Hi everyone. Our group is currently building a magneto-optical Kerr effect (MOKE) magnetometer to study magnetic thin films. We're calibrating the system by comparing our MOKE measurements to VSM measurements on the same samples.
We've been noticing a strange horizontal shift in the loop that looks almost like exchange bias (See image). However, we don't see this in VSM measurements, so I think it's a problem with our MOKE setup.
Has anyone seen something like this? Or do you have any ideas what could be causing this? We're measuring the field using a Hall probe / Gaussmeter positioned right next to the sample between our electromagnet poles.
You didnt see it in vsm? Did you condition the sample to show ex bias? (heat sample above neel/curie temperature, apply a mag field and cool under applied field?)
You didnt see it in vsm? Did you condition the sample to show ex bias? (heat sample above neel/curie temperature, apply a mag field and cool under applied field?)
Hi Jason. The sample isn't expected to show exchange bias, since it's just a relatively thick Fe film on MgO. I've spoken with some other researchers and they suggested demagnetizing our electromagnet before beginning a measurement.
We did this and it got rid of the shift. It appears that there was some remanent magnetization in the pole pieces that were causing the offset.
I am no specialist in Moke, but yes in VSM technique. We have a VSM made for us in the laboratory (intecin.fi.uba.ar), including a PPMS . But I put you in touch with our MOKE specialist, Dr. Vitali Bilovol: [email protected]
Regards
Marcelo
Hi Steven! There are some oscillations on the upper and lower branch of the hysteresis loop on your graph. Do you know where they come from? Right now Im working on a MOKE setup as well, and we have these oscillations too. When we change the magnetic field in smaller steps, the frequency of these oscillations increases.
Hi there,
As MOKE is more sensitive, it could be possible that these oscillations are related to the resistance within the film, the VSM would not normally see this. As your pole pieces I assume are Fe, the remanence after a measurement is neglible due to it being magnetically very soft. Usually I would use a Paladium sample for calibration of equipment as the problems that arise in ferromagnetism is that the magnisation is not linear, yet in paramagnets it is almost linear. Here is a nice paper using Fe films on MgO which seems to explain the effect that you are seeing here - Please get in touch if you require any more information.
http://digital.csic.es/bitstream/10261/15295/1/Mart%C3%ADnez-Boubeta,%20C.%20et%20al%20Appl.Phys.Lett._88_2006.pdf
If you have your gauss probe in between the magnet poles and close to the sample, I would assume that the hysteresis of the electromagnet core is not the problem because you are measuring the field directly, but you definitely need to check this. It is possible that if you measured with the VSM when the sample was fresh you did not have much of an oxide. If then you waited some time and measured with MOKE, you could have easily oxidized the sample and then you could be seeing exchange bias. I suggest that you get or make another film and cap it with something like aluminum or another non-magnetic metal, and try again. The capping layer can't be too thick, otherwise the laser light used for MOKE will not reach into the iron film.
There are two problems to be aware of:
- the remanence magnetization of the magnet poles is typically of the order of 40-60 Oe
- the Kerr effect shows the magnetization (more precisely: magnetic induction) at the surface of the sample, not too deeply inside it, while VSM shows sample's total magnetic moment. One should not expect uniform magnetization is a multilayer samples.
As it comes to the "oscillations" (mentioned by Maria Mansurova and well visible at your graph) I would seriously suspect the influence of the magnet core, and more specifically the eddy currents generated in them during field change. My own experience with 20cm diameter poles indicates that field, measured with NMR probe, stabilizes only after more than 2 minutes! The field range you need is pretty small, so maybe you should consider measurements in Helmholtz/Maxwell coils, with no ferromagnetic core at all, no matter how soft?
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