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The magnetic response related to the paramagnetic Meissner effect (PME) is studied in a high quality single crystal ZrB 12 with non-monotonic vortex-vortex interactions. We observe the expulsion and penetration of magnetic flux in the form of vortex clusters with increasing temperature. A vortex phase diagram is constructed, and shows that the PME can be explained by considering the interplay among the flux compression, the different temperature dependencies of the vortex-vortex and the vortex-pin interactions, and thermal fluctuations. Such a scenario is in good agreement with the results of magnetic relaxation measurements.

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In BCS-type superconductors Zr N B 12 (T C ≈ 6 K) and Lu N B 12 (T C ≈ 0.42 K) heat capacity C(T ) and magnetization measurements have been carried out on high quality single crystals with various boron isotopes (with N = 10, 11, and with natural composition of 10 B and 11 B). Parameters of the superconducting and normal states have been deduced from this study, allowing comparison between these two dodecaborides. It was shown that ZrB 12 is a type-II superconductor in which the Ginzburg-Landau parameter varies in the range κ = 0.8-1.12. A detailed analysis of specific heat in the normal state of R 10 B 12 , R 11 B 12 and R nat B 12 (where R = Zr, Lu) has revealed three Einstein type vibration modes of Zr 4+ -ions in ZrB 12 with characteristic energies θ E1 (Zr N B 12 ) ≈ 200 K and θ E2,3 (Zr N B 12 ) ≈ 450 K, N. Sluchanko ( ) •