Papers by Keyword: Rare Earth Intermetallics

Abstract: A study on the formation and stability of new quaternary compounds with the general chemical formula Gd₃TAl₃Ge₂ (T = Mn, Cu) has been undertaken by experimental investigations (SEM-EDX, DTA and XRD) and density functional theory (DFT) calculations. These compounds crystallize in the hexagonal Y₃NiAl₃Ge₂-type structure (hP9, P–62m, Z = 1) (an ordered, quaternary derivative of the ternary ZrNiAl or of the binary Fe₂P prototypes), with lattice parameters values a = 7.0239(2) Å and c = 4.2580(1) Å for Gd₃MnAl₃Ge₂ and a = 7.0434(1) Å and c = 4.2089(1) Å for Gd₃CuAl₃Ge₂. DTA suggests a peritectic reaction for the formation of these compounds (at 1245°C for Gd₃CuAl₃Ge₂). The existence and stability of these phases has been explained on the basis of DFT calculations, and a comparison of ground state properties of the studied compounds with the earlier known Gd₃CoAl₃Ge₂ phase is outlined. The negative formation energies in all three cases govern the stability of compounds from theory as well, predicting Gd₃MnAl₃Ge₂ as the most stable phase with highest formation energy (–13.01 eV/f.u.). The total DOS are generic in nature and suggest the robust magnetism, with the Gd-f moments of ≈7 μ_B. An antiparallel coupling among Gd-f and T-d states is observed for all compounds, as usually seen in rare earth (R) - transition metal (T) compounds. Preliminary magnetization measurements on Gd₃MnAl₃Ge₂ show two ferromagnetic/ferrimagnetic (FM/FIM) like transitions at T_C1 = 142 K and T_C2 = 97 K, with another anomaly seen at ≈15 K. Isothermal magnetization data show no hysteresis even at 5 K, and the magnetization does not saturate up to 50 kOe, further suggesting a possible FIM behavior.

Abstract: The ground state behavior of rare earth intermetallic compound TmPb_3, which crystallize in AuCu₃ type structure, has been examined using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. Very few study on structural and electronic properties of TmPb₃ compound has been available in the literature, which motivated us to perform the present study. The spin polarized calculations are carried out within the PBE-GGA and LSDA for the exchange correlation (XC) potential. Our calculated ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B’) are in good agreement with the experimental results. The value of bulk modulus of TmPb₃ is found to be 44.32 GPa and 55.01GPa by PBE-GGA and LSDA respectively. The electronic band structure (BS) and density of states (DOS) verify the metallic nature of this compound. The calculated density of states at the fermi level is found to be 0.16 states/eV and 19.50 states/eV for spin-up and spin-down modes respectively. The magnetic moment of TmPb₃ is found to be 0.95.

Abstract: The direct measurements of the magnetocaloric effect (MCE) and the magnetization for R₂Fe₁₇ (R = Y, Tb, Dy) compounds are reported. The maximal values of the MCE for different R₂Fe₁₇ compounds are almost the same at the Curie temperature and are equal to 0.8 – 0.85 K at ΛH = 13.5 kOe. The field dependencies of the magnetization and MCE and the dependence of MCE via magnetization deviates from the classical regularities and describes by the thermodynamic theory attracting the higher terms. Obviously the occurrence of the mixed exchange interactions in these compounds leads to the requirement of consideration both thermal and local fluctuations of the exchange integrals near the magnetic transition temperature.

Abstract: The direct measurements of the magnetocaloric effect (MCE) and the magnetization have been conducted for a large number of compounds based on RCo₂ (R = Tb, Ho) with substitutions by nonmagnetic elements (Y, Al, Ga) in rare earth and 3d- sublattices. The concentration dependencies of the Curie temperature and the value of MCE are discussed. The linear dependencies of the value of the MCE from the second power of the magnetization were find out at the region of the phase transition of the second type .

Abstract: NdScGe is a ferromagnet with TC = 194(2) K. The magnetic moments of the Nd substructure order along the tetragonal c-axis just below TC and undergo a gradual canting away from the c-axis upon cooling, commencing at around 165 K. We used neutron powder diffraction to show that the Nd magnetic moments lie 70(3)o off the c-axis at 4 K [1]. The apparent discrepancy between our neutron diffraction results and those obtained using PAC [2] (a canting angle of around 32o just below TC , increasing to 45o at 25 K) can be reconciled by placing the principal axis of the electric field gradient tensor in the tetragonal basal plane, rather than along the c-axis, as assumed in the PAC analysis. The unusual temperature dependence of the 111Cd PAC quadrupole frequency at the Sc site, which was interpreted in terms of a lattice softening occurring near the Curie temperature, is shown to be a consequence of the fact that one cannot determine the sign of the quadrupole frequency from a PAC experiment. Finally, we show that the Nd magnetic moments in the canted regime have a [110] basal component rather than [100].

Abstract: We report on structural and superconducting properties of La3-xRxNi2B2N3- where La is substituted by the magnetic rare-earth elements Ce, Pr, Nd. The compounds Pr3Ni2B2N3- and Nd3Ni2B2N3- are characterized for the first time. Powder X-ray diffraction confirmed all samples R3Ni2B2N3- with R = La, Ce, Pr, Nd and their solid solutions to crystallize in the body centered tetragonal La3Ni2B2N3 structure type. Superconducting and magnetic properties of La3-xRxNi2B2N3- were studied by resistivity, specific heat and susceptibility measurements. While La3Ni2B2N3- has a superconducting transition temperature Tc ~ 14 K, substitution of La by Ce, Pr, and Nd leads to magnetic pair breaking and, thus, to a gradual suppression of superconductivity. Pr3Ni2B2N3- exibits no long range magnetic order down to 2 K, Nd3Ni2B2N3- shows ferrimagnetic ordering below TC =17 K and a spin reorientation transition to a nearly antiferromagnetic state at 10 K.

Abstract: Direct measurements of the magnetocaloric effect (MCE) and magnetization have been conducted for the R(Co1-xAlx)2 (R = Tb, Ho) compounds. It was found that the small substitutions of Co by nonmagnetic Al lead to an increase both of the magnetic ordering temperature and the value of MCE. The following increase in Al content decreases the Curie temperature and the value of MCE. The analyses of the MCE effect is conducted considering the ferrimagnetic structure and the itinerant nature of these compounds.

Abstract: Magnetization curves and magnetic susceptibility of Tb1 xRxMn6Sn6 (R = Gd, Y) compounds have been measured for aligned powder samples. Concentration dependences of the spin-reorientation transition temperature and temperature dependences of critical fields of the field-induced magnetic transitions have been determined. For the compound Tb0.1Y0.9Mn6Sn6 with the spiral magnetic structure, we found that application of hydrostatic pressure facilitates the ferrimagnetic ordering. This unusual feature is supported by the observation of a large negative spontaneous volume magnetostriction for TbMn6Sn6. The obtained results are discussed in a model considering three different interlayer Mn-Mn exchange interactions in these compounds.

Abstract: The study of high pressure structural stability and equation of state of f-electron based binary intermetallics of type AXBY, where A belongs to either rare earth of actinide atom and B any other d or p block metal, is interesting from both basic as well as applied research point of view. These studies have lead to some general systematic patterns emerging. Firstly, the AB type of compounds in general stabilizes in NaCl type cubic structure and transform to CsCl type under the action of pressure. The AB2 type of compounds is very interesting and under pressure undergoes a series of structural transitions. However, the AB3 type systems are highly stable and do not show structural transitions under pressure up to as high as 30 GPa. We found that it is interesting and enlightening to explore: (i) the reason for their stability by examining the electronic structure and (ii) look for general trends in the structural transformations. In this paper, we have presented some of our studies on f-electron based intermetallics (f-IMCs), elaborate on the trends seen in the structural transitions and correlate the results obtained with the electronic structure calculations.