Electron paramagnetic resonance (EPR) under the conditions of insitu laser illumination of the sample in the microwave cavity, called photo-EPR is an excellent method to detect photo induced electron transfer in real time. We report Photo-EPR results on the formation of clusters of magnetic ions in 0.67Pb(Mg 1/3 Nb 2/3 )O 3 . 0.33PbTiO 3 (PMN-PT) on illumination with blue laser at room temperature .The Photo EPR signal with g=2.00, agrees with Pb 3+ formation. The PMN-PT crystals 2-6mm size grown using PbO flux method were clear and transparent: and the upper part of the melt yielded pink colored crystals containing a few hundred PPM of Fe 3+ as seen by EPR Signal at g=4.3. The large linewidth (~45 Gauss) of the photo-EPR signal and the presence of Fe 3+ in the sample suggests the possibility of magnetic ion cluster formation on blue laser excitation. When the photo-magnetic centers were produced in magnetic field of 7.5KG, the signal was 30% more than that produced under switch-OFF condition of magnet having a remnant field of only 50G. This critical observation is a clear pointer to the formation of photo-induced magnetic polarons at room temperature, which are essentially clusters of ferromagnetically, coupled Fe 3+ , Pb 3+ and trapped electrons. The cluster formation and decay exhibited fast optical response with growth and decay time ≤ 100msec. These observations show that PMN-PT having excellent electromechanical properties can also be used for photomagnetic switching and real time holography with fast grating response using Pb 2+ ↔ Pb 3+ process. PMN-PT would have additional advantage compared to other photo-refractive materials: due to possibility that the grating contrast can be manipulated by external magnetic field.
Barium titanate crystals were grown by top seeded solution growth technique, nominally pure and also 0.05% and 1% Cr 3+ impurity. We have conducted electron paramagnetic resonance (EPR) and photo-EPR studies at room temperature to investigate the role of Cr 3+ impurity in photoinduced electron transfer. Nominally pure crystals contained Fe 3+ as impurity, and its EPR is consistent with work reported by previous investigators. The Cr 3+ doped crystals also contained Fe 3+ impurities. It was observed that the site symmetry and the strength of the axial field parameter for Fe 3+ complex were significantly different in Cr 3+ doped crystals compared to nominally pure BaTiO 3 . The EPR spectra of Cr 3+ were distinguished using the hyperfine structure of odd isotope 53 Cr (I=3/2). By Photo-EPR technique we observe that in the presence of Cr 3+ , Fe 3+ is not significantly photosensitive. In contrast Cr 3+ exhibited higher photosensitivity in the presence of Fe 3+ . This was monitored by locking the magnetic field to 1/2↔1/2 transition of Cr 3+ , and recording intensity as a function of time, under insitu laser illumination. In lightly doped crystals the intensity of Cr 3+ signal is sharply reduced immediately after switching the laser OFF showing non-exponential decay. In heavily doped crystals photo-EPR signal clearly shows that the fast decay of Cr 3+ was followed by slow and steady build up of Cr 3+ signal. The growth of Cr 3+ signal was attributed to photoinduced decoupling of Cr 3+ dimers. Thus, by doping BaTiO 3 with Cr 3+ more efficient grating formation can be achieved and time dependent phenomena are observed.
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