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An applied magnetic field offers the opportunity to drive new transitions and tune competing interactions in complex materials. Here, we employ magnetoinfrared spectroscopy to probe magnetic ordering-induced local lattice distortions through the series of field-induced transitions in the frustrated Kagom\'e staircase material ${\text{Co}}_{3}{\text{V}}_{2}{\text{O}}_{8}$. These experiments reveal that the most important local lattice distortion involves Co center displacement, mainly along the direction perpendicular to the buckled Kagom\'e planes. This result is important for understanding the microscopic aspects of magnetically driven transitions in functional oxides.

The thermodynamic, magnetic, dielectric, and magnetoelectric properties of ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$ and ${\text{Ho}}_{0.5}{\text{Nd}}_{0.5}{\text{Fe}}_{3}{({\text{BO}}_{3})}_{4}$ are investigated. Both compounds show a second order Ne\'el transition above 30 K and a first-order spin reorientation transition below 10 K. ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$ develops a spontaneous electrical polarization below the Ne\'el temperature $({T}_{N})$ which is diminished in external magnetic fields. No magnetic-field induced increase of the polarization could be observed in ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$. In contrast, the solid solution ${\text{Ho}}_{0.5}{\text{Nd}}_{0.5}{\text{Fe}}_{3}{({\text{BO}}_{3})}_{4}$ exhibits both, a spontaneous polarization below ${T}_{N}$ and a positive magnetoelectric effect at higher fields that extends to high temperatures. The superposition of spontaneous polarization, induced by the internal magnetic field in the ordered state, and the magnetoelectric polarizations due to the external field results in a complex behavior of the total polarization measured as a function of temperature and field.

are consistent with the model of Allen and Perebeinosin observation of a strong resonance close to the JT gap of2 eV for the Raman phonon peaks at 496 JT antistretching ,611 −1JT stretching , and 655 cm modes, the last one as-signed to an oxygen breathing vibration. The same resonancehas been observed for the main peaks between 1000 and1400 cm

The pressure-temperature phase diagram of multiferroic ${\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is investigated for hydrostatic pressures up to $2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The stability range of the ferroelectric phase associated with the incommensurate helical spin order is reduced by pressure and ferroelectricity is completely suppressed at the critical pressure of $1.64\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at $6.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Thermal expansion measurements at ambient pressure show strong steplike anomalies of the lattice parameters associated with the lock-in transition into the commensurate paraelectric phase. The expansion anomalies are highly anisotropic; the related volume change is consistent with the high-pressure phase diagram.

Strong anomalies of the thermal expansion coefficients at the magnetic and ferroelectric transitions have been detected in multiferroic $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. Their correlation with anomalies of the specific heat and the dielectric constant is discussed. The results provide evidence for the magnetic origin of the ferroelectricity mediated by strong spin-lattice coupling in the compounds. Neutron scattering data for $\mathrm{Ho}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ indicate a spin reorientation at the two low-temperature phase transitions.

${\mathrm{GdFe}}_{3}({\mathrm{BO}}_{3}{)}_{4}$ exhibits a structural phase transition at $156\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, antiferromagnetic order of the ${\mathrm{Fe}}^{3+}$ moments at $36\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, followed by a spin reorientation phase transition at $9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The reorientation phase transition is studied through dielectric, magnetic, and heat capacity measurements under the application of external magnetic fields of up to $7\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The dielectric constant indicates the existence of two distinct anomalies at ${T}_{SR}=9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ that separate in temperature under external magnetic fields. The spin rotation phase transition is proven to be of the first-order nature through the magnetic analog of the Clausius-Clapeyron equation. Magnetodielectric effect of up to 1% is observed at $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $7\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The uniaxial magnetocaloric effect along the $c$ axis is observed below the spin reorientation phase transition of $9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

The dielectric constant of multiferroic hexagonal HoMnO_3 exhibits an unprecedented diversity of anomalies at low temperatures (1.8 K< T

The polarized Raman spectra of $\mathrm{Ho}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ and $\mathrm{Tb}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ are reported and discussed. Most of the Raman lines corresponding to $\ensuremath{\Gamma}$-point Raman modes $(13{A}_{g}+13{B}_{1g}+11{B}_{2g}+11{B}_{3g})$ have been observed. The temperature dependence of the $\mathrm{Ho}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ spectra provides no evidence for phonon anomalies associated with magnetic and ferroelectric transitions below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Evidence for a strong spin-lattice coupling in multiferroic HoMnO_3 is derived from thermal expansion measurements along a- and c-axis. The magnetoelastic effect results in sizable anomalies of the thermal expansivities at the antiferromagnetic (T_N) and the spin rotation (T_{SR}) transition temperatures as well as in a negative c-axis expansivity below room temperature. The coupling between magnetic orders and dielectric properties below T_N is explained by the lattice strain induced by the magnetoelastic effect. At T_{SR} various physical quantities show discontinuities that are thermodynamically consistent with a first order phase transition.

The field-induced re-entrant phase in multiferroic hexagonal $\mathrm{Ho}\mathrm{Mn}{\mathrm{O}}_{3}$ is investigated to lower temperatures by dc magnetization, ac susceptibility, and specific heat measurements at various magnetic fields. Two new phases have been unambiguously identified below the N\'eel transition temperature, ${T}_{N}=76\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for magnetic fields up to 50 kOe. The existence of an intermediate phase between the $P{\underset{}{\mathit{6}}}_{3}\underset{}{\mathit{c}}m$ and $P{\underset{}{\mathit{6}}}_{3}\mathit{c}\underset{}{m}$ magnetic structures (previously predicted from dielectric measurements) was confirmed and the magnetic properties of this phase have been investigated. At low temperatures $(Tl5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ a dome shaped phase boundary characterized by a magnetization jump and a narrow heat capacity peak was detected between the magnetic fields of 5 kOe and 18 kOe. The transition across this phase boundary is of first order and the magnetization and entropy jumps obey the magnetic analogue of the Clausius-Clapeyron relation. Four of the five low-temperature phases coexist at a tetracritical point at 2 K and 18 kOe. The complex magnetic phase diagram so derived provides an informative basis for unraveling the underlying driving forces for the occurrence of the various phases and the coupling between the different orders.