Session Q6: Focus Session: Coupling Polarization and Magnetism II

Magnetically induced ferroelectricity in single crystalline Lu$_2$CoMnO$_6$

We present pulsed-magnetic-field measurements on Lu$_2$CoMnO$_6$ single crystals. We are able to resolve electric polarization in single crystals for the first time. The bulk hysteretic magnetization couples to the electric polarization resulting in coupled, hysteretic, multiferroic behavior. The alternating $S=3/2$ Co$^{2+}$ and Mn$^{4+}$ ions sit in a corner-sharing octahedral oxygen environment. The Co-Mn-Co-Mn spins order in an up-up-down-down (uudd) arrangement along the $c-$axis. The ferroelectricity was believed to originate from the exchange striction due to the uudd spin arrangement. However, recent dielectric measurements suggest polarization along the $b-$ not the $c-$axis. Our results confirm that ferroelectricity is indeed observed along the $b-$axis and not along the uudd spin-ordering direction. This indicates a different origin for the multiferroic behavior. The frustrated spin system displays an incommensurate long-wavelength modulation that may play a role in inducing ferroelectricity.   

Multiferroicity in HoFeO$_{3}$

Rare-earth ferrites are becoming popular among multiferroic materials due to its spontaneous magnetic and electric orderings. After discovery of new family of multiferroic RFeO$_{3}$(R=Lu)\footnote{H Wang at al, Phys. Rev. B 90, 014436 (2014)}(LFO)which shows simultaneous ferroelectricty and weak ferromagnetic moment, HoFeO$_{3}$(HFO) is explored as a potential candidate in multiferroic materials. Combined theoretical and experimental study has been done to understand ferroelectric and ferromagnetic properties of HFO. Our calculations suggest that iron in HFO exhibits very large canting, about one order of magnitude, compared to canting of iron in LFO, which is consistent with experimental findings. Analysis of structural phase transitions and calculation of phonon modes governing this ferroelectric transition revealed that HFO is improper ferroelectric with ferroelectric polarization about 8.3 $\mu$C/(cm$^{2}$). These ferroic properties advocate HFO to be a useful multiferroic among other rare-earth ferrites.   

A study of the origin of large magnetic field coupled electric polarization in HoAl(BO$_{3}$)$_{4}$

The multiferroic system RAl(BO$_{3}$)$_{4}$ is known to exhibit a strong coupling of magnetic field to the electrical polarization. Recently a giant magnetoelectric effect was found in HoAl$_{3}$(BO$_{3}$)$_{4}$ system. This phenomenon is considered quite interesting because the value discovered is significantly higher than reported values of linear magnetoelectric or even multiferroic compounds. We are conducting detailed structural measurements to understand the coupling. We are exploring the local and long range structure in these systems using x-ray PDF, XAFS and single crystal diffraction measurement between 10 K and 400 K. Structural parameters including lattice parameters and ADPs are being determined over the full temperature range. This work is supported by DOE Grant DE-FG02-07ER46402.   

High Pressure Study on the Multiferroic State in Mn$_{1-x}$Co$_x$WO$_4$: x = 0.135 and x = 0.15

Mn$_{1-x}$Co$_x$WO$_4$ has an interesting and complex phase diagram where 2 multiferroic phases coexist at x = 0.15. For x $<$ 0.15 a spiral spin structure forms, while in contrast a conical spin is observed for x $>$ 0.15. High pressure polarization measurements on x = 0.135 observed a polarization flop and an increase in the polarization, indicating the a-c spiral is converted to the conical spin. An enhancement of the polarization by $\sim$ 400\% is seen for x = 0.15, at the highest pressure measured. High pressure neutron measurements found supporting results.   

Temperature and high-pressure dependent X-ray absorption of SmNiO$_{3}$ at K-Ni and Sm-L$_{3}$ edges

We report XANES and EXAFS measurements of SmNiO$_{3}$ from 20 K to 600 K and up to 38 GPa done at LNLS, Campinas, Brazil, in the DXAS energy dispersive beamline. SmNiO$_{3}$ undergoes an atmospheric pressure insulator to metal transition at T$_{IM}$ $\sim$ 400 K, orders magnetically at T$_{N}$ $\sim$ 205 K, and shares with all RNiO$_{3}$ (R$=$Rare Erath) a negative --dT$_{MI}$/dP slope. Ni white line peak energies show an abrupt 2.4 GPa and 8.11 GPa valence discontinuity at 300 K and at 20 K respectively, due to non-equivalent Ni sites with Ni$^{3+\delta}+$ Ni$^{3-\delta}$ charge disproportion in a monoclinic distortion turning at T$_{MI}$ into Ni$^{3+}$ in the orthorhombic Pbnm metal oxide phase. Increasing pressure induces Ni-O-Ni angle increments toward more symmetric Ni$^{3+}$ octahedra of rhombohedral R$^-$ 3c space group (metallic LaNiO$_{3})$. At 38 GPa, there is a clear split of the main EXAFS band according to cell volume decrease due to contraction. Pre-edge tail accounts for e$_{g}$-t$_{2g}$ splitting and a stronger band growing in intensity in the higher symmetry metallic phases associated to e$_{g}$ electron delocalization hopping conductivity. The Ni$^{3+}$ post-edge becomes smoother and intensity increased by the reduction of electron-phonon interaction as the pressure-induced phases set in. We also found that Sm L$_{3}$-edge does not show distinctive behaviors either at 300 K or 20 K up about 35 GPa.   

Driving the magnetic response of BiFeO$_{3}$ by hydrostatic pressure

BiFeO$_{3}$ exhibits ferroelectric and magnetic orders at room temperature, which makes it an ideal candidate for spintronics, electro-optics and data storage applications. Most of its properties are related to its ferroelectric character, especially studied under electric or magnetic fields, however the antiferromagnetism has not been extensively investigated, in particular under pressure. Here, we bring insight into the rich spin physics of BiFeO$_{3}$ in a detailed study of the dynamic magnetic response of bulk BiFeO$_{3}$ under pressure up to 12GPa measured by Raman spectroscopy. As pressure increases, multiple spin excitations associated to non-collinear cycloidal magnetism collapse into two excitations, which show jump discontinuities at some of the ensuing crystal phase transitions. Using effective hamiltonian simulations of both the structure and the magnetism and Ginzburg-Landau theoretical calculations we show that the pressure controls both the structural phase and the magnetic anisotropy that drives the spin excitations.   

Multiferroicity in Ni doped MnWO$_{4}$

In this work, we studied the multiferroic properties of Mn$_{1-x}$Ni$_{x}$WO$_{4}$ for $x$=0.05, 0.10, 0.15, 0.20 and 0.30 with ferroelectric, magnetic and heat capacity measurements. For $x$=0.05 and 0.10, the maximum polarization is 38 $\mu$C/m$^2$ and ferroelectric transition temperature ($T_c$) is 13.2 K which is 0.5 K higher than in parent compound. For other increasing values of $x$, the polarization decreases. For $x$=0.15, due to onset of AF4 collinear phase, the polarization becomes maximum and then decreases at lower temperature. A small polarization of 2.3 $\mu$C/m$^2$ is observed for $x$=0.20 with $T_c$ lowered at $\sim $9.5 K and the polarization is completely suppressed for $x$=0.30. Unlike in Co doped MnWO$_4$, neutron scattering reveals only two (AF2 and AF4) AF magnetic phases and the onset of the AF4 phase shifts to higher temperature with increasing value of $x$.   

A second ferroelectric transition induced by pressure in multiferroic GdMn$_{2}$O$_{5}$

In multiferroic materials different magnetic and ferroelectric states compete for the ground state and are usually close in energy. This results in a high sensitivity of the multiferroic state to external perturbations, such as magnetic fields or pressure, which has been demonstrated, e.g. in Ni$_{3}$V$_{2}$O$_{8}$, MnWO$_{4}$, and RMn$_{2}$O$_{5}$ (R $=$ Tb, Ho, Dy, Y). Here we report the results of a high-pressure study of the multiferroic and ferroelectric properties of GdMn$_{2}$O$_{5}$, the RMn$_{2}$O$_{5}$ compound with the highest value of the polarization. The ferroelectric polarization below 29 K is enhanced upon application of pressure. Above a critical pressure, a second ferroelectric transition at even higher temperature is detected through a sizable increase of the polarization and a second peak of the dielectric constant. The ferroelectric polarization at high pressures exhibits two step-like increases upon decreasing temperature.   

Evolution of weak ferromagnetism in BiFeO$_3$ under applied epitaxial strain

The magnetoelectric effect has been a focus of research in multiferroic materials due to potential applications in magnetic data storage, spintronics and memory devices. We perform first principles calculations based on density functional theory to explore the evolution of weak ferromagnetism under compressive and tensile strains in BiFeO$_3$. An isosymmetric phase transition between the rombohedral ground state and strained phases (Cc space group) is considered which confines the polarization vector to rotate in the (110) plane. For the ground state, we find that the easy spin axis is degenerate in a plane perpendicular to the polarization direction. This weak ferromagnetic moment also persists for the tested strain values (up to $\pm$5\%). Under compressive strains, the easy spin axis is also degenerate in a plane perpendicular to the polarization vector. On the other hand for tensile strain the weak ferromagnetic ordering is stabilized along the [1-10] direction. Further, calculated Dzyaloshinskii-Moria interactions help us to understand the stabilization of weak ferromagnetic moments in the tensile region. Our study thus offers useful insights for manipulating the magnetic response and utilizing the magnetoelectric effect.   

Control of oxygen octahedral rotation in BiFeO$_{3}$ films using modulation of SrRuO$_{3}$ bottom electrode layer

Oxygen octahedral rotation of multiferroic BiFeO$_3$ (BFO) has attracted great attention due to changes of electrical and magnetic properties. Coupling of octahedral rotation in BFO-bottom electrode layer interface remains unexplored. Recently, there have been reported the control of octahedral rotation in SrRuO$_3$ (SRO) film on SrTiO$_3$ (001) substrate by coherently controlling the oxygen pressure during growth and interfacial coupling [1]. Here we demonstrate that the octahedral rotation of BFO film is changed using tetragonal a$^0$a$^0$c$^-$ tilted-SRO bottom electrodes. In this work, BFO/SRO heterostructure is fabricated to SrTiO$_3$ (001) single crystal substrates by pulsed laser deposition at different oxygen partial pressures. The rotation pattern of FeO$_6$ and the structural symmetry are identified from half-integer reflections using high-resolution X-ray diffraction. The effects depending on octahedral tilting of BFO films on the magnetic and ferroelectric properties will be presented. \\[4pt] [1] Wenlai Lu \textit{et al}., Phys. Rev. B. \textbf{88}, 214115 (2013)   

Predicting a Ferrimagnetic-Ferroelectric Phase of Zn$_{2}$FeOsO$_{6}$ with Strong Magnetoelectric Coupling

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of novel memory devices that allow for electrical writing and non-destructive magnetic readout operation. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. BiFeO$_{3}$ has been the most heavily investigated single-phase multiferroic to date due to the coexistence of its magnetic order and ferroelectric order at room temperature. However, there is no net magnetic moment in the cycloidal (antiferromagnetic-like) magnetic state of bulk BiFeO$_{3}$, which severely limits its realistic applications in electric field controlled spintronic devices. Here, we predict that double perovskite Zn$_{2}$FeOsO$_{6}$ is a new multiferroic with properties superior to BiFeO$_{3}$. First, there are strong ferroelectricity and strong ferrimagnetism at room temperature in Zn$_{2}$FeOsO$_{6}$. Second, the easy-plane of the spontaneous magnetization can be switched by an external electric field, evidencing the strong magnetoelectric coupling existing in this system. Our results suggest that ferrimagnetic 3d-5d double perovskite may therefore be used to achieve voltage control of magnetism in future spintronic devices. \\[4pt] [1] P. S. Wang \textit{et al.}, arXiv:1409.8430 (2014).   

Prediction of electromagnons in BiFeO$_{3}$ from atomistic simulations

We developed a first-principles-based computational approach to study the finite-temperature complex dynamics in multiferroics. Application of this technique to one of the most well-known multiferroic, BiFeO$_{3}$, revealed that the most general form of the magnetoelectric coupling that is allowed in any multiferroic, may produce an electromagnon in antiferromagnetic ferroelectric. Such an electromagnon shares most of the features with the magnon, but is activated by an electric, rather than magnetic, field. We also found that the A$_{1}$ phonon mode is repelled by the magnon mode, while the E phonon modes is rather insensitive to the presence of (electro)magnons. We believe that our findings will aid to a deeper understanding of electromagnon modes and their origin in multiferroics as well as provide a computational methodology for further research.   

BaFe2Se3: A high TC magnetic multiferroic with large ferrielectric polarization

The iron-selenides are important because of their superconducting properties. Here, an unexpected phenomenon is predicted to occur in an iron-selenide compound with a quasi-one-dimensional ladder geometry: BaFe$_2$Se$_3$ should be a magnetic ferrielectric system, driven by its magnetic block order via exchange striction. A robust performance (high $T_{\rm C}$ and large polarization) is expected. Different from most multiferroics, BaFe$_2$Se$_3$ is ferrielectric, with a polarization that mostly cancels between ladders. However, its strong magnetostriction still produces a net polarization that is large ($\sim$$0.1$ $\mu$C/cm$^2$) as compared with most magnetic multiferroics. Its fully ferroelectric state, with energy only slightly higher than the ferrielectric, has a giant improper polarization $\sim$$2-3$ $\mu$C/cm$^2$. \\[4pt] [1] S. Dong, J.-M. Liu, E. Dagotto, Phys. Rev. Lett. 113, 187204, (2014)   

Magnetic structures in potential multiferroic GdCrO3

For the past decade, multiferroics materials have atracted a lot of attention in the condensed matter community because of potential applications for devices. A somewhat ambiguous addition to the multiferroics family was recently reported in the peroskite based GdCrO$_3$ in both bulk and thin film samples. Indeed, ferroelectricity was evidenced by a strong enhancement of the capacitance in a field but significant leakage and no well developed P-E hysteresis blurred the picture. Our own measurements clearly indicate the existence of a polar phase below 2K. To complete the understanding of this material, the determination of the magnetic structure is required but is hampered by the fact Gd is a strong neutron absorber. We will present some neutron diffraction data collected on an isotopic $^{160}$GdCrO$_3$ sample at the WISH diffractometer at ISIS which confirm the presence of three successive magnetic phases, previously only seen by magnetization, as a function of temperature. We will compare our determined structures against predictions based on group theoretical considerations and experimental work on other rare-earth ortho-chromates and discuss the mechanism for multiferroicity.