Magnetoresistive nanowires

In the last decade, one-dimensional (1D) magnetic nanowires have attracted much attention due to their unique physical properties and potential applications in magnetic recording, spin electronics, and sensor devices. In hard disc technology, recording surface density has increased about 100?000 times in thirty years, reaching 100 Gbits/inch2. The challenge is now to achieve 1 Tbits/inch2, which corresponds to an information bit of about 10 × 10 nm2. A lot of effort has been put into developing new recording media based on self-organized magnetic nanoparticles or patterned media composed of magnetic dot arrays. The development of more sensitive magneroresistive read heads is also a key ingredient for improving hard disc drive capacities. At present, read heads are composed of spin-valves that exhibit the giant magnetoresitance (GMR) effect. The GMR effect consists of a large variation of the electrical resistance through the application of an external magnetic field. This effect was first observed in multilayered materials composed of alternately magnetic and non-magnetic layers [1]. The change in resistance of this structure is due to the spin dependent scattering of electrons at the interfaces. In this mechanism, the scattering rate depends on the spin of the electrons and on the orientation of the magnetization of the magnetic layers. The reorientation of magnetization from an anti-parallel to parallel configuration induced by applying a magnetic field causes a decrease of the electrical resistance.

There are two possible geometries for current to pass through the layers: the Current-In-Plane (CIP) geometry and the Current-Perpendicular-to-Plane (CPP) geometry. The first one is used in spin-valve based magnetic sensors such as read heads, but the second is not yet used due to technological problems in 2D multilayered structures. However, it has been predicted by the Valet-Fert model [2] that in the CPP geometry the GMR effect is higher. The use of this geometry should be a good way to increase the sensitivity of hard drive read heads and thus the storage density. In this field, one-dimensional (1D) magnetic multilayered nanowires appear to be promising candidates [3]. These nanostructures exhibit a high aspect ratio (length/diameter), and provide an ideal opportunity to investigate the CPP-GMR. Among the different routes for the fabrication of such nanowires, template-based electrochemical deposition is widely used due to its cost effectiveness, versatility, and high deposition rate.

In this context, the precise control of such nanostructures is a new challenge and a detailed investigation at the ultimate scale becomes crucial for understanding and improving magneto-transport properties. Among the different systems being investigated, Cu/Co is a particularly good candidate. Firstly, it can be easily electrodeposited, and secondly it presents a high GMR value due to the low mismatch between the Cu-fcc and Co-fcc phases. In our studies, multilayered Cu/Co nanowires were grown by electrodeposition and characterized by conventional techniques. Atom probe tomography (APT) was used to investigate the nanowires at the atomic scale. APT, which has been developed in our group since the 90s [4] and [5], is based on pulsed-field evaporation of surface atoms and their identification by time-of-flight mass spectrometry. Thanks to the 3D imaging capability and the high spatial resolution of this technique, a new and more precise characterization of Cu/Co nanowires should be possible. Thus, the morphology and the local chemical compositions of Cu/Co interfaces could be characterised. This technique is very powerful, but it requires the manipulation of a single nanowire, and this represents a real challenge.

This month's cover image was obtained via scanning electron microscopy, and shows an assembly of multilayered Cu/Co nanowires. They were grown into an alumina template by electrodeposition. Because of the inhomogeneous filling rate of nanopores, some nanowires emerge from the template allowing a lateral growth which leads to the formation of semi-spherical objects at the alumina surface.

Further Reading
[1] M.N. Baibich et al. Phys Rev Lett, 61 (2472) (1988), p. 21
[2] T. Valet et al. Phys Rev B, 48 (7099) (1993), p. 10
[3] A. Fert et al. J Magn Magn Mater, 200 (338) (1999), pp. 1–3
[4] D. Blavette et al. Rev Sci Instrum, 64 (2911) (1993), p. 10
[5] D. Blavette et al. Nature, 363 (432) (1993), p. 6428