Superconductor Trapped Field and its AC Attenuation: Weekend Reading

I'll be doing some reading this weekend trying to get some more information behind the experimental results I've seen this week. The rough observations this week were:

1. Levitation force seems to go down for the same peak magnetic field as frequency increases.
2. The levitation force can be increased at a given frequency by increasing the peak magnetic field.

I haven't found much research that measures the levitation force between an AC electromagnet and a superconductor. I have found some interesting papers on the attenuation of magnetic flux trapped within a superconductor when an AC magnetic field is applied to the superconductor. Two of the more interesting papers I've found are

Ueda, H., Itoh, M., & Ishiyama, a. (2003). Trapped field characteristic of HTS bulk in AC external magnetic field. IEEE Transactions on Appiled Superconductivity, 13(2), 2283-2286. doi:10.1109/TASC.2003.813075

and

Ogawa, J., Iwamoto, M., Yamagishi, K., Tsukamoto, O., Murakami, M., & Tomita, M. (2003). Influence of AC external magnetic field perturbation on trapped magnetic field in HTS bulk. Physica C: Superconductivity, 386, 26-30. doi:10.1016/S0921-4534(02)02114-7

At first glance, the results from these papers seem to contradict each other.

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