United States

In this work we study the influence of interface parameters on the perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (DMI) of Pd(111)/Co/CoO/Pd thin epitaxial films with different oxidation depths. Epitaxial Si(111)/Cu(2 nm)/Pd(2 nm)/Co(1 nm) films were grown by molecular beam epitaxy in ultrahigh vacuum chamber. After deposition of ferromagnetic layer, films were exposed the procedure of artificial oxidation with dry oxygen. We varied the oxide thickness and this way we changed the remanent ferromagnetic thickness. Determination of the thickness of the magnetic layers carried out by measuring the saturation magnetic moment with vibrating samples magnetometry. We also used SQUID-magnetometer for low-temperature measurements. Magnetic structure was investigated by a magneto-optical Kerr effect (MOKE) microscope which was equipped with a hand-made coil applying out-of-plane magnetic fields and an in-plane electromagnet.&lt;br/&gt;Significant elastic strains appear on Pd/Co interface lead to induce strong perpendicular magnetic anisotropy [1]. Artificial oxidation of magnetic layer leads to symmetry breaking on the bottom and top magnetic interfaces in epitaxial films, which reduces compensation of interfacial DMI. It should be noted that including of oxide layer is eliminate the asymmetrical contribution of chiral dumping in comparing with symmetrical Pd/Co/Pd [2] structures, it following from symmetrical parabolic shape of domain walls velocity curves as shown on figure 1(a) and allow determine the DMI field by minimal velocity field H&lt;sub&gt;min&lt;/sub&gt;[3]. Moreover, Pd/Co/CoO thin films demonstrate strong exchange bias in out-of-plane magnetic field at a low temperature, as shown on figure 1(b).&lt;br/&gt;&lt;br/&gt;The reported study was partially funded by Council for Grants of the President of the Russian Federation (project - MK-1384.2021.1.2), the Russian Ministry of Science and Higher Education (the state task 0657-2020-0013) and grant of the Government of the Russian&lt;br/&gt;Federation (Project Proposal No.2020-220-08-4899)&lt;br/&gt;.

![](https://assets.underline.io/uploads/markdown_image/1/image/065806aae110d0b3e47020289f3a6c9f.jpg)

Dzyaloshinskii-Moriya Interaction and exchange bias in epitaxial Pd/Co/CoO films

2022 Joint MMM-INTERMAG

Dzyaloshinskii-Moriya Interaction and exchange bias in epitaxial Pd/Co/CoO films

**WELCOME TO THE 2021 CAP VIRTUAL CONGRESS UNDERLINE PLATFORM**

The CAP staff and our organizing and program committees are pleased to welcome you to our 2021 Congress filled with plenary talks, technical sessions/talks, student presentations and competition sessions, workshops, and several symposia offered by the CAP divisions and its partners.  

You will also have opportunities to network with other delegates through the Gathertown platform accessed by selecting the lounge or posters button on the left.

Registered delegates should use the schedule button to call up the details of the session/event they wish to attend.  Each sessions includes a link to access the online platform where the event is taking place, either within Underline or through other Congress-related Zoom meetings.  

Invited speaker talks are identified with an (I), and student competitors include a (G*) or (U*) to identify graduate or undergraduate presenters.

Please take the time to check out the information about our sponsors and exhibitors, request further information, or leave a note of thanks for their support.

General information about the Congress can be found on the CAP website Congress pages at https://www.cap.ca/congress/2021.

Looking for a specific talk?   A full searchable list is available on our Indico Congress site at https://indico.cern.ch/event/985448/contributions/

We look forward to sharing this exciting week with you!


CAP Virtual Congress 2021

In this work we study the influence of interface parameters on the perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (DMI) of Pd(111)/Co/CoO/Pd thin epitaxial films with different oxidation depths. Epitaxial Si(111)/Cu(2 nm)/Pd(2 nm)/Co(1 nm) films were grown by molecular beam epitaxy in ultrahigh vacuum chamber. After deposition of ferromagnetic layer, films were exposed the procedure of artificial oxidation with dry oxygen. We varied the oxide thickness and this way we changed the remanent ferromagnetic thickness. Determination of the thickness of the magnetic layers carried out by measuring the saturation magnetic moment with vibrating samples magnetometry. We also used SQUID-magnetometer for low-temperature measurements. Magnetic structure was investigated by a magneto-optical Kerr effect (MOKE) microscope which was equipped with a hand-made coil applying out-of-plane magnetic fields and an in-plane electromagnet. Significant elastic strains appear on Pd/Co interface lead to induce strong perpendicular magnetic anisotropy [1]. Artificial oxidation of magnetic layer leads to symmetry breaking on the bottom and top magnetic interfaces in epitaxial films, which reduces compensation of interfacial DMI. It should be noted that including of oxide layer is eliminate the asymmetrical contribution of chiral dumping in comparing with symmetrical Pd/Co/Pd [2] structures, it following from symmetrical parabolic shape of domain walls velocity curves as shown on figure 1(a) and allow determine the DMI field by minimal velocity field Hmin[3]. Moreover, Pd/Co/CoO thin films demonstrate strong exchange bias in out-of-plane magnetic field at a low temperature, as shown on figure 1(b). The reported study was partially funded by Council for Grants of the President of the Russian Federation (project - MK-1384.2021.1.2), the Russian Ministry of Science and Higher Education (the state task 0657-2020-0013) and grant of the Government of the Russian Federation (Project Proposal No.2020-220-08-4899) .

![](https://assets.underline.io/uploads/markdown_image/1/image/065806aae110d0b3e47020289f3a6c9f.jpg)

Dzyaloshinskii-Moriya Interaction and exchange bias in epitaxial Pd/Co/CoO films

poster

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Manipulations of micro-sized magnetic particles by an external field have been widely studied for the applications in MEMS and biofluidics. A popular way of manipulation is forming a microchain containing several particles in a liquid environment. The chain can be well controlled by a designated external field to perform rotation [1] or oscillation [2] for desired purposes. Nevertheless, the induced hydrodynamic drag would affect the formation and motion of the chain, such as the well studied chain rupture in a dynamic field. In the present paper, study of motion synchronicity of a micro-magnetic-particle chain in a rotating field is focused. To achieve synchronous rotation with the external field, force balance acting on the chain is required. Experiments are performed firstly to identify actual rotating frequency of an un-ruptured chain, denoted as fc, subjected to various field frequency fH. As the results shown in figure, for a 3-particle chain, motion synchronicity can only preserved up to a critical field frequency. Beyond this critical field frequency, the average rotating frequency of the chain starts to decay. The critical field frequency increases if magnetic field strength H is raised. The mechanism of this asynchronous motion can be understood by the competition of magnetic torque and hydrodynamic torque. The predicted critical frequencies, based on two approximations reported in the literatures [3,4], i.e., fc1 and fc2 shown in the figure, agree well with the experiments. In addition, temporal dynamical motion is also presented to illustrate the asynchronous motion.

![](https://assets.underline.io/uploads/markdown_image/1/image/fa77da4a4f02a4fcfe4bf92f2f888ada.jpg)

Actual rotating frequency of a 3-particle chains fc subjected to various field frequency fH. Marks of circle (O) and cross (X) are the predicted critical frequency for correspondent field strength H. The linear lines inductae synchronous motion of the chain and external field. Beyond a critical frequency, the actual rotating frqeuency of chain fc starts to drop. The predicted critical frquencies for different field strength H agree well with the experiments.

Motion Synchronicity of a Micro-magnetic-particle Chain in a Rotating Field

Magnetite is one of the oldest known magnetic materials. At room temperature, magnetite exhibits an inverse spinel structure with Fe3+ ions split between tetrahedral and octahedral sites, and Fe2+ ions occupying only octahedral sites. As it is well known, with decreasing temperature, magnetite undergoes a first order magnetic transition accompanied by a metal-insulator transition: the so-called Verwey transition (~120 K) [1]. However, there is another magnetic transition taking place at even lower temperatures (&lt; 50 K), whose underlaying nature is still not well understood. In bulk magnetite, a few studies have attributed this transition to different causes, such as the freezing of conduction electrons or rearrangement of electrons within domain walls [2,3]. Of particular interest is the case of magnetite nanoparticles, whose magnetic properties have been extensively studied in the last decades due to their multiple applications, especially in biomedicine [4]. However, a clear understanding on the presence of this transition in these nanoparticles has remained elusive [5]. Therefore, this work has been focused on the study of the low T transition in several types of magnetite nanoparticles, with different sizes, shapes, and purities. These nanoparticles have been structurally and morphologically characterized by using a combination of X-ray Diffraction and Transmission Electron Microscopy. Different oxidation states of the nanoparticles (i.e., into maghemite) have been revealed. Their magnetic response has been analyzed both as a function of the temperature (M vs. T, 5-300 K) and the magnetic field (M vs. H, up to 5 T). Some of these nanoparticles have exhibited a magnetic response close to the bulk, with saturation magnetization ~90 Am2/kg (300 K) and a clear Verwey transition. The low T transitions have been studied by AC susceptibility. In all cases, we have observed 1 or 2 magnetic transitions below 50 K, even for single domain nanoparticles. The nature of these transitions has been further investigated by using different spin relaxation models. Arrhenius law fittings suggest electron hopping freezing (τ0 = 10-8 – 10-9 s, and EB/kB = 200-300 K) for the nanoparticles with magnetic behavior closer to the bulk.

Magnetite Nanoparticles: A Detailed Analysis of the Low Temperature Magnetic Transitions

Over the past few years, the outstanding magnetic properties of highly crystalline iron oxide (Fe3O4) nanorods (NRs) have been extensively investigated by us for advanced magnetic hyperthermia [1], as well as spintronics applications [2]. Building upon this success, we propose an effective thermal treatment approach to creating a novel class of iron oxide core/shell structured nanorods. In this study, we demonstrate how Fe3O4/α-Fe2O3 core/shell structures can be formed in these nanorods through a controlled annealing process. The highly crystalline Fe3O4 nanorods were hydrothermally grown first and then annealed at 250° C for different periods of time (1, 3, 5, 7, and 9 h) to form Fe3O4/α-Fe2O3 core/shell structures, which are characterized by XRD and TEM. Magnetometry measurements indicate that the Verwey transition becomes sharper with increasing the annealing duration, which is attributed to the improved crystallinity of the Fe3O4 phase. Relative to the as-synthesized NRs, the saturation magnetization of the annealed NRs is reduced considerably, which is resulted from the reduced volume fraction of the Fe3O4 core and the formation of the antiferromagnetic α-Fe2O3 shell. When the annealing duration is extended to 5 h, a sharp drop in magnetization is observed in the temperature-dependent magnetization around 260 K which is associated with the Morin transition of the α-Fe2O3 layer. Interestingly, zero-field-cooled and field-cooled magnetic hysteresis loops show a negligible horizontal shift but a significantly large enhancement in magnetization was observed when the core/shell NRs are cooled down in a magnetic field of 1T from the room temperature, suggesting the presence of an intermediate layer composed of canted/disordered spins in between the core and shell. These findings indicate that the Fe3O4/α-Fe2O3 core/shell nanorods would be an excellent model system for probing interfacial magnetism at the nanoscale [3].

Coexistence of Verwey and Morin transitions and emerging magnetic phenomena in iron oxide core/shell nanorods

Multifunctional nanomaterials are an essential requirement to prepare smart sensor- or actuator-based devices [1]. Some of multifunctional materials can be found in the large group of Heusler alloys. Ni2FeGa and Ni2MnGa-based Heusler alloys are well known for their magnetocaloric and shape memory behavior [2], [3]. Moreover, their properties depend on the overall chemical composition, which makes them tunable [4]. Fabrication of magnetocaloric and shape memory Heusler alloys-based nanowires might be the next step to prepare a nanodimensional sensing and actuation device. Furthermore, these materials show a multifunctional behavior. It makes them suitable for preparation of a simultaneous sensing and actuation device. This research aims to describe the magnetocaloric and ferromagnetic shape memory behavior of electrochemically fabricated Heusler alloy-based nanowires with the chemical composition of Ni-Fe-Ga and Ni-Mn-Ga (Figure 1). Preparation of the Ni-Fe-Ga and Ni-Mn-Ga nanowires in an array introduces a magnetostatic interaction between the adjacent nanowires into the system. Combined with their shape anisotropy, it is possible to prepare nanowires with a highly efficient magnetocaloric behavior. The magnetic entropy change of &gt;0.2 J/kg.K was observed for the Ni-Fe-Ga nanowires (Figure 2). The Ni-Mn-Ga nanowires show an inverse magnetocaloric effect with the magnetic entropy change of even higher values. The magnetic field with the strength of up to 2 T was applied in the parallel direction with respect to the individual nanowires' longitudinal axes for both the Ni-Fe-Ga and Ni-Mn-Ga nanowires. Moreover, the structural transformation region associated with the magnetocaloric and shape memory processes of the Ni-Fe-Ga and Ni-Mn-Ga nanowires was observed above room temperature. Therefore, application of the prepared nanowires can be found in the temperature region of the human body or at the working temperatures of the majority of electrotechnical devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/b29b34a7e23d9b52b1b711bb6310561a.jpg)

Fig.1: SEM micrograph of the electrochemically prepared Ni-Mn-Ga nanowires

![](https://assets.underline.io/uploads/markdown_image/1/image/5274a388b3f35e6f19e87abe71b08bd8.jpg)

Fig.2: Magnetic entropy change of the magnetocaloric Ni-Fe-Ga nanowires at different applied magnetic fields

Magnetocaloric Effect and Ferromagnetic Shape Memory Behavior of Electrodeposited Nanowires Array

As the number of biomedical applications of superparamagnetic iron-oxide nanoparticles (SPION) grows, having a detailed understanding of their magnetic state is increasingly important. It is commonly assumed that dipole-dipole interactions are not relevant for superparamagnetic nanoparticles because of thermal fluctuations. Here we show evidence that dipole-dipole interactions do indeed affect the collective magnetic state of a SPION sample. Ferromagnetic resonance spectroscopy of iron-oxide nanoparticles with a diameter of 10 nm reveals that the configuration of the particles has a strong effect on their effective magnetic anisotropy on short time scales. Fitting the experimental spectra using a theoretical model [1] enables the quantification of these anisotropy fields. Nanoparticles in suspension with a surfactant exhibit an easy-axis anisotropy due to field-induced alignment, whereas condensed dry nanoparticles (which no longer have the surfactant) exhibit an easy-plane anisotropy due to agglomeration. The difference between the effective uniaxial anisotropies in suspension versus the dry state is on the order of 0.1 T. The effective uniaxial anisotropies exhibited by the nanoparticles in these configurations are an order of magnitude larger than the intrinsic magneto-crystalline anisotropy. This suggests that dipole-dipole interactions can be important to the collective magnetic state of superparamagnetic systems despite thermal fluctuations and should be considered for high-frequency applications.

Ferromagnetic resonance of superparamagnetic iron-oxide nanoparticles: quantification of dipole-dipole interactions

The Ga2-xFexO3 (GFxO) compound, is a quite interesting material due to simultaneous exhibit magnetic and electric orderings [1,2]. However, the physical properties of GFxO at reduced grain sizes remain underexplored due to the challenges associated with their synthesis with a proper control over the crystalline phase. In addition, the effect of the Fe/Ga ratio on the magnetic properties of the nanosized GFxO system has not yet been completely established. In this work, therefore, we prepare a series of GFxO nanoparticles with 0.8 ≤ x ≤ 1.2 using the sol-gel method to systematically study the effect of the Fe/Ga ratio on their structural and magnetic properties. All the samples of GFxO with 0.8 ≤ x ≤ 1.2 have been characterized by the powder XRD measurements and Rietveld refinements. All the observed peaks can be indexed by reflections of the Pc21n orthorhombic structure of GFxO, and no impurity phase was found. The determined lattice parameters are found to display a monotonic increase with the Fe content x. Composition of the GFxO nanoparticles was confirmed by energy dispersive X-ray spectra. The evolution of the Néel temperature (TN) of GFxO samples versus their Fe-content (x). It is found that with increasing Fe-content, up to 1.2, the TN increases from 279 to 350 K. Moreover, the magnetic hysteresis curves measured below TN exhibit two-phase-like magnetic behavior consisting of soft and hard magnetic phases. The results could be explained by the site-disorder of the cations in GFxO nanoparticles.

Compositional dependence of magnetic properties of multiferroic Ga2-xFexO3 nanoparticles

The exchange bias (EB) phenomenon, generally quantified by the shift of the hysteresis loop along the magnetic field axis, has been observed in several Mn-rich Heusler alloys [1, 2, 3]. The fundamental science that drives EB and its technological impact are both of great interest [4, 5]. The EB effect in Mn-rich Heusler alloys is primarily related to the coexisting ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Interestingly, the Mn atoms are responsible for both the FM and AFM interactions in these materials. In the Heusler alloy Mn(2)NiGa, both FM and AFM interactions are present. However, the material does not exhibit any EB effect. Interestingly, a strong EB was observed in Ga-deficient/Ni-rich Mn(2)Ni(1.4)Ga(0.6), which demonstrated the importance of crystalline site occupancies of the Mn atoms in driving EB [6]. Here, we have performed an experimental study on the effect of Fe doping on the magnetic and EB properties of Mn(2-x)Fe(x)Ni(1.6)Ga(0.4) (0 ≤ x ≤ 0.35). The goal of the study was to explore the effect of Fe doping on the FM and AFM interactions in the system. The samples were fabricated by arc melting, drop-casting, and annealing techniques. X-ray diffraction measurements and the scanning electron microscopy images showed that the alloys were phase segregated with two different lower symmetry structures. For all values of x, EB was observed in the materials. An EB field of as large as ~4000 Oe and a coercive field of ~14 kOe were observed in the materials. The structural, magnetic, and EB properties of all the alloys in the Mn(2.x)Fe(x)Ni(1.6)Ga(0.4) system will be presented.

The Magnetic and Exchange Bias Properties of Phase Segregated Mn2-xFexNi1.6Ga0.4

Self-Assembled Magnetic Nanoparticle (MNP) structures have a wide range of applications in industry, chemistry, and medicine[1]. It has been shown through experiments and computational simulation that various patterns of MNPs can be deposited on a substrate [2,3]. One mechanism for self-assembly is the gradient of an applied field exerting a force on the MNPs. For example, a single perpendicular transition of magnetization (from up alignment to down) in a computer hard drive can produce a gradient sufficient to promote a single line with a width of multiple MNPs assembling on the substrate. Therefore, the chosen magnetic structure of the substrate is a determining factor in creating the desired end structure. While simulations have been performed to understand this assembly, they lack key features found in experimental systems. This work builds upon existing computational models [3] by including small reversed regions in the magnetization, multiple transition regions in the simulated area, and changes to the total magnetic field created by the addition of a soft underlayer found in hard disk drives, as seen in Fig. 1. Along with new geometry, we also investigate the effect of varying the transition separation (distance d in Fig. 1) on the final structure. Our simulations investigate the trajectories of 27 nm diameter magnetite nanoparticles using a model based on Langevin dynamics including translation and rotation. The simulation results match those found experimentally both qualitatively and also in quantitative measures of the hexagonal ordering of assembled MNPs.

![](https://assets.underline.io/uploads/markdown_image/1/image/c5671c00cf66a50726252709d063f817.jpg)

Fig. 1. The magnetic profile of the hard and soft layers of the hard drive. With constants t1, t2, a, and d being the hard layer thickness, soft layer thickness, width of reversed magnetization, and separation of regions respectively. Above the substrate is the deposited nanoparticle structure found through simulation results using this geometry.

![](https://assets.underline.io/uploads/markdown_image/1/image/1d865be38544da32adc5e3193282044a.jpg)

Fig. 2. Scanning Electron Microscope image of magnetic nanoparticles assembled on substrate with spacing of 500 nm between the center of the reversed magnetized regions.

Self-Assembled Magnetite Nanoparticles above a hard drive: the effect of the Soft Underlayer and Changing the Separation of Reversed Magnetic Regions

Pt/Co/Ir-based synthetic antiferromagnets (SAFs) are key building blocks in high performance magnetic memory devices due to their large perpendicular magnetic anisotropy (PMA) and interlayer exchange coupling (IEC) energies, respectively, and tolerance to thermal processing [1-2]. Fortuitously, the IEC field and PMA field both can be enhanced significantly via judicious tuning of the Ir thickness [3]. Further gains in IEC and PMA may be required in emerging memory devices. And while the materials system may appear to be well optimized, additional refinement could be realized through the exploration of alternative processing approaches. In particular, the replacement of Argon with a heavier gas during sputtering (e.g. Krypton) is known to enhance the PMA of Co/Pt multilayers due to reduced bombardment of the growing film from gas species during deposition, reduced intermixing of layers and improved columnar film growth, overall enhancing the PMA [4]. To the best of our knowledge, this remains an unexplored approach for SAFs in general, and particularly not for the Pt/Co/Ir system. We investigated the role of sputter gas in the properties of a SAF structure grown by DC magnetron sputtering in an ultrahigh vacuum chamber (base pressure &lt; 10-7 Pa) on a thermally oxidized Si wafer with the following film stacking structure (thicknesses in parentheses in nm): Ta(5)/Pt(5)/[Co(0.4)/Pt(0.6)]x5 /Co(0.4)/Ir(t Ir )/[Co(0.4)/Pt(0.6)]x3 /Ru(2)/Ta(3). We varied the Ir spacer thickness between 0.3 nm and 0.7 nm to cover the first IEC peak around 0.5 nm. The sample sputtered in an 6 mTorr Kr environment exhibited a peak IEC field of 1.7 T, exceeding its Ar counterpart by over 35% (see Figure 1). This same thickness is also associated with the largest PMA (µ0HK = 5.4 T), which exceeded the Ar-sputtered counterpart by over 30% (see Figure 2). A 400°C post-annealing treatment in vacuum (&lt; 4 x 10-6 Pa) only causes a moderate reduction in the IEC field to 1.5 T. These results suggest that using Krypton as the sputtering gas could lead to superior performing SAFs for emerging memory devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/3890c9e476423eca20805db63c667207.jpg)

Out-of-plane magnetic hysteresis curves for Pt/Co/Ir SAF sputtered in 6 mTorr (a) Ar or (b) Kr

![](https://assets.underline.io/uploads/markdown_image/1/image/6b14d60039618d0b815c660023e772ea.jpg)

In-plane magnetic hysteresis loops for Pt/Co/Ir SAF

Perpendicular Magnetic Anisotropy, Interlayer Exchange Coupling and Annealing Stability in Krypton-sputtered Pt/Co/Ir Synthetic Antiferromagnets

Transition-metal oxide (TMO) based heterostructures are the fertile playground to explore or functionalize novel quantum states. In this regard, the combination of 3d and 5d TMOs have gained special interest because of the strong competition between strong spin-orbit coupling and electron correlation at the interface of those heterostructures. Additionally, the inversion symmetry breaking may result in a distinct interfacial Dzyaloshinskii-Moriya interaction (DMI) and the formation of non-collinear magnetic texture in case of magnetic TMOs. Among the 5d TMOs, SrIrO3 (SIO) has gained significant attention in recent years. It displays large spin-orbit coupling and the semi-metallic ground state is highly susceptible to structural distortions. In this study, we have grown crystalline trilayer structures consisting of SIO and a 3d ferromagnetic insulator (FMI), i. e., (SIO/FMI/SIO or FMI/SIO/FMI). In-situ observation of RHEED oscillations and x-ray diffraction indicate layer-by-layer epitaxial growth. We will report in detail on the electronic transport and magnetic properties of the these heterostructures.

Next from 2022 Joint MMM-INTERMAG

Motion Synchronicity of a Micro-magnetic-particle Chain in a Rotating Field

Similar lecture

Functionalized surfaces of graphene field effect transistors for gas sensing