Hyperfine interactions at Ta impurities in cobalt and cobalt-hafnium intermetallic compounds.

Abstract

The magnetic and electric hyperfine interaction of $^{181}\mathrm{Ta}$ impurities on substitutional sites of cobalt metal has been investigated by \ensuremath{\gamma}\ensuremath{\gamma} perturbed angular correlations (PAC) in samples prepared by melting and ion implantation of $^{181}\mathrm{Hf}$. In molten samples the saturation value of the magnetic hyperfine field of Ta is ${\mathit{H}}_{\mathit{H}\mathit{F}}$(Ta:Co(hcp); 9 K)=-40.7(6) T for hexagonal Co and \ensuremath{\Vert}${\mathit{H}}_{\mathit{H}\mathit{F}}$${(}^{181}$Ta:Co(fcc); 9 K)\ensuremath{\Vert}=36.3(5) T for cubic Co. In ion-implanted samples radiation damage causes a slight reduction of the hyperfine field. The magnetic hyperfine field of $^{181}\mathrm{Ta}$ has also been determined in Fe: \ensuremath{\Vert}${\mathit{H}}_{\mathit{H}\mathit{F}}$${(}^{181}$Ta:Fe; 9 K)\ensuremath{\Vert}=61.7(9) T for a polycrystalline sample prepared by $^{181}\mathrm{Hf}$ implantation. The temperature dependence of ${\mathit{H}}_{\mathit{H}\mathit{F}}$ in Co has been measured between 9 and 1410 K. ${\mathit{H}}_{\mathit{H}\mathit{F}}$(T) in the hexagonal phase (T700 K) suggests a Curie temperature for hcp Co of ${\mathit{T}}_{\mathit{C}}$(hcp)=975(50) K. In the cubic phase, ${\mathit{H}}_{\mathit{H}\mathit{F}}$(T) of $^{181}\mathrm{Ta}$ is close to the magnetization curve of the host with ${\mathit{T}}_{\mathit{C}}$(fcc)=1398(5) K, in contrast to the anomalous temperature dependence of ${\mathit{H}}_{\mathit{H}\mathit{F}}$ of most other impurities in Co investigated up to now. The electric-field gradient of $^{181}\mathrm{Ta}$ on substitutional sites of hexagonal Co is \ensuremath{\Vert}${\mathit{V}}_{\mathit{z}\mathit{z}}$${(}^{181}$Ta:Co; 9 K)\ensuremath{\Vert}=1.23(4)\ifmmode\times\else\texttimes\fi{}${10}^{17}$ V/${\mathrm{cm}}^{2}$.The PAC spectra reflect the transformation of the solid solution of 0.2 at. % of Hf in elemental Co into the Co-Hf intermetallic compounds ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ and ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$ in the temperature range 900\ensuremath{\le}T\ensuremath{\le}1320 K. Upon cooling from 1400 K, elemental Co starts to transform into ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$ at about 1320 K and into ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ at about 1220 K. At 1100 K the evolution of the phase equilibrium between the solid solution and ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ was observed to occur with a half-life of ${\mathit{T}}_{1/2}$=5.5(4) h. The temperature dependence of the hyperfine-interaction parameters suggests magnetic-ordering temperatures of 450 K and 750 K for ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ and ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$, respectively.