DOI: 10.1002/adfm.200700444 Sub-20 nm Magnetic Dots with Perpendicular Magnetic Anisotropy** By Holger Stillrich, Andreas Frömsdorf , Sabine Pütter, Stephan Förster, and Hans Peter Oepen* 1. Introduction Controlling the structure of matter at the nanometer scale and assembling nanoparticles into arrays and networks in a controlled manner are the keys to new technologies. The appli- cation of magnetic nanostructure arrays as storage medium is a big issue for years. The fabrication of nanoparticle arrays, how- ever, with nanoscale precision remains a formidable task. The problem is threefold. At first nanostructures have to be fabri- cated, their scale dependent properties have to be studied and manipulated, and at last they have to be arranged in a perfect periodicity in a certain structure. [1–3] The latter condition is best suited by the traditional “top- down” techniques, i.e. lithography, which offer the highest flex- ibility. The problem here is that direct writing is a serial process and structuring of large areas takes very long time and is very expensive. [3] A way out of this dilemma is to create masks or molds via electron beam lithography which can be used to cre- ate replica in multiple ways. [4–6] An alternative route to nano- structures and arrays is the so-called “bottom-up” approach utilizing self-assembly of organic and inorganic material at sur- faces which can offer advantages including experimental sim- plicity, the possibility of three-dimensional assembly and the potential for low-cost mass fabrication. Research in this field is gathering speed and different pathways to nanostructures and arrays have been successfully tested. [1,3,7] The problem is that the periodicity is usually not as good as for artificial structuring methods. A solution for this problem has been found by “meso- scopic” prestructuring of the surface. [8,9] Recently, the self-assembly of block copolymers on surfaces has attracted attention for the production of nanomagnet ar- rays. [10–12] Under certain conditions the block copolymers phase separate building nanoscale periodic structures that are used as mask in succeeding etching or deposition steps. A fabrication procedure has been worked out that was successfully used to create magnetic nanostructure arrays. [11,13] This fabrication pro- cess, however, turns out to be very complex which is in dis- agreement with the often claimed simplicity of “bottom-up” approaches. The magnetic nanodots of Ref. [13] exhibit uniax- ial perpendicular anisotropy and thus meet a main requirement for magnetic data media. This method offers a high flexibility in tuning magnetic properties of the nanodots. A cognate method uses self assembly of micelles that are built from di- block copolymers. The micelles, which are filled with ferromag- netic material, are deposited on the surface. The polymer shell is removed via an oxygen plasma process and magnetic nano- particles are generated after further preparation steps. [14–16] Some obstacles show up concerning the magnetic properties as there are no aligning forces to orient the magnetic axes. In gen- eral a random orientation of the easy axes of magnetization is found. To overcome the deficiencies of both methods we have de- veloped a different method that relies also on micellar self as- sembly but which incorporates a limited number of fabrication steps after coating with micelles, similar to a method described before. [17] In the beginning we will explain how we create a mask for ion milling from micellar self assembly and will dis- cuss in detail the size and shape of the attained nanodots. The magnetic properties will be discussed in the succeeding part which show unambiguously that the nanodots are ferromag- netic with a perpendicular easy axis of magnetization. 2. Results and Discussion Nanostructure arrays in this study are made of (Co 0.7 nm / Pt 2 nm /Co 0.7 nm /Pt 3 nm ) multilayer films (6.4 nm total thickness) with perpendicular magnetic anisotropy on a 4.1 nm platinum 76 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 76–81 – [*] Prof. H. P. Oepen, H. Stillrich, Dr. S. Pütter Institut für Angewandte Physik, Universität Hamburg Jungiusstr. 11, 20355 Hamburg (Germany) E-mail: oepen@physik.uni-hamburg.de; Holger.Stillrich@physik.uni-hamburg.de Dr. A. Frömsdorf, Prof. S. Förster Institut für Physikalische Chemie, Universität Hamburg Grindelallee 117, 20146 Hamburg (Germany) [**] Financial support by the “Deutsche Forschungsgemeinschaft” via SFB 508 is gratefully acknowledged. A new and simple method for the preparation of magnetic dot arrays is introduced. Diblock copolymer micelles with a silica core are used as template for the generation of nanostructure arrays. The silica cores are utilized as mask for ion milling prepa- ration. The morphology and size of the silica and magnetic dot arrays are discussed. The magnetic dots are made from Co/Pt multilayer films. Ferromagnetic dots with a diameter well below 20 nm and perpendicular easy axis of magnetization are creat- ed. The switching behavior changes from domain wall motion, dominant in the film, to single domain particle switching in the dots. The magneto-optic saturation signals and the evolution of magnetic anisotropy are discussed. FULL PAPER