Subject classification: 75.25.+z; 75.50.Ee; 76.80+y; S4 Recently it was shown [1] that the hyperfine magnetic field (B hf ) for non-magnetic sp atoms in metallic magnets could be highly anisotropic. The anisotropic polarization of the valence electrons arising through hybridization with the electrons of the neighbor magnetic atoms was treated in the frameworks of the spin-dipolar approach. According to the proposed model [1], the magnitude of the anisotropic contribution to B hf depends strongly on the magnetic moment orientation relative to valence bond directions. In general, the anisotropic transferred field does not vanish even when the net moment of the neighbor magnetic atoms is equal to zero. For Sn impurity in the UGa 3 type-II antiferromagnet, good agreement between theory and experiment has been obtained [1]. In this case, the hyperfine field at the Sn site is completely anisotropic and is equal to about 3 T. In a general case, the relation between the anisotropic contribution and the isotropic one should depend strongly on the peculiarities of the electron density distribution and on the type of magnetic structure. These results are essential for both hyperfine data interpretation and application of these data for studying the spin structure of metallic magnets.
Systems in which the alignment of the magnetic moments changes with temperature are particularly well-suited to reveal the anisotropic hyperfine field component. The UPdSn antiferromagnet provides an opportunity to explore the behavior of B hf under conditions when isotropic and anisotropic contributions to B hf coexist. The Ne ´el point is equal to about 43 K. The crystal and magnetic properties of UPdSn were investigated in detail ([2-4] and references therein). UPdSn crystallises in the ordered GaGeLi-type structure in which Sn atoms center the trigonal prisms of six U atoms. The magnetic moments of four neighboring U atoms cancel pairwise, but the net magnetic moment of the nearest environment of the Sn atom is nonzero. UPdSn is a noncollinear antiferromagnet with a uranium magnetic moment of about 2 m B . The magnetic structure can be described in terms of two canting angles q and j. q is the angle between the magnetic moment and the c axis of the orthorhombic cell; j is the angle between the in-plane component of the magnetic moment and the b axis. At low temperatures the angles q and j are equal to 54 and 45 , respectively. The angle q is almost constant over the whole temperature range, while the angle j decreases continuously up to a temperature of about 35 K, where it appears to rise again [2]. According to the model [1], this moments rotation should lead to a change in the anisotropic hyperfine field.
We carried out studies of the hyperfine field for 119 Sn in UPdSn by the Mo ¨ssbauer spectroscopy technique. Mo ¨ssbauer spectra have been measured over temperature ranges from 5 to 60 K. For the detection of Mo ¨ssbauer radiation a resonance detector has been used. Since the quadrupole and magnetic interactions were of comparable magnitude, the spectra were analyzed by diagonalization of the full Hamiltonian. It has been found that the principal component of the electrical field gradient (EFG) is oriented along the orthorhombic c axes, therefore the asymmetry parameter of the EFG is equal to zero. The hyperfine field vector lies in the a-b plane (or very close to this plane). At 5 K, B hf = 4.95(3) T. The quadrupole constant is equal to -1.18(3) mm/s, in good agreement with the quadrupole splitting value in the paramagnetic phase (at 60 K). The negative sign of the quadrupole constant corresponds to a positive sign of the EFG. The isomer shift is equal to 1.83(3) mm/s at 5 K. phys. stat. sol. (b) 225, No. 1, R1-R2 (2001) 1