A variety of factors affect the performance of transformers and inductors including winding and the size, shape, and construction of the core. But with literally dozens of possible choices, core material is also a critical factor in the performance of magnetics and one that is becoming increasing important in meeting the stringent requirement of GaN switching applications.
The major problem is core loss, which in a magnetic device can by due to hysteresis—shifts in the magnetic domains within the core material—or to eddy currents, the formation of circulating current loops within the core. In both cases, valuable electrical energy is converted to problematic heat, making this a double loss.
There is no best material for all applications, and you’ll get the most efficient magnetics by working with a provider well versed in core material options and specifications and by giving that provider as much information as possible about the factors like switching frequency and operating temperature of your device.