Magnetoresistance anisotropy in a hexagonal lattice of Co antidots obtained by thermal evaporation Alessandro Chiolerio a,Ã , Paolo Allia b , Edvige Celasco b , Paola Martino a , Federico Spizzo c , Federica Celegato d a Physics Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, IT-10129 Torino, Italy b Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, IT-10129 Torino, Italy c Physics Department and CNISM, Universit  a di Ferrara, Building C, Via Saragat 1, IT-44100 Ferrara, Italy d Electromagnetism Division, INRIM, Strada delle Caccie 91, IT-10135 Torino, Italy article info Available online 15 September 2009 Keywords: Thin film Patterned material Anisotropic magnetoresistance abstract Patterned soft magnetic materials are eligible for use in magnetic random access memories. A hexagonal-lattice pattern of circular antidots was produced by optical lithography in a Co film. In order to test the effect of geometry on the local magnetisation configuration of such a structure, we performed room-temperature angle-resolved magnetisation measurements aimed to check the pinning of domain walls by the pattern’s lattice. Magnetoresistance (MR) room-temperature measurements were performed at various angles between the magnetic field direction and the macroscopic electrical current vector, to clarify whether and how the local current density configuration affects the MR response. We found that the magnetoresistance is of anisotropic type (AMR) and has a local origin. Furthermore, the largely unsaturating behaviour of MR at high fields may be explained only by considering that tiny portions of the pattern constitute highly frustrated regions and align their magnetisation at rather high fields. A simplified model based on a local anisotropy term is shown to account for the experimental results for both M and MR. & 2009 Elsevier B.V. All rights reserved. 1. Introduction and experimental Large-area hexagonal-lattice antidot patterns were obtained by optical lithography on a thin Co film thermally evaporated on a Si (10 0) substrate. Morphological analysis performed by Tapping Mode Atomic Force Microscopy (AFM) allowed us to evaluate, with high resolution, dots morphology, thickness (65 nm), surface roughness (0.2 nm), hole diameter (900 nm) and hole-to-hole distance (300 nm). Magnetic domains were observed by means of Lift Mode Magnetic Force Microscopy (MFM) with a DI NanoScope III, using a commercial MFM tip (MESP Co–Cr coated). Sample scans were taken in air at room temperature. A straight probe for electrical measurements, consisting of a 15-nm-thick Cu chevron- like bridge, deposited across the patterned area, was specifically designed and realised after scanning probe characterisation. Angle-resolved MR measurements were done, at room tempera- ture, with the four-contact technique by rotating the sample along its out-of-plane axis and applying the magnetic field at a = 01, 301, 601 and 901 with respect to the in-plane macroscopic current density vector. The DC current density was kept constant at 30 mA. Angle-resolved magnetisation measurements were performed under the same conditions through a Superconducting Quantum Interference Device (SQUID) magnetometer (max. field 50 kOe). A Finite Element Method (FEM) simulation has been run in a Comsol Multiphysics TM environment, reproducing the sample geometry and evaluating the local current density distribution. 2. Results and discussion An enhanced image of the antidot pattern is shown in Fig. 1a; it has been obtained by the superposition of two subsequent scans acquired with mutually orthogonal fast scan axis, as described in [1]. The static magnetic structures, investigated by MFM, are shown in the enhanced image of Fig. 1b (corresponding to a demagnetised sample state). Magnetic information derived from the MFM images was enhanced by means of a numerical procedure [1], which has been shown to compensate possible artefacts like instrumental distor- tions and to improve the signal-to-noise ratio. The samples were demagnetised before each MFM measurement, by applying a reducing alternate magnetic field, both in-plane and out-of-plane. Each image was recorded at crossed scanning tip directions (01, 901) to check the possible magnetic probe interactions with the ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.044 Ã Corresponding author. Tel.: +39 0115647381; fax: +39 0115647399. E-mail address: alessandro.chiolerio@polito.it (A. Chiolerio). Journal of Magnetism and Magnetic Materials 322 (2010) 1409–1412