ABSTRACT
Simultaneously enhancing the uniaxial magnetic anisotropy ([Formula: see text]) and thermal stability of [Formula: see text]-phase Fe[Formula: see text]N[Formula: see text] without inclusion of heavy-metal or rare-earth (RE) elements has been a challenge over the years. Herein, through first-principles calculations and rigid-band analysis, significant enhancement of [Formula: see text] is proposed to be achievable through excess valence electrons in the Fe[Formula: see text]N[Formula: see text] unit cell. We demonstrate a persistent increase in [Formula: see text] up to 1.8 MJ m[Formula: see text], a value three times that of 0.6 MJ m[Formula: see text] in [Formula: see text]-Fe[Formula: see text]N[Formula: see text], by simply replacing Fe with metal elements with more valence electrons (Co to Ga in the periodic table). A similar rigid-band argument is further adopted to reveal an extremely large [Formula: see text] up to 2.4 MJ m[Formula: see text] in (Fe[Formula: see text]Co[Formula: see text])[Formula: see text]N[Formula: see text] obtained by replacing Co with Ni to Ga. Such a strong [Formula: see text] can also be achieved with the replacement by Al, which is isoelectronic to Ga, with simultaneous improvement of the phase stability. These results provide an instructive guideline for simultaneous manipulation of [Formula: see text] and the thermal stability in 3d-only metals for RE-free permanent magnet applications.
ABSTRACT
New (1-x)Bi0.5Na0.5TiO3 + xCaFeO3-δ solid solution compounds were fabricated using a sol-gel method. The CaFeO3-δ materials were mixed into host Bi0.5Na0.5TiO3 materials to form a solid solution that exhibited similar crystal symmetry to those of Bi0.5Na0.5TiO3 phases. The random distribution of Ca and Fe cations in the Bi0.5Na0.5TiO3 crystals resulted in a distorted structure. The optical band gaps decreased from 3.11 eV for the pure Bi0.5Na0.5TiO3 samples to 2.34 eV for the 9 mol% CaFeO3-δ-modified Bi0.5Na0.5TiO3 samples. Moreover, the Bi0.5Na0.5TiO3 samples exhibited weak photoluminescence because of the intrinsic defects and suppressed photoluminescence with increasing CaFeO3-δ concentration. Experimental and theoretical studies via density functional theory calculations showed that pure Bi0.5Na0.5TiO3 exhibited intrinsic ferromagnetism, which is associated with the possible presence of Bi, Na, and Ti vacancies and Ti3+-defect states. Further studies showed that such an induced magnetism by intrinsic defects can also be enhanced effectively with CaFeO3-δ addition. This study provides a basis for understanding the role of secondary phase as a solid solution in Bi0.5Na0.5TiO3 to facilitate the development of lead-free ferroelectric materials.
ABSTRACT
Two-dimensional (2D) structures that exhibit intriguing magnetic phenomena such as perpendicular magnetic anisotropy and its switchable feature are of great interests in spintronics research. Herein, the density functional theory studies reveal the critical impacts of strain and external gating on vacancy-induced magnetism and its spin direction in a graphene-like single layer of zinc oxide (ZnO). In contrast to the pristine and defective ZnO with an O-vacancy, the presence of a Zn-vacancy induces significant magnetic moments to its first neighboring O and Zn atoms due to the charge deficit. We further predict that the direction of magnetization easy axis reverses from an in-plane to perpendicular orientation under a practically achievable biaxial compressive strain of only ~1-2% or applying an electric field by means of the charge density modulation. This magnetization reversal is mainly driven by the strain- and electric-field-induced changes in the spin-orbit coupled d states of the first-neighbor Zn atom to a Zn-vacancy. These findings open interesting prospects for exploiting strain and electric field engineering to manipulate magnetism and magnetization orientation of 2D materials.
ABSTRACT
We systematically investigate the effects of having Pt as a substrate and/or capping layer on the magnetism and magnetocrystalline anisotropy (MCA) of 3d transition metal (TMs; Cr, Mn, Fe, and Co) monolayers (MLs) by using a first-principles calculationl method. We found that Fe and Co MLs are ferromagnetic (FM) on a Pt(001) surface, but Mn and Cr MLs are antiferromagnetic (AFM). The magnetic moments are quite robust with additional Pt-capping. Furthermore, Pt-capping enhances the small perpendicular MCA (meV) of Fe/Pt(001) significantly to 4.44 meV. Our electronic structure analyses indicate that strong hybridization between Pt-5d and TM-3d orbitals plays a crucial role in determining magnetic ordering and MCA. For comparison we also calculated magnetism and MCA of 3d TM MLs on Ta(001) with and without Ta-capping.
