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Start of the AFM/MFM measurements on Grain Oriented Samples
The ESIEE Amiens and the Université Picardie Jules Verne have started to measure the topography (AFM) and the magnetic structure (MFM) of two samples: a Floppy disk taken as a reference sample and a GO SiFe sample provided by SEPSA. This is the first stage of a two-stage analysis. Results will be compared with the measurements to be made on the same sample after the laser scribing process in order to measure its impact.
(a)Topography of Floppy disk , (b) magnetic structure of Floppy disk, (c) Topography of non-scribed GO SiFe sample from SEPSA, (d) magnetic structure of non-scribed GO SiFe sample from SEPSA.
Atomic Force microscope (AFM) is a technique to visualize the surface topography of a sample at extremely high resolution (nanometer scale). This technique is a type of local probe microscope: a probe with a conducting cantilever mounted on his top by a sharp tip is placed in the scanner head of the instrument. In semi-contact mode, the tip oscillates near its resonance frequency. The magnitude of oscillations is influenced by the attractive-repulsive forces between the tip and the sample. Therefore a feedback is provided to maintain the magnitude of oscillation at constant reference. The feedback controls the height of the probe that follows the surface topography. The motion of the tip is sensed by an optical system using a laser beam focused on the top of probe and the reflected beam is captured by a photodiode detector to deduce the variation in magnitude during the scanning.
Magnetic force microscopy (MFM) is an extension of the atomic force microscopy that measures the interacting magnetic forces between the tip and the sample. This mode needs a specific probe with a tip coated by a thin layer of magnetic material. It gives the imaging of the magnetic surface structures of the sample: the distribution of magnetic domains and domain walls. MFM uses two pass techniques during the scanning: in the first pass the tip scans the surface topography (as AFM semi-contact mode). In the second pass the tip-sample distance is increased based on the data of topography line obtained from the first pass. So the influence of Van Der Waals forces on the tip become negligible at certain height and the tip is affected in majority by the magnetic force; this allows the scanning of the magnetic surface information. When the tip is influenced by an attractive force the magnitude of oscillation is reduced, this leads to a dark contrast of the image while when it is influenced by a repulsive force the magnitude of oscillation increases and gives the bright contrast of the image. A feedback is maintained to keep a constant magnitude during the scan. An optical system using a laser beam focused on the top of probe and the reflected beam is captured by a photodiode detector to determine the change in magnitude and in phase of oscillation.
For AFM measurements probes should be without magnetic coating. These probes could have different resonant frequency, cantilever stiffness, tip shape and cantilever length. The choose of the probe depends on the mode of scanning (contact, semi-contact or non-contact mode). For MFM measurement magnetic coating probes have different properties such as cantilever stiffness, resonant frequency, thickness of the magnetic coating (CoCr layer), coercivity, and magnetic moment. We will use three types of probes: hard magnetic probes with average moment, hard magnetic with low moment soft magnetic with low coercivity. The hard magnetic probes are sensitive to magnetic domains. This type of probes will keep its direction of magnetization during the scanning and we choose average and low moment to test which moment will result in clear resolution of the imaging depending on the thickness of the insulating coating. While a soft magnetic probe with low coercivity acquires the direction of domain magnetization during the scan so it is more sensitive to the domain walls of the sample surface.
During our first measurements, we started measuring the topography (AFM) and the magnetic structure (MFM) of a Floppy disk considered as reference sample to test the efficiency of the probe and the choice of measurement conditions: we used a probe with hard magnetic coating and low moment. The topography image in Figure (a) is completely different from the MFM image in Figure (b) and this confirms that the magnetic information of the sample surface is presented in the measurement. The dark and bright contrasts are clearly shown and defined strictly in the figure (b) which defines the distribution of up and down magnetic domains on the floppy surface. The domain size measured is about 2.5 μm in average.
The GO SiFe sample from SEPSA is polished to remove the coating layer before MFM measurement. The topography image in Figure (c) is still completely different from the MFM image in Figure (d) and this confirms that the magnetic information of the sample surface is presented in the measurement. As expected, the dark and bright contrast shows bigger magnetic domains on GO SiFe sample.