Download Abstract_Kee Hoon Kim

Magnetic Field Induced Classical and Quantum Critical Points in Multiferroics
Kee Hoon Kim
Center for Novel States of Complex Materials Research, Department of Physics and
Astronomy, Seoul National University, Seoul 151-747, Korea
(Tel: +82-2-880-9068, Fax: +82-2-888-0769, [email protected])
Multiferroics, wherein several ferroic orders such as ferroelectricity and magnetism are
cross-coupled, provide a novel possibility of employing high magnetic fields to tune the
frustrated spin system into either classical or quantum critical regime and thus
investigating magnetic and/or electric critical phenomena. In this talk, two multiferroic
materials BiMn2O5 and Ba2CoGe2O7 are discussed in this context. In the former, wherein
a spontaneous ferroelectric polarization P develops due to the exchange striction of the
frustrated Mn3+-Mn4+ spin network, magnetic field H ~18 T produces a reversal of P. We
show this concomitantly occurs with a crossover of spin-configuration into the spinflopped phase at temperatures above ~1 K and at a lower temperature, a critical end point
is formed [1]. The existence of such a critical end point proximity to zero temperature
emerges as divergent dielectric constant and magnetic susceptibility with a characteristic
power law, thus constituting experimental evidences of a multiferroic critical end point.
In the latter Ba2CoGe2O7, wherein a new p-d hybridization model can generate P [2], a
spontaneous antiferromagnetic order of Co2+ (3d7) spins sitting in the center of tetrahedra
network forming a quasi-2D square lattice, is suppressed to T=0 by application of high H
~ 39 T. Here, we present convincing experimental evidences that such high magnetic
fields of 39 T induces a second order magnetic transition from a canted antiferromagnetic
to a collinear state at zero temperature. Nearby the transition, we uncover that the
dielectric properties show a unique power-law behavior as well as a characteristic phase
boundary expected near a magnetic quantum critical point. We show by the free energy
description based on the p-d hybridization model that the observed polarization change
under H can be explained and the physical condition of a material is uniquely formed in
such a way that dielectric constant can be directly proportional to antiferromagnetic
susceptibility near the quantum critical point. Based on the power-law behavior, we also
infer the universality class of this quantum critical point. Our findings point to a new
avenue of researches for investigating quantum criticality of magnetic order that is
strongly coupled to the lattice degrees of freedom.
[1] J. W. Kim et al., Proc. Nat’l. Acad. Sci. 106, 11573 (2009).
[2] H. Murakawa et al., Phys. Rev. Lett. 105, 137202 (2010).
[3] J. W. Kim et al., Nature Comm. 5, 4419 (2014).
In close collaboration with J. W. Kim, S-W. Cheong, J. H. Han, C. Batista