Magnetic anisotropy induced in NiCo granular nanostructures by ZnO nanorods deposited on a polymer substrate

Abstract

Hetero-nanostructures made of NiCo and ZnO nanostructures have been deposited on a polymer substrate through a two-step method. First a ZnO nanorod array was grown on polydimethylsiloxane (PDMS) substrate by a hydrothermal method. NiCo granular nanostructures were then deposited on the ZnO nanorod array through an electroless polyol process with precursors of Ni complex and Co complex in a mole ratio of 50 : 50. The effect of deposition time (t) on the structure and properties of the hetero-nanostructures has been studied. Ni-rich nanostructures are observed when t < 30 min. With t increasing, the atomic ratio (at.%) of Ni to Co approaches 50 : 50. In addition, a magnetic field (10 kOe) was applied on the hetero-nanostructures, which was parallel and perpendicular to the ZnO nanorods. The magnetization (M_s) of the NiCo nanostructures deposited on ZnO nanorods when t = 60 min is measured at 93.4 emu g⁻¹. The results of magnetic measurements and X-ray absorption near edge structure spectroscopy (XANES) suggest that the slightly lower magnetizations of the hetero-nanostructures may be caused by the oxidation of Co and Ni on the surface of the granular films. The values of squareness ratio (M_r/M_s) of the hetero-nanostructures with t are far below 0.5. Moreover, the coercivity (H_c) value of the hetero-nanostructures measured in the direction perpendicular to the ZnO nanorods is higher than that in the parallel direction. Therefore, the deposited NiCo granular nanostructures are perpendicular to the plane of the PDMS substrate, and show perpendicular magnetic anisotropy. These results clearly demonstrate that the rod-shaped non-magnetic nanomaterials can significantly affect the magnetic anisotropy of the hetero-nanostructures.