Magnetic M ( H ext ) loops of ( Fe 3 + ) [ Fe 3 + Co 2 + ] O 4 2 − , ( Fe 3 + ) [ Fe 3 + Cu 2 + ] O 4 2 − , ( Fe 3 + ) [ Fe 3 + Fe 2 + ] O 4 2 − , and ( Fe 3 + ) [ Fe 3 + Ni 2 + ] O 4 2 − at temperature T = 850 K.