Data Storage Systems Center

FeCo Films

Media Overview

In order to maximize density, tracks in Heat Assisted Magnetic Recording(HAMR) drives will be packed as closely as possible. This tight packing will almost certainly result in the edges of the hot spot overlapping tracks adjacent to the track being written.

In the simplest picture, the overlap will reduce the stability of the written tracks due to a drop in KuV/kbT. We are investigating whether other properties of the media change as well by measuring the distribution of energy barriers to thermal flipping as a function of temperature.

In the simple picture, the energy barrier distribution would scale with the anisotropy but its percentage width would remain constant. However, the percentage width of the distribution may become wider with increasing temperature due to a drop in intergranular exchange or due to a distribution of Curie temperatures in the media.

We have measured energy barrier distributions in several types of perpendicular media. Extracting the intrinsic barrier distribution is complicated by the effects of demagnetizing field. We are currently attempting to confirm our choice of demagnetizing field correction.

The project focuses on the growth of the FeRh films and the study of their switching from Antiferromagnetic state to Ferromagnetic state. The change in the switching behavior by changing various parameters like deposition temperature, type of substrates (single crystal or polycrystalline), capping layers, and post-deposition annealing is studied. The objective is to optimize these parameters to get sharp transition from the Antiferromagnetic to Ferromagnetic state at a temperature well below the Curie temperature and away from room temperature. The switching behavior being highly sensitive to composition, substrate bias is used to alter the composition of the films. Also the use of Platinum as a dopant to alter the switching temperature is being studied. Finally, the effect of the microstructural aspects on the switching is studied using the Transmission electron microscopy technique. The growth of FePt on this layer and the study of the combined switching of the stack will be work for the future as a part of implementing the Binary Anisotropy Media (BAM)

The goal of this project is to engineer an ultra-high density recording media using L10 FePt, a material with high uniaxial magnetocrystalline anisotropy. Because the size of magnetic grains is approaching the superparamagnetic limit in current perpendicular media, it is necessary to produce thin film media made with magnetic alloys with larger anisotropy energies to achieve higher recording densities. Due to its high anisotropy field and good environmental stability, FePt (L10) is the most promising media for achieving such ultra-high recording densities. In order to use FePt as a high-density recording media, very small (less than 5nm), uniform, highly-ordered, and isolated FePt (L10) columnar grains with excellent perpendicular texture are desired.

In our work, perpendicular L10 ordered FePt-oxide two-phase thin films with an average grain size of 5 nm have been prepared by alternate sputtering of FePt and oxide at 475 oC. Very uniform and well-isolated columnar grains have been obtained with coercivity as high as 7 kOe. For 10 nm columnar grains, the coercivity can be 15 kOe. Our current method is also able to produce columnar 2.9 nm grains with 3.5 kOe coercivity.

The long-range order (LRO) parameter for the <111> fiber grains in binary FePt alloy films with compositions in the range of 39.3-55.3 at% Pt was determined using x-ray diffraction intensity data. The LRO parameter showed a maximum for the compositions where the kinetic ordering temperature and the c/a ratio were the lowest. However, there were two surprising upturns at the terminal compositions. The measured order parameters were lower, sometimes significantly lower, than reported by others using variants of the “c/a” method. The films were sputter deposited from elemental targets onto oxidized silicon wafers coated with an adhesion layer. The films were 500 nm thick and were annealed to 200 °C above the kinetic ordering temperature for L10 phase formation measured for one-micron thick, free-standing films. The kinetic ordering temperatures, taken as the peak temperature at a heating rate of 40 °C/min for the exothermic A1 to L10 transformation, were measured by differential scanning calorimetry.

The goal of this project is to engineer an ultra-high density recording media using L10 FePt, a material with high uniaxial magnetocrystalline anisotropy. Because the size of magnetic grains is approaching the superparamagnetic limit in current perpendicular media, it is necessary to produce thin film media made with magnetic alloys with larger anisotropy energies to achieve higher recording densities. Due to its high anisotropy field and good environmental stability, FePt (L10) is the most promising media for achieving such ultra-high recording densities. In order to use FePt as a high-density recording media, very small (less than 5nm), uniform, highly-ordered, and isolated FePt (L10) columnar grains with excellent perpendicular texture are desired.

In our work, perpendicular L10 ordered FePt-oxide two-phase thin films with an average grain size of 5 nm have been prepared by alternate sputtering of FePt and oxide at 475 oC. Very uniform and well-isolated columnar grains have been obtained with coercivity as high as 7 kOe. For 10 nm columnar grains, the coercivity can be 15 kOe. Our current method is also able to produce equiaxed 2.9 nm grains with 3.5 kOe coercivity.

The nucleation and growth of the L10 phase in a series of FePt and FeCuPt alloy films was investigated using isothermal and non-isothermal differential scanning calorimetry (DSC) . The transformation was modeled using variants of the Johnson-Mehl-Avrami-Kolmogorov formulation for isothermal and non-isothermal annealing conditions. Comparison of experimental and calculated DSC traces showed that the only nucleation scenario that is consistent with the combination of isothermal and non-isothermal DSC results for the alloys investigated (FePt with compositions in the range of 45-55 at % Fe, and FeCuPt) is one of athermal nucleation, i.e., one wherein the L10 nuclei are pre-existing in the deposited film.