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\ $RT_2$B$_2$C and $RT$BC, with $R$ being (essentially) a rare earth, and $T$ a transition metal. We assume a single band in which electrons feel a local attractive interaction (negative Hubbard-$U$) on sites representing the $T$B layers, while U=0 on sites representing the $R$C layers; the multi-band structure is taken into account minimally through a band offset $\epsilon$. 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 ($L=0$) the only source of magnetostriction is believed to be the exchange striction (ES). If the systems, for instance \chem{Gd^{3+}}-based compounds ($S=7/2$, $L=0$), 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 ($L=0$) symmetry-breaking distortions below the Néel 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