Searcharxiv⌕ Search

arXiv subjects

P. Esquinazi

Publications and source records attributed to P. Esquinazi.

At least 37 records · Page 2Linked to original sources
arXiv2012

Comment on: "Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite", by M. Sepioni, R.R. Nair, I.-Ling Tsai, A.K. Geim and I.V. Grigorieva, EPL 97 (2012) 47001

This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.

cond-mat.str-el↗
arXiv2012

Length dependence of the resistance in graphite: Influence of ballistic transport

Using a linear array of voltage electrodes with a separation of several micrometers on a $20 $nm thick and 30 $μ$m long multigraphene sample we show that the measured resistance does not follow the usual length dependence according to Ohm's law. The deviations can be quantitatively explained taking into account Sharvin-Knudsen formula for ballistic transport. This allows us to obtain without free parameters the mean free path of the carriers in the sample at different temperatures. In agreement with recently reported values obtained with a different experimental method, we obtain that the carrier mean free path is of the order of $\sim 2 μ$m with a mobility $μ\sim 10^7 $cm$^{2}$V$^{-1}$s$^{-1}$. The results indicate that the usual Ohm's law is not adequate to calculate the absolute resistivity of mesoscopic graphite samples.

cond-mat.mtrl-sci↗
arXiv2011

Comment on "Dirac cones reshaped by interaction effects in suspended graphene" by Elias et al., Nature Physics 7, 701 (2011)

Two different points of view are available to understand the behavior of graphene at low energies. One is considering a large $N_F$ that makes graphene a semimetal, and another for small $N_F < 2.5$ that would make graphene a narrow gap semiconductor (D. T. Son, Phys. rev. B 75, 235423 (2007)), a prediction supported independently by Monte Carlo simulations (J. E. Drut and T. A. Lähde, Phys. Rev. Lett. 102, 026802 (2009), see also Phys. Rev. B 79, 241405(R) (2009)). Taking into account recently obtained experimental evidence for weakly coupled graphene layers, i.e. graphite (N. García et al., arXiv/1106.0437), we tend to support the last one.

cond-mat.str-el↗
arXiv2011

Experimental Study of the Intrinsic and Extrinsic Transport Properties of Graphite and Multigraphene Samples

This work deals with the intrinsic and extrinsic properties of the graphene layers inside the graphite structure, in particular the influence of defects and interfaces. We discuss the evidence for ballistic transport found in mesoscopic graphite samples and the possibility to obtain the intrinsic carrier density of graphite, without the need of free parameters or arbitrary assumptions. The influence of internal interfaces on the transport properties of bulk graphite is described in detail. We show that in specially prepared multigraphene samples the transport properties show clear signs for the existence of granular superconductivity within the graphite interfaces. We argue that the superconducting-insulator or metal-insulator transition (MIT) reported in the literature for bulk graphite is not intrinsic of the graphite structure but it is due to the influence of these interfaces. Current-Voltage characteristics curves reveal Josephson-like behavior at the interfaces with superconducting critical temperatures above 150K.

cond-mat.mtrl-sci↗
arXiv2011

Hydrogen-induced ferromagnetism in ZnO single crystals investigated by Magnetotransport

We investigated the electrical and magnetic properties of low-energy hydrogen-implanted ZnO single crystals with hydrogen concentrations up to 3 at.% in the first 20 nm surface layer between 10 K and 300 K. All samples showed clear ferromagnetic hysteresis loops at 300 K with a saturation magnetization up to 4 emu/g. The measured anomalous Hall effect agrees with the hysteresis loops measured by superconducting quantum interferometer device magnetometry. All the H-treated ZnO crystals exhibited a negative magnetoresistance up to the room temperature. The relative magnitude of the anisotropic magnetoresistance reaches 0.4 % at 250 K and 2 % at 10 K, exhibiting an anomalous, non-monotonous behavior and a change of sign below 100 K. All the experimental data indicate that hydrogen atoms alone in a few percent range trigger a magnetic order in a ZnO crystalline state. Hydrogen implantation turns out to be a simpler and effective method to generate a magnetic order in ZnO, which provides interesting possibilities for future applications due to the strong reduction of the electrical resistance.

cond-mat.str-el↗
arXiv2011

Evidence of defect-induced ferromagnetism in ZnFe$_{2}$O$_{4}$ thin films

X-ray absorption near-edge and grazing incidence X-ray fluorescence spectroscopy are employed to investigate the electronic structure of ZnFe$_{2}$O$_{4}$ thin films. The spectroscopy techniques are used to determine the non-equilibrium cation site occupancy as a function of depth and oxygen pressure during deposition and its effects on the magnetic properties. It is found that low deposition pressures below 10$^{-3}$ mbar cause iron superoccupation of tetrahedral sites without Zn$^{2+}$ inversion, resulting in an ordered magnetic phase with high room temperature magnetic moment.

cond-mat.mtrl-sci↗
arXiv2011

Hydrogen mediated ferromagnetism in ZnO single crystals

We investigated the magnetic properties of hydrogen plasma treated ZnO single crystals by SQUID magnetometry. In agreement with the expected hydrogen penetration depth we found ferromagnetic behavior located at the first 20 nm of the H-treated surface of ZnO with magnetization at saturation up to 6 emu/g at 300 K and Curie temperature T$_c$ $\gtrsim $ 400 K. In the ferromagnetic samples a hydrogen concentration of a few atomic percent in the first 20 nm surface layer was determined by nuclear reaction analysis. The saturation magnetization of H-treated ZnO increases with the concentration of hydrogen.

cond-mat.str-el↗
arXiv2011

Enhancement of the ferromagnetic order of graphite after sulphuric acid treatment

We have studied the changes in the ferromagnetic behavior of graphite powder and graphite flakes after treatment with diluted sulphuric acid. We show that this kind of acid treatment enhances substantially the ferromagnetic magnetization of virgin graphite micrometer size powder as well as in graphite flakes. The anisotropic magnetoresistance (AMR) amplitude at 300 K measured in a micrometer size thin graphite flake after acid treatment reaches values comparable to polycrystalline cobalt.

cond-mat.str-el↗
arXiv2010

Ballistic transport at room temperature in micrometer size multigraphene

The intrinsic values of the carriers mobility and density of the graphene layers inside graphite, the well known structure built on these layers in the Bernal stacking configuration, are not well known mainly because most of the research was done in rather bulk samples where lattice defects hide their intrinsic values. By measuring the electrical resistance through microfabricated constrictions in micrometer small graphite flakes of a few tens of nanometers thickness we studied the ballistic behavior of the carriers. We found that the carriers' mean free path is micrometer large with a mobility $μ\simeq 6 \times 10^6 $cm$^2$/Vs and a carrier density $n \simeq 7 \times 10^8 $cm$^{-2}$ per graphene layer at room temperature. These distinctive transport and ballistic properties have important implications for understanding the values obtained in single graphene and in graphite as well as for implementing this last in nanoelectronic devices.

cond-mat.mtrl-sci↗
arXiv2010

Uncompensated magnetization and exchange-bias field in La$_{0.7}$Sr$_{0.3}$MnO$_3$/YMnO$_3$ bilayers: The influence of the ferromagnetic layer

We studied the magnetic behavior of bilayers of multiferroic and nominally antiferromagnetic o-YMnO$_3$ (375~nm thick) and ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.67}$Ca$_{0.33}$MnO$_3$ ($8 \ldots 225~$nm), in particular the vertical magnetization shift $M_E$ and exchange bias field $H_E$ for different thickness and magnetic dilution of the ferromagnetic layer at different temperatures and cooling fields. We have found very large $M_E$ shifts equivalent to up to 100\% of the saturation value of the o-YMO layer alone. The overall behavior indicates that the properties of the ferromagnetic layer contribute substantially to the $M_E$ shift and that this does not correlate straightforwardly with the measured exchange bias field $H_E$.

cond-mat.mtrl-sci↗
arXiv2010

ZnO:Co Diluted Magnetic Semiconductor or Hybrid Nanostructure for Spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie-Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.

cond-mat.mtrl-sci↗
arXiv2010

The role of hydrogen in room-temperature ferromagnetism at graphite surfaces

We present a x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon $π$ states, also hydrogen-mediated electronic states exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top $\approx$10 nm of the irradiated sample where the actual magnetization reaches $ \simeq 15$ emu/g at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin as the one found in proton bombarded graphite.

cond-mat.mtrl-sci↗
arXiv2010

The influence of Ga$^+$-irradiation on the transport properties of mesoscopic conducting thin films

We studied the influence of 30keV Ga$^+$-ions -- commonly used in focused ion beam (FIB) devices -- on the transport properties of thin crystalline graphite flake, La$_{0.7}$Ca$_{0.3}$MnO$_3$ and Co thin films. The changes of the electrical resistance were measured in-situ during irradiation and also the temperature and magnetic field dependence before and after irradiation. Our results show that the transport properties of these materials strongly change at Ga$^+$ fluences much below those used for patterning and ion beam induced deposition (IBID), limiting seriously the use of FIB when the intrinsic properties of the materials of interest are of importance. We present a method that can be used to protect the sample as well as to produce selectively irradiation-induced changes.

cond-mat.mtrl-sci↗
arXiv2009

Andreev reflection measurements in nanostructures with amorphous WC$_x$ superconducting contacts

Point-contact Andreev reflection measurements of Co/ and Cu/tungsten-carbide (WC$_x$) contacts are presented. Metallic thin films were patterned by e-beam-lithography and lift-off; tungsten carbide superconducting tips were grown directly on the pre-patterned samples by decomposition of a metallo-organic vapour (tungsten hexacarbonyl) under a focused Ga$^+$-ion beam (FIB). Current-voltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. The experimental conductance-voltage curves were analyzed within the Blonder-Tinkham-Klapwijk theory. The results highlight the possibilities, advantages and disadvantages of using FIB-produced amorphous WC$_x$ tips for point-contact spectroscopy in metallic nanostructures.

cond-mat.supr-con↗
arXiv2009

Changes in the electrical transport of ZnO under visible light

Complex impedance spectroscopy data in the frequency range 16Hz < f < 3 MHz at room temperature were acquired on pure ZnO single crystal and thin film. The measured impedance of the ZnO samples shows large changes with time after exposure to or covering them from visible light. At fixed times Cole-Cole-diagrams indicate the presence of a single relaxation process. A simple analysis of the impedance data allows us to obtain two main relaxation times. The behavior for both, ZnO crystal and thin film, is similar but the thin film shows shorter relaxation times. The analysis indicates the existence of two different photo-active defects with activation energies between ~0.8 eV and ~1.1 eV.

cond-mat.mtrl-sci↗
arXiv2009

On the origin of the electric carrier concentration in graphite

We investigate the dependence of the electrical resistivity of $\sim 60 $nm thick single crystalline graphite samples on the defect concentration produced by proton irradiation at very low fluences. We show that the resistivity decreases few percent at room temperature after inducing defects at concentrations as low as $\sim 0.1 $ppm due to the increase in the carrier density, in agreement with theoretical estimates. The overall results indicate that the carrier densities measured in graphite are not intrinsic but related to defects and impurities.

cond-mat.mtrl-sci↗
arXiv2009

Disordered Electrical Potential Observed on the Surface of SiO$_2$ by Electric Field Microscopy

The electrical potential on the surface of $\sim 300$ nm thick SiO$_2$ grown on single crystalline Si substrates has been characterized at ambient conditions using electric field microscopy. Our results show an inhomogeneous potential distribution with fluctuations up to $\sim 0.4 $V within regions of $1 μ$m. The potential fluctuations observed at the surface of these usual dielectric holders of graphene sheets should induce strong variations in the graphene charge densities and provide a simple explanation for some of the anomalous behaviors of the transport properties of graphene.

cond-mat.mtrl-sci↗