7. Conclusions

In this chapter, we have shown several approaches to tailor the high-frequency permeability of ferromagnetic materials: shapes, particle size distribution, heat treatments, coating and spin transfer torque effect. Micromagnetic simulations are used to explain the origins of multi-peaks in permeability spectra of nanowire array. In addition, just like the simultaneous excitation method (pulse H field + STT effect) shown here, we want to propose the other possibilities (pulse H field + the other excitation) to tailor μ f spectra and to realize negative imaginary parts of permeability (μ″ <sup>&</sup>lt; 0), such as femtosecond laser pulses or photomagnetic pulses.
