By employing highly sensitive millikelvin SQUID magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1-xMnxN. The interpretation of the results in the frame of tight binding theory and of Monte Carlo simulations, allows us to assign the spin interaction to ferromagnetic superexchange and to benchmark the accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.
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