engleski

Magnetic resonance study of rare-earth titanates

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 YTiO3. Measurements of single-crystalline samples of (Y, Ca, La)TiO3 in a wide range of isovalent substitution (La) and hole doping (Ca) reveal several unusual features in the paramagnetic state of these materials. 89Y NMR demonstrates a clear discrepancy between the static and dynamic local magnetic susceptibilities, with deviations from Curie-Weiss behavior far above the Curie temperature TC. No significant changes are observed close to TC, but a suppression of fluctuations is detected in the NMR spin-lattice relaxation time at temperatures of about 3×TC. 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 t2g orbitals. The value of the gap closely follows TC 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.

NMR ; ESR ; rare-earth titanates ; magnetizm

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