Fabrication and magnetic reversal behaviors of FePt-MgO percolated perpendicular medium
Date Issued
2007
Date
2007
Author(s)
Tsai, Yun-Chih
DOI
zh-TW
Abstract
In the present granular perpendicular medium (GPM), the transition jitter noise is essentially determined by the grain size. In order to reduce signal-to-noise ratio (SNR), the magnetic grain size must be scaled down. But it would cause a degradation of the medium thermal stability. As a consequence, the largest area density of GPM is expected to be below 500 Gbits/in2. To overcome the problem and further increase the area density, a new type of recording medium referred to as percolated perpendicular medium (PPM) is proposed. The PPM, consisting of densely distributed nonmagnetic pinning sites, ensure relatively smooth transition boundaries and reduced SNR. Besides, the ferromagnetic exchange coupling between the grains provides sufficient strength to resist thermal fluctuation.
In this study, percolated perpendicular FePt-MgO films were prepared by alternating magnetron sputtering Fe, Pt, MgO on MgO(001) underlayer at 530 and 580℃ in an ultra-high vacuum chamber. The MgO content was varied between 0 to 10.07 vol.%. X-ray diffraction (XRD) using Cu-Kα radiation and high-resolution transmission electron microscopy (HRTEM) were used to study the crystalline phases and microstructure of the films. A vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) were used to measure the magnetic properties.
When the deposition temperature of FePt-MgO was 530℃ and MgO content was increasing from 0 to 0.76 vol.%, the ordering parameter (Sorder) was maintained at about 0.96. The out-of-plane squareness (S⊥) remained at 0.88, but in-plane squareness (S∥) was increased from 0.35 to 0.45. However, the out-of-plane coercivity (Hc⊥)was only increased from 9.5 to 9.7 kOe. The anisotropy field (Hk) and the crystalline anisotropy constant (Ku) were increased from increased from 77.5 to 92.5 kOe and 2.76×107 to 3.21×107 erg/cm3 respectively. The α value, which gives the degree of coupling between magnetic grains, was decreased from 2.3 to 2.17. Further increasing MgO content, the FePt(002) peak shifted to low angle accompanied with the rapid reduction of Sorder and S⊥ although S∥ was always kept at 0.45. For example, Sorder was 0.89 and 0.75 for 2.61 and 4.43 vol.% and S⊥ changed from 0.87 to 0.84 from 2.61 to 4.43 vol.%. Hc⊥ was decreased to 9.2 and 8.7 kOe. Hk was decreased to 74.5 and 69 kOe. Ku was decreased to 2.55×107 and 2.32×107 erg/cm3, and α value was decreased to 1.99 to 1.87.
Consequently, the PPM effect of FePt-MgO film deposited at 530℃ was not conspicuous. However, when the deposition temperature was increased to 580℃, the PPM effect becomes much more apparent. Hc⊥ raised largely from 10.6 to 13 kOe when the MgO content was increased from 0 to 1.23 vol.% and the magnetization reversal process transfers from domain wall motion to magnetization rotation . It demonstrated that the MgO formed as pinning sites hindered the motion of domains. The values of Sorder, S⊥, Hk, Ku and α were all increased with a reduction of S∥. From the microstructure analysis, FePt formed as a continuous film without MgO additive. With increasing MgO content, MgO columns and flakes were present in the FePt matrix with the decreased size of FePt grains. However, the FePt-MgO film will transfer into granular film as the MgO content was increased above 6.13 vol.%, losing the unique properties of PPM.
Thus, in order to distribute more MgO pinning sites in FePt matrix and avoid forming the FePt-MgO granular film, alternating sputtered Fe, Pt, MgO at lower temperatures with a rapid post thermal annealing should be adopted. The densely distributed MgO pinning sites in the FePt matrix is expected to markedly decrease SNR and increase the area density of medium.
Subjects
展透性
percolated
Type
thesis
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