Magnetic resonance study of rare-earth titanates

Abstract

We present a nuclear magnetic resonance (NMR) and electron spin resonance (ESR) study of rare-earth titanates derived from the spin-1/2 Mott insulator YTiO${}_3$. Measurements of single-crystalline samples of (Y,Ca,La)TiO${}_3$ in a wide range of isovalent substitution (La) and hole doping (Ca) reveal several unusual features in the paramagnetic state of these materials. ${}^{89}$Y NMR demonstrates a clear discrepancy between the static and dynamic local magnetic susceptibilities, with deviations from Curie-Weiss behavior far above the Curie temperature $T_C$. No significant changes are observed close to $T_C$, but a suppression of fluctuations is detected in the NMR spin-lattice relaxation time at temperatures of about $3\times T_C$. Additionally, the nuclear spin-spin relaxation rate shows an unusual peak in dependence on temperature for all samples. ESR of the unpaired Ti electron shows broad resonance lines at all temperatures and substitution/doping levels, which we find to be caused by short electronic spin-lattice relaxation times. We model the relaxation as an Orbach process that involves a low-lying electronic excited state, which enables the determination of the excited-state gap from the temperature dependence of the ESR linewidths. We ascribe the small gap to Jahn-Teller splitting of the two lower Ti $t_{2g}$ orbitals. The value of the gap closely follows $T_C$ and is consistent with the temperatures at which deviations from Curie-Weiss fluctuations are observed in NMR. These results provide insight into the interplay between orbital and spin degrees of freedom in rare-earth titanates and indicate that full orbital degeneracy lifting is associated with ferromagnetic order.

References in corpus (16)

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• Nature of the ferromagnetic-antiferromagnetic transition in Y${}_{1-x}$La${}_x$TiO${}_3$
• Two-component electronic phase separation in the doped Mott insulator Y${}_{1-x}$Ca${}_x$TiO${}_3$
• Growth and characterization of large (Y,La)TiO${}_3$ and (Y,Ca)TiO${}_3$ single crystals
• Raman scattering study and lattice-dynamics calculations in YTiO3: Precursor of the magnetic phase transition in the phonon anomalies