Requirements of magnetic medium:
In order to use the magnetic materials in a recording medium, these materials should have the following basic requirements (see Figure 31.1) for high density recording:
1. Coercivity (HC ): A high coercivity in the materials is necessary to accommodate very sharp transitions. Coercivities of the hard magnetic materials are often determined by magnetocrystalline anisotropy, meaning that high anisotropy materials, such as CoCrPt, FePt based alloys, are required. However, the coercivity of the materials should not exceed the writing capability of the currently available write heads, which does not allow efficient writing in the medium.
2. Magnetization at remnant state (M R ): High values of remnant magnetization (>95 % of Saturation magnetization) in small thickness (δ) films are preferred to obtain adequate readback signals with the minimum thickness spacing loss. For MR heads, the product 
should match that of the MR element (or spin valve free layer).
3. Negative nucleation field: This is defined as the reversing field (preferably in the second quadrant of a hysteresis loop) at which the magnetization starts to drop from its value at saturation. The more negative the nucleation field, the more stable the remnant magnetic state should be, since a larger reversing field is required to alter the magnetization. The value of the nucleation field should also scale with the component of anisotropy in the direction of the applied magnetic field.
4. Nearly squared M-H loop is very much important (see eqn.(14.15)) to achieve a sharp transitions and satisfactory overwrite ratio.
5. As shown in Figure 31.1b, fine and well isolated single domain particles with uniform size distributions and large anisotropy are required to obtain sufficient coercivity and to reduce the switching field distribution. This is needed to reduce the noises in the media.
6. Very smooth surface on top of the disk and reliable mechanical stability are needed to attaining small magnetic spacing with the acceptable tribological performance.
References:
[1]. K.G. Ashar, Magnetic Disk Drive Technology, Heads, Media, Channel, Interfaces and Integration, IEEE Press, New York, 1997.
[2]. A. Perumal et al., FePtAg-C nanogranular films fabricated on a heat resistant glass substrate for perpendicular magnetic recording, Journal of Applied Physics 108 (2010) 083907.