Magnetoelectric Phase Research Articles

Magnetoelectric effect and phase transitions in CuO in external magnetic fields.

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.

Crystal structure, incommensurate magnetic order, and ferroelectricity inMn1−xCuxWO4 (0 ≤x≤ 0.19)

We have carried out a systematic study on the effect of Cu doping on nuclear, magnetic, and dielectric properties in ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{WO}}_{4}$ for $0\ensuremath{\le}x\ensuremath{\le}0.19$ by a synergic use of different techniques, viz, heat capacity, magnetization, dielectric, and neutron powder diffraction measurements. Via heat capacity and magnetization measurements we show that with increasing Cu concentration magnetic frustration decreases, which leads to the stabilization of commensurate magnetic ordering. This was further verified by temperature-dependent unit cell volume changes derived from neutron diffraction measurements which was modeled by the Gr\"uneisen approximation. Dielectric measurements show a low temperature phase transition below about 9--10 K. Furthermore, magnetic refinements reveal no changes below this transition indicating a possible spin-flop transition which is unique to the Cu doped system. From these combined studies we have constructed a magnetoelectric phase diagram of this compound.

Electric polarization of magnetic domain walls in magnetoelectrics

Two prominent magnetoelectrics MnWO4 and CuO possess low-temperature commensurate paraelectric magnetically ordered phase. Here using Monte Carlo simulations we show that the walls between the domains of this phase are ferroelectric with the same electric polarization direction and value as those in the magnetoelectric phases of these compounds. We also suggest that experimental observation of electric polarization of domain walls in MnWO4 should help to determine the macroscopic interactions responsible for its magnetoelectric properties.

Room temperature polarization in the ferrimagnetic Ga2−xFexO3 ceramics

The effect of the Fe/Ga ratio on the magnetic and electric properties of the multiferroic Ga2−xFexO3 compound has been studied in order to determine the composition range exhibiting magnetic and electric orders coexistence and their critical temperatures. A magnetoelectric phase diagram, showing the evolution of both the Néel magnetic ordering temperature TN and the electric ordering temperature Tc, versus the iron content has been established for 0.9≤x≤1.4. While the ferrimagnetic Néel temperature increases with the iron content, the electric ordering temperature shows an opposite trend. The electric polarization has been found to exist far above room temperature for the x=1.1 composition which shows the highest observed electric ordering temperature of Tc≈580K. The x=1.3 and 1.4 compounds are ferrimagnetic–electric relaxors with both properties coexisting at room temperature.

Room-temperature ferromagnetism in thin films of LaMnO3 deposited by a chemical method over large areas.

Hole-doping into the Mott insulator LaMnO3 results in a very rich magneto-electric phase diagram, including colossal magnetoresistance and different types of charge and orbital ordering. On the other hand, LaMnO3 presents an important catalytic activity for oxygen reduction, which is fundamental for increasing the efficiency of solid-oxide fuel cells and other energy-conversion devices. In this work, we report the chemical solution (water-based) synthesis of high-quality epitaxial thin films of LaMnO3, free of defects at square-centimeter scales, and compatible with standard microfabrication techniques. The films show a robust ferromagnetic moment and large magnetoresistance at room temperature. Through a comparison with films grown by pulsed laser deposition, we show that the quasi-equilibrium growth conditions characteristic of this chemical process can be exploited to tune new functionalities of the material.

Thermodynamic and neutron diffraction studies on multiferroic NdMn2O5

Magnetically frustrated RMn2O5 oxides have attracted considerable attention in recent years, because most of the members of this family show spin ordering induced dielectric polarization along with strong magneto-electric coupling. Although the true origin of the ferroelectricity is still a matter of debate, it has been observed that the magneto-electric phase diagram can be substantially tuned with the variation of rare earth elements. In this work, we have chosen NdMn2O5 as the compound of our interest since it lies exactly in between the ferroelectric and non-ferroelectric members of this family and also, because there are few investigations performed on RMn2O5 systems with large rare earth atoms . With the combination of heat capacity, magnetic susceptibility, dielectric permittivity, powder X-ray diffraction, and powder neutron diffraction measurements, it has been found that NdMn2O5 undergoes an incommensurate magnetic ordering around 30K followed by a possible ferroelectric-like transition at ∼26K. Another lock-in kind of magnetic transition appears when the temperature is decreased to ∼15K. With further lowering of temperature, an antiferromagnetic ordering, which is presumably associated with the Nd3+, is achieved near 4K. This study thus sheds light on a new compound of the RMn2O5 series presenting different multiferroic properties.

High pressure and multiferroics materials: a happy marriage.

The community of material scientists is strongly committed to the research area of multiferroic materials, both for the understanding of the complex mechanisms supporting the multiferroism and for the fabrication of new compounds, potentially suitable for technological applications. The use of high pressure is a powerful tool in synthesizing new multiferroic, in particular magneto-electric phases, where the pressure stabilization of otherwise unstable perovskite-based structural distortions may lead to promising novel metastable compounds. The in situ investigation of the high-pressure behavior of multiferroic materials has provided insight into the complex interplay between magnetic and electronic properties and the coupling to structural instabilities.

Magnetic and dielectric behavior of the spin-chain compound Er2BaNiO5 well below its Néel temperature

We have recently reported that the Haldane spin-chain system, Er2BaNiO5, undergoing antiferromagnetic order below (TN=) 32 K, is characterized by the onset of ferroelectricity near 60 K due to magnetoelectric coupling induced by short-range magnetic-order within spin-chains. We have carried out additional magnetic and dielectric studies to understand the properties well below TN. We emphasize here on the following: (i) A strong frequency dependent behaviors of ac magnetic susceptibility and complex dielectric properties have been observed at much lower temperatures (<8 K), that is, “reentrant multiglass-like” phenomenon, naturally suggesting the existence of an additional transition well below TN. (ii) “Magnetoelectric phase coexistence” is observed at very low temperature (e.g., T = 2 K), where the high-field magnetoelectric phase is partially arrested on returning to zero magnetic field after a cycling through metamagnetic transition.

Giant spin-driven ferroelectric polarization in TbMnO3 under high pressure.

The recent research on multiferroics has provided solid evidence that the breaking of inversion symmetry by spin order can induce ferroelectric polarization P. This type of multiferroics, called spin-driven ferroelectrics, often show a gigantic change in P on application of a magnetic field B. However, their polarization (<~0.1 μC cm(-2)) is much smaller than that in conventional ferroelectrics (typically several to several tens of μC cm(-2)). Here we show that the application of external pressure to a representative spin-driven ferroelectric, TbMnO3, causes a flop of P and leads to the highest P (≈ 1.0 μC cm(-2)) among spin-driven ferroelectrics ever reported. We explain this behaviour in terms of a pressure-induced magnetoelectric phase transition, based on the results of density functional simulations. In the high-pressure phase, the application of B further enhances P over 1.8 μC cm(-2). This value is nearly an order of magnitude larger than those ever reported in spin-driven ferroelectrics.

Electric-field-induced Skyrmion distortion and giant lattice rotation in the magnetoelectric insulator Cu2OSeO3.

Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected magnetic spin vortexlike objects, display a magnetoelectric coupling and can be manipulated by externally applied electric (E) fields. Here, we explore the E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the response using neutron scattering. Giant E-field induced rotations of the Skyrmion lattice are achieved that span a range of ∼25°. Supporting calculations show that an E-field-induced Skyrmion distortion lies behind the lattice rotation. Overall, we present a new approach to Skyrmion control that makes no use of spin-transfer torques due to currents of either electrons or magnons.

Novel room temperature multiferroics on the base of single-phase nanostructured perovskites

The theoretical description of the nanostructured Pb(Fe1/2Ta1/2)x(Zr0.53Ti0.47)1−xO3 (PFTx-PZT(1−x)) and Pb(Fe1/2Nb1/2)x(Zr0.53Ti0.47)1−xO3 (PFNx-PZT(1−x)) intriguing ferromagnetic, ferroelectric, and magnetoelectric properties at temperatures higher than 100 K are absent to date. The goal of this work is to propose the theoretical description of the physical nature and the mechanisms of the aforementioned properties, including room temperature ferromagnetism, phase diagram dependence on the composition x with a special attention to the multiferroic properties at room temperature, including anomalous large value of magnetoelectric coefficient. The comparison of the developed theory with phase diagrams allow establishing the boundaries between paraelectric, ferroelectric, paramagnetic, antiferromagnetic, ferromagnetic, and magnetoelectric phases, as well as the characteristic features of ferroelectric domain switching by magnetic field are performed and discussed. The experimentally established absence of ferromagnetic phase in PFN, PFT and in the solid solution of PFN with PbTiO3 (PFNx-PT(1−x)) was explained in the framework of the proposed theory.

Magnetic field reversal of electric polarization and magnetoelectric phase diagram of the hexaferrite Ba1.3Sr0.7Co0.9Zn1.1Fe10.8Al1.2O22

Low magnetic field reversal of electric polarization has been demonstrated in the multiferroic Y-type hexaferrite Ba1.3Sr0.7Co0.9Zn1.1Fe10.8Al1.2O22 single crystal. The maximum magnetoelectric coefficient at 200 K reaches 1065 ps/m near zero magnetic field. By a systematic investigation of magnetic field dependence of magnetic and dielectric responses at various temperatures, we obtained the magnetoelectric phase diagram describing the detailed evolution of the spin-induced ferroelectric phases with temperature and magnetic field. Below 225 K, the transverse spin cone can be stabilized at zero magnetic field, which is responsible for the reversal behavior of electric polarization. Our study reveals how to eventually achieve magnetic field reversal of electric polarization in hexaferrites at room temperature.

Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry

The majority of magnetoelectric crystals show complex temperature–magnetic field or temperature–pressure phase diagrams with alternating antiferromagnetic incommensurate, magnetoelectric, and commensurate phases. Such phase diagrams occur as a result of successive magnetic instabilities with respect to different order parameters, which usually transform according to different irreducible representations (IRs) of the space group of the crystal. Therefore, in order to build a phenomenological theory of phase transitions in such magnetoelectrics one has to employ several order parameters and assume the proximity of various instabilities on the thermodynamic path. In this work we analyze the magnetoelectrics MnWO4, CuO, NaFeSi2O6, NaFeGe2O6, Cu3Nb2O8, α-CaCr2O4 and FeTe2O5Br using the praphase concept and the symmetry of the exchange Hamiltonian. We find that in all the considered cases the appearing magnetic structures are described by IRs entering into a single exchange multiplet, whereas in the cases of MnWO4 and CuO by a single IR of the space group of the praphase structure. Therefore, one can interpret the complex phase diagrams of magnetoelectrics as induced by a single IR either of the praphase or of the symmetry group of the exchange Hamiltonian. Detailed temperature–magnetic field phase diagrams of MnWO4 and CuO for certain field directions are obtained and the magnetic structures of the field-induced phases are determined.

First-order ferroelastic transition in a magnetoelectric multiferroic: CuCrO2

We present the magnetic phase diagram of the multiferroic geometrically frustrated antiferromagnet CuCrO${}_{2}$ determined using dielectric constant and ultrasonic velocity measurements with the magnetic field parallel to the $[1\overline{1}0]$ direction. According to these measurements, at zero field the magnetoelectric phase induced by a proper screw magnetic ordering is observed below ${T}_{N1}=23.4$ K, while the velocity measurements reveal another transition at ${T}_{N2}=24.3$ K. As the dielectric and velocity measurements were performed simultaneously, our results confirm the presence of an intermediate nonferroelectric magnetic state between the magnetoelectric and paramagnetic phases. Moreover, our observations indicate that this intermediate phase persists up to 7 T for a field applied along the $[1\overline{1}0]$ direction. Based on similar observations obtained on the multiferric compounds CuO and MnWO${}_{4}$, this intermediate state is most likely a collinear antiferromagnetic phase. Apart from two phases observed at zero field, the ultrasonic velocity measured in the magnetoelectric phase as a function of the magnetic field reveals a spin-flop transition at ${H}_{\mathrm{flop}}=5$ T at 23 K, which is attributed to the flop of the spin spiral from the (110) plane to the $(1\overline{1}0)$ plane. In this paper, we also present a detailed analysis of the elastic properties of CuCrO${}_{2}$. The analysis of the data using a Landau-type free energy indicates that CuCrO${}_{2}$ undergoes a first-order pseudoproper ferroelastic transition leading to a $\overline{3}m\ensuremath{\rightharpoonup}2/m$ structural transition at ${T}_{N2}$. According to the model, the order parameter of the ferroelastic-antiferromagnetic transition at ${T}_{N2}$ belongs to the ${E}_{g}$ irreducible representation of the trigonal $\overline{3}m$ point group, and the magnetic moments must act as a secondary order parameter.

Novel dielectric anomalies due to spin-chains above and below Néel temperature in Ca3Co2O6

We bring out novel dielectric behavior of a spin-chain compound, Ca3Co2O6, undergoing Néel order at (TN = ) 24 K. It is found that the virgin curve in the plot of isothermal dielectric constant (ε') versus magnetic-field lies outside the ‘butterfly-shaped’ envelope curve well below TN (e.g., 2.6 K), with a signature of a partial arrest of the high-field magnetoelectric (ME) phase in zero-field after travelling through magnetic-field-induced magnetic transitions. This behavior is in contrast to that observed in the isothermal magnetization data. Thus, this work brings out a novel case for ‘phase-coexistence phenomenon’ due to ME coupling. Another strange finding is that there is a weak, but a broad, peak in ε' around 85–115 K well above TN, attributable to incipient spin-chain magnetic ordering. This finding should inspire further work to study ME coupling on artificial assemblies of magnetic chains, also keeping in mind miniaturization required for possible applications.

Ferromagnetic behavior in mixed valence europium (Eu2+/Eu3+) oxide EuTi1−x M xO3 (M = Al3+ and Ga3+)

We have investigated the Ti-site substitution effect on the magnetic properties of antiferromagnetic insulator EuTiO3 with a Néel temperature of ∼5 K. Partial substitution of Ti4+ with heterovalent Al3+ or Ga3+ turns the corresponding amount of magnetic Eu2+ into non-magnetic Eu3+. Both EuTi1−xAlxO3 (0.05 ≤ x ≤ 0.10) and EuTi1−xGaxO3 (0.05 ≤ x ≤ 0.10) exhibit ferromagnetic (FM) insulating behavior below ∼4 K. The Eu2+/Eu3+ mixed valence state probably contributes to the emergence of the FM behavior. Fine control of the magneto-electric (ME) phases of EuTi1−xAlxO3 and EuTi1−xGaxO3 would lead to intriguing ME phenomena such as giant ME effect.

Electric-field-induced generation and reversal of ferromagnetic moment in ferrites

The prospect of controlling the magnetization (M) of a material solely with an external electric field (E) could enable the development of low-power spintronics. Although there has been some success towards this end, most approaches involve controlling interactions at the interface between two different materials rather than switching of a single bulk phase. Here we report the ability to exert complete control over the generation and reversal of the bulk spontaneous M of the single-component multiferroics RFeO3 (R = Dy0.70Tb0.30, Dy0.75Gd0.25) with an E alone. We achieve this by controlling the anisotropic character of rare-earth magnetism and exploiting the competition between different magnetoelectric phases. We also show that whether M is reversed or retained on the E-induced polarization reversal depends on the E modulation speed. This is ascribed to the different dynamical characteristics of ferroelectric and multiferroic domain walls governed by the reversal dynamics of rare-earth moments and iron spins, respectively. The ability to modify a material’s magnetization with an electric field could enable lower-power electronic devices. Such ‘magnetoelectric’ behaviour is usually only seen at the interface between magnetostrictive and electrostrictive materials, but has now been observed in the bulk of single-component rare-earth ferrites.

Microstructural and magnetotransport properties of La1 − xCaxMnO3 (0.45 ≤ x ≤ 0.60) thin films

In the present work we focus on the coexistence of magnetic-electronic phases and hence phase separation in La1−xCaxMnO3 thin films in the composition range of 0.45≤x≤0.60. The oriented polycrystalline thin films were prepared by nebulized spray pyrolysis technique on single crystalline substrate LaAlO3 [LAO (001)]. The lattice parameters are observed to decrease with increasing in Ca content and the structural/microstructural disorder is also seen to increase concomitantly. As the Ca content is increased from x=0.45 to 0.55, the paramagnetic insulator (PMI) to ferromagnetic metal (FMM) transition temperatures (TC/TIM) are suppressed. This is accompanied by (i) appreciable suppression of the TC, (ii) sharp reduction in the magnetic moment, (iii) occurrence of a peak in the magnetization below TC, (iv) stronger bifurcation of the ZFC–FC magnetization, and (v) decline in the magnetization in the lower temperature regime. At x=0.60, the FMM disappears and a weak FM insulator like phase is observed. In the vicinity of x=0.55, the magnetic ground state resembles spin cluster glass. This is also confirmed by the occurrence of a hysteresis loop in the resistivity of the x=0.55 measured during the cooling and warming cycle. The transition broadening up to x=0.55 and the vanishing of the two transitions in x=0.60 film along with the simultaneous decrease in the magnetic moment and enhanced resistivity show that the observed variations are due to the increase in the antiferromagnetic-charge ordered (AFM-CO) phase fraction at higher Ca content. This is also confirmed by the decrease in magnitude of the magnetoresistance and its temperature dependence. Our results demonstrate that, there is strong phase separation in the overdoped region (0.50<x≤0.60) due to the presence of multiple magneto-electric phases.

Praphase concept for the phenomenological description of magnetoelectrics

Complex cascades of phase transitions with the alternation of antiferromagnetic, incommensurate, and magnetoelectric phases are observed in most crystals having magnetoelectric properties. The entire variety of observed phase transitions in numerous recent works is described by introducing several order parameters which are sequentially “condensed” with a decrease in temperature. The possibility of introducing the praphase concept into consideration to reduce the number of order parameters and interpret rich phase diagrams due to the smaller number of instabilities is demonstrated by the examples of MnWO4 and CuO magnetoelectrics. This approach is also used in the phenomenological description of the phase transitions in the BiFeO3 ferroelectromagnetic with the cubic phase of the perovskite structure as a starting point.

Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an pseudoproper ferroelastic transition []. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two-dimensional irreducible representation Eg (x2 − y2, xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at ωEg = 352 cm−1 and ωA1g = 692 cm−1, while these modes are detected at ωEg = 457 cm−1 and ωA1g = 709 cm−1 in CuCrO2. The analysis of the temperature dependence of the modes in both compounds shows that the frequencies of all modes increase with decreasing temperature. This typical behavior is attributed to anharmonic phonon–phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transitions observed at the Néel temperature TN1. Finally, a broad band at about 550 cm−1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be associated with magnons.