16 research outputs found
Magnetic Phase Diagram of GdNi2B2C: Two-ion Magnetoelasticity and Anisotropic Exchange Couplings
Extensive magnetization and magnetostriction measurements were carried out on
a single crystal of GdNi2B2C along the main tetragonal axes. Within the
paramagnetic phase, the magnetic and strain susceptibilities revealed a weak
anisotropy in the exchange couplings and two-ion tetragonal-preserving
alpha-strain modes. Within the ordered phase, magnetization and
magnetostriction revealed a relatively strong orthorhombic distortion mode and
rich field-temperature phase diagrams. For H//(100) phase diagram, three
field-induced transformations were observed, namely, at: Hd(T), related to the
domain alignment; Hr(T), associated with reorientation of the moment towards
the c-axis; and Hs(T), defining the saturation process wherein the exchange
field is completely counterbalanced. On the other hand, For H//(001) phase
diagram, only two field-induced transformations were observed, namely at: Hr(T)
and Hs(T). For both phase diagrams, Hs(T) follows the relation
Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In
contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than
along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where
Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the
magnetoelastic interaction and the anisotropic exchange coupling are important
perturbations and therefore should be explicitly considered if a complete
analysis of the magnetic properties of the borocarbides is desired
A layering model for superconductivity in the borocarbides
We propose a superlattice model to describe superconductivity in layered
materials, such as the borocarbide families with the chemical formul\ae\
BC and BC, with being (essentially) a rare earth, and a
transition metal. We assume a single band in which electrons feel a local
attractive interaction (negative Hubbard-) on sites representing the B
layers, while U=0 on sites representing the C layers; the multi-band
structure is taken into account minimally through a band offset . The
one-dimensional model is studied numerically through the calculation of the
charge gap, the Drude weight, and of the pairing correlation function. A
comparison with the available information on the nature of the electronic
ground state (metallic or superconducting) indicates that the model provides a
systematic parametrization of the whole borocarbide family.Comment: 4 figure
Mossbauer relaxation and thermodynamic properties of spin cluster-triad in EuMg_5
Quantum Matter and Optic
Magnetoelastic paradox: Absence of symmetry-breaking distortions below
Phase transitions are often associated with symmetry breaking.
In case of magnetic order time reversal symmetry is broken and
this leads to magnetostriction. For magnetic systems without
orbital moment () the only source of magnetostriction is
believed to be the exchange striction (ES). If the systems, for
instance \chem{Gd^{3+}}-based compounds (, ), order
ferromagnetically (fm) no lattice distortions are expected from the
standard model of rare-earth magnetism, whereas in the
antiferromagnetically (afm) ordered compounds symmetry-breaking
lattice distortions should occur. These latter prediction of the
theory is in complete contrast to all available experimental data on
\chem{Gd^{3+}} antiferromagnets. They show in many cases large
magnetostrictive effects, but no symmetry breaking. Thus we can
formulate the “magnetoelastic paradox”: in afm systems without
orbital moment () symmetry-breaking distortions below the
NĂ©el temperature are expected, but have not been found.
New experimental data indicates, that the magnetoelastic paradox is only
present in zero field and may be lifted by a small magnetic field