850 research outputs found

    Emergent half-metal with mixed structural order in (111)-oriented (LaMnO3)2n|(SrMnO3)n superlattices

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    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 A2A_2La2_2NiW2_2O12_{12} (AA = Sr, Ba)

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    We report on the geometrically frustrated two-dimensional triangular-lattice magnets A2A_2La2_2NiW2_2O12_{12} (AA = Sr, Ba) studied mostly by means of neutron powder diffraction (NPD) and muon-spin rotation and relaxation (μ\muSR) 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 R3ˉR\bar{3} space group reproduces the NPD patterns better than the previously reported R3ˉmR\bar{3}m space group. Both compounds adopt the same magnetic structure with a propagation vector k=(0,0,0)\boldsymbol{k} = (0, 0, 0), in which the Ni2+^{2+} magnetic moments are aligned ferromagnetically along the cc-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 Ni2+^{2+} moments. The small longitudinal muon-spin relaxation rates, in both the ferromagnetic- and paramagnetic states of A2_2La2_2NiW2_2O12_{12}, indicate that spin fluctuations are rather weak. Our results demonstrate that chemical pressure indeed changes the superexchange interactions in A2A_2La2_2NiW2_2O12_{12} compounds, with the FM interactions being dominant.Comment: 8 pages, 7 figures; accepted by Phys. Rev. Material

    Crystal structure and magnetic properties of spin-1/21/2 frustrated two-leg ladder compounds (C4_4H14_{14}N2_2)Cu2X6_2X_6 (XX= Cl and Br)

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    We have successfully synthesized single crystals, solved the crystal structure, and studied the magnetic properties of a new family of copper halides (C4_4H14_{14}N2_2)Cu2X6_2X_6 (XX= Cl, Br). These compounds crystallize in an orthorhombic crystal structure with space group PnmaPnma. The crystal structure features Cu2+^{2+} 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-1/21/2 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 (J0/kB≃116J_0/k_{\rm B} \simeq 116 K and 300 K), weak leg coupling (J′′/kB≃18.6J^{\prime\prime}/k_{\rm B} \simeq 18.6 K and 105 K), and equally weak diagonal coupling (J′/kB≃23.2J^{\prime }/k_{\rm B} \simeq 23.2 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-CrTe2_2

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    The two-dimensional 1T-CrTe2_2 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-CrTe2_2 was recently found to exhibit zigzag antiferromagnetism with the easy axis oriented at 70∘70^\circ 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-CrTe2_2. Moreover, the Ruderman-Kittle-Kasuya-Yosida interaction can also be extracted from the dispersion calculations, which explains the metallic behavior of 1T-CrTe2_2. Our results demonstrate that 1T-CrTe2_2 is potentially a rare metallic Kitaev material

    Intrinsic and tunable quantum anomalous Hall effect and magnetic topological phases in XYBi2Te5

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    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

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    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

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    Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de Lectura: 22-03-2023La era de la información en la que nos encontramos reclama cada día 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 tecnologías 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 estímulos 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 capítulos. 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 anisotropía 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 mayoría 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

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    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

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    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 Na2_2Mn3_3Cl8_8 with an undistorted Mn2+^{2+} kagome network. Using neutron-diffraction and bulk magnetic measurements, we show that Na2_2Mn3_3Cl8_8 hosts two different incommensurate magnetic states, which develop at TN1=1.6T_{N1} = 1.6 K and TN2=0.6T_{N2} = 0.6 K. Magnetic Rietveld refinements indicate magnetic propagation vectors of the form q=(qx,qy,32)\mathbf{q} = (q_{x},q_{y},\frac{3}{2}), 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-q\mathbf{q} states. Our results expand the known range of magnetic behavior on the kagome lattice.Comment: 13 pages, 8 figure
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