Magnetic properties of NpGa3 at high pressures.

Abstract

High-pressure studies on ${\mathrm{NpGa}}_{3}$ were performed using $^{237}\mathrm{Np}$ M\"ossbauer spectroscopy, resistivity measurements, and x-ray diffraction, up to 9.2 GPa between 1.3 and 130 K, up to 25 GPa between 1.3 K and room temperature, and up to 40 GPa at room temperature, respectively. The cubic ${\mathrm{AuCu}}_{3}$ crystal structure at ambient pressure is preserved up to 40 GPa. The bulk modulus ${\mathit{B}}_{0}$ is 75(2) GPa with ${\mathit{B}}_{0}^{\ensuremath{'}}$=6(2). ${\mathrm{NpGa}}_{3}$ orders antiferromagnetically at ambient pressure at ${\mathit{T}}_{\mathit{N}}$=67 K. The magnetic ordering temperature increases up to \ensuremath{\sim}200 K at 25 GPa. At 51 K and at ambient pressure a first-order antiferro- to ferromagnetic (AF-F) transition occurs with a sudden reduction of the magnetic hyperfine field ${\mathit{B}}_{\mathrm{hf}}$ by \ensuremath{\sim}15% when entering the AF phase. At elevated pressure this transition is no longer observed. At 4.2 K ${\mathit{B}}_{\mathrm{hf}}$, the value of the electric quadrupole coupling constant |${\mathit{e}}^{2}$qQ| and the isomer shift S slightly but continuously decrease with reduced volume. Above ${\mathit{T}}_{\mathit{N}}$ a negative logarithmic resistivity slope d\ensuremath{\rho}/d ln T0 is present at ambient pressure and disappears at 3 GPa. The slight decrease of ${\mathit{B}}_{\mathrm{hf}}$ at 4.2 K, the variation of ${\mathit{e}}^{2}$qQ and S with pressure, and the suppression of d\ensuremath{\rho}/d ln T at \ensuremath{\sim}3 GPa indicate 5f electron delocalization. This delocalization, however, is much less pronounced than in band magnets, as, e.g., in ${\mathrm{NpOs}}_{2}$. A magnetic phase diagram is suggested that consistently explains the pressure variation of \ensuremath{\rho}, ${\mathit{B}}_{\mathrm{hf}}$ and ${\mathit{T}}_{\mathrm{ord}}$. We discuss the properties of ${\mathrm{NpGa}}_{3}$ in terms of the Kondo interaction and the Doniach phase diagram, and alternatively, within a model which includes 5f electron delocalization effects. This second model seems to be promising. \textcopyright{} 1996 The American Physical Society.