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.

Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)/Fe-C/MgO(100)/Fe(100) magnetic tunnel junctions, as a function of the magnetic state. The junctions show large tunnel magnetoresistance (185% at 300K and 330% at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe-C interface. The shot-noise shows a Poissonian character. This demonstrates a pure spin dependent direct tunneling mechanism and validates the high structural quality of the MgO barrier. PACS numbers:Magnetic tunnel junctions (MTJs) [1,2] are nowadays one of the most active areas of material science and spintronics. Recent theoretical predictions [3,4] and experimental demonstration [5,6,7,8,9] of coherent spin-dependent tunneling in single crystal Fe(100)/MgO(100)/Fe(100) MTJs revolutionized this area providing new ways to create devices with room temperature Tunneling Magneto-Resistance (RT-TMR) exceeding 100%. The large TMR at low bias is mostly due to fully spin polarized ∆ 1 bulk electron states in Fe(001), reflected for antiparrallel ferromagnetic electrodes configuration (AP) or well transmitted for the parallel (P) state [3,4]. However, the tunneling mechanism gets more complex when taking into account the electronic structure of the interfaces[10] and when biasing the junction. Therefore, for finite bias polarities the antiparallel conductance may exceed the parallel one, resulting in TMR suppression [8] or its sign reversal [10]. By engineering the chemical and electronic structure of the Fe/MgO interface, the voltage variation of the TMR in amplitude and sign can be skilfully manipulated. It has been recently demonstrated that the carbon-doping of the bottom Fe/MgO interface leads to strongly asymmetric TMR vs bias, providing a root for creation of high-output voltage device applications [9].Our Letter presents a first study of dynamical conductance and TMR in a large bias window, up to 2.7 V, for Fe(100)/Fe-C/MgO(100)/Fe(100) MTJs. The shot noise analysis in different magnetization configuration is performed at voltages up to 1V. The experiments are done at room temperature (300K) and low temperature (4K-10K). The measured TMR ratio increases from 185% at 300K to 330% at 4K, mostly due to the strong temperature variation of the tunnel conductivity in the antiparallel (AP) state. Moreover, our tunneling spectroscopy experiments show a clear maximum in the AP conductivity for a finite bias and a multiple TMR sign inversion. These experiments demonstrate the role of the minority spin Fe interface resonance state (IRS) to the tunneling. Furthermore, in both parallel (P) and antiparallel magnetization configuration, the shot noise measurements demonstrate an uncorrelated direct tunneling mechanism across the MgO barrier. The shot noise analysis and the large breakdown voltage of the junctions (up to 3V) demonstrates the high quality of our MgO barriers (i.e. absence of...

We have studied a series of Pt/Co/M epitaxial trilayers, in which Co is sandwiched between Pt and a non magnetic layer M (Pt, Ir, Cu, Al). Using polar magneto-optical Kerr microscopy, we show that the field- induced domain wall speeds are strongly dependent on the nature of the top layer, they increase going from M=Pt to lighter top metallic overlayers, and can reach several 100 m/s for Pt/Co/Al. The DW dynamics is consistent with the presence of chiral N\'eel walls stabilized by interfacial Dzyaloshinskii-Moriya interaction (DMI) whose strength increases going from Pt to Al top layers. This is explained by the presence of DMI with opposite sign at the Pt/Co and Co/M interfaces, the latter increasing in strength going towards heavier atoms, possibly due to the increasing spin-orbit interaction. This work shows that in non-centrosymmetric trilayers the domain wall dynamics can be finely tuned by engineering the DMI strength, in view of efficient devices for logic and spitronics applications.Comment: 5 pages, 4 Figure

We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations.

We report on room temperature 1/f noise in fully epitaxial Fe(45nm)/MgO(2.6nm)/Fe(10nm) magnetic tunnel junctions (MTJs) with and without carbon doping of the Fe/MgO bottom interface.We have found that the normalized noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. Both types of MTJs exhibit record low Hooge factors being at least one order of magnitude smaller than previously reported.