44 research outputs found
Magnetic fluctuations and superconductivity in Fe pnictides probed by electron spin resonance
The electron spin resonance absorption spectrum of Eu^{2+} ions serves as a
probe of the normal and superconducting state in Eu_{0.5}K_{0.5}Fe_2As_2. The
spin-lattice relaxation rate 1/T_1^{\rm ESR} obtained from the ESR linewidth
exhibits a Korringa-like linear increase with temperature above T_C evidencing
a normal Fermi-liquid behavior. Below 45 K deviations from the Korringa-law
occur which are ascribed to enhanced magnetic fluctuations within the FeAs
layers upon approaching the superconducting transition. Below T_C the
spin-lattice relaxation rate 1/T_1^{\rm ESR} follows a T^{1.5}-behavior without
the appearance of a coherence peak.Comment: 5 pages, 5 figure
Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4
We report on magnetization and electron spin resonance (ESR) measurements of
SrVO with orthorhombic symmetry. In this dimer system the
ions are in tetrahedral environment and are coupled by an antiferromagnetic
intra-dimer exchange constant 100 K to form a singlet ground
state without any phase transitions between room temperature and 2 K. Based on
an extended-H\"{u}ckel-Tight-Binding analysis we identify the strongest
exchange interaction to occur between two inequivalent vanadium sites via two
intermediate oxygen ions. The ESR absorption spectra can be well described by a
single Lorentzian line with an effective g-factor = 1.89. The temperature
dependence of the ESR intensity is well described by a dimer model in agreement
with the magnetization data. The temperature dependence of the ESR linewidth
can be modeled by a superposition of a linear increase with temperature with a
slope = 1.35 Oe/K and a thermally activated behavior with an
activation energy = 1418 K, both of which point to spin-phonon
coupling as the dominant relaxation mechanism in this compound.Comment: 5 pages, 4 figure
Weak ferromagnetism and glassy state in kappa-(BEDT-TTF)2Hg(SCN)2Br
Since the first observation of weak ferromagnetism in the charge-transfer
salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840
(1992)], no further evidence of ferromagnetism in this class of organic
materials has been reported. Here we present static and dynamic spin
susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak
ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the
molecular dimers suppress long-range order, forming a spin-glass ground state
in the insulating phase
Complex nature of magnetic field-induced ferroelectricity in GdCrTiO5
This work shows an unconventional route for spin-driven ferroelectricity
originating from a metastable magnetic field-induced canting of chromium
sublattice in the presence of gadolinium moments in GdCrTiO5 at low
temperatures. Compared to the isostructural neodymium compound, significant
differences of magnetism and magnetoelectric effects are seen. We present the
results of thorough investigations of temperature and magnetic field dependent
magnetization as well as ac and dc magnetic susceptibility. These bulk
measurements are complemented by local-probe spectroscopy utilizing
electron-spin resonance and muon-spin rotation/relaxation for probing the
chromium moments. Ferroelectric order is inferred from pyro- and magnetocurrent
measurements. GdCrTiO5 shows a pyrocurrent signal around 10 K, only if the
system is cooled in an applied magnetic field exceeding 10 kOe. A distinct
spin-driven ferroelectric order is revealed in this state for temperatures
below 10 K, which can be switched by changing magnetic-field direction and the
polarity of the electric field. But, the magnetic measurements reveal no clear
signature of long-range magnetic ordering. The presence of such
meta-magnetoelectric-type behaviour in the absence of any meta-magnetic
behavior is rare in the literature. Our microscopic spectroscopy results
indicate significant changes of the magnetic properties around 10 K. Probably
there is an exchange frustration between Gd and Cr moments, which prevents
long-range magnetic ordering at further high temperature. Below 10 K, weak
magnetic ordering occurs by minimizing frustration due to lattice distortion,
which helps in magnetodielectric coupling. However, the non-polar distortion
attains appreciable values after application of magnetic fields above 10 kOe to
break the spatial inversion symmetry, which creates ferroelectricity.Comment: accepted in PR
Nonequilibrium relaxation study of the anisotropic antiferromagnetic Heisenberg model on the triangular lattice
Effect of exchange anisotropy on the relaxation time of spin and vector
chirality is studied for the antiferromagnetic classical Heisenberg model on
the triangular lattice by using the nonequilibrium relaxation Monte Carlo
method. We identify the Berezinskii-Kosterlitz-Thouless (BKT) transition and
the chiral transition in a wide range of the anisotropy, even for very small
anisotropy of 0.01%. As the anisotropy decreases, both the critical
temperatures steeply decrease, while the BKT critical region becomes
divergently wide. We elucidate a sharp "V shape" of the phase diagram around
the isotropic Heisenberg point, which suggests that the isotropic case is
exceptionally singular and the associated Z vortex transition will be isolated
from the BKT and chiral transitions. We discuss the relevance of our results to
peculiar behavior of the spin relaxation time observed experimentally in
triangular antiferromagnets.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study
We report a comprehensive study of magnetic correlations in LaNiO, a
parent compound of the recently discovered family of infinite-layer (IL)
nickelate superconductors, using multiple experimental and theoretical methods.
Our specific heat, muon-spin rotation (SR), and magnetic susceptibility
measurements on polycrystalline LaNiO show that long-range magnetic order
remains absent down to 2 K. Nevertheless, we detect residual entropy in the
low-temperature specific heat, which is compatible with a model fit that
includes paramagnon excitations. The SR and low-field static and dynamic
magnetic susceptibility measurements indicate the presence of short-range
magnetic correlations and glassy spin dynamics, which we attribute to local
oxygen non-stoichiometry in the average infinite-layer crystal structure. This
glassy behavior can be suppressed in strong external fields, allowing us to
extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the
intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures,
in analogy to doped cuprates that possess robust non-local spin fluctuations.
The distinct temperature dependence of the intrinsic susceptibility of
LaNiO can be theoretically understood by a multi-method study of the
single-band Hubbard model in which we apply complementary cutting-edge quantum
many-body techniques (dynamical mean-field theory, cellular dynamical
mean-field theory and the dynamical vertex approximation) to investigate the
influence of both short- and long-ranged correlations. Our results suggest a
profound analogy between the magnetic correlations in parent (undoped) IL
nickelates and doped cuprates.Comment: 18 pages, 14 figure
Electron spin resonance in Eu based Fe pnictides
The phase diagrams of EuFeCoAs and
EuFeAsP are investigated by Eu
electron spin resonance (ESR) in single crystals. From the temperature
dependence of the linewidth of the exchange narrowed ESR line the
spin-density wave (SDW) and the normal metallic regime are clearly distinguished. At the isotropic
linear increase of the linewidth is driven by the Korringa relaxation which
measures the conduction-electron density of states at the Fermi level. For the anisotropy probes the local ligand field, while the coupling
to the conduction electrons disappears. With increasing substitution or
the transition temperature decreases linearly accompanied by a
linear decrease of the Korringa-relaxation rate from 8 Oe/K at down to
3 Oe/K at the onset of superconductivity at or at , above which it remains nearly constant. Comparative ESR measurements on
single crystals of the Eu diluted SDW compound EuSrFeAs
and superconducting (SC) EuSrFeCoAs
corroborate the leading influence of the ligand field on the Eu spin
relaxation in the SDW regime as well as the Korringa relaxation in the normal
metallic regime. Like in EuKFeAs a coherence peak is
not detected in the latter compound at K, which is in agreement
with the expected complex anisotropic SC gap structure
Softened magnetic excitations in the s = 3/2 distorted triangular antiferromagnet alpha-CaCr2O4
The spin dynamics and magnetic excitations of the slightly distorted
triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman
spectroscopy and electron spin resonance (ESR) to elucidate its peculiar
magnetic properties. Two-magnon excitations in circular RL symmetry show a
multi-maximum structure with a dominant spectral weight at low energies. The
temperature dependence of the ESR linewidth is described by a critical
broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5)
for temperatures above T_N = 42.6 K. The exponent is much smaller than that of
other s = 3/2 triangular lattices. This is ascribed to soft roton-like modes,
indicative of the instability of a helical 120{\deg} phase. As an origin we
discuss a complex spin topology formed by four inequivalent nearest neighbor
and sizable next-nearest neighbor interactions.Comment: 7 pages, 4 figure