Hyperfine interactions in titanates: Study of orbital ordering and local magnetic properties

Abstract

1 1. INTRODUCTIONOxides of transition metals with orbital degeneracy are on the focus of attention of researchers. "Orbital physics" [1] appeared as a new field in the solid state physics that describes the unusual properties of these materials taking into account features caused by orbital degeneracy (orbital structure, Jahn-Teller effect, orbitally dependent exchange interaction). Per ovskite oxides RMO 3 (where R = Y, La and other trivalent rare earth ions and M is a 3d ion) are typical objects for this field. Titanates RTiO 3 belonging to this class of compounds seem to be key materials for the study of the strong coupling of the lattice, spin, and orbital degrees of freedom.Titanates with one d electron in the t 2g shell are convenient objects for answer to the questions: whether a static orbital structure [2,3] exists in the titanium sublattice or the orbital liquid state is imple mented [4,5], what is the role of Jahn-Teller distor tions, why LaTiO 3 is a G type antiferromagnet with T N = 140 K [6], whereas isostructural YTiO 3 [7] is a ferromagnet with T C = 25 K.The first attempt to clarify the microscopic nature of the unusual properties of titanates was made by Khalliullin and Maekawa [4]. They proposed the orbital liquid model that explains the reduction of the magnetic moment of titanium of about 0.5μ B in LaTiO 3 [5] and an isotropic spectrum of spin waves [8]. However, the model contradicts the NMR experimen tal results for 47,49 Ti [9], some calculations in the crys tal field model [3,10], and band calculations [11]. The results of calculations show that the ground level of the Ti 3+ ion is an orbital singlet separated from the The NMR experiments on the 47,49 Ti nuclei allow the detection of the local orbital order, whereas NMR on 139 La( 89 Y) depends on the entire picture of the orbital and magnetic orders. For this reason, investiga tion of hyperfine fields on La and Y nuclei can provide a more complete picture of the orbital and magnetic orders in titanates.In this work, the magnetic fields induced on the La and Y nuclei in LaTiO 3 and YTiO 3 , respectively, are calculated. The dependence of these fields on the crys tal, orbital, and magnetic structures of the compounds under study is analyzed. Comparison with the known experimental data is performed. HYPERFINE FIELDS IN TITANATES AND THEIR RELATION TO THE CRYSTAL, ORBITAL, AND MAGNETIC STRUCTURESThe problem of the calculation of hyperfine fields induced on the nuclei of nonmagnetic ions in mag netic materials is solved for a long time using various methods [14,15]. This problem in application to titanates is still topical because there are no theoretical Abstract-Hyperfine magnetic fields induced on the nuclei of nonmagnetic ions 139 La and 89 Y in LaTiO 3 and YTiO 3 , respectively, have been microscopically calculated. The dependence of the hyperfine fields on the orbital and magnetic structures of the compounds under study has been analyzed. The comparative analysis of the calculated and known experimental data confirms the existence o...