Magnetic gas sensing: working principles and recent developments

Gas sensors work on the principle of transforming the gas adsorption effects on the surface of the active material into a detectable signal in terms of its changed electrical, optical, thermal, mechanical, magnetic (magnetization and spin), and piezoelectric properties. In magnetic gas sensors, the change in the magnetic properties of the active materials is measured by one of the approaches such as Hall effect, magnetization, spin orientation, ferromagnetic resonance, magneto-optical Kerr effect, and magneto-static wave oscillation effect. The disadvantages of different types of gas sensors include their chemical selectivity and sensitivity to humidity and high-temperature operation. For example, in the case of chemiresistive-type gas sensors, the change in the sensor resistance can drastically vary in the real environment due to the presence of other gas species and the overall electrical effect is quite complex due to simultaneous surface reactions. Further, it is not easy to make stable contacts for powdered samples for the conventional electrical property-based gas sensors. Fire hazard is another issue for the electrical property-based hydrogen gas sensors due to their flammable nature at higher operating temperature. In this regard, to solve these issues, magnetic gas sensor concepts have emerged, in which the magnetic properties of the materials get modified when exposed to gas molecules. In this review article, the working principles, fundamentals, recent developments, and future perspectives in magnetic gas sensors are reviewed. Finally, the prospects and opportunities in these exciting fields are also commented upon based on their current progress.


Introduction
A gas-sensing system or electronic nose can qualitatively or quanticationally detect specic gases, which is important in various applications such as industrial pollutant gas leakage detection (e.g., H 2 , NO 2 , NH 3 , H 2 S, CO, and SO 2 ), environmental monitoring, medical care, food industry, and homeland security. [1][2][3][4][5][6][7] The performance of an ideal gas sensor is evaluated by its high responsivity, fast response/recovery times, great stability/ repeatability, good selectivity, room-working temperature, low cost, and easy fabrication for practical applications. [1][2][3][4][5][6][7] Gas sensors work on the principle of transforming gas adsorption on the surface of the active material into a detectable signal in terms of its changed electrical, optical thermal, mechanical, magnetic (magnetization and spin), and piezoelectric properties. Depending on these principles, many types of gas sensors with different transductions forms are developed, which include chemiresistors, eld-effect transistors (FETs), Schottky and junction diode sensors, solid-state electrochemical sensors (SSES), quartz crystal microbalance (QCM), gas capacitors, surface acoustic wave (SAW), and plasmonic and surfaceenhanced Raman spectroscopy (SERS) sensors. In chemiresistive-type gas sensor devices, the change in the system resistance or conductance due to gas adsorption is measured. [1][2][3][4][5][6][7] In magnetic gas sensors, the change in the magnetic properties of the active materials is measured by one of the approaches such as Hall effect, magnetization, spin orientation, ferromagnetic resonance, magneto-optical Kerr effect, and magnetostatic wave oscillation effect. 8,9 Surface acoustic wave gas sensing utilizes the advantage of the piezoelectric effect of the sensor material, whose frequency gets shied by the adsorption of analyte gas molecules. [10][11][12][13] Optical gas sensors detect the gas species by monitoring their optical properties such as optical absorption, transmission, refractive index, and surface plasmon effects due to the interaction of the gas molecules with the active materials. [11][12][13] New opportunities for sensing devices involve hot topics such as the internet of things (IoT) devices, wearable, exible, and self-powered devices. Hence, research activities in these areas will lead to the fabrication of low cost, low power, small size, long term stability, and selective gas sensors for the detection of dangerous gas species. [11][12][13][14] The disadvantages of different types of gas sensors include their chemical selectivity and sensitivity to humidity and hightemperature operation. For example, in the case of chemiresistive-type gas sensors, the change in the sensor resistance can drastically vary in the real environment due to the presence of other gas species and the overall electrical effect is quite complex due to simultaneous surface reactions. [11][12][13][14] Further, it is not easy to make stable contacts for powdered samples for the conventional electrical property-based gas sensors. Fire hazard is another issue for the electrical propertybased hydrogen gas sensors due to their ammability at higher operating temperature. In this regard, to solve these issues, magnetic gas sensor concepts have emerged, in which the magnetic properties of the materials get modied when exposed to gas molecules. These magnetic gas sensors are compatible with the processes involved in the fabrication of silicon devices and hence allows the integration of the sensors with on-chip devices. [15][16][17] Systematic changes in the magnetic properties and exchange coupling, when exposed to gases, are monitored by different laboratory magnetometric techniques and equipment such as superconducting quantum interference devices (SQUID), vibrating sample magnetometers (VSM), polarized neutron reectivity, X-ray resonant magnetic scattering (XRMS), Hall effect, optical Kerr effect, and ferromagnetic resonance setups. [15][16][17] Moreover, nanomaterials also play a signicant role in the further improvement of the gas sensor's performance. The nanomaterials show exceptional physical, chemical, as well as optoelectronic properties compared to their bulk counterparts. [18][19][20][21][22] The high sensing performance comes from their high surface area and controlled grain size. Both these parameters expose more surface volume ratio for the gas sensing process. Their high surface area and controlled grain size play a signicant role in achieving high sensing performance. In this review article, the working principles, fundamentals, and recent developments in magnetic gas sensors-based on different materials are reviewed. Furthermore, the future perspectives of magnetic gas sensors are discussed in the conclusion section.
2 Overview, theories, working principles, and recent developments of magnetic gas sensing Several effects of magnetism such as the Hall effect, Kerr effect, magnetization, spin change effects, the ferromagnetic resonance (FMR) effect, magneto-plasmonic effect, and magnetostatic spin-wave (MSW) effect are employed for magnetic gas sensing applications (Fig. 1). [22][23][24][25][26] Powder, thin lms, and nanomaterials with magnetic (ferro-and antiferromagnets), diluted magnetic semiconducting (DMS) properties, and Pd alloys with transition metals (Co, Ni, Fe, Mn, and Cu) have been employed for magnetic gas sensing applications using different magnetic effects. Compared to their electrical property-based gas sensor counterparts, magnetic gas sensors have emerged as the more attractive candidate due to the following reasons: (i) no electrical contacts are needed to detect the gas, which lowers the risk of explosion due to re when used in hydrogen-powered vehicles or in the presence of reactive chemicals or pollutants, (ii) magnetic response is much faster compared to chemiresistive sensors, (iii) the working temperature of the sensors can be room-temperature and can be tuned to a very low or very high temperature by choosing magnetic materials with different Curie temperatures (T c ). [27][28][29][30] 2.1 Magnetization change-based magnetic gas sensors 2.1.1 Overview, theories, and working principles. Metal oxide-based materials are known for their gas sensing applications since their magnetic parameters such as M s (saturation magnetization), M r (remanence magnetization), and H c (coercivity) are very sensitive to reducing or oxidizing gases. [29][30][31] A schematic illustration of the experimental set-up employed for magnetic gas sensing using the change in the magnetic properties is shown in Fig. 2. Usually, the set-up consists of a VSM capable of producing variable magnetic eld H and accessories for gas ow arrangements. High-temperature heating units and mass ow controllers connected to gas cylinders are arranged for controlled sensing measurements in the presence of the gases at a different temperatures under the application of magnetic gas sensing. It was observed that for a temperature < 475 K, the chemisorbed water present on the nanoparticle surface inhibits particle-gas interaction, whereas for temperature > 475 K, the chemical reduction of the gas molecules by the oxygen species on the surface resulted in an increase in the carrier concentration. Further, the Sn 0.95 Fe 0.05 O 2 nanoparticles showed stable ferromagnetic behavior aer successive gassensing cycles, suggesting their good stability and long cycling performance. Saturation magnetization and remanence of the nanoscale antiferromagnetic haematite showed an increase of one to two orders of magnitude in the presence of H 2 gas at concentrations in the range of 1-10% at 575 K. 30 The M s increased from 0.30 to 18 emu per g and M r increased from 0.10 to 5 emu per g with a varied H 2 gas (10%) ow from 0 to 200 mL min À1 . Similarly, H c increased from 60 to 250 Oe for 60 mL min À1 whereas no further change was observed with increased gas ow. X-ray photoelectron spectroscopy (XPS) analysis of the Fe 2 O 3 samples before and aer hydrogen treatment suggested that the observed change in the magnetization is due to the creation of oxygen vacancies. Density functional calculations of the MnN 4 moiety embedded graphene (MnN 4 -Gp) upon interaction with gas molecules (CO, CO 2 , NO, NO 2 , N 2 O, SO 2 , NH 3 , H 2 O, H 2 S, CH 4 , O 2 , H 2 , and N 2 ) showed obvious changes in its electronic and magnetic properties. 32 The magnetic moment of MnN 4 -Gp decreased from 3.01 m B to 0.13, 1.01, and 2.01 m B aer NO, CO, and NO 2 absorption with recovery times of 2.5 Â 10 14 s, 1.4 s, and 8275 s, respectively.
Glover et al. 33 showed the adsorption of sulfur dioxide by CoFe 2 O 4 nanoparticles and the corresponding changes in magnetism. Sulfur dioxide forms a sulfate upon adsorption on the particle surface by the chemisorption mechanism. Fig. 3a shows (transmission electron microscopy) the TEM images of CoFe 2 O 4 magnetic nanoparticles (MNPs). Fig. 3b showing the Fourier transform infrared (FTIR) spectra of the native MNPs before exposure to sulfur dioxide as well as MNPs aer exposure (test 1) and MNPs aer exposure but without desorption (tests 2 and 3). The spectra of all the three tests are nearly identical, signifying that sulfur dioxide is retained on the surface of the MNPs. The magnetic susceptibility curves for MNPs samples before and aer sulfur dioxide adsorption experiments are shown in Fig. 3c. As compared to the unexposed MNPs, the saturation magnetization decreases by 20%, the remnant magnetization decreases by 23%, and the coercivity decreases by 9% for CoFe 2 O 4 MNPs aer sulfur dioxide adsorption. The changes induced by gas adsorption are attributed to the magnetic metal cations at the surface layer of the nanoparticles that are coordinated with sulfur dioxide, which reduces the spin-orbital coupling and surface anisotropy.
The saturation magnetization and perpendicular anisotropy energy (K p ) of Co/Pd multi-layered lms are reported to change the reversibility with H 2 gas adsorption and desorption. [34][35][36][37][38] The change in M s and magnetic anisotropy are attributed to electronic transfer to the Pd band near the Co/Pd interfaces when H 2 is adsorbed. Electron transfer from Pd to the transition metal lowered the difference between the density of states of the spin-up and spin-down electrons at the Fermi level, leading to a net change in its magnetic properties. [36][37][38] Similarly, magnetic coupling in Fe/Nb and Fe/V multi-layered lms can be changed by hydrogen absorption, which was conrmed by SQUID magnetization measurements. [39][40][41] Though the method is effective, the high cost and tricky optimization of several setup parameters limit its application. 2.2 Hall effect-based magnetic gas sensors 2.2.1 Overview, theories, and working principles. Hall effect is the production of a voltage difference known as "Hall voltage" across an electrical conductor transverse to an electron current owing through the conductor and to an applied magnetic eld perpendicular to the current. Hall effect is clas-sied into ordinary Hall effect (OHE) and extraordinary Hall effect (EHE) depending on its origin in magnetic materials. Due to their advantageous properties, both OHE and EHE concepts are employed for the fabrication of magnetic gas sensors. 15,42,43 In the OHE approach, the change in the Hall voltage with and without target gas is measured. 43 The sensor responses for oxidizing and reducing gases are dened as: For oxidizing gas, For reducing gas, Where V a and V g are the Hall voltages in air and in the target gas, respectively. If only the OHE is considered, the Hall voltage due to the Lorentz force acting on the moving charge carriers is dened by eqn (3)-(5). 43 Since for the semiconductor, the Hall resistance, with b ¼ m n /m p where n and p are electron and hole concentration, m n and m p are the electron and hole mobility, respectively, and e is the elemental charge (electron). From the law of mass action, p ¼ n i 2 /n, where n i is the electron and hole concentration in an undoped semiconductor material. Using a normalized electron concentration, By using these relations, the Hall co-efficient for the active sensor materials in the presence and absence of the gases can be evaluated.
Gerber et al. 15 employed the EHE effect for magnetic H 2 gas sensing applications of the Pd/Co lm as the sensor material. By following their approach, the electric current owing along the magnetic lm generates a voltage in the direction perpendicular to the current direction.
Here, I is the current, t is the thickness of the lm, and B and M are components of magnetic induction and magnetization perpendicular to the lm, respectively. R OHE is the ordinary Hall effect coefficient related to the Lorentz force acting on moving charge carriers and R EHE is the extraordinary Hall effect coefficient associated with a break in the right-le symmetry at spin-orbit scattering in magnetic materials. Since the EHE term is signicantly high, i.e., R EHE [ R OHE , which implies that the Hall signal m 0 M with Hall resistivity, and r $ R H /t and drH dB indicate the dominance of the right hand over the le-hand spin-orbital scattering. The Co/Pd lms rich in Co exhibited +ve polarity, whereas the Pd-rich sample exhibited Àve polarity. The sensitivity of the magnetic gas sensors based on the EHE contribution can be calculated by the formula where Hammond et al. 44 designed a tin-oxide-based sensor, viz., the Hall effect sensor as shown in Fig. 4a and its cross-sectional view is presented in Fig. 4b. The sensor is made up of four layers, wherein the bottom silicon layer (0.3 mm thick) served as a support for the remaining layers. The insulator layer of silicon dioxide between the silicon substrate and the remaining sensor component is nearly 1 mm thick. The third rectangular layer of tin oxide (0.5 mm Â 2.0 mm Â 1100Å) shows good adhesion to the SiO 2 surface during deposition and operation. The fourth layer was made up of 3000Å-thick platinum, consisting of electrodes, in order to measure the conductivity, temperature, and hall voltage. The Hall effect is the induction of transverse voltage in a current-carrying conductor when the conductor is exposed to the magnetic eld. Here, the magnetic eld is applied perpendicular to the tin oxide surface using a 5403 electromagnet system. The Hall voltage (V H ) was measured before exposure to H 2 in air and aer the sensor reached the maximum conductivity level when exposed to H 2 in air. The values for conductivity (s), electron density (h), and Hall mobility (m H ) in air as a function of the operating temperature are shown in Fig. 4c. The Hall mobility remains stable compared to the variation in the conductivity and electron density. All the lms have Hall mobility value in the range of 0.9 cm 2 V À1 s À1 to 1.83 cm 2 V À1 s À1 . The experimental results indicated that the Hall mobility sensitivity slightly increased with the operating temperature and weakly depends on the H 2 concentration in air.
2.2.2 Recent developments. Undoped and doped metal oxide lms based on Hall effects are reported for selective and high-performance magnetic gas sensing applications. 21,43,45 Lin et al. 43 reported NO 2 sensing properties of WO 3 lms based on the ordinary Hall effect. The sensor displayed a selective response of 3.27 for 40 ppm NO 2 with the response and recovery times of 36 s and 45 s, respectively, based on the sensor working principles described in eqn (1)- (5). The output Hall voltage increased for oxidizing gas such as NO 2 , whereas it decreased for reducing gases such as NH 3 and H 2 S. The surface adsorbed oxygen ion species (O 2 À and O À ) played a crucial role in the enhanced magnetic NO 2 sensing performance of WO 3 . 46 Due to the n-type semiconducting nature of WO 3 (with n > n i ) and the presence of surface-adsorbed oxygen species, the following NO 2 sensing mechanisms are expected to occur.
NO 2(gas) + e À / NO 2(ads) À (10) Due to these surface reactions, the electron concentration on the surface of WO 3 decreases when the electron concentration is greater than 1.14n i . This process helped to increase the Hall coefficient, correspondingly increasing the Hall voltage in the presence of NO 2 gas. 43 Based on similar working principles, the SnO 2 nanowires-based Hall sensor showed good H 2 sensing properties with a response of 7.81 towards 1000 ppm H 2 at 125 C. 21 From the experimental observations, it has been demonstrated that the Hall coefficient of the gas sensor devices was dependent on the gas concentration and the high surface area of the nanostructures helped to achieve enhanced gassensing performance.
Lin et al. 47 reported the detection of H 2 S at room temperature using ZnO sensors based on the Hall effect. The electrical and sensing characteristics of the sensors (Fig. 5a) were detected through the bench system under ambient temperature, as shown in Fig. 5b. The sensor was placed in a magnetic eld perpendicular to the current, which is moving through the sensor. A constant voltage of 5 V was continuously supplied to the sensor circuit from a power supply and the Hall voltage of the sensor was measured using different concentrations of the gas. At room temperature, the response of the sensor increases exponentially along with increasing gas concentration (Fig. 5c). For 100 ppm H 2 S, the recovery and response time was 82 s and 35 s, respectively. As the concentration increased, the response time becomes shorter and the recovery time becomes longer (Fig. 5d). From Fig. 5e, it was observed that the Hall effect sensor displayed a negligible response to acetone and ethanol. Also, the sensor showed a higher response to 60 ppm H 2 S than that at 200 ppm NH 3 and 60 ppm NO 2 . The behavior of the sensor based on the Hall effect to H 2 S is associated with a change in the electron concentration and has potential applications in actively detecting toxic gas without using any electrical power.
Shaei and his group fabricated conductometric sensors based on an oxidized liquid metal galinstan layer and investigated their sensitivity towards NO 2 , NH 3 , and CH 4 gases. 48 Fig. 6a and b indicates the negative Hall coefficients as a function of the magnetic eld for Sensor 2. Also, a linear relationship between the sheet carrier density, ns, and magnetic eld is given. Fig. 6c indicates the higher response of Sensor 2 than Sensor 1, which is attributed to better lm coverage (as conrmed by scanning electron microscopy (SEM)) and a larger active surface area for gas interaction. A response of 7.7% and 5.4% was recorded for 12 ppm NO 2 using Sensor 2 and Sensor 1, respectively (Fig. 6d). The Hall effect measurements revealed that the p-type oxide lm inuenced the sensing response towards different gases.
Extra-ordinary Hall phenomena (EHE, eqn (6)-(9)) are employed in ferromagnetic Co/Pd lms for hydrogen gas sensing applications. 15,49 Due to the high solubility of hydrogen in Pd, it is usually chosen to make stacked layers of Pd with the transition metal for H 2 sensing. The palladium lattice expands signicantly with the absorption of H 2 (0.15% for a-phase and 3.4% for b-phase), which leads to the formation of palladium hydride; this phenomenon is usually employed for the fabrication of H 2 sensors based on Co-Pd alloys and multi-layered lms. 15,[50][51][52] Field dependence of the Hall resistivity of the Co x -Pd 1Àx lm (0 # x # 0.4) displayed that the polarity reverses between x ¼ 0.15 and x ¼ 0.2. Samples with richer Co content were found to exhibit positive polarity while samples with richer Pd content showed negative polarity. Hall effect resistance as a function of the magnetic eld for the Co x Pd 1Àx lms in H 2 (4%) showed changes in the hysteresis loop area. The Hall effect (EHE) in the optimized sample (Co 0.17 Pd 0.83 ) showed a sensitivity > 240% per 10 4 ppm at H 2 concentration below 0.5% in the hydrogen/nitrogen atmosphere, which was 2 orders of magnitude higher than that of the conductance-based sensor. From the detailed H 2 sensing studies, it was demonstrated that depending on the lm composition, thickness, and eld of operation, enhanced EHE response of the Co/Pd lms could be achieved and these types of materials emerge as an attractive candidate for magnetic gas sensing. 15,49 Hall effect-based sensors depend on the operating temperature, p-or n-type conductivity of the material, as well as on the lm composition, thickness, and eld of operation.

Kerr effect-based magnetic gas sensors
2.3.1 Overview, theories, and working principles. Magnetooptic Kerr effect (MOKE), which is related to the change in the polarization and intensity of reected light from a magnetized surface, is employed for gas sensing applications. 9,16,22,53,54 This type of magnetic gas sensing measurement is carried out by a MOKE magnetometer with the application of a very small magnetic eld ($50 Oe) at room temperature. The MOKE technique offers great advantages due to its high surfacesensitive properties and the signal is not affected by the paramagnetic and diamagnetic contributions of the substrates. A typical MOKE experimental set up along with in situ X-ray analysis and four-point probe electrical measurement are  shown in Fig. 7a-d. 16 By this approach, the change in the re-ected light beam and magnetic properties of the materials, i.e., M s , H c , and squareness (M r /M s ) of the hysteresis loop along with the perpendicular and horizontal (in-plane) directions are monitored by rotating the incident polarized light in the presence of the gas. The sensitivity of the magnetic sensor can be evaluated by 55 S ¼ (B air À B gas )/B gas (12) Where magnetic induction B ¼ M/M s . The change in the Kerr intensity of the reected light in positive and negative directions (+m and Àm) as a parabolic function of the analyzer angle (4) is represented by where E p is the p component of the reected light. Also, the relative change in the Kerr rotation angle is dened by which is used to evaluate the magnetic gas sensing by MOKE, where q k,s is the saturation Kerr rotation signal in the gas. In some reports on MOKE-based gas sensors, it is demonstrated that the changes obey 54 Dq k q k0 fP gas 1=2 (15) 2.3.2 Recent developments. ZnO nanorods arrays with different sizes grown on Co-layers have been utilized for magnetic gas sensing applications by MOKE. 22 By the MOKE magnetometer, the change in the polarization of a linearly polarized light aer reection on the magnetized sample (ZnO:Co) surface is measured. For magnetic gas sensing, the signals are collected and monitored by an Si-photoanode continuously under a constant applied eld of $50 Oe and in different concentrations of gases. Ciprian et al. 22 reported the comparative magnetic gas sensing properties of the ZnO nanorod lms with different sizes [MGS1: 100-200 nm long with irregular tips, MGS2: $1 mm long, average dia $ 20-30 nm] (Fig. 8). The sensor showed good sensitivity to all the tested gases even at the lowest concentration with a linear increase in the signal with an increase in the concentration. The sensitivity of the sensor is found to be greater than three times the standard deviation of the background noise and the detection limits are 10, 5, and 4 ppm C 3 H 6 O, CO, and H 2 , respectively. The sensors displayed response and recovery times less than 60 s with good stability and recyclability performance. The mechanism involved in magnetic gas sensing is summarized in Fig. 8e. It is demonstrated that the structural defects and oxygen vacancies in ZnO played an important role and enhanced the internal stress. The piezoelectric properties as well as the polarizability of the ZnO nanorods get modied, which resulted in the release of free electrons during the absorption of the gas molecules. These effects changed the magneto-elastic and magneto-electric contributions to the system anisotropy, transduced by Co, and led to a decrease in the magnetization. Manera et al. 56 presented a novel combination of materials, namely, a magneto-plasmonic Au/Co/Au trilayer with TiO 2 and its remarkable capabilities as a gas sensor by magneto-optical surface plasmon resonance measurements.
Pd/transition metal (Co, Fe, and Ni) bilayer, multilayer, and alloyed lms have been investigated for magnetic gas sensing by the MOKE approach. 54,55,57,58 Since upon gas exposure, the extinction angle of MOKE gets shied and the Kerr intensity signicantly increases, thus, by choosing the proper analyzer angle, enhanced sensing performance could be achieved. In a comparative MOKE study of Pd covered with Fe, Co, and Ni magnetic layers, it was demonstrated that the magneto-optical enhancement could reach up to 35-70% with hydrogen absorption depending on the magnetic layer. An enhancement of 40%, 35%, and 50-70% was obtained for Pd/Fe, Pd/Co, and Pd/Ni bilayers in 1 Â 10 5 Pa H 2 . 58 For the Pd/Co/Pd trilayer lm, hydrogenation not only increased the Kerr signal but also signicantly enhanced the Hc by 17%. 59 Lin et al. 55 reported a more pronounced reduction in the magnetization in the presence of H 2 gas for Co 14 Pd 86 because of the large Pd content around the Co-atoms. Hydrogenation introduced structural defects due to the lattice expansion and contraction of hydrogen absorption and desorption. Upon exposure of H 2 gas, the squareness of the hysteresis loop both in the perpendicular and in-plane directions showed a large transition from approximately 10% to 100%, and the saturation Kerr signal reduced to nearly 30% of the pristine value. The reversibility of transition from the hysteresis loops and the analysis of the change in the intensity of reected light happened with a response time of $2-3 s. These observations indicated the formation of palladium hydride, which transformed the short-range coupled and disordered magnetic state of the Co 14 Pd 86 lm to a long-range ordered ferromagnetic state and induced a decrease in the magnetic moment. Further, the enhancement in long-range ordering and the reduction in the magnetic moment were attributed to the change in the electronic structure in the Co 14 Pd 86 lm due to hydrogen absorption. A similar type of observation is reported for the Co (0.5 nm)/Pd (3 nm) multilayer lm. 60 For a continuous lm, H c was enhanced by 47% and the Kerr intensity was signicantly reduced to 10% of the pristine value aer hydrogenation. For the nanodots, hydrogenation led to a 25% reduction in H c , whereas for nanodot chains, the shape of the magnetic hysteresis loop could be modulated. In another report, the MOKE effect was observed in magnetic hysteresis loops of the lms with a Pd content of 61%, 76%, and 86%, whereas no observable changes were obtained in the MOKE hysteresis loops for Co-rich alloy lms. [61][62][63] For the Co 30 Pd 70 alloy lms, considerable hydrogen-induced reduction in the magnetic coercivity by a factor of 1/5 in the longitudinal direction and the enhancement in magnetic resonance to a saturation ratio from 60% to 100% were reported. 64,65 For the Co 40 Pd 60 /Cu layered lms, the M s (induced resistance change) dropped (increased) by a factor of 5 in an H 2 pressure of 75 kPa. 16 In the case of perpendicularly-magnetized Pd/Co/Pd trilayers, the hydrogenation not only increased the Kerr signal but also signicantly enhanced the Hc by 17%. 59 Lin et al. 58 reported on the reversible change of MOKE in the Pd-covered magnetic thin lms by controlling the H 2 absorption. This absorption induced reversible MOKE enhancement in Pd-covered Fe, Co, and Ni thin lms. As shown in Fig. 9a, the Kerr intensity drastically increased with H 2 exposure and saturated at $5 Â 10 3 Pa, while Pd/Co and Pd/Fe were saturated at $8 Â 10 3 and $11 Â 10 3 Pa, respectively. Fig. 9b indicates that H 2 adsorption does not affect the magnetization processes of the magnetic lms and the magneto-optical enhancement should originate from H 2 -induced change in the optical properties of Pd. Fig. 9c represents the reversibility of this H 2 adsorption effect in the Pd/Ni bilayer. The Kerr intensity reached a saturation value within a few minutes and decreased slowly aer the recovery of the vacuum. Fig. 9d indicates the time constant when the Kerr intensity recovered 80% of the H 2induced enhancement. These results indicate that the extinction angle of MOKE becomes shied and correspondingly, the Kerr intensity is signicantly increased aer H 2 exposure. This MOKE augmentation originates from the change in the optical properties of the hydrogenated Pd layer.
Hydrogenation-induced MOKE studies revealed that by tuning the thickness of the Pd layer in Pd/Fe bilayers, the MOKE extinction angle can be gradually shied by 0.6%. 66 The enhancement in the Kerr intensity reached 35-40% by exposure of 1 atm hydrogen with a different analyzer angle. Mudinepalli et al. 67 investigated the annealing effect on the hydrogenationrelated magnetic property changes of the Pd/Fe bilayer lms. It was revealed that Pd-Fe inter diffusion, i.e., alloy formation occurred at about 700 K and the H c increased by 2-3 times with an annealing temperature up to 700 K. Hydrogenation effect was observed in the 710 K annealed sample with an increase in  the in-plane H c by 10% when the hydrogen pressure was above 200 mbar. With further annealing in the range of 740-800 K, the hydrogenation effect on H c became nearly unobservable. Hydrogenation induced changes in the magnetic properties of the Fe-V nanoclusters and the bilayer lms were investigated by the MOKE effect. 24,68,69 It was revealed that the lattice constant and electronic properties of the vanadium-intermediated nanoclusters get modied by the hydrogen content, which directly inuences the exchange coupling between the Fe nanostructures. 68 Hsu et al. 25 revealed that by selecting a suitable Pd thickness under the application of a magnetic eld perpendicular to the easy axis of the bottom Fe layer, two wellsplitted hysteresis loops with almost zero Kerr remanence can be obtained. The split of the hysteresis into two double loops is reported to be due to the 90 rotation of the top-Fe moment. By the exposure to hydrogen gas, the separation of the two minor loops was increased due to the formation of the Pd-hydride formed in the space layer. These investigations suggested that the Pd space layer mediated the magnetic interlayer coupling, which was sensitive to the hydrogen atmosphere, demonstrating Fe/Pd multilayers as an emerging material for a great magnetic resistance (GMR)-type sensor for H 2 sensing.

FMR-based magnetic gas sensors
2.4.1 Overview, theories, and working principles. Perpendicular magnetic anisotropy (PMA) is known to be induced at the interface between a ferromagnetic metal (FM) layer and a non-magnetic (NM) heavy metal such as Pt or Pd in an FM/NM bilayer or multilayer lms. 17,70,71 The origins of the interface PMA are identied to be due to the (i) breaking of the crystal symmetry at the interface, (ii) interface alloying, (iii) and the effect of magnetostriction. 72,73 The rst two effects are termed as electronic contribution due to their electronic nature. The third contribution is due to the indirect elastic strain at the interface on the magnetization of the FM layer due to the NM layer. 74 This term due to PMA is usually termed as "magneto-elastic". Several reports have demonstrated that the high frequency, resonant magnetization dynamics within the NM/FM interface such as Pd/Co lms can be exploited for gas (H 2 ) sensing. 17,70,71 For example, the functionality of the Pd/Co lm-based H 2 sensor device prototype is based on a modication of the strength of PMA at the interface between Co and Pd layers upon adsorption of H 2 by Pd. 75 FMR represents an eigen excitation in the magnetic materials, which exists in the microwave range and is originated as the resonant absorption of microwave power by a magnetic sample when the frequency of the applied microwave source is equal to the resonance frequency of the material. 76 For this type of a gas sensor device, FMR is used to measure the change in PMA. By this approach, the frequency of the input signal drives the FMR constant and in the presence of a gas, the FMR peak position for the Co-layer shis to a lower applied eld H, which is due to the decrease in the PMA in the presence of the analyte gas molecule. The magnitude of the FMR peak shi is used to determine the sensitivity of the sensor. The typical experimental set up used for the FMR-based gas sensor measurements is shown in Fig. 10. 77 During the FMR measurement, the substrate with the sample is placed on top of the microstrip line with the sputtered lm facing the microstrip. The radiofrequency signal transmitted through the microstrip line is measured to register the FMR absorption. A reduction in the transmitted power at a given frequency under the application of an external magnetic eld signies the functionality of the sensor device. To improve the signal-tonoise ratio, a eld-modulated FMR method is employed, which is shown in Fig. 10b. A further enhancement in the signal-to-noise ratio is achieved by employing a microwave interferometric receiver, as shown in Fig. 10a. Typical eldresolved FMR traces obtained in pure nitrogen and H 2 gas are shown in Fig. 10c, which are used for the analysis of the gassensing performance.
2.4.2 Recent developments. By employing the FMR change effects, the PdCo bilayer or multi-layered lms are employed for selective H 2 sensing applications. 17,23,71,75 Lueng et al. 23 studied the FMR response of the Co x Pd 1Àx alloy lms (x ¼ 0.65, 0.39, 0.24, and 0.14) in the presence of H 2 gas. It was demonstrated that without any special processing, the lms with x ¼ 0.39 and 0.24 demonstrated promising H 2 gas sensing properties in a very broad concentration range from 0.05% to 100%. For x ¼ 0.24, a continuous lm of Co x Pd 1Àx , showing a 1.7 times change in the FMR peak shi when the H 2 concentration changed from 10% to 67%, was observed. Further, it is demonstrated that by properly adjusting the magnetic eld, a wide range of H 2 gas concentrations (0.2-100%) can be detected by the Pd/Co bilayer lm with better sensing performance. 77 Across this broad H 2 concentration range, the sensitivity varied by 80 times from 0.45 mV % À1 (at low concentration) to 0.01 mV % À1 (for high concentration $100%) with no signal saturation. This reveals that a proper adjustment of the magnetic eld is needed to achieve better sensing performance.
The observed decrease in the resonance linewidth upon hydrogen charging for the Pd/Co bilayer lm is explained by different possible contributions. 76 One of the proposed effects is the spintronic effect of spin pumping at the interface due to the Pd layer. 78 Upon hydrogen absorption, the conductivity of the Pd layer decreases, which results in a reduction in the spin pumping contribution to the resonance linewidth of FMR. The second contribution is proposed to be due to the variation in the Gilbert damping by the variation in the d-d hybridization at the interface. 79,80 The third effect is believed to be due to eddy current losses at the resonance linewidth upon the reduction in the conductivity of the Pd layer. 81,82 It is further demonstrated that the nano-patterning of the Pd/Co/Pd multilayered lms results in a higher sensitivity to hydrogen gas and a much faster desorption rate without any applied external magnetic eld. 17 Recently, Causer et al. 83 reported the interface-resolved materials characterization of Co (5 nm)/Pd (8 nm) magnetic H 2 gas sensors in operando during hydrogen gas cycling and revealed the physical mechanism. Combined observations from interface-sensitive polarized neutron reectometry (PNR) with in situ FMR measurements and density functional theory (DFT) calculations revealed that the spin polarization at the Fermi level of the Co/Pd structure gets modied upon H 2 gas absorption (Fig. 11). FMR experiments showed a broad spectrum that reached resonance at an applied eld of H res ¼ 819 Oe, whereas H 2 absorption changed the resonance position to a reduced eld of H res ¼ 707 Oe. DFT calculations supported the experimental ndings and indicated that an out-of-plane expansion of the Pd layer ($7.5% increase in the overall thickness of the Pd layer upon exposure to H 2 ) and electronic modication of the Co/Pd lm occurs due to H 2 absorption change in the total density of states and modied spin polarization at the Fermi level was observed, which provides insights to the magnetic H 2 sensing based on FMR (Fig. 11g-i).
2.5 Magnetostatic surface spin-wave (MSSW) oscillator type magnetic gas sensors 2.5.1 Overview, theories, and working principles. Magnetostatic spin-wave (MSW)-based approaches along with the combination of a magnetic material as a sensitive layer are demonstrated to detect low concentrations of gases. 26,84 Tunable MSW oscillators possess more than sufficient sensitivity to accurately measure weak variations induced in the magnetic characteristics of the sensitive material. YIG sphere oscillators based on the yttrium iron garnet (YIG) has been used in the MSSW instrument for the development of planar spin-wave technology. 85 Based on these principles, Saniger and co-workers developed magnonic gas sensors based on magnetic (CuFe 2 O 4 ) nanoparticles lms coated on the YIG lm. 22,84 The fabrication details of the sensor devices are reported in their report, which is summarized in Fig. 12. 26 The oscillation frequency (f) of the device can be approximated by where f 0 is the unperturbated oscillation frequency, dF SL is the frequency shi due to the interaction between the sensitive magnetic layer, and the toxic gas, where g ¼ 2.8 MHz Oe À1 is the gyromagnetic constant, H SL is the static magnetic eld to be induced by the sensitive layer, and dH SL is the variation of the magnetic eld induced by the interaction between the sensitive magnetic layer and toxic gas. The experimental set-up for MSSW-based gas sensing is shown in Fig. 12d.

Recent developments.
Based on the MSSW principles and CuFe 2 O 4 nanoparticles as the magnetic layer, magnetic gas sensors have been developed. 26,84 The sensor displayed good sensing performance in terms of the sensitivity, short response time, and good reproducibility, which could detect low concentrations of different volatile compounds such as dimethylformamide, isopropanol, and ethanol or aromatic hydrocarbons such as benzene, toluene, and xylene at room temperature.
The observed frequency shis in the presence of gas vapors were attributed due to the changes in the magnetic properties of the CuFe 2 O 4 nanoparticles. Further, Matatagui et al. 84 investigated the comparative magnetic gas sensing performance of magnetic nanoparticle layers of CuFe 2 O 4 , MnFe 2 O 4 , ZnFe 2 O 4 , and CoFe 2 O 4 (Fig. 13). Typical frequency response curves in the presence of VOCs, dynamic response, and the response change in different concentrations of gas vapors are provided in Fig. 13. The applications of MSSW type sensors are mainly limited due to the complicated setup and high cost. 26,84 The advantageous features of these emerging magnetic gas sensors along with their key limitations are summarized in Table 1.

Conclusion and future perspectives
We have reviewed the basic working principles, sensing mechanisms, and recent developments in magnetic gas sensing. Compared to their electrical property-based gas sensor counterparts, magnetic gas sensors have emerged as a more attractive candidate due to the following reasons. (i) No electrical contacts are needed to detect the gas, which lowers the risk of explosion due to re when used in hydrogen-powered vehicles or in the presence of reactive chemicals or pollutants, (ii) the magnetic response is much faster compared to chemiresistive sensors, (iii) the working temperature of the sensors can be room temperature and it can be tuned to a very low or very high temperature by choosing magnetic materials with different Curie temperature (T c ). We have provided a detailed description on the several effects used for magnetic gas sensing, which includes the Hall effect, Kerr effect, magnetization, spin change effects ferromagnetic resonance (FMR) effect, magnetoplasmonic effect, and magneto-static spin-wave (MSW) effect are employed for magnetic gas sensing applications. We have also reviewed recent developments on different materials used for magnetic sensing, which includes powder, thin lms, and nanomaterials with magnetic (ferro-and antiferromagnets), diluted magnetic semiconducting (DMS) properties, and Pd alloys with transition metals (Co, Ni, Fe, Mn, and Cu). Subsequently, we have discussed the origin of enhanced sensitivity and working principles of magnetic gas sensors.
Although magnetic gas sensors based on different materials perform well, there is still a long way to go before it can be applied for further practical applications and there is plenty of room to investigate the gas sensing performances of several emerging advanced magnetic 2D materials. For example, black phosphorous, MXenes, and other groups of TMDs are not yet investigated for magnetic gas sensor device applications. Doping and modifying these 2D materials with magnetic materials can enable them to tune their magnetic properties for gas sensing applications. Tuning the number of layers and the modication of these 2D materials by different other approaches such as defect and vacancy engineering, alloying, intercalation, tuning the materials in x-y and z directions, and fabrication of heterostructures of 2D materials with a different orientation, by which the properties of the materials can be further tuned to achieve enhanced magnetic gas sensing performance. More investigations are needed to inspire efforts to address challenges such as a compatible and low-power operation in different interfering environments, stability, selectivity, speed, and multifunctionality. Piezotronic and piezophototronic effects are emerging areas in gas sensors research in recent years, which may be applied to magnetic gas sensor devices by choosing appropriate magnetic materials. Further, the fabrication of exible, wearable, and self-powered gas sensor design is another important research area that needs to be explored in magnetic gas sensor devices. Further, research in this eld by using active materials with high mechanical exibility will create new dimensions and possibilities in the area of wearable magnetic gas sensor systems. In this regard, novel emerging materials in magnetic gas sensor device conguration and technologies need to be explored.

Conflicts of interest
The authors declare no conict of interest.