Matteo Staffaroni

HAMR thermal modeling including media hot spot

All previous thermal modeling on heat exchange between hard disk drive (HDD) head and media treated media as a heat sink with uniform ambient temperature. However in heat-assisted magnetic recording (HAMR) system, the media temperature is no longer uniform and the temperature at the hot spot center can reach as high as 800 to 1000 K depending on recording layers Curie temperature. In this paper, both media hot spot and the air bearing flow are included in formulating heat transfer across the head-media interface. Numerical results for a waveguide only slider and a slider with a metallic near-field transducer (NFT) are presented to illustrate the effect of HAMR media temperature.

Thermal Aspects and Static/Dynamic Protrusion Behaviors in Heat-Assisted Magnetic Recording

The pressure of increasing the areal density of magnetic storage media demands the leap to new storage technologies. Among the prime candidates, available is heat-assisted magnetic recording technology. Among many aspects available in this paper, we will focus on newly introduced thermal aspects and protrusions due to additional heat sources in the recording head and recording media. As needed with the existing head protrusions such as thermal flying height control and writer pole tip protrusion, we will present the data, which is based on thermomechanical modeling and experiments during flying and static conditions. The ongoing debate of comparing flying versus nonflying conditions remains a critical issue but needs to be accepted as an imperfect but valuable way of exploring the new protrusion effects. We will address the important issue of the differences between the pure thermal and the associated mechanical time constants.

Heat Assisted Magnetic Recording With Laser Pulsing

In conventional heat assisted magnetic recording, a thermal write assist is provided by a near field transducer which is constantly energized by a laser during writing. Here, we report experimental and theoretical data where the laser is pulsed, whereby the pulsing can be synchronous or asynchronous to the write clock. It is found that the recording performance of pulsed heat assisted recording is slightly deteriorated when compared with conventional dc operation. This is explained by the combined effect of an increased magnetic-to-thermal offset and fast freezing dynamics. The deterioration of the recording performance can be eliminated by an increase of pulsing frequency so that the inherent low-pass behavior of the thermal response of the medium removes the temperature variations that occur in the recording layer.

Curie Temperature Distribution in FePt Granular Media

The Curie temperature distribution is an important parameter affecting transition noise in heat-assisted magnetic recording. In this paper, we follow up on our recent report on a technique to evaluate the Curie temperature distribution and provide modeling that validates the method as well as provides the experimental bounds for its validity. Thermal modeling is used to determine whether the technique is sensitive to extrinsic sources of grain temperature variations, such as distributions in thermal boundary resistance. The technique is applied to a variety of FePt granular media films of varying alloy composition and chemical ordering, and we find the Curie temperature distribution to depend primarily on grain ordering kinetics. We also present results of grain switching probability as a function of the applied magnetic field and find a non-trivial dependence on the alloy magnetization.

A New AFM-Based Technique to Detect the NFT Protrusion on HAMR Head

Heat assisted magnetic recording (HAMR) is anticipated to increase the areal density in hard disk drives to multiple Tb/in2. However, the newly introduced protrusions caused by additional heat sources must be taken into account in head-disk spacing control. The light absorption by the near field transducer (NFT) and the heat dissipation into the slider through the laser delivery system can result in the temperature rise well over a hundred degrees Celsius. In response to such a big temperature change, an instantaneous protrusion at the NFT region is expected. This protrusion has a direct impact on the head-magnetic spacing (HMS) and therefore affects writing performance. Due to its small lateral size below 1 micrometer, the NFT protrusion is difficult to probe. In this paper, we introduce a new technique, which is based on atomic force microscopy (AFM) and allows for a simultaneous topography imaging and NFT protrusion mapping on the air bearing surface (ABS) with a nano-scale resolution. The measured NFT protrusion profile of a nonflying head indicates the local min-fly point at the location predicted by our simulation results. A further quantitative analysis proves the dependence of NFT protrusion on the laser power, which agrees very well with modeling and can offer a good guidance to the head-disk interface management during recording.

Predicting scattering scanning near-field optical microscopy of mass-produced plasmonic devices

Scattering scanning near-field optical microscopy enables optical imaging and characterization of plasmonic devices with nanometer-scale resolution well below the diffraction limit. This technique enables developers to probe and understand the waveguide-coupled plasmonic antenna in as-fabricated heat-assisted magnetic recording heads. In order validate and predict results and to extract information from experimental measurements that is physically comparable to simulations, a model was developed to translate the simulated electric field into expected near-field measurements using physical parameters specific to scattering scanning near-field optical microscopy physics. The methods used in this paper prove that scattering scanning near-field optical microscopy can be used to determine critical sub-diffraction-limited dimensions of optical field confinement, which is a crucial metrology requirement for the future of nano-optics, semiconductor photonic devices, and biological sensing where the near-field character of light is fundamental to device operation.

Nanoscale Thermal Mapping of HAMR Heads Using Polymer Imprint Thermal Mapping

A number of systems require high-resolution, non-perturbative thermal mapping. This need is especially apparent for heat-assisted magnetic recording (HAMR), a technology developed to increase the areal density of magnetic storage. In HAMR, plasmonic antennas are used to heat magnetic media by hundreds of degrees in a nanoscale region. Understanding the antennas temperature is critical, but the available methods for high-resolution thermal mapping are dominated by experimental artifacts. In order to measure the temperature distributions of a plasmonic antenna without a major perturbation, a new technique, polymer imprint thermal mapping (PITM), was developed, which relies on the thermal response of a thin polymer film to report temperature. The polymer permanently cross-links upon heating, so the thermally induced height change of the polymer film is directly correlated with the sample temperature. It is experimentally shown that PITM creates thermal maps that are far superior to conventional scanning thermal microscopy. In addition, the modeled thermal distributions show remarkable agreement with the measured PITM thermal maps.

Visualizing the bidirectional optical transfer function for near-field enhancement in waveguide coupled plasmonic transducers

We report visualizations of the bidirectional near-field optical transfer function for a waveguide-coupled plasmonic transducer as a metrology technique essential for successful development for mass-fabricated near-field devices. Plasmonic devices have revolutionized the observation of nanoscale phenomena, enabling optical excitation and readout from nanoscale regions of fabricated devices instead of as limited by optical diffraction. Visualizations of the plasmonic transducer modes were acquired both by local near-field excitation of the antenna on the front facet of a waveguide using the focused electron beam of a scanning electron microscope as a probe of the near-field cathodoluminescence during far-field collection from the back facet of the waveguide, and by local mapping of the optical near-field for the same antenna design using scattering scanning near-field optical microscopy as a probe of the near-field optical mode density for far-field light focused into the back facet of the waveguide. Strong agreement between both measurement types and numerical modeling was observed, indicating that the method enables crucial metrological comparisons of as fabricated device performance to as-modeled device expectations for heat-assisted magnetic recording heads, which can be extended to successful development of future near-field-on-chip devices such as optical processor interconnects.

Near-Field Transducer Material Q: Assumptions and Reality

In heat-assisted magnetic recording (HAMR), it is common for near-field transducers (NFTs) to comprise gold (Au), which is modeled as having textbook optical properties. The latter practice is traditionally justified by optical metrology on Au thin films employed in HAMR NFTs, reporting material quality factors comparable to the literature values from the likes of Johnson and Christy or Palik. We find models assuming that these textbook Au optical properties can recover the experimentally verified link between high thermal gradient and good recording performance; however, the models fail to reproduce experimentally observed NFT power requirements and absolute thermal gradient values. The model-experiment gap can be closed by revising the optical properties of the NFT Au in the model such that the material quality factor is reduced by a factor of 3–4. Physically, this downward revision may be attributed to the ubiquitous use of metallic adhesion layers between NFT Au and the surrounding dielectrics. The reduction in the optical quality of the NFT Au has significant implications for NFT design strategy as high thermal gradients remain attainable but the path to attaining them involves unconventional NFT design choices. It is also worth noting that the significant reduction in the material quality factor of current gold-based NFTs opens the door for the use of other materials previously scorned for being substantially optically inferior to gold.