. The saturation moment at 5 K is found to be 10.7μ_B. The compound undergoes two magnetic transitions below Curie temperature T_C = 48 K and shows a reversible second-order magnetic transition around T_C. A magnetic entropy change ΔS = 6.9 J/(kg K) is observed for a magnetic field change from 0 to 5 T. The full width at half maximum of the ΔS peak is found to be about 38 K.     

Magnetic phase transition in Ti-doped ErCo2 alloys

The homogeneity range of U₂Co_17−xSi_x system with the hexagonal Th₂Ni₁₇-type crystal structure extends from x = 1 to 3.4. The variation of the magnetic properties within the homogeneity range was studied on single crystals. All the compounds are ferromagnetic, M_s and T_C decrease monotonously with increasing Si content. The strongly modified magnetic anisotropy of U₂Co_17−xSi_x, as compared to isostructural Lu₂Co_17−xSi_x with the non-magnetic Lu, points to a magnetic state of U up to x = 3.0. The U contribution to K₁ decreases with increasing Si content and vanishes at x = 3.4 that can be treated as a transition from magnetic to non-magnetic state of U. Spin reorientation was observed with varying temperature in compounds with x ≤ 3 due to a competition of the U and Co sublattices anisotropies which occurs as two second-order phase transitions of the “plane–cone” and the “cone–axis” type.     

The crystal structure and magnetic properties of the Gd6Cr4Al43 compound

The crystal structure of intermetallic compound Gd₆Cr₄Al₄₃ has been investigated by means of X-ray diffraction data (Ho₆Mo₄Al₄₃ structure type, space group P6₃/mcm, Pearson symbol hP106, a = 10.9144(7) Å, c = 17.7361(13) Å).

SQUID magnetic measurements carried out for the title compound point to the existence of two antiferromagnetic phase transitions observed at T_N1 = 19.0(1) K and T_N2 = 6.8(1) K, respectively.     

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.     

Gd Mössbauer effect and magnetic properties of GdNi2Sb2, GdCu2Sb2 and GdAl2Ga2

We have investigated the magnetic properties and the ¹⁵⁵Gd Mössbauer spectra of the ThCr₂Si₂-type compounds GdNi₂Sb₂, GdCu₂Sb₂ and GdAl₂Ga₂. These three compounds were found to order antiferromagnetically, with T_N=6.5 K, 15.8 K, and 42.4 K respectively. The electric field gradient V₂₂ derived from the quadrupole splitting of the Mössbauer spectra gives rise to a sign change at the end of the T3d series in GdT₂Sb₂, as was observed previously also for the corresponding compounds with Si and Ge. This behaviour was explained in terms of decreasing hybridization between the Gd valence electron states and the d electron states of the T atoms.     

Study of structure and magnetic properties of the perovskite Tb0.5Sr0.5CoO3

The perovskite compound Tb_0.5Sr_0.5CoO₃ has been prepared and studied for the first time. We report here the structural and magnetic properties of the compound using the DC magnetization and powder neutron diffraction techniques. The compound is found to be orthorhombic with Pbnm space group. The magnetic ground state of the system is ferromagnetic with T_c = 120 K. The ordered magnetic moment is found to be 1.57 (4) μ_B/Co ion at 12 K along the crystallographic b-axis. The observed effective paramagnetic moment μ_eff = 2.54 μ_B/f.u. The role of ionic size effect on the magnetic and transport properties of the compound through the variation of CoOCo bond angles is highlighted.     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Influence of HDDR treatment on magnetic properties of TbFe2- and DyFe2-based intermetallics

Magnetic properties of TbFe₂, DyFe₂, Tb(Fe_0.8M_0.2)₂ and Dy(Fe_0.8M_0.2)₂ with M=Co, Al, Si, Ga alloys affected by the Hydrogenation-Decomposition-Desorption-Recombination processing have been studied. After hydrogen treatment the coercive force H_c grows sharply, so HDDR-powders can be used as isotropic permanent magnets with the energy product up to 26 MG Oe at T=77 K. In Dy(Fe_0.8Al_0.2)₂ at T<10 K the stepwise magnetic reversal has been observed. The HDDR-treatment increases the critical field of magnetic reversal from 11 up to 18.4 kOe at 4.2 K. It is shown that the effect of the stepwise magnetic reversal is caused by a heat release in a sample during an avalanche motion of narrow domain walls.     

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.     

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.     

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).     

Synthesis and magnetic structure of the YbMn2Sb2 compound

A neutron diffraction investigation has been carried out on the trigonal La₂O₃-type (hP5, space group , No. 164; also CaAl₂Si₂-type) YbMn₂Sb₂ intermetallic. A two-step synthesis route has been tried in this work, and successfully utilised to prepare single phase samples of this compound. This study shows that YbMn₂Sb₂ presents antiferromagnetic ordering below 120 K. The magnetic structure of this intermetallic consists of antiferromagnetically coupled magnetic moments of the manganese atoms, in the Mn1 (1/3, 2/3, Z_Mn) and Mn2 (2/3, 1/3, 1 − Z_Mn) sites; the direction of magnetic moments of manganese atoms forming a φ and a θ angle, respectively with the X- and the Z-axis. At 4 K the magnetic moment of the Mn1 atom is μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 62(4)°, whilst the Mn2 atom has a magnetic moment μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 242(4)°. On the other hand, in this compound no local moment was detected on the Yb site.     

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.     

Structure and anelasticity of Fe3Ge alloy

The structure and anelastic properties of Fe-27 at.%Ge alloy are studied. Long-term annealing of the as-cast alloy at 1273 K leads to homogenising and several transformations take place below 873 K. These low temperature transitions are studied by several methods: X-ray diffraction, calorimetry, vibrating-sample magnetometry and internal friction, and are related to magnetic transitions in the different phases. A high stability of the hexagonal (D0₁₉) phase at room temperature is recorded. The hexagonal β (B8₁) phase is also detected in the alloy at room temperature, while the presence of the ′ and phases is doubtful. A broad internal friction relaxation peak with the relaxation strength of Δ = 0.0036, the activation energy of about 1.78 eV and the preexponential relaxation time of τ₀ = 2 × 10⁻¹⁷ s was discovered and classified as the Zener peak in both the and β phases.