Molecular field theory analysis of R3Co11B4 compounds

The temperature dependence of magnetization of the R₃Co₁₁B₄ compounds has been analysed using the two-sublattice molecular field theory. The molecular field coefficients, n_CoCo, n_RCo, n_RR, have been calculated by a numerical fitting process. The analytic form of the exchange field H_R(T) varying with temperature for each of the R₃Co₁₁B₄ compounds is presented, and some results are discussed.     

Magnetic properties of R2Pt compounds with R = Gd,Tb, Dy,Ho, Er and Tm

Magnetic properties of polycrystalline samples of R₂Pt compounds (R = Gd, Tb, Dy, Ho, Er and Tm) are presented. The Gd, Td, Dy, Ho based compounds are ferromagnetic with Curie temperatures ranging between 155 and 17 K. Er₂Pt and Tm₂Pt are antiferromagnetic with Néel temperatures of 9 and 5 K respectively. The observed properties are discussed considering indirect exchange interactions and crystal field effects acting on the rare earth ions which lies in very low symmetry sites.     

Effect of hydrogenation on the magnetic and magnetoelastic properties of R 2Fe14B compounds (R = Nd, Gd, Er, Lu)

The temperature dependences of the magnetization σ(T), magnetostriction λ(T), and linear thermal expansion coefficient α(T) of R ₂Fe₁₄B intermetallic compounds (R = Nd, Gd, Er, Lu) and of their hydrides R ₂Fe₁₄BH_2.5 are studied. The magnetization was measured with a pendulum magnetometer within the temperature interval 77–700 K in a magnetic field H = 500 Oe. Magnetostriction and thermal expansion were measured using the tensometric technique in the temperature interval 77–420 K. It was established that Gd₂Fe₁₄BH_2.5 undergoes a spin-reorientational (SR) transition at T _SR = 235 K. In compounds with Nd and Er, anomalies associated with the SR transition were found in the σ(T), λ(T), and α(T) curves. The SR transition temperatures were refined and magnetic phase diagrams were constructed for the compounds studied. The α(T) curves of the R ₂Fe₁₄BH_2.5 hydrides (R = Nd, Er) revealed anomalies of a nonmagnetic origin associated with hydrogen ordering in the crystal lattice of these compounds.     

Molecular field theory analysis of R2Fe17C (R=PR,Nd, Gd,Tb, Dy,Ho, Er)

The temperature dependence of magnetization is analysed for R₂Fe₁₇C via the two-sublattice molecular field theory. The molecular field coefficients n_FF, n_RF and n_RR are obtained, by which T_C was calculated. Using the least-squares method, the fitted-form of H_R(T) varying with temperature for each compound is presented. The results are analysed. In addition, the parameters F=M_Fe²(0)n_FF/T_C was calculated for each R₂Fe₁₇C. By F, some phenomena different from the normal view were explained.     

Magnetic properties of R3Rh2 compounds with R=Gd, Tb,Dy, Ho and Er

Bulk magnetic measurements performed on polycrystalline samples of the tetragonal compounds R₃Rh₂ with R=Gd, Tb, Dy, Ho and Er are presented. All the compounds are ferromagnetic at low temperature. However in Tb₃Rh₂ an antiferromagnetic behaviour is observed between 14 and 24 K. In Gd₃Rh₂, where the magnetocrystalline anisotropy must be negligible, it seems that the magnetic structure is not collinear. In the other compounds the observed properties essentially result from indirect exchange interactions and crystal field effects acting on the rare earth ions which lie in low symmetry sites.     

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.     

Determination of magnetic anisotropy of complex rare-earth compounds from Mössbauer and NMR spectra

Experimental data and theoretical papers on the magnetic anisotropy (MA) of rare-earth-transition metal intermetallic compounds are reviewed. Discrepancies between the experimental data obtained by different authors, as well as between these data and the theoretical calculations of the MA constants, are indicated. A technique is proposed for determining the crystal-field parameters and the effective charges Q _i ^* of ions in intermetallic compounds. Using experimental Mössbauer and NMR spectroscopic data, possible values of Q _i ^* are determined for R ₂T_17?x and R ₂T_17?x Ti_x compounds, which allow one to find the MA constants of these systems with different R and T in a unified way. The problem of the sign of the contribution from the rare-earth metal sublattice to the MA is discussed. The heavy x dependence of this contribution in the R ₂T_17?x Ti_x system is explained to be due to the contribution to the crystal field from Ti ions in the dumbbells.     

A note on exchange and crystal field interactions in R2Fe14B compounds: Yb2Fe14B

The R₂Fe₁₄B phase has been found to exist for R=Yb. The magnetic properties presented in this paper complete the characterization of the compounds in this series for which the Stevens α_J coefficient of the R³⁺ ion is positive. ⁵⁷Fe Mössbauer spectroscopy establishes the existence of a magnetization reorientation at 115 K of the type observed in Er and Tm compounds associated with a small Fe magnetization anisotropy. From the neutron diffraction measurements obtained at 4.2 K with and without an applied magnetic field, the easy direction of magnetization was found to be along the [100] direction, in the basal plane of the tetragonal structure. These results show that in all compounds where α_J>0 for the R³⁺ ion, the easy direction of magnetization in the plane is determined by the second order crystal field terms and rare earth-Fe exchange interactions and is independent of the sign of the 4th order crystal field terms.     

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.     

The intersublattice molecular field in the rich Fe−4f intermetallics

Some experimental evidence is given to show that the3d-4f spin coupling parameter is rather insensitive to the kind of the rare earth spin and iron composition in the R _n Fe _m intermetallics. The intersublattice molecular field has been calculated for the rich Fe–4f compounds on the basis of a value of 6.72 K for this parameter as derived from high magnetic field studies. This field affects the R magnetic moment and originates from iron sublattice in the R₂Fe₁₇, R₆Fe₂₃, RFe₃ and RFe₂ compounds. The values of the field for the new magnetic materials of the R₂Fe₁₄B type have also been calculated.     

Magnetic properties of ternary RMn2Si2 and RMn2Ge2 compounds

Magnetic properties of RMn₂Si₂ and RMn₂Ge₂ compounds, where R is a rare earth metal, have been investigated by magnetometric measurements. RMn₂Ge₂ (where R is a light rare earth) and LaMn₂Si₂ are ferromagnets. Remaining compounds have antiferromagnetic properties. DyMn₂Si₂ and ErMn₂Si₂ show ferromagnetic properties at low temperatures. It was confirmed that the value of Curie (or Néel) temperature for the Mn sublattice decreases with increasing c constant.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

Influence of Si and Co substitutions on magnetoelastic properties of R2Fe17 (R=Y, Er and Tm) intermetallic compounds

The magnetostriction of the off-stoichiometric R₂Fe₁₇-type intermetallic compounds based on R₂Fe_14−xCo_xSi₂ (R=Y, Er, Tm and x=0, 4) was measured, using the strain gauge method in the temperature range 77-460 K under applied magnetic fields up to 1.5 T. All compounds show sign change and reduction in magnetostriction values compared to the R₂Fe₁₇ compounds by Si substitution. For Y₂Fe₁₄Si₂ and Er₂Fe₁₄Si₂, saturation behaviour is observed near magnetic ordering temperature (T_C), whereas for Tm₂Fe₁₄Si₂, saturation starts from T>143 K. Also, Co substitution has different effects on the magnetostriction of R₂Fe₁₄Si₂ compounds. In Er₂Fe₁₀Co₄Si₂ and Tm₂Fe₁₀Co₄Si₂, saturation occurs below the spin reorientation temperature (T_SR). In addition, in Er₂Fe₁₄Si₂, a sign change occurs in the anisotropic magnetostriction (Δλ) as well as the volume magnetostriction (ΔV/V) at their T_SR values. The volume magnetostrictions of the Tm-containing compounds show an anomaly around their T_SR. In R₂Fe₁₄Si₂ compounds, parastrictive behaviour is also observed in ΔV/V near their T_C values. In addition, the magnetostriction of the sublattices is investigated. Results show that in R₂Fe₁₄Si₂ compounds, the rare-earth sublattice contribution to magnetostriction is negative and comparable to the iron sublattice, whereas, in R₂Fe₁₀Co₄Si₂ compounds, the rare-earth sublattice contribution is positive and larger than Fe sublattice. These results are discussed based on the effect of Si and Co substitutions on the anisotropy field of these compounds. Influence of the spin reorientation transition on the magnetostriction of these compounds is discussed in terms of the anisotropic sublattice interactions.     

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.     

R2Fe17-xAlx化合物中R-T交换耦合常量的计算

讨论了利用分子场近似结合中子衍射或X射线衍射的实验结果计算R₂Fe₁₇型稀土过渡族化合物中稀土磁矩与过渡族磁矩之间交换耦合常量的方法,并据此计算了R₂Fe_17-xAl_x(R=Tb,Dy,Ho,Er,Gd,x=7或8)化合物中稀土磁矩与过渡族磁矩之间的交换耦合常量.计算结果与实验值符合较好. 关键词： 2Fe_17-xAl_x化合物')" href="#">R₂Fe_17-xAl_x化合物 交换耦合常量     

Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

The heat capacities of the compounds DyAl₂, ErAl₂ and LuAl₂ were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl₂ and ErAl₂ show C_P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C_L was obtained from the C_P data of LuAl₂ by a method of interpolation which gave values of C_L for an arbitrary R Al₂ compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1₂ phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T^{$\text{3}\text{2}$} dependence above this temperature.Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl₂ on the ⁴I_{$\text{15}\text{2}$} ground state multiplet of the Er³⁺ ion. It is concluded that the magnetic ordering in ErAl₂ takes place within the Γ₈³ quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.     

EPR of Spin Transitions in Complexes of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

Polymer chain complexes [Cu(hfac)₂L^R] _n exhibit thermally and light-induced magnetic anomalies in many aspects similar to a spin crossover. These compounds attracted significant attention in the field of molecular magnetism and have been extensively studied by electron paramagnetic resonance (EPR) during the last several years. All compounds studied so far were based on copper(II) ions bridged by pyrazolyl-substituted nitronylnitroxides. The present work reports the first EPR study of complexes of Cu(hfac)₂ with tert-butylpyrazolylnitroxides—a new type of nitroxide ligand expected to modify exchange interaction pathways and physical properties of the crystals. The Q-band EPR spectra of three representative novel compounds are principally different from those studied previously, supporting the assumption that the magnetic motif of the compound has changed. Dominant intercluster exchange interactions are now found along the structural polymer chains. This complicates the EPR detection of phase transitions to some extent; however, theoretical modeling of the observed spectral changes allows for unambiguous assignment of different spin states and transitions between them. The magnitudes of intercluster exchange interaction were estimated to be ca. 0.1–1.5 cm^?1 for the studied compounds.     

3d magnetism in R2Fe14B compounds

An investigation of the magnetic properties of R₂Fe₁₄B compounds with R = Y, La, Ce, Gd, Lu and Th has been carried out. In all compounds the value of the Fe moment is in the range 2.0–2.2μ_B, close to that of metallic Fe. This shows that effects of electron transfer and hybridization are weaker than in binary R-Fe compounds. The variations of the Curie temperatures of the different compounds is interpreted in terms of the dependence of the magnetic interactions on distance and on R. In all compounds, except Th₂Fe₁₄B, the temperature dependence of the anisotropy exhibits a maximum at T/T_c ≃ 0.4. This effect is ascribed to competition between contributions from different Fe atomic sites and/or to a change in the crystal field interactions associated with the magnetovolume anomaly which occurs in R₂Fe₁₄B compounds below T_c.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Formation and magnetic properties of R2Fe17−xGaxC2 compounds prepared by arc-melting

The single phase compounds of R₂Fe_17−xGa_xC₂ (R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm;x = 2 and 3) with the rhombohedral Th₂Zn₁₇-type or hexagonal Th₂Ni₁₇-type structures were prepared by arc-melting. Their formation, structure and magnetic properties were studied. The substitution of Ga in the R₂Fe₁₇C₂ results in the increase of unit cell volume and the decrease of saturation magnetization. Curie temperatures have a small change when x ≤ 2, then decreases with x The Sm₂Fe_17−xGa_xC₂ compounds with x = 2 and 3 are found to have a uniaxial magnetocrystalline anisotropy and show a room-temperature anisotropy field of 95 and 93 kOe, respectively. Spin reorientation transitions are observed in the Er₂Fe_17−xGa_xC₂ and Tm₂Fe_17−xGa_xC₂ compounds. The substitution of Ga for some Fe in the R₂Fe₁₇C₂ compounds with R = Er and Tm increases the uniaxial anisotropy of the R sublattices, resulting in the increase of spin reorientation temperature.