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First-principles study of magnetoelectric materials
Electrically manipulating magnetization is crucial to state-of-the-art information technology. Recently, magnetoelectric antiferromagnets have attracted considerable attention due to their applications in voltage-controlled magnetization, which helps reduce the energy consumption but increases the processing speed. However, the N\'eel temperatures (TN) of magnetoelectrics are usually low. For practical applications, it is desirable to increase TN. First-principles calculations are used to explore the possibility of enhancing the TN of Cr2O3 by substitutional doping or strain. The electronic structure of transition-metal (V, Ti, Mn, Fe, Co, Ni) and anion (N, B) impurities are described, and the effect on the exchange interaction are also evaluated. While transition-metal impurities and N are likely to reduce TN, B increases it. Boron introduces impurity states mediating strong hybridization between neighboring Cr ions and thereby enhances the exchange interactions. In addition, compressive epitaxial strain has a positive effect on TN. Strategies towards higher TN in magnetoelectric Fe2TeO 6 are explored to reinforce superexchange. Substitution of larger ions like Zr or Hf for tellurium increases the superexchange angles. The compensating O vacancies tend to form bound complexes with Zr dopants, which do not degrade the band gap. Substitution of charged N3– for O is favorable due to the decreased charge-transfer gap. In addition, compressive epitaxial strain tends to enhance TN. Magnetism on (110) surface is also investigated. Besides raising TN, the exchange-driven magnetoelectric susceptibility α is formulated using a microscopic model Hamiltonian coupling the spins to lattice displacements and electric field. Electronic and ionic contributions are sorted out, and the latter is resolved into a sum of contributions from different normal modes. This method is applied to analyze the temperature dependence of the longitudinal magnetoelectric susceptibility of Cr2 O3A substantial electronic contribution is found, which is opposite to the ionic part. We also found that non-Heisenberg interactions are too weak to account for the sign change of α∥ of Cr2O 3. The piezomagnetic effect in transition-metal fluorides is investigated. Λ 36 is evaluated from the stress-induced symmetry breaking of the exchange parameters, while Λ14 is driven by the spin-orbit coupling. The effect of anisotropy on the exchange-driven piezomagnetic response is also studied.
Electromagnetics|Condensed matter physics|Materials science
Mu, Sai, "First-principles study of magnetoelectric materials" (2014). ETD collection for University of Nebraska - Lincoln. AAI3667139.