P. J. Czoschke

Areal-Density Limits for Heat-Assisted Magnetic Recording and Perpendicular Magnetic Recording

Differences in the areal-density capability limits for heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) are explored using spinstand measurements, drive footprinting, and micromagnetic modeling. The written track curvature is measured with a special technique that mitigates the cross-track averaging effects due to a finite read sensor width. Tracks written with HAMR heads are shown to have more curvatures compared with those written with modern PMR writers. Mitigation of written track curvature is demonstrated with two different HAMR writer designs. The curvature effect appears to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density (LD). Experimental measurements of a constant bit error rate for different LDs and track densities (TDs) indicate a significant opportunity for high TD recording using HAMR. The difference appears to be related to the ability for HAMR to address high track pitches with a minimal increase in risk of adjacent track interference compared with PMR.

Time resolved scanning Kerr microscopy of hard disk writer structures with a multilayered yoke

Partially built hard disk writer structures with a multilayered yoke formed from 4 repeats of a NiFe(∼1 nm)/CoFe(50 nm) bilayer were studied by time and vector resolved scanning Kerr microscopy. Dynamic images of the in-plane magnetization suggest an underlying closure domain equilibrium state. This state is found to be modified by application of a bias magnetic field and also during pulse cycling, leading to different magnetization rotation and relaxation behavior within the tip region.

Real-time direct measurement of field rise time and dynamic instability of perpendicular writers

A method is presented to measure the dynamic write field of a perpendicular recording head directly in the time domain using a tunneling magnetoresistive read sensor. The method is used to measure the magnetic field rise time of two different writer designs and real-time measurements of the write field without averaging are demonstrated, enabling investigation of transient switching behavior that would not be observed with stroboscopic or frequency-domain techniques. A dynamic instability is observed and characterized in a particular writer design and is attributed to the insertion of an antiferromagnetic coupling layer in the writer pole.

Time- and vector-resolved Kerr microscopy of hard disk writers

Time-resolved scanning Kerr microscopy has been used to make wafer level measurements of magnetization dynamics within the yoke and pole piece of partially built hard disk writer structures. Three Cartesian components of the vector magnetization were recorded simultaneously using a quadrant photodiode polarization bridge detector. The rise time, relaxation time, and amplitude of each component has been related to the magnetic ground state, the initial torque, and flux propagation through the yoke and pole piece. Dynamic images reveal “flux-beaming” in which the magnetization component parallel to the symmetry axis of the yoke is largest along that axis.

Self-organization of Pb islands on Si(111) caused by quantum size effects

Growth of metallic Pb islands on Si(111) by vacuum deposition was studied in real time using synchrotron x-ray diffraction. The islands coarsen and order, maintaining a nearly uniform interisland distance but without angular correlation. The resulting interisland structure is akin to a two-dimensional liquid. Over a wide temperature range, the interisland ordering is well correlated with the development of “magic” island heights caused by energy minimization of the Pb electrons. The results demonstrate quantum confinement effects as a driving force for self-organization, as opposed to strain effects that generally govern the formation of semiconductor quantum dot arrays.

Imaging the equilibrium state and magnetization dynamics of partially built hard disk write heads

Four different designs of partially built hard disk write heads with a yoke comprising four repeats of NiFe (1 nm)/CoFe (50 nm) were studied by both x-ray photoemission electron microscopy (XPEEM) and time-resolved scanning Kerr microscopy (TRSKM). These techniques were used to investigate the static equilibrium domain configuration and the magnetodynamic response across the entire structure, respectively. Simulations and previous TRSKM studies have made proposals for the equilibrium domain configuration of similar structures, but no direct observation of the equilibrium state of the writers has yet been made. In this study, static XPEEM images of the equilibrium state of writer structures were acquired using x-ray magnetic circular dichroism as the contrast mechanism. These images suggest that the crystalline anisotropy dominates the equilibrium state domain configuration, but competition with shape anisotropy ultimately determines the stability of the equilibrium state. Dynamic TRSKM images were acquire...

Effect of Coil Position on Magnetization Dynamics of Multilayered Hard Disk Writer Yokes

Three partially built hard disk writer structures with active coil windings located at different positions relative to the yoke were studied by time resolved scanning Kerr microscopy. All three Cartesian components of the magnetization were recorded simultaneously. Time resolved signals recorded near the pole tip showed that the component of magnetization parallel to the driving field had the same rise time irrespective of the location of the coil winding, but largest amplitude when the active coil was located near the middle of the yoke. Dynamic images obtained from three devices showed “flux beaming” in a channel parallel to the driving field. The magnetic contrast was strongest when the active coil was located near the centre of the yoke, while relaxation after removal of the excitation was most complete when the active coil was located near the confluence region. These results confirm the need for a multiturn coil to ensure effective flux propagation along the entire length of the yoke.

Imaging of isolated magnetic cluster switching in thin CoCrPt films

Thin film permanent magnet materials are a vital component of magnetic recording read elements. However, local variations in the magnetic microstructure inherent in such devices can have numerous consequences on the magnetic state of the films. In this study, magnetic force microscopy is used to image the domains in thin nanocrystalline CoCrPt films that are part of a patterned read sensor device. The films were imaged before and after being subjected to stress fields of 1000–2000Oe (less than the sheet-film coercivity of the CoCrPt) transverse to the original magnet set direction. Subtraction of the images reveals that the magnetization of isolated magnetic clusters irreversibly rotates in the film. These data show that the mechanism for net moment rotation in such films is not a uniform grain moment rotation. The change in net magnetization occurs in discrete local moment switching similar to Barkhausen jumps, where moments of weakly coupled grains irreversibly rotate at fields that are lower than the b...

Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads

To meet growing data storage needs, the density of data stored on hard disk drives must increase. In pursuit of this aim, the magnetodynamics of the hard disk write head must be characterized and understood, particularly the process of “flux beaming.” In this study, seven different configurations of perpendicular magnetic recording (PMR) write heads were imaged using time-resolved scanning Kerr microscopy, revealing their detailed dynamic magnetic state during the write process. It was found that the precise position and number of driving coils can significantly alter the formation of flux beams during the write process. These results are applicable to the design and understanding of current PMR and next-generation heat-assisted magnetic recording devices, as well as being relevant to other magnetic devices.

Time resolved imaging of magnetization dynamics in hard disk writer yokes excited by bipolar current pulses

A partially built hard disk writer structure with a NiFe/CoFe/Ru/NiFe/CoFe synthetic antiferromagnetic (SAF) yoke was studied by time and vector resolved scanning Kerr microscopy. All three time dependent components of the magnetization were recorded simultaneously as a bipolar current pulse with 1 MHz repetition rate was delivered to the coil. The component of magnetization parallel to the symmetry axis of the yoke was compared at the pole and above a coil winding in the centre of the yoke. The two responses are in phase as the pulse rises, but the pole piece lags the yoke as the pulse falls. The Kerr signal is smaller within the yoke than within the confluence region during pulse cycling. This suggests funneling of flux into the confluence region. Dynamic images acquired at different time delays showed that the relaxation is faster in the centre of the yoke than in the confluence region, perhaps due to the different magnetic anisotropy in these regions. Although the SAF yoke is designed to support a sin...