Epitaxially Grown Single-Crystalline Cobalt Nanowires with High Coercivity

Abstract

I. INTRODUCTIONAlthough ferromagnetic particles can nowadays be grown in various size and shapes, controlling the crystallinity and microstructure parameters have remained challenging.[1,2] Nanomagnets with single-crystalline structures are particularly important because they typically exhibit reduced crystalline defects. Motivated by these prospects, our group has been established efforts in producing single-crystal and single-domain nanowires (NWs) through chemical process.[3-5] The single-domain structure led to changes in the magnetization reversal mechanism from the complicated domain-wall-motion-related process to a magnetic moment rotation mechanism that found to results in record high coercivity. Having a technique at hand that could extend growth to oriented magnetic NW array films would be highly desirable.Thin films of magnetic materials with high perpendicular anisotropy and coercivity have great application potential in ultrahigh-density perpendicular magnetic recording, permanent magnets, and spintronic and magnetoresistance sensors.[6] However, to synthesize and organize magnetic NWs to a vertically oriented structure is still a challenging task because of its dipolar interaction between neighboring NWs. In this work, we demonstrate bottom-up epitaxial solution growth of ferromagnetic Co NW arrays on crystalline metal surfaces. It is found that the coercivity of the NW arrays is strongly related to the wire diameter, film thickness, and orientation. This study provides a simple procedure to achieve recording media with ultra-high-recording densities.II. EXPERIMENTAL SECTIONSingle step chemical approach was used to fabricate the vertically aligned Co NW array films. In a typical synthesis, silicon substrates sputtered with Pt (111) film were kept in the solution of 1,2-butanediol, cobalt (II) laurate, and hexadecylamine (HDA). The growth was allowed for 12 h at 180 C in a Teflon-lined stainless-steel hydrothermal reactors. The diameter of Co NWs was controlled by varying the Pt film thickness from sub-10 nm to 100 nm.III. RESULTS AND DISCUSSIONFigures 1a-c show top view scanning electron microscopy image of Co NW arrays having wire diameter of 12-20 nm. The NW array film is highly dense, and nanowires are uniformly distributed over large wafer-scale area (25 x 25 mm) with an areal density up to 5.6 × 1012 wires/in2. The cross-sectional view (inset of Figures 1a-c) of the as-grown film demonstrates that the NWs are vertically aligned. High-resolution TEM imaging and X-ray diffraction (XRD) measurements demonstrate the epitaxial growth of cobalt on platinum, with c-axis along the long-axis of NWs (i.e perpendicular to the film). As one would expect, the 6-fold symmetry of the Pt(111) surface favors Co crystal growth along the hexagonal (0001) wire basal planes and hence with a single in-plane orientation.Figures 1d-e shows the room temperature magnetic hysteresis loops of the Co NWs arrays of wire diameters 12-20 nm. It is found that the coercivity for the NW arrays along the wires axis () increases from 3.4 kOe to 7 kOe with the decrease of diameter. In addition, it can also be seen from Figures 1d-e that the squareness in hysteresis loop and coercivity in the parallel direction of NWs are larger than those in the perpendicular direction, which reveals that the easy axis of the Co NWs is parallel to the wire axis. Although the NW array film shows a typical perpendicular anisotropic behavior with high coercivity along NW axis, the squareness ratio of MR/MS < 0.9. The low value of squareness could be due to the closely packed NW array, which induces a demagnetizing field along the plane of the film.The effective magnetic anisotropy of the NWs in an array film can results from the interplay of three major effects:1) the macroscopic demagnetization field, Hd = -6πMsp, where p is the porosity of the film and Ms is bulk value of the saturation magnetization for Co; 2) the shape anisotropy of individual nanowire Hsh= 2πMs; and 3) magnetocrystalline anisotropy field, Hm = -2K1/Ms, where K1 is the magnetocrystalline anisotropy constant for Co.[7] Thus, the theoretical effective coercive field of the nanowires is given by = Hd + Hsh + Hm. The calculated effective anisotropy field for Co NWs is ~12.5 kOe, which is much higher than the experimental coercivity values. The observed low coercivity in the prepared Co NW arrays is due to the strong magnetostatic coupling among the NWs, which is in agreement with those reported in the literature.[8]IV. CONCLUSIONSIn conclusion, highly ordered Co NW arrays with coercivity comparable to magnetocrystalline anisotropy have been successfully fabricated via a simple chemical synthesis process. Magnetic measurements show that the as-prepared Co NWs with different diameters display magnetic anisotropy with the easy magnetization direction parallel to the axis of Co NW. The coercivity measured along the wire axis increases with decreasing nanowires diameter, which reveals the role of shape anisotropy in enhancing coercivity. In addition, this study opens an interesting option in tuning the magnetic properties of NW arrays and their applications in perpendicular recording media. **