Obviousness

The US patent 9928864 claims a magnetic recording medium designed to overcome the "writeability problem" in high-density recording by separately adjusting the coercive field and thermal energy barrier. The core invention, as described in independent claim 1, involves a specific multilayer structure where a hard magnetic storage layer is formed on an underlayer, and a softer "nucleation host" is formed directly on the hard magnetic storage layer. A critical feature is that this nucleation host itself "comprises ferromagnetic layers with increasing anisotropy constant K from layer to layer," and is exchange coupled to the hard magnetic storage layer.

An obviousness analysis under 35 U.S.C. § 103 requires identifying prior art references that disclose the elements of the claim and establishing a motivation for a person having ordinary skill in the art (POSITA) to combine those references to arrive at the claimed invention.

Obviousness Argument: Combination of Victora and Shen (2005) with Principles derived from Hagedorn (1970)

Primary Reference: Victora and Shen (2005), "Composite Media for Perpendicular Magnetic Recording".
As described in the background of US9928864, Victora and Shen proposed "magnetic multilayer structures composed of magnetically hard and magnetically soft layers" for "Perpendicular Magnetic Recording." This reference clearly teaches:

1. A magnetic recording medium for perpendicular recording.
2. The use of a non-magnetic substrate and an underlayer (standard in such media, as illustrated by FIG. 1 of US9928864 as an "existing disk of recording medium").
3. An exchange-coupled magnetic multilayer structure.
4. This structure includes both magnetically hard and magnetically soft layers, which correspond to the hard magnetic storage layer (H_s > 0.5 T) and a nucleation host (H_n < H_s) as claimed in US9928864.
5. The goal of such structures was to improve magnetic recording performance, implicitly addressing issues like the writeability problem.

Victora and Shen thus disclose elements related to the overall structure, the presence of hard and soft exchange-coupled layers, and their application in perpendicular recording.

Secondary Reference/Principle: Hagedorn (1970), "Analysis of Exchange-Coupled Magnetic Thin Films", as interpreted and described in US9928864.
The patent 9928864 itself provides strong motivation and teaching for the critical element of the nucleation host: "The field, required to overcome the pinning field to push a domain wall from the softer layer to the hard layer depends on the difference between the anisotropy constants of these layers as described by a formula by Hagedorn et al. If the number of layers is increased, this difference can be decreased leading to a reduction of the pinning and coercive field."
The patent further explains: "For example, if the anisotropy of the m-th layer assumes the value K m =(m−1)K hard /(N−1), the difference between the anisotropy constants of adjacent layers is K hard /(N−1). Such an architecture leads to a coercive field of the whole structure of H c =1/(4N−4)×2 K hard /J s ."

This passage, directly from US9928864, explicitly teaches:

1. The concept of using multiple layers (increasing the "number of layers").
2. Arranging these layers such that their anisotropy constants increase from layer to layer (as implied by K_m = (m-1)K_hard / (N-1), where K increases with 'm').
3. The direct benefit of this arrangement: "reduction of the pinning and coercive field."

Motivation for Combination:
A POSITA, working in the field of high-density perpendicular magnetic recording media around the priority date of US9928864 (June 17, 2006), would be acutely aware of the "superparamagnetic limit" and the associated "writeability problem" described in the patent's background. This problem arises because increasing anisotropy (K) to ensure thermal stability also increases coercivity (H_c), making the media difficult to write with existing recording heads.

Victora and Shen (2005) provided a foundational approach using exchange-coupled hard/soft multilayer structures to address these challenges in perpendicular recording. Building upon such a known system, a POSITA would be motivated to seek further improvements in reducing the coercive field without sacrificing thermal stability.

The detailed explanation within US9928864 regarding Hagedorn's work offers a clear path. A POSITA would recognize that by applying Hagedorn's principle—that a gradual increase in anisotropy across multiple layers reduces the coercive field—to the soft nucleation host of an exchange spring medium (such as those described by Victora and Shen), they could further reduce the field required to write information. Instead of a single soft layer, the nucleation host could be composed of multiple ferromagnetic layers, each with a progressively higher anisotropy constant, thereby creating a smooth anisotropy gradient. This gradient would facilitate domain wall propagation during reversal, leading to a lower overall coercive field while the hard magnetic storage layer continues to provide the necessary thermal stability.

Furthermore, regarding the stacking order in claim 1 where the nucleation host is formed on the hard magnetic storage layer, US9928864 explicitly states in its detailed description: "Although the hard magnetic storage layer is shown on top of the nucleation layer [in FIG. 1], the order of the layers can also be reversed." This indicates that positioning the hard magnetic storage layer below the nucleation host was a known and contemplated arrangement for such media.

Therefore, combining the known exchange-coupled hard/soft multilayer perpendicular recording media (Victora and Shen) with the established principle of reducing coercive fields through a gradual, increasing anisotropy gradient in multiple layers (derived from Hagedorn) would have been an obvious design choice for a POSITA seeking to improve writeability. The specific arrangement of increasing anisotropy within the nucleation host directly addresses the known problem of high coercivity in hard magnetic materials, making the combination an obvious solution.