A universal DC characterisation system for hard and soft magnetic materials

A fully automatic system has been designed for the accurate measurement of the DC magnetic properties of soft and hard ferromagnetic materials utilising discrete calibrated instruments in order to provide a traceable calibration route separate from the transfer of standard magnetic test samples. Custom written software is used to operate the system in one of three modes, constant dH/dt, variable dH/dt and a second quadrant demagnetisation curve mode. The first two of these modes are utilised for soft magnetic materials with the second mode varying dH/dt in order to keep dB/dt relatively constant. Both modes use cycle times of between 60 and 300 s and may utilise a variety of test configurations including a bar permeameter, electromagnet, ring samples or Epstein frame. The minimum cycle time and the most appropriate mode is dependent on the particular sample and the effect of this on materials with a large dB/dH is significant. Measurements on soft materials include major BH loop, minor BH loops, first-order reversal curves, remanence, coercivity, normal magnetisation curve, peak permeability and loop area. The third mode is used with an electromagnet to measure the demagnetisation curve of hard magnetic materials up to a maximum demagnetisation field of 1.6 MA/m. The measurement algorithm modulates dH/dt depending on dB/dt and incorporates holdback in order to accommodate rare earth materials which exhibit high viscosity.