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Mechanisms and origins of half-metallic ferromagnetism in CrO2

Chromium dioxide (CrO2) offers a rare example of metallic ferromagnetism among stoichiometric transition-metal oxides. What makes it even more remarkable is the half-metallic electronic structure. Today, CrO2 is widely used in magnetorecording and regarded as a promising spintronic material. Nevertheless, the key question "Why is it ferromagnetic?" remains largely unanswered, despite general interest to the problem and practical importance of CrO2. In the present work we challenge this question by combining first-principles electronic structure calculations with the model Hamiltonian approach and modern many-body methods for treating electron correlations. Our analysis demonstrates that the problem is indeed highly nontrivial: at the first glance, the ferromagnetism in CrO2 can be easily explained by Hund's rule related exchange processes in the narrow t2g band. However, the electron correlations, rigorously treated in the frameworks of dynamical mean-field theory, tend to destabilize this state. The ferromagnetism reemerges if, besides conventional kinetic energy changes in the t2g band, to consider other mechanism, involving direct exchange and magnetic polarization of the oxygen band. We show how all these contributions can be evaluated using first-principles electronic structure calculations. Our results explain the overall stability of the ferromagnetic state and provide the firm microscopic basis for understanding the magnetism of CrO2.