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Interesting papers from colleagues around the world, recently appeared on arXiv.org

Field driven magnetostructural transitions in GeCo2O4

X. Fabrèges, E. Ressouche, F. Duc, S. de Brion, M. Amara, C. Detlefs, L. Paolasini, E. Suard, L.-P. Regnault, B. Canals, P. Strobel, V. Simonet
(Submitted on 28 Sep 2016)

arXiv:1609.08805 [cond-mat.str-el]

Abstract

In the spinel compound GeCo_2O_4, the Co^{2+} pyrochlore sublattice
presents remarkable magnetic field-induced behaviors that we unveil through
neutron and X-ray single-crystal diffraction. The N\'eel ordered magnetic phase
is entered through a structural lowering of the cubic symmetry. In this phase,
when a magnetic field is applied along a 2-fold cubic direction, a spin-flop
transition of one fourth of the magnetic moments releases the magnetic
frustration and triggers magnetostructural effects. At high field, these
ultimately lead to an unusual spin reorientation associated to structural
changes.

Emergent ultrafast phenomena in correlated oxides and heterostructures

M. Gandolfi, L. Celardo, F. Borgonovi, G. Ferrini, A. Avella, F. Banfi, C. Giannetti
(Submitted on 23 Sep 2016)

arXiv:1609.07394 [cond-mat.str-el]

Abstract

The possibility of investigating the dynamics of solids on timescales faster
than the thermalization of the internal degrees of freedom has disclosed novel
non-equilibrium phenomena that have no counterpart at equilibrium. Transition
metal oxides (TMOs) provide an interesting playground in which the correlations
among the charges in the metal d-orbitals give rise to a wealth of intriguing
electronic and thermodynamic properties involving the spin, charge, lattice and
orbital orders. Furthermore, the physical properties of TMOs can be engineered
at the atomic level, thus providing the platform to investigate the transport
phenomena on timescales of the order of the intrinsic decoherence time of the
charge excitations.
Here, we review and discuss three paradigmatic examples of transient emerging
properties that are expected to open new fields of research: i) the creation of
non-thermal magnetic states in spin-orbit Mott insulators; ii) the possible
exploitation of quantum paths for the transport and collection of charge
excitations in TMO-based few-monolayers devices; iii) the transient wave-like
behavior of the temperature field in strongly anisotropic TMOs.

Electromagnon resonance in a collinear spin state of a polar antiferromagnet Fe2Mo3O8

T. Kurumaji, Y. Takahashi, J. Fujioka, R. Masuda, H. Shishikura, S.
Ishiwata, and Y. Tokura
(Submitted on 23 Sep 2016)

arXiv:1609.07230 [cond-mat.str-el]

Abstract

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magneto electric effect.

NanoSQUIDs: Basics & recent advances

M. J. Martinez-Perez and D. Koelle
(Submitted on 20 Sep 2016)

arXiv:1609.06182 [cond-mat.supr-con]

Abstract

Superconducting Quantum Interference Devices (SQUIDs) are one of the most
popular devices in superconducting electronics. They combine the Josephson
effect with the quantization of magnetic flux in superconductors. This gives
rise to one of the most beautiful manifestations of macroscopic quantum
coherence in the solid state. In addition, SQUIDs are extremely sensitive
sensors allowing to transduce magnetic flux into measurable electric signals.
As a consequence, any physical observable that can be converted into magnetic
flux, e.g., current, magnetization, magnetic field or position, becomes easily
accessible to SQUID sensors. In the late 1980's it became clear that downsizing
the dimensions of SQUIDs to the nanometric scale would encompass an enormous
increase of their sensitivity to localized tiny magnetic signals. Indeed,
nanoSQUIDs opened the way to the investigation of, e.g., individual magnetic
nanoparticles or surface magnetic states with unprecedented sensitivities. The
purpose of this review is to present a detailed survey of microscopic and
nanoscopic SQUID sensors. We will start by discussing the principle of
operation of SQUIDs, placing the emphasis on their application as
ultrasensitive detectors for small localized magnetic signals. We will continue
by reviewing a number of existing devices based on different kinds of Josephson
junctions and materials, focusing on their advantages and drawbacks. The last
sections are left for applications of nanoSQUIDs in the fields of scanning
SQUID microscopy and magnetic particle characterization, putting special stress
on the investigation of individual magnetic nano particles.

Superconductivity and spin-orbit coupling in non-centrosymmetric
materials: a review

M. Smidman, M. B. Salamon, H. Q. Yuan, and D. F. Agterberg
(Submitted on 19 Sep 2016)

arXiv:1609.05953 [cond-mat.supr-con]

Abstract

In non-centrosymmetric superconductors, where the crystal structure lacks a
centre of inversion, parity is no longer a good quantum number and an
electronic antisymmetric spin-orbit coupling (ASOC) is allowed to exist by
symmetry. If this ASOC is sufficiently large, it has profound consequences on
the superconducting state. For example, it generally leads to a superconducting
pairing state which is a mixture of spin-singlet and spin-triplet components.
The possibility of such novel pairing states, as well as the potential for
observing a variety of unusual behaviours, led to intensive theoretical and
experimental investigations. Here we review the experimental and theoretical
results for superconducting systems lacking inversion symmetry. Firstly we give
a conceptual overview of the key theoretical results. We then review the
experimental properties of both strongly and weakly correlated bulk materials,
as well as two dimensional systems. Here the focus is on evaluating the effect
of ASOC on the superconducting properties and the extent to which there is
evidence for singlet-triplet mixing. This is followed by a more detailed
overview of theoretical aspects of non-centrosymmetric superconductivity. This
includes the effects of the ASOC on the pairing symmetry and the
superconducting magnetic response, magneto-electric effects, superconducting
finite momentum pairing states, and the potential of non-centrosymmetric
superconductors to display topological superconductivity.

Multiferroic phases of the frustrated quantum spin chain compound
linarite

(Submitted on 20 Sep 2016)
K. Yu. Povarov, Y. Feng, A. Zheludev

arXiv:1609.06087 [cond-mat.str-el]

Abstract

The dielectric properties of the prototypical frustrated ferromagnetic spin
chain compound PbCuSO_4(OH)_2 known as linarite, are studied across its
strongly anisotropic magnetic phase diagram on single crystal samples. The
ferroelectric character of the principal low-field spin spiral phase is
confirmed. The measured polarization is fully consistent with the previously
proposed magnetic structure. Spontaneous polarization is also detected in two
other field-induced phases, but in some cases is incompatible with previously
suggested models for the spin arrangement.

Magnetic anisotropy and the phase diagram of chiral MnSb2O6

J. Werner, C. Koo, and R. Klingeler, A. N. Vasiliev, Y. A. Ovchenkov,
A. S. Polovkova, G. V. Raganyan, and E. A. Zvereva
(Submitted on 20 Sep 2016)
arXiv:1609.06105 [cond-mat.str-el]

Abstract

The magnetic phase diagram and low-energy magnon excitations of structurally
and magnetically chiral MnSb2O6 are reported. The specific heat and the static
magnetization are investigated in magnetic fields up to 9 and 30 T,
respectively, while the dynamicmagnetic properties are probed by X-band as well
as tunable high-frequency electron spin-resonance spectroscopy. Below TN = 11.5
K, we observe antiferromagnetic resonance modes which imply small but finite
planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are
well described by means of an easy-plane two-sublattice model with the
anisotropy field BA = 0.02 T. The exchange field BE = 13 T is obtained from the
saturation field derived from the pulsed-field magnetization. A crucial role of
the small anisotropy for the spin structure is reflected by competing
antiferromagnetic phases appearing, at T = 2 K, in small magnetic fields at BC1
\approx 0.5 T and BC2 = 0.9 T. We discuss the results in terms of spin
reorientation and of small magnetic fields favoring helical spin structure over
the cycloidal ground state which, at B = 0, is stabilized by the planar
anisotropy. Above TN, short-range magnetic correlations up to >=60 K and
magnetic entropy changes well above TN reflect the frustrated triangular
arrangement of Mn2+ ions in MnSb2O6.

Symmetry Analysis of Multiferroic Co_3TeO_6.

A. B. Harris

arxiv:1202.3631[cond-mat.str-el]

Abstract

A phenomenological explanation of the magnetoelectric behavior of Co_3TeO_6 is developed. We explain the second harmonic generation data and the magnetic field induced spontaneous polarization in the magnetically ordered phase below 20K.

Ferroelectricity induced by interatomic magnetic exchange interaction.

Xiangang Wan, Hang-Chen Ding, Sergey Y. Savrasov, Chun-Gang Duan

arxiv.org:1202.3381[cond-mat.str-el]

Abstract

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science[1-9]. However, the coexistence of magnetism and conventional ferroelectricity is physically unfavoured[10]. Recently several remedies have been proposed, e.g., improper ferroelectricity induced by specific magnetic[6] or charge orders[2]. Guiding by these theories, currently most research is focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature, consequently far from the practical application. Simple collinear magnets, which can have high magnetic transition temperature, have never been considered seriously as the candidates for multiferroics. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic (AFM) insulator MnO, can display a magnetically induced ferroelectricity under a biaxial strain.

Spin wave measurements over the full Brillouin zone of multiferroic BiFeO3

Jaehong Jeong, E. A. Goremychkin, T. Guidi, K. Nakajima, Gun Sang Jeon, Shin-Ae Kim, S. Furukawa, Yong Baek Kim, Seongsu Lee, V. Kiryukhin, S-W. Cheong, Je-Geun Park

arxiv:1201.0436[cond-mat.str-el]

Abstract

Using inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO3 single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO3 provides a fundamental ingredient for understanding of the novel magnetic properties of BiFeO3.

Collective magnetism at multiferroic vortex domain walls

Yanan Geng, Nara Lee, Y. J. Choi, S-W. Cheong, Weida Wu

arxiv:1201.0694[cond-mat.str-el]

Abstract

Topological defects have been playgrounds for many emergent phenomena in complex matter such as superfluids, liquid crystals, and early universe. Recently, vortex-like topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal manganites. Numerous vortices are found to form an intriguing self-organized network. Thus, it is imperative to find out the magnetic nature of these vortices. Using cryogenic magnetic force microscopy, we discovered unprecedented alternating net moments at domain walls around vortices that can correlate over the entire vortex network in hexagonal ErMnO3 The collective nature of domain wall magnetism originates from the uncompensated Er3+ moments and the correlated organization of the vortex network. Furthermore, our proposed model indicates a fascinating phenomenon of field-controllable spin chirality. Our results demonstrate a new route to achieving magnetoelectric coupling at domain walls in single-phase multiferroics, which may be harnessed for nanoscale multifunctional devices.

On the multiferroic and magnetoelectric nature of GaFeO3, AlFeO3 and related oxides

Ajmala Shireen, Rana Saha, Sharmila N. Shirodkar, Umesh V. Waghmare, A. Sundaresan, C. N. R. Rao

arxiv:1112.5848[cond-mat.str-el]

Abstract

GaFeO3, AlFeO3 and related oxides are known to be ferrimagnetic exhibiting magnetodielectric effect. There has been no evidence to date for the occurrence of ferroelectricity and hence multiferroicity in these oxides. We have investigated these oxides for possible ferroelectricity by carrying out pyroelectric measurements. These measurements establish the occurrence of ferroelectricity at low temperatures below the \'Neel temperature in these oxides. They also exhibit significant magnetoelectric effect. We have tried to understand the origin of ferroelectricity based on symmetry arguments and first-principles calculations.

Magnetic and electric properties in the distorted tetrahedral spin chain system Cu3Mo2O9

T. Hosaka, S. Hachiuma, H. Kuroe, T. Sekine, M. Hase, K. Oka, T. Ito, H. Eisaki, M. Fujisawa, S. Okubo, H. Ohta

arxiv:1112.5805[cond-mat.str-el]

Abstract

We study the multiferroic properties in the distorted tetrahedral quasi-one dimensional spin system Cu_3Mo_2O_9, in which the effects of the low dimensionality and the magnetic frustration are expected to appear simultaneously. We clarify that the antiferromagnetic order is formed together with ferroelectric properties at T_{\rm N}=7.9 K under zero magnetic field and obtain the magnetic-field-temperature phase diagram by measuring dielectric constant and spontaneous electric polarization. It is found that the antiferromagnetic phase possesses a spontaneous electric polarization parallel to the c axis when the magnetic field H is applied parallel to the a axis. On the other hand, there are three different ferroelectric phases in the antiferromagnetic phase for H parallel to the c axis.

Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5

N. Leo, D. Meier, R. V. Pisarev, N. Lee, S.-W. Cheong, M. Fiebig

arxiv.1112.1810[cond-mat.str-el]

Abstract

Three independent contributions to the magnetically induced spontaneous polarization of multiferroic TbMn2O5 are uniquely separated by optical second harmonic generation and an analysis in terms of Landau theory. Two of them are related to the magnetic Mn3+/4+ order and are independent of applied fields of up to 7 T. The third contribution is related to the long-range antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application of a magnetic field and mediates the change of sign of the spontaneous electric polarization in TbMn2O5. The close relationship between the rare-earth long-range order and the non-linear optical properties points to isotropic Tb-Tb exchange and oxygen spin polarization as mechanism for this rare-earth induced ferroelectricity.

Ordering process and ferroelectricity in a spinel derived from FeV2O4

Q. Zhang, K. Singh, F. Guillou, C. Simon, Y. Breard, V. Caignaert, V. Hardy

arxiv:1112.1738[cond-mat.str-el]

Abstract

The spinel FeV2O4 is known to exhibit peculiar physical properties, which is generally ascribed to the unusual presence of two cations showing a pronounced interplay between spin, orbital and lattice degrees of freedom (Fe2+ and V3+ on the tetrahedral and octahedral sites, respectively). The present work reports on an experimental re-investigation of this material based on a broad combination of techniques, including x-ray diffraction, energy dispersive and M\"ossbauer spectroscopies, as well as magnetization, heat capacity, dielectric and polarization measurements. Special attention was firstly paid to establish the exact cationic composition of the investigated samples, which was found to be Fe1.18V1.82O4. All the physical properties were found to point out a complex ordering process with a structural transition at TS = 138 K, followed by two successive magnetostructural transitions at TN1 = 111 K and TN2 = 56 K. This latter transition marking the appearance of electric polarization, magnetization data were analysed in details to discuss the nature of the magnetic state at T< TN2. An overall interpretation of the sequence of transitions was proposed, taking into account two spin couplings, as well as the Jahn-Teller effects and the mechanism of spin-orbit stabilization. Finally, the origin of ferroelectricity in Fe1.18V1.82O4 is discussed within the framework of an extended spin-current model [Phys. Rev. B 83, 174432, (2011).].

Temperature-dependent multi-k magnetic structure in multiferroic Co3TeO6

S. A. Ivanov, R. Tellgren, C. Ritter, P. Nordblad, R. Mathieu, G. Andre, N. V. Golubko, E. D. Politova, M. Weil

arxiv:1110.6776 [cond-mat.str-el]

Abstract

A complex magnetic order of the multiferroic compound Co3TeO6 has been revealed by neutron powder diffraction studies on ceramics and crushed single crystals. The compound adopts a monoclinic structure (s.g. C2/c) in the studied temperature range 2 K - 300 K but exhibits successive antiferromagnetic transitions at low temperature. Incommensurate antiferromagnetic order with the propagation vector k1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a second antiferromagnetic structure with k2 = (0, 0, 0) takes place at 21.1 K. Moreover, a transition to a commensurate antiferromagnetic structure with k3 = (0, 0.5, 0.25) occurs at 17.4 K. The magnetic structures have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Different coordinations of the Co2+ ions involved in the low-symmetry C2/c structure of Co3TeO6 render the exchange-interaction network very complex by itself. The observed magnetic phase transformations are interpreted as an evidence of competing magnetic interactions. The temperature dependent changes in the magnetic structure, derived from refinements of high-resolution neutron data, are discussed and possible mechanisms connected with the spin reorientations are described.

Transport, magnetic, and thermal properties of non-centrosymmetric Yb2Co12P7

J. J. Hamlin, M. Janoschek, R. E. Baumbach, B. D. White, M. B. Maple

arxiv:1110.6423 [cond-mat.str-el]

Abstract

We report magnetization and specific heat measurements down to 2 K and electrical resistivity down to millikelvin temperatures on polycrystalline samples of the non-centrosymmetric compound Yb2Co12P7. In addition to the previously reported ferromagnetic ordering of the cobalt sub-lattice at T_C = 136 K we find a magnetic transition below T_M = 5 K that is likely associated with ordering of the Yb ions. The broad nature of the specific heat anomaly suggests disordered magnetism and possible short range correlations well above T_M.

Mg substitution in CuCrO2 delafossite compounds

Emmanuel Guilmeau, Maria Poienar, Stefan Kremer, Sylvain Marinel, Sylvie Hébert, Raymond Frésard, Antoine Maignan

arxiv:1110.5730 [cond-mat.str-el]

Abstract

A detailed investigation of the series CuCr(1-x)MgxO2 (x = 0.0 - 0.05) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg2+ for Cr3+ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x ~ 0.02 - 0.03, indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr2O4 spinels as soon as x = 0.005. The thermopower is discussed in the temperature-independent correlation functions ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.

Weak antiferromagnetic ordering and pure magnetic reflections induced by Dzyaloshinskii-Moriya interaction in MnSi-type crystals

Vladimir E. Dmitrienko, Viacheslav A. Chizhikov

arxiv:1110.4968 [cond-mat.str-el]

Abstract

Symmetry analysis of the Dzyaloshinskii-Moriya (DM) interaction in MnSi-type cubic crystals demonstrates that the magnetic moments are tilted periodically, producing a weak antiferromagnetic pattern, when the helix is unwound by magnetic field. The tilt angles of four Mn sublattices are determined by a component of the DM vector perpendicular to that one responsible for helical spiraling; both components have been evaluated using a simple model. It is shown that the tilting should induce pure magnetic reflections 00l, l=2n+1 in neutron or x-ray magnetic scattering, and the structure factors of these "forbidden" reflections are calculated for arbitrary field orientations.

Origin of the large polarization in multiferroic YMnO_3 thin films revealed by soft and hard x-ray diffraction

H. Wadati, J. Okamoto, M. Garganourakis, V. Scagnoli, U. Staub, Y. Yamasaki, H. Nakao, Y. Murakami, M. Mochizuki, M. Nakamura, M. Kawasaki, Y. Tokura

arxiv:1109.2366 [cond-mat.str-el]

Abstract

We investigated the magnetic structure of an orthorhombic YMnO3 thin film by resonant soft x-ray and hard x-ray diffraction. We observed a temperature-dependent incommensurate magnetic reflection below 45 K and a commensurate lattice-distortion reflection below 35 K. These results demonstrate that the ground state is composed of coexisting E-type and cycloidal states. Their different ordering temperatures clarify the origin of the large polarization to be caused by the E-type antiferromagnetic states in the orthorhombic YMnO3 thin film.

Magnetic structure of CuCrO2: a single crystal neutron diffraction study

M. Frontzek, G. Ehlers, A. Podlesnyak, H. Cao, M. Matsuda, O. Zaharko, N. Aliouane, S. Barilo, S.V. Shiryaev

arXiv:1109.1747v1 [cond-mat.str-el]

Abstract

This paper presents results of a recent study of multiferroic CCO by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at TN1=24.2 K and TN2=23.6 K. The low temperature magnetic structure below TN2 is unambiguously determined to be a fully 3-dimensional proper screw. Between TN1 and TN2 antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the H,K) plane, whereas nearest neighbors along the L-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.

Hidden order and disorder in a quasi-two-dimensional frustrated hybrid magnet

K. Iida, S.-H. Lee, S.-W. Cheong
(Submitted on 2 Sep 2011)

arXiv:1109.0474v1 [cond-mat.str-el]

Abstract

In magnetic materials frustration arises when the pairwise interactions between the magnetic moments cannot be satisfied simultaneously, leading to exotic properties such as spin liquid or spin ice at low temperatures. Some cases, however, exhibit spin glass behaviors, and it has been theoretically and experimentally challenging to understand whether or not the spin glass state is an intrinsic ground state. Here we have studied a quasi-two dimensional triangular lattice of bi-pyramids with dominant antiferromagnetic nearest neighbor interactions. We show that the magnetic problem can be mapped onto a problem of two independent degrees of freedom, tri-color and binary sign: the color represents the director of the collinear spins of each bi-pyramid, while the binary sign represents spins within each bi-pyramid that are either parallel or antiparallel to the director. At the mean-field level, the tri-color orders long range in a \surd3 \times \surd3 structure, while the binary sign has numerous degenerate ground states. When combined, the two degrees of freedom yield the collinear bi-pyramid spin ground state for the hybrid lattice. The infinite number of coplanar bi-pyramid spin states can be systematically generated by collective rotations of spins from collinear states. For the long range ordered collinear and coplanar states, we identify 'partial but extensive' spin zero-energy modes of excitations that are qualitatively different from the 'local' zero-energy excitations found in the spin liquid states of the pure two-dimensional kagome and pure three-dimensional pyrochlore lattices. We argue that due to the infinite ground state degeneracy and the unique characteristics of the zero-energy excitations, the ground state of the quasi-two-dimensional hybrid antiferromagnet becomes a spin glass.

Evidence for charge and orbital order in the doped titanates RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu)

A. C. Komarek, M. Reuther, T. Lorenz, A. Cousson, P. Link, W. Morgenroth, D. Trots, C. Baehtz, M. Braden
(Submitted on 1 Sep 2011)

arXiv:1109.0234v1 [cond-mat.str-el]

Abstract

Combining macroscopic and diffraction methods we have studied the electric, magnetic and struc- tural properties of RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu) focusing on the concentration range near the metal-insulator transition. The insulating phase, which is stabilized by a smaller rare-earth ionic ra- dius, exhibits charge order with a predominant occupation of the dxy orbital. The charge and orbital ordering explains the broad stability range of the insulating state in RE_(1-x)Ca_xTiO_3 with smaller rare-earth ions. The strong modulation of the Ti-O bond distances indicates sizeable modulation of the electric charge.

Magnetic excitations in the geometric frustrated multiferroic CuCrO_2

M. Frontzek, J. T. Haraldsen, A. Podlesnyak, M. Matsuda, A. D. Christianson, R. S. Fishman, A. S. Sefat, Y. Qiu, J. R. D. Copley, S. Barilo, S. V. Shiryaev, G. Ehlers
(Submitted on 31 Aug 2011)

arXiv:1108.6288v1 [cond-mat.mtrl-sci]

Abstract

In this paper detailed neutron scattering measurements of the magnetic excitation spectrum of CuCrO_2 in the ordered state below T_{\rm}=24.2 K are presented. The spectra are analyzed using a model Hamiltonian which includes intralayer-exchange up to the next-next-nearest neighbor and interlayer-exchange. We obtain a definite parameter set and show that exchange interaction terms beyond the next-nearest neighbor are important to describe the inelastic excitation spectrum. The magnetic ground state structure generated with our parameter set is in agreement with the structure proposed for CuCrO_2 from the results of single crystal diffraction experiments previously published. We argue that the role of the interlayer exchange is crucial to understand the incommensurability of the magnetic structure as well as the spin-charge coupling mechanism.

Magneto-electric effect in NdCrTiO5

J. Hwang, E. S. Choi, H. D. Zhou, J. Lu, P. Schlottmann
(Submitted on 29 Aug 2011)

arXiv:1108.5754v1 [cond-mat.mtrl-sci]

Abstract

We have measured the dielectric constant and the pyroelectric current of orthorhombic (space group Pbam) NdCrTiO_5 polycrystalline samples. The dielectric constant and the pyroelectric current show features associated with ferroelectric transitions at the antiferromagnetic transition temperature (T_N = 21 K). The effect of magnetic fields is to enhance the features almost linearly up to the maximum measured field (7 T) with a spontaneous polarization value of ~ 3.5 \mu C/m^2. Because of the collinear spin structure of NdCrTiO_5 the possible and likely mechanism for the observed multiferroicity (antiferromagnetism + ferroelectricity) is magnetostriction.

Induced magnetoelectric response in Pnma perovkites

Eric Bousquet, Nicola Spaldin
(Submitted on 30 Aug 2011)

arXiv:1108.6073v1 [cond-mat.mtrl-sci]

Abstract

We use symmetry analysis to show that the G, C and A-type antiferromagnetic Pnma perovskites can exhibit magnetoelectric (ME) responses when a ferroelectric instability is induced with epitaxial strain. Using first-principles calculations we compute the values of the allowed ME response in strained CaMnO3 as a model system. Our results show that large linear and non-linear ME responses are present and can diverge when close to the ferroelectric phase transition. By decomposing the electronic and ionic contributions, we explore the detailed mechanism of the ME response.

Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres

A. Slablab, L. Le Xuan, M. Zielinski, Y. de Wilde, V. Jacques, D. Chauvat, J.-F. Roch
(Submitted on 26 Aug 2011)

arXiv:1108.5256v1 [cond-mat.mes-hall]

Abstract

We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.

Domain wall conduction in multiaxial ferroelectrics

Eugene A. Eliseev, Anna N. Morozovska, George S. Svechnikov, Peter Maksymovych, Sergei V. Kalinin
(Submitted on 26 Aug 2011)

arXiv:1108.5344v1 [cond-mat.mtrl-sci]

Abstract

The conductance of domain wall structures consisting of either stripes or cylindrical domains in multi-axial ferroelectric-semiconductors is analyzed. The effects of the domain size, wall tilt and curvature, on charge accumulation, are analyzed using the Landau-Ginsburg Devonshire (LGD) theory for polarization combined with Poisson equation for charge distributions. Both the classical ferroelectric parameters including expansion coefficients in 2-4-6 Landau potential and gradient terms, as well as flexoelectric coupling, inhomogeneous elastic strains and electrostriction are included in the present analysis. Spatial distributions of the ionized donors, free electrons and holes were found self-consistently using the effective mass approximation for the respective densities of states. The proximity and size effect of the electron and donor accumulation/depletion by thin stripe domains and cylindrical nanodomains are revealed. In contrast to thick domain stripes and thicker cylindrical domains, in which the carrier accumulation (and so the static conductivity) sharply increases at the domain walls only, small nanodomains of radius less then 5-10 correlation length appeared conducting across entire cross-section. Implications of such conductive nanosized channels may be promising for nanoelectronics.

Progress in Neutron Scattering Studies of Spin Excitations in High-Tc Cuprates

Masaki Fujita, Haruhiro Hiraka, Masaaki Matsuda, Masato Matsuura, John M. Tranquada, Shuichi Wakimoto, Guangyong Xu, Kazuyoshi Yamada

arXiv:1108.4431v1 [cond-mat.supr-con]

Abstract

Neutron scattering experiments continue to improve our knowledge of spin fluctuations in layered cuprates, excitations that are symptomatic of the electronic correlations underlying high-temperature superconductivity. Time-of-flight spectrometers, together with new and varied single crystal samples, have provided a more complete characterization of the magnetic energy spectrum and its variation with carrier concentration. While the spin excitations appear anomalous in comparison with simple model systems, there is clear consistency among a variety of cuprate families. Focusing initially on hole-doped systems, we review the nature of the magnetic spectrum, and variations in magnetic spectral weight with doping. We consider connections with the phenomena of charge and spin stripe order, and the potential generality of such correlations as suggested by studies of magnetic-field and impurity induced order. We contrast the behavior of the hole-doped systems with the trends found in the electron-doped superconductors. Returning to hole-doped cuprates, studies of translation-symmetry-preserving magnetic order are discussed, along with efforts to explore new systems. We conclude with a discussion of future challenges.

Resonant X-Ray Diffraction Study of Strongly Spin-Orbit-Coupled Mott Insulator CaIrO3

Kenya Ohgushi, Jun-ichi Yamaura, Hiroyuki Ohsumi, Kunihisa Sugimoto, Soshi Takeshita, Akihisa Tokuda, Hidenori Takagi, Masaki Takata, Taka-hisa Arima

arXiv:1108.4523v1 [cond-mat.str-el]

Abstract

We performed resonant x-ray diffraction experiments at the L absorption edges for the post-perovskite-type compound CaIrO_ with (t_)^5 electronic configuration. By observing the magnetic signals, we could clearly see that the magnetic structure was a striped order with an antiferromagnetic moment along the c-axis and that the wavefunction of a t_ hole is strongly spin-orbit entangled, the J_{\rm eff} =1/2 state. The observed spin arrangement is consistent with theoretical work predicting a unique superexchange interaction in the J_{\rm eff} =1/2 state and points to the universal importance of the spin-orbit coupling in Ir oxides, irrespective of the local coordination and lattice topology. We also propose that the non-magnetic resonant scattering is a powerful tool for unraveling an orbital state even in a metallic iridate.

Complex magnetism and magnetic field-driven electrical polarization in Co_3TeO_6

M. Hudl, R. Mathieu, S. A. Ivanov, M. Weil, V. Carolus, Th. Lottermoser, M. Fiebig, Y. Tokunaga, Y. Taguchi, Y. Tokura, P. Nordblad

arXiv:1108.4186 [cond-mat.mtrl-sci]

Abstract

The magnetic and electrical properties of Co_3TeO_6 single-crystals with corundum related structure reveal a magnetic-field induced polarization below 21 K. A sharp peak in the specific heat at \approx 18 K indicates a reconstructive-type first-order phase transition. From second-harmonic generation (SHG) measurements breaking of inversion symmetry is evident and the point-group symmetry was determined as m. The temperature and magnetic-field dependence of the magnetic and electrical polarizations are discussed in the light of the SHG results.

Determination of Intrinsic Ferroelectric Polarization in Orthorhombic Manganites with E-type Spin Order

Y. S. Chai, Y. S. Oh, L. J. Wang, N. Manivannan, S. M. Feng, Y. S. Yang, L. Q. Yan, C. Q. Jin, Kee Hoon Kim

arXiv:1108.3458 [cond-mat.mtrl-sci]

Abstract

By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we accurately determined the remanent ferroelectric polarization Pr of orthorhombic RMnO3 (R = Ho, Tm, Yb, and Lu) compounds below their E-type spin ordering temperatures. We found that LuMnO3 has the largest Pr of 0.17 uC/cm^2 at 6 K in the series, indicating that its single-crystal form can produce a Pr of at least 0.6 \muuC/cm^2 at 0 K. Furthermore, at a fixed temperature, Pr decreases systematically with increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong correlation with the variations in the in-plane Mn-O-Mn bond angle and Mn-O distances. Our experimental results suggest that the contribution of the Mn t2g orbitals dominates the ferroelectric polarization.

First Principle Study of Magnetism and Magneto-structural Coupling in Gallium Ferrite

Amritendu Roy, Rajendra Prasad, Sushil Auluck, Ashish Garg

arXiv:1108.2773v1 [cond-mat.mtrl-sci]

Abstract

We report a first-principles study of the magnetic properties, site disorder and magneto-structural coupling in multiferroic gallium ferrite (GFO) using local spin density approximation (LSDA+U) of density functional theory. The calculations of the ground state A-type antiferromagnetic structure predict magnetic moments consistent with the experiments whilst consideration of spin-orbit coupling yields a net orbital moment of ~ 0.025 Bohr magneton/Fe site also in good accordance with the experiments. We find that though site disorder is not spontaneous in the ground state, interchange between Fe2 and Ga2 sites is most favored in the disordered state. The results show that ferrimagnetism in GFO is due to Ga-Fe site disordering such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. The effect of spin configuration on the structural distortion clearly indicates presence of magneto-structural coupling in GFO.

Phase transition close to room temperature in BiFeO3 thin films

J. Kreisel, P. Jadhav, O. Chaix-Pluchery, M. Varela, N. Dix, F. Sanchez, J. Fontcuberta

arXiv:1107.5801v1 [cond-mat.mtrl-sci]

Abstract

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, displays a reversible temperature-induced phase transition at about 100\circ, thus close to room temperature.

Domain walls in helical magnets.

Fuxiang Li, Thomas Nattermann, Valery L. Pokrovsky

arXiv:1107.5753v1 [cond-mat.mtrl-sci]

Abstract

The structure of domain walls determines to a large extent the properties of magnetic materials, in particular their hardness and switching behavior, it represents an essential ingredient of spintronics. Common domain walls are of Bloch and Neel types in which the magnetization rotates around a fixed axis, giving rise to a one-dimensional magnetization profile. Domain walls in helical magnets, most relevant in multiferroics, were never studied systematically. Here we show that domain walls in helical magnets are fundamentally different from Bloch and Neel walls. They are generically characterized by a two-dimensional pattern formed by a regular lattice of vortex singularities. In conical phases vortices carry Berry phase flux giving rise to the anomalous Hall effect. In multiferroics vortices are charged, allowing to manipulate magnetic domain walls by electric fields. Our theory allows the interpretation of magnetic textures observed in helical magnetic structures.

Multiferroic coupling in nanoscale BiFeO3.

arXiv:1107.5133v1 [cond-mat.mtrl-sci]

Abstract

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ~22 nm particles of BiFeO3, exhibits a jump by ~30% around the magnetic transition point T_N (~635 K) and a suppression by ~7% under 5T magnetic field at room temperature. These results confirm presence of strong multiferroic coupling even in nanoscale BiFeO3 and thus could prove to be quite useful for applications based on nanosized devices of BiFeO3.

Molecular-spin dynamics study of electromagnons in multiferroic RMn_2O_5

arXiv:1107.3349v1 [cond-mat.mtrl-sci]

Abstract

We investigate the electromagnon in magnetoferroelectrics RMn_2O_5 using combined molecular-spin dynamics simulations. We identify the origin of the electromagnon modes observed in the optical spectra and reproduce the most salient features of the electromagnon in these compounds. We find that the electromagnon frequencies are very sensitive to the magnetic wave vector along the a direction. We further investigate the electromagnon in magnetic field. Although the modes frequencies change significant under magnetic field, the static dielectric constant electromagnon does not change much in the magnetic field.

Spiral ground state against ferroelectricity in the frustrated magnet BiMnFe2O6

arXiv:1107.2972v1 [cond-mat.mtrl-sci]

Abstract

The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN 220 K. The magnetic structure is characterized by the propagation vector k=[0,beta,0] with beta 0.14 and the P22_12_11'(0beta0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b-axis. The magnetic moments lie in the ac plane and rotate about pi*(1+beta) 204.8 deg angle between the adjacent magnetic atoms along b. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the ab plane. The antiferromagnetic coupling along the c-axis leads to the cancellation of electric polarization, and results in the lack of ferroelectricity in BiMnFe2O6.

Polar modes in multiferroic Ba_2Mg_2Fe_(12)O_(22) hexagonal Y-type hexaferrite

arXiv:1102.0664v1 [cond-mat.mtrl-sci]

Abstract

Spectra of transmission and reflection coefficients of single crystalline Y-type hexaferrite Ba_(2)Mg_(2)Fe_(12)O_(22) are measured at Terahertz and infrared frequencies, 3 - 4500 c--1, and at temperatures 10 to 300 K, for polarization E prp c. Oscillator parameters of nineteen phonon lines allowed by the R3m crystal lattice symmetry are determined. Absorption lines assignment to vibrational modes of the lattice structural fragments is performed. Below the temperatures of 195 K and 50 K, corresponding to the zero field phase transitions to the proper screw and to the longitudinal-conical spin states, respectively, new absorption lines are discovered whose origin is assigned to the electric-dipole active magnetic excitations.