164 research outputs found
Commensurate Dy magnetic ordering associated with incommensurate lattice distortion in orthorhombic DyMnO3
Synchrotron x-ray diffraction and resonant magnetic scattering experiments on
single crystal DyMnO3 have been carried out between 4 and 40 K. Below TN(Dy) =
5K, the Dy magnetic moments order in a commensurate structure with propagation
vector 0.5 b*. Simultaneous with the Dy magnetic ordering, an incommensurate
lattice modulation with propagation vector 0.905 b* evolves while the original
Mn induced modulation is suppressed and shifts from 0.78 b* to 0.81 b*. This
points to a strong interference of Mn and Dy induced structural distortions in
DyMnO3 besides a magnetic coupling between the Mn and Dy magnetic moments.Comment: submitted to Phys. Rev. B Rapid Communication
Microscopic Model and Phase Diagrams of the Multiferroic Perovskite Manganites
Orthorhombically distorted perovskite manganites, RMnO3 with R being a
trivalent rare-earth ion, exhibit a variety of magnetic and electric phases
including multiferroic (i.e. concurrently magnetic and ferroelectric) phases
and fascinating magnetoelectric phenomena. We theoretically study the phase
diagram of RMnO3 by constructing a microscopic spin model, which includes not
only the superexchange interaction but also the single-ion anisotropy (SIA) and
the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the
Monte-Carlo method reproduces the experimental phase diagrams as functions of
the R-ion radius, which contain two different multiferroic states, i.e. the
ab-plane spin cycloid with ferroelectric polarization P//a and the bc-plane
spin cycloid with P//c. The orthorhombic lattice distortion or the
second-neighbor spin exchanges enhanced by this distortion exquisitely controls
the keen competition between these two phases through tuning the SIA and DMI
energies. This leads to a lattice-distortion-induced reorientation of P from a
to c in agreement with the experiments. We also discuss spin structures in the
A-type antiferromagnetic state, those in the cycloidal spin states, origin and
nature of the sinusoidal collinear spin state, and many other issues.Comment: 23 pages, 19 figures. Recalculated results after correcting errors in
the assignment of Dzyaloshinsky-Moriya vector
Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO and TbMnO due to 4f crystal-field excitations
We present measurements of the thermal conductivity and the thermal
expansion of NdMnO and TbMnO. In both compounds a splitting of
the multiplet of the ion causes Schottky contributions to
. In TbMnO this contribution arises from a crystal-field splitting,
while in NdMnO it is due to the Nd-Mn exchange coupling. Another
consequence of this coupling is a strongly enhanced canting of the Mn moments.
The thermal conductivity is greatly suppressed in both compounds. The main
scattering process at low temperatures is resonant scattering of phonons
between different energy levels of the multiplets, whereas the complex 3d
magnetism of the Mn ions is of minor importance.Comment: 9 pages including 6 figure
The multiferroic phases of (Eu:Y)MnO3
We report on structural, magnetic, dielectric, and thermodynamic properties
of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the
multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the
interference of magnetic contributions of the 4f-ions. In addition, it offers
the possibility to continuously tune the influence of the A-site ionic radii.
For small concentrations x <= 0.1 we find a canted antiferromagnetic and
paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric
polarization coexists with the features of a long wavelength incommensurate
spiral magnetic phase analogous to the observations in TbMnO3. In the
intermediate concentration range around x = 0.2 a multiferroic scenario is
realized combining weak ferroelectricity and weak ferromagnetism, presumably
due to a canted spiral magnetic structure.Comment: 8 pages, 8 figure
The Effect of - Magnetic Coupling in Multiferroic MnO Crystals
We have established detailed magnetoelectric phase diagrams of
(EuY)TbMnO () and
(Eu,Y)GdMnO (), whose average ionic radii of
-site (: rare earth) cations are equal to that of Tb, in order to
reveal the effect of rare earth 4 magnetic moments on the magnetoelectric
properties. In spite of the same -site ionic radii, the magnetoelectric
properties of the two systems are remarkably different from each other. A small
amount of Tb substitution on sites () totally destroys
ferroelectric polarization along the a axis (), and an increase in Tb
concentration stabilizes the phase. On the other hand, Gd substitution
() extinguishes the phase, and slightly suppresses the
phase. These results demonstrate that the magnetoelectric properties of
MnO strongly depend on the characteristics of the rare earth 4
moments.Comment: 10 pages, 5 figures Submitted to Journal of the Physical Society of
Japa
X-ray Resonant Scattering Study of the Order Parameters in Multiferroic TbMnO
We report on an extensive investigation of the multiferroic compound
TbMnO. Non-resonant x-ray magnetic scattering (NRXMS) revealed a dominant
-type domain. The temperature dependence of the intensity and wavevector
associated with the incommensurate magnetic order was found to be in good
agreement with neutron scattering data. XRS experiments were performed in the
vicinity of the Mn and Tb edges in the high-temperature collinear
phase, the intermediate temperature cycloidal/ferroelectric phase, and the
low-temperature phase. In the collinear phase resonant satellites were
found at the Mn edge associated with -type but also -type peaks. The
azimuthal dependence of the -type satellites (and their absence in the NRXMS
experiments) indicates that they are most likely non-magnetic in origin. We
suggest instead that they may be associated with an induced charge multipole.
At the Tb edge resonant - and -type satellites () were
observed in the collinear phase. These we attribute to a polarisation of the Tb
5 states by the ordering of the Mn sublattice. In the
cycloidal/ferroelectric phase a new set of resonant satellites appear
corresponding to -type order. These appear at the Tb edge only. In
addition to a dominant component in the channel, a
weaker component is found in the pre-edge with
polarization. Calculations of the XRS were performed using the code
showing that the unrotated component of the Tb
-type peaks appearing in the ferroelectric phase contains a contribution
from a multipole that is odd with respect to both space and time, known in
various contexts as the anapole.Comment: Phys. Rev. B (In press
Non-Fermi-Liquid Scaling in Ce(Ru_{0.5}Rh_{0.5})_2Si_2
We study the temperature and field dependence of the magnetic and transport
properties of the non-Fermi-liquid compound Ce(Ru_{1-x}Rh_x)_2Si_2 at x=0.5.
For fields 0.1T the experimental results show signatures of the
presence of Kondo-disorder, expected to be large at this concentration. For
larger fields, however, magnetic and transport properties are controlled by the
coupling of the conduction electrons to critical spin-fluctuations. The
temperature dependence of the susceptibility as well as the scaling properties
of the magnetoresistance are in very good agreement with the predictions of
recent dynamical mean-field theories of Kondo alloys close to a spin-glass
quantum critical point.Comment: 4 pages, 4 figures. Improved discussion. To appear in Phys. Rev. Let
Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs
We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds
in relation to crystal structure. The two compounds crystallize in two
structural polymorphs, both tetragonal. The high temperature polymorph (HTP)
adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low
temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying
polycrystals prepared by arc melting we have observed that the rapidly cooled
samples retain the HTP even at room temperature (RT) and below. Annealing such
samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP
and LTP were subsequently studied with respect to magnetism and
superconductivity by electrical resistivity, magnetization, AC susceptibility
and specific heat measurements. The HTP and LTP of both compounds respectively,
behave as Pauli paramagnets. Superconductivity has been found exclusively in
the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in
LaIr2Si2). The relations of magnetism and superconductivity with the electronic
and crystal structure are discussed with comparing experimental data with the
results of first principles electronic structure calculations
Fermi-liquid instabilities at magnetic quantum phase transitions
This review discusses instabilities of the Fermi-liquid state of conduction
electrons in metals with particular emphasis on magnetic quantum critical
points. Both the existing theoretical concepts and experimental data on
selected materials are presented; with the aim of assessing the validity of
presently available theory. After briefly recalling the fundamentals of
Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models
and their lattice versions is described. Next, the scaling concepts applicable
to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of
quantum phase transitions is described in detail. The breakdown of the latter
is analyzed in several examples. In the final part experimental data on
heavy-fermion materials and transition-metal alloys are reviewed and confronted
with existing theory.Comment: 62 pages, 29 figs, review article for Rev. Mod. Phys; (v2) discussion
extended, refs added; (v3) shortened; final version as publishe
Decoupling between Field-instabilities of Antiferromagnetism and Pseudo-metamagnetism in Rh-doped CeRu2Si2 Kondo Lattice
Doping Kondo lattice system CeRu2Si2 with Rh-8% (Ce(Ru0.92Rh0.08)2Si2) leads
to drastic consequences due to the mismatch of the lattice parameters between
CeRu2Si2 and CeRh2Si2. A large variety of experiments clarifies the unusual
properties of the ground state induced by the magnetic field from longitudinal
antiferromagnetic (AF) mode at H = 0 to polarized paramagnetic phase in very
high magnetic field. The separation between AF phase, paramagnetic phase and
polarized paramagnetic phase varying with temperature, magnetic field and
pressure is discussed on the basis of the experiments down to very low
temperature. Similarities and differences between Rh and La substituted alloys
are discussed with emphasis on the competition between transverse and
longitudinal AF modes, and ferromagnetic fluctuations.Comment: 10 pages, 21 figures, accepted for publication in J. Phys. Soc. Jp
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