Session F02: Multiferroics and Magnetoelectric Couplings

A Strain-enabled Novel Multiferroic State in Barium Hexaferrite through Suppression of Quantum Paraelectricity

Barium hexaferrite (BaFe₁₂O₁₉) is predicted to harbor a novel multiferroic phase within which frustrated antiferroelectricity and ferrimagnetism coexist. However, experimental studies have shown that in bulk barium hexaferrite, quantum fluctuations instead lead to a quantum paraelectric ground state at low temperature [1-2]. Strain is a popular method for tuning functional properties in epitaxial thin films and has previously been used to coax ferroic electric dipole order, for example in SrTiO₃ [3]. Similarly, recent theoretical exploration has suggested that strain may enhance antiferroelectricity in barium hexaferrite [4]. In this theoretical work, we combine first-principles calculations with quantum Monte Carlo simulations to explore the temperature- and strain-dependent properties of barium hexaferrite. We find good agreement between our model and available bulk experimental data. In the investigation of strained barium hexaferrite, we find that a modest compressive strain of approximately 1% enables the transition to a frustrated antiferroelectric phase with a critical temperature of greater than 10 K. Our results suggest that further investment in epitaxy and development of compressive substrates for hexaferrites may provide a promising route towards room temperature multiferroics.

[1] Shen et al. Phys. Rev. B 90, 180404(R) (2014)

[2] Zhang et al. Phys. Rev. B 101, 104102 (2020)

[3] Haeni et al. Nature 430 758–761 (2004)

[4] Wang et al. Phys. Rev. X 4, 011035 (2014)   

Controlling magnetocrystalline anisotropy energies of magnetic dopants in ferroelectric oxides via local crystal-field tuning

To date, most work on combining and controlling magnetism with ferroelectric distortions has been in systems with long-range order for both. Here instead, we create a magnetoelastic material with long-range electric order but localized magnetic order by doping a ferroelectric material with dilute amounts of magnetic ions. The crystal structure of the ferroelectric host is controllable by external electric fields, which can then change the local crystal field environment around the magnetic defect. The spin orientation of the defect is coupled to the local crystal field environment via the spin-orbit interaction. This offers a pathway to control the magnetic properties with an external electric field. Presently, there is no clear understanding of which crystal field environments will allow for favorable magnetic properties, such as reorientation  of the spin easy-axis with a 180-degree reversal of the electric polarization, or high magnetocrystalline anisotropy energies (MCAE). To address this, we consider the tetragonal, rhombohedral, and orthorhombic phases of prototypical ferroelectric oxide BaTiO₃ doped with Fe³⁺. We explore how the MCAE surfaces for these systems depend on crystalline symmetries and electronic structure through first-principles density functional theory (DFT) calculations and propose design principles for tailoring the electric control of single spin sites in ferroelectric oxide hosts.   

Effect of Cr and La doping on the physical properties of Aurivillius Bi5Ti3FeO15

Lead-based materials are the most used piezoelectric ceramics due to their high efficiency. Unfortunately, lead is toxic to humans and other living beings. Thus, there is significant interest in layered lead-free ceramics which show large piezoelectric effect at room temperature. This project studies layered Aurivillius perovskites of the form Bi₄Bi_n-3Ti₃Fe_n-3O_3n+3 where n = 4, specifically, the compounds Bi₅Ti₃FeO₁₅, Bi₄LaTi­₃FeO₁₅, and Bi₅Ti₃Fe_0.5Cr_0.5O₁₅ to understand the effect of doping the Bi-site with La and the Fe-site with Cr. We have prepared our samples using solid-state method and studied the crystal structure using synchrotron X-ray diffraction, thereby determining the space group of A21am for all three compositions. Further, we present the local crystal structure features using pair distribution function analysis. Our Raman scattering experiments reveal the vibrational modes and contrast the three compounds. The X-ray photoelectron spectroscopy results confirm the nominal valence of cationic species in the three compounds. All the three compounds show an antiferromagnetic nature of the magnetic ground state and are characterized using magnetometry and specific heat. We observe that the bulk properties of the three compounds are resilient to doping induced changes while only observable changes are to the local structure.    

Magnetic ordering in new polar magnetoelectric ErCrWO6

Much effort has been expended in the last two decades in exploring magnetoelectrics and multiferroics due to their novel functionality for potential application in magnetoelectric RAM or gate devices. To date, many of the most studied magnetoelectrics suffer from weak coupling between ferroic orders, even those with strong—even above room temperature—orderings such as BiFeO₃. One appealing avenue toward higher magnetoelectric coupling is in the emerging family of polar magnets in which the ferroelectric distortion and magnetic ordering are centered on the same ion. Here we discuss our recent results on the new polar magnetoelectric compound ErCrWO₆, which crystallizes in the noncentrosymmetric space group Pna2₁. Neutron diffraction experiments have been performed on the WAND2 diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory revealing commensurate antiferromagnetic order canted slightly in the c-direction below a transition temperature of T_N ~ 22 K. We report on the magnetic interactions as studied via inelastic neutron scattering as well as dielectric and pyroelectric properties at measured at low temperature.   

Correlation between ferroic orders in the polar magnet system RFeWO6

Multiferroics are materials with more than one ferroic order. We are interested, in particular, in compounds with coexisting magnetic and ferroelectric orders the so called magnetoelectric multiferroics. The coupling between the orders is technologically relevant in applications like MERAMS but it is not completely understood. They are classified according to their coupling strength of the orders from independent origins (type I) to the strong coupling limit (type II) wherein the electric polarization (EP) emerges via symmetry-breaking by the magnetic ordering (MO). Recently a new class of multiferroics, the polar magnets is described to have a polar structure in the paramagnetic phase but the EP emerges only below MO. One such system is RFeWO₆ (R: smaller rare earths) which crystallizes in the aeschynite structure. The EP sets in at the MO temperature around 15-20 K depending on R. Although ferroelectric and magnetic properties have been determined, detailed magnetic and crystal structure investigations to determine the origin and coupling of the ferroic orders are lacking. In this work we utilize neutron scattering techniques, detailed bulk property measurements and theoretical modeling to address these issues for several members. Our results indicate that the EP emerges from multiple origins governed by the underlying magnetic interactions.   

Polar vs chiral magneto-structural coupling in cubic quadruple perovskites

We report the high-pressure, high-temperature synthesis and characterisation of the quadruple perovskite CaMn₃(Cr₃Mn)O₁₂. Neutron powder diffraction experiments showed that below T_N = 155 K the B transition metal sublattice adopts long range G-type antiferromagnetic order while the A' transition metal sublattice remains paramagnetic until T₂ = 55 K, below which it also develops G-type order. Unlike the related multiferroic compound LaMn₃Cr₄O₁₂, which displays magnetically-induced ferroelectricity below T₂, we found that CaMn₃(Cr₃Mn)O₁₂ remained non-polar down to the lowest measured temperatures. We propose a phenomenological model of magneto-structural coupling based on competing free-energy terms that couple polar and chiral distortions, respectively, depending on the global direction of magnetic moments. The model naturally explains why some compounds in this family become ferroelectric below T₂, while others do not. The phenomenology is applicable to all cubic Im-3 quadruple perovskites that support antiferromagnetic G-type order on both A’ and B sites, implies a novel approach towards multiferroic functionality, and can be generalised to other multi-sublattice systems where the magnetic interaction between sublattices is prohibited by spatial inversion.   

Revisit of multiferroic perovskite metal-organic frameworks [C(NH2)3]M(HCOO)3 (M = Cr, Cu) by first-principles calculations

Multiferroic metal-organic framework (MOF) [C(NH₂)₃]M(HCOO)₃ (M = Cr, Cu) adopts a hybrid perovskite ABX₃ structure. Jahn-Teller distortion is responsible for both the weak-ferromagnetism and ferroelectricity in these materials. Unlike the M = Cu analogue which has been well studied, the [C(NH₂)₃]Cr(HCOO)₃ has been synthesized very recently [1] overcoming the rapid oxidation of Cr²⁺. The measurements of its magnetic properties following the successful synthesis exhibit quantitative deviation from the previous theoretical estimations [2]. In order to resolve this discrepancy, we performed the density functional theory calculations and the Monte Carlo simulations to improve the previous estimations. In addition, we suggest two unusual aspects of the electronic and magnetic properties of these materials. In terms of the Landau free energy theory, the polar distortion mode in these materials appears by the trilinear coupling to two non-polar modes (hybrid improper ferroelectricity) [2]. Interestingly, the combination of the two non-polar modes, i.e., the hybrid mode, is the main origin of polarization in these materials [3]. In the M = Cu case, we found that the orbital magnetic moment is a main contribution to the weak-ferromagnetism [1].

[1] K. Yananose et al., Inorg. Chem. 2023, 62, 42, 17299–17309

[2] A. Stroppa et al., Adv. Mater. 2013, 25, 2284–2290.

[3] K. Yananose et al., arXiv.2202.06537, in preparation.   

Electronic Ferroelectricity Resulting in a Valence Bond Solid in the Triangular Lattice Organic Mott Insulator κ-(BEDT-TTF)2Cu2(CN)3

A triangular lattice of dimer molecular sites (BEDT-TTF)₂ in a Mott insulator κ-(BEDT-TTF)₂Cu₂(CN)₃ (BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene) can simultaneously show magnetic and ferroelectric properties. Each site carries one electron, with S=1/2 spins interacting antiferromagnetically, while a non-equilibrium probability of the electrons to reside on individual molecules of dimer sites results in ferroelectricity. We use Raman scattering spectroscopy to identify the low temperature state of the system which is a consequence of the interactions of magnetic, charge, and lattice degrees of freedom. We identified electric dipole fluctuations within molecular dimer sites, emerging below 40 K and freezing below 20 K, through line-shape analysis of charge-sensitive molecular vibrations. We demonstrate that the charge dipoles are coupled to the lattice, so that freezing of the fluctuations is observed through anisotropic broadening of lattice phonons related to whole-molecule motion. We argue that this freezing of electric dipoles leads to spins freezing into a disordered valence bond solid state at 6 K, with ferroelectric domain walls carrying orphan spins.   

Magnetoelectric coupling in molecules

Materials with magnetoelectric (ME) coupling exhibit behavior wherein applied electric field controls the magnetization or applied magnetic field controls the electric polarization. These have many potential and realized applications including electric control of spin qubits or coupling between qubits in molecular magnets. Molecular magnets are particularly promising materials to search for novel ME behavior because of their soft lattices, which can result in exceptionally high coupling constants – deformations on the order of 1% are common. This talk will cover a few of our recent examples: one involves a spin crossover with a valence tautomerization transition where the change in Co spin state causes an electron to migrate across the molecule and toggle the existence of an electric dipole. Another involves the evolution of a frustrated configuration of antiferromagnetic moments that couple to electric polarization via magnetostriction. We will discuss the quantum magnetism of each compound and its symmetry, the evolution of the magnetic states with temperature and magnetic field, and finally the mechanism to couple to electric polarization and dielectric constant. We will discuss measurements of magnetic and electrical properties at low and high magnetic fields, and theoretical calculations.   

Surface and interfacial effects in Spin crossover molecular systems.

Surface and interfacial effects are known to give rise to phenomenon which do not occur in the bulk. In other words, the surface is different from the bulk. In this regard, we report on the surface properties of spin crossover molecular systems namely [Fe(qsal)₂][Ni(dmit)₂] and [Fe(qsal)₂][Pd(dmit)₂], where, (qsalH = N-(8-quinolyl)salicylaldimine, dmit^2- = 1,3-dithiol-2-thione-4,5-dithiolato). We employ angle resolved X-ray photoemission spectroscopy (XPS) to study surface segregation in thin films of the said molecular complexes. Angle resolved XPS is also employed to study the spin state transition on the surface and in the bulk. It is seen that due to surface effects, the spin state on the surface is robust to temperature variations as compared to the bulk. We also report on a spin crossover/2-D semiconductor interface between [Fe(H₂B(pz)₂)₂(bipy)], where,  (pz=(pyrazol-1-yl)-borate and bipy = 2,2’-bipyridine) and WSe₂. Using angle resolved XPS, we are able to report on the core level shift for W and Se at and away from the interface. This study ties to earlier reported surface and interfacial phenomenon in Spin crossover molecular systems. However, the surface segregation in spin crossover systems has not been reported earlier.     

Heterogeneously integrated freestanding oxide membrane with multiferroicity

Transition metal oxides exhibit diverse electrical and magnetic characteristics, which are influenced by their order parameters. Ferroic orderings offer a wide range of important scientific phenomena and practical uses. To create multiferroic oxides successfully, a promising approach involves combining ferroelectric (FE) and ferromagnetic (FM) materials. The development of freestanding, heterogeneous membranes containing these multiferroic oxides is highly desirable.

In this study, we employed pulsed laser epitaxy to create freestanding bilayer membranes made of BaTiO₃ and La_0.7Sr_0.3MnO₃ [1]. These membranes display ferroelectric and ferromagnetic properties at room temperature, along with a measurable magnetoelectric (ME) coupling coefficient. Our research highlights the importance of achieving freestanding heterostructures in determining their structural and emerging qualities. When not clamped by the substrate, the magnetic orientation axis undergoes reorientation, resulting in perpendicular magnetic anisotropy due to changes in the orbital occupancy of the magnetic layer. These findings open the door to integrating these flexible membranes into electronic applications.   

Co/Pt Multilayer System: Exploring Magnetic Properteis for Advanced Applications

Recent research has focused on layered systems containing 3d,4d, or 5d transition metals for various magnetic applications, such as high-density recording media and magnetic random access memories. In particular, Co/Pt(111) multilayer structures have attracted considerable interest due to their exceptional attributes, which include remarkable thermal stability, low critical spin switching current density, and perpendicular magnetization characteristics. Despite the considerable attention these systems have received, there are still significant gaps in our understanding of their magnetic properties, particularly with respect to magnetic anisotropy and the distinct roles played by individual atoms within the system. Therefore, we carried out a systematic analysis of the magnetic properties of Co/Pt(111) multilayers using density functional theory, taking into account the spin-orbit interaction. We examined the changes in magnetic anisotropy energy in terms of magnetic crystalline anisotropy and magnetic shape anisotropy based on the number of Co and Pt layers. The main reason for the changes in physical properties with varying Co/Pt thickness is attributed to interfacial effects. By precisely controlling individual layer thicknesses, we have demonstrated the potential to tailor magnetic properties for specific applications   

Electric Field Control of Magnetoresistance in Epitaxial Fe0.75Co0.25 in Composite Multiferroics

Composite multiferroics that integrate ferroelectric and magnetic properties can function above room temperature and exhibit improved magnetoelectric (ME) coupling compared to single-phase multiferroic materials, making them desirable candidates for both studying the fundamental physics of ME coupling and applications for future novel devices.¹ Further, these composite multiferroics present an opportunity to alter magnon generation and propagation via electrical methods. In this study, we investigate the magnetization dynamics of MBE-grown Fe_0.75Co_0.25, a metallic ferromagnet with low damping and high magnetoelastic constant, epitaxially grown on ferroelectric materials.^2–4 Electric field manipulation of magnetoresistance is observed and further characterized using transport and optical techniques.

References

1. Pradhan, D. K., Kumari, S. & Rack, P. D. Magnetoelectric Composites: Applications, Coupling Mechanisms, and Future Directions. Nanomaterials (Basel) 10, 2072 (2020).

2. Edwards, E. R. J., Nembach, H. T. & Shaw, J. M. Co₂₅Fe₇₅ Thin Films with Ultralow Total Damping of Ferromagnetic Resonance. Phys. Rev. Appl. 11, 054036 (2019).

3. Lee, A. J. et al. Metallic ferromagnetic films with magnetic damping under 1.4 × 10−3. Nat Commun 8, 234 (2017).

4. Schwienbacher, D. et al. Magnetoelasticity of Co25Fe75 thin films. J Appl Phys 126, https://doi.org/10.1063/1.5116314 (2019).