Pnictide-height dependent ferromagnetism in CuFeAs and CuFeSb

Electronic structures and magnetism properties of CuFeAs and CuFeSb are investigated by using first-principles calculations. We found that CuFeAs and CuFeSb share similar electronic structures and magnetic properties. Unlike the antiferromagnetic isostructure LiFeAs, the ground state of both compounds is ferromagnetic state driven by the Stoner ferromagnetic instability. Their ground state is very sensitive to the height of anion (As or Sb), translating from the ferromagnetic state to the stripe antiferromagnetic ordering when the anion height is smaller than a critical value. Such magnetic phase transition can be understood by the J1−J2 Heisenberg model. Reducing the anion height will decrease the nearest-neighbor interaction J1 but increase the next-nearest-neighbor interaction J2. The competing between the anion height dependent antiferromagnetic superexchange mediated by As(Sb) and the ferromagnetic direct exchange between Fe results the variations of magnetic structure with anion height.