Magnetoresistance and magnetization reversal of single Co nanowires R. A. Silva, T. S. Machado, G. Cernicchiaro, A. P. Guimarães, and L. C. Sampaio Centro Brasileiro de Pesquisas Físicas, Rua Dr. Xavier Sigaud, 150, Rio de Janeiro 22.290-180, RJ, Brazil Received 23 December 2008; revised manuscript received 26 March 2009; published 29 April 2009 The magnetization reversal in Co single wires was investigated through magnetoresistance measurements and micromagnetic simulations. We developed a model to calculate the magnetoresistance based on the mag- netization structure obtained by the solution of the Landau-Lifshitz-Gilbert equation. It allowed us to under- stand details of the magnetoresistance curves, including the jumps that are related to the magnetization reversal process. Depending on the angle between the wire and the applied magnetic field, simulations show that the magnetization structure exhibits curling or uniform rotation modes in the magnetization reversal process. In the curling mode, the magnetization structure exhibits a vortex along the wire with its core displaced from the wire axis. For angles larger than 50° the vortex core is close to the wire surface and disappears at 80°, changing the mode from curling to uniform rotation. Our model for computing the magnetoresistance revealed to be a useful tool in the understanding of magnetic properties of nanostructures. DOI: 10.1103/PhysRevB.79.134434 PACS numbers: 75.60.-d I. INTRODUCTION Recently, the search for new kinds of electronic devices using nanowires has attracted a huge interest. Among the main motivations one can cite, for instance, the shrinking of the electronic circuits, new functionalities such as optoelectronics, 1 and applications as sensor elements. 2 Prob- ably, nanowires are emerging as building blocks for a new generation of nanoelectronic devices. 3 Other applications have been proposed which take into account spin transfer effects. Within certain conditions the domain wall movement can be controlled by a spin polarized current, a property that can be used either to create a nonvolatile memory 4 or to perform logical operations. 5 The magnetization reversal in individual small particles and nanowires has been discussed for a long time. Studies based on magnetic measurements of such small objects are difficult to be performed due to the tiny magnetic signal. Only a few results were reported so far using microsupercon- ducting quantum interference device micro-SQUID magnetometry 6 and magnetic force microscopy MFM. 7 An alternative approach to probe the magnetic properties of nanowires consists of the measurement of the electronic transport in the presence of a magnetic field. The magnetore- sistance, mainly the anisotropic magnetoresistance AMR, is very sensitive to small changes in the magnetization and has become a useful tool to study the magnetization reversal in nanowires 8–12 and in other nanoscale systems. In this paper we explore by experiments and numerical simulations the magnetization reversal in Co nanowires. We have calculated the magnetic structure for a single Co nano- wire by micromagnetic modeling and the corresponding magnetoresistance. The focus of this work is to discuss whether the magnetization reversal occurs by curling, by uni- form rotation, or by a wall displacement in the limit of long wire and the relationship between these magnetization rever- sal modes with the magnetoresistance. The model developed to calculate the magnetoresistance revealed to be a useful tool to understand details of the magnetoresistance experi- mental curves. As we shall see, the existence of vortices in the magnetization reversal explains the observed behavior in the experiments. The paper is organized as follows. First, in Sec. II the nanowire preparation and the electric transport measurements in single nanowires are detailed. Section III describes the model to simulate the magnetic structure and to calculate the magnetoresistance, taking into account the local magnetization. In the remaining sections, the results obtained from the experiments and simulations are discussed. II. EXPERIMENT Co wires were grown by electrodeposition using porous polycarbonate membranes as template. The membranes are metalized on both sides with a Au film; in one side the film is thick enough 300 nm to close the pores and on the other side the film is relatively thin 50 nm keeping the pores open. The membranes are positioned in front of the counter- electrode CE with the thicker Au film on the backside of the membrane, which acts as working electrode WEsee Fig. 1. A potential difference of -1 V between WE and CE electrodes is applied, measured in relation to the Ag/AgCl reference. We have used membranes commercialized by GE Water & Process. 13 The solution used in the electrodeposi- FIG. 1. Color online Scheme of the wire growth process. PHYSICAL REVIEW B 79, 134434 2009 1098-0121/2009/7913/1344346 ©2009 The American Physical Society 134434-1