Sensor arrays used to detect electrophysiological signals from the brain are paramount in neuroscience. However, the number of sensors that can be interfaced with macroscopic data acquisition systems currently limits their bandwidth. This bottleneck originates in the fact that, typically, sensors are addressed individually, requiring a connection for each of them. Herein, we present the concept of frequency-division multiplexing (FDM) of neural signals by graphene sensors. We demonstrate the high performance of graphene transistors as mixers to perform amplitude modulation (AM) of neural signals in situ, which is used to transmit multiple signals through a shared metal line. This technology eliminates the need for switches, remarkably simplifying the technical complexity of state-of-the-art multiplexed neural probes. Besides, the scalability of FDM graphene neural probes has been thoroughly evaluated and their sensitivity demonstrated in vivo. Using this technology, we envision a new generation of high-count conformal neural probes for high bandwidth brain machine interfaces.

We report on the magnetic and structural properties of ferromagnetic-insulating La 2 CoMnO 6 thin films grown on top of (001) STO substrates by means of RF sputtering technique. Careful structural analysis, by using synchrotron X-ray diffraction, allows identifying two different crystallographic orientations that are closely related to oxygen stoichiometry and to the features (coercive fields and remanence) of the hysteresis loops.Both Curie temperature and magnetic hysteresis turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen content of the films, therefore controlling their microstructure and magnetic properties. PACS: 75.70.Ak, 81.15.Cd 4 Bulk La 2 CoMnO 6 (LCMO) double perovskite has been the subject of strong interest during the last years [ 1,2,3,4,5,6,7] due to its ferromagnetic insulating character.Besides its perspectives as magnetodielectric material, LCMO is a good candidate for active insulating barriers in spin filters. For these devices, insulating barriers must be thin enough to enable tunneling conduction. The properties of epitaxial LCMO thin films have been the subject of some theoretical and experimental works in the recent years [ 8,9,10,11]. Previous experimental reports based on films prepared by pulsed laser deposition (PLD) suggest that magnetic properties are strongly dependent on growth conditions. When samples are grown under low oxygen pressure the Curie temperature (T C ) is around 100K while increasing oxygen pressure (200 mTorr and above) T C values around 230 K can be achieved. Nevertheless, there is no clear consensus on whether this variation of T C comes from differences on the Co/Mn cationic ordering [ 10] or from changes in the oxygen stoichiometry [ 9]. On the other hand, low temperature hysteresis loops reported in the literature present anomalies and "biloop" features that have been attributed to the existence of a bidomain structure in the films. However, these "biloop" features of hysteresis cycles are present up to T C =230K and therefore, cannot be linked to the persistence of regions with low T C phase [ 10].Previous studies report low temperature hysteresis loops with a saturation magnetization close to 6  B /f.u. [ 8,9,10] [ 12,13,14] are antiferromagnetic (AF). Therefore, it is expected that the existence of antisites will reduce the saturation magnetization from 6  B /f.u.. Thus, the departure of the saturation magnetization of a given sample from this saturation value can be interpreted as a measure of the degree of Co/Mn disorder in the structure.The effect of Co/Mn disorder on the magnetic properties has been previously studied in bulk samples (where it can be precisely quantified by means of neutron powder diffraction) [ 6]. These studies show that hysteresis loops become wider (with higher coercive field and lower remanence) in the presence of disorder [ 2,4,6]. Co/Mn ordering temperature is around 1125ºC and the ordering process is blocked below 1000ºC due to extremely large relaxation...

Ferromagnetic insulating La 2 CoMnO 6- (LCMO) epitaxial thin films grown on top of SrTiO 3 (001) substrates present a strong magnetic anisotropy favoring the out of plane orientation of the magnetization with a large anisotropy field (70 kOe for film thickness of about 15 nm). Diminishing oxygen off-stoichiometry of the film enhances the anisotropy. We attribute this to the concomitant shrinkage of the out of plane cell parameter and to the increasing of the tensile strain of the films. Consistently, LCMO films grown on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 and LaAlO 3 substrates (with a larger out-ofplane lattice parameter and compressive stress) display in-plane magnetic anisotropy. Thus, we link the strong magnetic anisotropy observed in LCMO to the film stress: tensile strain favors perpendicular anisotropy and compressive stress favors in plane anisotropy. We also report on the thickness dependence of the magnetic properties. Perpendicular anisotropy, saturation magnetization and Curie temperature are maintained over a large range of film thickness. PACS: 75.30.Gw,75.50.Dd, 75.47.Lx, 75.70.Ak I IntroductionSpintronic devices rely on the use of the spin degree of freedom of the electrons as control variable and are presently based on the generation and control of highly spinpolarized currents in ferromagnetic metals [1] or, more recently, on controlling pure spin currents [2]. In this context, ferromagnetic insulators may play an important role as spin sources or spin conductors [3]. In addition, they also have emerged as potential candidates for magnetically active barriers or spin filters [1]. Ferromagnetic insulating materials are scarce since in many cases ferromagnetic interactions are of exchangetype and driven by carriers. Among these rare materials, La 2 CoMnO 6 and La 2 NiMnO 6 double perovskites have been reported to be ferromagnetic insulators when cationic ordering (Co,Ni/Mn) is achieved [4,5]. Ferromagnetic ordering in these materials relies on the Goodenough-Kanamori-Anderson rules that predict a ferromagnetic interaction between Co 2+ or Ni 2+ (t 2g 5 e g 2 and t 2g 6 e g 2 respectively) and Mn 4+ (t 2g 3 ) cations when they are ordered in a fully alternating way in the lattice [6,7,8]. In addition, they have been intensively investigated recently because of claims of magnetodielectric response 2 [9,10], which could be highly interesting for the implementation of new devices including tunable filters, magnetic sensors, and spin-charge transducers [11].Much interest is added when the magnetization of these materials is found to be out-ofplane, as the possibility of controlling perpendicular magnetic anisotropy (PMA) opens the door to the implementation of high density magnetic memory devices [12,13]. Actually, in some cases, magnetic anisotropy appears to be sensitive to lattice distortion and point towards promising spintronic applications based on the ability to control magnetic properties through modification of strain conditions. It has been proved in several material systems that t...

Multiple spin functionalities are probed on Pt/La2Co0.8Mn1.2O6/Nb:SrTiO3, a device composed by a ferromagnetic insulating barrier sandwiched between non-magnetic electrodes. Uniquely, La2Co0.8Mn1.2O6 thin films present strong perpendicular magnetic anisotropy of magnetocrystalline origin, property of major interest for spintronics. The junction has an estimated spin-filtering efficiency of 99.7% and tunneling anisotropic magnetoresistance (TAMR) values up to 30% at low temperatures. This remarkable angular dependence of the magnetoresistance is associated with the magnetic anisotropy whose origin lies in the large spin-orbit interaction of Co2+ which is additionally tuned by the strain of the crystal lattice. Furthermore, we found that the junction can operate as an electrically readable magnetic memory device. The findings of this work demonstrate that a single ferromagnetic insulating barrier with strong magnetocrystalline anisotropy is sufficient for realizing sensor and memory functionalities in a tunneling device based on TAMR.

X-ray absorption spectroscopy was used to determine the valence state in La 2 Co 1-x Mn 1+x O 6 (x≈0.23) thin films. We found that in spite of the non-stoichiometry, Co is in a divalent state while Mn ions show a mixed valence state. The relation of this finding with the magnetic properties of the films is discussed. X-ray magnetic circular dichroism measurements prove that magnetic anisotropy originates from Co spin-orbit coupling and it is strain-dependent: a strong increase of the angular contribution to the magnetic moment is found when in-plane (out-of-plane) and cell parameters get expanded (compressed). This behavior is reproduced by first order perturbation theory calculations.