conjunction with another technique to be described below can extend this frequency range even further to the vicinity of 60 ghz in many applications where ferrimagnetic resonators are used the strength of the applied magnetic field is varied systematically over time to produce a swept frequency output in these circumstances the hysteresis of the electromagnet presents a problem because the frequency of the resonator depends on the history of the applied magnetic field as well as on the current state of the applied magnetic field for this reason magnets of some alloys such as 80 nickel and 20 iron that have a low hysteresis are desirable for many applications however there is in general a correlation between hysteresis and permeability so that there is always a trade off between the materials that are best for minimizing hysteresis and those that are best for maximizing the strength of the magnetic field before saturation occurs the trade off between saturation properties and hysteresis properties can be eased by the use of a pole tip that has at its terminal end a final layer of an alloy of the higher hysteresis higher saturation type referring now to fig3 a small layer of cobalt iron alloy 50 to 50 is seen as a cap 325 on the beveled tip 315 portion of pole piece 310 the small size of this layer minimizes the negative hysteresis effects while its location at the point where the magnetic field must be most concentrated optimizes the benefit from using it to increase the flux density where it is most.