H. Reuther

Fe implanted ferromagnetic ZnO

Room-temperature ferromagnetism has been induced within ZnO single crystals by implant-doping with Fe ions.The four samples implanted initially have been analyzed using SQUID (superconducting quantum interference device) magne-tometry. It was found that only two of them, i.e. the HFHT and the LFLT samples exhibit a pronounced hysteresis loop upon magnetization reversal at 5 K and 300 K. As was found using synchrotron X-ray diffraction, transmission electron microscopy and conversion electron Mossbauer spectroscopy (CEMS) the origin of the ferromagnetic properties of the HFHT-sample are tiny alpha-Fe-nanoparticles with a mean diameter of 8 nm. At 180 keV Fe implanted ZnO single crystals can develop ferromagnetic properties that are either caused by alpha-Fe nanoparticles or an indirect coupling of the Fe ions in a DMS system, depending on the details of ion fluence and implantation temperature. In any case sophisticated structural characterization is required in order to rule out secondary phases.

Fe-implanted ZnO: Magnetic precipitates versus dilution

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier-mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present article, we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences, temperatures, and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to Fe concentration and process temperature are found: (1) Disperse Fe2+ and Fe3+ at low Fe concentrations and low processing temperatures, (2) FeZn2O4 at very high processing temperatures, and (3) an intermediate regime with a coexistence of metallic Fe (Fe0) and ionic Fe (Fe2+ and Fe3+). Ferromagnetism is only observed in the latter two cases, where inv...

Crystallographically oriented magnetic ZnFe2O4 nanoparticles synthesized by Fe implantation into ZnO

In this paper, a correlation between structural and magnetic properties of Fe-implanted ZnO is presented. High fluence Fe+ implantation into ZnO leads to the formation of superparamagnetic α-Fe nanoparticles. High vacuum annealing at 823 K results in the growth of α-Fe particles, but the annealing at 1073 K oxidizes the majority of the Fe nanoparticles. After a long term annealing at 1073 K, crystallographically oriented ZnFe2O4 nanoparticles are formed inside ZnO with the orientation relationship of ZnFe2O4(1 1 1)[1 1 0]//ZnO(0 0 0 1)[11 2 0]. These ZnFe2O4 nanoparticles show a hysteretic behaviour upon magnetization reversal at 5 K.

Ga1−xMnxN epitaxial films with high magnetization

We report on the fabrication of pseudomorphic wurtzite GaMnN grown on GaN with Mn concentrations up to 10% using molecular beam epitaxy. According to Rutherford backscattering the Mn ions are mainly at the Ga-substitutional positions, and they are homogeneously distributed according to depth-resolved Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A random Mn distribution is indicated by transmission electron microscopy, no Mn-rich clusters are present for optimized growth conditions. A linear increase of the c-lattice parameter with increasing Mn concentration is found using x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting quantum-interference measurements showing saturation magnetizations of up to 150 emu/cm^3.

Ferromagnetism and suppression of metallic clusters in Fe implanted ZnO -- a phenomenon related to defects?

We investigated ZnO(0 0 0 1) single crystals annealed in high vacuum with respect to their magnetic properties and cluster formation tendency after implant-doping with Fe. While metallic Fe cluster formation is suppressed, no evidence for the relevance of the Fe magnetic moment to the observed ferromagnetism was found. The latter along with the cluster suppression is discussed with respect to defects in the ZnO host matrix, since the crystalline quality of the substrates was lowered due to the preparation as observed by x-ray diffraction.

Absence of ferromagnetism in V-implanted ZnO single crystals

The structural and magnetic properties of V doped ZnO are presented. V ions were introduced into hydrothermal ZnO single crystals by ion implantation with fluences of 1.2×1016–6×1016cm−2. Postimplantation annealing was performed in high vacuum from 823to1023K. The ZnO host material still partly remains in a crystalline state after irradiation and is partly recovered by annealing. The V ions show a thermal mobility as revealed by depth profile Auger electron spectroscopy. Synchrotron radiation x-ray diffraction revealed no secondary phase formation which indicates the substitution of V onto Zn site. However, in all samples no pronounced ferromagnetism was observed down to 5K by a superconducting quantum interference device magnetometer.

The effect of flash lamp annealing on Fe implanted ZnO single crystals

The effect of flash lamp annealing applied to ZnO single crystals implanted with 3.6 at. % Fe has been studied. For intermediate light power, the implantation-induced surface defects could be annealed without creation of secondary phases within the implanted region. At the same annealing temperatures, however, ion-beam-induced open volume defects start to increase in size. Recrystallization is initiated for the highest light power applied, i.e., the ion-beam-induced lattice disorder reflected by the minimum channeling yield of Rutherford backscattering spectroscopy decreases from 76% to 46% and the open volume defects are annealed. At the same time, the Fe3+ fraction increases at the cost of the Fe2+ states. Weak ferromagnetic properties that are mainly associated with nanoparticles are induced.

Suppression of secondary phase formation in Fe implanted ZnO single crystals

Unwanted secondary phases are one of the major problems in diluted magnetic semiconductor creation. Here, the authors show possibilities to avoid such phases in Fe implanted and postannealed ZnO(0001) single crystals. While α-Fe nanoparticles are formed after such doping in as-polished crystals, high temperature (1273K) annealing in O2 or high vacuum before implantation suppresses these phases. Thus, the residual saturation magnetization in the preannealed ZnO single crystals is about 20 times lower than for the as-polished ones and assigned to indirect coupling between isolated Fe ions rather than to clusters.

Ion beam synthesis of Fe nanoparticles in MgO and yttria-stabilized zirconia

To form embedded Fe nanoparticles, MgO(001) and YSZ(001) single crystals have been implanted at elevated temperatures with Fe ions at energies of 100keV and 110keV, respectively. The ion fluence was fixed at 6×1016cm−2. As a result, γ- and α-phase Fe nanoparticles were synthesized inside MgO and YSZ, respectively. A synthesis efficiency of 100% has been achieved for implantation at 1273K into YSZ. The ferromagnetic behavior of the α-Fe nanoparticles is reflected by a magnetic hyperfine field of 330kOe and a hysteretic magnetization reversal. Electron holography showed a fringing magnetic field around some, but not all of the particles.

Fe nanoparticles embedded in MgO crystals

Iron nanoparticles embedded in MgO crystals were synthesized by Fe+ ion implantation at an energy of 100 keV and varying fluences from 3×1016 to 3×1017 cm−2. Investigations of structural and magnetic properties of Fe nanoparticles have been performed using magnetometry, x-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy, as well as by theoretical Preisach modeling of bistable magnetic systems. It has been found that α- and γ-Fe nanoparticles are formed for all fluences. The content of the α-Fe phase increases at higher fluences and after annealing. The influence of postimplantation annealing at 800 °C in vacuum and under enhanced hydrostatic pressure on the formation of nanoparticles has been analyzed.