#
2009 APS March Meeting

## Volume 54, Number 1

##
Monday–Friday, March 16–20, 2009;
Pittsburgh, Pennsylvania

### Session J30: Focus Session: Multiferroic Manganites

11:15 AM–1:51 PM,
Tuesday, March 17, 2009

Room: 334

Sponsoring
Units:
DMP GMAG

Chair: Despina Louca, University of Virginia

Abstract ID: BAPS.2009.MAR.J30.1

### Abstract: J30.00001 : Order Parameters and Phase Diagram of Multiferroic RMn2O5

11:15 AM–11:51 AM

Preview Abstract
Abstract

####
Author:

A. Brooks Harris

(University of Pennsylvania)

\def\rhov{{\mbox{\boldmath{$\rho$}}}}
\def\tauv{{\mbox{\boldmath{$\tau$}}}}
\def\Lambdav{{\mbox{\boldmath{$\Lambda$}}}}
\def\sigmav{{\mbox{\boldmath{$\sigma$}}}}
\def\xiv{{\mbox{\boldmath{$\xi$}}}}
\def\chiv{{\mbox{\boldmath{$\chi$}}}}
\def\oh{{\scriptsize 1 \over \scriptsize 2}}
\def\ot{{\scriptsize 1 \over \scriptsize 3}}
\def\of{{\scriptsize 1 \over \scriptsize 4}}
\def\tf{{\scriptsize 3 \over \scriptsize 4}}
Recently there has been great interest in systems which display
phase transitions at which incommensurate magnetic order
and a spontaneous polarization develop simultaneously.
Perhaps the most puzzling and seemingly complicated behavior
occurs in the series of compounds RMn$_2$O$_5$, where
R=Y, Ho, Er, Tb, Tm, and Dy. (For references to experimental
data,
see [1].) The sequence of magnetoelectric
phases of the type I systems R=Tb, Ho, and Dy is slightly
different from that of the type II systems R= Y, Tm, and Er.
At about 45K both types develop essentially collinear
modulated magnetic order into a ``high-temperature ordered"
(HTO)
phase with a wave vector ${\bf q} = (1/2-\delta , 0, 1/4 +
\epsilon)$
where $\delta$ and $|\epsilon|$ are of order 0.01 and the
spontaneous polarization is zero. There is a lower-temperature
phase transition to a ferroelectric phase in which transverse
magnetic
order appears and produces a magnetic spiral with
$\delta=\epsilon=0$.
In type I systems, this transition occurs
directly from the HTO phase, whereas for type II
systems, there is an intervening ferroelectric phase in which
$\epsilon=0$, but $\delta$ remains nonzero.
%At low ($<10$K) temperature the classification into types I
and II
%breaks down and each system requires its own specific
description.
I will discuss a Landau free energy[1]
which allows both type I and type II sequences of phase
transitions.
This theory is couched in terms of the uniform
polarization vector ${\bf P}$ and two complex-valued
magnetic order parameters $\sigma_1({\bf q})$ and $\sigma_2
({\bf q})$
whose symmetry follows from the magnetic structure analyses.[2]
The magnetoelectric coupling and the competition between
commensurate and incommensurate phases are analyzed.
\\[4pt]
[1] A. B. Harris, A. Aharony, and O. Entin-Wohlman, Phys. Rev.
Lett.
{\bf 100}, 217202 (2008) and J. Phys. Condens. Mat. {\bf 20},
434202 (2008).
\\[0pt]
[2] A. B. Harris, Phys. Rev. {\bf 76}, 054447 (2007); A. B.
Harris, M. Kenzelmann, A. Aharony, and O. Entin-Wohlman, Phys.
Rev. B {\bf 78}, 014407 (2008).

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.MAR.J30.1