In the twenty-first century, scanning probe microscopy characterization techniques have seen significant progress and are capable of probing complex structures and devices for a variety of near-surface features and phenomena with nanometer scale resolution. With modest customization, we can deploy these techniques for industrial metrology purposes in a simultaneous and multimethod system capable of shedding light on device function and failure modes, as well as determining the most efficient methods for data collection. To demonstrate this concept with a current, complex industrial device under development, several scanning probe microscopy techniques advantageous to the progress of heat-assisted magnetic recording heads were selected. This work describes simultaneous and multimethod approaches for performing heat-assisted magnetic recording head characterization using atomic force microscopy with scattering scanning near-field optical microscopy simultaneously performed with magnetic force microscopy or photo-induced force microscopy that could be extended to applications of other complex nanoscale devices. We demonstrate that the optical and magnetic fields are overlapping for fabricated heads, which is necessary for performing heat-assisted magnetic recording. We also observed that the multimethod atomic force microscopy methods show strong agreement between the measured optical and magnetic fields and the locale of their associated parts on the head.

Since 2000, scattering scanning near-field optical microscopy (sSNOM) has seen prominent development and application,1 in particular, for probing the near-fields of plasmonic resonances in basic nanostructures,2 two-dimensional materials,3 and complex industrial devices,4–6 such as heat-assisted magnetic recording (HAMR) heads for hard disk drives (HDDs). More recently, multifrequency atomic force microscopy (AFM) and photo-induced force microscopy (PiFM), which also uses a multifrequency AFM platform, have each seen their own prominent development and application.7–14 A PiFM platform is coupled to a coherent light source and measures the dipole induced at or near the surface of a sample by detecting the dipole–dipole forces that exists between the sample and the AFM tip.15 The magnitude of this interaction is directly affected by the optical polarizability of the sample. PiFM provides spectral absorption contrast,13 mapping of local electric fields,16,17 Raman for chemical imaging,18 and pump-probe transitions19 with spatial imaging resolution on the order of or smaller than the imaging tip, typically 10 nm.

The dipole force is governed by a 1/Z4 dependence, requiring that the measurement is taken within 1−2 nm of the sample surface,20 shown in Fig. 1(a). PiFM measures the dipole force gradient requiring that the AFM be operated in a dynamic mode with a non-contact operating setpoint. Using the multi-flexural nature of an AFM cantilever,7 PiFM utilizes one mode for topographic imaging and a second mode for the dipole–dipole response. The eigenmode nature of these two mechanical modes ensures that they do not couple the topographic response with the optical response. The condition for PiFM requires that the imaging light source is modulated such that it satisfies the difference or sum frequency, ωm, of the two flexural eigenmodes, ω0 and ω1, shown in Figs. 1(b) and 1(c).

FIG. 1.

Photo-induced force microscopy (PiFM) is a multimodal AFM technique, where two mechanical resonances of the imaging cantilever are used. When the imaging probe is within a few nanometers (a) of the sample surface, coupling between the tip and optical fields occurs. An external coherent light source is used to excite the sample and is modulated at a frequency ωm. The frequency for ωm is chosen such that it satisfies either the sum or the difference of the first mechanical mode ω0 and the second mechanical mode ω1 (b) and (c). The mixing of the polarization of the sampling frequency ωm with the topographic imaging frequency results in a photo-induced force signal on the first mechanical mode ω0. The photo-induced force signal is further amplified by this resonant mode, where the amount of amplification is governed by the quality factor of the cantilever.

FIG. 1.

Photo-induced force microscopy (PiFM) is a multimodal AFM technique, where two mechanical resonances of the imaging cantilever are used. When the imaging probe is within a few nanometers (a) of the sample surface, coupling between the tip and optical fields occurs. An external coherent light source is used to excite the sample and is modulated at a frequency ωm. The frequency for ωm is chosen such that it satisfies either the sum or the difference of the first mechanical mode ω0 and the second mechanical mode ω1 (b) and (c). The mixing of the polarization of the sampling frequency ωm with the topographic imaging frequency results in a photo-induced force signal on the first mechanical mode ω0. The photo-induced force signal is further amplified by this resonant mode, where the amount of amplification is governed by the quality factor of the cantilever.

Close modal

HAMR has long been the primary candidate for the future of HDD technology and enables increased areal density through heating a small spot of the magnetic media with a plasmonic antenna.4,21,22 The plasmonic antenna is located adjacent to and strongly interacts with the head's magnetic write pole as well as the magnetic media during drive operation.5 In addition, HAMR technology is strongly dependent on the combined effects of the magnetic and plasmonic elements and their adjacent fields for the successful recording of data. While characterization of the plasmonic behavior of the head has been demonstrated using sSNOM, it is useful to more fully characterize multiple properties and fields of the head simultaneously. Combining sSNOM with magnetic force microscopy (MFM) can shed light on the magnetoplasmonic nature of the plasmonic antenna and nearby magnetic write pole. Combining sSNOM with PiFM promises to directly compare two methods of observing the optical response of the plasmonic antenna. Both approaches offer the desired multimethod and single-pass, simultaneous characterization strategies desired for developing complex devices that have multiple interacting and coupled nanoscale properties.

Scanning electron microscopy cathodoluminescence (SEM-CL) and sSNOM have been previously demonstrated as successful techniques for the characterization of HAMR heads,4–6 and both have advantages and disadvantages for this application. The generation of the evanescent near-field in sSNOM is most similar to the method of excitation for the head during its intended use, and custom sample holders have also been configured to measure fully assembled parts referred to as head gimbal assemblies (HGAs, Fig. S1 in the supplementary material),5 which include a mounted laser diode. Therefore, it is possible to use sSNOM, PiFM, MFM, and AFM topography simultaneously for characterization of heads that have already experienced drive operation tests, and thus providing enlightenment to possible failure mechanisms as well as to determine the geometric and material characteristics for particularly successful heads. However, any scanning probe technique used will have one key disadvantage for characterizing HAMR heads: the plasmonic antenna's near-field cannot be accessed when the magnetic media is present, and since the strong polarizability of the media does indeed alter the resonance of the plasmonic antenna, all results observed are not true to what the media actually sees during drive operation. One possible correction that can be made to help bridge this gap would be the use of an atomic force microscopy (AFM) tip coated in magnetic material (in this case cobalt) having a polarizability similar to the magnetic media during sSNOM measurements. This cobalt-coated tip also enables magnetic force microscopy (MFM) of the head's magnetic components (i.e., the magnetic write pole) through measurement of the magnetic dipole–dipole interaction between the tip and the sample.14,23–25 Here, the magnetic write pole's pulsed field is synchronized with the plasmonic antenna's near-field generation, which is also a pulsed field from the 830 nm diode laser.

Simultaneous AFM, sSNOM, and MFM measurements were performed using a pulsed write head current at a frequency resonant with the magnetic tip's oscillation (ω1) for a head containing an E-shaped plasmonic antenna, also referred to as a near-field transducer (NFT) (Figs. 2 and 3).4–6,21 A 100 nm full width at half maximum (FWHM) near-field spot generated by a 60 nm FWHM E-antenna notch was observed in the 3ω0 adjacent to the location of the maximum magnetic field generated by the write pole. When the maps are overlaid, the relative locations of the notch, magnetic write pole, near-field spot, and magnetic field maximum can be seen [Fig. 3(d)],5 and only one scan is necessary to find closure between as-fabricated heads and their magnetic and optical simulation.6 The position of the maximum scattering amplitude from the sSNOM measurement occurs at the end of the E-antenna notch as measured in the sample topography, as is predicted by numerical simulation and detailed in prior works.4–6,21 As is similarly expected, the maximum of the magnetic force is observed in the region of the write pole. The resolution of the MFM image is dominated by the character of the magnetic fields arising from the write pole, which is much larger than the notch of the E-antenna and in the absence of hard drive media is expected to have a softer falloff without the magnetic focusing effects of a large planar region of magnetically polarizable material. While there are mild differences from the operating configuration of a HAMR write head, due to the missing planar media layers, our approach enables direct and precise measurement of the adjacency of the magnetic and optical fields, a crucial figure of merit for HAMR performance.

FIG. 2.

Conceptual setup for the simultaneous PiFM and sSNOM or MFM and sSNOM measurements. The hard drive slider (Fig. S1 in the supplementary material) mechanism is mounted to an X, Y, and Z piezo sample stage. The laser diode for the NFT is modulated (ωm) using a square wave voltage source (the output power of the diode is controlled by the upper voltage level of the square wave) in the case for PiFM measurements. For MFM measurements, the write pole is modulated with the same scheme. An AFM cantilever is placed on top of the NFT region, and the sample is scanned to generate the 3D image. For sSNOM measurements, the scattered near-field light is collected by a long working distance objective positioned ∼45° from the sample and is incident on a silicon avalanche photodiode (APD). During scanning, three lock-ins are used to demodulate the topography, PiFM or MFM, and sSNOM signals, all simultaneously. The 200 nm scale bar applies to all three scan images. The color scales are linear with the AFM colorbar being comparable to that of Figs. 3 and 4, and the sSNOM and PiFM or MFM colorbars being of arbitrary units.

FIG. 2.

Conceptual setup for the simultaneous PiFM and sSNOM or MFM and sSNOM measurements. The hard drive slider (Fig. S1 in the supplementary material) mechanism is mounted to an X, Y, and Z piezo sample stage. The laser diode for the NFT is modulated (ωm) using a square wave voltage source (the output power of the diode is controlled by the upper voltage level of the square wave) in the case for PiFM measurements. For MFM measurements, the write pole is modulated with the same scheme. An AFM cantilever is placed on top of the NFT region, and the sample is scanned to generate the 3D image. For sSNOM measurements, the scattered near-field light is collected by a long working distance objective positioned ∼45° from the sample and is incident on a silicon avalanche photodiode (APD). During scanning, three lock-ins are used to demodulate the topography, PiFM or MFM, and sSNOM signals, all simultaneously. The 200 nm scale bar applies to all three scan images. The color scales are linear with the AFM colorbar being comparable to that of Figs. 3 and 4, and the sSNOM and PiFM or MFM colorbars being of arbitrary units.

Close modal
FIG. 3.

Simultaneous topography (a), sSNOM (b), and MFM (c) of the NFT region. The scattering sSNOM image is generated by demodulation of the 3ω0. The laser diode for the NFT is modulated to minimize overheating the NFT structure. The modulation frequency was chosen to not interfere with the mixing products of the MFM measurement. The MFM image (c) is generated by modulating the write pole such that it satisfies the difference between the first and second mechanical modes. Image (d) gives a combined overlay of the three images to aid in visualization of the overlapping areas of maximum intensity for electric fields and magnetic fields. Cross-sectional scan profiles show the FWHM of the notch (black line) and near-field spot (red dashed) as well as their relative positions with the magnetic field profile (blue dotted). The 200 nm scale bar applies to all images with the sSNOM and MFM color bars being linearly scaled and of arbitrary units.

FIG. 3.

Simultaneous topography (a), sSNOM (b), and MFM (c) of the NFT region. The scattering sSNOM image is generated by demodulation of the 3ω0. The laser diode for the NFT is modulated to minimize overheating the NFT structure. The modulation frequency was chosen to not interfere with the mixing products of the MFM measurement. The MFM image (c) is generated by modulating the write pole such that it satisfies the difference between the first and second mechanical modes. Image (d) gives a combined overlay of the three images to aid in visualization of the overlapping areas of maximum intensity for electric fields and magnetic fields. Cross-sectional scan profiles show the FWHM of the notch (black line) and near-field spot (red dashed) as well as their relative positions with the magnetic field profile (blue dotted). The 200 nm scale bar applies to all images with the sSNOM and MFM color bars being linearly scaled and of arbitrary units.

Close modal

Using the same setup in Fig. 2, PiFM measurements were substituted for MFM measurements and a gold-coated silicon tip replaced the cobalt-coated tip. A single pass of the scanning probe revealed the topography (AFM) of the E-antenna (NFT), the near-field spot as measured by sSNOM with a FWHM of 60 nm, and the photo-induced force on the cantilever as measured by PiFM with a FWHM of 35 nm [Figs. 4(a)4(d)]. The photo-induced force was shown to be highly correlated in its location to the simultaneously measured sSNOM near-field spot [Fig. 4(d) left line plots where maxima for the two signals occur at the same position, which are shifted to the side of the notch due to notch asymmetry seen in Fig. 3(a)], which suggests that the intense near-field, which is pulsed at ωm, is in fact responsible for the high force measured on the AFM tip. Furthermore, the near-field spot as measured by PiFM is more sharply resolved [Figs. 4(d) and 4(e)] than that measured by sSNOM when using the exact same tip at the exact same time. We believe this is primarily due to less efficient coupling of the sSNOM signal due to the long working distance objective's ∼45° position (vs a more glancing angle relative to the sample) and the use of the 3ω0 rather than a higher harmonic, in light of the reduced amount of collected signal for the objective position in the present measurements. Prior investigations using sSNOM signal collection at a more grazing angle have demonstrated significantly sharper resolution at higher harmonics for devices with similar plasmonic structures.4–6 PiFM is also a simpler technique in practice as it is simply a force measurement and does not require the additional optical detection hardware that sSNOM requires, which adds to its advantages including potentially higher speed image acquisition. Additionally, in complex material systems, such as the the magnetoplasmonic devices investigated in this work, evaluation of the electrostatic interactions between the tip and the sample can be significant,26 and similarly to the methods employed for PiFM and MFM herein, signals arising from electrostatic interactions, as in Kelvin probe force microscopy (KPFM) or electrostatic force microscopy (EFM), should be simultaneously measurable by similar means. Finally, while not shown here, PiFM is capable of hyperspectral infrared absorption measurements for chemical imaging13 that would make it possible to detect surface materials and chemistry of a complex nanoscale device like a HAMR head, including the possible presence of contaminants after testing of heads in a mock hard disk drive, which offers an additional advantage to employing the PiFM technique.

FIG. 4.

AFM, sSNOM, and PiFM of a HAMR head. Simultaneous topography (a), sSNOM (b), and PiFM (c) of the NFT region. The sSNOM image is generated by demodulation of the 3ω0. The laser diode for the NFT is modulated such that it satisfies the difference between the first and second mechanical modes. Images (d) and (e) give a combined overlay of the three imaging modes to aid in visualization of the areas of maximum intensity for the measured fields. In (d), the cross-sectional lines of the topography (black line), sSNOM (red dashed), and PiFM (green dotted) show the relative sizes of the NFT notch and its measured fields. In (e), the dashed concentric circles represent the size and location of the topography (outermost), sSNOM field (middle), and PiFM field (innermost). The 125 nm scale bar in (a) applies to (b)–(d) as well. The sSNOM and PiFM color bars are linearly scaled and of arbitrary units.

FIG. 4.

AFM, sSNOM, and PiFM of a HAMR head. Simultaneous topography (a), sSNOM (b), and PiFM (c) of the NFT region. The sSNOM image is generated by demodulation of the 3ω0. The laser diode for the NFT is modulated such that it satisfies the difference between the first and second mechanical modes. Images (d) and (e) give a combined overlay of the three imaging modes to aid in visualization of the areas of maximum intensity for the measured fields. In (d), the cross-sectional lines of the topography (black line), sSNOM (red dashed), and PiFM (green dotted) show the relative sizes of the NFT notch and its measured fields. In (e), the dashed concentric circles represent the size and location of the topography (outermost), sSNOM field (middle), and PiFM field (innermost). The 125 nm scale bar in (a) applies to (b)–(d) as well. The sSNOM and PiFM color bars are linearly scaled and of arbitrary units.

Close modal

In conclusion, simultaneous, multimethod characterization of HAMR heads was performed using AFM and sSNOM combined with MFM and PiFM to demonstrate the usefulness of the combined approach for nanoscale, near-surface characterization of complex nanoscale devices. The described methods can be applied to other structures and systems with magnetoplasmonic, optical, electronic, and multimaterial, near-field properties, using the same, similar, or other scanning probe approaches. Finally, we also demonstrated the utility of the PiFM approach for the specific application of measuring optical near-fields generated by a (magneto)plasmonic antenna.

A PiFM—AFM platform (VistaScope—Molecular Vista Inc.) was used to evaluate the local electric field and magnetic field distribution of the NFT region. [Previous internal studies have shown similar results between HGAs (Fig. S1 in the supplementary material) and the row bar state of manufacture with an external laser projected into the waveguide.] The microscope demodulates both the AFM, sSNOM, and PiFM or MFM signals simultaneously. For PiFM, the integrated laser is modulated (ωm) at the difference of the two flexural modes (ω0 and ω1) to satisfy ωm = ω1 ± ω0 (Fig. S2 in the supplementary material). For MFM, the write pole is also modulated at ωm to satisfy the difference between the first and second mechanical modes; this measures the gradient of the magnetic force providing better spatial resolution compared to the standard MFM mode.27 The first eigenmode of the cantilever was excited to dither at 30 nm peak-to-peak, while the second mode was not dithered, only monitored for quality factor (Q) enhancement and detection of the PiFM or MFM signals, respectively, using a heterodyne mixing scheme and lock-in detection. The sample is raster scanned under the tip which is held in a fixed location. Tip–sample spacing is maintained by Z motion of the sample, allowing the tip to remain fixed in Z, removing unwanted tip motion from affecting the optical alignment of the sSNOM detection. The optical signal for sSNOM is collected by a long working distance objective lens (Optem 10×, 0.45 NA) positioned ∼45° from the sample surface plane and projected onto a silicon diode detector (Thorlabs, APD110A). The 3ω0 was chosen as a good compromise between SNR and resolution. Figure 2 shows the experimental setup.

The simultaneous AFM, sSNOM, and MFM measurements were acquired with an MFM specific cantilever (BudgetSensors—MagneticMulti75-G). The images in Fig. 3 are 256 × 256 pixels imaged at 1.0 Hz linescan speeds.

The simultaneous AFM, sSNOM, and PiFM measurements were acquired with a gold-coated cantilever (Nanosensors—NCHAu). The images in Fig. 4 are 128 × 128 pixels imaged at 0.3 Hz linescan speeds.

See the supplementary material for a brief description of hard disk drive industry language for its subcomponents to aid in understanding how the experiments were performed using HAMR heads and a brief description of the scan settings and cantilever resonance used in the simultaneous multimethod measurements.

L.M.O. and D.N. contributed equally to this work.

L.M.O. acknowledges the National Science Foundation Graduate Research Fellowship Program (No. 00039202).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material