1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a continuous-media type of perpendicular magnetic recording disk with a granular recording layer having controlled grain size.
2. Description of the Related Art
In a “continuous-media” perpendicular magnetic recording disk, the recording layer is a continuous layer of granular cobalt-alloy magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the write head writes on the magnetic material. Continuous-media disks, to which the present invention is directed, are to be distinguished from “bit-patterned-media” (BPM) disks, which have been proposed to increase data density. In BPM disks, the magnetizable material on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to continuous-media disks wherein a single “bit” may have multiple magnetic grains.
FIG. 1 is a schematic of a cross-section of a prior art perpendicular magnetic recording continuous-media disk. The disk includes a disk substrate and an optional “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In the example of FIG. 1, the material for the recording layer (RL) is a granular ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of a nonmagnetic segregant, which is depicted in FIG. 1 as an oxide, including oxides of Si, Ta, Ti, Nb, B, C, and W. These oxides (Ox) tend to form at the grain boundaries as shown in FIG. 1, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. An optional capping layer (CP), such as a granular Co alloy without added oxides or with smaller amounts of oxides than the RL, is typically deposited on the RL to mediate the intergranular coupling of the grains of the RL, and a protective overcoat (OC) such as a layer of amorphous diamond-like carbon is deposited on the CP.
The Co alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hexagonal-close-pack (hcp) crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, intermediate layers of ruthenium (Ru1 and Ru2) are located below the RL. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that induce the proper growth of the RL. An optional seed layer (SL) may be formed on the SUL prior to deposition of Ru1.
The enhancement of segregation of the magnetic grains in the RL by the additive oxides as segregants is important for achieving high areal density and recording performance. The intergranular Ox segregant material not only decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on the Ru2 layer that exhibits columnar growth of the Ru or Ru-alloy grains. The amount of Ox segregants inside the RL needs to be sufficient to provide adequate grain-to-grain separation, but not too high to destroy the thermal stability of the RL. The typical content of the Ox segregants is about 20% in volume, and the mean grain boundary thickness is typically between about 1.0 and 1.5 nm.
FIG. 2 is a transmission electron microscopy (TEM) image of a portion of the surface of a prior art CoPtCr—SiO2 RL from a disk similar to that shown in FIG. 1. FIG. 2 shows well-segregated CoPtCr magnetic grains separated by intergranular SiO2 (white areas). However, as is apparent from FIG. 2, there is a relatively wide variation in the size of the magnetic grains. FIG. 2 also illustrates the randomness of grain locations, which results in a wide variation in the grain-to-grain distance or grain “pitch”. Because the nucleation sites during the sputtering deposition are randomly distributed by nature, there is no control of the grain locations. A wide grain size distribution and wide grain pitch distribution are undesirable because they can cause wide distributions in magnetic anisotropy strength and intergranular exchange, which contribute to noise in the readback signal.
To achieve high areal density of 1 to 5 Terabits/in2 and beyond, it is desirable to have high uniformity (or tighter distribution) of the grains within the RL, mainly for the structural parameters of grain diameter (i.e., the diameter of a circle that would have the same area as the grain), and grain-to-grain distance or grain pitch (i.e., the distance between the centers of adjacent grains). Narrower distribution of grain diameter and grain pitch will lead to narrower distributions of magnetic exchange interaction and magnetic anisotropy strength, both of which are desirable.
Thus the prior art RL shown in FIG. 2 is far from ideal. First, the grains have an irregular polygonal shape with a large size distribution. The average grain diameter is about 8-11 nm with a relatively wide distribution of over 20%. Second, the location of the grain centers is highly random, with a wide distribution of 20-23% of the mean. The distribution information is obtained by measuring neighboring grain-to-grain distances in high resolution scanning electron microscopy (SEM) or TEM images and then fitting with a log normal function. Distribution value as referred to in this application shall mean the width of the lognormal function.
A magnetic recording disk with a Co alloy and oxide-segregant RL and a nanoparticle template layer below the Ru underlayer is described in application Ser. No. 13/772,110 filed Feb. 20, 2013 and assigned to the same assignee as this application. The nanoparticles are nanoparticle cores with polymer ligands that self-assemble in a regular pattern across the disk substrate. The Ru underlayer generally replicates the surface topology of the nanoparticle template and the Ru “bumps” above the nanoparticles serve as nucleation sites for the growth of the Co alloy grains of the RL. The result is a disk with a Co alloy RL having reduced grain diameter distribution and reduced grain pitch distribution.
While narrower grain pitch distribution has been achieved due to the mechanism of self-assembly and uniformity in size of the nanoparticles, “rafts” of nanoparticles with long range ordering are also formed. Post-deposition annealing of the nanoparticles is commonly used to lower the grain pitch distribution, but this tends to increase raft size dramatically. Raft boundaries are the regions between rafts where the nanoparticles are not arranged in a regular pattern. Since the sizes and locations of the Co alloy magnetic grains are defined by the initial nanoparticle locations on a one-to-one basis, large rafts and long raft boundaries in the nanoparticle template will lead to formation of large magnetic rafts and raft boundaries, which will degrade media performance.
Therefore, for a granular cobalt alloy RL with additive oxides formed on a nanoparticle template, it is desirable not only to reduce the grain pitch distribution but to control the long range ordering of the grains without enlarging rafts and raft boundaries. This will enable a continuous-media perpendicular magnetic recording disk with a narrow distribution of grain pitch but with controlled long range ordering of the grains.