Magnetic studies of RCo12B6 compounds (R=Y,Ce, Pr,Nd, Sm,Gd and Dy)

Magnetization of the RCo₁₂B₆ borides (R=Y, Ce, Pr, Nd, Sm, Gd and Dy), which crystallize in a rhombohedral structure of the SrNi₁₂B₆-type, has been measured in the temperature range 4.2–300 K. All compounds were found to order magnetically with Curie temperatures ranging from 154 to 177 K. Saturation moments at 4.2 K were found to be 6.5, 5.4, 8.4, 8.8, 6.8, 2.1 and 5.9μ_B/f.u. for R=Y, Ce, Pr, Nd, Sm, Gd and Dy, respectively. These results imply a ferromagnetic coupling of Co and rare earth moments for light rare earths and an antiferrimagnetic coupling for heavy rare earths in these compounds. A spin-compensation effect is observed in GdCo₁₂B₆ alloys at T_comp=46 and 72 K, respectively. Results suggest that in CeCo₁₂B₆ the Ce ion exists in the quadripositive state. It is clear that RCo₁₂B₆ materials are not of interest for permanent magnet applications.     

Magnetic and structural characteristics of the RInCu2 compounds in the Heusler L21 structure

Magnetic and structural behaviour of materials of composition RInCu₂ (R = rare earth) were investigated. All the compounds (R = La-Lu) were found to be isotypic and to crystallize with the Heusler L2₁ type structure. The magnetic behaviour of the compounds was studied in the temperature range 2–300 K. The RInCu₂ (R = Sm, Gd, Tb, Dy) compounds are antiferromagnetically ordered at low temperatures T_N = 12 K for GdInCu₂, Ce in CeInCu₂ is in a pure trivalent state. LaInCu₂ is Pauli paramagnetic and LuInCu₂ is diamagnetic. No superconductivity was observed for temperatures as low as 1.8 K. The observed magnetic properties result from indirect exchange interaction and crystal field effects and compared to the isomorphous MnInCu₂.     

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.     

Novel RCo5Ga7 intermetallic compound

The novel RCo₅Ga₇ (R=Y, Tb, Dy, Ho and Er) intermetallic compounds have been synthesized, and their crystallographic and magnetic properties have been studied using X-ray diffraction and magnetic measurement. RCo₅Ga₇ crystallizes in an orthorhombic structure with ScFe₆Ga₆ type. The space group is Immm, and Z=2. According to the structural refinement result, the 2a, 4e, 4f, 4g, 4h, and 8k crystal positions are occupied by 2R, 4Ga^I, 4(Ga^II, Co^I), 4Ga^III, 4(Ga^IV,Co^II), and 8(Co^III,Ga^V), respectively. The RCo₅Ga₇ intermetallic compound can be stabilized in the range of the radius ratio of R_Re/R_(Co,Ga)<1.36. The RCo₅Ga₇ compound exhibits a paramagnetic behavior. The magnetization at 5 K ranges from 28.93 to 40.62 emu/g.     

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

The magnetic susceptibility of RB₂C₂ has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R³⁺ ions. The RB₂C₂ (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB₂C₂ and TbB₂C₂. The compounds, DyB₂C₂ and HoB₂C₂, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.     

Magnetic studies on RCo3B2 (R=Sm,Gd, Er)

Compounds RCo₃B₂ withR=Sm, Gd, and Er were prepared and identified structurally by X-ray diffraction. Magnetic properties of these materials were studied down to liquid He temperature. The materials order ferromagnetically (e.g.,T _c for GdCo₃B₂ is 58 K). The magnitude of saturation moments indicates that Co is nonmagnetic in this environment. Bulk specimens of the compound SmCo₃B₂ show extraordinarily high coercive forces at 4.2 K (H _c =50 kOe). The second-order crystal field term of Sm in this compound is calculated on the point charge crystal field model and is found to be considerably larger than the one of Sm in SmCo₅. This is mainly due to the unusual ratio of the lattice constants. This work was supported by a grant from the National Science Fundation.     

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.     

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.     

Magnetic properties and structural chemistry of ternary silicides (RE,Th, U)Ru2Si2 (RE = RARE EARTH)

Ternary silicides (RE, Th, U)Ru₂Si₂ have been synthesized from the elements. All the compounds (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were found to be isotypic and to crystallize with the structure type of ThCr₂Si₂ (ordered derivative of the BaAl₄-type). The magnetic behavior of these alloys was studied in the temperature range 1.5 K < T < 1100 K. Magnetic susceptibilities at temperatures T > 300 K closely follow a typical Van Vleck paramagnetism of free RE³⁺-ions. In the case of CeRu₂Si₂ susceptibilities are well described for 20 K < T < 1100 K by a Van Vleck paramagnetism of widely spaced multiplets; the observed effective paramagnetic moment μ_eff = 2.12 BM indicates a high percentage (85%) of Ce³⁺. SmRu₂Si₂ yields an effective moment μ_eff = 0.54 BM, which compares reasonably well with the Hund's rule J = 5/2 ground level for free Sm⁺ and a low-lying excited level with J = 7/2. For temperatures T > 15 K the magnetic susceptibility as a function of temperature follows the “Van Vleck behavior” for free Sm³⁺. At low temperatures ferromagnetic ordering was encountered for (Pr, Nd, Ho, Er, Tm)Ru₂Si₂, whereas antiferromagnetic ordering was observed for (Sm, Gd, Tb, Dy)Ru₂Si₂. The ordering temperatures are generally below 55 K. No superconductivity was found for temperatures as low as 1.8 K.     

Magnetic and structural investigations of GdMn2-GdNi2 intermetallic compounds

Magnetic and structural properties of the intermetallic system Gd(Mn_1-xNi_x)₂ were investigated. The 0?x?0.4 and 0.7<x?1 compounds are Laves phases of the MgCu₂ type. The remaining compounds 0.4?x?0.7. crystallize with the hexagonal structure. For single crystal GdMn_0.8Ni_1.2 the P6₃/mmc space group and the lattice parameters a=5.175 Å and c=16.731 Å are obtained. The magnetic properties of investigated series are investigated in stationary and in pulsed magnetic fields in the temperature range 2.6–700 K. For 0.4?x?0.8 two magnetic transitions are observed. In the paramagnetic range the χ^-1_M (T) is described by a Néel type variation and the molecular field coefficients N_{i j} are calculated. The localized character of the magnetic moment of Mn is observed.     

Magnetic and structural studies of the alloy system REIn0.5Ag0.5

The structural and magnetic properties of the alloy system REIn_0.5Ag_0.5 [RE = Gd, Tb, Dy, Ho, Er, Tm and Yb] are reported. All these alloys (except that of Yb) crystallize in a cubic CsCl type structure at room temperature. Low temperature X-ray diffraction data does not reveal any structural phase transformation down to 8 K. On the basis of magnetic susceptibility data at a different temperature (3–300 K) and applied magnetic field (2 × 10⁵ to 8 × 10⁶ A m^-1, it has been concluded that GdIn_0.5Ag_0.5 is ferromagnetic (T_c = 118 K), TbIn_0.5Ag_0.5 and DyIn_0.5Ag_0.5 are meta magnetic (T_N = 66 and 30 K, respectively) and alloys involving Ho, Er, Tm and Yb are ferrimagnetic with Néel temperatures (T_N) equal to 24, 22, 21 and 20 K, respectively. The evaluated effective magneton number (p) is found to be slightly larger compared to theoretical values for tripositive ions of Gd, Tb and Dy and a bit smaller for Ho, Er, Tm and Yb. The results have been qualitatively explained using appropriate theories.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Magnetic properties of RAu2Si2 rare earth compounds

The magnetic properties of the R Au₂Si₂ compounds with R = Ce-Er have been investigated. It was found that the compounds for which R = Ce, Sm, Gd, Tb and Dy are antiferromagnetically ordered at temperatures ranging from 5.7 to 15.9°K. PrAu₂Si₂ and NdAu₂Si₂ exhibit paramagnetic behavior for temperatures as low as 4.2°K. The magnetic structure is ferrimagnetic for the compounds in which R = Eu, Ho, and Er. The Eu compound is in the divalent state. The Néel and Curie points for this system do not follow the De-Gemnes function. Curie-Weiss Behavior is exhibited by all the compounds with effective moments in good agreement with that of a free tripositive lanthanide ion. The difference in magnetic properties between R Au₂Si₂ and the isomorphous R Fe₂Si₂ series is discussed.     

Neutron diffraction studies of the magnetic ordering in the rare earth compounds TbNi2Si2, HoCo2Si2 and TbCo2Si2

Neutron diffraction studies and magnetic measurements on the compounds TbNi₂Si₂ (1), HoCo₂Si₂ (2) and TbCo₂Si₂ (3) revealed a collinear antiferromagnetic order below T_N = 10 ± 1 K (1), T_N = 13 ± 1 K (2) and T_N = 30 ± 2 K (3) with the rare earths moments oriented along the c-axis [m₀ = 8.8 ± 0.2 μ_B (1), m₀ = 8.1 ± 0.2 μ_B (2), m₀ = 8.8 ± 0.2 μ_B (3)] and the corresponding wavevector are $\text{k}\text{= [}\text{1}\text{2}\text{1}\text{2}\text{0] (1)}\text{and}\text{k}\text{= [ 0 0 1] (2) (3)}$. The magnetic structure of the compounds HoCo₂Si₂ and TbCo₂Si₂ consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically (+?+?) while for TbNi₂Si₂ the ordering within (0 0 1) plane is antiferromagnetic and the planes (0 0 1) are indeed decoupled.     

Magnetic properties of MgCo2, MgNi2 and Mg2Ni

The magnetic properties of MgCo₂, MgNi₂ and Mg₂Ni were investigated in the range 4–400 K. The compound MgCo₂ is ferromagnetic below T_c = 321 but gives rise to a first order ferromagnetic-antiferromagnetic transition near 45 K. The compounds MgNi₂ and Mg₂Ni are both Pauli paramagnetic; they have an almost temperature-independent magnetic susceptibility. For all compounds the lattice constants were determined.     

Magnetization studies on ErCo3-hydride and TmCo3-hydride

The compounds ErCo₃ and TmCo₃ are ferrimagnetic with Curie temperatures of 401 K and 370 K, respectively. These absorb hydrogen to form ErCo₃H_4.3 and TmCo₃H_3.3. From magnetization studies on these as well as other RCo₃ hydrides in the temperature interval 4.2 to 300 K, it is inferred that hydrogen absorption leads to a reduction in magnetic moment on cobalt and a weakening of the R-Co interaction (R = rare earth). Except in the case of the GdCo₃-hydride, saturation in magnetization is not achieved at 4.2 K in applied fields up to 21 kOe. This suggests the possibility of fanning of rare earth moments. The RCo₃-hydrides investigated earlier with R = Gd, Dy and Ho and the ErCo₃-hydride and TmCo₃-hydride all appear to be magnetically ordered at room temperature.     

Paramagnetic and spin-glass properties of pyrochlore-like oxides Ln2Mn2/3Mo4/3O7 (Ln=Sm, Gd, Tb, or Y)

The magnetic properties of complex oxides Ln ₂Mn_2/3Mo_4/3O₇ (Ln=Sm, Gd, Tb, or Y) with a pyrochlore-type structure are studied in the temperature range 2–300 K. For all compounds in the paramagnetic state, the temperature dependence of the magnetic susceptibility is described by a generalized Curie-Weiss law with a temperature-independent component of ∼10⁻⁶ cm³/g and with a Weiss constant Θ<0 and |Θ|<16 K. At low temperatures (T<10–12 K), the compounds have spin-glass properties; they exhibit magnetic and temperature hysteresis and the typical dependences of the imaginary and real parts of the dynamic magnetic susceptibility on temperature and the frequency of an ac magnetic field in a wide range of magnetization relaxation times. The data obtained suggest that d electrons are responsible for the formation of frustrated exchange interactions in the compounds and that 4f electrons in the compounds with Sm or Tb provide strong magnetic-anisotropy effects. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 2, 2004, pp. 287–295. Original Russian Text Copyright ? 2004 by Korolev, Bazuev.     

Magnetism and structural chemistry of ternary silicides: (RE,Th, U)Pt2Si2 (RE = rare earth)

Ternary silicides (RE, U, Th)Pt₂Si₂ have been prepared from the elements. All the compounds (RE= Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and U, Th) were found to be isotypic and crystallize with the primitive tetragonal CePt₂Si₂-type structure closely related to the CaBe₂Ge₂-type. The magnetic properties of these alloys were studied in the temperature range 1.5 K < T < 1100 K and in fields up to 1.3 T revealing a typical Van Vleck paramagnetism of free RE³⁺-ions for temperatures T > 200 K. A nonmagnetic ground state is reflected from the magnetic susceptibility data of CePt₂Si₂, which are interpreted in terms of interconfiguration fluctuations (ICF). The magnetic results of SmPt₂Si₂ (μ_eff = 0.7 BM) compare well with the ideal Van Vleck behavior of Sm₃₊ ions with a $\text{J =}\text{5}\text{2}$ ground state and a low-lying excited first level $\text{J =}\text{7}\text{2}$. At temperatures below 40 K antiferromagnetic ordering is found for (Gd, Tb, U)Pt₂Si₂; whereas in case of (Dy, Ho, Er, Tm)Pt₂Si₂ the onset of ferromagnetism is indicated below 4 K. None of the samples exhibited a superconducting transition above 1.8 K.     

Magnetic structures of PrCO2Si2 and TbCo2Si2 compounds

Neutron diffraction studies of polycrystalline PrCo₂Si₂ and TbCo₂Si₂ compounds were carried out at 4.2 and 293 K. Both samples have collinear antiferromagnetic order below T_N(31(1) and 46(1) K for Pr and Tb compound respectively), with their magnetic moments parallel to the c axis. The ordered magnetic moment values of Pr and Tb at 4.2 K (3.19 and 9.12 μ_B respectively), are close to the saturation value of the free ions. The corresponding magnetic space group P_l4/mnc (Sh⁴¹⁰₁₂₈) is body-anticentered ($\text{k =}\text{111}\text{222}$ refering to P_l cell).