Origin of magnetic moments in defective TiO2 single crystals

In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, \emph{i.e.} Ti$^{3+}$ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO$_{6-x}$) stretching Raman mode. We elucidate that Ti$^{3+}$ ions with one unpaired 3d electron provide the local magnetic moments.Comment: 4 pages, 4 figures, to be published at Phys. Rev.

Spinel ferrite nanocrystals embedded inside ZnO: magnetic, electronic and magneto-transport properties

In this paper we show that spinel ferrite nanocrystals (NiFe2O4, and CoFe2O4) can be texturally embedded inside a ZnO matrix by ion implantation and post-annealing. The two kinds of ferrites show different magnetic properties, e.g. coercivity and magnetization. Anomalous Hall effect and positive magnetoresistance have been observed. Our study suggests a ferrimagnet/semiconductor hybrid system for potential applications in magneto-electronics. This hybrid system can be tuned by selecting different transition metal ions (from Mn to Zn) to obtain various magnetic and electronic properties.Comment: 12 pages, 14 figs. accepted for publication at PR

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 paper we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences and 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 the Fe concentration and the process temperature are found: 1) Disperse Fe$^{2+}$ and Fe$^{3+}$ at low Fe concentrations and low processing temperatures, 2) FeZn$_2$O$_4$ at very high processing temperatures and 3) an intermediate regime with a co-existence of metallic Fe (Fe$^0$) and ionic Fe (Fe$^{2+}$ and Fe$^{3+}$). Ferromagnetism is only observed in the latter two cases, where inverted ZnFe$_2$O$_4$ and $\alpha$-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier mediated coupling. However, their separation is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison investigations of Fe-implanted epitaxial ZnO thin films are presented.Comment: 14 pages, 17 figure

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 alpha-Fe nanoparticles. High vacuum annealing at 823 K results in the growth of alpha-Fe particles, but the annealing at 1073 K oxidized the majority of the Fe nanoparticles. After a long term annealing at 1073 K, crystallographically oriented ZnFe2O4 nanoparticles were formed inside ZnO with the orientation relationship of ZnFe2O4(111)[110]//ZnO(0001)[1120]. These ZnFe2O4 nanoparticles show a hysteretic behavior upon magnetization reversal at 5 K.Comment: 21 pages, 7 figures, accepted by J. Phys. D: Appl. Phy

Ferromagnetic transition metal implanted ZnO: a diluted magnetic semiconductor?

Recently theoretical works predict that some semiconductors (e.g. ZnO) doped with magnetic ions are diluted magnetic semiconductors (DMS). In DMS magnetic ions substitute cation sites of the host semiconductor and are coupled by free carriers resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid-solubility limit of magnetic ions in semiconductor host. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5-10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism. Due to their different crystallographic orientation with respect to the host crystal these nanocrystals in some cases are very difficult to be detected by a simple Bragg-Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films ranges between 10 and 40 at.% using different growth conditions pulsed laser deposition. Such diluted, Co-doped ZnO films show paramagnetic behaviour. However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d exchange interaction as compared to Co-implanted ZnO single crystals.Comment: 27 pages, 8 figure

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

We investigated ZnO(0001) 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 for 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.Comment: 20 pages, 6 figure

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

We investigated ZnO(0001) 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

In situ observation of secondary phase formation in Fe implanted GaN annealed in low pressure N2N_2 atmosphere

The formation of secondary phases in Fe implanted GaN upon annealing in low pressure N$_2$-atmosphere was detected by means of in situ x-ray diffraction and confirmed by magnetization measurements. A repeatable phase change from Fe$_3$N at room temperature and Fe$_3$−$_x$N at 1023 K was observed in situ. The phase transformation is explained by the change of lattice site and concentration of nitrogen within nitrides. The diffusion of Fe toward sample surface and oxidation with increasing annealing cycles limits the availability of secondary phase and hence the repeatability. At high temperature GaN dissolves and Ga as well as Fe oxidize due to presence of residual oxygen in the process gas. The ferromagnetism in the samples is related to nanometer sized interacting Fe$_3$−$_x$Ncrystallites.GaN is a wide band gap semiconductor that has been studied intensively in the last years because of its potential field of applications like in optoelectronics, plasmonics, as well as for high power electronics. By doping with transitional metals like Fe it might also be a diluted magnetic semiconductor (DMS) with a Curie temperature above room temperature (RT) and could then be used for spintronics.1 There are many experimental studies reporting ferromagnetism at RT in Fe doped GaN. In a real DMS, with magnetic atoms randomly substituting cation sites, ferromagnetic coupling is supposed to be due to the indirect exchange coupling between magnetic impurities mediated by holes.1–4 Experimental observation of strong-coupling effects in a DMS Ga$_1$−$_x$Fe$_x$Nwas reported by Pacuski et al.5 Robust ferromagnetism in the region of insulator-to-metal transition was predicted for high hole densities. However, there are also other possible sources of ferromagnetism like spinodal decomposition of Fe or ferromagnetic secondary phases. The detection of those is rather difficult. Bonanni et al.6 prepared GaN:Fe layers by metalorganic chemical vapor deposition (MOCVD) and observed ferromagnetism that was partially accounted to the spinodal decomposition and non-uniform distribution of Fe-rich magnetic nanocrystals. Kuwabara et al.7 reported the formation of nanoclusters and superparamagnetic behavior in GaN:Fe epilayers prepared by rf-plasma-assisted molecular beam epitaxy. In case of ion implantation the reports from different groups are quite controversial. Theodoropoulou et al.8 and Shon et al.9,10 did not relate ferromagnetic response to secondary phases after transition ion implantation into semiconductors. In our experiments, however, the formation of α-Fe nanoclusters, that were responsible for ferromagnetic response, was observed.11 Though the appearance of such precipitates is not desired in a DMS they might be useful for certain applications.12 Li et al.13 detected co-occurrence of α-Fe and ε-Fe$_3$Nin MOCVD prepared GaN:Fe films and pointed out the role of nitrogen pressure and structural disorder in the formation of Fe-rich phases. Bonanni et al.14 have shown that the controlled aggregation of magnetic ions in a semiconductor can be affected by the growth rate and doping with shallow impurities.Recently we reported predominant formation of epitaxially oriented α-Fenanoclusters if Fe-doped samples were annealed in a N$_2$ flow at 1.1 bar pressure.11 In this paper we report the formation of ε-Fe$_3$−$_x$N with x<1 that builds up during annealing at 1073 K in 0.5 bar N2 and the reversible transformation to ε-Fe$_3$Nduring cooling down to RT.P-type (Mg) doped (∼2×10$^{17}$cm$^{−3}$)single crystalline wurtzite GaN(001) films of about 3μm thickness epitaxially grown by metal organic vapor phase epitaxy on sapphire (001) were used. Samples, 7° tilted relative to the ion beam to avoid channeling, were implanted with 195 keV $^{57}$Fe ions with fluence Φ=4×10$^{16}$cm$^{−2}$ (peak Fe concentration of 4 at. % at the projected range R$_p$=85nm according to TRIM$^{15}$), keeping the samples at RT. In order to reduce the implantation damage and to investigate the formation of secondary phases the implanted samples were annealed at 1073 K in a low pressure N$_2$-atmosphere(0.5 bar) within several minutes. The annealing experiments along with the in situ x-ray diffraction characterization were performed at the Rossendorf beamline at the ESRF in Grenoble with a x-ray wavelength of λ=0.124nm. The annealing chamber was equipped with a boron nitride heater, controlled by a Eurotherm controller, gas inlet and a half sphere beryllium dome. The temperature was measured by a PtRh/Pt thermocouple placed on top of the heater. The gas pressure in the chamber was limited to 0.5 bar with a flow of about 40l⋅min$^{−1}$. The purity of the N$_2$gas (99.9999%) was limited by the setup with an oxygen contamination in the ppm range.A Pilatus 100 K two-dimensional (2D) pixel detector was used to record 2D diffraction pattern. Additionally, a scintillation counter was used for 2θ-ω-scans. Generally, for clusters in the range of some nm the signal to background ratio is very low. In order to increase the signal to background ratio to an acceptable level the acquisition time of the 2D detector was set to 10 s and those of the scintillation counter to 5 s per point. The investigations of the magnetic properties were performed with a Quantum Design MPMS superconducting quantum interference device magnetometer. For the evaluation of the 2D detector exposures rectangular areas with dimensions 2θ from 32.3° to 36.5° and χ=±0.2° (angle ⊥ to the scattering plane) were integrated over χ and assumed as line scans. Those line scans are represented over annealing/cooling time with the color/grayscale coded intensity in Fig. 1(a). Single examples of line scans are given in Figs. 1(b) and 1(c) for RT and 1073 K, respectively. Prior to the annealing procedure (time t=0 at RT) no reflexes from secondary phases were detected [see Fig. 1(a)]. After about 200 s annealing, at 1073 K, a broad reflex starts to evolve in the region between 32.5° and 35° with a local maximum at about 33.3°. The peak shift due to the lattice expansion from RT to 1073 K is in the order of 0.1° and can be neglected. The position of the maximum fits well to the pattern of FeN(200) (33.46° at RT). However, because of the broadness of the reflex, other nitrides like disordered ε-Fe$_3$−$_x$N(002) or ζ-Fe$_2$N(102) can also be taken into account. Disordered in this sense means the redistribution of mainly N atoms within the structure and can be described by the transfer of N from Fig. 2(b) and 2(c) Wyckoff site position in the ε-Fe$_3$−$_x$N-phase.16 From symmetry reasons the formation of ε-Fe$_3$−$_x$N is more probable since it features the same type of structure (wurtzite) as GaN. The strain caused by the lattice mismatch is supposed to relax by generating misfit dislocations, as was shown in Ref. 13