I measure the magnetic properties of thin film at two different maximum H field by VSM and obtained different coercivity for each of them.
To be sure of what answer is correct, one would have to look at the entire magnetization curves. Magnetization curves with hysteresis normally possess a so-called irreversability point. This is, where the hysteresis vanishes (at large field).
If your max. field is larger than the irreversability field in both cases, than - as Murat points out - the coercive field should be independent of the maximum applied field. In case one or both max fields being below the irreversability field, coercive field shall depend on max. field.
No. According to my knowledge, the problem you encounter should be an instrument mistake.
To be sure of what answer is correct, one would have to look at the entire magnetization curves. Magnetization curves with hysteresis normally possess a so-called irreversability point. This is, where the hysteresis vanishes (at large field).
If your max. field is larger than the irreversability field in both cases, than - as Murat points out - the coercive field should be independent of the maximum applied field. In case one or both max fields being below the irreversability field, coercive field shall depend on max. field.
I have simulated hysteresis loops with a simple macrospin model, and sometimes in loops where the reversal process occurs via two steps, the second step is nearly invisible in the longitudinal curve, but obvious in the transverse loop. In this case you can measure minor loops, as Kai already stated, even without recognizing it.
You could measure with a series of different maximum fields, and in case of such a minor loop problem, there should be a jump in the coercivity at a certain maximum field.
Talking about instrumental errors, as Murat already stated - do you use a Hall probe to detect the field or only the magnet's characteristics (which will also depend on the maximum fields)?
Yes it does, if your thin film is isotropic (or contains isotropic regions). In this case the coercive field is defined as the field where the derivative dM/dH of the loop has a maximum rather than as the field where the magnetization becomes zero. The two definitions give the same result only when magnetic regions are well aligned, subjected to a field applied antiparallel to its magnetization direction. However they can differ strongly when, in addition to irreversible processes, reversible rotation becomes important (which is the case for isotropic magnetic materials).
Dear Amir,
then it looks, as Kai has already stated, like a minor loop problem rather than a problem of the data acquisition. I would suggest to measure hysteresis loops with a series of maximum external field values and to check whether there is a sudden jump in the coercivity.
Good luck,
Andrea
Yes, the coercivity generally depend on the intensity of applied field for the hard magnetic material when the maximum H is smaller than the saturation field. For some hard magnetic materials, the saturation field would be large as 60 T (6000000 Oe), it is mostly hard to achieve such field using generally measuring equipment.
However, for most hard magnetic materials, the changes of coercivity is very small when the maximum field is far larger than the coercicity field.
In additions, for film, it is also should conside the shape anisotropy.
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