Intra-grain perpendicular percolated L1₁ CoPt thin films

The areal storage density of hard drives has increased rapidly with the development of magnetic recording technology. Thermal stability of the magnetic media is an increasingly significant issue due to downsizing of the magnetic grains, especially for media with a conventional granular microstructure. The reason for this is that isolation of the grains greatly reduces the effective volume V in the term K_uV/k_BT, which represents the energy barrier for thermal demagnetization [1, 2]. Bit-patterned media, a well-known solution proposed more than a decade ago, in which each patterned bit contains a number of strongly exchange coupled grains and the bits form an ordered array, solves the problem by increasing V and thus promises to push the areal density beyond 1 Tbit in⁻² [3–8]. Nevertheless, an economic and efficient method to fabricate such a large area array of nanostructures with good uniformity is still not available. Recently, an innovative recording mechanism using percolated perpendicular media (PPM) was proposed [9–11]. The media is composed of interconnected, strongly coupled magnetic grains as a matrix dispersed uniformly with non-magnetic domain wall pinning sites that penetrate through the whole depth of the film. The pinning sites could be holes or non-magnetic precipitates of oxides or metals. The thermal stability of the bits recorded under this regime is no longer determined by the factor K_uV/k_BT, which is greatly enhanced by the strong exchange coupling of the matrix, but rather by the pinning strength of non-magnetic defects to domain walls. PPM is also expected to give reduced transition jitter noise, because the width of the transition region between recording bits, which is determined by the diameter and pitch of the pinning sites, is narrower than in the case of conventional granular media, where it is determined by the grain size.

Experimentally, different approaches have been proposed to realize PPM. One is to use oxide columns precipitated at the grain boundaries of interconnected hard magnetic grains by alternating sputter deposition or post annealing segregation [12–14]. However, the alternating deposition process is rather complex and difficult to control to obtain good uniformity over a large area and the post segregation approach tends to embed the spherical precipitants inside the films, failing to form a percolated structure [13, 14]. Another method to achieve a percolated microstructure is to use porous templates such as anodized aluminum oxide (AAO) and ZrO₂ [15–17] or patterned defects [18]. The fabricated percolated films show enhancement in their magnetic properties and the corresponding magnetic reversal behavior is consistent with the theoretical predictions [19]. Nevertheless, due to the requirement for a low-temperature deposition process for the purpose of optimizing the magnetic performance, the magnetic film used to demonstrate the PPM with porous templates is a Co/Pt multilayer [15–18]. The value of K_u for the Co/Pt multilayer is on the order of 10⁶ erg cm⁻³, which is an order of magnitude smaller than the value required for recording media with a density exceeding 1 Tbits in⁻². The challenges in using high-K_u materials such as Co–Cr–Pt, L1₀ FePt and CoPt on porous templates will be texture control and a high-temperature process. In addition, the high surface roughness is another intrinsic problem in using porous templates for practical applications.

In this study, we present a method of fabricating nano-scale intra-grain perpendicular anisotropic percolated films with the high-K_u material L1₁ CoPt, a rhombohedral metastable phase with a K_u of approximately 2−4 × 10⁷ erg cm⁻³ [20–24]. Compared with other approaches towards forming percolated microstructures, the method presented is much simpler, giving the required high K_u of the order of 10⁷ erg cm⁻³, showing uniform dispersion of fine precipitants on the scale of 2–7 nm in diameter, with significant enhancement of the magnetic properties and ideal magnetic reversal behavior, and achieving a reduced surface roughness in the range of a few angstroms. The results confirm the proposed route is a promising one for realizing PPM.

The L1₁ CoPt:MgO films were deposited by radio frequency magnetron co-sputtering on MgO(111) substrates. The background vacuum was better than 5 × 10⁻⁹ Torr and the working pressure was fixed at 10 mTorr. High-purity targets of Co, Pt and MgO were used. The composition of the samples was controlled by fine tuning the output power of the individual sources. Before deposition, the MgO(111) substrate was heated to 600 °C for 1 h for surface cleaning and structural reconstruction. The substrate temperature was held at 350 °C during deposition, after which a post annealing was subsequently applied at temperatures (T_a) in the range 350–650 °C for 1 h for the purpose of MgO precipitation. The thickness of the samples varied from 2 to 20 nm. The structure of the samples was analyzed by x-ray diffractometry (XRD). The microstructure was studied by field-emission transmission electron microscopy (FE-TEM). The surface roughness was characterized by atomic force microscopy (AFM). Magnetic properties were characterized by an alternating gradient magnetometer (AGM) and a vibrating sample magnetometer (VSM) at room temperature with maximum magnetic field strengths of 1 T and 2 T, respectively.

To realize PPM in an L1₁ CoPt:MgO film, understanding its basic structural and magnetic properties is necessary before performing microstructure engineering. The effect of MgO content on the structure is first studied in the as-deposited state. The deposition temperature of the samples is set to 350 °C, for which the L1₁ ordering is optimized [23]. Figure 1 shows the XRD patterns of as-deposited CoPt:MgO films of 20 nm in thickness containing different volumes of MgO. The CoPt films grown on MgO(111) exhibit a strong texture of close-packed planes. The binary CoPt sample forms an L1₁ structure in the given conditions, as indicated by the superlattice peak L1₁(111) at an approximate 2θ of 21°. With the addition of MgO, the integrated intensity of L1₁(111) increases and reaches a maximum at MgO = 23.1 vol.%. The peak width increases with increasing intensity, indicating that the doping of MgO broadens the distribution of the L1₁ lattice parameter. Further increasing the MgO content above 26.5 vol.% seems to compromise the order structure, as indicated by the disappearance of the L1₁(111) peak. The results reveal that the addition of MgO with an appropriate content facilitates the formation of the CoPt L1₁ phase. Reflection fringes obtained around the fundamental peak for most of the samples reveal that the surface of the films is quite smooth. The result is consistent with the small root-mean-square value of surface roughness, of only a few angstroms (peak-to-peak value of ∼1 nm), measured by AFM.

Zoom In Zoom Out Reset image size

Since the L1₁ ordering is optimized by the addition of MgO at 23.1 vol.%, the magnetic properties and microstructure on that sample are investigated to clarify the role of MgO in the film. Figures 2(a) and (b) show in-plane and out-of-plane hysteresis loops of 20-nm-thick CoPt and thin films deposited at 350 °C, respectively. Both films show perpendicular magnetic anisotropy (PMA). The anisotropy field H_k of the two films is around 2 T at RT; however, the CoPt:MgO film has a smaller in-plane remanence and in-plane coercivity H_c∥, and a sharper magnetic reversal process, indicative of better easy-axis alignment and a lower resistance to domain wall motion. Corresponding cross-sectional TEM images showing different microstructures are shown in figures 2(c) and (d). The texture of close-packed planes is confirmed as induced by the MgO(111) surface, although the interface is wavy, with a roughness of a few monolayers. The lattice image in figure 2(c) indicates that the binary film contains structural defects, including stacking faults, dislocations and lattice distortions. In the CoPt:MgO film, segregation of MgO is observed. The MgO precipitants form spherical clusters with coherent interfaces to the L1₁ CoPt matrix with diameters in the range from 1 to 2 nm. The coherent precipitation preserves the texture but produces considerable lattice strain around the precipitants due to the large difference in lattice parameters between MgO and CoPt, as shown in figure 2(d). The variation in lattice constants explains the broadened superlattice peak in figure 1. Since the segregated MgO did not penetrate through the whole depth of the film, the domain wall pinning effect is thus limited, leading to a small perpendicular coercivity H_c⊥ of 0.23 kOe.

Zoom In Zoom Out Reset image size

In order to form a percolated structure, the film thickness of the CoPt was decreased to 2 nm to match the size of the MgO precipitants. In the CoPt L1₁ film deposited at 350 °C, the decrease of thickness down to 2 nm results in a higher in-plane remanence and reduces H_k to about 13 kOe, as shown in figure 3(a). As indicated by the cross-sectional TEM images shown in figures 3(b) and (c), the PMA film has a continuous layer structure, which is different from the (001)-textured L1₀ FePt or CoPt films with the same thickness, which exhibit an island-like morphology [25–27]. The difference in microstructure results from the deposition temperature; the formation temperature of L1₀ FePt or CoPt increases drastically with decreasing film thickness [28–30] to over 600 °C, leading to island-growth, while the ordering temperature of L1₁ CoPt seems independent of film thickness and thus preserves a continuous structure even at 2 nm thickness. Prevention of island-like structures is crucial for PPM because it decouples magnetic grains, resulting in degraded thermal stability and an independent magnetic reversal process similar to granular films. The advantages of L1₁ CoPt include a low ordering temperature, good PMA at very low thicknesses, and strong exchange coupling between the interconnected grains, thus manifesting itself as an ideal material for PPM.

Zoom In Zoom Out Reset image size

The precipitation of MgO during the 350 °C-deposition is incomplete, leading to the formation of clusters in the sub-nanoscale, which gives rise to considerable residual strain. In order to facilitate the segregation as well as relax/decrease the lattice distortion and structural defects to form the percolated structure, post annealing was applied to the L1₁ CoPt–MgO 23.1 vol.% films after deposition. Figure 4 shows the in-plane and out-of-plane hysteresis loops of the 2-nm-thick L1₁ CoPt:MgO films post annealed at various temperature (T_a), as measured by AGM at RT. The magnetization values are normalized to the saturation values in the out-of-plane direction. Compared to the CoPt sample shown in figure 3(a), the as-deposited CoPt:MgO film demonstrates a remanence ratio close to unity and a significantly reduced in-plane magnetic component (figure 4(a)), which greatly enhances the effective perpendicular magnetic anisotropy (the area enclosed by the two magnetization curves). H_k obtained by extrapolation is about 15 kOe. Nevertheless, the value of H_c⊥ is only about 0.2 kOe, due to the incomplete phase separation of MgO from the CoPt matrix. H_c⊥ increases gradually with T_a in the samples subjected to post annealing. In the films with T_a = 350 and 400 °C, an increased longitudinal magnetization and a reduced perpendicular remanence ratio is observed, indicative of an intermediate state of MgO phase separation. A square hysteresis loop and a remanence ratio of unity are obtained in the film with T_a = 450 °C. H_c⊥ rises to 4 kOe in the sample with T_a = 500 °C. The extrapolated value of H_k exceeds 20 kOe and the estimated K_u reaches 1.6−2.3 × 10⁷ erg cm⁻³, which are sufficient for ultrahigh-density recording. The sharp magnetic reversal signals the mechanism of domain wall motion and the linear magnetization curve in the in-plane direction confirms perfect alignment of the magnetic easy-axis. As T_a increases to 550 °C, H_c⊥ reaches a maximum of 4.4 kOe. Nevertheless, the drop in magnetization at low field obtained in both in-plane and out-of-plane hysteresis loops indicates the formation of a soft magnetic phase, which is a disordered A1 phase. High-temperature annealing triggers the phase transformation from L1₁ to A1 that has been described previously [23]. On further increasing T_a to 700 °C, order–disorder transformation occurs again, forming a high-K_uL1₀ structure with a strong (111) texture, as evidenced by the increased coercivity and tilt of the magnetic easy-axis. The in-plane and out-of-plane hysteresis loops become similar in shape and coercivity (∼2.7 kOe) and magnetic reversal is broadened.

Zoom In Zoom Out Reset image size

The microstructure of the CoPt–MgO 23.1 vol.% film with T_a = 550 °C exhibiting maximum H_c⊥ was studied to explain the magnetic properties. The plan-view TEM image shown in figure 5(a) indicates the precipitation of MgO grains inside the interconnected L1₁ CoPt grains. Most of the precipitants are round, with diameters in the range 2–7 nm and mean distances between the MgO particles similar to the diameter. The size of the non-magnetic pinning sites is much smaller than previously reported results [13–18]. The cross-sectional image (figure 5(b)) shows the disconnected profile of the film, revealing that the non-magnetic precipitants penetrate through the whole depth of the CoPt film. The microstructure results confirm that the observed coercivity enhancement originates from the segregation of MgO, forming a percolated morphology within a continuous matrix rather than a granular structure with isolated grains. The intra-grain segregation facilitates sharp magnetic reversal, as observed in figures 4(c)–(f), because the propagation field of domain walls in a single-crystal region is very small. Sharp reversal of PPM has been predicted theoretically [9, 10]. Compared to the presented results, PPM formed by inter-grain segregation of the non-magnetic phase shows a rather broad magnetic reversal process [13, 14]. This is due to the boundary precipitation somewhat decreasing the exchange coupling of the magnetic grains, leading to discontinuous propagation of the domain walls. The interface between the L1₁ CoPt matrix and MgO particles proves to be coherent, as presented in figure 5(c). This explains the high remanence ratio and sharp magnetic reversal exhibited, the combination of which is rarely achieved in reported results. The coherent interface optimizes the alignment of the L1₀ magnetic easy-axis near the heterogeneous interfaces, which greatly narrows the angular distribution of magnetization near the pinning sites in the remanent state, leading to a high out-of-plane remanence ratio, negligible in-plane remanence, and a narrowed pinning field distribution. It also stabilizes the hard magnetic phase of the matrix in the present case. In porous PPM, in contrast, magnetic misalignment near pinning sites is unavoidable due to the high surface roughness and structural relaxation of the magnetic films near the pinholes, which tilts the magnetization away from the direction normal to the plane.

Zoom In Zoom Out Reset image size

The pinning mechanism of PPM has been investigated theoretically by means of micromagnetic simulation [19]. In that paper the pinning energy of a domain wall in a PPM is described as $E_{{\rm pin}}\approx 4NDt\sqrt {AK_{{\rm u}}}$, where N is the number of pinning defects, D is the diameter of the defects, t is the film thickness, and A is the exchange constant (in the present case A = 10⁻⁶ erg cm⁻¹ [31]). A maximum E_pin is shown at a ratio δ₀/D = 1, where δ₀ is the Bloch wall parameter, which is related to the domain wall width $\delta = \pi \delta _{0}=\pi \sqrt {A/K_{{\rm u}}}$. The value of δ in this study is between 6.5 and 7.5 nm and the corresponding δ₀/D ratio is in the range 0.34–1.1, which covers the plateau of pinning energy [19]; the strong pinning effect of the presented microstructure is thereby confirmed.

Figure 6 shows the dependence of H_c⊥ on T_a of CoPt–MgO 23.1 vol.% films with different thickness. Coercivity enhancement does not occur in CoPt films with thicknesses of 10 and 20 nm even after high-temperature post annealing. H_c⊥ remains lower than 0.5 kOe, although a good PMA is obtained in samples with T_a < 600 °C. In the thicker films, the MgO precipitants are embedded in the CoPt films rather than forming pinning sites passing through the whole depth of the film or developing a granular structure. Similar results have been observed in the Co–Cr–Pt:SiO₂ system [12, 14]. The CoPt film remains continuous, therefore magnetic hardening originating from domain wall pinning does not occur.

Zoom In Zoom Out Reset image size

In summary, we have demonstrated a simple approach to achieve very thin nano-scale intra-grain percolated films using high-K_uL1₁ CoPt:MgO on MgO(111) substrates with thicknesses of 2 nm. Through an appropriate post annealing, the MgO added via co-sputtering segregates into the nano-grains with a coherent interface to the matrix of L1₁ CoPt. The coherent segregation and the small size of the precipitants, as well as their separation, yield a remarkable enhancement in coercivity and a sharp magnetic reversal, both of which are favorable to PPM. In addition to forming ideal pinning sites for domain walls, the containment of MgO assists the development of the L1₁ phase and optimizes the perpendicular magnetic properties of the L1₁ phase by improving the easy-axis alignment. From the processing aspects, the advantages of the presented method are the avoidance of specialized engineering of substrates, complex deposition processes for the magnetic film, and multilayer structures, making it more feasible for practical production. From the aspect of film characteristics, the achievement of smooth surfaces, high K_u, suitable coercivity, sharp magnetic reversal, and the ultrathin nature of the film fulfills the requirements for a magnetic medium with an extremely high recording density.

Acknowledgment