Hello Junmoni,

me again ;-) 

The easiest - and most common - methods to measure magnetic anisotropies are Brillouin Light Spectroscopy (BLS), Ferromagnetic Resonance (FMR) and similar methods, measuring frequencies which can be converted into anisotropies.

However, it is possible to extract anisotropies from angle-resolved measurements of hysteresis loops - but not as directly as in the above mentioned methods. Some of our articles deal with simulations of angle-dependent hysteresis loops and their correlation with (mostly fourfold) anisotropies: A. Ehrmann and T. Blachowicz, Influence of fourfold anisotropy form on hysteresis loop shape in ferromagnetic nanostructures, AIP Advances 4, 087115 (2014); T. Blachowicz and A. Ehrmann, Micromagnetic simulations of anisotropies in coupled and uncoupled ferromagnetic nano-wire systems, Sci World J 2013, 472597 (2013); A. Ehrmann, T. Błachowicz,

Adjusting exchange bias and coercivity of magnetic layered systems with varying anisotropies, J. Appl. Phys. 109, 083923 (2011); or A. Tillmanns, T. Błachowicz, M. Fraune, G. Güntherodt, and I. K. Schuller, Anomalous magnetization reversal mechanism in unbiased Fe/FeF2 investigated by means of the magneto-optic Kerr effect, J. Magn. Magn. Mater. 321, 2932-2935 (2009) (they are available here or on my private website). As you will see, you can verify a certain anisotropy form which you expect, and you can even calculate anisotropy constants - but if the form differs from the one which you expect, there's a lot of trial-and-error waiting for you.

Dear Junmoni,

For example, you can act as we did in our paper 

Article Magnetic anisotropy in Fe films deposited on SiO2 /Si(001) a...

Thanks for your answers.

Dear Komogortsev sir, is the method given in your paper is applicable for polycrystalline bulk samples?

Dear, Junmoni, I am able to say a few things about bulk polycrystalline samples, but will restrict myself to speaking of ~80%Ni permalloys. The quick answer is that it is not immediately possible to interpret hysteresis loops in terms of anisotropy. This is because increasing grain size can cause changes to hysteresis loops that mimic changes that result from lowering total magnetic anisotropy. By way of example I refer to two papers. In Müller et al (1998, “Grain structure, coercivity and high-frequency noise in soft magnetic Fe-81Ni-6Mo alloys” JMMM 177-181, 231=232) they have used an inappropriate heat treatment resulting in high anisotropy. Magnetic properties correlate with grain sizes in unusual ways. Magnetic domains are large and irregular – a sign of high anisotropy. In Couderchon et al (1989, “MAGNETIZATION PROCESS IN NiFe ALLOYS WITH VANISHING ANISOTROPIES”, IEEE MAG-25, 3973-3975), almost the same material has been properly heat treated to very low anisotropies. But like Müller et al again they found strong correlations of magnetic hysteresis properties with grain size. In some cases the two groups of results are opposed, but that is an artifact of the two different heat treatments. Several papers by Herzer at Vacuumschmelze address this issue, as does Vacuumschmelze’s recent book “Magnetic Materials”, by Rainer Hilzinger and Werner Rodewald. Both effects operate by changing the shape of magnetic domains within the material. Increasing grain size reduces the magnetostatic energy internal to the specimen. Decreasing anisotropy reduces domain wall areal energy density. Both of these cause the shape of domains to become thinner relative to their overall size, and this effects what you see in the hysteresis loop.

Measuring the anisotropy field from the M(H) curve of a polycrystalline sample can be quite problematic. For such cases, a very suitable method is the SPD (Singular Point Detection) technique, that was developed starting from the '80s specially for this purpose. In the two following references you can find: 1) an example of a recent application of SPD; 2) a review about SPD.