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The ideal frequency characteristic of inductance is where the impedance rises in proportion to the frequency. However, in the above measurement results, the impedance drops after exceeding 3MHz.
One of the causes of this is the effect of stray capacitance generated between coils.

It is also possible that impedance characteristics are degraded due to coil loss.
The loss of a coil wound around a magnetic material can be broadly classified into

1) loss due to the magnetic material (core) and
2) loss due to the winding.

Iron loss (core loss), which is the loss due to the magnetic material (core), increases as the frequency increases.
On the other hand, copper wire loss due to winding (copper loss) decreases as the frequency increases. This is because the impedance of the coil increases proportionately to the resistance value of the coil.

From the above, we see that iron loss has a large effect on coil loss at high frequencies.

When a current flows through the coil, a magnetic field is generated inside the coil, and the eddy current that is generated when the magnetic flux passes through the inside of the magnetic body (core) lowers the apparent permeability.
This becomes eddy current loss and lowers the impedance. In this way, the generation of eddy currents weakens the strength of the magnetic field inside the magnetic material.

The hysteresis characteristic indicates a state in which different trajectories are drawn for going and returning. The figure above is a B-H curve showing the magnetic field H on the horizontal axis and the magnetic flux density B on the vertical axis.
The hysteresis characteristic indicates the state in which the magnetic flux density does not become zero when a strong magnetic field H is applied to the magnetic material and the magnetic field is returned to zero from the state in which the magnetic flux density B is saturated. The area derived from these curves is the hysteresis loss.

The sum of this hysteresis loss and overcurrent loss is iron loss.

Based on the above, the effect of the deterioration of high-frequency impedance characteristics can be shown as follows by expressing the relationship between the actual diagram and the equivalent circuit.