IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 12, DECEMBER 2011 4769 Recording Behavior of Exchange Coupled Composite Media H. J. Richter , G. Choe , and B. D. Terris Hitachi Global Storage Technologies, San Jose Research Center, San Jose, CA 95135 USA Hitachi Global Storage Technologies, Media Development, San Jose, CA 95119 USA The recording behavior of exchange coupled composite (ECC) media is investigated. It is found experimentally that the change of the written track width is less pronounced as a function of linear density when the write-ability is controlled by varying the vertical exchange coupling between the various layers in the medium stack as opposed to varying the lateral exchange coupling between the grains. The results are explained using a simple three-spin model which captures the essential physics. It is pointed out that the traditional concept of an effective field that separates out the field angle effect does not apply to the recording process of ECC media. Index Terms—CGC media, ECC media, effective field, perpendicular recording. I. INTRODUCTION R ECENTLY, exchange coupled composite media (ECC media) have received much attention for the further ad- vancement of high-density magnetic recording media [1]–[4]. Initially, the concept of ECC media was suggested as a means to reduce grain size with the aim to keep the number of grains per bit constant while increasing recording density [5]–[7]. To achieve sufficient thermal stability of small grains, the media have to be made of a material with very high magnetic anisotropy. However, the available head fields are insufficient to record on media with very high anisotropy and thus the design space becomes limited, which is also referred to as the “tri-lemma” of magnetic recording [8]. The fundamental idea of ECC media is to combine a magnet- ically soft material with a magnetically hard material, whereby the switching field is reduced while maintaining thermal sta- bility. This implies that the magnetization reversal process has to be incoherent when a magnetic field is applied (that is, during recording) and coherent in zero applied field (that is, during information storage) [8]. Clearly, a very strong exchange cou- pling between the soft and the hard layer(s) cannot lead to a functional ECC medium, and therefore the exchange between the various layers has to be carefully controlled to tune the switching behavior [1]–[3]. It is important to note that the “cor- rect” amount of exchange coupling between the layers depends on the anisotropy contrast between the various layers. If the anisotropy contrast between the materials is strong, there may not be a need to artificially reduce the coupling between the in- dividual layers. In other words, there is an interplay between the optimum layer couplings and the anisotropy grading, i.e., the variation of the anisotropy throughout the stack. Current recording media have a “cap” structure where the topmost layer has a considerably stronger exchange coupling in the lateral direction than the other layers. The class of media where lateral exchange coupling has been introduced by a Manuscript received March 24, 2011; revised June 04, 2011; accepted June 07, 2011. Date of publication June 23, 2011; date of current version November 23, 2011. Corresponding author: H. J. Richter (e-mail: hans.richter@hitachigst. com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2160274 separate layer has been termed composite granular composite (CGC) [9] and is in general use today, albeit with a lower level of lateral exchange coupling than initially envisioned. It is well known that introducing lateral exchange coupling improves write-ability [8], but this goes along with an increased tendency of the written magnetization to form clusters and consequently a poorer signal-to-noise ratio (SNR). In current media, both the CGC and the ECC effects are present and it is important to optimize them independently. Inspecting the development of recent perpendicular media, it is readily evident that the concept of ECC media is widely used but yet the grain size has not been reduced. The paper tries to explain why there are advantages for ECC media beyond the grain size argument. In the literature, advantages for ECC media beyond the grain size argument have been mentioned, such as an increased effective field gradient in the cross-track direction [10] and the fact that the field gradient in the bottom part of the stack may be rather weak without sacrificing performance [11]. The paper consists of two parts: the first part describes recording measurements on two series of samples where the strength of the exchange coupling in both the lateral and the vertical direction is systematically varied. In the second part, the recording results are interpreted using a modified effective field concept, which provides an enhanced understanding of the recording process. II. EXPERIMENTAL SETUP A. Samples The experiments were conducted on two series of disks. In se- ries A, the thickness of the cap layer was varied between 3.7 and 6.2 nm which results in a variation of the lateral exchange (CGC series). In series B, the exchange coupling in the vertical direc- tion between the main storage layer and cap layer was varied by changing the thickness of the exchange coupling layer (ECL) between 0 and 1.63 nm (ECC series). The ECL layer consists of an alloy with a very low saturation magnetization. Fig. 1 shows a sketch of the medium structure where the details irrelevant for this study were omitted. First, it was confirmed that series A and B exhibit the ex- pected signatures of a CGC and an ECC variation. For the CGC series, one expects that for increasing exchange, the nucleation field remains constant whereas both the coercive and the saturation field decrease [1], [12] as illustrated in Fig. 2 on the 0018-9464/$26.00 © 2011 IEEE