Ferromagnetic instability in the heavy-electron compoundURu2Si2doped with Re or Tc

Abstract

We report the first ferromagnetic instability observed in a heavy-electron system. Solid solutions of ${\mathrm{URu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{M}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$, where M=Re or Tc, have been found to undergo a ferromagnetic transition in the concentration range 0.4≲x≲1.4. Magnetization M measurements at 5 K reveal a small saturation moment that attains maximum values of 0.44${\mu }_B$/U atom at x≊0.8 for Re and 0.27${\mu }_B$/U atom at x≊1 for Tc. The Curie temperature ${\mathrm{FTHETA}}_C$ vs x in ${\mathrm{URu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Re}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$ also exhibits a maximum at x≊0.8. The electrical resistivity of both the Re- and Tc-substituted systems contains a $T^2$ contribution below ${\mathrm{FTHETA}}_C$, indicative of electron-magnon scattering. The low-temperature coefficient of the electronic specific heat, γ, in the ${\mathrm{URu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Re}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$ system initially increases with x, attains a value of 160 mJ/mol ${\mathrm{K}}^2$ at x=0.6, and then decreases with further increase of x. X-ray diffraction analyses indicate a crystallographic transformation in the ${\mathrm{URu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{M}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$ (M=Re, Tc) systems from tetragonal to cubic symmetry at x≊1. The strong sensitivity of M, ${\mathrm{FTHETA}}_C$, and γ to Re or Tc composition coupled with the large value attained by γ suggests that the ferromagnetism is associated with a narrow feature in the density of states of a band of strongly correlated electrons.