Are Fe and Co implanted ZnO and III-nitride semiconductors magnetic?

The chemical nature, lattice site locations and magnetic behaviour of Fe and/or Co ions implanted in nitrides (GaN, AlN, and InN) and in ZnO have been investigated using Mössbauer spectroscopy and vibrating sample magnetometer (VSM) techniques. Mössbauer data on nitride and $^{56}$Fe pre-implanted ZnO samples were obtained from emission Mössbauer spectroscopy (eMS) measurements at the ISOLDE facility, CERN, following the implantation of radioactive $^{57}$Mn$^{*}$ which $\beta$$^{-}$decays to the 14.4 keV Mössbauer state of $^{57}$Fe. In addition, conversion electron Mössbauer spectroscopy (CEMS) data were collected on ZnO single crystals co-implanted with $^{57}$Fe + $^{56}$Fe and $^{57}$Fe + $^{59}$Co ions in a box profile. Emission Mössbauer spectra obtained for GaN and AlN reveal magnetic structure in the ‘wings’ assigned to high spin Fe$^{3+}$ weakly coupled to the lattice showing spin-lattice relaxation effects. The observed spin-relaxation rate (τ$^{-1}$) closely follows a ${T}^{2}$ temperature dependence, typical of a two-phonon Raman process expected in the high temperature region. The spectra for InN did not display any magnetic structure, presumably due to the absence of high spin Fe$^{3+}$. The central region of the spectra for all nitride samples showed an angle dependence reflecting the crystalline nature of the environment of most $^{57}$Fe probes while a small fraction was attributed to implantation induced damage in isolated amorphous regions. The absence of recoil produced interstitials in the nitride samples could be explained by displacement energies of III sublattice atoms being greater than the recoil imparted to the $^{57}$Fe ions in the $^{57}$Mn$^{*}$ $\beta$$^{-}$decay, resulting in the daughter $^{57}$Fe maintaining the substitutional lattice site occupied by $^{57}$Mn$^{*}$ upon implantation. For the $^{56}$Fe pre-implanted ZnO samples, magnetic structure is visible in the ‘wings’ of the spectra as observed previously for virgin ZnO. The spin-lattice relaxation rates of Fe$^{3+}$ in the pre-implanted ZnO samples increase with increasing fluence at equivalent temperatures. Strong magnetic features are evident in the CEMS spectrum of ZnO:$^{57}$Fe+$^{56}$Fe after annealing at 973 K which are attributed to different Fe sites in $\epsilon$-Fe$_{2}$O$_{3}$ nanoparticles. This result is supported by VSM measurements conducted at 4 K and 293 K showing ferromagnetic signals with coercive fields of ~0.08 T and ~0.02 T, and corresponding remanent magnetisation fields which are ~62% and ~34% of the saturation magnetisation at respective temperatures. The spectrum for ZnO:$^{57}$Fe+59Co did not reveal any magnetic feature at 973 K, but shows a switch to hyperfine parameters consistent with Fe$^{3+}$ in different local environments. The absence of any magnetic structure in the spectra suggests the absence of $\epsilon$-Fe$_{2}$O$_{3}$, attributed to low concentration of Fe atoms. The spin-lattice relaxation rates of Fe$^{3+}$, lattice site locations, annealing behaviour and variation of hyperfine parameters of spectral components as a function of temperature in these materials are discussed. In addition, a report on the development of a new parallel plate avalanche counter (PPAC) based on an FeAl single line electrode is presented.