Spontaneous magnetostriction in R2Fe14B (R=Y,Nd, Gd,Tb, Er)

Thermal expansion anomalies of R₂Fe₁₄B (R=Y, Nd, Gd, Tb, Er) stoichiometric compounds were studied by X-ray diffraction with high-energy synchrotron radiation using a Debye–Scherrer geometry in temperature range of ∼10–1000 K. A large invar effect with a corresponding large temperature dependence of lattice parameters ∼10–15 K above their Curie temperatures (T_c) are observed. The a-axes show a larger invar effect than the c-axes in all compounds. The spontaneous magnetostrain of the lattices and bonds are calculated. The iron sublattice is shown to dominate the volumetric spontaneous magnetostriction of the compounds and the contribution from the rare-earth sublattice is roughly proportional to the spin magnetic moment of the rare earths. The bond-length changes are consistent with the theoretical spin-density calculation. The average bonds magnetostrain around Fe sites is approximately proportional to their magnetic moments.     

Synthesis and magnetic properties of ternary carbides R2Fe14C (R = Pr,Sm, Gd,Tb, Dy,Ho, Er,Tm, Lu) with Nd2Fe14B structure type

The R₂Fe₁₄C compounds were prepared from the elements by arc melting and long-time annealing (more than 480 hrs) at temperatures below 1200 K. They crystallize with the tetragonal Nd₂Fe₁₄B structure type as shown by X-ray powder diffraction analysis. All samples contain at least 70 vol.% of the R₂Fe₁₄C phase. Both saturation magnetization values (42–133 Am²/kg) and Curie temperatures (493–613 K) are lower than those measured on the corresponding borides.     

57Fe and 166Er Mössbauer and magnetic studies of RFe4B (R = Er,Tm, Lu) compounds

Magnetic and Mössbauer studies have been carried out on a series of ternary borides RFe₄B (R = Er, Tm, Lu) which have the hexagonal CeCo₄B type structure. These compounds are found to be magnetically ordered at room temperature. Magnetization studies in the temperature range from 5 to 300 K reveal the presence of compensation temperatures in Er and Tm compounds and indicate antiferromagnetic coupling between the rare earth and Fe moments. Room temperature ⁵⁷Fe Mössbauer studies yield values of hyperfine fields at the two Fe sites as 246 and 185 kOe in ErFe₄B and TmFe₄B, and 204 and 145 kOe in LuFe₄B. The ¹⁶⁶Er Mössbauer studies give nearly free-ion hyperfine field at the Er sites which indicates that the exchange interaction in ErFe₄B is much stronger than crystal field interaction.     

Spin reorientations in R2Fe14-xCoxB systems (R = Pr,Nd and Er)

The effect of cobalt substitution on spin reorientation phenomena in R₂Fe_14-xCo_xB systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2–1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x ⩾ 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x ⩾ 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.     

Optical properties of the charge-density-wave polychalcogenide compounds R 2Te 5 (R=Nd, Sm and Gd)

We investigate the rare-earth polychalcogenide R₂Te₅ (R=Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical reflectivity measurements. We obtain the optical conductivity through Kramers-Kronig transformation of the reflectivity spectra. From the real part of the optical conductivity we then extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTe_n (n=2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.     

Effect of hydrogenation on the magnetic properties of the intermetallic compound Er2Fe14B with single-crystal and nanocrystalline structures

Hydrogenated single crystals Er₂Fe₁₄Bh _x with different hydrogen contents are grown and their magnetic properties are studied for the first time. It is established that both the Curie temperature and the temperature of the spin-reorientation phase transition increase with an increase in the hydrogen content. In the Er₂Fe₁₄B single crystal, the contributions of the rare-earth metal and iron sublattices to the magnetic anisotropy decrease upon hydrogenation. However, their compensation occurs at a temperature higher than that in the initial compound Er₂Fe₁₄B due to the enhancement of the Fe-Fe and R-Fe exchange interactions. The effect of hydrogenation on the magnetic characteristics of the Er₂Fe₁₄B compound with a nanocrystalline structure is investigated. It is revealed that the hydrogenation leads to an increase in the coercive force and the residual magnetization of these alloys.     

Mössbauer effect study of R2Fe14C (R=Dy and Er) compounds

The results of⁵⁷Fe Mössbauer effect measurements and magnetic studies performed on Dy₂Fe₁₄C and Er₂Fe₁₄C compounds are reported. The magnetic contributions of different iron sites are estimated and compared with those of corresponding boron compounds. The magnetic behaviour of the studied compounds is analysed in the molecular field approximation considering a two sublattices ferrimagnet.     

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.     

Magnetic properties of some RPd2Si2 compounds (R=Gd,Tb, Dy,Ho and Er)

The magnetic susceptibility of the ternary compounds, RPd₂Si₂ (where R=Gd, Tb, Dy, Ho and Er) has been measured. GdPd₂Si₂ order antiferromagnetically at 13 and 20 K respectively; the rest of the compounds do not show clear ordering down to 4.2 K. Palladium carries no moment in these compounds. The De Gennes formula is not obeyed indicating that the exchange interaction between the 4f moments via conduction electrons is not isotropic     

Mössbauer study of Nd2Fe14B and Nd2(Fe1−xCox)14B compounds

The Mössbauer spectra of Nd₂Fe₁₄B and Nd₂(Fe_1?xCo_x)₁₄B compounds (x=0.0.05, 0.10 and 0.16) have been investigated at room temperature and at 77 K. Taking advantage of combined ME and NMR investigations of the Nd₂Fe₁₄B compund, the hyperfine field values and their assignment to the six Fe sites have been determined to be the following sequence: 378(j₂), 346(k₂), 334(j₁), 325(k₁), 322(c), 306(e) kOe. The substitution of Co for Fe decreases the hyperfine fields at all Fe sites. The intensity variations of the subspectra with Co content show that Co atoms have a strong preference to occupy the k₂ site, but have a rather less tendency to enter the j₂ site, which is preferred by Fe atoms.     

Magnetic properties of the compounds R2Sc3Si4 (R=Gd, Tb, Dy, Ho, Er)

The magnetization of R₂Sc₃Si₄ compounds is measured in static magnetic fields up to 14 kOe in the temperature range 77–300 K. It is established that all compounds in the given series are paramagnetic at these temperatures. The paramagnetic Curie points are determined, and the effective magnetic moments are calculated. The measurements are performed on polycrystalline samples. Fiz. Tverd. Tela (St. Petersburg) 41, 1804–1805 (October 1999)     

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 RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

Effect of hydrogenation on spin-reorientation phase transitions and magnetic anisotropy constants of RFe11Ti single crystals (R=Lu, Ho, and Er)

The magnetic anisotropy and spin-reorientation phase transitions in single crystals of the RFe₁₁Ti (R=Lu, Ho, and Er) compounds and their hydrides are investigated. Measurements are carried out on capacitance and torque magnetometers. The magnetic anisotropy constants K ₁ and K ₂ are determined by the mathematical processing of experimental magnetization curves in terms of the phenomenological theory of the anisotropic ferromagnet magnetization. It is demonstrated that the hydrogenation strongly affects the magnitude and the sign of magnetic anisotropy constants, as well as the spin-reorientation phase transitions. The hydrogenation of the HoFe₁₁Ti compound leads to the change in sign of the magnetic anisotropy constant K ₁. The inference is made that a change in the atomic volume and the axial ratio c/a cannot result in the observed effects. A change in the magnetic anisotropy constants upon hydrogenation is primarily due to the change in the interaction of the quadrupole moment of a 4f electron subshell of rare-earth ions with surrounding ions of the crystal lattice and also with valence and conduction electrons.     

Magnetic Characteristics of Ball-Milled Nd2Fe14B - Gd

Mössbauer measurements using ⁵⁷Fe and ¹⁵⁵Gd have been made on samples of ball-milled Nd₂Fe₁₄B in a gadolinium matrix in order to determine the magnetisation of each phase separately. Although magnetometer measurements did not show any coercivity, both measurements showed the phases were in intimate magnetic contact. The spectra of each phase differed considerably from those of the respective original materials and provide information on the conduction electron polarisation through the transferred hyperfine fields.