53 research outputs found
Magnetic-field induced multiferroicity in a quantum critical frustrated spin liquid
Dielectric spectroscopy is used to check for the onset of polar order in the
quasi one-dimensional quantum spin system Sul-Cu2Cl4 when passing from the
spin-liquid state into the ordered spiral phase in an external magnetic field.
We find clear evidence for multiferroicity in this material and treat in detail
its H-T phase diagram close to the quantum-critical regime.Comment: 5 pages, 4 figures. Revised according to suggestions of referee
Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4
We report on infrared, Raman, magnetic susceptibility, and specific heat
measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the
nearest-neighbor and next-nearest neighbor exchange constants from the magnetic
susceptibility and extract the spin-spin correlation functions obtained from
the magnetic susceptibility and the magnetic contribution to the specific heat.
By comparing with the frequency shift of the infrared optical phonons above TN
, we derive estimates for the spin-phonon coupling constants in these systems.
The observation of phonon modes which are both Raman and infrared active
suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in
agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74,
060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon
modes could be detected which are both Raman and infrared active indicating the
conservation of inversion symmetry in the low temperature phase.Comment: 11 pages, 13 figure
Relaxor ferroelectricity and the freezing of short-range polar order in magnetite
A thorough investigation of single crystalline magnetite using broadband
dielectric spectroscopy and other methods provides evidence for relaxor-like
polar order in Fe3O4. We find long-range ferroelectric order to be im-peded by
the continuous freezing of polar degrees of freedom and the formation of a
tunneling-dominated glasslike state at low temperatures. This also explains the
lack of clear evidence for a non-centrosymmetric crystal structure below the
Verwey transition. Within the framework of recent models assuming an intimate
relation of charge and polar order, the charge order, too, can be speculated to
be of short-range type only and to be dominated by tunneling at low
temperatures.Comment: 16 pages, 4 figures, final version with revisions according to
referee demand
Multiferroicity in an organic charge-transfer salt: Electric-dipole-driven magnetism
Multiferroics, showing simultaneous ordering of electrical and magnetic
degrees of freedom, are remarkable materials as seen from both the academic and
technological points of view. A prominent mechanism of multiferroicity is the
spin-driven ferroelectricity, often found in frustrated antiferromagnets with
helical spin order. There, similar to conventional ferroelectrics, the
electrical dipoles arise from an off-centre displacement of ions. However,
recently a different mechanism, namely purely electronic ferroelectricity,
where charge order breaks inversion symmetry, has attracted considerable
interest. Here we provide evidence for this exotic type of ferroelectricity,
accompanied by antiferromagnetic spin order, in a two-dimensional organic
charge-transfer salt, thus representing a new class of multiferroics. Quite
unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly
simultaneously. This can be ascribed to the loss of spin frustration induced by
the ferroelectric ordering. Hence, here the spin order is driven by the
ferroelectricity, in marked contrast to the spin-driven ferroelectricity in
helical magnets.Comment: 8 pages, 9 figures (including 4 pages and 6 figures in supplementary
information). Version 2 with minor errors corrected (legend of Fig. 3c and
definition of vectors e and Q
Magnetic and superconducting transitions in BaKFeAs studied by specific heat
We report on specific heat measurements in BaKFeAs
(). For the underdoped sample with both the spin-density-wave
transition at K and the superconducting transition at 23 K can be
identified. The electronic contribution to the specific heat in the
superconducting state for concentrations in the vicinity of optimal doping
can be well described by a full single-gap within the BCS limit.Comment: 5 pages, 4 figures, 2 table
Chain Formation by Spin Pentamers in eta-Na9V14O35
The nature of the gapped ground state in the quasi-one-dimensional compound
eta-Na9V14O35 cannot easily be understood, if one takes into account the odd
number of spins on each structural element. Combining the results of specific
heat, susceptibility and electron spin resonance measurements we show that
eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a
linear chain. These objects - pentamers - consist of five antiferromagnetically
arranged spins with effective spin 1/2. Their spatial extent results in an
exchange constant along the chain direction comparable to the one in the
high-temperature state.Comment: 6 pages, 5 figure
Optical Spectroscopy in CoO: Phonons, Electric, and Magnetic Excitations
The reflectivity of single-crystalline CoO has been studied by optical
spectroscopy for wave numbers ranging from 100 to 28,000\wn and for
temperatures 8 325 K\@. A splitting of the cubic IR-active phonon mode
on passing the antiferromagnetic phase transition at = 289 K has been
observed. At low temperatures the splitting amounts to 15.0\wn. In addition, we
studied the splitting of the cubic crystal field ground state of the Co
ions due to spin-orbit coupling, a tetragonal crystal field, and exchange
interaction. Below , magnetic dipole transitions between the
exchange-split levels are identified and the energy-level scheme can be well
described with a spin-orbit coupling \lambda = 151.1\wn, an exchange constant
J = 17.5\wn, and a tetragonal crystal-field parameter D = -47.8\wn. Already
in the paramagnetic state electric quadrupole transitions between the
spin-orbit split level have been observed. At high frequencies, two electronic
levels of the crystal-field-split -manifold were identified at 8,000 and
18,500\wn.Comment: 12 pages, 8 figures, 2 table
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FeCrâ‚‚Sâ‚„ in magnetic fields: possible evidence for a multiferroic ground state.
We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K
Relaxations as key to the magnetocapacitive effects in the perovskite manganites
We present a detailed dielectric study of the relaxation effects that occur
in several perovskite rare-earth manganites, including the multiferroics TbMnO3
and DyMnO3. We demonstrate that the strong magnetocapacitive effects, observed
for electrical fields E||c, are nearly completely governed by magnetic-state
induced changes of the relaxation parameters. The multiferroic materials, which
undergo a transition into a spiral magnetic state, show qualitatively different
relaxation behavior than those compounds transferring into an A-type
antiferromagnetic state. We ascribe the relaxations in both cases to the
off-center motion of the manganese ions, which in the multiferroic systems also
leads to the ferroelectric ordering.Comment: 4 pages; 4 figures; final version, revised according to suggestions
of referee
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
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