Investigate in magnetic micro and nano structures by Magnetic Force Microscopy (MFM)

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1 Investigate in magnetic micro and nano Related Topics Magnetic Forces, Magnetic Force Microscopy (MFM), phase contrast imaging, vibration amplitude, resonance shift, force Principle Caution! - Set up your system on a very steady table - Do your experiments in a calm, vibrational free environment Magnet interaction between the tip and sample is used to image magnetic structures with Atomic Force Microscopy (AFM). The principle of magnetic force microscopy can be shown scanning an backup tape (DAT). For this a magnetic tip is used and is magnetized with an ultra-magnet along a certain direction to be sensitive on different direction of magnetic field lines. After imaging the topography in contact mode the cantilever is retracted a few 10 nm. At this distance magnetic force dominates the interaction between cantilever and tape. Scanning the same area it leads to stripe-like structures showing the magnetization of the tape. Also magnetic structures for data storage on floppy disks (ZIP or others) or hard disk down to a structure size of a few ten nanometer can be imaged and analyzed. Equipment 1 Compact Atomic Force Microscope (AFM) Material analysis upgrade Additionally needed 1 PC, Windows XP or higher Double sided scotch tape 1 Strong permanent magnet (ultra magnet) 1 Small compass Other magnetic storage samples (DDS tape, ZIP disk, floppy disk, older hard disks) Figure 1: PHYWE Atomic Force Microscope P25385 PHYWE Systeme GmbH & Co. KG All rights reserved 1

2 Investigate in magnetic micro and nano PHYWE Compact AFM set Fig. 2: Content of Tasks 1. Set-up the microscope, magnetize the magnetic coated tip along a certain direction and approach the tip to the sample in phase imaging mode. Take a topography image. 2. Retract the tip a few 10 nm to do a MFM measurement for different distances. Magnetize the tip to another direction and compare and interpret the results. 3. Image the magnetic structures of different samples, e.g. floppy disk, ZIP floppy disk, and hard disk. Set-up and Procedure Task 1 and 2: The general handling and preparation of the AFM device incl. cantilever is described in the operating manual. A short introduction is also given in the experimental guide P25380 "Basic methods in micro and nano imaging of structures with AFM". The aim of Magnetic force microscopy (MFM) is to image a spatial distribution of a magnetic field. For MFM measurements magnetic cantilevers like Budget Sensors Multi75M-G are required. These are Silicon cantilever which are coated with a ferromagnetic material e.g. Iron. The cantilevers are usually already magnetized in the pointing direction of the tip. 2 PHYWE Systeme GmbH & Co. KG All rights reserved P25385

3 Investigate in magnetic micro and nano Typical samples for MFM measurements are different digital data storage (DDS) devices like magnetic tapes, hard drive disks (HDD) or magneto-optical disks. However, basically any sample showing magnetic features can be imaged in MFM, as long as it fits inside the AFM and its surface is flat enough. Magnetic tapes can be preared by cutting a piece and mount with the shiny side pointing upwards on a spare sample holder using double-sided scotch tape. Pieces of disks can be mounted in the same way. The following examples show different samples of magnetic tape and hard drive disk samples which show magnetic features of different spatial sizes. Figure 3 1 shows example fotographs of the digital data storage devices used for the following scans. Figure 3 Fotographs of a HP DDS Tape (left), Exabyte Data Cartridge (center) and an opened HDD (right). Imaging Investigate the topography of the samples in phase imaging mode. As shown in figure 4 the two tapes show similar topographies, showing the magnetic film deposited on the tape. In contrast the sample of HDD shows lengthy features which come from its fabrication or the magnetic reading head scratching over its surface. However, no magnetic bits and bytes can be imaged in this way. Figure 4 Topography of the Exabyte 8MM Data Cartridge-tape (left), HP DDS Tape (center) and a HP GB HDD. NOTE It is recommended to use a standart cantilever instead of a magnetic cantilever for topographic imaging in order not to damage the magnetic coating when imaging P25385 PHYWE Systeme GmbH & Co. KG All rights reserved 3

4 Investigate in magnetic micro and nano Figure 5 (left) Forces in MFM: Force gradient of a point dipole. (right) MFM imaging in dynamic mode Theoretical Background When performing MFM measurements the tip shouldn't come into mechanical contact with the surface as the influence of the mechanical contact is usually stronger than the magnetic effects. Therefore during MFM the tip is scanned at a defined distance above the sample. The magnetic force which is measured in MFM can be approximated by the force gradient of a point dipole seen by the magnetic tip (see figure 5 (left)), where is the unit vector normal to the cantilever plane and the magnetic force When the AFM is operated in dynamic mode, e. g. when the tip is vibrated in -direction (vertically to the sample) during the scanning, the influence of the magnetic force in this direction is dominant and thus determines the contrast the MFM image (see figure 5 (right)). The magnetic field influences the spring constant of the cantilever probe in the following way: where is the original spring constant. Therefore a force gradient pointing away from the tip decreases the spring constant and a force gradient pointing towards the tip increases the spring constant. Like in dynamic mode, this can be related to a decrease in height of the topography for the lower spring constant and an increase in height for a higher spring constant. The effect of the magnetic force on the amplitude and phase signal are also comparable to the dynamic mode topography imaging. For the amplitude the change of the spring constant induces a shift of the resonance peak and the difference in the amplitude is thereby related to the magnitude of the magnetic field (see figure 6 (left)). The same shift is also observed in the phase signal (see figure 6 (right)). The advantage of the phase signal over the amplitude signal is its local linearity and the thus simpler and linear relation between the force and the phase shift: where is the -factor of the cantilever probe. For this reason, we will use phase images for imaging the magnetic samples structures. 4 PHYWE Systeme GmbH & Co. KG All rights reserved P25385

5 Investigate in magnetic micro and nano Figure 6 Amplitude (left) and phase shift (right): The magnetic tip sample interaction influences the spring constant of the cantulever probe and provokes a shift in resonance frequency. The resonance shift can be detected in the amplitude signal (left) and phase signal (right). Technical Implementation The most important change compared to the standard dynamic mode is that the probe needs to be scanned at a defined distance above the surface. This is realized by performing measurements in constant height mode as shown in figure 7. In this mode, the feedback loop is switched off. Figure 7 Linear scan at a given height over the sample surface Preparing the Measurement 1. Mount the MFM Probe on the AFM. Usually, new MFM cantilevers are magnetised. However, you might neew need to magnetise the tip with a strong permanent magnet as shown in figure Magnetisation of the Tip (figure 8). 2. Determine the north-pole of the permanent magnet (for example using a compass). 3. Mark the north-pole, in order to be able to magnetise the tip in a reproducible direction. 4. Bring the pole of the magnet to within 1-2 mm distance of the tip. 5. Move the magnet away from the tip in a direction perpendicular to the cantilever surface. 6. Select cantilever type in the software. If the present cantilever type is not there create a new cantilever type according to the specifications of the manufacturer. 7. If not done yet, select the phase contrast imaging mode. P25385 PHYWE Systeme GmbH & Co. KG All rights reserved 5

6 Investigate in magnetic micro and nano Figure 8 Magnetization of the tip Approaching the Surface Set the free vibration amplitude to 2mV. Clear the check box Auto. start imaging in the SPM Parameter -> Approach panel as shown in figure Auto Start (figure 9). A direct start could harm the tip. Approach the surface. Figure 9 Auto start imaging CAUTION Only change the amplitude calibration value when the tip is witdrawn far from the sample. Otherwise you could damage the tip. Calibrating the Vibration Amplitude The calibration value of the vibration amplitude depends on the cantilever type, because of changes in the length and resonance frequency of the cantilever. As it is important to know the vibration amplitude it needs to be calibrated. This needs to be done only when a cantilever was changed. Use the following procedure to calibrate the vibration amplitude: Measure a vibration amplitude vs. distance curve in spectroscopy mode as shown in figure Amplitude Calibration (figure 10). Determine the slope with the measure length tool and calculate the calibration value. Withdraw the tip from the surface again. Set the free vibration amplitude to a value in order of the magnetic feature size you intend to measure. The feature size can be determined by using the Measure Length tool. An example is given in Task 3. Starting the measurement Ensure that the check box Auto. start scanning is still cleared. Approach the surface. 6 PHYWE Systeme GmbH & Co. KG All rights reserved P25385

7 Investigate in magnetic micro and nano Figure 10 Amplitude-distance curve with indicated measure length tool. Auto adjust the scanning slope by pressing the Adjust slope button in the SPM Parameter -> Imaging panel (figure 11). This will make sure the tip is scanned parallel to the surface and will not crash. Activate the constant height mode by checking the box Enable Constant Height Mode also found in the SPM Parameter -> Imaging panel (figure 12). Start at about and decrease the distance manually until you reach the optimum contrast, an example is given in Task 3. The scanning time per line can be set to low values of as the tip is not touching the sample and therefore will not be harmed by a fast scanning speed. Figure 11 Auto start imaging Figure 12 Enable constant heigt mode CAUTION In the constant height box a negative value means a displacement away from the surface. A positive value will crash the tip into the sample. P25385 PHYWE Systeme GmbH & Co. KG All rights reserved 7

8 Investigate in magnetic micro and nano Task 3: Image the magnetic micro structures of different samples In the following the samples of magnetic tapes and a hard drive disk are imaged in MFM. Selecting the Vibration Amplitude The magnitude of the MFM (phase) signal, the phase noise and the resolution of the MFM signal depend on the vibration amplitude. When the amplitude is too small, the noise is too high, when the amplitude is too large, the resolution becomes low, and ultimately the signal will also become too small. It can be shown that the vibration amplitude should be of a similar magnitude as the smallest details you wish to see. The accurate vibration amplitude can be calculated by multiplying the previously measured value from the spectroscopy with the mean feature size. In the measurement of a magnetic tape (Exabyte 8MM Data Cartridge) shown in figure 13, the aproximate feature size is, the amplitude calibration gave, therefore the corresponding recommended vibration amplitude would be However, such large amplitudes exceed the maximum possible virbration amplitude of the cantilever. In this case the the largest possible amplitude should be used. Nevertheless, the results show a good resolution. Figure 13 MFM measurements of a Exabyte 8MM Data Cartridge-tape using different scan sizes. The mean feature size is. Selecting the Relative Tip-Position In Order to achieve the best contrast, the distance to the sample must be as small as possible. At a certain point, however the tip will touch the surface and will be harmed when the contact is to strong. A safe procedure is to start from a safe tip to sample distance (e.g. ) and reduce it step wise until the tip begins to feel the surface. Usually, this is the case at a distance well below. Immediately increase the distance until the topography features disappear from the phase signal. 8 PHYWE Systeme GmbH & Co. KG All rights reserved P25385

9 Investigate in magnetic micro and nano Figure 14 shows the measurement of another magnetic tape (Hewlett Packard Digital Data Storage 4GB). Here we see a different magnetic stucture with a mean feature size of.. Furthermore we see, there are 2 parallel aligned tracks. By using the Measure Angle tool we can determine the tracks are tilted by with respect to each other. The reason for this is that there are each two read and write heads in DDS drives. Figure 14 MFM measurements of a Hewlett Packard Digital Data Storage 4GB-tape using different scan sizes. The mean feature size is. In figure 15 the magnetic structure of a hard drive disk (Samsung, 40 GB) is shown. The mean features size is around. Figure 15 MFM measurements of a Samsung size is. hard drive disk using different scan sizes. The mean feature The measurements show that not only the magnetic feature size is growing smaller with increasing storage capacity but also there are different structural arrangements for the respective techniques. Further- P25385 PHYWE Systeme GmbH & Co. KG All rights reserved 9

10 Investigate in magnetic micro and nano more, these arrangements are not visible in topographic AFM imaging but only in MFM measurements probing the local magnetization of the data storage devices. In currently used Data storage devices the feature size usually is already very small, such that it is only possible to image the magnetic structure using high end MFM devices. 10 PHYWE Systeme GmbH & Co. KG All rights reserved P25385