PHYSICAL REVIEW B 83, 245323 (2011) GaN/Fe core/shell nanowires for nonvolatile spintronics on Si Cunxu Gao, * Rouin Farshchi, Claudia Roder, Pinar Dogan, and Oliver Brandt Paul-Drude-Institut f¨ ur Festk¨ orperelektronik, Hausvogteiplatz 5–7, D-10117 Berlin, Germany (Received 25 March 2011; published 29 June 2011) We explore the relationship between the structural and magnetic properties of GaN/Fe core/shell nanowires grown epitaxially on Si substrates. The magnetic properties are consistent with the coexistence of two magnetic contributions: a ferromagnetic response from the single-crystalline Fe particles formed at the nanowire tips, and a superparamagnetic response originating from the granular Fe clusters grown on the nanowire sidewalls, giving them a corncob-like morphology. We show that our interpretation of the origin of the magnetic behavior can be confirmed by the viscous decay of magnetic remanence in the nanowires. Ferromagnetic remanence is observed both parallel and perpendicular to the nanowire axis, making such structures appealing as high-density nonvolatile spintronic components on Si. DOI: 10.1103/PhysRevB.83.245323 PACS number(s): 61.46.Km, 68.37.−d, 75.50.−y, 81.15.Hi I. INTRODUCTION Core/shell nanowires (NWs) have captivated the imagina- tion of many researchers over the past decade due to their novel functionalities over a range of potential applications. 1–5 In particular, high-quality III-V semiconductor (SC) core/shell structures of GaAs, GaP, and GaN NWs on Si substrates are finally bridging the lattice incompatibility gap between Si electronics and III-V optical components that has plagued thin films for decades, 6 hence allowing for III-V NW light-emitting diodes (LEDs) 7 and lasers 8,9 to be grown on Si substrates. The introduction of a ferromagnetic (FM) component as the shell adds even more versatility to core/shell NWs. 10–12 Injection of spin-polarized electrons from FMs into SCs and their subsequent manipulation in gated structures or conversion into circularly polarized light in spin-LEDs and spin vertical- cavity surface-emitting lasers (VCSELs) are active research areas in the field of SC spintronics, 13 which aims to harness the spin information of electrons. Synthesis of such structures in NW form is advantageous for the realization of high- density memory elements, directional sources of circularly polarized light that can operate in magnetic remanence, and functional multilayer architectures such as spin-torque devices that cannot be realized as thin films in a bottom-up approach. A GaN core paired with an Fe shell is of particular interest, since the Fe/GaN interface has been shown to be thermodynamically stable. 14 Moreover, epitaxial GaN films have been grown on Fe substrates, 15 representing one of the rare exceptions of a successful SC-on-metal epitaxy, potentially allowing for FM/SC/FM heterostructure NWs. Here, we synthesize GaN/Fe core/shell NWs on a Si substrate by molecular beam epitaxy (MBE) and find that the structural properties of the Fe shell result in the coexistence of FM and superparamagnetic (SP) contributions, in strong contrast to Fe/GaN thin-film heterostructures. Furthermore, the NWs exhibit a nonvanishing remanence in both parallel and perpendicular field config- urations, making them attractive for nonvolatile spintronic applications. II. EXPERIMENTAL The epitaxial growth of the GaN/Fe core/shell NWs was performed in a custom-built plasma-assisted MBE system equipped with solid-source effusion cells for Ga and Fe. Active nitrogen was provided by a radiofrequency N 2 plasma source. Nucleation and growth were monitored in situ by reflection high-energy electron diffraction (RHEED). GaN NWs were grown directly on Si(111) 2 ′′ wafers under N-rich conditions (N/Ga = 3) at a substrate temperature of 800 ◦ C. Directly after growth of the GaN NWs, the substrate was cooled down to 350 ◦ C and Fe deposition was initiated with an Fe flux of 1.7 × 10 13 atoms/cm 2 s, while keeping the chamber pressure at the low 10 −9 mbar range. Both during GaN and Fe growth, the substrates were continuously rotated to ensure homogeneous conditions across the wafer. To assess the NW morphology, the samples were cleaved into smaller pieces for investigation by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the latter through the standard mechanical thinning and ion-milling processes. A reference sample consisting of an Fe film with a thickness of 24 nm was grown at a substrate temperature of 350 ◦ C on a GaN layer. Finally, the magnetic properties of the NWs and the reference sample were measured in a superconducting quantum interference device magnetometer (SQUID). All magnetization data presented here are corrected for the diamagnetic background of the substrate. III. RESULTS AND DISCUSSION We focus on two sets of samples, each comprised of a sample with low and high Fe coverage and essentially differing in the length of the NWs. Figure 1 depicts SEM images taken in top-view of samples S1–S4. In all cases, a high-density NW array, with an estimated density of more than 10 10 NWs/cm 2 , is observed. The diameter distributions are broad with average values increasing with the growth times of the GaN NWs as well as the Fe deposition (see below for quantitative results). Side-view SEM images of samples S1–S4 are shown in Fig. 2 and reveal that the NWs are vertically aligned and exhibit an average height solely determined by the growth time of the GaN NWs. The presence of Fe becomes clearly evident in images with higher magnification, as displayed in Fig. 3 for samples S1–S4. While bare GaN NWs exhibit atomically smooth (10 ¯ 10) side and (0001) top facets, the Fe deposition results in a corncob-like morphology of the NWs side facets and pronounced hats 245323-1 1098-0121/2011/83(24)/245323(5) ©2011 American Physical Society