What is the fine-grained topic of the following text: Tunability studies of the magneto-dielectric PNCs were carried out by adding the PNC to cavity and microstrip linear resonator devices, and passing frequencies of 1-6 GHz through them in the presence of transverse external magnetic fields of up to 4.5 kOe, provided by an electromagnet. Only return the topic.
Tunability studies of magneto-dielectric PNCs in cavity and microstrip linear resonator devices using external magnetic fields.