Patent Publication Number: US-RE38474-E

Title: CoCrPtB alloys with increased boron content and method of producing same

Description:
FIELD OF THE INVENTION 
     This invention relates generally to magnetic alloy layer structures used for high density magnetic data storage media, such as rigid magnetic recording disks, and in particular to magnetic layer structures comprised of a layer of CoCrPtB alloy. 
     BACKGROUND 
     Advances in high density magnetic storage systems depend largely on improvements in the magnetic data storage media. The properties of the media are determined by the composition and the structure of the active magnetic storage material and contiguous layers within the media structure. For high density longitudinal recording media the important characteristics are coercivity, signal-to-noise ratio, remanent magnetization, coercivity squareness, layer thickness and surface roughness. 
     It is known, for longitudinal magnetic recording with a ring head that the linear recording density (LARD) of the magnetic media is governed by the following relationship: 
     
       
         LRD 1/a  
       
     
     where a is a parameter given approximately by the relation:        a   ≅           M   r          t   ·   d         π   ·     H   c                           
     where M r  is the remnant magnetization, t is the thickness of the magnetic layer, d is the distance from the pole tip bottom of the magnetic write or read device to the top of the magnetic layer and H c  is the coercivity. Furthermore, it has also been established that the output is related to M r  and t as follows: 
     
       
         OUTPUT M r ·t  
       
     
     where M r ·t is called the magnetization thickness product. Hence, in designing a magnetic data storage media having a high recording density, it is necessary to increase H c  while maintaining a low M r t value that is still sufficient for the required output application. According to prior art, to support longitudinal magnetic recording at densities close to 10 Gbit/inch 2  the coercivity H c  of the magnetic media must range between 2,500-4,500 Oe and the material grain size should be approximately 100 Angstroms; Glijer et al., “Structural Design of CoCrPt(Ta,B)/Cr Magnetic Thin Film Media for Ultra High Density Longitudinal Magnetic Recording”, Scripta Metallurgica et Materialia, Vol. 33, Nos. 10/11, 1995, pp. 1575-84. 
     Small grain sizes are important for the next generation magnetic data storage media because small grain sizes can significantly increase the media signal to noise ratio (SNR). The media SNR is a ratio of the output signal over media noise, and can be described by the equation; 
     
       
         SNR WB/D 2    
       
     
     Here W is the track width, B is the bit size and D is the in-plane grain diameter of the active magnetic layer. Hence, the SNR of the magnetic media varies inversely as the square of the in-plane grain diameter and a significant reduction in the magnetic media noise can be achieved by reducing the grain size. 
     Traditionally, making magnetic data storage media involves coating magnetic particles in a resin or binder. Recent efforts to achieve high density magnetic media have focused on the thin layer alloy magnetic structures. The thin layer alloy magnetic media are made by several methods including sputtering, vapor deposition, ion-beam deposition and electroplating. 
     Thus far, the most successful thin layer magnetic alloy materials for data storage are based on cobalt-chromium alloys with additions of platinum, tantalum, boron and alloys thereof. Examples of such magnetic media are described by Murayama et al. in U.S. Pat. No. 5,478,661, by Oka et al. in U.S. Pat. No. 5,494,722 and by N. Tani et al. in “High Coercivity Hard Disk with CoCrPtB/Cr Media”, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 4736-38. The composite alloys, described in the references above, are generally deposited on a chromium or a chromium alloy under-layer with a body-centered-cubic (bcc) crystal lattice structure. Additionally, the chromium or the chromium alloy under-layer usually has a substantial [100] or [110] crystallographic orientation normal to the plane of the deposition substrate. This [100] or [110] crystallographic orientation of the bcc chromium or chromium alloy is well known in the art to be a useful under-layer for subsequent deposition of cobalt based alloys. 
     Cobalt-chromium-platinum with additions of less than 7% boron (% here and throughout this text is understood to mean atomic percent) are particularly useful for longitudinal magnetic recording media. Addition of boron to cobalt-chromium-platinum composite alloys has been proposed to enhance the anisotropic growth on the chromium under-layer causing some grains to be oriented with the c-axis in the plane of the film. However, this has not been observed for all compositions of cobalt-chromium-platinum alloys. Achieving this preferred orientation is important for these materials to be used in high density longitudinal magnetic media because the c-axis is the easy direction to magnetize and, therefore, it is preferably to have grains with their c-axes parallel to the plane (in-plane) of the longitudinal media for efficient write and read operations. 
     The addition of boron (less than 7 atomic %) to the composite alloy cobalt-chromium-platinum also increases the coercivity H c  and the signal to noise ratio of the magnetic media as recognized by Doerner et al. in U.S. Pat. No. 5,523,173 where additions of 2-10 percent of B have been examined; M. Oka et al., U.S. Pat. No. 5,494,722. Further studies of this alloy have also been published by L. W. Song et al. in “Magnetic Properties and Recording Performance of Multi-layer Films of CoCrTa, CoCrPtTa, and CoCrPtTa with CoCrPtB”, IEEE Transactions on Magnetics, Vol. 30, No. 6, November 1994, pp. 4011-13; P. Glijer et al. in “Magnetic Force Microscopy (MFM) Studies of Micromagnetic Structures of High Coercivity CoCrPt/Cr and CoCrPtB/Cr Thin Films”, IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995, pp. 2842-2844. 
     Prior art teaches the advantages of cobalt-chromium-platinum-boron composite alloys containing less than 10% boron for magnetic structures deposited on bcc chromium or chromium alloy under-layers with substantial [100] or [110] crystallographic orientation normal to the plane of the deposition substrate. The benefits include, anisotropic grain growth (for some composition alloys) with the c-axis along in plane, increases in coercivity, and improved signal-to-noise ratio. There is a continued need to discover new compositions of magnetic materials for high density longitudinal magnetic media that have smaller grain size for improved signal to noise ratios while maintaining the high coercivity values. 
     OBJECTS AND ADVANTAGES 
     Accordingly, it is a primary object of the present invention to disclose a new magnetic recording media comprised of a layer of CoCrPtB containing B&gt;10% and a method for producing the same. The advantages of the magnetic layer structure are high coercivity H c  values and reduced grain sizes. 
     Increasing the boron content from 10 to 20% in the CoCrPtB composite alloys to decrease grain sizes while maintaining useful magnetic properties is accomplished by providing a nucleation layer of Co composite alloy, where B&lt;10, between a magnetic layer of CoCrPtB composite alloy, where B&gt;10%, and bcc metal or bcc metal alloy under-layer having substantially [100] or [110] crystallographic orientation normal to the plane of the deposition substrate. The resultant grains of the magnetic structure are well defined and isolated. The magnetic structures described have high coercivities (H c ), greater than 2,000 Oe, and have the crystallographic c-axis of the magnetic layer in-plane of the bcc metal or metal alloy under-layer. Additional objects and advantages of the invention are set forth in the specification. 
     SUMMARY 
     These objects and advantages are attained by depositing a bcc metal or bcc metal alloy under-layer on a suitable substrate. The preferred under-layer is chromium or chromium alloy that is deposited with a negative bias voltage applied to the substrate. Typically the substrate is a rigid disk that is aluminum or aluminum coated with a plated nickel phosphorus over coating but can also be glass, silicon, ceramic, quartz or any substrate suitable for deposition. A nucleation layer of a Co alloy, where B≦10% and ranging from 1 to 50 Angstroms in thickness is deposited on top of the bcc metal or alloy under-layer. A layer comprising of CoCrPtB, where B&gt;10% and ranging from 5 to 500 Angstroms in thickness is deposited on the nucleation layer, thus providing a magnetic layer structure. The under-layer, the nucleation layer and the magnetic layer can be deposited by sputtering, ion-beam deposition and laser deposition. 
     The bcc metal or bcc alloy under-layer presents a template for the subsequent depositions of a cobalt or cobalt alloy layer such that the cobalt or cobalt alloy layer will grow epitaxially to the bcc metal or bcc alloy under-layer with the c-axis residing in the plane of the film. The under-layer is preferably deposited with [100] or [110] crystallographic orientation. The nucleation layer provides a means for a compatible growth surface between the bcc metal or metal alloy under-layer and the Co composite alloy with B&gt;10%. The magnetic structure with a magnetic layer of CoCrPtB alloy, where B&gt;10% also has small grain sizes with the crystallographic c-axis oriented in the plane of the structure surface. 
     The magnetic films of this invention have grain sizes between 20 and 200 Angstroms. The grains are well defined and segregated by an amorphous phase. In the applications of magnetic structures of this invention, such as longitudinal magnetic recording media, the magnetic layer contains grains that are ferromagnetic. Furthermore, the magnetic layer structures produced by this invention have a high magnetic coercivity (H c ) between 2,000 and 5,000 Oe. The invention is described in detail in the below detailed description in reference to the appended drawing figures. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross sectional view showing a film of a CoCrPtB alloy made with a nucleation layer according to the invention. 
     FIG.  2 (a-d) are TEM images showing the grain structure in the CoCrPtB composite alloy containing 0% B, 3% B, 6% B and 12% B. 
     FIG. 3 is a graph of the X-ray data of the magnetic structure Co 71 Cr 17 Pt 12  and 12% B deposited on a Co 71 Cr 17 Pt 12  nucleation layer. 
     FIG. 4 is a graph of the X-ray data of the magnetic structure Co 71 Cr 17 Pt 12  and 12% B deposited without a nucleation layer. 
     FIG. 5 is a graph comparing the coercivities H c  of magnetic structures made of a CoCrPtB composite alloys by methods in prior art and made of a CoCrPtB composite alloys according to the invention. 
    
    
     DETAILED DESCRIPTION 
     A layered magnetic structure of this invention is visualized in FIG. 1. A substrate  12  is a flat surface or a rigid disk that is aluminum or aluminum plated with a nickel phosphorus over coating, but can also be glass, silicon, quartz, ceramic or any other substrate that is suitable for deposition. If a substrate other than nickel phosphorus coated aluminum disk is used, a preparatory layer maybe necessary to make the substrate suitable for depositing a bcc metal or a bcc metal alloy layer  14 . For example, if glass is the substrate a nickel-aluminum layer is coated on the glass prior to depositing an under-layer  14 . 
     The preferred under-layer  14  is chromium or a chromium-titanium alloy. A negative bias voltage is typically applied to the substrate during the deposition of the under-layer  14 . The conditions under which the under-layer  14  is deposited, including pressure, temperature and bias voltage, will affect the magnetic properties of the resultant magnetic films. Conditions are chosen so that the under-layer  14  is deposited with a substantial [100] or [110] crystallographic orientation providing an epitaxial growth surface for subsequent deposition growth steps. The thickness T 1  of the under-layer  14  optimized to obtain the desirable magnetic properties and is preferably in the range of 10 to 1000 Angstroms. 
     A nucleation layer  16 , is a Cobalt alloy preferably with a composition of CoCr x Pt y B z , where 15≦x≦40, 0≦y≦15 and 0≦z≦10 atomic percent and 25≦(x+y+z)≦50. The preferred thickness T 2  of the nucleation layer  16  is 1 to 50 Angstroms. The nucleation layer  16  provides nucleation sites for a subsequent deposition of a CoCr x Pt y B z  layer  22  containing B&gt;10%. 
     The magnetic layer  22  layer is deposited on layer  16  and is composed of CoCr x Pt y B z , wherein 0≦x≦30, 0≦y≦40 and 10≦z≦25 atomic percent and 25≦(x+y+z)≦60. The layer  22  is preferably 5 to 500 Angstroms in thickness T 3 . The under-layer  12 , the nucleation layer  16 , the magnetic layer  22  are deposited by sputtering, ion-beam deposition or laser deposition methods. 
     FIGS. 2 (a-d) display the plan view TEM images of the surfaces of the magnetic structures made according to the invention. FIGS. 2 (a-d) are magnetic layer structures made with a magnetic layer  22  of CoCrPtB, where B=0, 3, 6, and 12% boron, respectively. For this example, the magnetic layer  22  is comprised of [(Co 71 Cr 17 Pt 12 )B] and the nucleation layer  16  is comprised of Co 71 Cr 17 Pt 12 . The grains are isolated by an amorphous phase  28  (see FIG. 1) in the TEM images. Additionally, the grain sizes are reduced by almost an order of magnitude with the inclusion of boron from 0 to 12% in the magnetic layer. 
     FIG. 3 is the X-ray analysis of a magnetic layer structure of FIG. 1 comprising the magnetic layer  22  composed of [(Co 71 Cr 17 Pt 12 )B] with B=12% and the nucleation layer  16  composed of (Co 71 Cr 17 Pt 12 ). The X-ray data of this structure reveal a strong reflection at 2θ≅72 deg  32  corresponding to the [110] crystallographic planes the of the hexagonally-close-packed cobalt-chromium based alloy. The [110] reflection is observed when grains of the magnetic layer  22  are oriented with the c-axis parallel to the plane (in-plane) of the magnetic recording media; as stated previously, this is the preferred orientation. 
     FIG. 4 is the X-ray analysis of a magnetic layer structure with magnetic layer composition of [(Co 71 Cr 17 Pt 12 )B] with B=12% but without a nucleation layer  16 . The absence of a refection at 2θ≅72 deg  32  and the presence of a strong refection at 2θ≅42 deg  35 , corresponding to the [002] crystallographic planes of the cobalt-chromium-platinum alloy, shows that the grains of the magnetic layer  22  are predominantly aligned with the c-axis perpendicular to longitudinal plane of the magnetic media. Perpendicular orientation of the c-axis is not preferred for longitudinal magnetic media because of reasons stated previously. 
     Additionally, FIGS. 3-4 display that both X-ray data sets have strong peak at 2θ≅64 deg  30  correspond to the [200] crystallographic planes of the chromium under-layer that are deposited with bias voltage of −450 Volts applied to the substrate. The presence of a peak at 2θ≅64 deg signify that the bcc under-layers have substantially [100] or [110] crystallographic orientation normal to the plane of the substrate. 
     FIG. 5 plots coercivity H c  values of magnetic layer structures with ranges of boron percent in the magnetic layer  22  without a nucleation layer and with a nucleation layer  16 . All samples contain a chromium under-layer deposited with bias voltage of −450 Volts applied to the substrate. For these examples, the magnetic layers  22  are composed of [(Co 71 Cr 17 Pt 12 )B] B=12 atomic % [( Co   71   Cr   17   Pt   12 ) 100-x   B   x   ] B=x atomic  % ( x= 0  to  20   ). For samples measured in curve  36 , the nucleation layer  16  is composed of (Co 71 Cr 17 Pt 12 ). The coercivity curve versus percent boron  33  shows that the coercivity values for magnetic layer structures containing CoCrPtB alloy decrease rapidly as the boron percent exceeds 7% when the nucleation layer  16  is not incorporated in the fabrication. The coercivity curve  36  shows that the coercivity values of layered magnetic structures containing CoCrPtB alloy increase with addition of boron up to 12 atomic % when the nucleation layer  16  is incorporated in the fabrication process. After 12% incorporation of boron into magnetic layer  22  the coercivity decreases even with a nucleation step. However, magnetic structures produced by the invention continue to have useful magnetic properties up to 20% boron because of the relatively high coercivity values and the reduced signal to noise ratio achieved by the small grain sizes. 
     Based on the above description, a person of ordinary skill in the art will be able to make a magnetic layer structure comprised of a layer of CoCrPtB containing boron in excess of 10 atomic % useful for high density longitudinal magnetic media. Additionally, anyone of ordinary skill in the art will appreciate that many variations and alterations to the above details are within the scope of the invention. For example, the invention can include a plurality of magnetic layers with B&gt;10% or additional layers could be deposited on the substrate prior to depositing the bcc metal or bcc metal alloy under-layer. Therefore, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.