RESUMO
We combined synchrotron-based infrared absorbance and Raman scattering spectroscopies with diamond anvil cell techniques and a symmetry analysis to explore the properties of multiferroic (NH4)2FeCl5·H2O under extreme pressure-temperature conditions. Compression-induced splitting of the Fe-Cl stretching, Cl-Fe-Cl and Cl-Fe-O bending, and NH4+ librational modes defines two structural phase transitions, and a group-subgroup analysis reveals space group sequences that vary depending upon proximity to the unexpectedly wide order-disorder transition. We bring these findings together with prior high-field work to develop the pressure-temperature-magnetic field phase diagram uncovering competing polar, chiral, and magnetic phases in this system.
RESUMO
We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu(pyz)2(2-HOpy)2](PF6)2 and [Cu(pyz)1.5(4-HOpy)2](ClO4)2, under different external stimuli in order to explore the microscopic aspects of spin-lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic â fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF6- compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO4- system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin-lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu(pyz)1.5(4-HOpy)2](ClO4)2 because of its ladderlike character. Although spin-lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO4- compound may be ferroelectric.
RESUMO
We combine magnetic circular dichroism and photoconductivity with prior optical absorption and first principles calculations to unravel spin-charge interactions in the high Curie temperature magnet CoFe2O4. In addition to revising the bandgap hierarchy, we reveal a broad set of charge transfer excitations in the spin down channel which are sensitive to the metamagnetic transition involving the spin state on Co centers. We also show photoconductivity that depends on an applied magnetic field. These findings open the door for the creation and control of spin-polarized electronic excitations from the minority channel charge transfer in spinel ferrites and other earth-abundant materials.