Figure 2. Experimental methodology.

(a) Schematic illustration of the layer structure and of the circuit used for the measurements. Capacitors containing gate oxide and semi-insulating wurtzite Ga_1−xMn_xN film as insulators are biased by a low-frequency a.c. electric field applied between the gate and the back contact. The structure is placed in the loop of a superconducting quantum interference device (SQUID) magnetometer; changes of the magnetic flux in phase with the electric field are detected by a lock-in amplifier connected to the output of SQUID electronics. (b,c) In the case of the c axis parallel to the growth direction and Ga-face polarity, the application of a positive voltage to the gate (E>0) stretches the Ga_1−xMn_xN films along the c axis affecting in this way the single-ion uniaxial magnetic anisotropy.