Graphene-based synthetic antiferromagnets and ferrimagnets

Gargiani, Pierluigi; Cuadrado, R.; Vasili, Hari Babu; Pruneda, Miguel; Valvidares, Manuel

doi:10.1038/s41467-017-00825-9

Cited by 45 publications

(46 citation statements)

References 79 publications

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“…The absence of hybridization also prevents direct RKKY coupling of the ferromagnetic electrodes via graphene considered in Refs. [41,42]. In our vdW structures, the distance between two ferromagnetic layers, ~7.4Å is almost twice longer than in epitaxial structures [41,42], therefore, the interlayer RKKY interaction in our devices can be neglected, in agreement with the observed independent switching of the polarization of the two ferromagnetic films in our Co-G-Py devices.…”

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confidence: 83%

“…There is consensus between theoretical studies [10,12,38] that whether or not hybridization occurs depends strongly on two related factors, lattice matching and the distance, d FM/G , between the metal and carbon atoms. Hybridization is likely to occur for d FM/G 2 Å in latticematched structures, such as epitaxial G-Co films [41], or epitaxial G-Ir intercalated by Co [42]). For example, a recent STM and ARPES study of graphene in direct contact with Ni [43] emphasized that precise lattice matching is needed for a shorter d FM/G and hybridization.…”

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confidence: 99%

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Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

et al. 2017

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Graphene is hailed as an ideal material for spintronics due to weak intrinsic spin-orbit interaction that facilitates lateral spin transport and tunability of its electronic properties [1-3], including a possibility to induce magnetism in graphene [4-9]. Another promising application of graphene is related to its use as a spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive devices [10-20], the most prominent class of spintronic devices widely used as magnetic sensors. In particular, few-layer graphene was predicted [10-12] to act as a perfect spin filter. Here we show that the role of graphene in such devices (at least in the absence of epitaxial alignment between graphene and the FMs) is different and determined by proximity-induced spin splitting and charge transfer with adjacent ferromagnetic metals, making graphene a weak FM electrode rather than a spin filter. To this end, we report observations of magnetoresistance (MR) in vertical Co-graphene-NiFe junctions with 1 to 4 graphene layers separating the ferromagnets, and demonstrate that the dependence of the MR sign on the number of layers and its inversion at relatively small bias voltages is consistent with spin transport between weakly doped and differently spin-polarized layers of graphene. The proposed interpretation is supported by the observation of an MR sign reversal in biased Co-graphene-hBN-NiFe devices and by comprehensive structural characterization. Our results suggest a new architecture for vertical devices with electrically controlled MR. Following the successful development of graphene-based lateral spintronic structures [1-8], the implementation of graphene as a spacer in vertical magnetic tunnel junctions (MTJ) has become a subject of intense interest [13-20]. Up to now, theoretical proposals [10-12] for graphene's role in MTJs focused on the so-called 'K-point spin filtering' expected in ideally lattice-matched single-crystalline ferromagnet-graphene-ferromagnet (FM-G-FM) structures and attributed to matching spin-polarized bands in the ferromagnet and the electronic states in the graphene treated as a tunnelling barrier. This mechanism was also used to interpret the MR sign inversion observed in conventional Ni-Al 2 O 3 -Co [13] and Ni-MgO-Co [14] tunnel junctions where the Ni electrode was passivated by CVD grown (epitaxial) mono-[13] or few-layer [14] graphene. However, despite several attempts,

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confidence: 99%

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

et al. 2017

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“…XAS-XMCD. The XAS and magnetic circular dichroism experiments were carried out at the BOREAS beamline of the ALBA synchrotron using the fully circularly polarized X-ray beam produced by an apple-II type undulator (13). The base pressure during measurements was ~1×10 -10 mbar.…”

Section: Arxiv:180307443 [Cond-matmtrl-sci] 20 Mar 2018mentioning

confidence: 99%

Unraveling Dzyaloshinskii–Moriya Interaction and Chiral Nature of Graphene/Cobalt Interface

et al. 2018

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A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, that is, Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption, and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films and of the existence of chiral left-handed Néel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the spin orbit coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in (i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect face-centered cubic film, pseudomorphic with Pt and (ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates.

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“…A third example is the area of magnetism, where there are many possible applications [5]. A particularly exciting one is the creation of spring magnets using a 2D material such as graphene as a sharp and stable interface between atomically-thin films of magnetic metals [6]. In almost all of these applications, it is desirable to maximize the contact area between the metal and the 2D material, i.e.…”

Section: Introductionmentioning

confidence: 99%

Shapes of Fe nanocrystals encapsulated at the graphite surface

et al. 2020

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We describe and analyze in detail the shapes of Fe islands encapsulated under the top graphene layers in graphite. Shapes are interrogated using scanning tunneling microscopy. The main outputs of the shape analysis are the slope of the graphene membrane around the perimeter of the island, and the aspect ratio of the central metal cluster. Modeling primarily uses a continuum elasticity (CE) model. As input to the CE model, we use density functional theory to calculate the surface energy of Fe, and the adhesion energies between Fe and graphene or graphite. We use the shaft-loaded blister test (SLBT) model to provide independent stretching and bending strain energies in the graphene membrane. We also introduce a model for the elastic strain in which stretching and bending are treated simultaneously. Measured side slopes agree very well with the CE model, both qualitatively and quantitatively. The fit is optimal for a graphene membrane consisting of 2-3 graphene monolayers, in agreement with experiment. Analysis of contributions to total energy shows that the side slope depends only on the properties of graphene/graphite. This reflects delamination of the graphene membrane from the underlying graphite, caused by upward pressure from the growing metal cluster. This insight leads us to evaluate the delamination geometry in the context of two related, classic models that give analytic results for the slope of a delaminated membrane. One of these, the point-loaded circular blister test model, reasonably predicts the delamination geometry at the edge of an Fe island. The aspect ratio also agrees well with the CE model in the limit of large island size, but not for small islands. Previously, we had speculated that this discrepancy was due to lack of coupling between bending and stretching in the SLBT model, but the new modeling shows that this explanation is not viable. metal. This is challenging, since 2D materials have intrinsically low surface energies, so metals tend to grow on top of them as 3D clusters [7]. It is therefore attractive to consider synthesis strategies wherein metal morphologies are kinetically-limited [8], or the metal morphology is constrained (and stabilized) by intercalation, to promote the 2D morphology.Elsewhere, we have reported that metal nanoclusters can be synthesized at the surface of the 3D van der Waals material, graphite, in an intercalated form if two conditions are met [9][10][11][12]. First, the graphite surface must be ion bombarded to introduce defects that can act as entry portals for the metal atoms. Second, the graphite must be held at relatively high temperature while the metal is deposited, so that portals do not become blocked by growing metal clusters. Under these two conditions, we observe stable metal nanoclusters that are encapsulated beneath the graphite surface. Depending on the metal, they are a few atomic layers to hundreds of atomic layers tall, and about ten to hundreds of nm wide. We have reported the growth conditions and characteristics of such clusters in detail fo...

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Graphene-based synthetic antiferromagnets and ferrimagnets

Cited by 45 publications

References 79 publications

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

Unraveling Dzyaloshinskii–Moriya Interaction and Chiral Nature of Graphene/Cobalt Interface

Shapes of Fe nanocrystals encapsulated at the graphite surface

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