Ordered magnetic antidot arrays

Resumen

Nanoscale patterned magnetic films are envisaged for technological purposes profiting of the possibility of tuning the local distribution of magnetization in a controlled way. That makes them advantageous in comparison with their continuous thin films counterpart. The applications of the antidot arrays go from magnetic recording media to sensor and magnonic devices. Antidot arrays have been commonly prepared by different nanolithography techniques with size in the order of few hundred of nm or ¿m. Recently, an alternative route has been being increasingly employed, which is based on a combination of different techniques of electrochemical processes and physical deposition (e.g. sputtering). The templates are prepared by a controlled double anodization process, whereby self-assembled nanopores grow during the anodization of high purity aluminum films. Magnetic anisotropy in antidot films, either in-plane or out-of-plane, can be tuned by suitable selection of geometrical parameters. While there is a broad documentation about the magnetic behavior for lithographed antidots, there is still a lack of detailed studies on antidot arrays prepared following the ordering of precursor anodic templates. Specifically, antidot arrays of Permalloy grown over anodic aluminum templates have shown interesting magnetic properties. They lack crystalline and magnetoelastic anisotropies, and consequently their magnetic behavior are essentially determined by the surface anisotropy modified by the presence of local stray fields induced by the antidots. The discontinuities or periodic defects originate dipoles or charges that change strongly the internal shape effective anisotropy. These properties, such as coercivity, can overcome up to two orders of magnitude that the measured in continuous films or in the bulk of the same composition. This shape anisotropy can conveniently vary with the geometry of the arrays. They can even show biphase magnetic behavior in spite of being constituted by single alloy component. A study of the magnetic properties of Permalloy antidot thin films was made. These arrays were grown by sputtering onto anodic alumina templates replicating their hexagonal order and the micrometric geometric domains. The study consists in the measurements of magnetic properties with different geometric parameters, such as diameter, separation of the nanoholes and film thickness.Many techniques were employed in the magnetic study, such as Vibrating Sample Magnetometer, Magneto Optical Kerr Effect Magnetometer, Magnetic Force Microscopy, and a special study was performed using imaging to study the magnetic domain structures using an advanced technique of high spatial sensitivity: X-Ray Photoemission Electron Microscopy. The soft-magnetic character and the almost cero magnetocrystalline constant of the Permalloy, are reflected in the magnetic behavior of the antidot arrays, thus the magnetization reversal process is a direct result on the geometric dimensions of the nanoholes, and depends strongly in the properties of the thin films, such as the thickness. However, the most interesting result was the biphase behavior shown when the arrays have the largest diameter of nanoholes. The magnetic behavior on the surface of the arrays is different that the behavior present in the whole system. At the bottom of the films, the topological properties of the membranes and the sputtering conditions cause an extravagant behavior, producing in consequence different magnetic domain structures in the whole array. Using the results of the Permalloy arrays with different geometrical parameters, antidot arrays of Co were studied. These arrays in principle are expected to exhibit a harder behavior; besides gives a relatively high anisotropy and magnetization saturation. Subsequently, antidots of Co/Py bi-magnetic systems sputtered over anodic alumina membranes were prepared, with the aim to investigate the influence of the presence of antidots in the magnetic behavior. These bi-layer systems show an interesting exchange spring effect.