Patent Grant
8119263
The present invention relates to improved, high recording performance magnetic recording media comprising at least a weakly exchanged coupled magnetic layer and a strongly exchanged coupled magnetic layer, preferably in contact with each other. More particularly, the invention relates to hard disk recording media providing increased signal to noise ratio during magnetic recording.
Thin film magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the magnetic recording layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains (bits) of the magnetic grains in the magnetic recording layer.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hcr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. Thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic layer and V is the volume of the magnetic grain. V depends on the magnetic layer thickness (t); as t is decreased, V decreases. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media.
For high signal to noise ratio (SNR) magnetic recording media, it is desirable to have small uniformly sized magnetic particles or grains; and a moderately low, uniform exchange coupling between the particles or grains. For longitudinal, perpendicular and tilted magnetic recording media, the optimal exchange coupling value is different (e.g., typically, a higher exchange coupling is desired for perpendicular recording media), but for each case, a constant, moderate value is desired between each neighboring grain.
Generally, low exchange coupling is desired so that magnetic switching of neighboring grains does not become too highly correlated. Reducing exchange coupling decreases the size of the magnetic particle or magnetic switching unit. Cross-track correlation length and media noise are correspondingly reduced. However, near-zero exchange coupling between magnetic grains produces a very low squareness sheared hysteresis loop, a broad switching field distribution, less resistance to self demagnetization and thermal decay, and low nucleation field (Hn) in perpendicular media designs. Non-uniform exchange coupling allows some grains to act independently while other grains act in clusters, resulting in broad distributions of magnetic particle size and anisotropy field.
Conventionally used storage media contain a magnetic recording layer having Co—Cr—Pt—B and Co—Cr—Ta alloys where B and Ta are mainly used to improve the segregation of Cr in the magnetic layer. The exchange coupling is controlled by preferentially forming non-ferromagnetic material at the boundaries between magnetic particles. Non-ferromagnetic material is commonly formed during sputter deposition of CoPtCrB containing alloys on high temperature substrates by preferential surface diffusion of Cr and B to grain boundaries. The Co concentration varies between the grain center and the grain boundary such that there is a transition from magnetic to non-magnetic composition. Exchange coupling in such media is controlled by changing parameters such as the Cr and B concentrations, and the substrate temperature.
Non-ferromagnetic material can also be formed at magnetic grain boundaries during sputter deposition of CoPt containing alloys on low temperature substrates, by addition of a metal oxide to the sputter target or by reactively sputtering the target in a sputter gas containing oxygen. Exchange coupling in these media is controlled by changing parameters such as sputter gas pressure, oxygen concentration in the sputter gas, and oxide concentration in the sputter target.
In any case, a better segregation profile of non-ferromagnetic material leads to a sharper transition between the magnetic grains and the non-magnetic Co-depleted grain boundaries. Correspondingly, the recording media can have higher Co concentration inside each grain.
However, there are several problems when sputter parameters are adjusted to control the amount of non-ferromagnetic material to create a significantly, but not completely, exchange decoupled magnetic particles. A magnetic material having a significantly, but not completely, exchange decoupled magnetic particles is termed as moderately exchange coupled.
One problem with such an exchange decoupling method is that the exchange between two grains is extremely sensitive to the arrangement of a very small number of atoms at the grain boundary between each pair of adjacent grains. Thus, some grains are much more strongly coupled than others. A second problem is that radial diffusion profiles depend upon the size of the magnetic grains. Thus, larger grains can have systematically different composition than smaller grains, and hence have systematically different exchange coupling and magnetic anisotropy. A third problem is that the composition of the entire film including the ferromagnetic particles, the weakly exchange coupled ferromagnetic regions between magnetic particles, and the exchange decoupling non-ferromagnetic regions between magnetic particles is substantially the same, except for the preferential lateral transport of some atomic species. Changing exchange coupling in the film generally changes the composition of the magnetic particles as well as the grain boundary material. Thus, it is difficult to separately optimize the properties of each component of the film. Also, a fourth problem exists related to the degraded mechanical robustness and corrosion resistance that arises from such prior art wherein high pressure sputter methods necessarily yield increased surface roughness. There is thus a need for a new method for the magnetic recording media that provides improved control of exchange coupling in the magnetic layer and improved recording performance while preserving media robustness.
The embodiments of the invention are directed to a longitudinal or perpendicular recording medium having an improved compositional segregation within the magnetic layers. One embodiment relates to a magnetic recording medium comprising a substrate, a first magnetic layer and a second magnetic layer, wherein an exchange coupling in the first magnetic layer is lower than an exchange coupling in the second magnetic layer. Preferably, the first and second magnetic layers are in a film stack so that magnetic grains in the first magnetic layer are exchange coupled by a pathway through the second magnetic layer.
Another embodiment relates to a magnetic recording medium comprising a substrate, a first magnetic layer and a second magnetic layer, wherein said first magnetic layer comprises regions of amorphous material and said second magnetic layer comprises substantially no amorphous material. Preferably, said first magnetic layer comprises regions of non-ferromagnetic material and said second magnetic layer comprises substantially no non-ferromagnetic material.
Yet another embodiment relates to a method of manufacturing a magnetic recording medium comprising obtaining a substrate, depositing a first magnetic layer with reactive oxidation or depositing a second magnetic layer without substantial reactive oxidation. Preferably, said first magnetic layer and said second magnetic layer comprise films deposited at different substrate temperature, further wherein the second layer comprises a film deposited at a substrate temperature greater than 50° C. Also, preferably, the depositing a first magnetic layer is at a first sputter gas pressure and the depositing a second magnetic layer is at a second sputter gas pressure, in this order, wherein the first sputter gas pressure is higher than the second sputter gas pressure, and an exchange coupling in the first magnetic layer is lower than an exchange coupling in the second magnetic layer.
As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.
Magnetic recording media having Co—Cr—Pt—B and Co—Cr—Ta alloys contain B and Ta to improve the segregation of Cr in the magnetic layer. A better segregation profile of Cr leads to a sharper transition between the magnetic grains and the non-magnetic Cr-rich grain boundaries, and thus, the recording media is expected to have higher saturation magnetization (Ms) and magnetocrystalline anisotropy (Ku) and narrower intrinsic switching field distribution. Magnetic recording media having Co—Cr—Pt—MO (M is a reactive metal with oxygen) contain MO to improve the segregation in the magnetic layer. A better segregation profile of oxide leads to a sharper transition between the magnetic grains and the non-magnetic oxide-rich grain boundaries, and thus, the recording media is expected to have higher saturation magnetization (Ms) and magnetocrystalline anisotropy (Ku) and narrower intrinsic switching field distribution.
For perpendicular recording media, the simplest quantity to estimate exchange coupling is Hc-Hn. For a selected media structure, larger values of the quantity Hc-Hn correspond to lower exchange coupling. For prior art media having Hc˜5000 Oe, a value of Hc-Hn below 2000 Oe could be considered to be strongly exchange coupled, while a media having Hn above 4000 Oe would be weakly exchange coupled. Values of Hc-Hn between approximately 2000-4000 Oe could be called moderately exchange coupled. A more accurate measurement of exchange coupling can be defined by an exchange field (Hex). Hex can be measured from hysteresis loop parameters including Hc and Hn. A good approximation of exchange field is given by the equation Hex=((1-N)*4*Pi*Ms)-(Hc-Hn). N is the demagnetization shape anisotropy coefficient of a magnetic grain that depends upon grain size and film thickness. N is known in the prior art, and has a value between 0.15 and 0.3 for most current perpendicular media designs. Low exchange coupling may be defined by Hex below about 1000 Oe. Moderate exchange coupling may be between about 1000-3000 Oe, and high exchange coupling may be above about 3000 Oe.
The embodiments of the invention comprise a method and apparatus for a magnetic recording media including a weakly exchange coupled magnetic layer and a strongly exchange coupled magnetic layer. The weakly exchange coupled magnetic layer has sufficient non-ferromagnetic material between magnetic grains that exchange coupling is lower than is optimal for a magnetic recording media. The strongly exchanged coupled magnetic layer has high exchange coupling such that substantially all of the magnetic grains are strongly exchange coupled. The strongly exchange coupled and weakly exchanged coupled magnetic layers are adjacent in the film stack so that grains in the weakly exchanged coupled magnetic layer are moderately exchange coupled by a pathway through the strongly exchanged coupled magnetic layer. That is, exchange coupling between grains in the weakly coupled magnetic layer is increased toward an optimal moderate value by their coupling connection through the strongly coupled magnetic layer.
The embodiments of the present invention are based upon the recognition that 1) exchange between the magnetic grains in the weakly exchanged coupled magnetic layer can be controlled by changing the thickness of the strongly exchanged coupled magnetic layer, without changing the composition of either layer; 2) exchange coupling through the strongly coupled magnetic layer does not depend as much upon grain size of the weakly coupled magnetic layer; 3) exchange coupling through the strongly coupled magnetic layer does not depend as much upon preferential transport of selected elements; 4) Exchange coupling through the interface between the weakly coupled magnetic layer and the strongly coupled magnetic layer involves more atoms than exchange coupling between pairs of grains in the weakly exchange coupled layer; and 5) thickness control is precise, so that the small number of atoms involved in exchange tuning is relatively easy to control.
The embodiments of the invention provide magnetic recording media suitable for high areal recording density exhibiting high SMNR. A perpendicular recording media embodiment of the invention achieves such technological advantages by forming a soft underlayer. A “soft magnetic material” is a material that is easily magnetized and demagnetized. As compared to a soft magnetic material, a “hard magnetic” material is one that neither magnetizes nor demagnetizes easily.
The underlayer is “soft” because it is made up of a soft magnetic material, which is defined above, and it is called an “underlayer” because it resides under a recording layer. In a preferred embodiment, the soft layer is amorphous. The term “amorphous” means that the material of the underlayer exhibits no predominant sharp peak in an X-ray diffraction pattern as compared to background noise. The term “amorphous” encompasses nanocrystallites in amorphous phase or any other form of a material so long the material exhibits no predominant sharp peak in an X-ray diffraction pattern as compared to background noise. The soft magnetic underlayer can be fabricated as single layers or a multilayer. The amorphous soft underlayer is relatively thick compared to other layers. The amorphous soft underlayer materials include a Cr-doped Fe-alloy-containing alloy, wherein the Fe-alloy could be CoFeZr, CoFeTa, FeCoZrB and FeCoB.
An adhesion layer could be optionally included in the embodiments of this invention. An adhesion layer is a layer lying in between the substrate and the soft underlayer. A proper adhesion layer can also control anisotropy of the soft underlayer by promoting a microstructure that exhibits either short-range ordering under the influence of a magnetron field, or different magnetostriction. An adhesion layer could also alter local stresses in the soft underlayer.
Preferably, in the soft underlayer of the perpendicular recording medium of the embodiments of the invention, an easy axis of magnetization is directed in a direction substantially transverse to a traveling direction of the magnetic head. This means that the easy axis of magnetization is directed more toward a direction transverse to the traveling direction of the read-write head than toward the traveling direction. Also, preferably, the underlayer of the perpendicular recording medium has a substantially radial or transverse anisotropy, which means that the domains of the soft magnetic material of the underlayer are directed more toward a direction transverse to the traveling direction of the read-write head than toward the traveling direction.
Amorphous soft underlayer materials lack long-range order. Without long-range order, amorphous alloys have substantially no magnetocrystalline anisotropy. The use of amorphous soft underlayer could be one way of reducing noise caused by ripple domains and surface roughness. Amorphous soft underlayers could produce smoother surfaces as compared to polycrystalline underlayers. Therefore, amorphous soft underlayer could be one way of reducing the roughness of the magnetic recording media for high-density perpendicular magnetic recording.
The surface roughness of the amorphous soft underlayer is preferably below 1 nm, more preferably below 0.5 nm, and most preferably below 0.2 nm.
In accordance with embodiments of this invention, the substrates that may be used in the embodiments of the invention include glass, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
A preferred embodiment of the perpendicular recording media of the invention further includes an interlayer. The interlayer can be made of more than one layer of non-magnetic materials. The interlayer prevents an interaction between the amorphous soft magnetic underlayer and recording layer. The interlayer also promotes the desired microstructural properties of the recording layer. In some embodiments the interlayer comprises an hcp material with strong <0001> growth orientation. In a preferred embodiment the interlayer includes a Ru-containing alloy. In some embodiments a Ru-containing alloy interlayer is sputter deposited under argon pressure (PAr) greater than 20 mTorr. In some embodiments the Ru-containing interlayer comprises at least two Ru-alloy containing films sputter deposited at different PAr.
The soft underlayer and magnetic recording layer could be sequentially sputter deposited on the substrate, typically by magnetron sputtering. A carbon overcoat could be typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically less than about 20 Å thick.
The embodiments of magnetic recording media comprise at least two magnetic layers including at least one weakly exchange coupled magnetic layer having lower exchange than is optimal for a single-layer magnetic recording medium, and at least one strongly exchange coupled magnetic layer having exchange coupling higher than is optimal for a single-layer magnetic recording medium. The at least one weakly exchange coupled magnetic layer and the at least one strongly exchange coupled magnetic layer are deposited successively such that they are adjacent in the film stack and in direct physical contact. The weakly exchanged coupled magnetic layer is generally a cobalt and platinum containing (Co-Pt) alloy that also includes other nonmagnetic (non-ferromagnetic) elements. The weakly exchange coupled magnetic layer is deposited so as to form a compositionally segregated microstructure wherein the magnetic particles comprise higher concentrations of the Co and Pt, while the boundaries between magnetic particles comprise higher concentrations of non-magnetic elements and lower cobalt concentration, such that the boundary material is substantially non-magnetic. The strongly exchanged coupled magnetic layer has high exchange coupling such that substantially all of the magnetic grains are strongly exchange coupled. The strongly exchanged coupled magnetic layer often also comprises a Co-containing alloy. The strongly exchanged coupled magnetic layer is deposited so as to form a non-compositionally segregated microstructure or one less segregated than the weakly exchanged coupled magnetic layer. That is, the strongly exchange coupled magnetic layer has less or in some preferred embodiments, substantially no non-ferromagnetic material between ferromagnetic grains. The overall concentration of non-magnetic elements and specifically the concentration of nonmagnetic materials at magnetic particle boundaries is lower than in the weakly exchanged coupled magnetic layer case.
In one embodiment, the method of manufacturing comprises depositing adhesion layers, soft underlayers, and interlayers that optimize growth of the magnetic layer; depositing a Co-alloy magnetic layer having higher concentrations or volumes of nonmagnetic grain boundary material; depositing a Co-alloy layer having little or no non-magnetic grain boundary material; and depositing a protective overcoat.
The media of this invention have the following experimentally demonstrated advantages:
All samples described in this disclosure were fabricated employing DC magnetron sputtering. Carbon overcoat films were ion-beam deposited.
Oxide Perpendicular Recording Media Embodiments
Some embodiments of the perpendicular recording media of the invention include the following properties and steps:
Other embodiments of the perpendicular recording media of the invention include steps similar to 1-4 of the previously described Oxide Perpendicular Recording Media Embodiments, except as modified below:
Some embodiments of the longitudinal recording media of the invention include the following properties and steps:
In other embodiments, the order of performing steps 2 and 3 could be reversed.
This application discloses several numerical range limitations that support any range within the disclosed numerical ranges even though a precise range limitation is not stated verbatim in the specification because this invention can be practiced throughout the disclosed numerical ranges. Finally, the entire disclosure of the patents and publications referred in this application are hereby incorporated herein in entirety by reference.
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