Magneto-electric multiferroicity in quantum magnetic insulators

Michel Kenzelmann

ETH Zurich & Paul Scherrer Institute

I will talk about magnetic insulators in which the symmetry of the spin
interactions leads to strong fluctuations and qualitatively new ground
states. Of particular interest are frustrated quantum magnets in which
long-range magnetic order is impeded because of competing interactions. The
proximity of such systems to quantum critical points can lead to strong
cross-coupling between magnetic order and the nuclear lattice. Case in point
is a new class of multiferroic materials in which the magnetic and
ferroelectric order parameters are directly coupled, and a magnetic field
can suppress or switch the electric polarization [1]. Our neutron
measurements reveal that ferroelectricity is induced by magnetic order and
emerges only if the magnetic structure creates a polar axis [2-5]. The spin
dynamics and the field-temperature phase diagram of the ordered phases
provide evidence that competing ground states are essential but not
sufficient for ferroelectricity. The origin of the magneto-electric coupling
is not understood at present, but it may arise from strain derivatives of
the isotropic exchange interactions or anisotropic exchange couplings such
as Dzyaloshinskii-Moriya interactions.

## Magneto-electric multiferroicity in quantum magnetic insulators

## Michel Kenzelmann

## ETH Zurich & Paul Scherrer Institute

I will talk about magnetic insulators in which the symmetry of the spin

interactions leads to strong fluctuations and qualitatively new ground

states. Of particular interest are frustrated quantum magnets in which

long-range magnetic order is impeded because of competing interactions. The

proximity of such systems to quantum critical points can lead to strong

cross-coupling between magnetic order and the nuclear lattice. Case in point

is a new class of multiferroic materials in which the magnetic and

ferroelectric order parameters are directly coupled, and a magnetic field

can suppress or switch the electric polarization [1]. Our neutron

measurements reveal that ferroelectricity is induced by magnetic order and

emerges only if the magnetic structure creates a polar axis [2-5]. The spin

dynamics and the field-temperature phase diagram of the ordered phases

provide evidence that competing ground states are essential but not

sufficient for ferroelectricity. The origin of the magneto-electric coupling

is not understood at present, but it may arise from strain derivatives of

the isotropic exchange interactions or anisotropic exchange couplings such

as Dzyaloshinskii-Moriya interactions.

## References

[1] T. Kimura et al, Nature 426, 55 (2003).[2] G. Lawes et al, Phys. Rev. Lett. 95, 087205 (2005).

[3] M. Kenzelmann et al, Phys. Rev. Lett. 95, 087206 (2005).

[4] M. Kenzelmann et al, Phys. Rev. B 74, 014429 (2006).

[5] M. Kenzelmann et al, Phys. Rev. Lett. 98, 267205 (2007).