W. Berlinger

Electron paramagnetic resonance of Mn4+ in BaTiO3

Abstract The Electron Paramagnetic Resonance (EPR) of Mn 4+ in the rhombohedral low- temperature phase of BaTiO 3 has been observed at 19.3 and 13.0 GHz. The spectra have axial symmetry along 〈111〉 directions. Intensity measurements prove that the Mn 4+ EPR reflects {111} domain orientation. The spin-Hamiltonian constants are g ‖ =1.9968(19), g ‖ =2.0015(5), A ‖ =67.48(5)×10 −4 cm −1 , A ‖ =73.04(15)×10 −4 cm −1 , and D=0.65(1)cm −1 at 4.2 K. The very large D value is more than 20 times that found for the isoelectronic Cr 3+ . The latter ion remains centered in its surrounding octahedral oxygen cage, whereas the Mn 4+ is off-center with similar coordinates as Ti 4+ . This result is deduced from a superposition-model analysis.

Monodomain strontium titanate

Abstract Monodomain SrTiO 3 crystals were found in less than 0.3 mm thick (110) plates below the cubic-to-tetragonal phase transition. Up to 98.5% monodomain samples were observed for rectangular plates with edges 7 mm // [001] and 2 mm // [110]. Quantitative results were obtained by monitoring EPR spectra of Fe 3+ impurities. Optical dichroism was also observed. Uniaxial stress experiments to yield large volume monodomain crystals are also reported, and confirm that c/a > 1 below the transition.

Electron spin resonance and optical identification of Fe4+-V0 in SrTiO3

Abstract The electron spin resonance of the minority defect Fe 4+ -V 0 has been observed in SrTiO 3 . It is the first pair center identified with a two-fold positive charge with respect to the lattice. For Fe 4+ it occurs in the unusual high spin S = 2 state. Optically-induced conversion to the neighbor Fe 3+ -V 0 charge state served to identify the acceptor transfer bands, showing the potentiality of the conversion-rate method.

Photochromic Fe5+ (3d3) in SrTiO3 evidence from paramagnetic resonance

Abstract Using paramagnetic resonance it is shown that a photochromic center in SrTiO3 is Fe5+ (3d3). Its resonance parameters are g = 2.0131 ±0.0008, A( 57 Fe) = (8.6 ± 0.1) x 10 −4 cm −1 . As a function of valency the positive g-shift of Fe5+ lies nearly on a straight line joining those of the isoelectronic Cr3+ and Mn4+. The hyperfine fields of V2+, Cr3+, Mn4+ and Fe5+ are close to one another (Geschwinds rule). Simultaneous observation of Fe3+ and Fe4+ proves indirectly the existence of Fe3+ which can act as a hole trap.

Identification of Fe4+ and Fe5+ charge-transfer photochromic absorption bands in SrTiO3

Optical absorption and the enhancement and bleaching of Fe3+ and Fe5+ electron paramagnetic resonance in SrTiO3: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O2- valence states to Fe4+ and Fe5+. They occur at 2.09 and 2.82 eV for Fe4+ and at 1.99 and 2.53 eV for Fe5+.

Holes trapped near Mg2+ and Al3+ impurities in SrTiO3

Abstract From SrTiO 3 doped with Al or Mg, ESR signals are identified arising from holes captured in O 2− p (π) orbitals near Al 3+ or Mg 2+ ions substitutional on Ti 4+ sites. These centers are involved in photochromic processes in SrTiO 3 .

Paramagnetic Ru5+ and Ru3+ centers in TiO2:Ru

Abstract Paramagnetic resonance on TiO2 single crystals doped with ruthenium have shown the presence of both Ru5+(4d3) and Ru3+(4d5) in oxygen octahedral coordination.

ESR in quasi 2-d ferromagnets: The coexistence of paramagnetic excitations and magnetostatic spin waves and new interference effects

Abstract The quasi 2-dimensional layer structures (CnH2n+1NH3)2CuCl4 with n = 1,2 have been investigated by electron spin resonance. New phenomena arise for samples with well defined natural surfaces: (i) well resolved magnetostatic volume and surface spin wave modes at 4.2 K comparable in quality with spectra of YIG, (ii) a magnetostatic spin wave spectrum which is coincident with the paramagnetic resonance over a wide temperature range, (iii) interference effects between these two types of resonances.

Anisotropic microwave absorption and dc resistance in magnetic fields of granular superconducting aluminum films

Abstract The microwave absorption and critical resistive behaviour of superconducting granular aluminum films, which are metallic in their normal state, have been examined. Microwave absorption is appreciable in these films and can be accounted for by dissipation of flux motion. Perpendicular flux readily penetrates the film while parallel flux enters more gradually. Film resistance is sensitive to field direction because of the perpendicular demagnetizing field, weak pinning of perpendicular flux and inhibited nucleation of parallel fluxons.

The temperature dependence of the linewidth of iron group ions in MgO

The linewidths of the paramagnetic resonance lines of Fe1+, Fe3+ and Mn2+ in MgO decrease as a function of increasing temperature in the helium range. This decrease appears to result from a motional narrowing of the contribution to the linewidth from dipolar interaction with Fe2+ ions as the Fe2+ relaxation rate becomes comparable with the frequency width of the line studied.