High-frequency Magnetic Force Microscopy Research Articles

HF-MFM imaging of stray fields from perpendicular write heads

The high-frequency magnetic force microscopy (HF-MFM) technique has been employed for the measurement of stray fields emanating from traditional longitudinal write heads. Here, we show that it is also possible to apply the HF-MFM technique to perpendicular write heads. The situation is different in so far, that there is no magnetic flux between two poles. In contrast, there is only one pole and the flux closure is obtained in combination with the writing medium. An ac current of 50 mA with a carrier frequency ranging between 200 MHz and 2 GHz fed in into the write head causes an observable contrast in the HF-MFM image. Not only the write pole itself but also the magnetic shielding around the pole are strongly influenced by the HF-current.

Theoretical Description of the High-Frequency Magnetic Force Microscopy Technique

We present a theoretical description of the high-frequency magnetic force microscopy (HF-MFM) technique. There are currently two operation modes: 1) the measurement of high-frequency currents via their magnetic stray fields and 2) the measurement of stray fields emanating from hard disk writer poles. The advanced HF-MFM technique employs the amplitude-modulation technique in order to convert the HF signal to the frequency range of the cantilever. Using the point-dipole approximation, we calculate the resulting force onto the cantilever for both cases.

Different types of ferrite thin films as magnetic cantilever coating for magnetic force microscopy

Different types of ferrites are employed in the form of thin films as magnetic coating on cantilevers for magnetic force microscopy (MFM) use. This is especially needed for cantilevers employed in high-frequency MFM (HF-MFM), where stray fields of hard disk recording heads are investigated. Our experiments show that we can operate HF-MFM successfully at carrier frequencies up 2 GHz using such ferrite-coated cantilevers. Thin films of two ferrites, NiZnFe 2O 4 spinel ferrite and Co 2 Z-type hexaferrite (Ba 3Co 2Fe 24O 41, BCFO) were prepared by RF sputtering. As a basis for these probes, we employ commercial micromachined silicon cantilevers. Additionally, films on Si (1 0 0) and Si (1 1 1)-oriented substrates with a thickness up to 100 nm were prepared for analysis purposes, enabling the optimization of the sputter process. For a high spatial resolution of MFM, however, thinner magnetic coatings are required. Therefore, the third type, c, was prepared by laser-ablation with a thickness of 30 nm, also directly onto the Si without additional buffer layer.

High-frequency properties of stray fields emanating from hard disk writer poles up to 2 GHz

Using high-frequency magnetic force microscopy (HF-MFM), we investigate the stray fields emanating from hard disk writer poles. An amplitude-modulated high-frequency current is sent to the writer coil, and the resulting stray fields are imaged by HF-MFM, employing carrier frequencies up to 2 GHz. This advance is possible due to ferrite-coated MFM cantilevers. We observe that, as a function of frequency, the emanating field is forming two peaks around the gap area, and the maximum field is shifting from the centre position in the gap towards one side of the writer pole.

High-frequency MFM characterization of magnetic recording writer poles

By means of high-frequency magnetic force microscopy (HF-MFM), different magnetic recording writer poles are characterized in the frequency range 200– 1500 MHz using the dual-vibrational HF-MFM technique. The stray fields emanating from hard disk writer poles stemming from different makers are compared to each other, and the field distributions obtained at different carrier frequencies are analysed. At low carrier frequencies, the maximum field is located directly at the gap, whereas at high frequencies two distinct maxima before and after the gap are observed.

Advanced Cantilevers for Magnetic Force Microscopy and High Frequency Magnetic Force Microscopy

In order to improve the spatial resolution achieved by magnetic force microscopy (MFM) technique and its derivatives, we employ here advanced MFM tips fabricated by means of focused ion beam (FIB) milling. The magnetic coating applied on these tips is a CoCr film of 10 nm thickness. The MFM measurements on hard disk test samples reveal the achieved high resolution, and the measurement on a garnet film demonstrates the low invasiveness. High-frequency MFM (HF-MFM) is a development of the MFM technique to observe the HF stray fields emerging from magnetic recording writer poles at their operating conditions. By means of HF-MFM, magnetic recording writer poles are characterized in the frequency range 100-1,000 MHz. Up to now, all HF-MFM experiments conducted were using standard MFM cantilevers. From the HF-MFM images obtained using the advanced MFM cantilevers, it is clearly seen that the spatial resolution is considerably improved over the images obtained using standard MFM tips. However, the 10 nm thick magnetic coating of the cantilevers is found to work properly only at frequencies of up to about 500 MHz.

Preparation of ferrite-coated MFM cantilevers

Ferrite-coated magnetic force microscopy (MFM) cantilevers were prepared for the use with a high-frequency MFM (HF-MFM) setup. The ferrite coatings were fabricated by means of radio frequency (RF) magnetron sputtering directly on the Si surface. Two types of ferrites were employed in this study: NiZnFe 2O 4 spinel and Co 2 Z-type hexaferrite (Ba 3Co 2Fe 24O 41, BCFO). The typical thickness of the coatings was 50 nm. For comparison, ferrite samples on (1 0 0) and (1 1 1)-oriented Si substrates (analogous to the surfaces of the cantilevers) were prepared. Successful HF-MFM imaging was performed with both types of cantilevers using harddisk writer poles as samples. The HF-MFM images obtained by ferrite-coated cantilevers evidently reveal more details of the magnetic field distribution of the writer poles up to the GHz range than conventional CoCr-coated MFM cantilevers.

Optimization of the HF-MFM technique

To optimize the performance of the high-frequency magnetic force microscopy (HFMFM) technique, we investigate the effect of varying the frequency of the amplitude modulation on the magnetic contrast achieved. As samples, we employ hard disk writer poles. We find that the optimum condition for the HF-MFM operation is the use of a modulation frequency being about half of the cantilever resonant frequency. Together with optimized cantilevers, a clear improvement of the magnetic imaging is obtained. Furthermore, we can deduce that the optimum parameters for imaging must be determined for each experimental system (hard disk writer pole and cantilever) individually.

Optimization of high-frequency magnetic force microscopy by ferrite-coated cantilevers

High-frequency magnetic force microscopy (HF-MFM) is, e.g., employed for the analyses of harddisk write heads. To improve the performance of HF-MFM, we developed and analyzed cantilevers coated by (Ni, Zn) ferrites. The magnetic coatings were prepared by RF sputtering with a typical thickness of 50 nm. We systematically compared the HF-MFM images of harddisk write heads obtained by ferrite- and CoCr-coated cantilevers. The ferrite-coated cantilevers were found to produce increased HF-MFM signals up to the GHz regime, whereas the contrast in the images taken by the conventional CoCr-coated cantilevers appear to be strongly reduced at frequencies above 400 MHz. The improvement of the HF-MFM performance due to the ferrite-coated cantilevers is discussed by considering the specific imaging mechanism.

Dual vibrational high frequency magnetic force microscopy

The ability to evaluate high frequency writer performance is crucial to the write-head designs for high data rate application. To quantify the write-head performance at high frequency (∼1 GHz), a new high frequency detection approach has been implemented. The essence of the method is to utilize a two-step signal mixing process to produce the force harmonics, which are not only proportional to the amplitude of the deep-gap field but also fall within the active bandwidth of the probe spectrum. Furthermore, a dual-vibration detection scheme is also described, which can provide superior high spatial resolution and enhance the sensitivity of attractive mode force microscopy as well at high frequencies.