P. Novák

Character of the excited state of the Co3+ ion in LaCoO3

Generalized gradient approximation (GGA)+U calculations for LaCoO3 confirm the existence of a stationary state composed of a mixture of the low spin (LS) and high spin (HS) Co3+ ions in a 1:1 ratio, which is of insulating character. At low temperatures, this state is located about 25 meV above the homogeneous LS ground state if CoLS–O–CoHS bond length optimization is taken into account. The energy difference decreases with the lattice dilatation and the LS+HS(1:1) phase with bond length optimization becomes stable at some intermediate temperature. These results, together with previous electron spin resonance evidence for HS excitations in the LS ground state, lead to the conclusion that the diamagnetic–paramagnetic transition in LaCoO3 at 50–150 K is caused by a gradual population of HS Co3+ ionic states, provided that neighbouring sites are in the LS state. Consistency between experimental data for the paramagnetic susceptibility and anomalous thermal expansion is achieved for an HS–LS energy difference of 16 meV that corresponds to 180 K. With increasing HS population, the antiferromagnetic (AF) interactions HS–LS–HS become effective. The LS+HS(1:1) phase is saturated above 150 K and the susceptibility acquires a simple Curie–Weiss behaviour in which AF interactions are characterized by θ = −160 K.

Crystal field and magnetism of Pr3+ and Nd3+ ions in orthorhombic perovskites

Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO3 perovskites (R=Pr, Nd, M=Ga, Co) are calculated using a first-principles electronic structure and the Wannier projection. The method contains a single adjustable parameter that characterizes the hybridization of R(4f) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of the trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the 4f electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within a few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO3 the theory is also compared with the magnetic measurements on a single crystal presented by Luis et al (1998 Phys. Rev. B 58 798). Good agreement between the experiment and theory is found.

Magnetocrystalline anisotropy of magnetite

The spin reorientation temperature T(SR) of stoichiometric Fe(3)O(4), as well as of magnetite with a small number of vacancies and magnetite containing a low concentration of Ti, Zn, Al and Ga was measured on single-crystal samples using the ac susceptibility. In the same experiment the temperature T(V) of the Verwey transition was also found. The results show that a correlation between T(SR) and T(V) exists. The electronic structure of the compounds studied was determined using the density-functional-based GGA + U method. For stoichiometric magnetite the first and second cubic anisotropy constants were calculated, while for magnetite with defects the distribution of electron density using the atoms in molecules approach was determined. Based on a combination of experimental results with the electronic structure calculations an explanation of the temperature dependence of the magnetocrystalline anisotropy of magnetite is suggested.

NMR spectra of57Fe in hexagonal ferrites with magnetoplumbite structure

The NMR spectra of57Fe in domains of MFe12O19 (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe3+ ions in 2b and 4f2 sites while leaving other lines essentially unchanged; the significant role of different Fe3+−M2+ bonding was found. The dipolar broadening of lines in BaFe12O19 caused by random and static displacement of bipyramidal Fe3+ ions from the mirror plane is calculated and the results are compared with the experiment.

On the transferred hyperfine interaction in substituted yig

NMR of57Fe is studied in a number of (MxY3−x) Fe5O12 garnets for small concentrations of M (M is either trivalent RE ion −Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M3+−O2−-Fe3+ vs. Y3+−O2−-Fe3+ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M3+ and Fe3+ ions and not with the local deformation around the impurity.

Applicability of NMR to impurities detection in YIG thin films

Abstract An impurity in the vicinity of the 57 Fe nucleus induces a specific structure of satellite lines in the NMR spectrum, the intensities of which are proportional to the impurity concentration. Recently achieved sensitivity makes it possible to detect as little as 0.02% impurity. Spectra of nominally pure YIG epitaxial films, containing Pb, Ga, Al, Si, Pt, Ca impurities and ‘antisite’ defects are presented to illustrate the approach.

Defects in yttrium-iron garnet studied by NMR on iron nuclei

Abstract The NMR spectra of 57 Fe in nominally pure YIG were measured and a rich satellite structure caused by defects was found. We identified the satellites caused by antisite defects (yttrium on iron sites, and vice versa) and satellites caused by Pb 2+ impurities. From amplitudes of the satellites the concentration of the antisite defects was determined.

NMR study of La3+ substituted hexagonal ferrites with magnetoplumbite structure

Abstract Domain 57 Fe NMR spectra of La 3+ substituted hexagonal ferrites Pb 1− x La x Fe 12 O 19 and Ba 1− x La x Fe 12 O 19 have been measured at 4.2 K using the spin echo technique. Besides the five main lines, the satellite lines caused by the substitution were observed. The assignment of the satellite lines to the Fe 3+ sublattices was made using the dependence of the NMR spectrum on the external magnetic field. The experimental results are explained assuming the existence of Fe 2+ ions in the 2a sublattice.

Ground-state properties of the mixed-valence cobaltites Nd0.7Sr0.3CoO3, Nd0.7Ca0.3CoO3 and Pr0.7Ca0.3CoO3

The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed: TC = 130 K, 55 K and 25 K and m = 0.68 μB, 0.34 μB and 0.23 μB for Nd0.7Sr0.3CoO3, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3, respectively. The moment reduction with respect to the moment of the conventional ferromagnet La0.7Sr0.3CoO3 (T(C) = 230 K, m = 1.71 μB) in the so-called low spin/intermediate spin (IS/LS) state for Co(3+)/Co(4+) was originally interpreted using a phase-separation scenario. Based on the present results, mainly the analysis of the Schottky peak originating from Zeeman splitting of the ground-state Kramers doublet of Nd(3+), we find, however, that the ferromagnetic phase in Nd0.7Ca0.3CoO3 and likely also in Pr0.7Ca0.3CoO3 is uniformly distributed over the whole sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov et al (2009 Phys. Rev. B 80 024423). The ground state of Nd0.7Sr0.3CoO3 with an intermediate cobalt moment is inhomogeneous due to competition between the LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for 4f(3) (Nd(3+)), 4f(2) (Pr(3+)) and 4f(1) (Ce(3+) or Pr(4+)) are calculated and their magnetic characteristics are presented.

Anisotropy of the Hyperfine Field on 57Fe in Pure and Substituted Yttrium Iron Garnet

Using NMR, anisotropy of the hyperfine field on 57Fe nuclei was measured in Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (Y3+ - yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were performed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the 57Fe nuclei in an ideal environment and in an environment where one of Fe3+ nearest cation neighbours is replaced by the nonmagnetic impurity is given. When the Fe3+ on which the NMR is measured is on the a site and the impurity is Ga3+(d), the results may be interpreted in terms of the superposition model. On the other hand, the results for Fe3+ on the d site and Y3+(a) are in clear disagreement with the predictions of this model.