After more than 30 years of research, hard disk drives using perpendicular recording are finally commercially available. This review is a follow-up of a review written in 1999 and addresses the basic physics of perpendicular recording with special emphasis on the read and the write process and the magnetic aspects of the recording media. The paper also surveys various technical difficulties which prevented an earlier implementation of perpendicular recording. The paper closes with a short overview of alternative technologies that allow even higher storage densities.

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A comprehensive analysis of the areal density potential of bit-patterned media recording shows that the recording performance is dominated by written-in errors. The statistical fluctuations of the magnetic properties and the locations of the individual bits lead to error probabilities so that some dots are either not recorded at all or cannot be recorded in the time window necessary to ensure synchronized writing. The highest areal densities are obtained with a combination of a pole head, a soft magnetic underlayer, and a storage medium of the composite type. Areal density scenarios of up to 5 Tbits/ in. 2 are analyzed.

Using numerical and analytical micromagnetics we calculated the switching fields and energy barriers of the composite (exchange spring) magnetic recording media, which consist of layers with high and low magnetocrystalline anisotropy. We demonstrate that the ultimate potential of the composite media is realized if the interfacial domain wall fits inside the layers. The switching occurs via domain wall nucleation, compression in the applied field, de-pinning and propagation through the hard/soft interface. This domain wall assisted switching results in a significant reduction of the switching field without substantial decrease of the for thermal activation energy barrier. We demonstrate that the Domain Wall Assisted Magnetic Recording (DWAMR) offers up to a three-fold areal density gain over conventional single layer recording.Areal density is one of the most important figures of merits for the information storage technologies. With the increase of the recording density the dimensions of the magnetic media grains have to be reduced to maintain the signal-to-noise ratio. Reducing grains dimensions results in the decrease of the magnetic energy barrier which leads to the thermally activated loss of the stored information. To increase the thermal stability the magnetic anisotropy of the grains can be increased, however the maximum anisotropy is limited by the write head fields. The described "trilemma" [1] constitutes the main physical limit that magnetic recording faces in the attempt to achieve the higher recording densities.As a means to resolve this problem, Victora and Shen proposed the so-called composite media [2], which are made of two layers, one of which ("hard") possesses high magneto-crystalline anisotropy, while the other ("soft") layer has very low anisotropy. It was shown in Refs. [2,3] that for the optimum exchange coupling across the layers interface the switching field of such a structure may be substantially reduced compared to a single layer film. The concept of exchange spring media, utilizing noncoherent magnetization rotation in hard/soft multilayers to reduce the switching field, was independently introduced by Suess et al. [4,5]. Guslienko et al. [6] analyzed the switching field in dual layer media taking into account non-uniform magnetization rotation, and the results were shown [4,7] to differ from the "two-spin" (rigid layers) approximation.In this Letter we will show that the ultimate potential of the composite (exchange spring) media is realized when the interfacial domain wall fits inside soft and hard layers. We will analyze the potential of the Domain Wall Assisted Magnetic Recording (DWAMR) using numerical micromagnetics as well as analytical formulas derived in Refs. [8,9] for exchange spring magnets.In our micromagnetic simulations the hard and soft layers are discretized down to atomic scale (0.2 nm) in the vertical (perpendicular to the film plane) direction. We include the external, anisotropy and vertical exchange fields, however, for simplicity, we do not take into account t...

Three mechanisms by which edges induce stress relaxation in GeSi strained stripes are described and their relative importance is discussed. Relaxation of stresses in the middle of the layers with I/h( =half-width/thickness) varying from 3 to 100 is calculated including the efFect of the two mechanisms which are important in this range. The values calculated in this manner agree with our recent finite element calculations. Since the stresses in the stripes in the two orthogonal directions are not

Composite grains, consisting of a subgrain with high anisotropy field coupled with a subgrain with zero anisotropy, are analyzed using a two-spin model. An analytical expression is given for the exchange coupling between the subgrains that minimizes the field required to reverse the magnetically hard layer. The results of the two-spin model are compared with those of a spin chain that represents an “exchange spring magnet.” The limitations of the two-spin model are worked out.

Recently magnetorheological fluids with nanosized magnetic ferrite particles have become available. Their composition, rheological and magnetic properties are described. A comparison with conventional MR fluids based on micron-sized particles is given. The yield stress of nano-MR fluids can be increased by a moderate magnetic field (0,2 T) by 4000 Pa. It can be modulated by the magnetic field with a response time of less than 5 ms. Details are given on the long term thermal stability at 150 °C, on flow properties at elevated temperatures and at high shear rates. Design principles for MR fluid actuator design are outlined.