850 research outputs found
Emergent half-metal with mixed structural order in (111)-oriented (LaMnO3)2n|(SrMnO3)n superlattices
Using first-principles techniques, we study the structural, magnetic, and electronic properties of (111)-oriented (LaMnO3)2n|(SrMnO3)n superlattices of varying thickness (n=2,4,6). We find that the properties of the thinnest superlattice (n=2) are similar to the celebrated half-metallic ferromagnetic alloy La2/3Sr1/3MnO3, with quenched Jahn-Teller distortions. At intermediate thickness (n=4), the a-a-a- tilting pattern transitions to the a-a-c+ tilting pattern, driven by the lattice degrees of freedom in the LaMnO3 region. The emergence of the Jahn-Teller modes and the spatial extent needed for their development play a key role in this structural transition. For the largest thickness considered (n=6), we unveil an emergent separation of Jahn-Teller and volume-breathing orders in the ground-state structure with the a-a-c+ tilting pattern, whereas it vanishes in the antiferromagnetic configurations. The ground state of all superlattices is half-metallic ferromagnetic, not affected by the underlying series of structural transitions. Overall, these results outline a thickness-induced crossover between the physical properties of bulk La2/3Sr1/3MnO3 and bulk LaMnO3
Neutron scattering and muon-spin spectroscopy studies of the magnetic triangular-lattice compounds LaNiWO ( = Sr, Ba)
We report on the geometrically frustrated two-dimensional triangular-lattice
magnets LaNiWO ( = Sr, Ba) studied mostly by means of
neutron powder diffraction (NPD) and muon-spin rotation and relaxation
(SR) techniques. The chemical pressure induced by the Ba-for-Sr
substitution suppresses the ferromagnetic (FM) transition from 6.3 K in the
Ba-compound to 4.8 K in the Sr-compound. We find that the space
group reproduces the NPD patterns better than the previously reported
space group. Both compounds adopt the same magnetic structure with
a propagation vector , in which the Ni
magnetic moments are aligned ferromagnetically along the -axis. The
zero-field {\textmu}SR results reveal two distinct internal fields (0.31 and
0.10 T), caused by the long-range ferromagnetic order. The small transverse
muon-spin relaxation rates reflect the homogeneous internal field distribution
in the ordered phase and, thus, further support the simple FM arrangement of
the Ni moments. The small longitudinal muon-spin relaxation rates, in
both the ferromagnetic- and paramagnetic states of ALaNiWO,
indicate that spin fluctuations are rather weak. Our results demonstrate that
chemical pressure indeed changes the superexchange interactions in
LaNiWO compounds, with the FM interactions being dominant.Comment: 8 pages, 7 figures; accepted by Phys. Rev. Material
Crystal structure and magnetic properties of spin- frustrated two-leg ladder compounds (CHN)Cu (= Cl and Br)
We have successfully synthesized single crystals, solved the crystal
structure, and studied the magnetic properties of a new family of copper
halides (CHN)Cu (= Cl, Br). These compounds
crystallize in an orthorhombic crystal structure with space group . The
crystal structure features Cu dimers arranged parallel to each other
that makes a zig-zag two-leg ladder-like structure. Further, there exists a
diagonal interaction between two adjacent dimers which generates inter-dimer
frustration. Both the compounds manifest a singlet ground state with a large
gap in the excitation spectrum. Magnetic susceptibility is analyzed in terms of
both interacting spin- dimer and two-leg ladder models followed by exact
diagonalization calculations. Our theoretical calculations in conjunction with
the experimental magnetic susceptibility establish that the spin-lattice can be
described well by a frustrated two-leg ladder model with strong rung coupling
( K and 300 K), weak leg coupling
( K and 105 K), and equally weak
diagonal coupling ( K and 90 K) for Cl and
Br compounds, respectively. These exchange couplings set the critical fields
very high, making them experimentally inaccessible. The correlation function
decays exponentially as expected for a gapped spin system. The structural
aspects of both the compounds are correlated with their magnetic properties.
The calculation of entanglement witness divulges strong entanglement in both
the compounds which persists upto high temperatures, even beyond 370~K for the
Br compound.Comment: 13 pages, 9 figures, 2 table
Evidence of Kitaev interaction in the monolayer 1T-CrTe
The two-dimensional 1T-CrTe has been an attractive room-temperature van
der Waals magnet which has a potential application in spintronic devices.
Although it was recognized as a ferromagnetism in the past, the monolayer
1T-CrTe was recently found to exhibit zigzag antiferromagnetism with the
easy axis oriented at to the perpendicular direction of the plane.
Therefore, the origin of the intricate anisotropic magnetic behavior therein is
well worthy of thorough exploration. Here, by applying density functional
theory with spin spiral method, we demonstrate that the Kitaev interaction,
together with the single-ion anisotropy and other off-diagonal exchanges, is
amenable to explain the magnetic orientation in the metallic 1T-CrTe.
Moreover, the Ruderman-Kittle-Kasuya-Yosida interaction can also be extracted
from the dispersion calculations, which explains the metallic behavior of
1T-CrTe. Our results demonstrate that 1T-CrTe is potentially a rare
metallic Kitaev material
Intrinsic and tunable quantum anomalous Hall effect and magnetic topological phases in XYBi2Te5
By first-principles calculations, we study the magnetic and topological
properties of XYBi2Te5-family (X, Y = Mn, Ni, V, Eu) compounds. The strongly
coupled double magnetic atom-layers can significantly enhance the magnetic
ordering temperature while keeping the topologically nontrivial properties.
Particularly, NiVBi2Te5 is found to be a magnetic Weyl semimetal in bulk and a
Chern insulator in thin film with both the Curie temperature (~150 K) and full
gap well above 77 K. Ni2Bi2Te5, MnNiBi2Te5, NiVBi2Te5 and NiEuBi2Te5 exhibits
intrinsic dynamic axion state. Among them, MnNiBi2Te5 has a Neel temperature
over 200 K and Ni2Bi2Te5 even demonstrates antiferromagnetic order above room
temperature. These results indicate an approach to realize high temperature
quantum anomalous Hall effect and other topological quantum effects for
practical applications
Understanding and Tuning Magnetism in van der Waals Magnetic Compounds
The recently discovered two-dimensional (2D) magnetism has attracted intensive attention due to possible magnetic phenomenon arising from 2D magnetism and their promising potential for spintronics applications. The advances in 2D magnetism have motivated the study of layered magnetic materials, and further enhanced our ability to tune their magnetic properties. Among various layered magnets, tunable magnetism has been widely investigated in metal thiophosphates MPX3. It is a class of magnetic van der Waals (vdW) materials with antiferromagnetic ordering persisting down to atomically thin limit. Their magnetism originates from the localized moments due to 3d electrons in transition metal ions. So, their magnetic properties are strongly dependent on the choice of M. With this motivation, we synthesized metal-substituted MPX3 compounds such as Ni1-xMnxPS3 (0 ≤ x ≤ 1), Ni1-xCrxPS3 (0 ≤ x ≤ 0.09), and Fe1-xMnxPSe3 (0 ≤ x ≤ 1). The magnetic properties have been found to be very tunable with metal substitutions. Furthermore, we performed previously unexplored non-magnetic X substitution in MnPS3-xSex (0 ≤ x ≤ 3), FePS3-xSex (0 ≤ x ≤ 3), and NiPS3-xSex (0 ≤ x ≤ 1.3). Interestingly, such non-magnetic S-Se substitution also effectively modifies the magnetic exchange and anisotropy in MPX3¬. In addition to M and X substitutions, we conducted electrochemical intercalation of Li into NiPS3. We found the emergence of ferrimagnetism at low temperature in Li-intercalated NiPS3, which has never been observed due to substitution technique. Such efficient engineering of magnetism provides a suitable platform to understand low-dimensional magnetism and design future magnetic devices
Detailed temperature and angular investigation on magnetic properties of model transition-metal oxides: from flms grown on vicinal surfaces to bilayers exploiting interfacial exchange bias phenomena
Tesis Doctoral inédita leÃda en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de FÃsica de la Materia Condensada. Fecha de Lectura: 22-03-2023La era de la información en la que nos encontramos reclama cada dÃa más capacidad para procesar los datos, y una mayor velocidad y precisión a la hora de almacenar la información, a la vez que el tamaño y consumo energético se reducen. Estas exigencias han derivado en la búsqueda de tecnologÃas alternativas como la espintrónica, la cual se basa en estructuras magnéticas con las propiedades de transporte adecuadas en función de cada aplicación. Dentro de las estructuras magnéticas, aquellas que presentan transiciones metal-aislante, ofrecen la posibilidad de obtener diferentes comportamientos dentro de un mismo dispositivo en base a estÃmulos externos. Tener un completo conocimiento de las propiedades y efectos magnéticos de estos sistemas complejos es necesario para el desarrollo de los dispositivos espintrónicos del futuro.
En este trabajo se presenta un estudio sistemático de las propiedades magnéticas de dos óxidos complejos con transiciones metal-aislante controladas mediante la temperatura. Se utilizará un sistema MOKE con resolución vectorial y control de temperatura para analizar las propiedades magnéticas de ambos sistemas. Aunque en ambos casos la transición de fase se activa con temperatura, su naturaleza magnética y efectos son completamente diferentes; por ello los resultados experimentales se han dividido en dos capÃtulos. En primer lugar, se han estudiado láminas delgadas de LaSrMnO con distinta composición de Sr con el objetivo de encontrar las mejores condiciones para desarrollar un sensor magnético para aplicaciones biomédicas; este estudio se engloba dentro del proyecto Europeo ByAxon, en el cual nuestro grupo participa. El detallado estudio muestra como las propiedades magnéticas, principalmente la anisotropÃa y la temperatura de Curie, se ven afectadas por la composición y las terrazas del sustrato; de forma que se pueda maximizar el desempeño del sensor en función de las condiciones en las que va a ser utilizado. Además, se estudia el comportamiento del LSMO durante la transición de fase, remarcando la influencia en los mecanismo de inversión de imanación. Los datos experimentales se discuten dentro de los modelos de Stoner-Wohlfarth y pinning. En segundo lugar, se analiza el acoplamiento entre una lámina de V2O3 y otra capa ferromagnética de cobalto (Co). El creciente interés en los óxidos de vanadio se debe a la utilidad de la transición metal-aislante/aislante-metal en memristores, y como punto de partida para el desarrollo de la computación neuromórfica. Además, añadir a la estructura una capa de material ferromagnético permite estudiar los efectos del acoplamiento de canje entre las dos láminas sin que haya influencia de la rugosidad de la interfaz. Este acoplamiento de canje se usa en la mayorÃa de dispositivos espintrónicos, aunque aun hay contradicciones sobre su origen y efectos. Nuestros resultados resaltan la importancia que tiene en el fenómeno del acoplamiento de canje la estructura de dominios y los procesos de reversión de la imanación de la capa ferromagnética. El comportamiento del campo de acoplamiento (HE ) y la temperatura de bloqueo (TB ) se explican dentro del modelo de Malozemof y de los procesos de inversión de imanación. Por tanto, estos resultados aportan información fundamental dentro del nano magnetismo que permitirá mejorar los próximos dispositivos espintrónico
Metal-free Stoner and Mott-Hubbard magnetism in 2D polymers with honeycomb lattice
We computationally demonstrate Stoner-ferromagnetic half-metals and
antiferromagnetic Mott-Hubbard insulators in metal-free 2D polymers. Coupling
radicaloid (hetero)triangulene monomers via strong covalent bonds preserving
the in-plane conjugation of the electronic {\pi} system yields 2D crystals with
long-range magnetic order and magnetic couplings above the Landauer limit.
Dual-site honeycomb lattices produce both flat bands and Dirac cones. Depending
on the monomers, electron correlations lead to either a bandgap at the Dirac
points for antiferromagnetic Mott insulators, or Stoner ferromagnetism with
both spin-polarized Dirac cones and flat bands at the Fermi level. These
results pioneer a new type of Stoner and Mott-Hubbard magnetism emerging in the
electronic pi system of crystalline conjugated 2D polymers
Multiple Incommensurate Magnetic States in the Kagome Antiferromagnet Na2Mn3Cl8
The kagome lattice can host exotic magnetic phases arising from frustrated
and competing magnetic interactions. However, relatively few insulating kagome
materials exhibit incommensurate magnetic ordering. Here, we present a study of
the magnetic structures and interactions of antiferromagnetic
NaMnCl with an undistorted Mn kagome network. Using
neutron-diffraction and bulk magnetic measurements, we show that
NaMnCl hosts two different incommensurate magnetic states, which
develop at K and K. Magnetic Rietveld refinements
indicate magnetic propagation vectors of the form , and our neutron-diffraction data can be well
described by cycloidal magnetic structures. By optimizing exchange parameters
against magnetic diffuse-scattering data, we show that the spin Hamiltonian
contains ferromagnetic nearest-neighbor and antiferromagnetic third-neighbor
Heisenberg interactions, with a significant contribution from long-ranged
dipolar coupling. This experimentally-determined interaction model is compared
with density-functional-theory simulations. Using classical Monte Carlo
simulations, we show that these competing interactions explain the experimental
observation of multiple incommensurate magnetic phases and may stabilize
multi- states. Our results expand the known range of magnetic
behavior on the kagome lattice.Comment: 13 pages, 8 figure
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