Julius Hohlfeld

Heat-Assisted Magnetic Recording

Due to the limits of conventional perpendicular magnetic recording, it appears that alternative technologies are needed at areal densities >500 Gb/in2. Heat-assisted magnetic recording (HAMR) is a promising approach to extend areal densities to 1 Tb/in2 and beyond. All of the unique components necessary for a working HAMR system have been demonstrated. Although HAMR permits writing on high Hc media with lower magnetic fields and can produce higher write gradients than conventional magnetic recording, head/media spacing and the development of high Hc media with small grains remains challenging

Slow recovery of the magnetisation after a sub-picosecond heat pulse

The response of a magnetic spin system to pulsed laser heating on time scales in the picosecond regime is investigated using an atomic level classical spin Hamiltonian, the dynamics of which are based on the stochastic Landau-Lifshitz-Gilbert equation. It is found that the ferro- to paramagnetic phase transition can occur in less than one picosecond, in agreement with published experimental data. Calculating the spin temperature via the internal energy of the spin system we find that the system does not necessarily fully demagnetize even for spin temperatures above the Curie temperature. Our findings suggest that the spin system is far from thermal equilibrium so that the concept of a spin temperature has to be questioned on the time scale of picoseconds. Most importantly, the time for recovery of the magnetization can vary by orders of magnitude depending on the magnetic state after heating, a prediction which is verified by supporting pump-probe experiments.

Origins of the damping in perpendicular media : Three component ferromagnetic resonance linewidth in Co-Cr-Pt alloy films

The 9.41GHz ferromagnetic resonance field and linewidth have been measured as a function of the angle (θH) between the external magnetic field and film normal for a series of 17.5nm thick Co–Cr–Pt alloy films. The linewidths ranged from hundreds of Oersted to kiloersted, with different values at θH=0 and θH=90° and additional minima and maxima for θH-values from 16° to 64°. The profiles can be fitted with a combination of inhomogeneity line broadening, grain boundary two magnon scattering, and magnon-electron (m-e) scattering processes, with a notably small Gilbert damping α-value of 0.004 for the m-e term.

Athermal all-optical femtosecond magnetization reversal in GdFeCo

Magnetization reversal in GdFeCo by circularly polarized light is shown to occur at the femtosecond time scale. In contrast to the well-known laser-assisted magnetization reversal based on the laser heating, we here demonstrate that this femtosecond all-optical magnetization reversal is more efficient at lower temperatures. Both of these findings make all-optical recording on rare-earth transition metal alloys a promising technique for stable magnetic storage with high data rates.

Switching field distribution and transition width in energy assisted magnetic recording

In order to achieve higher areal density, magnetization transition width must be reduced. This requires small media switching field distribution (SFD). Here we explore SFD and transition width in heat assisted magnetic recording and microwave assisted magnetic recording. We reveal that for energy assisted magnetic recording, additional SFD broadening exists as compare to conventional perpendicular recording. We show the effect of SFD broadening on transition width and the implications to the magnetic recording system as the areal density increases.

Understanding Disk Carbon Loss Kinetics for Heat Assisted Magnetic Recording

An optical pump-probe set-up that allowed short dwell time ( ~ 500 μs) heating with a high-temperature ramp rate ( ~ 106 K/s) was used to detect the magnetization change in heat assisted magnetic recording (HAMR) media. The temperature of the media was monitored by observing the Kerr signal. The pump power at zero magnetization allowed the determination of the power needed to attain the Curie temperature of the media (the Curie temperature was determined with a high-temperature magnetometer before the pump-probe experiments). HAMR media was then irradiated with a pump power to obtain 480°C for increasing exposure times. Atomic force microscopy (AFM) of these media surfaces revealed depressions or holes in the media surface [within the carbon overcoat (COC) layer] that increased in extent with cumulative exposure time. Media surfaces exposed to somewhat lower temperatures (450°C) and for shorter times had a swollen region that surrounded a much smaller depression. High-spatial resolution Raman spectroscopy was used to analyze these irradiated areas. An increased D-band was observed within the swollen portion of the media surface, while the overall Raman signal intensity decreased within the small depressed area. Using time and temperature irradiations along with AFM analysis of the depressions the activation energy for COC loss was determined to be 0.6 eV. These observations were attributed to COC failure through graphitization and oxidation. The failure mechanism leading to these observed changes and the possible relationship of the present results to the HAMR media COC thermal stability are discussed.

Comparison of a near-field ferromagnetic resonance probe with pump-probe characterization of CoCrPt media

A near-field microwave technique is used to locally probe ferromagnetic resonance (FMR) in a series of CoCrPt alloys with varying perpendicular anisotropy (5kOe<Hk<15kOe). We observe broad FMR linewidths Δf on the order of a gigahertz and higher, indicating significant damping and sample inhomogeneities. These results are compared to time-domain measurements of the same samples using a pump-probe technique, where we observe relaxation times τ, considerably shorter than 500ps, using a damped sinusoid to model the magnetization dynamics. We find the local FMR measurements to yield time scales consistent with the pump-probe data, using the Fourier relationship, Δf=1∕πτ. Thus, this near-field technique is capable of quantitative characterization of high-anisotropy and highly damped magnetic systems, something that has not been demonstrated before with a local FMR technique.

HAMR head-disk interface - lubricant desorption and laser damage study

The HAMR head-disc interface is the subject of intense research due to its potential to extend the areal density of magnetic recording to the Tb/in2 and above range. We have studied possible material constituents of this interface to determine their robustness for this technology. Of specific interest are the effects of repeated high thermal excursions on these materials. Furthermore, it is expected that each bit may be irradiated ~107 times under normal operating conditions, thus placing a limit on accumulated effects due to this surface irradiation

HAMR Thermal Gradient Measurements and Analysis

The edge gradient of the thermal spot used to define bits in heat-assisted magnetic recording is a critical parameter in determining the quality of the magnetic transitions. We review the laser current modulation method used to measure this parameter and introduce the sideband ratio (SBR) approach as a fast, inexpensive technique for getting data on large volumes of heads or media. Mathematical derivation of the SBR method is given along with the details of implementation. Production level data on thousands of recording heads are used to illustrate its usefulness.

Carbon Overcoat Loss From the Surface of a Heat Assisted Magnetic Storage Disk due to Laser Irradiation

A pump-probe experimental technique that incorporated a 527nm wavelength pump laser and a 476nm probe laser was applied to a magnetic storage disk having a magnetic layer comprised of a FePt alloy and coated with a hydrogenated carbon overcoat (COC). The pump laser power was systematically increased while sweeping the applied field with an electromagnet to observe the temperature dependent magnetization, which is proportional to the change in the polarization of the reflected beam. In this way the laser power required to heat the media to the Curie temperature (Tc) was determined, with the Curie temperature of the media determined from a separate magnetometry measurement. Such a single point laser power-to-media temperature calibration allowed the determination of the media temperature over a small laser power range near Tc. The carbon over-coated FePt media was then irradiated for varying durations at temperatures pertinent to a Heat Assisted Magnetic Recording (HAMR) device [1]. The COC surface topography and carbon bonding structure within each irradiated zone was probed with AFM and micro-spot Raman. A subtle, systematic temperature and duration dependent change in the COC was observed. With increasing temperature and duration, the Raman D-peak became increasingly pronounced, signaling an increase of the sp2 (disorder) content in the film in the irradiated region. At incrementally higher temperatures, the loss of the carbon overcoat becomes apparent as a shallow depression in the COC film in the irradiated area. A clearer picture of the possible sensitivity and kinetics of the loss of COC on the HAMR media surface was obtained by measuring its loss over a range of irradiation temperatures and durations. The activation energy and COC loss rate were obtained and a possible mechanism for COC failure-loss was discussed within the bounds of the operating HAMR device [2].© 2013 ASME