C. Papusoi

Composite nanogranular films of FePt-MgO with (001) orientation onto glass substrates

For perpendicular magnetic recording, a design of using MgO as a seed layer and an intermediate layer is introduced. By annealing the multilayer film with a stack structure of MgO/[FePt/MgO]3 at 600 °C for 1 min, nanograins of FePt with a high perpendicular magnetic anisotropy are obtained on a glass substrate. This high magnetic anisotropy results from the (001) texture of L10 phase developed along the film normal. A thin MgO layer ranging 1.0 to 3.0 nm in thickness plays an important role in developing the (001) texture of L10 FePt and magnetically decoupling the FePt particles. Varying a layer thickness of the FePt layer enables to control the crystalline structure and the magnetic properties, resulting in the films with a controllable FePt grain size as small as several nanometers, a high remanence squareness, and a high coercivity.

(001) oriented FePt-Ag composite nanogranular films on amorphous substrate

By annealing the multilayer film with a stack structure of MgO 10 nm/Ag 20 nm/[FePt x nm/Ag 5 nm]5 at 550 °C for 1 h, (001)-oriented L10 FePt nanograins with high perpendicular magnetic anisotropy are obtained on an amorphous substrate. The 20-nm Ag underlayer plays an important role in improving (001) texture of FePt and decreasing the fct FePt ordering temperature. The switching volume and uniaxial magnetic anisotropy constant Ku of FePt–Ag composite films with a controllable FePt grain size are obtained by fitting the coercivity dependence on magnetic field sweeping rate employing coherent rotation model, taking into account the easy axis distribution.

Structural and magnetic studies of nanocomposite FePt-MgO films for perpendicular magnetic recording applications

Composite FePt–MgO films composed of L10 FePt nanograins were fabricated on glass substrates by annealing multilayer films of [FePt/MgO]3. Films with 2 nm FePt layer thickness that have Mr/Ms∼1.0 show a significantly improved (001) texture of L10 FePt with a narrow c axis texture dispersion resulting from insertion of 2 nm MgO layers between the FePt layers. Annealing the [FePt 2 nm/MgO 2 nm]3 film creates grains as small as 8.0 nm that confer a relatively high magnetic anisotropy constant Ku=5×106 erg/cc and thus fosters a high coercivity of approximately 10.0 kOe.

Transverse susceptibility of hard-disk media by dc methods

A torque method for measuring the transverse susceptibility of magnetic recording media is discussed. With respect to the previous methods involving an ac probe field HP, the present method introduces a concept of measurement involving a dc HP. The high precision of the proposed method makes it suitable for modern hard-disk media, unlike the previously reported methods. The measurement of the transverse susceptibility is performed in two different experimental conditions, denoted by zero-field blocked and field blocked, allowing a straightforward interpretation of the thermal relaxation effects on the transverse susceptibility. A theoretical interpretation of the experimental results is also proposed, that is based on the Stoner–Wohlfarth and two-level models. The proposed theory could be used for extracting the anisotropy field distribution from the experimental dc transverse susceptibility.

Anisotropy field distribution measurements for high-density recording media

Abstract The magnetic anisotropy fields of grains measured using different methods (rotational hysteresis, Flanders & Shtrikman method, SPD and H C ( t )) for advanced high density recording media, are discussed. The magnetic anisotropy field found using the SPD (stiff) method is more reliable as compared to other methods.

Study of the reversal mechanism in longitudinal hard disk media: The role of K2/K1

The validity of the coherent rotations mechanism of reversal in CoCrPtB longitudinal hard disk media is probed. The proposed method is based on a comparison between the K2/K1 ratios (K1 and K2 being the first and second order anisotropy constants) measured by a hard axis loop method, and that estimated using in-plane isothermal remanent magnetization and dc-demagnetization processes in the hypothesis of coherent reversal. The disagreement between the two values of K2/K1, noticed for all the investigated samples, indicates an incoherent reversal mechanism. The origin of this mechanism is believed to be the grain coupling. The high values of the switching field at π/2 angle between the applied field and the easy axis HSW(ψ=π/2), measured by hard axis loop, can be larger than the writing field. This may explain the experimentally noticed degradation of signal/noise ratio with increasing HSW(ψ=π/2).

Thermal stability and magnetic anisotropy dispersion in high-density hard-disk media

Abstract A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10%) with the most probable value of the anisotropy field distribution.

Influence of thermal relaxation on the irreversible magnetization processes for perpendicular recording media

The influence of grain interaction and thermal relaxation on the rotational hysteresis loss (RHL) for perpendicular recording media is investigated using Landau-Lifshitz-Gilbert and Monte Carlo simulations. The results indicate that both thermal relaxation and grain exchange interactions decrease the applied field value HM, corresponding to a vanishing RHL, with respect to the grain intrinsic anisotropy field HK(=2K/MS). In the case of weak grain exchange coupling, the thermal decrease of HM is scarcely affected by grain magnetostatic interactions. In this case, the ratio HM/HK can deliver the maximum grain size. This statement is checked on two samples of hard disk media with perpendicular (CoPtCrO) and longitudinal anisotropy (CoCrPtB).