Radek Lopusnik

High-frequency characterization of Permalloy nanosized strips using network analyzer ferromagnetic resonance

We report on the dynamic properties of Permalloy nanostrips at gagahertz frequencies. The thickness of the strips is 100nm, strip width is 300nm, strip spacing is 1μm, and length is 0.3–100μm; aspect ratios are 1:1, 1:2, 1:3, 1:5, 1:10, and 1:333. The dynamic behavior was studied by network analyzer ferromagnetic resonance (FMR) using Permalloy strips on a coplanar waveguide in flip-chip geometry. The FMR mode frequencies (fr) can be controlled by the aspect ratio as well as by the applied magnetic field (H). In longer strips (1:10 and 1:333), the excitation frequencies show a soft mode behavior (Heff=990Oe) when the field is along the hard axis. However, along the easy axis (along the strip length), fr increases with applied field. At a field of 3kOe, fr values are almost independent of aspect ratio along the easy axis except for the 1:1 strip. Along the hard axis, the frequencies are strongly dependent upon the aspect ratio. We also observed that the frequency linewidths of the strips are dependent on t...

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.

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.

The effect of FeF2 on the magneto-optic response in FeF2/Fe/FeF2 sandwiches

The room temperature optical constants n and k of MBE grown FeF2 films are reported. Because of poor chemical stability, FeF2 had to be coated with a protective Au layer. Reflection spectral ellipsometry in the photon energy range between 1.3 and 5.2 eV was performed on structures with a typical profile Au(0.5 nm)/FeF2(120 nm)/Au(30 nm)/Ag(20 nm)/Fe(0.6 nm) grown on GaAs(0 0 1) substrate. The spectra of n and k in FeF2 were subsequently employed in the design of FeF2/Fe/FeF2 sandwiches considered as magneto-optic (MO) sensors for weak microwave currents. Their MO response was evaluated using reflection MO (Kerr) spectroscopy at polar magnetization. The present results may be of interest in MO studies of magnetic nanostructures with Fe/FeF2/Fe, including MO magnetometry and MO magnetic domain imaging.

Magneto-optical AlN/Fe/AlN structures optimized for operation in the violet spectral region

The magneto-optical (MO) polar Kerr response of AlN/Fe/AlN/Cu/Si structures was optimized for a wavelength of 410 nm. Prior to the growth of the structures, the optimal thicknesses of the individual layers were determined by modelling to find the maximum MO figure of merit. Then the structures with different thicknesses of AlN layers were grown by sputtering. The optical properties of the AlN layers, grown by reactive sputtering, were characterized by ellipsometry. The MO polar Kerr spectroscopy was employed to measure the performance of the structures in the field of 3 kOe. The MO signal was scaled for saturation (factor 7) and saturation azimuth rotation greater than 20° was observed. The agreement between calculation and experimental data was remarkable, verifying the correct set of optical and MO parameters used in the model. These results provide a guide for the design of a MO sensor with optimal performance at a desired wavelength.

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.

Performance of U-Pole Writers With Straighter Field Contours

Curvature of transitions recorded onto a magnetic disc has become more pronounced as tracks narrow. Curvature creates interference between adjacent transitions recorded at high bit density, and broadens the read back signal pulse. Curved transitions have a shape similar to that of the field contours that write them. A concave write pole trailing surface, or “U-pole”, pushes the field contours near the pole edges farther down track. This straightens the contours by partially compensating for transition curvature. We used a track trimming technique to compare transition curvature of flat poles and U-poles, and verified that the U-pole reduced curvature by more than 30%. We also demonstrated that the more curved transitions written by flat poles generate ~ 1 nm more pulse broadening at the track edge relative to those written by U-poles. However, modeling predicts a ~ 10% loss in down track field gradient near the center of a U-pole. Measurements of bit error rate across the track confirm the corresponding loss in U-pole performance relative to a flat pole over all but the outer portions of a track.

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.