HAMR Media Research Articles

HAMR Media Jitter Relationships and Opportunities

HAMR Media Jitter Relationships and Opportunities

Effect of C Replacement on Magnetic Properties and Nanostructure of FePt-Nitride Granular Film for HAMR Media

Effect of C Replacement on Magnetic Properties and Nanostructure of FePt-Nitride Granular Film for HAMR Media

Microstructure Analysis on Size Distribution During Film Growth in HAMR Media

Grain size and size distribution reduction in L10 FePt granular thin film is becoming critical to increase the areal storage density and realize the full potential of heat-assisted magnetic recording media. FePt and amorphous carbon or mixture with other segregants are often deposited at elevated temperature in order to promote the L10 ordering in FePt. Due to the materials system and high temperature involved in the fabrication process, the bimodal distribution of the grains is often observed with a significant amount of small grains with diameter <3 nm. The bimodal distribution of the grains could potentially lead to a significant microstructure nonuniformity and thus grain-to-grain properties variation, such as Curie temperature (Tc), texture, ordering, and magnetic properties. In this paper, we performed a detailed analysis on microstructure evolution of FePt-C thin film during the growth process on different initial layer conditions to understand the origin of bimodal grain size distribution in the FePt-C media. Experimental modeling study of depositing FePt on continuous carbon layer at elevated temperature was conducted to understand the nucleation and growth process.

The effect of adding a magnetic oxide in the grain boundaries of HAMR media

Grain-to-grain Curie temperature (Tc) variation in media reduces the signal-to-noise ratio due to its contribution to the transition jitter noise, especially when the average grain size is decreased to increase the area storage capacity. Thermally insulating magnetic grain boundaries may suppress such grain-to-grain Tc variation, especially at small grain sizes. In this paper, we discuss the effect of adding a thermally insulating magnetic oxide, in particular, BaFexOy, to the grain boundaries of granular FePt-C heat-assisted magnetic recording media. It is found that BaFexOy is chemically inert with respect to FePt and the chemical ordering of FePt-BaFexOy-C media is similar to that of FePt-C media. By tuning the volume fraction of BaFexOy and C, well-separated FePt grains (average grain size = 6.8 nm) surrounded by BaFexOy shells enable the attainment of perpendicular Hc greater than 35 kOe. Chemical mapping by transition electron microscopy shows that the magnetic oxide appears to be crystalline and completely surrounds the FePt grains. Temperature-dependent magnetization measurements indicate an effective increase in the magnetic grain size at temperatures below the FePt Curie temperature. The pulsed laser pump-probe measurement indicates a measurable reduction of the Curie temperature variation for the FePt-BaFexOy-C media compared to the reference FePt-C media.

HAMR media based on exchange bias

In this work, we describe an alternative strategy for the development of heat assisted magnetic recording media. In our approach, the need for a storage material with a temperature dependent anisotropy and to provide a read out signal is separated so that each function can be optimised independently. This is achieved by the use of an exchange bias structure where a conventional CoCrPt-SiO2 recording layer is exchange biased to an underlayer of IrMn such that heating and cooling in the exchange field from the recording layer results in a shifted loop. This strategy requires the reorientation of the IrMn layer to allow coupling to the recording layer. This has been achieved by the use of an ultrathin (0.8 nm) layer of Co deposited beneath the IrMn layer. In this system, the information is in effect stored in the antiferromagnetic layer, and hence, there is no demagnetising field generated by the stored bits. A loop shift of 688 Oe has been achieved where both values of coercivity lie to one side of the origin and the information cannot be erased by a magnetic field.

Growth Mechanism of Columnar Grains in FePt–C Granular Films for HAMR Media Processed by Compositionally Graded Sputtering

The optimization of nanogranular FePt–C films for heat-assisted magnetic recording is carried out by varying processing conditions for compositionally graded sputtering. Microstructures and magnetic properties of the films were studied for various thicknesses ranging from 2 to 12 nm to observe the growth steps of the films. An excellent in-plane nanogranular microstructure is obtained for the films of thickness ≥8 nm. Below 6 nm in thickness, the grains are random in shape and the presence of fine grains deteriorates the magnetic properties on account of poor $L1_{0}$ ordering. During the nucleation and grain growth stages, the grain size does not change too much with increasing the film thickness. But, the coarsening of FePt grains occurs and the aspect ratio of the FePt grain changes in the films of thickness ≥8 nm. We achieve a minimum grain size of 6.2 nm having an aspect ratio of 1.9 with a large perpendicular coercivity of 3.9 T for $\sim 12$ nm thick FePt–C films.

Probing HAMR Media Thermal Properties With Pulsed Recording

A new technique for characterizing the thermal properties of heat-assisted magnetic recording media (HAMR) using pulsed near-field heating recording with a static tester is reported. It has the advantage over traditional recording techniques of enabling studies of the 1-100 ns transient thermal configurations rather than only thermal steady states. The technique is applied to HAMR media with different heatsink thicknesses. While each media has a different steady-state temperature profile, the heatsinking differences are eliminated or enhanced by tuning the pulse duration for each media. The technique is used to extract thermal properties of the media.

Characterization of the Magnetic Properties of HAMR Media Using the Temperature Dependence of the AC Susceptibility

The ac susceptibility of heat-assisted magnetic recording media was measured from room temperature to 750 K for the frequencies ranging from 500 Hz to 50 kHz. Using an analytical model for the ac susceptibility of weakly interacting uniaxial superparamagnetic particles, we were able to characterize the temperature dependence of the anisotropy energy ( $K$ ) distribution and the saturation magnetization ( $M_{s})$ near the Curie temperature ( $T_{c})$ . In addition, we were able to extract information about the dependence of Tc on the grain diameter.

Investigation of Heat-Assisted Magnetic Recording Media Films in Four Dimensions

Heat-assisted magnetic recording is one of the most promising approaches to achieve extremely high density magnetic recording. A laser is used to temporarily heat the recording media film to render its switching field to become lower than the magnetic field of the writing head. After writing, the film material is cooled down to room temperature for data storage. In this dynamic writing process, the challenge is to tailor and optimize the time resolved structural/thermal/magnetic properties of the media films and their correlations during/after the laser heating. In this paper, the four dimensions of current progress in HAMR media research, i.e., structure, magnetic properties, thermal properties and time, are briefly overviewed and discussed. Their correlations are also emphasized.

HAMR Adjacent Track Stability in the Presence of a Medium Curie Temperature Distribution

Evidence of a Curie temperature distribution has been observed in longitudinal media. These results were explained in terms of grain-to-grain variations in composition. Additives are under consideration for FePt HAMR media that may introduce such a distribution into HAMR media. Simple calculations are used to argue that a Curie temperature distribution is unlikely to affect adjacent track aging, but may widen transitions during the write process.