Session A41: Multiferroics and Magnetoelectric Couplings

Topologically protected unidirectional magntoelectric switching in multiferroic GdMn2O5

Electric control of magnetism and magnetic control of ferroelectricity can improve energy efficiency of magnetic memory and data processing devices. However, the necessary  magnetoelectric switching is hard to achieve, and requires more than just a coupling between spin and charge degrees of freedom. We show that an application and subsequent removal of a magnetic field reverses the electric polarization of the multiferroic GdMn₂O₅, thus requiring two cycles to bring the system back to the original configuration. During this unusual hysteresis loop, four states with different magnetic configurations are visited by the system, with one half of all spins undergoing unidirectional full-circle rotation in increments of ∼90^o. Therefore, GdMn₂O₅ acts as a magnetic crankshaft converting the back-and-forth variations of the magnetic field into a circular spin motion. This peculiar four-state magnetoelectric switching emerges as a topologically protected boundary between different two-state switching regimes. Our findings establish a paradigm of topologically protected

switching phenomena in ferroic materials.   

Magnetostructural studies of strongly correlated materials at 28-ID-1 (PDF), NSLS-II

Magnetic phase transitions (MPTs) are often accompanied by underlying structural modulations. Our recent studies indicate that MPTs can be triggered by changing temperature at zero field, or by applying a magnetic field isothermally staying close to the transition temperature. In most of the studied materials, evolution of the average diffraction pattern seems to be independent of the driving variable. However, it is not clear if the nature of the local structural response to the magnetic field, and temperature are the same as it is hidden in the diffuse background. In some materials, diminishing structural anomalies were observed in the vicinity of Curie temperature with increasing field. Another intriguing question is whether MPT drives the structural anomalies or vice versa. One of the newest capabilities of the NSLS-II Pair Distribution Function (PDF) beamline, provides a unique platform to study MPTs while simultaneously varying the magnetic field and temperature. The beamline setup enables PDF, Wide Angle X-ray Scattering, and Small Angle X-ray Scattering studies allowing exploration of the atomic structure on multiple length-scales. This presentation will discuss the results of our newest magnetostructural studies at 28-ID-1 and showcase the beamline capabilities.

    

Coupling between improper ferroelectricity and ferrimagnetism in the hexagonal LuFeO3 type ferrite systems

In magnetic oxide materials, the anti-symmetric anisotropic Dzyaloshinskii-Moriya (DM) exchange interactions between the localized spins are often weaker compared to their symmetric isotropic exchange counterpart. Therefore, they induce either weak magnetoelectric (ME) effect or the magnetization (M) in single-phase systems, thereby impairing their device applicability. Here, we aim to realize non-collinear ferrimagnetic orders having a considerably high M and their strong coupling with an improper ferroelectric (FE) order in hexagonal LuFeO₃ type systems which exhibit interesting topological orders [Nat. Commun. 5, 2998 (2014), Nature 537, 523–527 (2016), Nat Commun 11, 5582 (2020), Phys. Rev. Lett. 126, 157601 (2021)] driven by the DM interactions between the magnetic ions and their coupling with the FE order. The proposed two-sublattice magnetic system forms multiple energetically close, non-collinear ferrimagnetic orders, enabling the manipulation of the microscopic magnetic interactions and the triggering of spin-reorientation (SR) transitions by various efficient means. The electron doped hexagonal phase of LuFeO₃, where the two-sublattice structure was realized, exhibits electric polarization P  ~ 15 μC/cm² and M ~ 1.1 μ_B/Fe with magnetic transition near room temperature (~ 290 K). The microscopic mechanisms to achieve electric field E induced SR transitions and switching of the direction of M have been discussed here.   

Structural and magnetic properties of epitaxially-strained Pb2CoTeO6 from first-principles

B-site ordered double perovskites exhibit a range of structural, electronic, and magnetic orders that can coexist within one material. For example, Pb₂CoTeO₆ has been discussed as a multiferroic candidate due to the presence of lone pair-active Pb on the A-site and magnetic Co on the B-site. However, so far no polar phase has been identified in this compound, although a sequence of phase transitions between non-polar structures with different octahedral rotation patterns has been observed as a function of temperature and pressure. In this work, we combine group theoretic analysis with density functional theory calculations to investigate the energy landscape of bulk Pb₂CoTeO₆. We find that a P2₁/c phase (a^-a^-c⁺ octahedral rotations) with A-type antiferromagnetic order is the ground state, in agreement with the experiment, and identify a competing R-3 phase (a^-a^-a^- octahedral rotations) that is slightly higher in energy. We explore the impact of epitaxial biaxial strain on the structural and magnetic phases of Pb₂CoTeO₆ and investigate whether any polar instabilities arise under strain. Our results provide new insight into strain control of the structural and magnetic properties of Pb₂CoTeO₆.

    

Author not Attending

The unique coupling between ferroelectric and ferromagnetic orderings in multiferroics makes them promising materials for sustainable low-power microelectronics. As their compositional spaces have been limited to oxides and halides, a new group of materials is necessary for new potential applications that current traditional materials cannot meet. Among them are multiferroic nitrides. Nitrides in particular are sought-after for their stability at elevated temperatures and improved compatibility with other nitride semiconductor components. In this talk, I will discuss our ab initio predictions of new multiferroic nitrides using both chemical substitution and strain on already-reported compounds to stabilize the sought-after multiferroics phases. Finally, I will discuss the origins of multiferroism in this nitride family, and suggest design rules to identify further nitride candidates.   

Hybrid Improper Ferroelectricity and Tunable Ionic States in Bilayer Perovskites

Ferroelectricity was first discovered in 1920. The intrinsic binary states and electrically switchable nature enable wide applications of ferroelectric materials in electronics. Until now, the investigation of new ferroelectrics, unconventional ferroelectric mechanisms, and the coupling between ferroelectricity with other physical properties are still important topics in condensed matter physics. In bilayer perovskites, ferroelectricity can be induced by a combination of octahedron rotation and tilting, i.e., the hybrid improper ferroelectricity mechanism. In the first part of this talk, I will present the experimental demonstration of hybrid improper ferroelectricity in bilayer perovskite Sr₃Sn₂O₇ single crystal. The switching barrier is found to be remarkably low, resulting in a ferroelastic-coupled domain wall mobility. Theoretical and experimental evidence of a metastable antipolar intermediate phase as the possible root cause of low switching barrier will be discussed. The second part will focus on experimental studies of bilayer perovskite Li₂SrNb₂O₇ single crystal. Electron diffraction reveals a complex structure evolution, leading to anisotropic and temperature-strongly-dependent dielectric response. Moreover, due to intralayer Li ion activities, this material acts as a (anti)ferroelectric insulator or an ionic conductor (with memristive behavior and rectification effect) in different temperature and frequency regimes. These findings provide insight into the application of tunable ionic states in multifunctional quantum materials.

    

Magnetoelectrocaloric effect of multiferroic GdFeO3: electric-field-driven magnetic entropy change

Materials in which more than one type of caloric effects can be driven simultaneously by a single external field (magnetic, electric, or stress) have attracted much attention. An electric-field-driven magnetic entropy change termed magnetoelectrocaloric effect (MECE) is anticipated in multiferroic materials, where the magnetism and electricity are strongly correlated in a single material. Since no electric current is necessary, the energy dissipation of MECE due to Joule heating is expected to be negligibly small. We report an experimental demonstration of MECE in multiferroic GdFeO₃, which possesses the controllability of magnetism and ferroelectric polarization by an external electric field below the ferroelectric transition temperature. The temperature of the magnetic material changes when an external electric field of 25.6 kV/cm is suddenly applied or removed. Our experiments show the entropy change induced by MECE is largest just below the ordering temperature of Gd moments, suggesting that the ferroelectric transition affects MECE. The observed MECE is estimated to show an energy efficiency comparable or higher than typical magnetocaloric effects and adiabatic nuclear demagnetization. Our observation provides a proof-of-concept of MECE in multiferroics.

    

Experimental and theoretical investigation of magnetoelectric (ME) coupling in aligned multiferroic Janus fibers using second harmonic generation (SHG) polarimetry at different magnetic field orientations

Due to the absence of substrate clamping and high surface-to-volume ratio, heterostructure multiferroic fibers have been shown to support large ME coupling compared to their thin film counterparts. Here, the ME coupling in an ensemble of multiferroic Janus fibers consisting of ferrimagnetic cobalt ferrite (CFO) and ferroelectric barium titanate (BTO) is investigated using second harmonic generation (SHG) technique under different magnetic field orientations. We fit the experimental data by incorporating the known second order susceptibility tensor elements for bulk BTO and directly calculate the second order polarization. Our fits provide key information about the relative contribution of different ferroelectric BTO domains to the SHG polarization and help construct a 3D picture of how the domains redistribute upon changing the magnetic field orientation. We also find that the fibers in the ensemble behave similarly despite their inhomogeneity. We attempt to qualitatively relate the changes in the BTO to the expected changes in the CFO to gain insights into how the strain mediates the magnetoelectric (ME) coupling between the two constituents of multiferroic Janus fibers. 

    

Giant magnetoelectric effect and interactive control of magnetic anisotropy driven by a ferroelectric phase transition in ferromagnetic-piezoelectric heterostructure

Magnetoelectric materials and, to a larger extent, multiferroic heterostructures offer a novel route for electric field control of magnetism. We explored the induced magnetic anisotropy of an FeCo/Ag multilayer thin film deposited on the surface of 001 poled (Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ PIN-PMN-PT single crystal as it is electrically driven through a ferroelectric-ferroelectric phase transition. In PIN-PMN-PT an effective giant piezoelectric coefficient >10,000 pm/V occurs during the phase transition providing strain >0.25% along a principle strain direction. This effect translates into anisotropic strain to the magnetoelastic thin film inducing a giant magnetoelastic effect driven by the induced magnetic anisotropy along the principle strain direction. We measured an effective converse magnetoelectric coefficient ~1.4 x 10^-5 s/m when driven through the phase transition which was more than double the values obtained in the linear piezoelectric rhombohedral phase. The controllable and repeatable nature of the phase transition allows an interactive driving of the induced magnetization making the phenomenon attractive for application in spintronic devices.

    

Author not Attending

In Sr_0.98Mn_0.02TiO₃, successive spin (~34K) and structural glass (T_g~38K) transitions were previously found. The leading cause of magneto-electricity is the biquadratic coupling between antiferroelectrically paired dipoles (created by the off-centered occupation of Mn at Sr sites) with their corresponding antiferromagnetically paired Mn²⁺ spins. Here, to investigate the glassy magneto-electric correlation, the electrical third harmonic (|ε₃|) permittivity—the instantaneous dipolar response —has been measured across T_g by two ac field excitations, 1.4 and 4.2 kVm^-1, and each ac field measurement was performed under constant magnetic fields 0 T, 0.1 T, and 4.0 T. For this purpose, first, ac field-induced change is calculated for a particular magnetic field as δ|ε₃|_xT= (|ε₃|_4.2kV/m-|ε₃|_1.4kV/m)/|ε₃|_1.4kV/m, where x=0,0.1,4.0. Then the magneto-capacitance change is calculated as, Δ|ε₃|_xT= (δ|ε₃|_xT-δ|ε₃|_0T)/δ|ε₃|_0T, where x=0.1,4.0. Below the T_g, Δ|ε₃|_xT spectra show an anomaly in the 40-80 kHz range, especially for 0.1 T. The anomaly in Δ|ε₃|_xT spectra indicates the heterogeneous energy absorption by the biquadratically coupled spins and dipoles from the higher electric field is significantly influenced by the applied magnetic fields below the T_g.

    

Hidden magnetic octupoles: tensor decomposition and second-order magnetoelectric effect.

We discuss how magnetic octupoles provide a basis for interpreting the second-order magnetoelectric effect, in which a bilinear electric field induces a linear magnetization, or the product of magnetic and electric fields induces a polarization. Magnetic octupoles are the next order terms, beyond the well-established magnetoelectric multipoles responsible for the linear magnetoelectric effect, in the multipole expansion of a non-uniform magnetization density, μ(r). They are represented by the rank-3 tensor M_ijk = ∫ μ_i(r) r_j r_k d³r, which we begin by decomposing into its irreducible spherical components. We then use Cr₂O₃ as a case study and show that, in addition to the linear magnetoelectric effect caused by its magnetoelectric multipoles, it has a hidden second-order local magnetic

response caused by the anti-ferroic ordering of its magnetic octupoles.