Session E09: Multiferroic Oxides

Defect-induced hedgehog polarization states in multiferroics

Quasi-1D ferroic structures are nanoscale functional units that possess unique properties and hold great promise for the development of new device paradigms. In recent years, extensive efforts have been made towards fabricating and manipulating spin skyrmions or polarization vortices in ferromagnetics/ferroelectrics. Yet, another type of quasi-1D ferroelectric state known as polarization hedgehog, in which all the polarization vectors diverge from the center of the structure, remains unexplored. Here, we show the possiblity to stablize hedgehog polarization states by utilizing the interaction between ferroelectric polarization and deliberately introduced charged defects in the ferroelectric matrix. Atomic-scale scanning transmission electron microscopy imaging reveals that exotic polarization rotation patterns at the hedgehog polarization states can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Engineering defects thus may provide a new route for developing ferroelectic/multiferroic-based nanodevices.   

First Principles Study of (111)-Epitaxially Strained Multiferroic Perovskites

First principles calculations are used to explore the effect of (111) epitaxial strain in the spin-phonon and strain-polarization couplings of transition metal perovskite oxides. Using SrMnO₃ as a prototypical model system, we find that (111)-strain drives the antiferromagnetic-paraelectric cubic phase of SrMnO₃ to a previously unreported monoclinic multiferroic ferroelectric-ferromagnetic phase that is not present in either bulk or (001)-strained form. Under tensile (111)-strain, ferromagnetism stabilizes ferroelectricity at smaller strain values than the 001 case, and displays an out of plane polarization component P_s ~ 10 μC/cm². We further estimate magnetic ordering temperatures and compute lowest phonon frequencies as a function of (111)-strain.   

Multiferroic Properties of Sr1-xBaxMn1-yTiyO3 Perovskites

Multiferroics are the subject of intense interest with elaborate investigations designed to understand how such two disparate order parameters could interact, coexist and compete within the same host structure ultimately allowing the remote control of magnetic properties via the application of external electric fields or vice versa. I will discuss the achievement of robust ferroelectricity with T_F > 300 K and magnetism (T_N ~ 200 K) in Sr_1-xBa_xMnO₃ perovskites only if enough of the large Ba²⁺ ions were forced into the structure at the Sr²⁺ site with contents extending well beyond the normal solubility limit. Neutron and x-ray diffraction were successfully used to demonstrate the strong and tunable coupling of the two order parameters that originate exclusively at the same Mn⁴⁺ sublattice. The ferroelectric phase transition exhibits a crossover signature from displacive to order-disorder type as revealed by spectroscopic phonon measurements. Ti-substituted Sr_1-xBa_xMn_1-yTi_yO₃ have also been synthesized with small amounts of Ti that maintain the integrity of the long range ordered magnetic Mn sublattice. The doubly substituted materials exhibit ferroelectric properties above 400 K with spontaneous polarization properties significantly exceeding those of the classical titanates.   

Multiferroic order in BiFeO3 nanoparticles

Multiferroics exhibit simultaneous presence of magnetic and electric ordering. The archetypical multiferroic material, BiFeO₃ (BFO), has attracted much attention since it has both, high ferroelectric Curie temperature (1103 K) and, high antiferromagnetic Néel temperature (643 K) in bulk and thin film form. Furthermore, it has one of the highest electrical polarization (~90μC/cm2) among ferroelectric materials. We propose a new route to a magnetic control via nano-structuration in the form of nanoparticles (NPs). We have grown BFO NPs by a sol-gel process and found that their magnetism can be tuned from antiferromagnetic to ferromagnetic by controlling the particle size. Piezoelectric force microscopy and VSM measurements show that our NPs are ferroelectric and ferromagnetic, and therefore our results open new ways to achieve magnetoelectric coupling in BFO nano-structures.   

First-Principles Study on the Origin of Electrochromism in Ca-doped BiFeO3

Bismuth ferrite (BiFeO₃) has received much attention as a multiferroic material, which simultaneously exhibits both ferroelectric and antiferromagnetic properties. Cation doping has been widely used to improve fundamental properties such as the electronic, magnetic and ferroelectric properties of multiferroic oxides. Particularly, it has been reported that Ca-doped BiFeO3 exhibit a prominent electrochromic effect.
In this study, we performed first-principles hybrid functional calculations to understand the microscopic mechanism of electrochromism phenomena in Ca-doped BiFeO₃. Based on the defect study of calcium substitutional at the bismuth site (Ca_Bi) and oxygen vacancy defect (V_O), we compared electronic structures of V_O and Ca_Bi defects under thermodynamic stability. We found that the isolated neutral charged ((+)-charged) Ca_Bi defect induces a deep state in the band gap, indicating that the hole-induced trap state can be regarded as a small polaron (bipolaron). Our theoretical results from Ca dopant and oxygen vacancy in Ca-doped BiFeO₃ can provide an explanation for recent experimental observations and a correlation between hole polaron and electrochromism.   

First-principles designs of Two-dimensional Ferroelectrics/Multiferroics

Despite the past research on the diluted magnetic semiconductors and two-dimensional (2D) ferromagnetism, their potential applications are still hindered by their low Curie temperature, especially for the downscaling integrated circuit size close to nanoscale. Their bottlenecks have stimulated our research on 2D ferroelectrics/multiferroics that may combine semiconducting properties with non-volatile memories at nanoscale and ambient conditions and enable a wide range of applications. Here we will present a series of our predictions on 2D ferroelectrics/multiferroics starting from 2013, revealing that FE can be much more robust than FM in 2D with above room-temperature T_c and can also be integrated into semiconductor circuits. The couplings between different ferroics in 2D are also revealed, which can even be much more efficient than traditional multiferroics for data reading and writing.
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[2] ACS Nano 2017, 11, 6382
[3] J. Phys. Chem. Lett. 2017, 8, 1973
[4] Phys. Rev. B 2013, 87, 081406 (R)
[5] J. Am. Chem. Soc. 2012, 134, 14423; 2017, 139,11506.   

Catalytic Properties of BiFeO3 for Oxygen Reduction Reactions

First principles DFT studies are performed to determine the mechanism and catalytic activity of oxygen reduction on rhombohedral (R3C) BiFeO₃ surfaces with low index surface terminations at the solid/gas interface. The results show that the (111) surface termination is most favorable energetically for the chemisorption and dissociation of O₂ molecules. Furthermore chemisoption on Fe-O terminated sites is observed to be more favorable to adsoption than Bi-O terminated positions due to the greater number of partially filled valence states in Fe vs Bi.   

Electric Field Effects on Diffuse Scattering from PMN-30PT

Solid solutions of the relaxor PbMg_1/3Nb_2/3O₃ and the ferroelectric PbTiO₃ produce a system with a morphotropic phase boundary near the concentration of 30% PbTiO₃. At this phase boundary, the piezoelectric coefficient of the material is greatly enhanced, a useful bulk property likely arising from short-range correlations within the material. To investigate the connection between local correlations and electric fields, diffuse scattering experiments were performed on single crystals of PMN-30PT under a range of applied potentials and field directions. Different components of diffuse scattering were shown to behave differently under applied fields, providing insight into how the macroscopic piezoelectric behavior is determined by local structural correlations.   

Multiferroism in Iron-based Oxyfluoride Perovskites

Hybrid improper ferroelectricity is generated by the combination of layered cation ordering at the A-site and octahedral tilts, and unlike conventional ferroelectricity, it does not conflict with magnetism. Therefore, it provides a route to achieve multiferroic materials. In this work, we use anion engineering to design perovskite ferroelectrics with iron in +3 oxidation state to take advantage of its high magnetic-ordering temperature. To maintain charge neutrality in AA’Fe₂O₆ perovskites, previous Fe +3 perovskite ferroelectrics have been restricted to using A and A’ both having +3 oxidation state, which limits the polarization. Using first-principles density functional theory calculations, we show polarization as high as 19.3 μC/cm² in AA’Fe₂O₅F perovskites with A and A’ cations being +2 and +3, respectively. We also show strong superexchange interactions in these perovskites, leading to a new family of potential room-temperature multiferroics. Design rules to maximize the stable polarization as a function of the combination of A and A’ cations will also be discussed.   

Neutron Scattering Study of Spin-wave induced lattice modes in multiferroic BiFeO3

We have performed inelastic neutron scattering studies of the low-energy phonon and spin-wave modes in the multiferroic BiFeO3. We have observed an intensity enhancement near the lowest zone-center optic phonon increase upon cooling below TN and this enhancement is confined to a very narrow region in the energy-momentum space, where the spin wave spectral weight is expected to be small. An external magnetic field was found to affect the enhancement as well. Possible interactions between the lattice and spin dynamics will also be discussed.   

Structural study of the single ion Mn-based multiferroics Ba1-xSrxMn1-yTiyO3

Single-phase multiferroics exhibiting simultaneously ferroelectric (FE) and magnetic properties are very attractive for both fundamental research and the advanced technology applications. Such materials are very rare and usually are based on the two-independent structural sublattices. By using the tolerance factor arguments, we have designed and synthesized the unique multiferroic system based on the single Mn-ion. Here we report the synthesis and the x-ray powder diffraction structural studies of the Ba_1-xSr_xMn_1-yTi_yO₃ phase diagram. Compounds with x ~ 0.5 and y = 0 – 0.2 show large FE distortions below ~ 400 K that are identical to the tetragonal distortions of the prototypical nonmagnetic BaTiO₃ but with the much larger distortion size and the temperature hysteresis. Moreover, our compounds are also antiferromagnetic below T_N ~ 200 K due to the presence of magnetic Mn⁴⁺ ions. Extremly strong coupling between FE and magnetism is observed by the very large or complete suppression of FE distortion below T_N, which does not show the temperature hysteresis and can be controlled by the substitution levels x and y. Experiments using hydrostatic pressure show rapid suppression of FE transition while the magnetic T_N increases with pressure as confirmed by magnetic measurements.   

Multiferroic behavior at a spin state transition

We describe multiferroic behavior at a spin state transition (SST), instead of the more traditional approach using exchange-coupled order like ferromagnetism or spiral antiferromagnetism. SSTs involve large changes in ionic size, bonding and lattice, and so have potential for strong magnetoelectric coupling. We observe a magnetic field-induced electric polarization change at an SST that is within an order of magnitude of the record for any material. SSTs such as the S = 1 to S = 2 transition in Mn³⁺ occur when electrons change their occupation of magnetic partially-filed orbitals. SSTs can be sharp, first-order hysteretic phase transitions and they are an increasingly common functionality in inorganic-organic hybrid materials, persisting up to room temperature in some compounds. We study a Mn-based molecular system in which the high-spin state is Jahn-Teller active, and the JT distortions carry an electric dipole. We show that the magnetic field-induced SST induces ferroelectricity in both DC and pulsed magnetic fields. We use high magnetic fields at the NHMFL to study the magnetic and electric behavior of this system across a significant fraction of its T-H phase space, and compare to a phenomenological theoretical model.   

Raman Scattering Study of Multiferroicity under Pressure in CoCr2O4

In the multiferroic phase exhibited by spinel CoCr₂O₄ below T_S = 24K, the two contributing sublattices, Co²⁺ and Cr³⁺, are expected to be strongly coupled. With applied pressure, the coupling between the two sublattices can be enhanced, rendering the possibility of multiferroicity at temperatures higher than T_S. We present a temperature and pressure dependent Raman scattering study of the magnon and phonon spectrum to elucidate the microscopic details of intersublattice interactions and spin-lattice coupling in CoCr₂O₄. At temperatures higher than T_S, with increasing pressure (P > 30kbar), in addition to an existing T_1g magnon at ambient pressure, a second magnon at lower energies is observed, which could indicate increased single-ion anisotropy or spin-spin interactions. We also observe a splitting of the triply degenerate T_2g phonon, possibly indicating spin-induced lowering of crystal symmetry due to increased spin-lattice and intersublattice interactions. We are investigating whether these signatures reflect a multiferroic phase at T>T_S, which would indicate that pressure can be used to tune the coupling between lattice and spin dynamics to control the multiferroic behavior in CoCr₂O₄.   

Temperature Dependent Structural Studies of the Multiferroic RA3(BO3)4

The multiferroic systems RAl₃(BO₃)₄ (R=Ho) is known to exhibit significant electric polarization. However, the more general class containing Fe at the Al site shows only weak polarization. In this work we conduct detailed structural measurements of the full class of RA₃(BO₃)₄ (R=Sm, Nd, Ho, Gd, Eu, A=Fe, Al). Temperature-dependent single crystal measurements between 15 and 300 K are used to track the structure to enable modeling of the electronic properties. Direct comparisons between the RAl₃(BO₃)₄ and RFe₃(BO₃)₄ are made.
  

Giant Piezoelectricity Driven Magnetoelectric Coupling in a Multiferroic Membrane Heterostructure

Strain intermediated magnetoelectric coupling in thin film heterostructures is a promising paradigm for sensors and information storage devices, but it is hindered by substrate clamping. We prepared a freestanding, unclamped, 500 nm thick, (001) oriented [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (PMN-PT) membrane with a 35nm ferromagnetic Ni overlayer. Magneto-optic Kerr magnetometry shows that upon applying a bias voltage, an in-plane magnetic anisotropy of 5 kJ/m³ forms, resulting in Ni magnetization rotation. Additionally, the large strains in these devices cause macroscopic bending and flexing, which was characterized with laser Doppler vibrometry and white light interferometry.