APPLIED PHYSICS LETTERS 96, 143104 共2010兲
High resolution magnetic imaging of perpendicular magnetic recording head using frequency-modulated magnetic force microscopy with a hard magnetic tip Wei Lu,1,2,a兲 Zhenghua Li,2 Kodai Hatakeyama,2 Genta Egawa,2 Satoru Yoshimura,2 and Hitoshi Saito2,b兲 1
School of Materials Science and Engineering, Shanghai Key Laboratory of D&A for Metal-Functional Materials, Tongii University, Shanghai 200092, People’s Republic of China 2 Center for Geo-environment Science, Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan
共Received 19 November 2009; accepted 12 March 2010; published online 5 April 2010兲 High resolution imaging of ac magnetic field from a trailing-edge shielded perpendicular magnetic writing head was demonstrated by using frequency-modulated magnetic force microscopy 共FM-MFM兲 with a high-coercivity FePt MFM tip. The distribution of perpendicular magnetic field gradient of the recording head is presented and can be used to evaluate the recording performance of the head. A Fourier analysis of the images suggests that magnetic spectral features as small as 15 nm should be detectable by using the FM-MFM technique with a high coercivity tip. The enhancement in spatial resolution of FM-MFM is very crucial for the analysis of nanoscale magnetic features and to shed light on the development of next generation magnetic recording heads. © 2010 American Institute of Physics. 关doi:10.1063/1.3378977兴 To achieve high magnetic recording density in magnetic recording technology, key requirements for the recording head design are to provide large field magnitude and high field gradient in both down-track and cross-track directions.1,2 Currently, most studies of the magnetic field characteristics for recording heads were based on theoretical modeling and simulation3,4 with some experimental studies measuring the read/write performance of the combined heads/media by using spin-stand apparatus. However, simulating the exact conditions of heads in working drives proves difficult using theoretical modeling. The experimental spinstand measurements include not only the head but also the magnetic recording media and the electronics for operating the head. This method involves long evaluation cycles and the high cost associated with building individual sliders. Thus, it is of significance and interest to experimentally measure the real magnetic field distribution of the recording heads, as these provide valuable information for head design and analysis. Magnetic force microscopy 共MFM兲 has high spatial resolution for static magnetic field 共⬃25 nm in ambient conditions and ⬃10 nm in high vacuum environment兲 and has already been used to study the thin-film longitudinal magnetic recording heads5–7 共but very few on perpendicular magnetic recording heads兲. However, it is not easy to explain the magnetic field structure from the MFM images because the MFM image is not well resolved and the magnetization direction of MFM tip will be changed or moved during measurements due to high magnetic field strength from the head. As the recording densities increase, the geometrical size of writing pole in advanced head will be reduced to tens of nanometers. Therefore, it is of significant important to improve the spatial resolution of MFM technique for the chara兲
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acterization of advanced recording head in order to keep pace with industrial development. There are several ways to define resolution. From a model point of view the maximum detectable spatial frequency introduced in Ref. 7 is quite useful. This definition is based on a Fourier-based description of the imaging in MFM, e.g., Ref. 8. This description gives the response of the MFM 共force or force derivative兲 as a function of spatial frequency 共kx兲 in the stray field. With this criterion, MFM resolution can be estimated by the critical frequency 共kc兲 where the intensity of power spectrum reaches the white background level 共thermodynamic noise of cantilever兲.8–10 We define MFM resolution as the critical wavelength c 共=1 / kc兲. The merit of this definition is that it is easy to compare the experimentally obtained resolution with the theoretical ones based on the calculation of the tip’s transfer function.10,11 In this paper, high resolution magnetic imaging of a trailing-edge shielded perpendicular magnetic writing head has been carried out using a frequency-modulated magnetic force microscopy 共FM-MFM兲 共Ref. 12兲 with a highcoercivity FePt MFM tip. The frequency modulation detection method13 is used in the FM-MFM technique. A schematic diagram of the FM-MFM system is shown in Fig. 1. The system used in this method was based on a conventional JSPM-5400 共JEOL Ltd.兲 scanning probe microscope. The cantilever is oscillated by a piezoelectric element and the ac field frequency modulates the cantilever resonance. The cantilever deflections are sensed by laser beam deflection. The frequency modulated MFM signal is demodulated using a PLL circuit 共easyPLL, Nanosurf®兲. The signal out of PLL is fed into a lock-in amplifier and the ac voltage applied to a head is used as a reference signal. In-phase amplitude signals of the lock-in amplifier are obtained by adding an extra phase so that the out-of-phase signals become zero. In this case, the phase shift arising from the electronics can be compensated. The experiment was done in air atmosphere. The
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FIG. 1. 共Color online兲 Schematic diagram of the FM-MFM system.
oscillation frequency 共兲 of the piezoelectric element was near the resonant frequency of the tip 共0兲. The value of 0 was about 256 kHz and the value of Q was about 300. The sample is a trailing-edge shielded single-pole-type recording head. The head was driven by a sinusoidal ac current with a zero-to-peak amplitude of 20 mA and a frequency of 100 Hz. Tapping and lift mode AFM/MFM scans were carried out using a high-coercivity MFM tip 共SI-MF40-Hc, Nitto Optical Co., Ltd兲. The tip was coated with a 20 nm L10-FePt film and the radius is 50 nm. The magnetization direction of the tip is perpendicular to the sample surface. The tip has a coercivity of more than 10 kOe. As a result, we can consider that the magnetization direction of the MFM tip will not be changed or moved during measurement and the tip behaves as a monopole type tip. Since the magnetization direction of the tip is perpendicular to the sample surface, the monopole model gives the MFM response as a function of the gradient of magnetic field component 共Hz / z兲 which is perpendicular to the sample surface according to m
冋
册
Hz cos共mt兲 dz共t兲 d2z共t兲 + k0 + qtip + m␥ z共t兲 dt2 dt z = F0 cos共t兲,
共1兲
where z is the displacement of the tip in perpendicular direction, m is the effective mass of the tip, ␥ is the damping factor of the oscillation, k0 is the intrinsic spring constant of the cantilever, F0 is the force caused by a piezoelement, qtip is the magnetic charge at the tip end, is the oscillation frequency of a piezoelectric element and m is the frequency of an ac magnetic field from recording head. Figure 2共a兲 is a topographic image of the studied head around the main pole region. The main pole, the gap between main pole and trailing shield, and a nonmagnetic layer connected with main pole are clearly observed. Figure 2共c兲 is the corresponding MFM image of Fig. 2共a兲. This image was taken at a lift height of 10 nm. The magnetic field around the main pole is clearly visible and the gap position is also clearly seen. The estimated gap width from MFM image is about 50 nm, which is very close to that measured from scanning electron microscope image and the specification from manufacturer. The MFM images taken with a highcoercivity tip show MFM contrast of dark and bright, indicating that the magnetic force on the tip changed polarity. The return pole/trailing shield exhibits opposite magnetic field direction to the magnetic field of the main pole, with
FIG. 2. 共Color online兲 共a兲 Topographic image, 共b兲 3D image of 共c兲, 共c兲 FM-MFM image, and 共e兲 normal MFM image of the main pole measured at the same experimental condition with a lift height of 10 nm. 共d兲 and 共f兲 are the down track line profiles of MFM signal of the white line shown in 共c兲 and 共e兲, respectively.
trailing shield edge showing enhanced contrast. This indicates that both positive and negative components of magnetic field from head are captured. This is more clearly shown in the down track line profiles 关which correspond to the white line in Fig. 2共c兲兴 in Fig. 2共d兲. A principle feature of high-coercivity tip is the fixed magnetic moment direction during the measurement of head. In contrast to the fixed magnetic moment direction of high-coercivity tip, the magnetic moment direction of low-coercivity tip depends on the applied magnetic field. In current case, the magnetic field from head is in the order of several thousand oersted, which is much larger than the coercivity of conventional Co–Cr coated and soft magnetic film coated tips, the magnetization of the such tips is always along the field direction, so the net force is always attractive and we cannot get phase information of the measured magnetic field using such MFM tips. Figure 2共b兲 shows the corresponding three-dimensional 共3D兲 image of Fig. 2共b兲. From the 3D image, we can clearly observe the distribution of magnetic field emanated from the main pole. By extracting the down track line profiles and cross track line profiles from the MFM image 关Fig. 2共b兲兴, we can easily evaluate the recording performance of recording head such as magnetic writing width, adjacent track erasure and so on. Figure 2共e兲 shows the normal MFM image for dc magnetic field taken with the same FePt tip and the same conditions as Fig. 2共c兲. Figure 2共f兲 is the down track line profile corresponding to the white line in Fig. 2共e兲. It can be clearly observed that the signal to noise ratio of normal MFM image is worse than that of FM-MFM image. Figure 3共a兲 shows the spatial resolution result of FMMFM image 关Fig. 2共c兲兴. In this study, the resolution is the minimum magnetic spectral feature size that should be detectable while not the size of features which have necessarily been identified. The obtained spatial resolution is around 19 nm. We compare the spatial resolution of normal MFM and our FM-MFM images. Figure 3共b兲 shows the spatial resolution result 共around 61 nm兲 of normal MFM image 关Fig. 2共e兲兴. Results indicate that our FM-MFM has much better spatial resolution than normal MFM. This can be easily distinguished from the MFM images shown in Fig. 2. The resolution depends on the sample properties, the tip-to-sample distance, the type of the tip 共tip-transfer function兲 and the minimal measurable force of the force microscope instrumentation used.8 In FM-MFM, we use a lock-in amplifier,
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FIG. 3. 共Color online兲 Spatial resolution result of 共a兲 FM-MFM image 关Fig. 2共c兲兴 and 共b兲 normal MFM image 关Fig. 2共e兲兴; 共c兲 FM-MFM image with a lift height of 1 nm; 共d兲 FM-MFM image using FeNi soft magnetic tip with a lift height of 1 nm.
which enables FM-MFM to detect smaller ac force signal than normal MFM without a lock-in amplifier. In addition, the FM-MFM image has higher signal amplitude and better signal to noise ratio than normal MFM image which is captured by phase detection method. The smaller minimal measurable force signal, higher signal amplitude, and better signal to noise ratio result in a better resolution in FM-MFM technique than normal MFM technique. The best MFM spatial resolution could be obtained when the tips are scanned at nearly zero distance above the air bearing surface of recording head. Therefore, we analyze the spatial resolution of FMMFM image with a lift height of 1 nm, as shown in Fig. 3共c兲, in order to improve the spatial resolution of FM-MFM. It is seen that the spatial resolution can be improved to around 15 nm at ambient condition. In FM-MFM technique, we use the characteristic that the amplitude of tip oscillation does not change during the frequency modulation of cantilever resonance. With this characteristic, we can approach the MFM tip very close to the surface of samples without mixing the atomic force with the magnetic force in conventional MFM. The dependence of spatial resolution as a function of lift height is shown in the inset of Fig. 3共c兲. We also compare the spatial resolution of FeNi soft magnetic tip and FePt hard magnetic tip. Figure 3共d兲 shows the spatial resolution result of FM-MFM image using FeNi soft magnetic tip with a lift height of 1 nm. The spatial resolution of FeNi soft magnetic tip is around 26 nm, which is almost two times larger than that of FePt hard magnetic tip. For the soft magnetic tip, the magnetization of tip will be changed by the perpendicular magnetic field from the recording head and the position of
magnetization charge will be moved away from the tip end to cantilever direction during measurement. This means that the actual tip-sample distance 共the distance between the magnetization charge of tip and the sample兲 is increased. This may be the possible reason for the worse spatial resolution of soft magnetic tip compared with hard magnetic tip. In summary, a high resolution magnetic imaging technique was obtained for the study of magnetic field from a perpendicular magnetic writing head by using our FM-MFM with a high-coercivity FePt-coated MFM tip. It can be deduced that the FM-MFM technique with a high-coercivity tip can be applied to product engineering of next generation recording head, providing a high spatial resolution down to 15 nm at ambient condition. The enhancement in spatial resolution of FM-MFM is very crucial to preserve its significant role in the analysis of nanoscale magnetic features and to shed light on many open questions, understanding of which is essential for the progress of magnetic storage technologies. The authors would like to thank Dr. Y. Uehara from Fujitsu Ltd. for providing the single pole recording head. This research was supported by SENTAN, JST/Advanced Measurement & Analysis, the Storage Research Consortium and Akita prefectural government. 1
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