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Size-dependent vibronic coupling in α-Fe<sub>2</sub>O<sub>3</sub>
O'Neal, K. R.,Patete, J. M.,Chen, P.,Holinsworth, B. S.,Smith, J. M.,Lee, N.,Cheong, S.-W.,Wong, Stanislaus S.,Marques, C.,Aronson, M. C.,Musfeldt, J. L. American Institute of Physics 2014 The Journal of chemical physics Vol.141 No.4
<P>We report the discovery of finite length scale effects on vibronic coupling in nanoscale alpha-Fe2O3 as measured by the behavior of vibronically activated d-d on-site excitations of Fe3+ as a function of size and shape. An oscillator strength analysis reveals that the frequency of the coupled symmetry-breaking phonon changes with size, a crossover that we analyze in terms of increasing three-dimensional character to the displacement pattern. These findings demonstrate the flexibility of mixing processes in confined systems and suggest a strategy for both enhancing and controlling charge-lattice interactions in other materials. (C) 2014 AIP Publishing LLC.</P>
Georgia C. Papaefthymiou,Arthur J. Viescas,Jean-Marie Le Breton,Hubert Chiron,Jean Juraszek,Tae-Jin Park,Stanislaus S. Wong 한국물리학회 2015 Current Applied Physics Vol.15 No.3
Magnetic and Mössbauer characterization of single crystalline, sub-micron sized Bi2Fe4O9 cubes has been performed using SQUID magnetometry and transmission Mössbauer spectroscopy in the temperature range of 4.2 K ≤ T ≤ 300 K. A broad magnetic phase transition from the paramagnetic to the antiferromagnetic state is observed below 250 K, with the Mössbauer spectra exhibiting a superposition of magnetic, collapsed and quadrupolar spectra in the transition region of 200 K < T < 245 K. Room temperature Mössbauer spectra obtained in transmission geometry are identical to those recorded in back-scattering geometry via conversion electron Mössbauer spectroscopy, indicating the absence of strain at the surface. A small hysteresis loop is observed in SQUID measurements at 5 K, attributable to the presence of weak-ferromagnetism arising from the canting of Fe3+ ion sublattices in the antiferromagnetic matrix.
Surface phase transitions in BiFeO3below room temperature
Jarrier, R.,Marti, X.,Herrero-Albillos, J.,Ferrer, P.,Haumont, R.,Gemeiner, P.,Geneste, G.,Berthet, P.,Schü,lli, T.,Cevc, P.,Blinc, R.,Wong, Stanislaus S.,Park, Tae-Jin,Alexe, M.,Carpenter, M. A. American Physical Society 2012 Physical review. B, Condensed matter and materials Vol.85 No.18