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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Graphene Magnetic Field Sensors Simone Pisana, Patrick M. Braganca, Ernesto E. Marinero, and Bruce A. Gurney San Jose Research Center, Hitachi Global Storage Technologies, San Jose, CA 95135 USA Graphene extraordinary magnetoresistance (EMR) devices have been fabricated and characterized in varying magnetic fields at room temperature. The atomic thickness, high carrier mobility and high current carrying capabilities of graphene are ideally suited for the detection of nanoscale sized magnetic domains. The device sensitivity can reach 10 mV/Oe, larger than state of the art InAs 2DEG devices of comparable size and can be tuned by the electric field effect via a back gate or by imposing a biasing magnetic field. Index Terms—Extraordinary magnetoresistance (EMR), graphene, magnetic sensors.
I. INTRODUCTION
T
HE SPATIALLY localized detection of magnetic fields at nanoscale resolution is a considerable technological challenge, with potential applications ranging from sensors for magnetic storage to biosensing and scanning probe microscopy. The most common approaches employ magnetoresistive devices fabricated from ferromagnetic materials such as GMR and TMR spin valves and Hall devices. However, as the size of the magnetic domain to be detected decreases, the magnetic spacing and the size of the sensor need to be reduced accordingly in order to maintain high spatial resolution and signal-to-noise ratio (SNR). As an example, the readback signal attenuation from the surface of magnetic recording media with 20 nm feature size decays exponentially with characteristic decay length of 6.4 nm. This imposes strict limitations in data densities. magnetic recording beyond 1 GMR and TMR sensors are prone to thermal magnetic noise and spin-torque instabilities as their size is reduced. Hall devices require high mobility two-dimensional electron gas channels that typically lay more than 20 nm beneath the sensor surface and hence will suffer from signal loss at the nanoscale. Graphene, a two-dimensional honeycomb lattice of carbon atoms, is a material that provides a conductive layer only one atom thick that can be located at or near a sensor surface, and has high mobility and high current carrying capabilities [1]. These characteristics uniquely position graphene as a candidate material for high spatial resolution magnetic field sensors based on the Lorentz force. In this work, we study extraordinary magnetoresistance (EMR) [2] devices consisting of hybrid graphene-metal structures in which the redistribution of current flowing through a metal shunt and graphene channel in a magnetic field modulates the resistance of the device. In previous InAs based devices, this functionality was advantageously combined with a Hall-like effect to achieve high signal by the appropriate design of the lead configuration [3]. Using a similar design, our graphene
Manuscript received October 30, 2009; accepted January 12, 2010. Current version published May 19, 2010. Corresponding author: B. A. Gurney (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2041048
devices show sensitivities on par with state of the art Hall sensors of comparable area, and do so with a sense layer that is one atom thick. The EMR device’s sensitivity is proportional to the biasing current, and we show sustained operation at current densities of up to 10 A/cm and sensitivities reaching 1000 . The response of the graphene EMR devices is dependent on the carrier type (electrons or holes) and concentration, which we vary using a buried back gate electrode. Such a tunable feature, which regulates the channel resistance and sensor sensitivity, could be used at the device level to offset device-to-device variations that are difficult to control as the sensor size is reduced. Furthermore, the device’s response to magnetic field is found to be superlinear near minimum carrier concentration, which allows for even higher magnetic field sensitivities by superimposing a static magnetic field. II. DEVICE FABRICATION The graphene EMR devices were fabricated by exfoliating HOPG graphite on 1 cm n-doped crystalline Si substrates . The low resistivity covered by 300 nm thermally grown Si serves as a back gate electrode. Single layer and bilayer graphene flakes were first selected by optical microscopy and the number of layers confirmed by Raman spectroscopy [4]. The device fabrication was completed by defining electrodes and shunt by e-beam lithography through a PMMA mask, depositing Ta/Au (2.5/20 nm) and lifting-off. Fig. 1 shows typical single layer and bilayer EMR devices and the associated Raman spectra. The Raman spectrum contains the G peak near 1580 cm , associated with the zone center mode, and the 2-D peak near 2700 cm , associated with the second order breathing mode of the bonded carbon atoms in the honeycomb lattice. The first order breathing mode, referred to as the D peak, near 1350 cm requires a defect for its activation, and can be seen when probing the graphene flake edges or when structural defects are present. The lack of the D peak in our samples is an indication of good crystalline structure. The 2-D peak does not require defects to be Raman active and can be used to count the number of graphene layers in a flake. The presence of a single lorentzian peak (Fig. 1(c)) in the 2-D band indicates single layer graphene. On the other hand, the four lorentzian components in the 2-D band in Fig. 1(d) are indicative of bilayer graphene, as determined by their relative position and intensities [5].
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Fig. 1. Single layer (a) and bilayer (b) graphene EMR devices, scale bar 500 nm. The Raman spectra at 514 nm excitation associated with devices in (a) and (b) are shown in (c) and (d), respectively. The 2-D band in the bilayer device in (d) is composed of four lorentzian components, as shown.
III. ELECTRICAL CHARACTERIZATION Electrical measurements are taken at room temperature in order to prevent in a vacuum of better than contamination of the exposed graphene layers by water and other adsorbates, which are known to dope and change the carrier concentration. In the absence of extrinsic doping, the current flowing through a graphene device is minimized at zero , indicating that the Fermi level is at back gate voltage the crossing of the valence and conduction bands, where the electron and hole carrier concentrations are equal and at their minimum. However, most as-prepared devices show signs of extrinsic doping, as indicated by the inability to probe the high back resistivity charge neutrality point with small gate potentials. When significant extrinsic doping is present, the resistivity of the device can be dominated by contact resistance, due to the very low resistance of the graphene sheet, in which case no variation in current as function of the back gate voltage is observed [see inset in Fig. 2(a)]. We find that extrinsic doping can be removed by quickly ramping the potential across a pair of electrodes ( 0.1–0.5 V/s) A/cm is reached (as meauntil a current density of sured in cross section, assuming a graphene thickness of 0.34 nm). Following the ramp, reproducible and ohmic I–V characteristics are obtained, and the charge neutrality point can be observed within back gate potentials of 20 V [Fig. 2(a)]. The procedure is repeated for all electrodes in the four terminal EMR device. An estimate of the carrier mobility of the graphene layer for each EMR device can be obtained by measuring the back gate voltage dependence of the current flowing through two adjacent electrodes using the common expression for conductivity , where e is the electron charge, n is the carrier concentration and the mobility. The estimate is carried out by assuming that most of the current flows between a pair of adjacent electrodes without spreading to the shunt so that the electrode/graphene
Fig. 2. (a) Back gate voltage dependence of the current through a pair of adjacent electrodes in a single layer graphene EMR device after current annealing (30 mV bias). The dependence before current annealing is shown in the inset (100 mV bias). (b) Mobility extraction for the same data shown in (a).
overlap and the electrode spacing define the conducting channel width W and length L, respectively. The mobility can then be written as
(1) where is the bias voltage, I is the current flowing between the contact resistance. The carrier concenthe electrodes and tration n is obtained from the back gate voltage and the capacilayer. The contact resistance and motance of the 300 nm bility do not vary with the back gate voltage except near charge , therefore the mobility is obtained by neutrality choosing the contact resistance that renders the mobility constant away from charge neutrality. Fig. 2(b) shows the result of at the above procedure, yielding a mobility of 2500 cm cm and a contact resistivity an electron concentration of cm over the contact area of overlap between of the graphene sheet and electrode. As it can be seen in Fig. 2(b),
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
the hole branch of the mobility has a back gate voltage dependence, since the contact resistance for holes is slightly larger cm in this example). than that for electrons ( For all the samples studied here, the mobility estimates vary and the resistivity varies from from 1000 to 5000 cm 2 cm to 5 cm , in good agreement with data found in the literature. IV. MAGNETIC FIELD SENSITIVITY The EMR devices were tested in a uniform magnetic field perpendicular to the plane of the sample, in the IVIV or VIVI configuration in order to maximize the sensitivity [3]. The current leads are used to inject the current in a constant bias voltage or constant bias current operating regime, with the internal current lead at ground potential. The voltage leads are used to measure , with being the internal the EMR signal voltage lead. Fig. 3 shows the variation of the differential voltage as function of the magnetic field and back gate voltage for an additional single layer device. As seen in Fig. 3(b), the current is minimized at the charge neutrality point near zero Vg, where the graphene resistivity is at its peak and electron and hole compensation takes place. The increased differential voltage near charge neutrality is therefore explained in terms of higher sheet resistivity. The dependence of the differential voltage with magnetic field (Fig. 3(c)) can be divided into three regimes. (i) , conduction takes place through holes and When the graphene sheet resistivity is low. In this case the magnetic field response is linear and dominated by the Hall effect, since the shunting mechanism responsible for the EMR response is minimized when the shunt and semiconductor resistivities are , conduction takes place comparable. (ii) When through electrons, and the linear response with magnetic field is similar as in (i), but with opposite slope, due to the change in , the conduction is shared carrier charge. (iii) When among electrons and holes and the graphene sheet resistivity is at its maximum. In this regime, a longitudinal magnetoresistance is present due to the cancellation of the Hall field, with a response given by
(2) where is the resistivity and , with p the hole concentration [6]. Furthermore, the EMR response is maximized due to the increased difference in sheet resistivity 6000 and the metal shunt, between the graphene layer resulting in a magnetic field dependence of the form [7]. near charge neutrality The measured functional form for is non-linear for large variations of magnetic field. The response is approximately linear when the magnetic field to be sensed is of the order of 500 Oe or less, as it is commonly found in magnetic storage, scanning probe microscopy and biosensing applications. In this case, the magnetic field sensitivity can be tuned by varying the back gate voltage bias (see response in ), or alternatively by applying a static Fig. 3(c) near magnetic field bias , so that the magnetic field permeating
Fig. 3. (a) Differential voltage measured at the V leads in the IVIV configuration versus back gate voltage and magnetic field for the single layer graphene device pictured in Fig. 1(a). (b) Current flowing through the I leads at zero magnetic field measured concurrently with data displayed in (a). (c) Differential voltage versus magnetic field for the sample in (a), measured with finer magnetic field increments and in the VIVI configuration. The device is operated in constant 2.3 V bias.
the sensor is of the form . This case is commonly found in the detection of superparamagnetic beads for biosensing applications [8]. In Fig. 4(a) we compare the sensitivity as function of back gate voltage for an additional single
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Fig. 5. Peak Johnson noise-limited SNR near zero magnetic field as function of bias current density for different graphene EMR devices. Full symbols represent single layer devices, empty symbols are for bilayer devices. Several current bias conditions are indicated for the same device using the same symbol.
noise spectrum [10], making them good candidates for low noise magnetic field sensors. V. CONCLUSION We have fabricated graphene EMR devices and demonstrated high magnetic field sensitivity. The atomic thickness of the graphene layer is ideally suited for detecting magnetic fields at the nanoscale. Additionally, the tunable nature of the device allows optimizing the SNR after the device is fabricated via electric fields generated by a gate or with an additional static magnetic field.
Fig. 4. Sensitivity (a) and Johnson noise-limited SNR (b) versus back gate voltage for a single layer device operated around zero magnetic field and around a 1 T magnetic bias under constant 150 A current.
layer EMR device over the range 0.5 T (no bias) and 1 0.5 T (1 T bias). Here, the data is taken over a broad 0.5 T range for clarity of results, even though the response is not strictly linear. Nonetheless, the enhancement obtained by operating the device under a bias magnetic field near charge neutrality results in a 3X increase in the signal. Similarly, the Johnson noise-limited SNR is enhanced by the increased signal (Fig. 4(b)). Here the Johnson noise voltage is evaluated by including the increase in operating temperature as a result of Joule heating following [9]. The peak Johnson noise-limited SNR obtained for several single layer and bilayer devices is shown in Fig. 5. The devices A/cm remained stable for current densities up to 1.5 throughout the duration of the measurements ( 1 hr) without appreciable changes in electrical characteristics or magnetic field sensitivities. The lower sensitivity generally found in bilayer devices does not result in low SNRs with respect to single layer devices, due to the lowered sheet resistivity. Bilayer graphene devices have been shown to have a suppressed 1/f
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