Session M6: Focus Session: Coupling Polarization and Magnetism I

\textit{In-situ} Raman Spectroscopic Investigation of Relaxor Multiferroic Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$ under High Pressure and Temperature Conditions

The vibrational and structural properties of lead ferroniobate, Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$, have been investigated using Raman spectroscopy up to 40 GPa and up to 415 K at selected pressures. Three phase transitions were noted via the pressure evolution at 5.5, 8.7 and 24 GPa at room temperature, respectively. The temperature dependencies of the Raman spectra indicated two phase transitions at 1.5 GPa, 335 and 365 K, which support the appearance of an intermediate tetragonal \textit{P4mm} phase between the ferroelectric \textit{R3m} and paraelectric \textit{Pm-3m} phases. At 2.5, 3.9 and 7.4 GPa, the system showed one phase transition with temperature evolution at 337, 348 and 332 K respectively. With this set of data, a $P-T$ phase diagram was compiled to provide further insight into the magnetoelectric coupling and allow comparison to other systems in order to elucidate the impact of magnetic order on relaxor systems.   

Investigation of the multiferroic behavior in FeVO$_{4}$ single crystals

FeVO$_{4}$ is considered as a model system for understanding the magnetoelectric interaction mechanisms in low symmetry multiferroics. Bulk FeVO$_{4}$ exhibits two antiferromagnetic phase transitions at $T_{\mathrm{N1}} = $ 22 K and $T_{\mathrm{N2}} = $ 15 K. Below $T_{\mathrm{N2}}$, a noncollinear magnetic order develops, breaking the space inversion symmetry that induces ferroelectric order. Earlier measurements on polycrystalline samples of FeVO$_{4}$ doped with magnetic (e.g.: Chromium) as well as non-magnetic (e.g.: Zinc) ions, indicate the stability of the two antiferromagnetic transition temperatures, with a change of only 2{\%} corresponding to the doping concentration of 20{\%}. It also shows the ability of the FeVO$_{4}$ triclinic structure to accommodate such high doping levels. Working along the same line, we have prepared both doped and undoped single crystals of FeVO$_{4}$ by a flux method. Samples were characterized using XRD and Raman spectroscopy to track the changes in lattice parameters induced by different dopants. The magnetic and ferroelectric properties were investigated in order to understand the origin of magnetoelectric coupling in low symmetry multiferroics.   

Near Room-Temperature Magnetism and Enhanced Magnetic Moments in Multiferroic (LuFeO$_{3})_{\mathrm{m}}$/(LuFe$_{2}$O$_{4})_{\mathrm{n}}$ Superlattices

The development of room-temperature multiferroics is necessary to realize the potential of these materials in low-power energy, memory, and logic applications. Currently, there are only four potential single-phase multiferroics that exist at room-temperature, all of which have either antiferromagnetic or weakly ferromagnetic magnetic orderings. Here, we report on the magnetic properties of epitaxially grown superlattices composed of the ferroelectric, weakly ferromagnetic $h$-LuFeO$_{3}$ and the paraelectric, ferrimagnetic LuFe$_{2}$O$_{4}$. By inserting layers of $h$-LuFeO$_{3}$ ($T_{\mathrm{N}} = $147 K) into LuFe$_{2}$O$_{4}$, we increase $T_{\mathrm{C}}$ from 219 K for single-phase LuFe$_{\mathrm{2}}$O$_{4}$ to 270 K for (LuFeO$_{3}$)$_{7}$/(LuFe$_{2}$O$_{4}$)$_{1}$. Additionally, while the magnetic moment on the LuFe$_{2}$O$_{4}$ layers remains constant for $m$/($m+$2$n)$ \textless 0.5, it increases rapidly for $m$/($m+$2$n)$ \textgreater 0.5, resulting in magnetic moments orders of magnitude larger than the weak ferromagnetic room-temperature multiferroics. We will discuss the potential mechanisms for these enhanced transition temperatures and magnetic moments and the potential to increase $T_{\mathrm{C}}$ to above room temperature.   

RMO3 perovskite oxides with magnetic and ferroelectric polar structures

Enhancing the electrical polarization and the magnetic ordering transition temperatures constitutes a current research focus in multiferroics of fundamental and technological importance. Here we report on some progress on the RMO3 perovskites for novel routes to realize multiferroics, giving specific examples of rare earth and transition metal oxide materials. First principles computations, either supported by experimental results or awaiting for experimental verifications, are shown to offer useful guidance for the research of unconventional multiferroics. We hope to stimulate more efforts from experimentalists and theorists to work together for the future developments in fundamental science and device applications. These experimental and theoretical approaches will open up new possibilities for exploring, modeling, and exploiting novel electromagnetism and multiferroic materials.   

Excess-hole induced high temperature polarized state and its correlation with the multiferroicity in single crystalline DyMnO$_{3}$

Controlling the ferroelectricity and magnetism in multiferroic materials has been an important research topic. We report the formation of a highly polarized state in multiferroic DyMnO$_{3}$ single crystals which develops well above the magnetic transition temperatures, and we attribute it to the thermally stimulated depolarization current effect of excess holes forming Mn$^{4+}$ ions in the system. We also show that this high temperature polarized state intimately correlates with the lower temperature ferroelectric state that is induced by the incommensurate spiral magnetic order of Mn spins. This study demonstrates an efficient approach to tune the multiferroicity in the manganite system.   

Colossal Directional Dichroism in a Multiferroic CuB$_{2}$O$_{4}$

In multiferroic materials, electric and magnetic responses to light can interfere with each other, resulting in novel optical phenomena. One typical example is directional dichroism, where the absorption coefficient changes with the reversal of the propagating direction of light. It has been reported that CuB$_{2}$O$_{4}$ shows giant directional dichroism at 1.40 eV, which corresponds to the intratomic from d$_{\mathrm{x2-y2}}$ to d$_{\mathrm{xy}}$ transition of Cu$^{2+}$ hole. The optical absorption coefficient changes by a factor of three at low magnetic fields. In this study, we investigated magnetic field dependence of directional dichroism up to 52 T. We have observed colossal directional dichroism where the ratio of the optical absorption coefficient is as large as 1000{\%} at 52 T. The observed extraordinary large directional dichroism is explained by a modification of the electric and magnetic dipole transition moments due to the canting of the spin direction of Cu$^{2+}$ hole by the application of a high magnetic field.   

Magneto-orbital helices as a route to coupling magnetism and ferroelectricity in multiferroic CaMn7O12

In compounds with long-range ferromagnetic or antiferromagnetic ordering, the magnetic and structural degrees of freedom may couple through orbital ordering. It has long been hoped that this type of coupling could be exploited to create high-temperature multiferroics - materials in which both ferroelectricity and long-range magnetism coexist in a single phase, and may couple to give rise to spontaneous magneto-electric functionality. In this talk I will report a detailed experimental study of the multiferroic oxide CaMn7O12. Our complementary data from pyroelectric current, magnetometry, single crystal x-ray diffraction, and powder neutron diffraction experiments show that CaMn7O12 supports an unprecedented incommensurate magneto-orbital texture below the Neel temperature of 90 K. Furthermore, this magneto-orbital helix was found to give rise to a giant, magnetically induced, ferroelectric polarisation. I will discuss a phenomenological coupling model between orbital, antiferromagnetic, and ferroelectric degrees of freedom in this multiferroic compound. Importantly, our model is consistent with the presence of an experimentally observed second magnetic phase transition at 48 K to a ground state magnetic structure that appears to decouple from the orbital modulation, and at the time of writing, is yet to be fully solved.   

Ultraviolet Raman spectroscopy of hexagonal LuFeO$_{3}$ films

Hexagonal LuFeO$_{3}$ films grown by molecular-beam epitaxy on yttria-stabilized zirconia substrates were studied by variable temperature ultraviolet Raman spectroscopy. Hexagonal LuFeO$_{3}$ is a multiferroic that is isostructural to YMnO$_{3}$ at room temperature. LuFeO$_{3}$ spectra at room temperature are consistent with the polar hexagonal \textit{P6}$_{3}$\textit{cm} structure. The temperature evolution of the Raman spectra of a LuFeO$_{3}$ film measured in the range 10--1250 K indicate a transition to a non-polar (likely \textit{P6}$_{3}$\textit{/mmc}) phase; fitting of the temperature dependence of the Raman intensities yields a transition temperature of 1020 $\pm$ 50 K. We also observed a change in the slope of Raman intensity vs. temperature dependence of the most intense phonon peak around 400-450 K, which might indicate another structural transition, possibly to a structure with space group \textit{P6}$_{3}$\textit{mc} (also polar).   

Pressure induced spin-flop transition in multiferroic Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_{4}$

The effect of cobalt substitution in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_{4}$ results in the most complex multiferroic phase diagram with multiple polarization flops upon increasing Co content. Two critical concentrations occur at x$=$0.075 and x$=$0.15 that separates multiferroic phases with different spin structures and orientation of the ferroelectric polarization. For 0.075 \textless x \textless 0.15, neutron diffraction has revealed that the spin configuration forms an \textit{ac} spiral with associated electric polarization lying in the \textit{ac}-plane. With application of hydrostatic pressure, we have examined the evolution of spin structure for x$=$0.135 sample. We have observed a spin-flop transition to a conical spin structure similar to that of x\textgreater 0.15 samples. Our diffraction results are consistent with the pressure polarization measurement where a polarization flop to the $b$-axis is observed. The origin of pressure induced spin-flop transition is discussed.   

Composition and temperature dependences of electric-field control of nonvolatile magnetization in Co40Fe40B20/Pb(Mg1/3Nb2/3)(1-x)TixO3 multiferroic heterostructures

Recently, a large and nonvolatile bipolar-electric-field-controlled magnetization at room temperature (RT) was demonstrated in Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 structure, in which Pb(Mg1/3Nb2/3)0.7Ti0.3O3 is in the morphotropic phase boundaries (MPB) region. It is well-known that ferroelectrics with MPB can display excellent electromechanical properties. In order to investigate whether the MPB is important for the large nonvolatile behavior, we studied the electric-field-controlled magnetization in Co40Fe40B20/ Pb(Mg1/3Nb2/3)(1-x)TixO3 (x=0.17, x=0.30, x=0.34, x=0.38) heterostructures, of which Pb(Mg1/3Nb2/3)(1-x)TixO3 structure varies with increasing x from rhombohedral (R) phase, MPB, to tetragonal phase5. We found that the samples with Pb(Mg1/3Nb2/3)(1-x) TixO3 in R phase and MPB both display large and nonvolatile behavior, while it shows volatile behavior in tetragonal phase. These results indicate that it is not MPB but R phase that is vital for the nonvolatile behavior. We also studied the temperature effect of electric-field-controlled magnetization by varying temperatures from 200 K to 340 K, and found that the modulation display different behavior with varying temperature, which can be explained by the phase changes in Pb(Mg1/3Nb2/3)(1-x)TixO3.   

Directional dichroism in Co$_{3}$B$_{7}$O$_{13}$I

Directional dichroism in a well-known magnetoelectric multiferroic boracite Co$_{3}$B$_{7}$O$_{13}$I has been investigated. In the ferroelectric and weak ferromagnetic phase below 37K, we have found that the optical absorption of intra $d$-$d$ transitions at Co$^{2+}$ exhibits a fairly large directional dichroism, which shows up as a change in absorption by the application of a magnetic field in Voigt configuration. Magnetic-field dependence of the optical absorption shows a hysteresis like the magnetization curve, which indicates that the change in optical absorption should originate from the rotation of the magnetic moments of Co$^{2+}$.   

Iron borate multiferroic a new way to observe electromagnon?

In the last years, multiferroics have attracted much attention worldwide because of their large magnetoelectric effects. We used infrared scattering to probe the dynamical coupling between spin and lattice degrees of freedom in rare earth iron borates. These materials have interesting magnetic properties due to the subtle interactions between the rare earth and the iron moments. Among these materials, NdFe3(BO3)4 becomes multiferroics under magnetic field. At low temperature, a new mode appears at low frequency in the (ab) plane. This mode can be the signature of change in the lattice around the magnetic transition but no structural phase transitions are expected in this compound. This new peak has most probably a magnetic origin, electromagnon? Moreover, along the c direction the lowest phonon mode has its frequency decreasing with temperature and the total dielectric function for the c axis comes from this phonon. This is the typical behavior of a soft phonon mode in a ferroelectric compound. This material is a good candidate to observe the electromagnon.   

Strain effects on the ferroelectric polarization of hybrid organic inorganic perovskite compounds

Metal-organic frameworks (MOFs) are hybrid crystalline compounds composed of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic ABO$_3$ perovskites, have been shown to have interesting properties, \textit{i.e.} coexistence of ferroelectric and magnetic ordering [1]. In this present work, using first-principles density-functional theory, we have investigated the effect of strain on previously reported MOFs, such as $C(NH_2)_3 Cr(HCOO) _3$ and $(CH_3CH_2NH_3)Mn(HCOO)_3$. In these systems, a peculiar canted ordering of the organic A-cation dipole moments give rise to a \textit{weak} ferroelectric polarization. We show that compressive strain can substantially increase the ferroelectric polarization, as much as 200\%. Our study highlights the complex interplay between strain and dipole canting and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing that can have potential applications in organic electronics.\\[4pt] [1] Stroppa, A.; Barone, P.; Jain, P.; Perez-Mato, J. M.; Picozzi, S. Advanced Materials 2013, \textbf{25}, 2284-2290.