Magnetic properties of R2Zn17 compounds (R = rare earth)

Magnetization measurements have been performed on polycrystalline R₂Zn₁₇ compounds (R Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb) in the temperature range 1.5–300 K. In Yb₂Zn₁₇, the rare earth exhibits a valence of 2+ and the compound is not magnetic. All the other compounds order antiferromagnetically at low temperature. The main magnetic characteristics have been determined.     

Magnetic properties of R2CoGa3 (R = rare earth) compounds

R₂CoGa₃ compounds (R = Gd, Th, Dy, Ho, Er and Y) crystallize in the hexagonal P6,lmcm space group. In these compounds the Co atoms have practically no magnetic moment. The heavy rare earth compounds order ferromagnetically with relatively low Curie temperatures ( 3 K (Er) T_c 50 K (Gd)). The compounds with Tb and Dy, and to a smaller extent the Ho compound, present irreversibilities in their magnetization processes, and there are maxima in the zero-field-cooled susceptibility curves.     

Magnetic and electrical properties of the UCu2T3Al7 alloys

The UCu₂T₃Al₇-type compounds with T = Cr, Mn and Fe crystallize in the tetragonal ThMn₁₂-type structure (I4/mmm space group). Magnetic properties have been examined in magnetic fields up to 50 kOe in the temperature range 1.9–400 K using a SQUID magnetometer. The Cr based pseudoternary alloy is paramagnetic with the linear field dependence of the magnetization at 1.9 K. The temperature dependence of the magnetic susceptibility is described by a modified Curie–Weiss (MCW) law. The Mn containing compound is ferrimagnetic and the field dependence of the magnetization does not reach saturation while the magnetic susceptibility follows the MCW law at high temperature. The Fe alloy is ferromagnetic and at higher temperature the MCW law is fulfilled. The temperature dependence of the electrical resistivity of all materials was examined between 4.2 and 300 K showing metallic character. The Mn and Fe based compounds demonstrate some anomalies in the ρ(T) plot at low temperature. The results are discussed in relation to the anomalies observed in magnetic measurements.     

Synthesis and magnetic properties of R2Co17Nx type interstitial compounds

A series of interstitial ternary nitrides R₂Co₁₇N_x with 1.6 ≤ x ≤ 2.7 was prepared, with R Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Y. The nitrides have the same structures as the parent compounds, i.e. the Th₂Zn₁₇ or Th₂Ni₁₇ type, but the unit-cell volumes are about 6% larger. The Co magnetic moment and the R---Co coupling strength are found to be reduced by the nitrogenation. The diffusion of nitrogen in the R₂Co₁₇ compounds is remarkably slower than in R₂Fe₁₇. Decomposition of all R₂Co₁₇N_x compounds is found to start above 870 K.     

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.     

Neutron and resonant X-ray scattering studies of RNi2B2C (R=rare earth) single crystals

This family of intermetallic compounds is ideal for the study of the interplay between superconductivity and magnetism since, in several of these compounds (Ho, Er, Tm, Dy), superconductivity coexists with magnetic ordering. The most important findings of our scattering studies are (a) in the Ho-compound, a complex magnetic structure characterized by two incommensurate wave vectors, _a=0.585 ^* and _c=0.915 ^*, exists in the vicinity of 5 K, where the almost re-entrant behavior of this compound occurs (below 4.7 K, it is a commensurate antiferromagnet); (b) an incommensurate magnetic structure with wave vector along ^*, close to the zone boundary, is observed in several of these compounds (Ho, Er, Tb, Gd) and (c) pronounced soft-photon behavior was observed for both the acoustic and first optical Δ₄[ξ00] branches in the superconducting Lu and Ho compounds, a behavior characteristic of strongly coupled conventional superconductors. Furthermore, these phonon anomalies occur at wave vectors close to those of the incommensurate magnetically ordered structures observed in the magnetic compounds of this family (see (b)). This observation suggests that both the magnetic ordering and phonon softening originate from common nesting features of the Fermi surfaces of these compounds. Band theoretical calculations are in qualitative agreement with these results.     

Magnetocrystalline anisotropy of R2Fe17Hx (x=0, 3) single crystals

The magnetocrystalline anisotropy of R₂Fe₁₇ (R=Y, Gd, Tb, Ho and Er) and their hydrides are studied by analyzing the magnetization curves of single crystal samples in the temperature range 4.2–300 K in magnetic fields up to 140 kOe. There is no noticeable influence of hydrogenation on the magnetic anisotropy in the Y₂Fe₁₇ and Gd₂Fe₁₇ compounds. An easy-plane to easy-cone transition is detected for a Tb₂Fe₁₇H₃ single crystal after hydrogenation. A significant change of the magnetization process has been observed in the hydrides Ho₂Fe₁₇H₃ and Er₂Fe₁₇H₃. Hydrogenation induces a FOMP-type transition in the Ho₂Fe₁₇H₃ compound and, on the contrary, leads to the disappearance of the FOMP type transition in the Er₂Fe₁₇H₃ compound. The obtained results are discussed on the basis of a model based on the interaction of the quadrupolar moment and the magnetic moment of the 4f electron shell of the rare earth ion with the interstitial hydrogen. It is established that the orientation of the quadrupolar moment of the asymmetric 4f shell with respect to the direction of the resulting magnetic moment of 4f electrons plays an important role.     

Crystal structures of R2Pd2Pb (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds

The crystal structures of the R₂Pd₂Pb (R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds were determined using X-ray powder diffraction. The investigated compounds crystallize with Mo₂FeB₂ structure type (space group P4/mbm, Pearson code tP10). The importance of stabilization by polar intermetallic R–Pd bonding is underscored by a bonding analysis derived from electronic band structure calculations.     

New RRh5Ge3 compounds of the new SmRh5Ge3 structure type and their magnetic properties (R=Sm, Gd, Tb)

Powder X-ray diffraction results and macroscopic magnetic properties of new ternary RRh₅Ge₃ compounds (R=Sm, Gd, Tb) are reported. The compounds SmRh₅Ge₃ (a=2.2744(4) nm, c=0.3888(1) nm), GdRh₅Ge₃ (a=2.2711(5) nm, c=0.3872(1) nm) and TbRh₅Ge₃ (a=2.2628(7) nm, c=0.3851(1) nm) crystallize in the hexagonal SmRh₅Ge₃-type structure (space group P6₃/m; No. 176). The GdRh₅Ge₃ and TbRh₅Ge₃ compounds are Curie–Weiss paramagnets down to 5 K.     

CALCULATION OF THE EXCHANGE INTERACTION CONSTANT BETWEEN RARE-EARTH AND TRANSITION METAL SUBLATTICESIN R_2Fe_(17_x)Al_x(R=Tb, Gd AND Dy)COMPOUNDS

An approximate calculation of the exchange interaction constant JRT between the rare-earth sublattice and the transition metal sublattice in R2Fe17-xAlx (R=Tb, Gd, and Dy) compounds is given by the molecular-field model and the results of neutron diffraction. The calculated values, -JR,T/k, for Dy2Fe17-xAlx (x=5, 6, 7 and 8), Tb2Fe10Al7, Gd2Fe17-xAlx (x = 7, 8) compounds are 8.62K, 8.64K, 9.52K, 10.34K and 10.66K, 10.65K, and 9.85K, respectively, they are in agreement with the experimental values, -JR,T/k, of Dy2Fe17-xAlx (x=5, 6, 7 and 8), Tb2Fe10Al7 and Gd2Fe17-xAlx (x=7, 8) compounds, which are 8.77K, 9.25K, 10.1K, 10.9K and 10.35K, 10.1K, and 10.3K, respectively. The origins of the difference between the calculated and the experimental results are discussed.     

Thermoelectric properties of rare earth doped SrTiO3

The thermoelectric properties of Sr_0.9R_0.1TiO₃ (R=Y, La, Sm, Gd, Dy) have been measured from room temperature to 1073 K. The electrical conductivities and Seebeck coefficients are independent of the kind of rare earth elements in the temperature range, so the figure of merits are influenced by the difference in the thermal conductivities. The thermal conductivities decrease with doping according to the rare earth atomic mass and ionic radius. Sr_0.9Dy_0.1TiO₃ shows the highest figure of merit of the investigated samples, reaching 3.84×10⁻⁴ K⁻¹ at 573 K.     

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.     

Structural, magnetic and magnetocaloric properties in Pr0.5M0.1Sr0.4MnO3 (M = Eu, Gd and Dy) polycristalline manganites

Structural, magnetic and magnetocaloric properties of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) powder samples have been investigated by X-ray diffraction (XRD) and magnetic measurements. Our samples have been synthesized using the solid state reaction method at high temperature. Rietveld refinements of the X-ray diffraction patterns show that all our samples are single phase and crystallize in the distorted orthorhombic system with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT show that all our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to be 270 K, 258 K and 248 K for M = Eu, Gd and Dy, respectively. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of Pr_0.5M_0.1Sr_0.4MnO₃ (M = Eu, Gd and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS_M| and the relative cooling power RCP have been determined. In the vicinity of T_C, |ΔS_M| reached, in a magnetic applied field of 1 T, maximum values of 1.37 J/kg K, 1.23 J/kg K and 1.18 J/kg K for M = Eu, Gd and Dy, respectively.     

Neutron diffraction study of RNiC2 (R =rare earth) compounds

Low temperature neutron diffraction has been performed on the series of isostructural RNiC₂ compounds with R≡ Pr, Nd, Tb, Dy, Ho, Er and Tm (space group Amm2). The rare earth sublattices (with the exception of PrNiC₂) are found to order antiferromagnetically. Structural parameters and magnetic moment configurations have been determined. A systematic variation of the magnetic structures according to the sequence of rare earth elements is discussed.     

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.     

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.     

Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er)

The REFe₆Sn₄Ge₂ (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe₆Ge₆ structure although the parent ternary compounds REFe₆X₆ (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ T_N ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ T_t ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice.     

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.     

Magnetic properties and spin-reorientation transition in DyFe10−xNixSi2 compounds

The magnetic properties of DyFe_10−xNi_xSi₂ compounds with x = 0, 1, 2, 3, 4, 6, 9 and 10 have been investigated by means of X-ray diffraction and magnetic measurements. Substitution of Ni for Fe leads to a decrease in the lattice constants a, c and the unit-cell volume V. The Curie temperature reaches a maximum of 590 K at x = 2, then decreases strongly for x ≥ 2. The spin reorientations are observed for the compounds with x = 0, 1, 2 and 3. The spin reorientation temperature decreases strongly from 255 to 60 K as the Ni content is increased from x = 0 to 3. Below the spin reorientation temperature, the compounds exhibit ferrimagnetic ordering. For the Ni-rich compounds with x = 9 and 10, the magnetization of the Dy sublattice decreases strongly since the magnetization of the Dy sublattice is strongly affected by the molecular field produced by the 3d sublattice.     

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.