Antiferromagnétisme | EPFL Graph Search

L'antiferromagnétisme est une propriété de certains milieux magnétiques prédite par Louis Néelen 1936. Contrairement aux matériaux ferromagnétiques, dans les matériaux antiferromagnétiques, l’interaction d’échange entre les atomes voisins favorise un alignement antiparallèle des moments magnétiques atomiques. Dans l'état fondamental, les moments magnétiques moyens sur les sous-réseaux distincts peuvent être non-nuls mais se compenser à l'échelle macroscopique. L'aimantation totale du matériau est alors nulle. On parle alors d'ordre antiferromagnétique ou ordre de Néel. Tout comme les ferromagnétiques, ces matériaux deviennent paramagnétiques au-dessus d’une température de transition, appelée température de Néel (versus température de Curie pour les matériaux ferromagnétiques ou ferrimagnétiques). Du microscopique au macroscopique Structure cristalline et orientation de l'aimantation Dans un matériau antiferromagnétique, on peut diviser la structure cr

Canted antiferromagnetic phases in the candidate layered Weyl material EuMnSb2

Xiao Wang

EuMnSb2 is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline 151Eu M??ssbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below TNMn = 323(1) K with the ordered Mn magnetic moment ??Mn lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below TNEu1 = 23(1) K with the ordered Eu magnetic moment ??Eu canted away from the layer normal and ??Mn retaining its higher temperature order. ??Eu is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) Pn'm'a' with the chemical and magnetic unit cells having the same dimensions. Cooling below TNEu2= 9(1) K reveals a third AFM phase where ??Mn remains unchanged but ??Eu develops an additional substantial in-plane canting. This phase a' . We also find some evidence of short-range magnetic correlations associated with the Eu between 12 K ??? T ??? 30 K. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in EuMnSb2 and their potential for tuning topological properties.

AMER PHYSICAL SOC2022

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

Since their discovery, graphene and other 2D materials have become a subject of intense research in condensed matter physics. Especially the vast possibilities of combining those materials into heterostructures are promising for the discovery of novel physical phenomena. The heterostructures accessible through state-of-the-art techniques have suitably clean interfaces between the layers. Graphene has gained a lot of attention due to its remarkable mechanical stability and extremely high electron mobility. However, the zero bandgap of graphene is a major bottleneck for implementing this 2D material into most electronic applications. The ability to tune the properties of graphene by proximity effects has shifted the focus of graphene research towards the combination of graphene with other van der Waals materials. The main objective of the present thesis was to explore for two different types of graphene-based van der Waals heterostructure whether fingerprints of electronic proximity effects can be traced in their low temperature magnetotransport properties. The first part of the thesis deals with heterostructures of graphene and Bi2Te2Se (BTS). The strong spin-orbit coupling of BTS makes it a three-dimensional topological insulator with topological surface states which are protected by time-reversal symmetry. At the same time, BTS is promising to exert a pronounced spin-orbit proximity effect on graphene, and thereby to open a band gap and/or introduce a spin texture in the latter. The graphene/BTS heterostructures were fabricated by the direct growth of BTS on graphene to ensure a clean interface between both materials and correspondingly a good electronic coupling between them. Analysis of the weak localization effect observed in the magnetoconductivity revealed the presence of enhanced SOC in the proximitized graphene. The second part of the thesis focuses on heterostructures wherein graphene is combined with a-RuCl3 which has recently gained a lot of attention as a potential quantum spin liquid system. Previous studies on graphene/a-RuCl3 heterostructures found an unusual temperature evolution of the quantum oscillation amplitude, whose origin remained unclear. The magnetotransport data collected in this thesis point toward two possible origins for this behavior, namely spin fluctuations associated with the magnetic transition into an antiferromagnetic phase at the Néel temperature of approximately 7 K, and the hybridization-induced formation of heavy (flat) bands, both of which are likely to depend on the a-RuCl3 layer thickness. In addition, heterostructures comprising an a-RuCl3 monolayer were found to display a unusual gate dependence of the quantum oscillations, further corroborating the importance of the a-RuCl3 thickness.

EPFL2020

Realization and characterization of magnetic multilayer for MFM tips

High resolution magnetic force microscopy plays an important role in the development of magnetic data storage media. Particularly the envisioned increase of the storage density to 1TBit per sqinch within the next five years requires an ongoing development of magnetic force microscopy. At that time a lateral resolution of magnetic features of 10nm is routinely achieved with sophisticated high aspect ratio tips. However, the above described needs of the hard disc industry clearly require a further development of magnetic force microscope tips. Intending an improvement of MFM tips we have developed antiferromagnetic (afm) coupled layers by separating two ferromagnetic (fm) Co films by a Ru interlayer of 0.9nm. The principle idea is to coat the SPM tips by a thin and small stripe of this multilayer along the tip taper what would lead to a dipole-like magnetic charge distribution at the tip apex.

2006

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

EPFL2020