Multiferroics

Besides scientific interest in their physical properties, multiferroics have potential for applications as actuators, switches, magnetic field sensors and new types of electronic memory devices.

This work explained the origin of the contraindication between magnetism and ferroelectricity and proposed practical routes to circumvent it, and is widely credited with starting the modern explosion of interest in multiferroic materials.

[10] The graph to the right shows in red the number of papers on multiferroics from a Web of Science search until 2008; the exponential increase continues today.

The first known mention of magnetoelectricity is in the 1959 Edition of Landau & Lifshitz' Electrodynamics of Continuous Media which has the following comment at the end of the section on piezoelectricity: "Let us point out two more phenomena, which, in principle, could exist.

For example, in the prototypical ferroelectric barium titanate, BaTiO3, the parent phase is the ideal cubic ABO3 perovskite structure, with the B-site Ti4+ ion at the center of its oxygen coordination octahedron and no electric polarisation.

[5] This "d0-ness" requirement[5] is a clear obstacle for the formation of multiferroics, since the magnetism in most transition-metal oxides arises from the presence of partially filled transition metal d shells.

[14] In lone-pair-active multiferroics,[5] the ferroelectric displacement is driven by the A-site cation, and the magnetism arises from a partially filled d shell on the B site.

Such rotational distortions occur in many transition-metal oxides; in the perovskites for example they are common when the A-site cation is small, so that the oxygen octahedra collapse around it.

[19] Since the distortion is not driven by a hybridisation between the d-site cation and the anions, it is compatible with the existence of magnetism on the B site, thus allowing for multiferroic behavior.

[20] A second example is provided by the family of hexagonal rare earth manganites (h-RMnO3 with R=Ho-Lu, Y), which have a structural phase transition at around 1300 K consisting primarily of a tilting of the MnO5 bipyramids.

Larger polarizations occur when the non-centrosymmetric magnetic ordering is caused by the stronger superexchange interaction, such as in orthorhombic HoMnO3 and related materials.

[30] It remains a challenge to develop good single-phase multiferroics with large magnetization and polarization and strong coupling between them at room temperature.

[32] Recently an interesting layer-by-layer growth of an atomic-scale multiferroic composite has been demonstrated, consisting of individual layers of ferroelectric and antiferromagnetic LuFeO3 alternating with ferrimagnetic but non-polar LuFe2O4 in a superlattice.

[35] The mechanism of strong ME coupling is via magnetic exchange interaction between CFO and BFO across the core-shell interface, which results in an exceptionally high Neel-Temperature of 670 K of the BF-BKT phase.

There have been reports of large magnetoelectric coupling at room-temperature in type-I multiferroics such as in the "diluted" magnetic perovskite (PbZr0.53Ti0.47O3)0.6–(PbFe1/2Ta1/2O3)0.4 (PZTFT) in certain Aurivillius phases.

The prototypical example is BiFeO3 (TC=1100 K, TN=643 K), with the ferroelectricity driven by the stereochemically active lone pair of the Bi3+ ion and the magnetic ordering caused by the usual superexchange mechanism.

The prototypical example is TbMnO3,[42] in which a non-centrosymmetric magnetic spiral accompanied by a ferroelectric polarization sets in at 28 K. Since the same transition causes both effects they are by construction strongly coupled.

The combination of symmetry breakings in multiferroics can lead to coupling between the order parameters, so that one ferroic property can be manipulated with the conjugate field of the other.

[50] This quantity is important because it reflects the amount of time-reversal (and hence CP) symmetry breaking in the universe, which imposes severe constraints on theories of elementary particle physics.

[52] In particular, a proposed mechanism for cosmic-string formation has been verified,[52] and aspects of cosmic string evolution are being explored through observation of their multiferroic domain intersection analogues.

A number of other unexpected applications have been identified in the last few years, mostly in multiferroic bismuth ferrite, that do not seem to be directly related to the coupled magnetism and ferroelectricity.

[55] It is likely that the combination of ferroelectric polarisation, with the small band gap composed partially of transition-metal d states are responsible for these favourable properties.

Various films have been researched, and there is also a new approach to effectively adjust the band gap of the double perovskite multilayer oxide by engineering the cation order for Bi2FeCrO6.

Current research in this field is motivated both by the promise of new types of application reliant on the coupled nature of the dynamics, and the search for new physics lying at the heart of the fundamental understanding of the elementary MF excitations.

An increasing number of studies of MF dynamics are concerned with the coupling between electric and magnetic order parameters in the magnetoelectric multiferroics.

At the heart of the proposed technologies based on magnetoelectric coupling are switching processes, which describe the manipulation of the material's macroscopic magnetic properties with electric field and vice versa.

Ultrafast processes operating at picosecond, femtosecond, and even attosecond scale are both driven by, and studied using, optical methods that are at the front line of modern science.

The physics underpinning the observations at these short time scales is governed by non-equilibrium dynamics, and usually makes use of resonant processes.

[63] These are promising demonstrations of how the switching of electric and magnetic properties in multiferroics, mediated by the mixed character of the magnetoelectric dynamics, may lead to ultrafast data processing, communication and quantum computing devices.

Current research into MF dynamics aims to address various open questions; the practical realisation and demonstration of ultra-high speed domain switching, the development of further new applications based on tunable dynamics, e.g. frequency dependence of dielectric properties, the fundamental understanding of the mixed character of the excitations (e.g. in the ME case, mixed phonon-magnon modes – 'electromagnons'), and the potential discovery of new physics associated with the MF coupling.