Abstract:
Magnetic medium for storing information, includes at least two materials A and B which are connected with one another, material A being a hard magnetic material, wherein material B is a material which exhibits metamagnetic behavior in a magnetic field, the metamagnetic behavior of the material being such that, even after passing repeatedly through an external magnetic field from 0 to 10 tesla at least at a magnetic field strength below 3 tesla, an increase in the magnetization occurs as a function of the magnetic field, the increase being superproportional and having a positive curvature.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a magnetic information storage medium, which contains at least two materials A and B, which are connected with one another. 
     In magnetic data storage, the size of the individual bits is reduced further and further to increase the information density. This involves reducing the grain size; in order to avoid reaching the superparamagnetic limit and to maintain a thermally stable magnetization direction when this is done, hard magnetic materials are used which have greater magnetocrystalline anisotropy. However, this can increase the coercivity, so that switching is no longer possible with the usual writing heads, which can typically produce magnetic fields having a maximum strength of 2 T. 
     In order nevertheless to able to switch these materials, thermally assisted magnetic writing was developed. This involves heating the storage medium locally, so that the dynamic coercivity is reduced under the write field. Especially promising here are exchange coupled layers, which consist of FePt/FcRh, for example (J. Thiele, S. Maat, E. Fullteron, “FeRh/FePt antiferromagnet/ferromagnet exchange spring media for thermally assisted magnetic recording”, Appl. Phys. Lett. 82(17) (2003) 2859). Here the FePt, which is a hard magnetic material, stores the magnetization direction, and the FcRh has the very unusual property of exhibiting a first-order phase transition from the antiferromagnetic low temperature phase to a ferromagnetic high temperature phase. Whereas the remanent data storage takes place below the transition temperature, the storage medium is locally heated briefly above this temperature for the writing. This heating is carried out with an additional finely focussed laser beam, which is costly. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to create a magnetic information storage medium, with which it is possible to write on magnetic media of high coercivity with the usual writing heads and without additional local heating. 
     The inventive storage medium contains at least two materials A and B, which are connected with one another, material A being a hard magnetic material and material B being a material, which exhibits metamagnetic behavior in a magnetic field. The metamagnetic behavior of material B consists therein that, even after repeatedly passing through an external magnetic field from 0 to 10 tesla at least at a magnetic field strength below 3 tesla, a rise occurs in the magnetization as a function of the magnetic field, and this rise is superproportional and has a positive curvature. 
     Materials with such metamagnetic behavior are described in E. Stryjewski, N. Giordano “Metamagnetism”, Adv. Phys. 26(5) (1977) 48, 7 for example. 
     The inventive storage medium has some essential advantages over the known magnetic storage media. It is especially advantageous that the magnetization in the metamagnetic material, provided pursuant to the invention, can be influenced by an external magnetic field, so that the material changes over in a certain temperature range from a state of low magnetization (antiferromagnetic or paramagnetic) into a state of high magnetization (ferrimagnetic or ferromagnetic). Thus, it is possible to do without the costly additional heating with an additional laser beam, and the ferromagnetic state can be induced with conventional writing heads. After the writing process, the metamagnetic layer returns to its state of low magnetization, so that the thermal stability is not substantially reduced. 
     The inventive storage medium can be varied and advantageously and/or expediently designed as follows: 
     Material B has a first- or second-order phase transition induced by a magnetic field. 
     In the temperature range from 250° K to 400° K, material B has at least one phase transition which involves a change in the magnetic order and whose transition temperature can be changed, through an external magnetic field, by at least 1° K/tesla. 
     Material B consists at least 20 atom percent of elements which have magnetic orders at least at one temperature in the range of 1° K to 350° K. 
     Material B is based on Heusler alloys, manganates, or lanthanates. 
     Material B is based on the YCo 2  phase, with a variation of up to 10 atom percent in the composition. 
     Material B is based on the Gd 5 (Si x Ge 1-x ) 4  phase, with x&gt;0.8, with a variation of up to 10 atom percent in the composition. 
     Material B is based on the Hf 1-x Ta x Fe 2  phase, with x&lt;0.2, with a variation of up to 10 atom percent in the composition. 
     Material B is based on the Ni 2 MnGa phase, with a variation of up to 10 atom percent in the composition. 
     Material B contains up to 30 atom percent of alloys of elements, which have no ferrimagnetic or ferromagnetic order/orders above 350° K. 
     Material B contains up to 20 atom percent of alloys of elements which have magnetic orders at least at one temperature in the range from 1 K to 350° K. 
     Material B contains alloys made of elements from the group comprising Fe, Co, Ni, Cr, Mn or from the rare earth group of elements. 
     Material A is ferrimagnetic or ferromagnetic in the temperature range from at least 250° K to 320° K and has an anisotropy field strength of at least 1.5 tesla. 
     Material A contains the elements Fe, Co, Ni, or Cr, these elements combined constituting at least 20 atom percent. 
     Material A is formed as a monolayer or as a laminar composite. 
     Material A is based on at least 90 atom percent of the phases FePt, CoPt, FePd, MnAl, CrPt 3 , Co 3 Pt, Sm 2 Co 7 , Sm 1 Co 5 , Sm 1 Co 7 , Sm 2 Co 17 , Sm 2 Fe 17 N 3 , or Nd 2 Fe 14 B. 
     In the case of a laminar composite, material A consists of Co/Pd, Co/Pt, Fe/Pt, or Fe/Pd, the laminar composite having a periodicity of less than 3 nm. 
     Material A contains up to 30 atom percent of alloys of elements, which do not have any ferromagnetic or ferrimagnetic orders above 350° K. 
     Material A contains alloys of elements from the group comprising Pt, Cr, Zr, Ta, O, B, C, P, or N. 
     Material A contains up to 30 atom percent alloys of elements from the group comprising Fe, Co, Ni, Cr, Mn or from the rare earth group of elements. 
     Material A is in the form of a granular layer, this microstructure arising both during layer growth and also during the subsequent structuring methods. 
     Material A is not textured or has a texture, in which the minor magnetic axis is oriented in the direction of application. 
     Materials A and B are in the form of continuous monolayers or multilayers or they are in the form of granular monolayers or multilayers, in which materials A and B are also side by side in the plane of the layer. 
     Materials A and B are coupled with one another by means of exchange coupling. 
     Materials A and B are coupled with one another by means of magnetostatic interaction. 
     For coupling between material A and material B, one or more additional materials are present. 
     Materials A and B are connected with or covered by other functional materials common for magnetic storage media, such as soft magnetic base layers, seed layers, and corrosion-protection layers. 
     A nucleating layer of MgO is disposed between materials A and B. 
     Materials A and B are covered by a protective layer of diamond-like carbon. 
    
    
     
       BRIEF DESCRIPTION OF TILE DRAWINGS 
       The invention is explained in greater detail below by means of examples. 
         FIG. 1 : shows a diagrammatic representation of a magnetic medium for perpendicular recording; 
         FIG. 2 : shows a diagram with magnetization curves; 
         FIG. 3 : shows three illustrations of the switching behavior of the magnetic medium; 
         FIG. 4 : shows a diagrammatic representation of a magnetic medium with an additional seed layer; 
         FIG. 5 : is a diagrammatic representation of a magnetic medium for longitudinal recording; and 
         FIG. 6 : shows a diagrammatic representation of a magnetic medium built of a multilayer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Example 1 
     In the magnetic medium shown in  FIG. 1 , substrate  1 , which is made of glass, has a coherent metamagnetic layer  2  of Hf 0.7 Ta 0.3 Fe 2 , about 50 nm thick, which embodies material B. The material and its properties are described in detail in H. G. M. Duijn, E. Brück, A. A. Menovsky, K. H. J. Buschow, F. R. de Boer, R. Coehoorn, M. Winkelmann, and K. Siemensmeyer “Magnetic and transport properties of the itinerant electron system Hf 1-x Ta x Fe 2 ”, J. Appl. Phys. 81(8) (1997) 4218. 
     On the metamagnetic layer  2 , there is a granular layer  3 , which is about 10 nm thick and has hard magnetic behavior; this layer embodies material A. Granular layer  3  is textured for perpendicular recording, so that the minor magnetic axis (symbolized in  FIG. 1  by the double arrow) is oriented perpendicularly to substrate  1 . 
     The grains of the granular layer  3  consist of an FePt base alloy with 50 atom percent Fe and 50 atom percent Pt. If the FePt compound has an L 1   0  ordered structure, it is possible to achieve very high coercivity above 5 tesla (W. Weisheit, L. Schultz, S. Fähler “Textured growth of highly coercive L 1   0  ordered FePt thin films on single crystalline and amorphous substrates”, J. Appl. Phys. (2003), in press). To lower the temperature necessary for setting the order, this material A can also have 5 atom percent Cu and 2 atom percent B alloyed to it. These non-magnetic elements, which do not mix with FePt, can contribute to the magnetic decoupling of the grains, if they are located between the hard magnetic grains. 
     Laterally, the grains of granular layer  3  have a dimension of 5 nm, and thus lie just above the superparamagnetic limit. The grains are magnetically decoupled from one another by a narrow, about 1 nm wide gap. The material combination FePt and Hf 1-x Ta x Fe 2  has the advantage that both materials are relatively stable to oxidation, so that here it is possible, under certain circumstances, to do without a protective layer. 
     The information is stored in the magnetic medium shown in  FIG. 1  by a writing head (not shown) with its magnetic field H, the direction of which is symbolized by the double arrow above the granular layer  3 . 
       FIG. 2  shows the magnetization curves in material A of granular layer  3  which, as a textured hard magnet, has open hysteresis with high coercivity H c  and high remanence. Also plotted is a diagrammatic magnetization curve of material B of layer  2 , in which the magnetization rises in a superproportional manner to the field. Typical for a first-order transition is an inflection point in the magnetization curve, which is normally taken to be a transition field H 0 . Material B can also exhibit hysteresis. However, this does not change the qualitative behavior in the first quadrant. If the external magnetic field is sufficient to produce domain walls in material B, they can act, for example through exchange coupling (E. F. Kneller “The Exchange-Spring Magnet: A New Material Principle for Permanent Magnets”, IEEE Trans. Mag. 27(4) (1991) 3588), as nucleation cells in material A, and thus lead to switching. Thus, the coupling of materials A and B should reduce the switching field of the laminar structure from H c  to fields around H 0 . The transition field H 0 . should lie above the stray field of material A, which is limited by saturation magnetization, so that in the remanent state no magnetization worth mentioning is induced in material B. 
       FIG. 3  shows the switching behavior of the magnetic medium. The arrows illustrate the magnetization direction. The left picture shows the remanent state without an external magnetic field, in which only the hard magnetic material A has a remanent magnetization, which can withstand thermal fluctuations. In the middle picture, an external magnetic field of the order of magnitude of the transition field H 0  is applied for writing. This induces a magnetic moment in the metamagnetic material B. The right picture shows the switched state, which in the laminar structure has already taken place at fields clearly below the value of H c  of an individual layer of material A. 
     Example 2 
       FIG. 4  shows an alternative to the storage medium structure shown in Example 1, in which an additional seed layer  4  is present between a layer  5  of a hard magnetic material A and a layer  6  of a metamagnetic material B. Layer  5  consists of an FePt base alloy and layer  6  consists of Hf 0.7 Ta 0.3 Fe 2 . 
     Seed layer  4  is 3 nm thick and consists of MgO. It serves to form a (001) texture in the FePt layer  5 . Seed layer  4  also leads to a granular growth of isolated grains, since the MgO is not wetted by FePt. The coupling between the materials A and B takes place due to magnetostatic interaction. 
     The other combinations of materials A and B, which are provided pursuant to the invention, may also be provided as storage medium. Thus, material A can itself be built from multilayers. Material A can also consist of a repeated sequence of a monolayer of Co and two monolayers of Pd, for example. Such a sequence is described by G. H. O. Daalderop, P. J. Kelly, and M. F. H. Schuurmans “First-principles calculation of the magnetic anisotropy energy of (Co) n /(X) m  multilayers”, Phys. Rev. B 42(1990) 7270. The use of a separating seed layer also makes it possible for Ni 2 MnGa, which is relative reactive, to be used as material B. 
     Example 3 
     Material combinations, which are especially suitable pursuant to the invention, have many alloying elements in common. Accordingly, in this example, which relates to the arrangement shown in  FIG. 5 , the hard magnetic material A consists of SmCo 5  and the metamagnetic material B consists of YCo 2 . Using SmCo 5 , which has a very high anisotropy field of 35 T, allows grains with dimensions below 4 nm to withstand the superparamagnetic limit. In order to be able nevertheless to switch the magnetization, compounds derived from YCo 2  are used as the metamagnetic material (H. Yamada, “Metamagnetic Transition and Susceptibility Maximum in an Itinerant-Electron System”, Phys. Rev. B 47(17) (1993) 11211), the transition field H 0  of which can be reduced drastically by alloying a few percent of Al. 
     The texture of the SmCo 5  layer is such that the minor magnetic c-axis lies in the plane of the film. This is the growth direction preferred by this material, so that longitudinal writing geometry can be used. 
     The substrate may be an oxygen-free, high-strength aluminum alloy. 
     For protection against corrosion, the layers are given a protective layer, which here consist of a 2 nm thick diamond-like carbon layer. 
     In this example, the metamagnetic material B is located in a 3 nm wide area between the hard magnetic grains A. Thus, in the absence of an external magnetic field, the grains are magnetically decoupled from one another, and when an external magnetic field is applied a partial coupling occurs. 
     Example 4 
     The longitudinal storage medium shown in  FIG. 6  is built with antiferromagnetically coupled multilayers. As is known, multilayers achieve better thermal stability, without an increase in the coercivity (J. Lohau, A. Moser, D. T. Margulies, E. E. Fullerton, M. E. Schabes, “Dynamic coercivity measurements of antiferromagnetically coupled magnetic media layers”, Appl. Phys. Lett. 78(18) (2001) 2748). Here the information is read out by the stray field of the thicker top layer A. 
     Pursuant to the invention, the intermediate layer B in this storage medium is not paramagnetic, as is usually the case, but rather exhibits metamagnetic behavior. During writes, an external magnetic field induces magnetization in material B, so that the antiferromagnetic coupling of the two hard magnetic layers is no longer favored, but rather a parallel orientation of the magnetization This produces an unstable intermediate state which makes it easier for an external magnetic field to change the magnetization direction. This further reduces the write field.