119 research outputs found
Giant magnetic and optical anisotropy in cerium-substituted M-type strontium hexaferrite driven by 4 electrons
By performing density functional calculations, we find a giant
magnetocrystalline anisotropy (MCA) constant in abundant element cerium (Ce)
substituted M-type hexaferrite, in the energetically favorable strontium site,
assisted by a quantum confined electron transfer from Ce to specific iron (2a)
site. Remarkably, the calculated electronic structure shows that the electron
transfer leads to the formation of Ce and Fe at the site
producing an occupied Ce() state below the Fermi level that adds a
significant contribution to MCA and magnetic moment. A half Ce-substitution
forms a metallic state, while a full substitution retains the semiconducting
state of the strontium-hexaferrite (host). In the latter, the band gap is
reduced due to the formation of charge transferred states in the gap region of
the host. The optical absorption coefficient shows an enhanced anisotropy
between light polarization in parallel and perpendicular directions. Calculated
formation energies, including the analysis of probable competing phases, and
elastic constants confirm that both compositions are chemically and
mechanically stable. With successful synthesis, the Ce-hexaferrite can be a new
high-performing critical-element-free permanent magnet material adapted for use
in devices such as automotive traction drive motors.Comment: 10 pages, 6 figure
First-order ferromagnetic transitions of lanthanide local moments in divalent compounds: An itinerant electron positive feedback mechanism and Fermi surface topological change
Around discontinuous (first-order) magnetic phase transitions the strong
caloric response of materials to the application of small fields is widely
studied for the development of solid-state refrigeration. Typically strong
magnetostructural coupling drives such transitions and the attendant
substantial hysteresis dramatically reduces the cooling performance. In this
context we describe a purely electronic mechanism which pilots a first-order
paramagnetic-ferromagnetic transition in divalent lanthanide compounds and
which explains the giant non-hysteretic magnetocaloric effect recently
discovered in a EuIn compound. There is positive feedback between the
magnetism of itinerant valence electrons and the ferromagnetic ordering of
local -electron moments, which appears as a topological change to the Fermi
surface. The origin of this electronic mechanism stems directly from Eu's
divalency, which explains the absence of a similar discontinuous transition in
GdIn.Comment: 8 pages, 7 figure
Magnetic transition in Ni-Pt alloy Systems : Experiment and Theory
We report here the preparation and measurements on the susceptibility, sound
velocity and internal friction for Ni-Pt systems. We then compare these
experimental results with the first principle theoretical predictions and show
that there is reasonable agreement with experiment and theory.Comment: 9 pages, 5 figures. submitted to Journal of Magnetism and Magnetic
Material
Distinguishing erbium dopants in YO by site symmetry: \textit{ ab initio} theory of two spin-photon interfaces
We present a first-principles study of defect formation and electronic
structure of erbium (Er)-doped yttria (YO). This is an emerging
material for spin-photon interfaces in quantum information science due to the
narrow linewidth optical emission from Er dopants at standard telecommunication
wavelengths and their potential for quantum memories. We calculate formation
energies of neutral, negatively, and positively charged Er dopants and find the
configuration to be the most stable, consistent with experiment. Of the two
substitutional sites of Er for Y, the and , we identify the
former (with lower site symmetry) as possessing the lowest formation energy.
The electronic properties are calculated using the Perdew-Burke-Ernzerhof (PBE)
functional along with the Hubbard parameter {\color{black} and spin-orbit
coupling (SOC)}, which yields a 6 orbital and a 3
spin magnetic moment, and 11 electrons in the Er shell, confirming the
formation of charge-neutral Er. This standard density functional theory
(DFT) approach underestimates the band gap of the host and lacks a
first-principles justification for . To overcome these issues we performed
screened hybrid functional (HSE) calculations, including a negative for the
orbitals, with mixing () and screening () parameters. These
produced robust electronic features with slight modifications in the band gap
and the splittings depending on the choice of tuning parameters. We also
computed the many-particle electronic excitation energies and compared them
with experimental values from photoluminescence.Comment: 8 pages, 6 figure
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