Multiferroic Systems Research Articles

Optimization of the mechanochemical conditions for the synthesis of the xBiFeO 3–(1 − x)PbTiO 3 multiferroic system

The xBiFeO 3–(1 − x)PbTiO 3 solid solution is nowadays one of the most promising multiferroic systems. However, the obtaining of single phases with a high crystallinity is difficult by the conventional methods. In this work, the mechanochemical method has been optimized, testing different parameters in the reaction to obtain the suitable conditions of synthesis. The investigated parameters were: the atmosphere conditions and the material of the reaction medium. Single phases with compositions belonging to the whole system (0 ≤ x ≤ 1) have been obtained, being the mechanochemical reaction with tungsten carbide vessel the best synthesis medium. The structural characterization of the obtained powders has been carried out by XRD and microstructures of powdered samples have been studied by SEM and TEM.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Stress-based control of magnetic nanowire domain walls in artificial multiferroic systems

Artificial multiferroic systems, which combine piezoelectric and piezomagnetic materials, offer novel methods of controlling material properties. Here, we use combined structural and magnetic finite element models to show how localized strains in a piezoelectric film coupled to a piezomagnetic nanowire can attract and pin magnetic domain walls. Synchronous switching of addressable contacts enables the controlled movement of pinning sites, and hence domain walls, in the nanowire without applied magnetic field or spin-polarized current, irrespective of domain wall structure. Conversely, domain wall-induced strain in the piezomagnetic material induces a local potential difference in the piezoelectric, providing a mechanism for sensing domain walls. This approach overcomes the problems in magnetic nanowire memories of domain wall structure-dependent behavior and high power consumption. Nonvolatile random access or shift register memories based on these effects can achieve storage densities >1 Gbit/In2, sub-10 ns switching times, and power consumption <100 keV per operation.

Magnetic and dielectric properties of multiferroic Tb0.5Eu0.5MnO3

The structural, magnetic and dielectric properties of multiferroic compound Tb0.5Eu0.5MnO3 are reported near the boundary of the multiferroic phase of the Eu-doped Tb1−xEuxMnO3 system. The Mn3+ moments order in an incommensurate antiferromagnetic manner below TN = 50 K, followed by a spin canting-like response around 25 K. Below TN, extra hysteresis loops were observed for applied fields Ba around 1-2 T, with decreasing magnetic / K moment for increasing magnetic fields. The dielectric constant ϵ' showed a peak at 28, which was attributed to the ferroelectric transition in analogy with the parent compound TbMnO3. Structural analysis indicates the Mn-O2-Mn bond angle within the ab-plane is 145.9° suggesting that there exists a critical angle in the perovskite manganate multiferroic systems.

Eu doping in multiferroic BiFeO3 ceramics studied by Mossbauer and EXAFS spectroscopy

Bismuth ferrite ceramics (BiFeO3) are multifunctional materials classified as multiferroics for their special magnetic and electricproperties that can be modified by substitutional doping at the Bi and/or Fe sites. Understandingthe relation between magnetoelectric response and structural/electronic modification upondoping is a relevant issue. In this work, the structure of Eu-doped multiferroic systems (Bi1 − xEuxFeO3,x = 0, 0.5, 0.1, 0.15) as well as the valence state of Fe and Eu ions have been investigated combiningMossbauer and x-ray absorption fine structure (XAFS) spectroscopy techniques. TheEu3 + doping at the Bi site results in better magnetic properties. High temperature 57Fe Mossbauer data and Fe K-edge XAFS results show thatFeO6 octahedron distortionsreduce with Eu3 + doping. It is conclusively shown that the observed magnetic properties inBiFeO3 withchemical substitution (Eu) are mainly due to the structural distortions and not due to Fe multiple valence. 151Eu Mossbauer measurementsshow that the Eu3 + (Bi3 + ) site ismagnetically inactive in BiFeO3.

Hybrid functional study of proper and improper multiferroics

We present a detailed study of the structural, electronic, magnetic and ferroelectric properties of prototypical proper and improper multiferroic (MF) systems such as BiFeO(3) and orthorhombic HoMnO(3), respectively, within density functional theory (DFT) and using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). By comparing our results with available experimental data as well as with state-of-the-art GW calculations, we show that the HSE formalism is able to account well for the relevant properties of these compounds and it emerges as an accurate tool for predictive first-principles investigations on multiferroic systems. We show that effects beyond local and semilocal DFT approaches (as provided by HSE) are necessary for a realistic description of MFs. For the electric polarization, a decrease is found for MFs with magnetically-induced ferroelectricity, such as HoMnO(3), where the calculated polarization changes from approximately 6 muC cm(-2) using Perdew-Burke-Ernzerhof (PBE) to approximately 2 muC cm(-2) using HSE. However, for proper MFs, such as BiFeO(3), the polarization slightly increases upon introduction of exact exchange. Our findings therefore suggest that a general trend for the HSE correction to bare density functional cannot be extracted; rather, a specific investigation has to be carried out on each compound.

Magnetic-field control of charge structures in the magnetically disordered phase of multiferroicLuFe2O4

Using neutron diffraction, we have studied the magnetic-field effect on charge structures in the charge-ordered multiferroic material ${\text{LuFe}}_{2}{\text{O}}_{4}$. An external magnetic field is able to change the magnitude and correlation lengths of the charge valence order even before the magnetic order sets in. This affects the dielectric and ferroelectric properties of the material and induces a giant magnetoelectric effect. Our results suggest that the magnetoelectric coupling in ${\text{LuFe}}_{2}{\text{O}}_{4}$ is likely due to magnetic-field effect on local spins, in clear contrast to the case in most other known multiferroic systems where the bulk magnetic order is important.

Relaxations as Key to the Magnetocapacitive Effects in the Perovskite Manganites

We present a detailed dielectric study of the relaxation effects that occur in several perovskite rare-earth manganites, including the multiferroics TbMnO(3) and DyMnO(3). We demonstrate that the strong magnetocapacitive effects, observed for electrical fields E parallel c, are nearly completely governed by magnetic-state induced changes of the relaxation parameters. The multiferroic materials, which undergo a transition into a spiral magnetic state, show qualitatively different relaxation behavior than those compounds transferring into an A-type antiferromagnetic state. We ascribe the relaxations in both cases to the off-center motion of the manganese ions, which in the multiferroic systems also leads to the ferroelectric ordering.

A radical approach to promote multiferroic coupling in double perovskites

Double perovskites provide a unique opportunity to induce and control multiferroic behaviours in oxide systems. The appealing possibility to design materials with a strong coupling between the magnetization and the polarization fields may be achieved in this family since these magnetic insulators can present structural self-ordering in the appropriate growth conditions. We have studied the functional properties of La 2CoMnO 6 and Bi 2CoMnO 6 epitaxial thin films grown by pulsed laser deposition. Cation-ordered La 2CoMnO 6 films display a magnetic Curie temperature of 250 K while cation-disordered Bi 2CoMnO 6 films present ferromagnetism up to ∼800 K. Such high transition temperature for magnetic ordering can be further tuned by varying the strain in the films indicating an important contribution from the structural characteristics of the materials. Our approach might be generalized for other oxide systems. At this end, our results are compared with other multiferroic systems. The roles of various cations, their arrangements and structural effects are further discussed.

Computational quantum magnetism: Role of noncollinear magnetism

We are witnessing today a golden age of innovation with novel magnetic materials and with discoveries important for both basic science and device applications. Computation and simulation have played a key role in the dramatic advances of the past and those we are witnessing today. A goal-driving computational science—simulations of every-increasing complexity of more and more realistic models has been brought into greater focus with greater computing power to run sophisticated and powerful software codes like our highly precise full-potential linearized augmented plane wave (FLAPW) method. Indeed, significant progress has been achieved from advanced first-principles FLAPW calculations for the predictions of surface/interface magnetism. One recently resolved challenging issue is the role of noncollinear magnetism (NCM) that arises not only through the SOC, but also from the breaking of symmetry at surfaces and interfaces. For this, we will further review some specific advances we are witnessing today, including complex magnetic phenomena from noncollinear magnetism with no shape approximation for the magnetization (perpendicular MCA in transition-metal overlayers and superlattices; unidirectional anisotropy and exchange bias in FM and AFM bilayers; constricted domain walls important in quantum spin interfaces; and curling magnetic nano-scale dots as new candidates for non-volatile memory applications) and most recently providing new predictions and understanding of magnetism in novel materials such as magnetic semiconductors and multi-ferroic systems.

Erratum: Landau analysis of the symmetry of the magnetic structure and magnetoelectric interaction in multiferroics [Phys. Rev. B76, 054447 (2007)

This paper presents a detailed instruction manual for constructing the Landau expansion for magnetoelectric coupling in incommensurate ferroelectric magnets, including ${\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}$, $\mathrm{Mn}\mathrm{W}{\mathrm{O}}_{4}$, $\mathrm{Tb}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$, $\mathrm{Y}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$, $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_{2}$, and $\mathrm{Rb}\mathrm{Fe}{(\mathrm{M}{\mathrm{O}}_{4})}_{2}$. The first step is to describe the magnetic ordering in terms of symmetry adapted coordinates which serve as complex-valued magnetic order parameters whose transformation properties are displayed. In so doing, we use the previously proposed technique to exploit inversion symmetry, since this symmetry has seemingly been universally overlooked. Inversion symmetry severely reduces the number of fitting parameters needed to describe the spin structure, usually by fixing the relative phases of the complex fitting parameters. By introducing order parameters of known symmetry to describe the magnetic ordering, we are able to construct the trilinear magnetoelectric interaction which couples incommensurate magnetic order to the uniform polarization, and thereby we treat many of the multiferroic systems so far investigated. In most cases, the symmetry of the magnetoelectric interaction determines the direction of the magnetically induced spontaneous polarization. We use the Landau description of the magnetoelectric phase transition to discuss the qualitative behavior of various susceptibilities near the phase transition. The consequences of symmetry for optical properties such as polarization induced mixing of Raman and infrared phonons and electromagnons are analyzed. The implication of this theory for microscopic models is discussed.

Landau analysis of the symmetry of the magnetic structure and magnetoelectric interaction in multiferroics

This paper presents a detailed instruction manual for constructing the Landau expansion for magnetoelectric coupling in incommensurate ferroelectric magnets, including ${\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}$, $\mathrm{Mn}\mathrm{W}{\mathrm{O}}_{4}$, $\mathrm{Tb}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$, $\mathrm{Y}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$, $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_{2}$, and $\mathrm{Rb}\mathrm{Fe}{(\mathrm{M}{\mathrm{O}}_{4})}_{2}$. The first step is to describe the magnetic ordering in terms of symmetry adapted coordinates which serve as complex-valued magnetic order parameters whose transformation properties are displayed. In so doing, we use the previously proposed technique to exploit inversion symmetry, since this symmetry has seemingly been universally overlooked. Inversion symmetry severely reduces the number of fitting parameters needed to describe the spin structure, usually by fixing the relative phases of the complex fitting parameters. By introducing order parameters of known symmetry to describe the magnetic ordering, we are able to construct the trilinear magnetoelectric interaction which couples incommensurate magnetic order to the uniform polarization, and thereby we treat many of the multiferroic systems so far investigated. In most cases, the symmetry of the magnetoelectric interaction determines the direction of the magnetically induced spontaneous polarization. We use the Landau description of the magnetoelectric phase transition to discuss the qualitative behavior of various susceptibilities near the phase transition. The consequences of symmetry for optical properties such as polarization induced mixing of Raman and infrared phonons and electromagnons are analyzed. The implication of this theory for microscopic models is discussed.

Chiral Spin Pairing in Helical Magnets

A concept of chiral spin pairing is introduced to describe a vector-chiral liquid-crystal order in frustrated spin systems. It is found that the chiral spin pairing is induced by the coupling to phonons through the Dzyaloshinskii-Moriya interaction and the four-spin exchange interaction of the Coulomb origin under the edge-sharing network of magnetic and ligand ions. This produces two successive second-order phase transitions upon cooling: an O(2) chiral spin nematic, i.e., spin cholesteric, order appears with an either parity, and then the O(2) symmetry is broken to yield a helical magnetic order. Possible candidate materials are also discussed as new multiferroic systems.

Electrically Assisted Magnetic Recording in Multiferroic Nanostructures

We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

Multiferroic materials based on artificial thin film heterostructures

We discuss the use of thin film heterostructures for the generation of artificial multiferroic or magnetoelectric material systems. In this regard the transition metal oxides BiFeO3 and BiCrO3 are of particular interest. By using laser molecular beam epitaxy we have grown epitaxial thin films of these materials on SrTiO3 substrates in a two-dimensional layer-by-layer growth mode. The films show small surface roughness and excellent crystallographic quality with mosaic spread below 0.04°. In contrast to the bulk materials, both BiFeO3 and BiCrO3 films have tetragonal crystal structure due to epitaxial coherency strain. The magnetic properties have been studied by SQUID magnetometry. In contrast to a previous report on BiFeO3 we find only a small saturation magnetization below 0.05μ B/formula unit both for BiFeO3 and BiCrO3, which can be explained by a canting of the antiferromagnetically aligned Fe and Cr spins, respectively. Furthermore, no indication for a strain induced enhancement of magnetization was found.

Scanning low-temperature element-specific magnetic microscopy

We have developed a low-temperature element-specific magnetic microscopy instrument at beamline 4-ID-D of the Advanced Photon Source. The setup enables simultaneous chemical and magnetic characterization of materials with ∼1μm2 resolution at low temperature (>10K) under a moderate applied field (<0.8T). We demonstrate the potential of this apparatus by presenting results correlating chemical and magnetic local behavior in inhomogeneous layered manganites and multiferroic systems.