29 research outputs found
CdV2O4: A rare example of a collinear multiferroic spinel
By studying the dielectric properties of the geometrically frustrated spinel
CdV2O4, we observe ferroelectricity developing at the transition into the
collinear antiferromagnetic ground state. In this multiferroic spinel,
ferroelectricity is driven by local magnetostriction and not by the more common
scenario of spiral magnetism. The experimental findings are corroborated by
ab-initio calculations of the electric polarization and the underlying spin and
orbital order. The results point towards a charge rearrangement due to
dimerization, where electronic correlations and the proximity to the
insulator-metal transition play an important role.Comment: 4+ pages, 3 figure
Spin-State Transition and Metal-Insulator Transition in LaEuCoO}
We present a study of the structure, the electric resistivity, the magnetic
susceptibility, and the thermal expansion of LaEuCoO. LaCoO
shows a temperature-induced spin-state transition around 100 K and a
metal-insulator transition around 500 K. Partial substitution of La by
the smaller Eu causes chemical pressure and leads to a drastic increase
of the spin gap from about 190 K in LaCoO to about 2000 K in EuCoO, so
that the spin-state transition is shifted to much higher temperatures. A
combined analysis of thermal expansion and susceptibility gives evidence that
the spin-state transition has to be attributed to a population of an
intermediate-spin state with orbital order for and without orbital
order for larger . In contrast to the spin-state transition, the
metal-insulator transition is shifted only moderately to higher temperatures
with increasing Eu content, showing that the metal-insulator transition occurs
independently from the spin-state distribution of the Co ions. Around
the metal-insulator transition the magnetic susceptibility shows a similar
increase for all and approaches a doping-independent value around 1000 K
indicating that well above the metal-insulator transition the same spin state
is approached for all .Comment: 10 pages, 6 figure
Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)
We report measurements of the thermal expansion for two Eu- and two
Gd-based intermetallics which exhibit ferro- or antiferromagnetic phase
transitions. These materials show sharp positive (EuAgMg and GdAuMg) and
negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal
expansion coefficient which become smeared and/or displaced in an
external magnetic field. Together with specific heat data we determine the
initial pressure dependences of the transition temperatures at ambient pressure
using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure
dependences indicating strong spin-phonon coupling, in particular for GdAgMg
and EuAuMg where a quantum phase transition might be reached at moderate
pressures of a few GPa.Comment: 6 pages, 3 figure
Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic Mg intermetallics with , Eu, Gd, Yb and , Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic (, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized moments arising from the half-filled
shells of Eu or Gd. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.Comment: 12 pages, 7 figures include
Magnetoelectric effect due to local noncentrosymmetry
Magnetoelectrics often possess ions located in noncentrosymmetric
surroundings. Based on this fact we suggest a microscopic model of
magnetoelectric interaction and show that the spin-orbit coupling leads to
spin-dependent electric dipole moments of the electron orbitals of these ions,
which results in non-vanishing polarization for certain spin configurations.
The approach accounts for the macroscopic symmetry of the unit cell and is
valid both for commensurate and complex incommensurate magnetic structures. The
model is illustrated by the examples of MnWO4, MnPS3 and LiNiPO4. Application
to other magnetoelectrics is discussed.Comment: 11 pages, 2 figures, 2 table
Phase diagram and isotope effect in (PrEu)_0.7Ca_0.3CoO_3 cobaltites exhibiting spin-state transitions
We present the study of magnetization, thermal expansion, specific heat,
resistivity, and a.c. susceptibility of
(PrEu)CaCoO cobaltites. The measurements were
performed on ceramic samples with and . Based on these
results, we construct the phase diagram, including magnetic and spin-state
transitions. The transition from the low- to intermediate-spin state is
observed for the samples with , whereas for a lower Eu-doping level,
there are no spin-state transitions, but a crossover between the ferromagnetic
and paramagnetic states occurs. The effect of oxygen isotope substitution along
with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope
substitution (O by O) is found to shift both the magnetic and
spin-state phase boundaries to lower Eu concentrations. The isotope effect on
the spin-state transition temperature () is rather strong, but it is
much weaker for the transition to a ferromagnetic state (). The
ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by
the Co ions, which favor the formation of the intermediate-spin state of
neighboring Co ions.Comment: 13 pages, including 11 figures, to be published in Phys. Rev.