Elaborate your question:

Which sample you are measuring?

In which form it is?( NPs, Thin films, Bulk or...)

What is the magnetic field limit?

Better to share your BH curve image

Dear Cody,

The hysteresis loop don't use the susceptibility, only magnetization vs field. It is normal that the susceptibility decreases when you have saturation if you follow increasing the magnetic field, because the magnetization remains constant while the field increases.

It's not very clear that you are talking about magnetization or susceptibility. Sometimes magnetization has been found to decrease at high magnetic field amplitude if there is a diamagnetic capping over superparamagnetic nanoparticle. The diamagnetic contribution becomes significant at high field where magnetization due to superparamagnetism already saturates. So a decrease in magnetization is obvious. The amount of decrements mainly depend on capping thickness and applied field amplitude.

I suggest you might continue increasing the magnetic field and see what will happen. I have an experience with some samples that show necking in the hysteresis loop, meaning after reaching what looks like a saturation limit, the magnetization starts to increase again with increasing the applied field.

Dear Surojit,

Diamagnetism cannot be detected in a B-H hysteresis loop, even if you increase the applied magnetic field several orders of magnitude:

1. Superparamagnetism needs to have ferromagnetic elements (Fe, Co and Ni) which cannot have so properties. The highest diamagnet is Bi with a quite different electronic structure.

2. Usually the elements are even much poor metals because they are oxides for producing the ferromagnetic phase. Thus they are even worse diamagnets.

3.Last and more important, diamagnetism opposes to the applied field but no to the magnetization of the material.

Frankly, I only see a mistake in the interpretation of the measurements or due to a non stable configuration for the nanoparticles which could change them by the application of stronger magnetic fields. For instance, changing their dipolar interaction.

https://www.researchgate.net/post/Please_can_anyone_explain_me_why_in_this_hysteresis_loop_H_increasing_causes_a_decrease_of_B_in_the_proximity_of_saturation

Probably a very similar question. There, clearly M(H) loops, not B(H). It's probably the same here, considering the text (but for the probably wrong use of the word "susceptibility").