Interface-Induced Room-Temperature Ferromagnetism in Hydrogenated Epitaxial Graphene

Abstract

Due to the predominantly surface character of graphene, it is highly suitable for functionalization with external atoms and/or molecules leading to a plethora of new and interesting phenomena. Here we show ferromagnetic properties of hydrogenfunctionalized epitaxial graphene on SiC. Ferromagnetism in such a material is not directly evident as it is inherently composed of only non-magnetic constituents. Our results nevertheless show strong ferromagnetism, which cannot be explained by simple magnetic impurities. The ferromagnetism is unique to hydrogenated epitaxial graphene on SiC, where interactions with the interfacial buffer layer play a crucial role. We argue that the origin of the observed ferromagnetism is governed by electron correlation effects of the narrow Si-dangling-bond (Si-DB) states in the buffer layer exchangecoupled to localized states in the hydrogenated graphene layer. This forms a quasithree-dimensional ferromagnet with a Curie temperature higher than 300 K.Owing to its capability of ballistic transport over micrometer distances 1 , as well as its very long spin relaxation time and spin relaxation length 2, 3 , graphene represents a close-to-ideal material for spintronic applications 4 . In this context, considerable effort has recently been directed to rendering graphene ferromagnetic via chemical modification. Thus far, ferromagnetic order in graphene has been attained through covalent functionalization, involving the linkage of radical species like the spin-bearing carbon atom of an organic molecule or hydrogen atoms to the graphene layer [5][6][7][8][9][10][11][12][13][14][15][16][17] . Along these lines, functionalization of epitaxial graphene by aryl radicals has been reported to yield disordered magnetism, comprising a mixture of ferromagnetic, superparamagnetic and antiferromagnetic regions 18 .With the aid of combined atomic and magnetic force microscopy, it could be proven that these randomly dispersed regions are constituted by the attached moieties. This lack of a periodic functionalization pattern of the graphene sheet prevents the achievement of long range ferromagnetic order, thus limiting the use of such samples in spintronic devices.Furthermore, room temperature ferromagnetism has been detected in partially hydrogenated epitaxial graphene grown on silicon carbide (SiC), and attributed to hydrogen monomers bonded to the graphene sheet 12 . Despite these accomplishments, however, both the mechanism underlying the ferromagnetic ordering, and the role played by the SiC substrate used for the epitaxial graphene growth, has not yet been clarified. Here, we experimentally demonstrate that spin ordering within hydrogenated epitaxial graphene critically depends on the presence of the underlying buffer layer. In addition, it is shown that the created magnetic 3 areas are distributed over the entire graphene sheet, thus enabling to effectively tune the overall magnetization through the density of attached hydrogen atoms.To explore the ferromagnetism in epitaxial graphene, we use samples ...

d° Ferromagnetism of Magnesium Oxide

“…Vacancy mediated magnetism occurs in CuO [17] and ZnO [18,19]; oxygen vacancy induced magnetism in zinc peroxide [20] and CeO 2 [21]; and surface vacancy induced magnetism in NiO [22]. Thus, these studies reveal that defects in the form of vacancies are the source of this behavior [11][12][13][14][15][16][17][18][19][20][21][22], however, sustainability of this magnetism up to long-range order requires exigency of the mechanism of interaction among these sources. It was observed that this kind of magnetism is governed by the double exchange mechanism through p-p interactions in Mg-doped SnO 2 [23], as well as B-, C-, and N-doped BeO systems [24].…”

“…To date, this magnetism is observed in a variety of materials with different sources of origin and is classified accordingly [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25].…”

“…Carbon dangling bonds/vacancies are responsible for RTFM in polymers and carbon-based molecular materials [12][13][14]. RTFM in Ca-and Mg-stabilized zirconia is ascribed to oxygen vacancies [15].…”

“…In case of HfO 2 , high-spin defect states in isolated cation vacancy sites couple ferromagnetically with short-range magnetic interaction to establish a ferromagnetic ground state [26]. Thus d • ferromagnetism can not only be interpreted in terms of vacancies/defects, but understanding of appropriate mechanism revealing the interaction that leads to existence of such an effect upto understanding of appropriate mechanism revealing the interaction that leads to existence of such an effect upto long-range order is another aspect [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Hence, investigation of the nature of magnetic interaction, along with the nature of defects in these materials, are important to understand the concept of d° ferromagnetism.…”

Magneto-optical reflection spectroscopy on graphene/Co in the soft x-ray range

“…from localized electron states at grain boundaries which form arrays of point defects [4]. In single layer graphene grown on SiC ferromagnetic behavior at room temperature could be detected after hydrogenization [5]. For technical applications having graphene grown on magnetic, metallic substrates seems to be a fruitful approach [6,7].…”