A Study on the Microstructure and Magnetic Properties of Fe<sub>83.2</sub>Si<sub>5.33-0.33<i>x</i></sub>B<sub>10.67-0.67<i>x</i></sub>P<i><sub>x</sub></i>Cu<sub>0.8</sub> Nanocrystalline Soft Magnetic Alloys with Varying P Content