URu2Si2 is surely one of the most mysterious of the heavy-fermion compounds. Despite more than twenty years of experimental and theoretical works, the order parameter of the transition at T0 = 17.5 K is still unknown. The state below T0 remains called "hidden-order phase" and the stakes are still to identify the energy scales driving the system to this phase. We present new magnetoresistivity and magnetization measurements performed on very-high-quality single crystals in pulsed magnetic fields up to 60 T. We show that the transition to the hidden-order state in URu2Si2 is initially driven by a high-temperature crossover at around 40-50 K, which is a fingerprint of intersite electronic correlations. In a magnetic field H applied along the easy-axis c, the vanishing of this high-temperature scale precedes the polarization of the magnetic moments, as well as it drives the destabilization of the hidden-order phase. Strongly impurity-dependent magnetoresistivity confirms that the Fermi surface is reconstructed below T0 and is strongly modified in a high magnetic field applied along c, i.e. at a sufficiently-high magnetic polarization. The possibility of a sharp crossover in the hidden-order state controlled by a field-induced change of the Fermi surface is pointed out.
We report magnetization and magnetoresistivity measurements on the
isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields
up to 60 T and temperatures from 1.5 to 80 K. At low-temperature, a moment
polarization in UCoGe in a field $\mu_0\mathbf{H}\parallel\mathbf{b}$ of around
50 T leads to well-defined anomalies in both magnetization and
magnetoresistivity. These anomalies vanish in temperatures higher than 30-40 K,
where maxima in the magnetic susceptibility and the field-induced variation of
the magnetoresistivity are found. A comparison is made between UCoGe and URhGe,
where a moment reorientation in a magnetic field
$\mu_0\mathbf{H}\parallel\mathbf{b}$ of 12 T leads to field-induced reentrant
superconductivity.Comment: 4 pages, 4 figure
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