Controlling multiferroic behavior in materials will enable the development of a wide variety of technological applications. However, the exact mechanisms driving multiferroic behavior are not well understood in most materials. Two such materials are the spinels MnV 2O4 and Mn3O4, where mechanical strain is thought to play a role in determining magnetic behavior. Bulk studies of MnV2O4 have yielded conflicting and inconclusive results, due in part to the presence of mesoscale magnetic inhomogeneity, which complicates the interpretation of bulk measurements. To study the sub-micron-scale magnetic properties of Mn-based spinel materials, we performed magnetic force microscopy (MFM) on MnV 2O4 samples subject to different levels of mechanical strain. We also used a crystal grain mapping technique to perform spatially registered MFM on Mn 3O4. These local investigations revealed 100-nm-scale "stripe" modulations in the magnetic structure of both materials. In MnV2O4, the magnetization of these stripes is estimated to be M z ~ 10 5 A/m, which is on the order of the saturation magnetization reported previously. Cooling in a strong magnetic field eliminated the stripe patterning only in the low-strain sample of MnV 2O4. The discovery of nanoscale magnetostructural inhomogeneity that is highly susceptible to magnetic field control in these materials necessitates both a revision of theoretical proposals and a reinterpretation of experimental data regarding the low-temperature phases and magnetic-field-tunable properties of these Mn-based spinels.

Superconductivity evolves as functions of pressure or doping from charge-ordered phases in a variety of strongly correlated systems, suggesting that there may be universal characteristics associated with the competition between superconductivity and charge order in these materials. We present an inelastic light (Raman) scattering study of the structural changes that precede the pressuretuned charge-density-wave (CDW) to superconductor transition in one such system, ZrTe3. In certain phonon bands, we observe dramatic linewidth reductions that accompany CDW formation, indicating that these phonons couple strongly to the electronic degrees of freedom associated with the CDW. The same phonon bands, which represent internal vibrations of ZrTe3 prismatic rods, are suppressed at pressures above ∼10 kbar, indicating a loss of long-range order within the rods, specifically amongst intrarod Zr-Te bonds. These results suggest that the pressure-induced suppression of CDW order observed in ZrTe3 is structurally driven, and provide insights into the origin of pressure-induced superconductivity in this material.

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of one-and twomagnon excitations in MnV2O4 and Mn3O4. We observe a pair of q = 0 one-magnon modes at 74 cm −1 and 81 cm −1 in MnV2O4, which is in contrast with the single 80 cm −1 q = 0 magnon that has been reported for MnV2O4 based on previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below TN , which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.

The ferrimagnetic spinel Mn3O4 exhibits large and anisotropic changes in electronic and structural properties in response to an applied magnetic field. These changes are thought to result from the field-dependent tuning-via strong spin-lattice coupling-between two nearly degenerate magnetostructural phases. Recent variable-magnetic-field studies of Mn3O4 have been performed on melt-grown crystals, which can exhibit twin domains due to a Jahn-Teller structural transition below the melting temperature. Because of the near degeneracy of the magnetostructural phases, however, strain associated with the twin domains likely affects the magnetic responses of Mn3O4. In this report, we present a variable-magnetic-field Raman scattering study of untwinned Mn3O4 crystals grown out of a flux below the Jahn-Teller structural transition. We measure distinct q = 0 magnetic and vibrational excitation spectra for each isolated magnetostructural phase of untwinned Mn3O4 crystals and determine the symmetries of the observed excitations. We determine how the magnetostructural phase mixture changes in response to magnetic fields applied in the magnetic easy plane. Lastly, by comparing results on flux-and melt-grown Mn3O4 crystals, we show that the intrinsic mixture of the two magnetostructural phases is indeed strongly influenced by the presence of twin domains.

Magnetoelectric materials have generated wide technological and scientific interest because of the rich phenomena these materials exhibit, including the coexistence of magnetic and ferroelectric orders, magnetodielectric behavior, and exotic hybrid excitations such as electromagnons. The multiferroic spinel material, CoCr2O4, is a particularly interesting example of a multiferroic material, because evidence for magnetoelectric behavior in the ferrimagnetic phase seems to conflict with traditional noncollinear-spin-driven mechanisms for inducing a macroscopic polarization. This paper reports an inelastic light scattering study of the magnon and phonon spectrum of CoCr2O4 as simultaneous functions of temperature, pressure, and magnetic field. Below the Curie temperature (TC ∼ 94 K) of CoCr2O4 we observe a ω ∼ 16 cm −1 q = 0 magnon having T1g-symmetry, which has the transformation properties of an axial vector. The anomalously large Raman intensity of the T1g-symmetry magnon is characteristic of materials with a large magneto-optical response and likely arises from large magnetic fluctuations that strongly modulate the dielectric response in CoCr2O4. The Raman susceptibility of the T1g-symmetry magnon exhibits a strong magnetic-field dependence that is consistent with the magnetodielectric response observed in CoCr2O4, suggesting that magnetodielectric behavior in CoCr2O4 primarily arises from the field-dependent suppression of magnetic fluctuations that are strongly coupled to long-wavelength phonons. Increasing the magnetic anisotropy in CoCr2O4 with applied pressure decreases the magnetic field-dependence of the T1g-symmetry magnon Raman susceptibility in CoCr2O4, suggesting that strain can be used to control the magnetodielectric response in CoCr2O4.