Fe-implanted ZnO: Magnetic precipitates versus dilution

Abstract

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 Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

“…253 K) prohibits precipitation, however results in non-magnetism in our case, e.g. Fe implanted ZnO [54]. …”

Spinel ferrite nanocrystals embedded inside ZnO: Magnetic, electronic, and magnetotransport properties

“…Annealing was performed in a high vacuum (base pressure ≤10 −6 mbar) furnace at 1073 K for 60 minutes. In our previous study we have performed detailed annealing investigation for transition metal implanted ZnO single crystals 3,14,15 . Briefly, more metallic clusters formed when annealing at mild temperatures (823 K or 923 K), while the oxidation starts at around 1073 K. Keeping this high temperature, a longer annealing time results in the formation of ferrites in Fe implanted ZnO.…”

Paramagnetism and antiferromagnetic interactions in single-phase Fe-implanted ZnO

“…1] which are oxidized and then converted to the spinel ferrite ZnFe 2 O 4 [region (4) in Fig. 1] with increasing annealing temperature and duration [9,10,11,12]. Although this segregated region of the phase diagram of Fe-implanted ZnO is rather well established (continuous part of the arrow in Fig.…”

“…that it does not result from long-range magnetic order of randomly distributed magnetic dopants [1]. While limited experimental evidence exists that some DMS materials are intrinsically ferromagnetic at room temperature, a number of non-intrinsic sources of ferromagnetic-like behavior have been identified and are becoming increasingly well documented: magnetic contamination [2,3,4,5], instrumental artifacts [4,5,6] and chemical and/or structural segregation of the transition metal dopants into non-DMS phases [7,8,9,10,11,12,13,14,15,16,17].…”

“…the impurities precipitate in nanocomposites with a well defined interface with the host structure (e.g. in ZnO doped with Fe, Co and Ni [9,10,11,12,13,14,15,16,17]). …”