Defects in LiNbO3—I. experimental aspects

The electron spin resonance of F 3+ in single crystals of SrTiO 3 has been studied in the low temperature phase below 110°K. The resonance lines exhibited small splitting in addition to the usual tetragonal splitting. The orientation dependence of the splitting was measured at 77°K and at 4.2°K. The whole spectrum is explained to be due to the rotation of oxygen octahedron in the (100) plane, the angle of rotation being 1.4° at 77°K and 2.1° at 4.2°K. Corresponding splitting was also observed for the case of the `strong axial field' spectrum due to Fe 3+ accompanied by the nearest neighbor oxygen vacancy. The observed angles of rotation are about 30 percent smaller than those of the former case. The crystal structure deduced from the atomic positions of oxygen ions will be D 4 h 16 in the low temperature phase. Correlation with the resonance data of Gd 3+ in SrTiO 3 is discussed.

Electron Spin Resonance of Fe 3+ in SrTiO 3 with Special Reference to the 110°K Phase Transition

The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.

How is oxygen incorporated into oxides? A comprehensive kinetic study of a simple solid-state reaction with SrTiO3 as a model material.

THE DEFECT STRUCTUFE OF SrTi1−xFexO3−y (x = 0–0.8) INVESTIGATED BY ELECTRICAL CONDUCTIVITY MEASUREMENTS AND ELECTRON ENERGY LOSS SPECTROSCOPY (EELS)

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in these materials are discussed.

Surface spin disorder in ferrite nanoparticles (invited)

An axial electron paramagnetic resonance (EPR) spectrum in iron-doped (cubic) strontium titanate has been observed at 3.3 and 1.85 cm wavelength with effective $g$ values ${{g}_{\ensuremath{\perp}}}^{e}=5.993\ifmmode\pm\else\textpm\fi{}0.001$ and ${{g}_{\ensuremath{\perp}}}^{e}=5.961\ifmmode\pm\else\textpm\fi{}0.001$, respectively, and ${{g}_{\mathrm{II}}}^{e}={g}_{\ensuremath{\parallel}}=2.0054\ifmmode\pm\else\textpm\fi{}0.0007$. The same spectrum was found after charge displacement due to heat treatment in the dark or by reduction in crystals which contained, in addition to iron, other transition metal ions. The spectrum is attributed to $\ensuremath{\Delta}M=1$ transitions of the ${S}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ level of ${\mathrm{Fe}}^{3+}(3{d}^{5})$ in a strongly tetragonal electric crystalline field produced by local charge compensation at a nearest-neighbor oxygen site. The theory is developed for ions of half-integral spin in an axial field much greater than the Zeeman splitting. By applying the theory to the special case of $S=\frac{5}{2}$ and using the measured effective ${{g}_{\ensuremath{\perp}}}^{e}$ values and resonance magnetic fields ${g}_{\ensuremath{\perp}}=2.0101\ifmmode\pm\else\textpm\fi{}0.0008$ and a zero-field splitting parameter $|2D|$ of 2.85\ifmmode\pm\else\textpm\fi{}0.15 ${\mathrm{cm}}^{\ensuremath{-}1}$ is obtained. This is the largest splitting which has so far been observed for ${\mathrm{Fe}}^{3+}$ in any inorganic crystal and could be useful for submillimeter maser applications.

R. S. Rubins

Papers

STRONG AXIAL ELECTRON PARAMAGNETIC RESONANCE SPECTRUM OF Fe$sup 3+$ IN SrTiO$sub 3$ DUE TO NEAREST-NEIGHBOR CHARGE COMPENSATION