Can anyone tell that I am getting magnetic signals in MFM (magnetic domains) but not in SQUID measurement. What is the reason behind that ?
In case of dilute magnetic materials, I am getting magnetic domains in MFM, but I am not getting any magnetic signal in SQUID measurement.
Give the proper reasons and suggestions.
Also tell me that which instrument has more accuracy for magnetic signals
When done properly, the MFM does detect the occurrence of local stray fields with components along the direction your tip is being sensitive to. Typically, such stray fields emanate from domain boundaries. I am not an expert on this, but I have been told that MFM measurements may be tricky to interpret. So, in some way you need to make sure that what you see is actually magnetic contrast and not an artifact. I am not the person, though, to be able to suggest useful procedures for doing that, though.
What I can say, though, is that in order to interpret MFM, you need to model the tip and gauge the response accordingly. Then, hopefully, you can be able to cross-check whether the signals you get do make sense. If you're not an expert, I suggest you seek getting advice from someone who is an expert.
A properly set up and operated SQUID magnetometer is probably the most sensitive detector for small magnetic moments that I know of (maybe with the exception of some spectroscopic tools like x-ray magnetic dichroism, which however comes with quite a few requirements concerning your samples and is usually measured using synchrotron radiation and in ultrahigh vacuum). So, getting NO magnetic signal might be signalling that something went wrong. You should be able to detect the [oftentimes diamagnetic] "background" of the sample holder/substrate/host material. The moment relating to the DMS would come on top of that. Safely telling one contribution from the other might involve temperature dependent measurements (diamagnetism being nearly independent of temperature) and careful evaluation.
In order for anyone to give useful advice you should probably give more details on what your sample is, what the carrier and/or container of the sample is, at what temperature(s) you measured, what the expected magnitude of the total moment of the DMS would be and what is the level of uncertainty is at which you can claim that there is no magnetic signal from your sample.
You need to take into account and compare the characteristic sizes of the domains observed with the help of the MSM and the size of the local sample area for detecting signal in the squid-magnetometer. Perhaps magnetometry registers the average signal from multiple domains with opposite polarization, which can be close to zero.
If your MFM lift height is too low, you can get artefacts from the tip impacting the surface on the MFM lift mode pass. This introduces amplitude or phase shift because the tip has hit an obstacle. One way to avoid this is to ensure that your lift height is greater than any topography that you might encounter (e.g. if the particles that are on the surface are about 50 nm in diameter, use a 50 nm+ lift height to avoid tip impacts).
Are you using phase or amplitude shift to record the magnetic interactions with the tip and the surface? And what are the magnetude of the shifts that you are seeing. To be confident of the signal not being noise, you need at least 0.1-0.2 degrees phase shift to be able to attribute the change to magnetism over just general noise.
If it is phase shift, then you need to make sure that the resonance detection is centred on the centre of the peak, and not to the left of the peak on the steepest slope as is usual for pretty much all other types of AFM. If you are using phase shift, you can image and area, remove the tip and magnetise it in the opposite direction and re-scan the same area. If the signs on the magnetic domains are reversed (i.e. negative shift becomes positive and vice versa) then you are definately seeing magnetic domains. If they stay the same, then you are imaging artefacts.
If you are seeing stripes in the images, it is likely that this is due to the laser spot falling over one or both edges of your cantilever. If you have a very shiny surface (e.g. gold, silicon) this light can be reflected back to the detector, and appears as stripes in the magnetic contrast image. If you keep the same scan area (size and location), but alter the lift height between two images (e.g. 50 nm and 200 nm), the width of the stripes should change as you move the laser further from the reflective surface.
If you post the images, with a scale to show the magnitude of the shifts, I might be able to help more.
Jo
As everybody discussed, in MFM you may have a tip induced artifact which may give you false contrast in domain imaging. The quick check is obviously to do scan with different lift heights and with different orientation of the sample. Also you can do a field dependent MFM at least for the remanent loop to get the signal variation with applied field, which will also give you an idea about the expected result from squid measurement and what may be the draw back there.
The other way is to do micromagnetic simulation to know the detail of your sample. This all about to predict the squid result and what may be the problem in the measurement.
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