Abstract:
A method for producing an element including a substrate having a plurality of nanocylinders deposited thereon includes providing the substrate. The substrate is covered with a nanoporous Al 2 O 3  membrane so as to provide a covered substrate. The covered substrate is alternately vapor-deposited, at a vapor-deposition temperatures from 250° C. to 400° C., with atoms of a magnetic element and atoms of a non-magnetic element so as to provide the plurality of nanocylinders. Each nanocylinder includes at least four superposed layers including, alternatively, the atoms of the magnetic element and the atoms of the non-magnetic element. The nanoporous Al 2 O 3  membrane is then removed so that the nanocylinders remain on the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2007/003792, filed on Apr. 28, 2007, and claims the benefit of German Patent Application No. 10 2006 021 940.6, filed on May 11, 2006. The International Application was published in German on Nov. 22, 2007 as WO 2007/131617 A1 under PCT Article 221(2). 
     FIELD 
     The present invention relates to a method for producing an element, which includes a substrate onto which a multiplicity of nanocylinders are deposited, and to a method of using thereof as a magnetic storage medium, switching element or sensor. 
     BACKGROUND 
     Thin alloy films of metallic components on suitable substrates are used as magnetic storage media, switching elements or as sensors, in the case of storage applications, critical importance being placed on the orientation of the magnetic moments relative to the layer surface. 
     The bit storage size can be reduced by reducing the magnetic volume in which the information is stored. However, such a reduction inevitably results in a loss of thermal stability of the stored information (superparamagnetic limit, thermal excitation of remagnetization processes above a characteristic temperature), thereby limiting the storage density that is attainable at the present time. 
     By selecting appropriate classes of material and by employing patterning methods, it is possible to significantly increase the storage density. This requires using systems which exhibit a perpendicular magnetocrystalline anisotropy. At the present time, ternary material combinations, such as Co—Cr—Pt, which necessitate costly production methods to introduce the same, as well as a thermal treatment, are used for this purpose. 
     Yong Lei and Wai-Kin Chim,  Shape and Size Control of Regularly Arrayed Nanodots Fabricated Using Ultrathin Alumina Masks,  Chem. Mater. 2005, 17, pp. 580-585, describe the production of highly ordered semiconductor and metallic structures from nanoparticles having controllable sizes and shapes. These structures are deposited via nanoporous membranes of Al 2 O 3 , which are also described as ultrathin alumina masks or UTAMs, onto Si and Si/SiO 2  substrates. The size and shape of the structures can be controlled by the aspect ratio of the apertures of the traversing pores of these membranes, as well as by the quantity of material. These masks can also be used at higher temperatures for epitaxial growth. It has been possible up until now to employ this method to fabricate ordered structures of semiconductive nanoparticles having sizes of up to 20 nm. 
     In  Magnetic Properties of Ferromagnetic Nanowires Embedded in Nanoporous Alumina Membranes,  Journal of Magnetism and Magnetic Materials 249 (2002), pp. 241-245, M. Kröll, W. J. Blau, D. Grandjean, R. E. Benfield, F. Luis, P. M. Paulus and L. J. de Jongh describe producing iron, nickel and cobalt nanowires from Al 2 O 3  using nanoporous membranes. These nanowires can be adjusted in diameter (5-250 nm) and length (up to a few hundred micrometers). 
     In  Preparation of Fe/Pt Films with Perpendicular Magnetic Anisotropy,  Hyperfine Interactions (2005) 160, pp. 157-163, S. Kavita, V. R. Reddy, A. Gupta and M. Gupta describe the structures and magnetic properties of L1 0  ordered, thin equiatomic FePt films, which were produced by ion beam sputtering and subsequent annealing. 
     US 2002/0158342 A1 describes an element which includes a substrate onto which a multiplicity of nanocylinders are deposited, each nanocylinder having superposed layers alternately composed of atoms of a magnetic element and of a non-magnetic element. The element is fabricated using a diblock copolymer, upon which a metal layer is at least partially deposited, which, following orientation and removal of one component of the diblock copolymer, forms nanocylinders. 
     However, selected binary alloy systems, such as the Fe—Pt, Fe—Au or Co—Pt system, can likewise exhibit the desired properties. In the case of material synthesis preferably carried out by vapor deposition processes, kinetic inhibition prevents the ordering of the desired L1 0  phase having perpendicular magnetocrystalline anisotropy from being effectively controlled. Rather, the system crystallizes at room temperature in the disordered fcc phase, accompanied by random filling of the lattice sites. However, the desired orientation can be controlled by applying suitable vapor deposition parameters. As a result, material combinations can be artificially produced which do not provide a thermodynamically stable configuration for the L1 0  order. An example of this is the deposition of Fe and Au in monolayers. 
     SUMMARY 
     An aspect of the present invention is to provide a method for producing an element of this kind and to provide a method of using thereof that will address the aforementioned disadvantages and limitations. It is intended, in particular, that this method makes it possible to produce an element that exhibits an advantageous, lateral ordering of the structures, accompanied by adequate decoupling of the individual structural elements, that has a large-area array within the range of a few cm 2 , that exhibits a virtually perpendicular magnetic anisotropy, and that features a preferably monodisperse distribution of the structure sizes, the structure size being on the order of approximately 10 nm. 
     In an embodiment, the present invention provides a method for producing an element including a substrate having a plurality of nanocylinders deposited thereon. The method includes providing the substrate. The substrate is covered with a nanoporous Al 2 O 3  membrane so as to provide a covered substrate. The covered substrate is alternately vapor-deposited, at a vapor-deposition temperatures from 250° C. to 400° C., with atoms of a magnetic element and atoms of a non-magnetic element so as to provide the plurality of nanocylinders. Each nanocylinder includes at least four superposed layers including, alternatively, the atoms of the magnetic element and the atoms of the non-magnetic element. The nanoporous Al 2 O 3  membrane is removed so that the nanocylinders remain on the substrate. 
    
    
     DETAILED DESCRIPTION  
     Method step a) provides for first preparing a substrate, which is selected in accordance with the following criteria: All materials are suited as substrate materials, provided that they belong to one of the following two classes of material:
         amorphous systems, such as glass, for example, which are preferably suited for Fe—Pd, as well as Fe—Au; and   crystalline systems having a suitable lattice constant for mediating the growth of the MX films in that growth direction which makes possible a perpendicular orientation of the magnetization. Especially suited for FePt are MgO(001), GaAs (001) or SrTiO 3  (001).       

     The substrate is subsequently covered with a nanoporous membrane of Al 2 O 3  in accordance with method step b), the diameter, the interspacing and the configuration of the pores in the Al 2 O 3  membrane being selected in accordance with the parameters desired for the nanocylinders. 
     In the subsequent method step c), one or a plurality of atomic layers of atoms M of a magnetic element and atoms X of a non-magnetic element are alternately deposited onto the substrate that is covered with the nanoporous membrane until the desired value is reached for the height of the nanocylinders and the number of layers. The atoms are preferably deposited using an electron beam vaporizer (molecular beam epitaxy). The moderate vapor-deposition temperatures required therefor within the range of 250° C. to 400° C. render possible a virtually complete ordering of the deposited layers. Layers produced in this manner exhibit a virtually perfect L1 0  ordering. 
     Finally, in accordance with method step d), the Al 2 O 3  membrane is preferably removed from the surface of the substrate using a solvent suited for this purpose, such as water or alcohol, so that, instead of the pores of the membrane, nanocylinders remain on the substrate. 
     An element produced using the method according to the present invention is composed of a substrate onto which a multiplicity of nanocylinders, also referred to as nanodots, nanospots or nanocolumns, are deposited. In this case, each nanocylinder has four, five, six, seven, eight, nine, ten, eleven, twelve or more superposed layers. The layers, which make up the nanocylinders, are alternately composed in this case of atoms of a magnetic element M and of a non-magnetic element X. 
     In an embodiment, M=Fe, Co or Ni are selected as atoms of a magnetic element and X=Pd, Pt, Rh or Au as atoms of a non-magnetic element. Examples include the combinations Fe—Pt, Fe—Au, Co—Pt, Co—Au, Co—Ni and Co—Pd. 
     Each layer can include 1 to 10 atomic layers. For example, each layer can have the same number of atomic layers as those layers which are adjacent-but-one thereto. 
     The number and thickness of the layers can influence the magnetic properties of the nanocylinders. Thus, the L1 0 -ordered phase in the Fe—Pt system can be characterized by alternating atomic layers of Fe and Pt, and can also exhibits a very high coercitive field strength due to the exceptionally high magnetocrystalline anisotropy. However, this effect can necessitate very high remagnetization field strengths (&gt;500 mT) for a remagnetization in storage media that are difficult to apply to structure sizes on the order of a few nanometers. 
     The reason for this exceptional magnetocrystalline anisotropy might be the strong interaction between Fe and Pt atoms. However, this polarization may be utilized to control the coercitive field strength. To this end, the thickness of the Fe layer can be varied. For example, when two Fe monolayers are used, the coercitive field strength can be significantly reduced to practicable ranges (250 mT) and can be accompanied by an unvarying stability of the stored information in response to external fields and thermal excitation. By properly selecting the layer thicknesses and the vapor-deposition parameters, it can be possible to adjust a coercitive field strength of 50-1000 mT. The upper limit can be defined by the saturation magnetization of a completely ordered layer. A desired secondary effect of such a structure is the tilting of the magnetization vector by approximately 15° out of the layer plane. This tilting renders possible a simplified remagnetization process and a greater readout security when the element according to the present invention is used as a magnetic storage medium. 
     The nanocylinders can have a diameter of 10 nm to 100 nm, for example 20 nm to 50 nm, and an interspacing of 10 nm to 100 nm, for example 20 nm to 50 nm. The nanocylinders can be configured in the form of a two-dimensional cubic or hexagonal structure on the substrate. The lateral structure sizes are set by the diameter, the interspacings and the configuration of the pores in the Al 2 O 3  membrane. 
     Moreover, the nanocylinders can have a height of 2 to 500 nm, for example of 5 nm to 100 nm. The desired height value for the nanocylinders can be set by varying the deposited quantity of material within this broad range. 
     The elements produced using the method according to the present invention can be used as magnetic storage media, switching elements or as sensors. 
     In particular, the present invention has the advantages delineated in the following. 
     The use of self-ordered, nanoporous Al 2 O 3  masks makes it possible for the Fe—Pt nanocylinders produced to attain a high level of perfection. Once the masks are removed from the substrate, highly ordered arrays of L1 0 -ordered Fe—Pt nanocylinders remain having a diameter which is dependent on the diameter of the pores in the Al 2 O 3  masks used. This was verified by AFM (atomic force microscope) investigations which detected L1 0 -ordered Fe—Pt nanocylinders having a diameter of 15-50 nm. A further reduction in the diameter of the Fe—Pt nanocylinders may be effected by a further optimization of the Al 2 O 3  masks. 
     Under known methods heretofore, a large-area array of nanocylinders was able to be obtained over an area of approximately 500×500 μm 2 . Arrays on the order of a few cm 2  may be produced by further optimizing the masks. 
     Perpendicular anisotropy is a property of the L1 0  phase in the Fe—Pt system. Virtually any given orientation of the magnetization vector may be set as a function of the selected production conditions (vapor deposition rates, substrate temperature and layer thicknesses). A perpendicular orientation of the magnetization can be established by depth-selective Mössbauer spectroscopy (determination of the magnetic properties on an atomic level) and by SQUID measurements (characterization of remagnetization processes). 
     The present invention is explained in greater detail in the following with reference to exemplary embodiments. 
     To produce an element (array) of Fe—Pt nanocylinders in accordance with the present invention, substrates were prepared from MgO(001) monocrystals. Prior to deposition of the layers, the substrates were each provided with a nanoporous Al 2 O 3  mask. The pore size was 20 nm, the average interspacing of the pores, 25 nm. 
     Subsequently thereto, Fe or Pt atoms (purity in each case better than 99.95%) were deposited on the substrates by molecular beam epitaxy at defined substrate temperatures. 
     The substrate temperature and thus the deposition temperature were able to be adjusted through the use of a heating filament. The spacing between the material sources and the substrate was approximately 0.3 m. To achieve an ordered L1 0  phase, the layers were deposited at a deposition temperature of 350° C. at deposition rates of 0.3 nm/min (Fe), respectively 0.01 nm/min (Pt). The total layer thickness was approximately 4 nm. To carry out a deposition by atomic layers, the specimen holder and the material sources were each provided with a shutter, which was closed each time upon reaching the desired layer thickness. For deposition of the second element, the shutter in question was opened, and the respective element was deposited. 
     Following deposition of the layers and cooling of the substrate in ambient atmosphere, the nanoporous Al 2 O 3  mask was removed by rinsing with water or alcohol (isopropanol). 
     For the examples presented here, two iron monolayers and two platinum monolayers were successively vapor-deposited onto substrates of MgO (001) monocrystals until, altogether, eight or twelve superposed layers, alternately composed of two iron monolayers and of two platinum monolayers, were deposited. 
     The elements of FePt nanocylinders produced in accordance with the present invention exhibit a perpendicular magnetization orientation for all produced particle diameters (25 nm, 50 nm), accompanied by a slight tilting of approximately 10° to 20° from the layer normal. Higher growth temperatures may be used to achieve a complete orientation and thus to fully control the ordering of the layered system. However, a slight tilting is advantageous for a use as a storage medium, since, on the one hand, the high magnetocrystalline anisotropy makes it possible to store the desired information, and on the other hand, however, desired remagnetization processes may be facilitated by external fields, for example in the writing head.