Linear Magnetoelectric Effect Research Articles

Bi 1−xR xFeO 3 ( R=rare earth): a family of novel magnetoelectrics

Crystals of solid solutions Bi 1− x R x FeO 3, where R=La, Dy, Gd, were obtained with χ⩽0.7. Solid solutions of the stated rare earths, as χ is increased from 0 to 0.7, have one and the same sequence of five crystal structures (rhombohedral C 6 3v, triclinic C 1 1, orthorhombic D 6 2, orthorhombic D 5 2, orthorhombic C 9 2v). The ferroelectric-paraelectric transition occurs in rhombohedral and triclinic crystals at T=810−560° C. The high temoerature modifications are orthorhombic and cubic. The orthorhombic structure C 9 2v holds up to 1180°C. The ferroelectric domain structure was distinguished in all types of crystals. No magnetoelectric effect (MEE) was detected in the orthorhombic crystals with the D 2 (222) symmetry class. But the mm2 crystals were found to have both quadratic and linear MEE. The value of the quadratic effect is considerably smaller than that of the linear one. Magnetoelectric hysteresis takes place in the crystals. The tensorial properties of the obtained crystals are analyzed from the viewpoint of crystal symmetry.

On the linear magnetoelectric effect

A complete analysis of the linear magnetoelectric (ME) effect is carried out which exploits the PT symmetry of the ME susceptibility tensor alpha . The existing (or 'old') theory in effect considers only the time-reversal antisymmetry of alpha , with the result that it restricts the effect to magnetic crystals. It is shown that this presents only half the 'picture', and that consideration of the space-inversion antisymmetry of the tensor reveals, in accord with relativistic considerations, that the effect should also occur in certain paramagnetic electric (ferroelectric and antiferroelectric) crystals, which constitute the other half of the picture. A new classification of the eligible ME crystals is proposed. Among these crystals are BaCoF4 and Ti2O3, which, although prohibited by the old theory, have nonetheless been found to exhibit the effect. Other predicted candidates for the effect are presented.

Softening electromagnetic and sound waves at phase transitions in crystals with linear magnetoelectric effects

Whereas the order parameter of a phase transition is transformed either as the components of the electric or magnetic polarisation or as a linear combination of the deformation tensor components, the velocities of the electromagnetic waves and of sound, which propagate in a definite symmetry direction, become zero when approaching a critical point. A rather large class of such phase transitions of the second kind, the order parameter of which is transformed simultaneously as the electric and magnetic polarisations or as both polarisation and the components of the deformation tensor, is studied. In the latter case the bound electromagnetic and sound waves stand for softening hydrodynamic modes. It is shown that at such 'combined' phase transitions qualitatively new effects arise in the behaviour of the softening hydrodynamic waves. The strongest anomalies are observed at magnetoelectric phase transitions.

Magnetoelectric effect and low temperature transition of PbFe0.5Nb0.5O3 single crystal

Abstract Magnetic and magnetoelectric measurements were made on a single crystal of ferroelectric PbFe0.5Nb0.5O3. It was revealed that PbFe0.5Nb0.5O3 has a weak ferromagnetic moment and linear magnetoelectric effect below a transition point T m c of approximately 9 K. The magnetic class below and above T m c was determined to be m' and ml', respectively. A possible model of the transition which explains this symmetry change is proposed.

Linear and quadratic magnetoelectric effect in boracite Co3B7013Br

Abstract Linear and quadratic magnetoelectric (ME) effects have been simultaneously detected in Co-Br boracite(Co3B7O13 Br) ME coefficients a23, a32 and B311. have been measured versus temperature. The results are consistent with the symmetry m′m2′ down to 4 K.

Dipolar waves in ferromagnetic systems with linear magnetoelectric properties

The influence of the linear magnetoelectric effect on bulk and surface (Damon-Eshbach) magnetostatic modes is considered for uniaxial ferromagnetic systems. This effect is shown to give rise to a broadening of the band allowed for propagation of the bulk waves. It is shown that the linear magnetoelectric properties result also in change of the frequency and angle regions which are allowed for surface waves.

Polaritons in ferromagnets with linear magneto-electric properties

Dynamical electric, magnetic and magneto-electric susceptibilities of axial ferromagnets exhibiting linear magneto-electric effects are studied in the case of spin waves coupled resonantly with optical phonons. Both ionic and electronic contributions to the susceptibilities are calculated. Employing classical Maxwell equations these susceptibilities have been used to find the dispersion relations of mixed magnon-phonon polaritons. The coupling between spin waves and phonons is shown to imply the irreversible character of wave propagation.

Magnetic polaritons in electronic-type ferromagnetoelectrics

The influence of the linear magnetoelectric effect on the magnetic polariton spectrum is considered in simple uniaxial ferromagnets. Dispersion equations for the magnetic polaritons are obtained by two methods. (i) From the condition for the existence of a non-zero solution of Maxwell's equations in which the internal dynamics of the system is included through electric, magnetic and magnetoelectric susceptibilities. (ii) From the analysis of the linear response of the system to an external magnetic field. The linear magnetoelectric effect is shown to lead, in general, to the irreversible character of polariton propagation, as well as to broadening (by about ten per cent) of the band forbidden for propagation of right-hand circularly polarised waves. The light scattering cross section on the magnetic polaritons is also discussed.

Dielectric and magnetoelectric properties of Bi2Bi4Fe2Ti3O18

The temperature dependence of the permittivity of BTF 2 ceramic has been measured. Three singularities have been observed, of which two at 898°C and 752°C correspond to displacive phase transitions leading to the sequence paraelectricm ↔ ferroelectric ↔ ferroelectric. The third feature, around 400°C, is tentatively assigned to the onset of magnetic ordering. Poling and magnetizing have the same permanent effect on the permittivity. A large linear magnetoelectric effect has been detected at room temperature, even in non magnetized samples. We conclude that BTF 2 is femomagnetoelectric with a magnetic point group m'm2'.

Linear and bilinear magnetoelectric effects in magnetically biased magnetite (Fe3O4)

Linear and bilinear magnetoelectric effects in magnetically biased magnetite (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Fe</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>)

Structural Phase Transitions in BaMF4 Antiferromagnets

AbstractTwo‐dimensional antiferromagnets BaMF4 where M = Mn, Fe, Co, or Ni exhibit a linear magnetoelectric effect. It is known that this effect is forbidden by symmetry unless a structural phase transition takes place in these compounds. Recently such phase transition was discovered in BaMnF4. Using the group theory and the knowledge of magnetic structure and magnetoelectric effect, essential characteristics of this structural phase transition, i.e., symmetry of the phase transition parameter, possible symmetry changes, anomalous dielectric and elastic constants, are determined.

Raman study of soft zone-boundary phonons and antiferrodistortive phase transition in BaMnF 4

We have discovered that the 255 K phase transition in BaMnF 4 involves a doubling of the primitive unit cell, that the phase transition is second-order (or very nearly so), and that it is characterized by a soft or unstable optical phonon at the Brillouin zone boundary of the orthorhombic C 12 2 V phase. The soft mode has a low-temperature energy of about 40 cm -1; it remains underdamped below 210 K, but is heavily overdamped near T c = 255 K. The doubling of the chemical unit cell below 255 K, previously unknown, permits the possibility (below T N = 26 K) of a linear magneto-electric effect in this class of materials, which has been the subject of some controversy.

Magnetoelectric Evidence for the Attainability of Time-Reversed Antiferromagnetic Configurations by Metamagnetic Transitions in DyPO4

The observation of the linear magnetoelectric (ME) effect in the metamagnetic antiferromagnet DyPO4 is reported. Also presented is a new quantum-mechanical mechanism of the ME effect which is, in contrast to previous mechanisms, specifically designed for DyPO4 and other magnetically ordered materials containing rare-earth ions. These results are shown to provide evidence for the feasibility of isothermal forced switching between time-reversed antiferromagnetic configurations by means of metamagnetic transitions. The optically deduced1 magnetic structure of DyPO4 is shown to require 4′/m′mm′ symmetry so that the ME susceptibility tensor has only one independent component α. It is found experimentally that |α| decreases monotonically with increasing temperature from roughly 2×10−4 Gaussian units at 1.5°K to zero at the Néel temperature of about 3.38°K. Application of a sufficiently large, static, magnetic field Hc along the antiferromagnetic axis (which is the crystalline c axis) causes not only a metamagnetic increase of the magnetization to its saturation value but (due to the transition to 4/mm′m′ symmetry) also an even more abrupt decrease of |α| to zero. The relative sign of α is determined, rather remarkably, solely by the sign of that Hc which caused the most recent metamagnetic transition prior to the measurement of α. This new switching method is potentially applicable to an isothermal storage and retrieval of information in materials which are both metamagnetic and magnetoelectric. All the above-mentioned work is described more fully elsewhere.2 Subsequently, appropriate ME annealing produced a larger value of |α| (≈1×10−3 Gaussian units) so that a non-ME-annealed DyPO4 crystal is not a single antiferromagnetic domain.

Magnetoelectric Evidence for the Attainability of Time-Reversed Antiferromagnetic Configurations by Metamagnetic Transitions in DyPO4

The experimental observation and an atomic mechanism of the linear magnetoelectric effect in DyP${\mathrm{O}}_{4}$ are reported and shown to provide evidence for the feasibility of isothermal forced switching between time-reversed (i.e., moment-reversed) antiferromagnetic configurations by means of metamagnetic transitions.

Linear Magnetoelectric Effects in Gallium Iron Oxide at Low Temperatures and in High Magnetic Fields

The magnetoelectric (ME) experiments1 on monocrystals of the piezoelectric ferromagnet Ga2−xFexO3 (x≈1) are extended to low temperatures (T≳4.2°K) and high magnetic fields (H≲150 kOe). It is found that: (1) The linear ME susceptibility α, which has a maximum1 at some T=Tmax, exhibits a minimum at some T=Tmin satisfying 4.2°K&amp;lt;Tmin&amp;lt;Tmax; moreover, the value of α is almost as large at 4.2°K as at Tmax. (2) If H is sufficiently large so that the direction u=Ms/Ms of the spontaneous magnetization Ms is parallel to H, then the electric polarization Pb induced at constant T along the polar crystal axis (b axis) is given by Pb=ηu+αuH+βu H2, where βuH≪α for H≲150 kOe. The ``spontaneous ME polarization'' ηu is strongly u-dependent. However, αu is approximately u-independent, its u-dependent part being only about 2% at 145.2°K and 77.4°K and about 10% at 4.2°K. Since the (approximate) u-independence of αu indicates that the details of the spin canting assumed in the previous theoretical model1 are inapplicable to Ga2−xFexO3, a new thermodynamic description of ME effects is presented which does apply to ferromagnets having u ∥ H and satisfactorily describes (2). Finally, a thermodynamic calculation is given of a new mechanism of linear ME effects, namely the combined action of forced magnetostriction (i.e., a kind of piezomagnetism) and piezoelectricity. This mechanism, which ultimately arises from the strain dependence of exchange (and other) interactions, must exist in all unclamped piezoelectric ferromagnets having u ∥ H, and is probably significant in Ga2−xFexO3 near the Curie point.