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Scientific Reports Volume 6, Article Number: 21832 (2016) Cite this article

N-doped graphene with a Curie temperature higher than room temperature is a good candidate for nanomagnetic applications. Here, we report a ferromagnetic N-doped graphene with a high Curie temperature (>600 K). Four graphene samples were prepared by self-propagating high temperature synthesis (SHS), and the nitrogen content of the samples was 0 at.%, 2.53 at.%, 9.21 at.%, and 11.17 at.%, respectively. It has been found that the saturation magnetization and the coercive field increase as the nitrogen content in the sample increases. For the sample with the highest nitrogen content, its saturation magnetization reaches 0.282 emu/g at 10 K and 0.148 emu/g at 300 K; the coercivity reaches 544.2 Oe at 10 K and 168.8 Oe at 300 K . The decrease in magnetic susceptibility of N-doped graphene at ~625 K is mainly caused by the decomposition of pyrrole N and pyridine N. Our results indicate that the SHS method is an effective and high-throughput method for producing N-doped graphene with high nitrogen concentration, and the N-doped graphene produced by the SHS method is expected to be a good candidate for nanomagnetic applications.

Since Novoselov and Geim first separated graphene in 20041, graphene has attracted great attention, 2. It has been shown that graphene has many excellent properties in a wide range of fields such as energy materials, microelectronics, sensors, and superconductors, and it is worth looking forward to3,4. In recent years, researchers have discovered magnetism in doped or defective graphene or graphene oxide 5, 6, 7, which has stimulated widespread interest in the origins, influencing factors, and prospective applications of these two-dimensional materials. .

Magnetism in nanomaterials is a scientific discipline at the forefront of the rapidly emerging fields of nanoscience and nanotechnology. In current technical applications, magnetic materials are mainly d and f elements. Unexpected magnetism has been found in some low-dimensional materials. One-dimensional or multi-dimensional reduction usually leads to a reduction in the coordination number of atoms, thereby reducing the tendency of electrons to jump 8,9,10. In addition, the Coulomb interaction/bandwidth ratio is expected to increase, which contributes to the appearance of magnetism in materials of reduced size.

Research on the magnetism of graphene has established the possibility of developing magnetic materials with light weight, high strength, and high thermal conductivity. It has recently been shown that carrier-doped graphene has a very large susceptibility11. As the carrier doping of electrons or holes increases, the sensitivity decreases rapidly. Chen and Oleg V. Yazyev report that the modification of graphene with point defects leads to the realization of magnetism based on carbon nanostructures, in which ferromagnetic-antiferromagnetic transitions are possible13,14. At room temperature, in hydrogen-terminated graphene prepared by Birch reduction of graphite oxide, a small ferromagnetic signal was also observed with a magnetization of 0.006 emu/g15. Similarly, after partial reduction of graphene oxide using hydrazine, the room temperature saturation magnetization is reported to be 0.02 emu/g. It is reported that the magnetization of the sample composed of reduced graphene oxide at 300 K is 0.79 emu/g, and the value is increased to 1.99 emu/g by further annealing at 500 °C. It is interesting to find that graphene oxide (GO) has a high Curie temperature (>700 K) high room temperature ferromagnetic moment obtained by simple chemical activation with phosphoric acid followed by heat treatment, and its coercivity is less than 20 Oe6. Alternative doping is a promising method for adjusting the electronic and magnetic properties of graphene. According to reports, the room temperature magnetization of graphene embedded carbon (GSEC) films irradiated by 100 eV low-energy electrons can reach 0.26 emu/g18. According to reports, N-doped graphene can be synthesized by vacuum annealing the sandwich substrate at high temperature. Du et al. N-doped graphene has been prepared by annealing reduced graphene oxide in ammonia, which can increase its magnetization at relatively low temperatures (≤600°C)20. Li et al. It is pointed out that pyrrole N can produce a net magnetic moment of 0.95 μB/N, which is compared with pyridine N, which has less effect on edge state spin polarization. The synthetic route based on the stoichiometric dehalogenation of perhalogenated aromatic hydrocarbons and pyridine precursors by transition metals can form sp2 coordinated carbons with graphene domains, and can optionally incorporate nitrogen 22 at the pyrrole bonding site. According to reports, the Curie temperature of N-doped GO23 is about 100 K, and the magnetization at 2 K is 1.66 emu/g. The pyrrole N-doped graphene synthesized by the high-flux hydrothermal method has a doping concentration of 6.02 at.%, exhibits significant ferromagnetism, has a saturation magnetic moment (0.014 emu/g) and a narrow coercive force (181.4 Oe) 5. Therefore, the magnetism of graphene is a hot research topic because of its interesting properties and several advantages over traditional transition metal-based ferromagnetism.

The magnetic properties of graphene and related two-dimensional carbon materials can generally be explained by the presence of different types of defects 14, structural disorder, dangling bonds, or carbon edge termination 13, 24, 25, 26. It is generally believed that the ferromagnetism in the graphene system is caused by the indirect coupling between the local magnetic moments in the material, and is mediated by charge carriers 27, 28, 29, 30, 31, 32, 33. This coupling, called Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, exhibits unique behavior in graphene, which is different from that found in metal two-dimensional systems. In addition, the oscillation characteristics of the RKKY coupling between on-site impurities in graphene are controlled by the principle of relating the sign of the coupling integral to the sublattice, in which two magnetic impurities 35, 36, 37 are placed. These systems include nanoflakes with zigzag and armchair edges and two sublattices with equal numbers of carbon atoms and graphene with two types of edges 38, 39, 40. Defects in dense areas are expected to contribute to ferromagnetic coupling, while the number of defects located in adjacent locations should increase as the defect density increases. Therefore, defects can cause local moments and play an important role in the ferromagnetism of graphene. However, the long-range interactions between these local moments that cause ferromagnetism are still controversial. It is not clear why there is such a strong interaction in these N-doped samples, which leads to an increase in the Curie temperature. In addition, although the magnetic properties of graphene-based materials have been extensively studied, ferromagnetic graphene materials with a Curie temperature much higher than room temperature and higher coercivity are rare.

Self-propagating high-temperature synthesis (SHS) is a relatively simple, fast, low-cost and efficient new material production method, so it has received considerable attention. It is also used to produce certain advanced ceramics, composite materials, intermetallic compounds and carbon nanotubes 42, 43, 44. As an alternative to traditional furnace technology, SHS usually refers to an exothermic reaction caused by a short thermal pulse (ignition), which then propagates due to strong heat release and heat transfer from the hot part to the cold part, forming a combustion wave. The temperature of combustion may be very high (eg 5000 K), and the wave propagation speed may be very fast (eg 25 cm/s), so this process provides for the study of reactions under extreme thermal gradient conditions (eg 105 K/cm). SHS is widely used in the production of various materials, including carbon-free materials. This method has not yet been applied to synthesize CNTs45; as we all know, magnesium can be used as a carbon reducing agent to produce carbon with various solid structures; for example, magnesium has been used as a reduction in combustion synthesis It is also used to react with CaCO3 to produce thin-layer graphene 49 with PTFE flake graphite 46 and CO47 and CO2 (dry ice). However, as far as we know, our group first developed a high-throughput SHS method for synthesizing few-layer graphene and doping nitrogen into the few-layer graphene50, 51.

Here, we used a patented method based on self-propagating high temperature synthesis (SHS) to prepare four samples of few-layer graphene with different nitrogen content. We are particularly interested in the N-doped graphene sample because it has unexpected magnetic properties without adding 3d and 4f elements. The N-doped graphene samples produced by the SHS method exhibit high Curie temperature (above room temperature) and high coercivity. Therefore, it is considered a good candidate for nanomagnetic applications.

Magnesium (200 mesh, 99.0%), calcium carbonate (CaCO3, 99.5%), urea (CO(NH2)2, 99.5%) and carbon dioxide (99.9% purity) were used as starting materials. These materials were purchased from Sinopharm Chemical Reagent Co., Ltd.

Three N-doped graphene samples (samples 1 to 3) with different nitrogen content were synthesized by SHS patented method. The schematic diagram of the reaction device is shown in Figure 1. In a typical preparation process of sample 1, 8 grams of urea (30 grams for sample 2 and 21 grams for sample 3) was added to 14.4 grams of magnesium mixed powder to neutralize 33.3 grams of calcium carbonate, and then ground in a mortar for 20 minutes. The reaction is carried out in a crucible. The crucible is placed in a 21.2 L steel container and carried out under atmospheric carbon dioxide atmosphere, as shown in Figure 1. The reactant mixture is ignited by a direct current ignition device. The power supply and resistance wire heater use 22 A ignition current. The reaction spontaneously propagates through the mixture in the crucible from the top to the bottom in the form of a combustion wave, and terminates when the combustion wave reaches the bottom. The black crude product was then purified by dilute hydrochloric acid (10 v/v %) and stirred for two hours to remove MgO, CaO and remaining Mg metal. The product was then filtered and washed with deionized water and absolute ethanol. The thinner flakes in the product are further separated by centrifuging at 1000 rpm for 30 minutes using a centrifuge and filtering the supernatant. Finally, the sample was vacuum dried at 120 °C for 24 hours. For comparison, the original graphene sample (sample 0) was also prepared by SHS from 16 grams of magnesium and 33.3 grams of calcium carbonate, which is based on the reaction stoichiometric ratio: 2 Mg CaCO3 = 2 MgO CaO C; no urea was introduced in the reaction, the product Use the same method as above.

Schematic diagram of the reaction device.

The phase composition of the prepared powder was analyzed by powder X-ray diffraction (XRD) analysis (Philips X'Pert diffractometer) and CuKα radiation. Use environmental scanning electron microscope (ESEM, Helios Nanolab 600i) and high resolution transmission electron microscope (HRTEM JEM-2100) to observe the morphology of graphene sheets. TEM samples were prepared by dropping an ethanol/water (38 v/v%) solution containing 1 wt% N-doped graphene onto a copper mesh and drying at 100°C. The Raman spectrum was obtained using a Raman station (B&WTEK, BWS435-532SY). The 532 nm wavelength laser corresponds to 2.34 eV, and 30% laser power (total power: 240 mW) is used for the sample. X-ray photoelectron spectroscopy (XPS, Thermo Fisher) is used to determine the adhesion properties of the sample. All XPS peaks are calibrated based on the C 1 s peak (284.6 eV). Use X-ray fluorescence (XRF, AXIOS-PW4400) to confirm the composition to determine if any metal elements are present. Spread 2 mg of N-doped graphene powder on the surface of boric acid powder (99.0%, Sinopharm Chemical Reagent Co., Ltd.), and the effective test area is a disc surface with a diameter of 20 mm. Magnetism is measured using a Quantum Design MPMS magnetometer based on a superconducting quantum interference device (SQUID).

Figure 2(ac) shows the morphology of the original graphene and N-doped graphene sheets detected by SEM. The original graphene and N-doped graphene sheets have wrinkles and a 3D continuous structure. The 3D structure in Figure 2(a) is composed of many tiny flakes; for N-doped graphene sheets with higher nitrogen content (samples 2 and 3), the sheets are curved and more expanded, as shown in Figure 2(b) ) And Figure 2(c).

Characterization of N-doped graphene samples.

(Ac) SEM images of original and N-doped graphene sample 0, sample 2 and sample 3, respectively. (d) TEM image of sample 0. (e) TEM image of sample 2. (f) HRTEM of sample 2. (g) XRD of original and N-doped graphene samples. (h) Raman spectra of original and N-doped graphene samples. (i) The intensity ratio of ID/IG and I2D/IG in Raman spectra of original and N-doped graphene.

TEM observations are used to further study the high magnification morphology and crystal structure of pristine graphene and N-doped graphene sheets. A typical TEM image of pristine graphene sheets is shown in Figure 2(d). As shown in Figure 2(d), the graphene sheet is thin and wrinkled. In the TEM and HRTEM images of N-doped graphene (sample 2), a large number of edges of the sheet can be seen. According to the HRTEM observation in Figure 2(f), the number of layers of N-doped graphene is usually between 1 and 5.

The XRD spectrum of N-doped graphene in Figure 2(g) shows a broad diffraction peak at about 25.9°, which corresponds to the (002) diffraction of several layers of graphene. For samples 0 and 1, the diffraction at 42.7° The peak is a characteristic of (100) diffracted graphite. The weak peak intensity in Figure 2(g) indicates that the average number of layers of graphene sheets obtained is less than the number of layers 48,52 obtained before using similar methods.

The Raman spectra of the original and N-doped graphene sheets are shown in Figure 2(h). Three peaks generated by graphene can be clearly found in the spectra. According to previous studies, the peaks around 1585, 1340, and 2677 cm-1 correspond to G-band, D-band, and 2D-band 53,54, respectively. In Raman spectroscopy, the D-band, called disorder or defect mode, originates from the edge configuration in graphene, where the planar sheet configuration is destroyed55,56. The G-band is the result of the first-order scattering of the E2g mode in the sp2 carbon domain. Both bands will be affected by doping. Compared to the original graphene and sample 1, the D bands in samples 2 and 3 have been significantly enhanced, because pyrrole and pyridine N usually accompany defects or edges in the graphene sheet. Interestingly, the G-band of sample 2 and sample 3 moved to higher frequencies, which is a sign of high-density N-doping; in the work of Zhao et al., a similar trend in the G-band was also observed 57.

Both 2D and G-band features are highly correlated with the number of layers and defects. The D-band and G-band intensity ratio (ID/IG) is a measure of the number of defects58. The prepared sample showed a wider peak at 2677 cm-1, which is a characteristic of graphene19,59,60,61. As shown in Figure 2(i), the value of ID/IG in graphene increases with the increase of urea content in the reactant, which indicates that the defect concentration in N-doped graphene increases with the increase in N content in the reactant. Increase. sample. This is because the N atom destroys the sp2 C six-membered ring structure in graphene to form pyrrole or pyridine nitrogen 62. The 2D-band and G-band intensity ratio (I2D/IG) can be used to estimate the number of graphene sheets. The I2D/IG values ​​of sample 2 and sample 3 are both higher than that of sample 1, indicating that the thickness of sample 2 and sample 3 is thinner. In addition, the pyrrole N-bond configuration also weakens the 2D mode, because the strength of the 2D mode depends on the electron doping concentration, which leads to the inversion of I2D/IG and electrons. Therefore, the reason why the I2D/IG value of sample 3 is lower than that of sample 2 may be the higher concentration of pyrrole N in sample 3 (see also XPS analysis).

XPS is a common technique for measuring the doping concentration and N bond configuration in N-doped graphene. As shown in Figure 3(a), the XPS spectra of pristine graphene and N-doped graphene show a main C 1s peak near 284.4 eV, a weak O 1 s peak near 532.0 eV, and Ca 2p near 313.1 eV Peak, Mg 1 s peak is around 1225.1 eV, N 1 s peak in N-doped graphene is around 400.0 eV. The spectrum is analyzed by the X-peak software, and the Shirley algorithm is used to correct the background signal before the analysis of the curve63.

(a) XPS measurement spectra of sample 0 (black) and sample 2 (red). (b) XPS C 1 s spectrum and (c) XPS N 1 s spectrum of sample 2. (d) O content in N-doped graphene samples. (e) N content in N-doped graphene samples. (f) The ratio of pyrrole N to pyridine N in the N-doped graphene sample.

The C 1 s peak of the sample in XPS can be divided into three peaks, located at 284.3, 285.0, and 287.9 ​​eV (the fitting uses Gaussian decomposition and Lorentz decomposition). The main peak at 284.3 eV corresponds to the graphite-like sp2 hybrid carbon (CC) 53,64. The second peak at 285.0 eV is attributed to the sp3 hybrid C atom bonded to O, N or (C)3-N. , This may be derived from CO, pyrrole or graphite N-bond configuration. In addition, unlike N-doped graphene grown directly by a CVD system, a new weak peak at 287.9 ​​eV can be observed in our sample. This peak indicates that the sp2 hybrid C atom is bonded to N, and the CNC bond is derived from graphite or pyridine N bond configuration 65. Similarly, the high-resolution XPS spectrum of the N 1 s peak at 398.3 eV in Figure 3(c) is assigned to the sp2 hybrid aromatic N and two sp2 hybrid C neighbors, in the form C = NC (pyridine N) The sum peak at 400.4 eV is designated as tertiary N in the form of N-(C)3 or HN-(C)2 (pyrrole N). These allocations are consistent with previous reports66. Since another possible N bond configuration (graphite N bond, sp2 hybridized N atom and three sp2 hybridized C adjacent atoms) in the N-doped graphene lattice, the peak position will appear at about 402.0 eV, which indicates Graphite N-bond configurations are very limited in our research. N-doped graphene. The pyridine bond configuration refers to the bond between N and two C atoms at the defect or edge of N-graphene. The pyrrole N bond refers to the bonding of N atoms in a five-membered ring structure. Compared with the XPS spectra captured by the original graphene (sample 0), these changes observed for the C 1 s orbitals in the N-doped graphene indicate that the N-doping behavior does indeed occur in the graphene lattice to a certain extent.

Based on the XPS analysis, the N and O contents of the N-doped graphene sample are shown in Fig. 3(d, e). The N content of samples 1, 2 and 3 were about 2.53 atomic%, 9.21 atomic%, and 11.17 atomic%, respectively, and the O content was about 2.32 atomic%, 3.79 atomic%, and 4.02 atomic%, respectively. The ratio of pyrrole nitrogen to pyridine nitrogen is shown in Figure 3(e). It can be seen that as the nitrogen content increases from 2.53 at.% to 11.17 at.%, the ratio of pyrrole to pyridine N also increases from 1.69 to 2.67.

The magnetization and magnetic field (MH) curves of the original and N-doped graphene samples measured at room temperature (300 K) are shown in Figure 4(ad). All magnetic data are corrected for the background signal from the sample holder.

(ad) M and H curves of original and N-doped graphene at room temperature. (e) Hc of N-doped graphene at 10 K, 300 K, and 400 K. (f) Ms of N-doped graphene at 10 K, 300 K, and 400 K.

The hysteresis loop shown in Figure 4(ad) clearly shows the ferromagnetism of the original and N-doped graphene sheets. The coercive force (Hc) and saturation magnetization (Ms) can be obtained from the hysteresis loop in Figure 4(ad). The Ms values ​​of samples 0, 1, 2 and 3 at 10 K are 0.072, 0.275 and 0.318 emu/g, respectively; the Hc values ​​of samples 1, 2 and 3 at room temperature are 63.1, 143.7 and 168.8 Oe, respectively. The strange thing is that the sum of Hc and Ms (0.125 emu/g and 117.8 Os) of sample 0 is higher than that of sample 1 and lower than that of samples 2 and 3, which means that the original graphene has higher ferromagnetism than the sample. Samples with low N content (2.53 at%). Considering the result in Fig. 2(i), the result can be understood, which indicates that the ID/IG ratio of the original graphene is also higher than that of the sample, but lower than that of the samples 2 and 3. Since it is well recognized that defects play a major role in ferromagnetism, and the ratio of ID/IG corresponds to the defect concentration, the higher ferromagnetism of sample 0 can be understood from the perspective of defect concentration.

This result also shows that the introduction of urea can change the reaction temperature (the decomposition of urea requires a lot of energy, and the reaction enthalpy of the reaction with and without urea varies greatly) and the reaction atmosphere. There may be two reaction mechanisms with and without urea. different. Therefore, the reference value of sample 0 is limited.

The coercivity and saturation magnetization at different temperatures are shown in Figure 4 (e, f). It can be clearly seen that at each temperature, the coercivity of the three N-doped samples increases with the increase of the N content in the N-doped graphene. For sample 3 with the highest N content, the coercivity reached 544.2 Oe at 10 K and 168.8 Oe at 300 K. The coercivity of the three samples decreases with increasing temperature.

The Ms value and the coercivity value of the four samples have similar trends. They decrease with increasing N content and temperature. For sample 3, the Ms value reaches 0.282 emu/g at 10 K and 0.148 emu/g at 300 K, which is the same as the graphene obtained in free-standing MoS2 nanosheets or other dilute magnetic semiconductors without dopants quite. In our case, the key finding is the experimental observation of ferromagnetism in metal-free N-doped graphene at different temperatures. The results clearly show that the appearance of ferromagnetism and the values ​​of coercivity and remanence are positively correlated with the N content in N-doped graphene.

XRF measurement is used to check the content of ferromagnetic impurities in our samples. The specific ferromagnetic impurities are 8.9 ppm Fe and 5.0 ppm Ni, as shown in Figure S3, so the total content of ferromagnetic impurities in N-graphene is 13.9 ppm, which does not exceed 15 ppm. If it is assumed that all ferromagnetic impurities are in the form of bulk Fe metal, and its magnetization is 217.6 emu/g at room temperature, then for 15 ppm Fe, the expected ferromagnetic contribution is calculated to be 0.0033 emu/g, which is negligible of. This indicates that the d or f elements are not responsible for observing the ferromagnetism in N-doped graphene. Therefore, the existence of defects seems to be the main factor in the appearance of ferromagnetism in N-doped graphene.

In addition, it is reported that pyrrole N-doped graphene can be produced based on the stoichiometric dehalogenation of transition metals 22 and hydrothermal 5 based on perhalogenated aromatic hydrocarbons and pyridine precursors. Our work shows that the SHS method may also be a good candidate for the production of high N content pyrrole N-doped graphene.

The magnetization behavior of the four samples recorded at 3000 Oe is shown in Figure 5(a). It can be seen that all samples exhibit ferromagnetism, and the saturation magnetization does not change significantly in the temperature range of 10-400 K. In order to determine the Curie temperature of N-doped graphene, the magnetization behavior of sample 1 at 1000 Oe in the temperature range of 300-800 K, samples 2 and 3 at 500 Oe in the temperature range of 500 Oe and the corresponding M~T curve were further measured. As shown in Figure 5 (bd). The derivative of the M ~ T curve with respect to the temperature range of 550-700 K is plotted as the inset in Figure 5(bd) to more accurately determine the magnetization change of the sample. The inset shows only a large and broad peak at 625 K for sample 1. In contrast, sample 2 shows an intermediate peak at 621 K and a large peak at 678 K, while sample 3 shows a small peak at 620 K and a large peak at 673 K. These peaks reflect the decrease in magnetization with increasing temperature, which can provide information about changes in the Curie temperature or material structure.

The temperature dependence of the magnetic susceptibility of raw and N-doped graphene.

(a) Measured between 10 K and 400 K, H = 3000 Oe. (b) Measured between 300 K and 700 K, H = 1000 Oe. (c,d) Measured between 300 K and 800 K, H = 500 Oe. The illustration is the derivative of Ms with respect to temperature. (e) The C, O and N content of sample 2 heat-treated at different temperatures. (f) The thermogravimetric curves of sample 0 and sample 2.

In order to study the reason for the decrease in magnetic properties, take sample 2 as an example. We heated sample 2 at 600 K, 650 K, and 700 K in a vacuum for 5 minutes. XPS measurements were performed on samples heated at different temperatures. The C, O and N contents of sample 2 are shown in Figure 5(e). It can be seen that as the temperature increases from 300 to 650 K, the relative content of carbon increases, while the relative content of nitrogen and oxygen decreases; the content of C, O, and N tends to be stable between 650 and 700 K. The XPS results of the sample after heat treatment showed that the transformation process occurred between 600 and 650 K, which mainly corresponds to the loss of N element, and partly to the loss of O element in sample 2.

To further support the XPS results, thermogravimetric (TG) analysis was performed on sample 0 and sample 2. The TG curve is shown in Figure 5(f). The weight change of sample 0 is relatively gentle, while the weight change of sample 2 is relatively significant. It can be seen that the TG curve of sample 2 can be divided into three regions. For zone I (300 to 464 K), the weight of sample 2 dropped sharply and gradually increased, indicating the existence of special desorption and adsorption processes. For region II (464 to 609 K), the slope of sample 2 is similar to the slope of sample 0, which indicates that the two samples have undergone similar processes. For zone III (609 to 700 K), the weight loss of sample 2 is faster, while the weight loss of sample 0 is not obvious. Therefore, the decomposition of N-doped graphene occurs after 609 K. This result is very consistent with the XPS result in Figure 5(e), which also shows that the loss of N element occurs between 600 and 650 K.

Relating these results to the magnetic behavior of the sample, it is reasonable to conclude that the peak of the N-doped graphene sample at 625 K is caused by the decomposition of the N group, where the loss of the N group is the magnetic moment. The magnetization decreases rapidly. Therefore, the peak at 625 K is the result of the thermal instability of N-doped graphene between 600 and 650 K.

After 650 K, the relative contents of C, N, and O are stable. As shown in Figure 5(e), the magnetization change of the sample around 678 K corresponds to the Curie temperature, sample 2 is 678 K, and sample 2 is 673 K. For the sample 3. For sample 1, it is obvious that the magnetization changes caused by the decomposition of the N group and the Curie transformation are overlapped, so the Curie temperature is in the temperature range of 609-650 K.

As mentioned above, N plays an important role in the magnetic properties of N-doped graphene, and both Hc and Ms increase with the increase of N content in N-doped graphene. However, it is interesting to find that the Curie temperature of sample 3 is slightly lower than that of sample 2. We will discuss the meaning of this phenomenon.

In the case of the Curie temperature, although it is not clear why there is such a strong interaction in these N-doped samples, which leads to a high Curie temperature, we can speculate and discuss the influencing factors of the Curie temperature based on well-known physical principles. The principle is that the magnetic response of N-doped graphene can be determined by the competition between the RKKY interaction and the electronic shielding effect. We will discuss this issue from two aspects.

First, we should point out that pyrrole N plays an important role in the formation of magnetic moment, because as proposed by Li et al., pyrrole N can induce a net magnetic moment of 0.95 μB/N. 21 And pyrrole N has been proved to be the main defect of our N-doped graphene; while pyridine N and graphite N have little effect on spin polarization. In addition, the pyrrole N bond is usually accompanied by the generation of a large number of other defects in the N-doped graphene lattice, such as vacancies, disorder and edge defects, so other types of defects can also be used as the source of local magnetic moment13,14. In addition It should be mentioned that since sample 0 exhibits ferromagnetism at room temperature, the carbon defects generated during the SHS process also play a role in the formation of the magnetic moment.

Secondly, the ferromagnetism in N-doped graphene shows that the local magnetic moment from pyrrole N and defects can induce a ferromagnetic response through magnetic coupling. This coupling effect can be realized by the RKKY interaction of 32,35 delocalized electrons. Generally, as the defect concentration increases, the distance between the magnetic moment and the defect decreases, so the RKKY interaction can be enhanced. However, when the defect concentration is high, the shielding effect of electrons must be considered, which will weaken the interaction 69. As a result, as the N content increases, the interaction between the magnetic moments becomes stronger and the Curie temperature increases; however, when the defect concentration is too high, the shielding effect weakens the interaction and the Curie temperature decreases. Therefore, the Curie temperature should have a threshold corresponding to a threshold of N concentration. For our N-doped graphene, the Curie temperature of sample 3 is slightly lower than that of sample 2, indicating that the threshold of N concentration may be lower than 11.17 at.%.

It is worth emphasizing that the SHS method plays an important role in the formation of pyrrole N and defects in N-doped graphene. The SHS method used in this study is a process away from equilibrium technology, which uses the energy released by the exothermic combustion reaction of the starting material; the combustion reaction can instantly generate very high temperatures (up to 4000 K, generally higher than 2000 K) , And then quickly cool 12; the reaction time of the sample in this work is about 40 seconds. Our work shows that SHS has a good prospect in the high-throughput production of carbon-based ferromagnetic materials with high magnetic and controllable magnetic properties.

In summary, we produced several layers of pristine and N-doped graphene through the SHS method. Room temperature ferromagnetism has been discovered and is related to carbon defects and pyrrole N caused by nitrogen doping in N-doped graphene. It is also found that the saturation magnetization and coercive field increase with the increase of the nitrogen content in the sample. For N-doped graphene with high N content, the Curie temperature is 673 K~678 K. The decrease in magnetic properties between 600 and 650 K is caused by the thermal instability of N-doped graphene. The N-doped graphene samples produced by the SHS method exhibit high Curie temperature and high coercivity. This work proves that the SHS method is a promising high-throughput method for the preparation of N-doped graphene, which may have potential applications in electromagnetics.

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The author thanks the National Natural Science Foundation of China (No. 51471057), China Scholarship Council (No. 201206125006) and Harbin Key Technology Research and Development Program (2012DB2CP029) for their support.

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001

Miao Qinghua, Wang Lidong, Liu Zhaoyuan, Wei Bing, Xu Fubiao, Fei Weidong

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QM and LW designed the synthesis method; ZL designed the magnetic measurement process; BW and FX used the SHS method to produce samples; WF, QM and LW analyzed the results and wrote the main manuscript text. All authors reviewed the manuscript.

The author declares that there are no competing economic interests.

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Miao, Q., Wang, L., Liu, Z. etc. Magnetic properties of N-doped graphene with high Curie temperature. Scientific Report 6, 21832 (2016). https://doi.org/10.1038/srep21832

DOI: https://doi.org/10.1038/srep21832

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