As has been demonstrated previously, ion bombardment of magnetic films at optimized temperatures allows fine tuning of various magnetic properties like Curie-temperature, hysteretic behaviour, exchange bias fields as well as magnetically relevant structural phase transitions. In many cases, it is desirable to find a relation between certain types of defects or of disorder and corresponding changes of a specific magnetic property. For this purpose, a new low temperature (T > 6K) ac-susceptometer was developed and combined with the beam-line of a 350 keV ion accelerator.

The design of this system will be introduced and its performance demonstrated by first measurements on FeAl films. Within the chemically ordered B2 phase the FeAl films exhibit a paramagnetic behaviour. Due to iterative ion irradiation the structure of FeAl is modified into the A2 phase accompanied by a phase change from para- to ferromagnetism.