260 research outputs found
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
Coupling of phonons and electromagnons in GdMnO_3
The infrared and Terahertz properties of GdMnO_3 have been investigated as
function of temperature and magnetic field, with special emphasis on the phase
boundary between the incommensurate and the canted antiferromagnetic
structures. The heterogeneous incommensurate phase reveals strong
magnetodielectric effects, characterized by significant magnetoelectric
contributions to the static dielectric permittivity and by the existence of
electrically excited magnons (electromagnons). In the commensurate canted
antiferromagnetic phase the magnetoelectric contributions to the dielectric
constant and electromagnons are suppressed. The corresponding spectral weight
is transferred to the lowest lattice vibration demonstrating the strong
coupling of phonons with electromagnons.Comment: 5 pages, 4 figure
Uniaxial pressure dependencies of the phase transitions in GdMnO
GdMnO shows an incommensurate antiferromagnetic order below
K, transforms into a canted A-type antiferromagnet below K, and for
finite magnetic fields along the b axis ferroelectric order occurs below
K. From high-resolution thermal expansion measurements along all
three principal axes, we determine the uniaxial pressure dependencies of the
various transition temperatures and discuss their correlation to changes of the
magnetic exchange couplings in MnO ().Comment: 2 pages, 3 figures, submitted to JMMM (Proceedings of ICM'06, Kyoto
Low-energy Mott-Hubbard excitations in LaMnO_3 probed by optical ellipsometry
We present a comprehensive ellipsometric study of an untwinned, nearly
stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures
20 K < T < 300 K. The complex dielectric response along the b and c axes of the
Pbnm orthorhombic unit cell, \epsilon^b(\nu) and \epsilon^c(\nu), is highly
anisotropic over the spectral range covered in the experiment. The difference
between \epsilon^b(\nu) and \epsilon^c(\nu) increases with decreasing
temperature, and the gradual evolution observed in the paramagnetic state is
strongly enhanced by the onset of A-type antiferromagnetic long-range order at
T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap
excitation at 2 eV, there are opposite changes observed at higher energy at 4 -
5 eV, appearing on a broad-band background due to the strongly dipole-allowed O
2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the
observation of a pronounced spectral-weight transfer between low- and
high-energy features upon magnetic ordering, they are assigned to high-spin and
low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy
of the lowest-energy optical band and the spectral weight shifts induced by
antiferromagnetic spin correlations are quantitatively described by an
effective spin-orbital superexchange model. An analysis of the multiplet
structure of the intersite transitions by Mn e_g electrons allowed us to
estimate the effective intra-atomic Coulomb interaction, the Hund exchange
coupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3.
This study identifies the lowest-energy optical transition at 2 eV as an
intersite d-d transition, whose energy is substantially reduced compared to
that obtained from the bare intra-atomic Coulomb interaction.Comment: 10 pages, 14 figure
Spin-controlled Mott-Hubbard bands in LaMnO_3 probed by optical ellipsometry
Spectral ellipsometry has been used to determine the dielectric function of
an untwinned crystal of LaMnO_3 in the spectral range 0.5-5.6 eV at
temperatures 50 K < T < 300 K. A pronounced redistribution of spectral weight
is found at the Neel temperature T_N = 140 K. The anisotropy of the spectral
weight transfer matches the magnetic ordering pattern. A superexchange model
quantitatively describes spectral weight transfer induced by spin correlations.
This analysis implies that the lowest-energy transitions around 2 eV are
intersite d-d transitions, and that LaMnO_3 is a Mott-Hubbard insulator.Comment: 4 pages, 4 figure
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