Tuesday 9 July 2013, Pentland Room, 14:30-16:30
Magnetic oxides
Magnetic soft modes in the distorted triangular antiferromagnet α-CaCr2O4
S Tóth1, B Lake2, K Hradil3, T Guidi4, K Rule5, M Stone6, A Islam2 and C Rüegg1
1
Paul Scherrer Institut, Switzerland, 2Helmholtz-Zentrum Berlin, Germany, 3Technische Universität Wien, Austria,
ISIS, UK, 5Australian Nuclear Science and Technology Organisation, Australia, 6Oak Ridge National Laboratory, USA
4
α-CaCr2O4 is a spin-3/2, uniquely distorted triangular lattice Heisenberg antiferromagnet. It develops long-range
magnetic order below TN=42 K where the angles between nearest neighbor spins are 120° on the triangular planes.
The simplicity of this symmetric magnetic structure masks the complex pattern of exchange interactions [1]. The
magnetic excitation spectrum has been measured for the first time using inelastic neutron scattering on powder and
single crystal samples. It reveals unusual low energy modes forming roton-like minima, which can be explained by
linear spin-wave theory assuming a complex set of 1st and 2nd neighbor exchange interactions [2-3]. The fitted
direct exchange interactions correlate well with the Cr3+-Cr3+ distances and are in agreement with other chromium
delafossite compounds. The mode softening is due to the instability of the 120° structure. This is supported by the
calculated magnetic phase diagram, where the fitted exchange parameters put α-CaCr2O4 close to the phase
boundary of the 120° structure in exchange parameter space.
[1]
[2]
[3]
S. Toth, B. Lake et. al., Phys. Rev. B, 84, 054452 (2011)
S. Toth, B. Lake et. al., Phys. Rev. Lett., 109, 127203 (2012)
D. Wulferding, K. Choi et al., J. Phys.-Condens. Mat., 24, 435604 (2012)
Neutron scattering studies of electromagnon in multiferroics Ba2CoGe2O7
T Masuda1, M Soda1, M Matsumoto2, S Gvasaliya3, M Mansson4 and A Zheludev3
1
The university of Tokyo, Japan, 2Shizuoka university, Japan, 3ETH Zurich, Switzerland, 4Paul Scherrer Institute,
Switzerland
Ba2CoGe2O7 having the noncentrosymmetric crystal structure shows a staggered antiferromagnetic structure in the
(001) plane below TN=6.7 K. Below TN, a ferroelectric polarization is observed even at a magnetic field H=0, and
largely enhanced under H. Murakawa and co-workers have shown that the ferroelectricity is induced by the spindependent d-p hybridization mechanism. Furthermore, the 4 meV excitation, which is an electric-active mode
through the coupling between spin and electric-dipole, was observed in the electromagnetic wave absorption. This
electromagnon was also investigated theoretically. In the present studies, inelastic neutron scattering
measurements were carried out to clarify this novel excitation in Ba2CoGe2O7. Non-trivial magnetic excitations, which
can be explained by the multiboson spin wave theory, were observed at low temperature. In the multiboson spin
wave theory, this magnetic excitation is induced by the longitudinal fluctuation, which connects with fluctuation of
the electric polarization along the crystallographic c-axis in considering the d-p hybridization model.
ICNS 2013 International Conference on Neutron Scattering
Giant magnetoelastic effect in a hexagonal perovskite containing unstable Bi(IV) ions
C Ling1, B Kennedy1 and M Avdeev2
1
The University of Sydney, Australia, 2Australian Nuclear Science and Technology Organisation, Australia
The 4d and 5d transition metals have much more diffuse valence orbitals than their 3d (first-row) counterparts.
Quantum cooperative phenomena due to changes in the way these orbitals overlap and interact, such as
magnetoelasticity, are therefore rare in 4d and 5d compounds. We recently discovered an example in the 6Hperovskite Ba3BiIr2O9, which contains 5d Ir4+ (S = 1/2) in face-sharing Ir2O9 bi-octahedra. This compound exhibits a
giant magnetoelastic effect - the largest of any known in a 5d compound - coincident with the opening a spin-gap
at T* = 74 K. [1] In this paper we will present new QENS data that clearly show the spin-gap excitation for the first
time.
The first-order transition observed by NPD is characterised by a remarkable 4% increase in Ir-Ir distance on cooling,
driving a 1% negative thermal volume expansion. The transition is marked by a dramatic change in the interactions
among Ir 5d orbitals, and represents a crossover between two competing, ground states: one that optimises direct
Ir-Ir bonding (at high temperature); and one that optimises Ir-O-Ir magnetic superexchange (at low temperature).
A crucial feature is the presence of bismuth in a highly unusual and unstable 4+ valence state, which appears to act
as a charge reservoir facilitating the first-order transition. Nominally "Bi4+" oxides invariably disproportionate into
Bi3+/Bi5+, but in this case the disproportionation is frustrated by the hexagonal disposition of BiO6 octahedra. In this
paper we will present new evidence for Bi4+ from a high-pressure NPD experiment that shows the state collapsing in
another first-order transition at 5.5 GPa.
[1]
W. Miiller et al., J. Am. Chem. Soc. 134, 3265 (2012)
Magnetism, spin waves, and orbitons of cobalt oxide
R Cowley1, W Buyers2, C Stock3, Z Yamani2, D Prabhakaran1, J Taylor4, R Ewings4 and C Foot4
1
University of Oxford, UK, 2National Research Council of Canada, Canada, 3University of Edinburgh, UK, , 4ISIS, UK
Cobalt oxide is a cubic antiferromagnet with a Neel temperature of 290K. The interest and difficulty is that the low
energy states have both orbital and spin components while there are as many as 12 different domains for the low
temperature structure. Furthermore the magnetic structure at low temperatures is uncertain despite many different
experiments. In order to determine the magnetic excitations we have made neutron scattering measurements in
three different ways. High energy electronic excitations have been detected up to 3eV with the MAPS spectrometer
and show that the electronic excitations are insulating and consistent with calculations of the electronic structure
including the partially covalent bonding of the Co and O ions. The low energy modes with coupled spin and orbital
components have been measured with a triple axis, MARI and IRIS spectrometers using a powder sample of MgO
doped with 3% of CoO. These measurements enable us to deduce the spin-orbit interaction and the dominant
exchange interaction between the two next nearest neighbour Co ions. Surprisingly the measurements also showed
that in the dilute material there were many low energy modes, where low energy is in the range of 0.3 to 6 meV.
These modes are absent in both CoO and MgO and we suggest a possible origin for these modes. The third set of
measurements on a large single crystal of CoO were made with a triple axis spectrometer and with the MAPS and
MERLIN time-of-flight spectrometers. The results are consistent with one another and show that the magnetic gap is
20 mev and that the low energy excitations have a maximum energy of about 70meV. We shall compare these
results with our calculations.
ICNS 2013 International Conference on Neutron Scattering
(invited) Crystal and magnetic structures of novel A-site-ordered perovskites
Y Shimakawa
Kyoto University, Japan
A-site ordered perovskites with a chemical formula AA’3B4O12 contain transition-metal ions at both A’ and B sites,
and thus A’-A’ and/or A’-B interactions in addition to B-B interaction, which is usually seen in simple perovskites,
play important roles in giving rise to a large variety of physical and chemical properties. Precise analysis of the
crystal and magnetic structures by neutron diffraction gives us crucial information to reveal the unusual properties.
CaCu3(Ge,Ti,Sn)4O12 shows very unusual A’-site magnetism, in which either a ferromagnetic or G-type
antiferromagnetic structure of A’-site Cu2+ (S = 1/2) spins can be stabilized within a cubic S = 1/2 spin sublattice.
The ferromagnetic Cu–Cu direct-exchange interaction and the antiferromagnetic Cu–O–Ti–O–Cu superexchange
interaction are mutually competitive in the antiferromagnetic CaCu3Ti4O12. LaMn3V4O12, on the other hands,
contains magnetic Mn2+ ions at the A’ site and shows an antiferromagnetic transition at 44 K. The magnetic
structure of the compound consists of high-spin Mn2+ (S = 5/2) sublattices, in which the nearest neighboring Mn2+
spins align with each other with an angle of 120 degrees. The electrons of V at the B site are delocalized and do not
apparently contribute to the magnetic behavior. The results of crystal and magnetic structures of some other A-site
ordered perovskites will also be presented.
Bunched and butterfly modulations of the Fe-langasite magnetic structure as probed by neutron polarimetry
L Chaix1, V Simonet2, S Petit3, S de Brion2, E Ressouche4, L-P Regnault4, J Ollivier1, P Lejay2 and R Ballou2
1
Institut Laue Langevin, France, 2Institut Néel, CNRS & Université Joseph Fourier, France, 3Laboratoire Léon Brillouin,
France, 4Institut de Nanosciences et Cryogénie, France
The langasite Ba3NbFe3Si2O14 has recently attracted a lot of attention due to its unique chiral magnetic ground state
(Marty et al., Phys. Rev. Lett. 2008) and excitations (Loire et al., Phys. Rev. Lett. 2011) and its potential
magnetoelectric properties (Zhou et al. Chem. Mat. 2009, Marty et al. Phys. Rev. B 2010). The magnetic order of
the Fe3+ magnetic moments is made of a 120° arrangement in triangles, helically propagated in the perpendicular
direction. We report here new experimental evidence showing that this picture is incomplete. From single crystal
neutron diffraction measurements, with and without longitudinal polarization analysis, higher order and forbidden
weak magnetic and structural satellites have been evidenced. Moreover, a hybridization of the magnons branches
leading to extinction at the magnons crossing is observed by inelastic neutron scattering. I will show that these
features can be explained by the presence of single-ion anisotropy combined with the Dzyaloshinskii-Moryia
interaction, and by the loss of the 3-fold axis arising from a structural distortion. This results in a bunched
modulation of the helical order and an additional butterfly magnetization contribution perpendicular to the helix
plane. Moreover, the resulting symmetry lowering is a key parameter to understand the new kind of magnetoelectric
excitations evidenced in this material as well as its multiferroic properties (Chaix et al. submitted to Phys. Rev.
Lett.).
ICNS 2013 International Conference on Neutron Scattering