Lutetium dodecaboride LuB 12 is a simple weak-coupling BCS superconductor with critical temperature T c ≈ 0.42 K, whilst ZrB 12 is a strong-coupling BCS superconductor with the highest critical temperature T c ≈ 6.0 K among this group of materials. In case of lutetium substitution by zirconium ions in LuB 12 the crossover from weak-to strong-coupling superconductor can be studied. We have investigated the evolution of critical temperature T c and critical field H c in high-quality single crystalline superconducting samples of Lu 1-x Zr x B 12 (0 ≤ x 0.45) by measuring magnetic ac-susceptibility between ≈ 1 K and 50 mK. To obtain this kind of experimental data, a new susceptometer was designed, constructed and tested, which can work in a wide temperature range of 0.05 K -3 K in 3 He-4 He dilution refrigerator. The measurements with this new susceptometer revealed how T c (x) and H c (x) increases with increasing concentration of zirconium in Lu 1-x Zr x B 12 solid solutions as well as how their superconducting phase diagram develops.
Rare-earth tetraborides (REB 4) crystallize in tetragonal crystal structure, where the sublattice of rare-earth ions forms in ab-plane a geometrically frustrated Shastry-Sutherland lattice. In this work we constructed a detailed T N vs. B phase diagram of ErB 4 from data of temperature/field dependencies of resistance. We have applied hydrostatic pressure up to 2.88 GPa using a piston cylinder pressure cell and performed sensitive ac-resistance measurements in temperature range 1.8 K-300 K and in magnetic fields up to 6 T. The obtained results exhibit shifts of ordering temperature T N to higher values as well as shifts of boundaries between different magnetic phases. The effect of pressure on the interaction between magnetic ions in this compound is discussed and compared with the previous results obtained on TmB 4 .
TmB 4 is an anisotropic, metallic magnetic system with geometrical frustration of the Shastry-Sutherland type. Here an experimental study of the magnetocaloric effect (MCE) in Lu doped Tm 1-x Lu x B 4 (x = 0.06, 0.30), evaluated from the temperature dependence of heat capacity and magnetization curves at 2 K, is presented. The results are described within a theoretical model based on an extended Ising Hamiltonian which considers interactions up to the 4th next nearest neighbours. Model parameters were optimized to achieve the best match to the experimental results over the whole range of Lu 3+ ion concentrations. After optimisation a good quantitative agreement with the adiabatic temperature change and a good qualitative agreement with magnetization curves is obtained. Our study shows that the efficiency of the MCE can be tuned by dilution with non-magnetic Lu ions. The theoretical model developed could be used to design new magnetocaloric materials.
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