the successful isolation 
 of individual layers of graphene in 2004 has triggered an intense interest in its unique 
 structural and electronic properties . a 
 very high carrier mobility along with weak spin 
orbit and hyperfine 
 interactions predestinates graphene as a promising material for spintronics , 
 mainly if magnetic ordering can be introduced
. however , due to the delocalized  bonding network , ideal graphene 
 is intrinsically nonmagnetic .
therefore , developing effective methods 
 for synthesizing ferromagnetic ( fm ) graphene with high magnetization 
 is vital for applications in novel spintronic devices combining charge 
 and spin manipulation .
a number of factors including atomic vacancies , 
 zigzag edges , sp functionalization , and 
 chemical doping of foreign atoms can induce localized magnetic moments 
 in graphene , which are indispensable for the existence of magnetic 
 ordering . among these strategies , doping of the graphene lattice with noncarbon 
 atoms has been identified as a promising approach for imprinting magnetic 
 ordering into graphene while preserving / enhancing its electric / optical 
 properties , as desirable for spintronic , optoelectronic , and magnetooptical 
 applications .
substitutional doping of graphene by light elements 
 has recently 
 attracted much attention ( see , e.g. , refs ( 1114 ) ) .
in particular , 
 nitrogen doping of the graphene lattice has been extensively studied , 
 aimed at tuning the graphene electronic features , and hence its physical 
 properties , in order to meet the requirements of a given application . 

n - doped graphenes have been tested as active materials for li - ion 
 batteries , fuel cells , field - effect transistors , ultra- 
 and supercapacitors , and in the fields of photocatalysis and electrochemical biosensing .
chaban and prezhdo have argued that graphene structures containing 1/3 of nitrogen atoms or less 
 should be stable up to 1000 k provided n 
having similar atomic size and one 
 additional electron compared to a carbon atom , nitrogen acts as an n - type ( electron - donating ) dopant by increasing the number 
 of electrons when substituted into the graphene lattice , thus affecting 
 the graphene electric conductivity .
the substitution of carbon by 
 nitrogen atoms in the graphene lattice necessarily leads to changes 
 in the electronic density of states , and graphitic nitrogen can provide 
  electrons close to the fermi level of graphene .
if the itinerant 
 electrons occupy narrow bands at the fermi level , stoner magnetism 
 can emerge , as recently shown for graphene doped with sulfur .
indeed , it has been theoretically proposed that depending on the concentration and packing 
 geometry of doping nitrogen atoms , it is possible to induce a magnetic 
 response in graphene .
however , the physical mechanism governing the 
 emergence of magnetically ordered structures was not discussed . until 
 now , all the experimental attempts to imprint magnetism into graphene 
 through n - doping have failed . recently , a detailed theoretical and 
 experimental magnetic study revealed that introduction of pyrrolic 
 nitrogen into the graphene lattice decreases the magnetization values 
 compared to those observed for defective graphene .
% ) was identified to enhance ferromagnetism in highly oxidized 
 graphene ( i.e. , graphene oxide ) synthesized via hydrothermal reaction .
fm behavior has also been reported in other 
 nitrogen - doped graphene oxide systems . however , n - doped graphene oxides are unsatisfactory because a high 
 amount of oxygen ( sp functionalization ) 
 is regarded as a dominant source of magnetism , overwhelming the effects 
 of nitrogen doping itself .
moreover , oxygen - containing functional 
 groups drastically reduce the electric conductivity of graphene  the 
 main prerequisite for spintronic technologies .
thus , development of 
 fm n - doped graphene with negligible oxygen content and elucidation 
 of the effect of various nitrogen configurations ( pyrrolic , pyridinic , 
 graphitic , chemisorbed ) remains a key challenge in the fields of nanotechnologies , 
 magnetism , and spintronics . in this work ,
we first report the emergence of a magnetically 
 ordered 
 state in graphene doped solely with nitrogen .
% of nitrogen , the doping - induced paramagnetic 
 centers commenced to interact magnetically , resulting in establishment 
 of fm ordering at temperatures below 69 k. in the fm state , 
 the saturation magnetization reached 1.09 emu / g , which is 
 among the highest values reported so far for doped graphene - based 
 systems .
dft calculations demonstrated that graphitic nitrogen was 
 dominantly responsible for evolution of the magnetically active configurations . 

in contrast , pyridinic and chemisorbed nitrogen had a considerably 
 lower effect on imprinting the magnetism into graphene .
this finding 
 opens possibilities for an extensive research in spintronic and magnetooptical 
 technologies based on graphitic n - doped graphene .
graphite microparticles 
 ( 215 m , 
 99.9995% , alfa aesar ) were used for all syntheses . sulfuric acid ( 98% ) , 
 nitric acid ( 68% ) , potassium chlorate ( 99% ) , hydrochloric acid ( 37% ) , 
 silver nitrate ( 99.5% ) , and barium nitrate ( 99.5% ) were obtained from 
 penta , czech republic .
nitrogen ( 99.9999% purity ) and ammonia ( 99.9995% 
 purity ) were obtained from siad , czech republic .
briefly , sulfuric acid ( 98% , 87.5 
 ml ) and nitric acid ( 68% , 27 ml ) were added to a reaction flask ( pyrex 
 beaker with thermometer ) containing a magnetic stir bar .
graphite ( 5 g ) 
 was then added to the mixture under vigorous stirring and , keeping 
 the reaction flask in the ice bath , potassium chlorate ( 55 g ) was 
 slowly added . after a complete dissolution of potassium chlorate , 

the reaction flask was loosely capped to allow the escape of gas evolved 
 and the mixture was continuously stirred for 96 h at room temperature . 

the 
 obtained graphene oxide was then redispersed in hcl solution ( 5% , 
 3 l ) to remove sulfate ions and repeatedly centrifuged and redispersed 
 in deionized water until a negative reaction on chloride and sulfate 
 ions with silver and barium nitrate was achieved .
the resulting graphite 
 oxide was dried in a vacuum oven at 50 c for 48 h. nitrogen - doped graphenes 
 were prepared by combined exfoliation and 
 reduction of graphite oxide in an ammonia atmosphere .
briefly , 100 
 mg of graphite oxide was placed inside a quartz glass capsule connected 
 to a magnetic manipulator and mounted in a horizontal quartz glass 
 reactor .
subsequently , the sample was inserted in the 
 hot zone of the reactor while the ammonia flow was kept at 1000 ml 
 min and the pressure was 100 kpa .
the temperature 
 of the sample was held constantly for 12 min at 400 , 600 or 800 c . 

after removal from the hot zone of the reactor , the sample was cooled 
 in an ammonia atmosphere .
the gn0.015 sample was prepared at 400 
 c , gn0.033 sample was prepared at 600 c , and 
 gn0.051 sample was prepared at 800 c ; the subscript 
 reflects the level of nitrogen in the respective sample as identified 
 from the analysis of the respective survey x - ray photoelectron spectroscopy 
 patterns .
% in all 
 the samples studied . compared to thermally reduced graphene oxide 
 ( trgo ) , used as a blank undoped sample , a slight increase in the oxygen 
 content can be explained in terms of competing processes of nitrogen 
 incorporation and reduction of graphite oxide .
trgo sample was 
 prepared at 800 c by a similar procedure using only nitrogen 
 ( 1000 ml min ) as an exfoliating atmosphere .
the residual metal content 
 in the trgo and n - doped graphene samples was analyzed by inductively 
 coupled plasma mass spectrometry ( icp - ms ) .
an exact amount of sample 
 ( 10 mg ) was immersed in concentrated nitric acid ( 99.999% 
 trace metals basis ) and heated for 2 h at 100 c .
afterward , 
 the mixture was transferred into a 10 ml volumetric flask , diluted 
 with water and any undissolved graphene was separated using a 200 
 nm millipore filter .
the measured concentration of metals in the solution 
 was recalculated to the amount in the tested sample ( analogically , 
 diluted nitric acid was used as a blank ) . the atomic percent 
 of c , o , and n , and types of bonds were assessed by x - ray photoelectron 
 spectroscopy ( xps ) , employing a phi 5000 versaprobe ii xps system 
 ( physical electronics ) equipped with a monochromatic al k source ( 15 kv , 50 w ) with a photon energy of 1486.7 ev .
all the xps 
 patterns were measured in a vacuum of 1.4  10 pa and at room temperature ( 22 c ) . for high resolution xps 
 patterns , a pass energy of 23.500 ev and step size of 0.200 ev
xps patterns were evaluated with a multipak ( ulvac , phi , inc . ) 

all binding energy values were referenced to the c 1s peak 
 at 284.80 ev .
raman spectra were acquired using a dxr raman 
 spectroscope ( thermo 
 scientific , u.s.a . )
the respective sample was first deposited on a glass platform 
 and the excitation laser was focused on its surface .
the laser power on the sample was set to 5 mw and the exposition time 
 was 20 s. each measured raman spectrum was an average of 16 experimental 
 microscans .
thermogravimetric analysis ( tga ) and evolved gas 
 analysis ( ega ) 
 curves were recorded using a netzsch sta 449c jupiter system with 
 an adapted qms 403c aolos quadrupole mass spectrometer .
tga / ega 
 measurements were performed in an open -al2o3 crucible under an argon atmosphere ( 80 cm min ) .
a temperature program from 40 to 1000 c with 
 a heating rate of 5 k min was used .
magnetization 
 measurements of the trgo and n - doped graphene samples 
 were performed using a physical property measurement system ( ppms ) 
 equipped with a vibrating sample magnetometer ( vsm ) from quantum design , 
 u.s.a .
hysteresis loops were measured over the temperature range from 
 5 to 300 k and under static external magnetic fields ranging from 
 50 to + 50 koe .
temperature profiles of the mass magnetic susceptibility , 
 mass , were recorded in a sweep mode over a temperature 
 range from 5 to 300 k in a field of 1 koe after cooling in a field 
 of 1 koe .
magnetization values were corrected assuming the response 
 of the sample holder , sample capsule , and respective pascal constants .
atomistic calculations were performed 
 using a spin - polarized density functional theory ( dft ) and projected 
 augmented wave potentials ( paw ) representing atomic cores as implemented 
 in the vienna ab initio simulation package ( vasp ) .
electronic 
 exchange and correlation effects were treated by using the perdew , 
 burke , and ernzerhof ( pbe ) generalized - gradient 
 approximation ( gga ) with a plane wave cutoff of 600 ev .
brillouin 
 zone integrations were performed with a 6  6  1  
 point - centered monkhorst 
pack k - point mesh 
 per conventional 4  3 rectangular cell ( structure and cell optimization ) . 

the electronic density of states was calculated using the tetrahedron 
 method with a 21  21  1 k - point mesh .
partitioning of the ground state electronic 
 density into contributions attributed to the different atoms was performed 
 exploiting the bader analysis .
total magnetic moments were calculated 
 from the difference between the number of electrons in occupied majority- 
 and minority - spin states .
local magnetic moments were calculated by 
 projecting the plane - wave components of all the occupied eigenstates 
 onto spherical waves inside an atomic sphere and integrating the resulting 
 local density of states .
a full structural optimization was 
 performed using a quasi - newton algorithm until the residual atomic 
 forces were lower than 25 mev  .
simultaneously , 
 the electronic and magnetic degrees of freedom were converged to an 
 energy of less than 10 ev .
the stability of the 
 reported configurations was analyzed in terms of the formation energy , ef , of the n - doped complexes using the formula 
 given as ef = 1/n[edpd  egraph + n(c  n ) ] , where edpd and egraph stand for the total energies of the doped and pristine 
 graphene sheet , respectively , c and n are the chemical potentials of c and n atoms , respectively 
 ( here , approximated by the atomic energies of c and n ) , and n is the number of substituted atoms .
a positive ef indicates that the doping process is endothermic , 
 although it does not hinder the formation of thermodynamically stable 
 complexes .
the adsorption energy ( ead ) per n atom 
 ( n = 1 and 2 ) on a pristine layer was calculated 
 as the total energy difference between the energy of an adatom - graphene 
 complex , en / graph , a clean graphene layer , egraph , and an isolated n atom in the gas - phase , en , i.e. , ead = 1/n(en / graph  egraph  nen ) .
similarly , the adsorption energy of on - surface nitrogen 
 on n - doped graphene was calculated as the total energy difference 
 between the energy of a doped graphene with an adatom , en / dpd , a doped graphene sheet , edpd , and a gas - phase n atom , i.e. , ead = en / dpd  edpd  en .
to study the role of n - doping on imprinting 
 magnetic features into 
 graphene , three samples differing in nitrogen content were prepared ; 
 the level of doping was solely controlled by the temperature , while 
 all other synthetic parameters were kept constant .
the trgo sample 
 was synthesized as a reference blank sample for which no source of 
 nitrogen ( i.e. , ammonia atmosphere ) was used during the thermal reduction 
 of graphite oxide .
the reduction process was found to be highly efficient , 
 as evident from the very low content of oxygen ( 1 at .
% , see the survey 
 and high - resolution c 1s xps pattern in figure s1a , b in supporting information ) .
the paramagnetic / fm 
 response of graphene evolved due to defects and/or functionalization 
 is of several orders of magnitude lower compared to that of 3d - block metals , such as iron , cobalt , nickel , and manganese . 

hence , the results of magnetization measurements may be incorrectly 
 interpreted if these  strong  magnetic elements are 
 present in the system as a consequence of using reactants containing 
 3d - block metals and/or the sample handling .
thus , icp - ms was employed to quantify the presence 
 of 3d metal impurities and exclude their effect on 
 the magnetic properties of n - doped graphenes . the total concentration 
 of fe , ni , co , and mn , regarded as the main magnetic impurities in 
 trgo and n - doped graphene samples , was below 10 ppm ( see table s1 
 in supporting information ) . taking into 
 account the determined concentrations and magnetic moments of the 
 metal impurities ,
the total mass of fe , ni , co , 
 and mn was estimated to be of the order of 10 emu 
 g oe at 0 k and in a 1 koe 
 field . as mass values for trgo and n - doped graphene 
 systems reached orders from 10 down to 10 emu g oe in 
 a 1 koe field ( see below ) , the contribution of fe , ni , co , and mn 
 to the samples mass was assumed to be negligible 
 in measurements of temperature evolution of mass and hysteresis loops , thus definitely not overshadowing the magnetic 
 properties of graphene induced solely by nitrogen doping .
doping 
 of graphene with nitrogen was monitored by xps and raman 
 spectroscopy . in survey xps patterns recorded for n - doped graphene 
 samples , peaks belonging to c , n , and
o were clearly observed ( see 
 figure s2 and table s2 in supporting information ) .
the content of nitrogen was found to increase progressively with 
 the temperature at which the thermal reduction of graphite oxide in 
 the presence of ammonia was conducted ( i.e. , 400 c , 1.5 at . 

the presence of nitrogen was further evidenced in the high - resolution 
 c 1s xps profile , which showed the emergence of a peak at a binding 
 energy of around 285.5 ev corresponding to the c 
n spectral component increased 
 in area with the level of n - doping . a small shift in the maximum of 
 the c 
n peak witnessed for the three n - doped graphene samples 
 can be explained in terms of impossibility to distinguish differently 
 coordinated nitrogen atoms with carbon atoms in graphene with similar 
 binding energy values in the c 1s domain and the significant overlap 
 of the c  n and c  o spectral components .
high - resolution 
 n 1s xps patterns of n - doped graphene samples ( see figure 2 and figure s3 and table s3 
 in supporting information ) showed three 
 distinct peaks corresponding to nitrogen in different configurations 
 inside a graphene lattice or attached covalently to a graphene sheet 
 ( see figure 3 ) , i.e. , 
 pyridinic nitrogen ( at 398.3 to 398.5 ev ) , graphitic 
 nitrogen ( at 401.0 to 401.5 ev ) , and chemisorbed n / n2 ( at 404.5 to 405.5 ev ) . in contrast , no traces 
 of pyrrolic nitrogen , usually present at 400.0 ev , were observed 
 ( compare the high - resolution n 1s xps patterns shown in figure 2 and figure s3 in supporting information with figure 3 ) .
the pyridinic and graphitic nitrogen were viewed as nitrogen incorporated 
 inside the graphene lattice , whereas chemisorbed n / n2 was 
 viewed as nitrogen adsorbed as adatoms .
the presence of n / n2 was further confirmed by tga and ega techniques with gas electron 
 ionization mass spectrometry , indicating emission of a fragment with m / z = 28 ( for n / n2 ) above 300 
 c ( see figure s4 in supporting information ) .
in addition , as no fragments with m / z = 30 and 48 were detected , the samples were considered free of any 
 no and no2 species , respectively .
this agrees with the 
 analysis of the raman spectra of n - doped graphene samples as no raman 
 peaks around 1430 cm , characteristic for no and 
 no2 species , are observed 
 ( see figure 1b , d , f ) .
high - resolution 
 c 1s xps patterns of the ( a ) gn0.015 , ( c ) gn0.033 , ( e ) and gn0.051 sample with 
 bonds indicated .
raman spectra of the ( b ) gn0.015 , ( d ) 
 gn0.033 , and ( f ) gn0.051 sample with the id / ig ratio indicated .
high - resolution n 1s xps pattern of the gn0.051 sample 
 with peaks assigned to differently coordinated nitrogen .
scheme showing different bonding configurations of nitrogen 
 in 
 n - doped graphene and corresponding peaks in a simulated high - resolution 
 n 1s xps pattern .
raman spectra of the 
 gn0.015 , gn0.033 , gn0.051 , and trgo 
 sample are shown in figure 1b , d , f and figure s1c in supporting information .
it can be seen that on increasing 
 the level of n - doping , the d - to - g band intensity ratio , id / ig , increased .
it is known 
 that if nitrogen enters the graphene lattice , the intensity of the 
 d - band , i d , in the raman spectrum of graphene 
 is enhanced due to defects that emerge upon incorporation of nitrogen 
 into the graphene structure .
this provides 
 further evidence that under the synthetic conditions used , accommodation 
 of nitrogen into the graphene lattice was strongly favored over a 
 simple adsorption / addition process . if defects are introduced 
 into graphene , paramagnetic centers emerge 
 that may interact via suitable mediators ( i.e. , -conduction 
 electrons , overlapping orbitals favoring superexchange mechanism , 
 etc . ) to generate long - range magnetic ordering in the 2d lattice . 

under such conditions , the magnetic susceptibility of graphene ,  , 
 involves three contributions , i.e. ,  = dia + para + ferro , where dia is the diamagnetic term including orbital , landau and core 
 diamagnetic contributions , para is the paramagnetic 
 term including noninteracting ( isolated ) defect - induced paramagnetic 
 centers , pauli paramagnetic contribution from conduction electron 
 and van vleck contribution , and ferro is the fm 
 term describing the magnetic response of interacting defect - induced 
 paramagnetic centers .
as expected , pristine 
 trgo behaved in a diamagnetic manner with only a tiny paramagnetic 
 response at low temperatures ( see figure s1d in supporting information ) due to paramagnetic centers emerging 
 as a result of defects and/or the negligible content of oxygen functionalities , 
 which were most likely at the edges ; the profile of mass of trgo well matched the modified curie law , i.e. , mass = mass , dia + c / t , where mass , dia is the diamagnetic contribution , c is the curie constant , and t is the temperature .
similarly , the temperature evolution of mass measured 
 for the gn0.015 and gn0.033 sample also obeyed 
 the modified curie law ( see figure 4a , b ) .
the number of paramagnetic centers increased 
 upon increasing the level of n - doping , as expected and evidenced by 
 an enhanced paramagnetic curie contribution ( see insets in figure 4a , b ) .
however , n - doping 
 at such levels did not imprint any magnetic configuration ( as confirmed 
 by theoretical calculations discussed below ) and the induced paramagnetic 
 centers were far from each other , precluding the establishment of 
 a long - range magnetic ordering . thus , the gn0.015 and gn0.033 sample showed dominant diamagnetic behavior , as also 
 witnessed from the isothermal magnetization curves measured at 5 k 
 ( see insets in figure 4a , b ) .
temperature evolution of the mass magnetic susceptibility , mass , of the ( a ) gn0.015 and ( b ) gn0.033 sample recorded under an external magnetic field of 1 koe .
the insets 
 in panel ( a ) and ( b ) show the temperature profile of mass after subtraction of the diamagnetic component and behavior of the 
 5 k hysteresis loop of the gn0.015 and gn0.033 sample , respectively .
( c ) temperature evolution of mass for the gn0.051 sample recorded under an external magnetic 
 field of 1 koe with the curie temperature , tc , indicated .
the insets show the trend of mass at low temperatures and the temperature profile of the fm contribution , 
 mass , ferro , derived from fitting the measured mass .
( d ) hysteresis loop of the gn0.051 sample 
 measured at a temperature of 5 k. the insets show the behavior of 
 the hysteresis loop around the origin with the coercivity marked and 
 field - dependent profiles of magnetization for the ferromagnetic , mferro , and paramagnetic , mpara , component derived from fitting the measured isothermal 
 magnetization curve .
( e ) scheme showing different magnetic fractions 
 in the gn0.051 sample and their respective profiles of 
 mass .
interestingly , for the gn0.051 sample , the temperature 
 profile of mass was drastically different compared 
 to the gn0.015 and gn0.033 sample as it could 
 not be described by the modified curie law .
on lowering the temperature , 
 mass showed a saturation tendency followed by an 
 abrupt increase , indicating two contributions , i.e. , fm and paramagnetic 
 ( see figure 4c  e ) ; 
 contrary to the graphene systems with a lower n - doping , the diamagnetic 
 term was found to be negligible here .
the presence of both magnetic 
 fractions can be explained by assuming that the n - doped graphene sheets 
 within the sample contained nitrogen in different structural configurations , 
 which have a different impact on the magnetism of graphene , as predicted 
 by the theory discussed below .
moreover , it is known that even in 
 sheets showing fm behavior , isolated paramagnetic centers may evolve 
 owing to the presence of vacancies and defects of topological and 
 edge nature . in order to fit the temperature evolution of mass
, the paramagnetic term was fitted using the curie function ( i.e. , 
 mass = c / t ) over 
 the whole temperature interval , whereas a model function involving 
 a combination of the curie 
weiss law ( i.e. , mass = c/(t   ) , where 
  is the weiss temperature ) at high temperatures and brillouin 
 function within the mean - field approximation at low temperatures was 
 constructed to describe the fm contribution ( see inset in figure 4c ) .
the fitting yielded 
   69 k , angular momentum number j  
 1.19 , and a weight - normalized ratio of ferromagnetic - to - paramagnetic 
 contribution equal to 1.66 . note that the value of j is not an integral multiple of 0.5 as expected and must be treated 
 as an average over all the n - doped graphene sheets in the gn0.051 sample as the sheets can have different c  n configurations / arrangements 
 with different net magnetic moments ( see below ) , as already reported 
 for n - doped and s - doped graphene systems .
furthermore ,  can be assigned to the curie temperature , tc , marking the transition from the paramagnetic 
 to fm regime on lowering the temperature .
thus , in accordance with 
 the theory , if the concentration of nitrogen exceeds a threshold value 
 ( > 5 at .
% of n ) , magnetically active motifs inside the graphene 
 lattice 
 evolve , eventually leading to a magnetically ordered ( i.e. , fm ) state 
 at low temperatures .
% of n to the low - temperature 
 fm regime was further confirmed by a series of hysteresis loops measured 
 over the temperature range from 5 to 300 k ( see figure 4d and figure s5 in supporting information ) . at 5
k , the isothermal magnetization curve showed 
 hysteresis with a coercivity of 92 oe ( see inset in figure 4d ) and saturation 
 magnetization reaching 1.09 emu / g . the derived value of the saturation 
 magnetization is of identical order as reported for s - doped graphenes and vertical graphenes , which are regarded as the magnetically strongest carbon - based systems 
 prepared to date .
the nonzero value of the coercivity implies that 
 n - doping imprints magnetic anisotropy on the graphene lattice , establishing 
 an easy axis of magnetization along which the magnetic moments of 
 the generated paramagnetic centers energetically prefer to lie .
following 
 the mathematical procedures of liu et al . and tuek et al . and assuming 
 the j value and weighted ferromagnetic - to - paramagnetic 
 ratio derived from the mass vs t profile , we next attempted to separate the paramagnetic and fm contribution 
 in the 5 k isothermal magnetization curve ( see inset in figure 4d ) .
the fitting yielded a saturation 
 magnetization of 0.65 emu / g and 0.37 emu / g for the 
 fm and paramagnetic contributions , respectively .
on raising the temperature , 
 the coercivity decreased to zero ( see inset in figure s5b in supporting information ) and the hysteresis was 
 lost at a temperature of 70 k ( see figure s5a in supporting information ) , indicating a transition 
 from the fm to paramagnetic regime .
thus , the experimental results 
 suggested that when the concentration 
 of nitrogen was increased and it became firmly embedded in the crystal 
 lattice of graphene , the number of induced paramagnetic centers increased , 
 eventually forming magnetically active motifs with conduction electrons 
 providing interaction pathways between them and establishing long - range 
 magnetic ordering upon decreasing the temperature . it was hypothesized 
 that upon further increasing the nitrogen concentration in the graphene 
 lattice , magnetic interactions would be strengthened , as reflected 
 by a shift of tc to higher temperature . 

moreover , the presence of more paramagnetic centers would enhance 
 the saturation magnetization , approaching the values observed for 
 s - doped and vertically oriented graphenes , i.e. , systems where the magnetic features are imprinted by defects . 
 here
% of n was probably a consequence of the different nitrogen motifs 
 identified in the samples by xps ( graphitic , pyridinic , and chemisorbed 

importantly , we did not identify pyrrolic nitrogen 
 in the samples , which has previously been shown to cause a fall in 
 magnetization values . to decipher 
 the effect of nitrogen in various bonding configurations 
 ( as identified by xps , figure 2 and figure s3 in supporting information ) on the magnetic properties of graphene
, we performed a first - principles study of the structural , electronic , and magnetic properties of 
 n - doped graphenes .
the  magnetic contributions  of individual 
 n - motifs were also addressed in details .
we used a rectangular graphene 
 cell containing 96 atoms , which was computationally tractable and 
 enabled the experimentally determined total and relative ( among different 
 bonding configurations ) content of nitrogen to be followed closely . 

specifically , we considered chemisorbed nitrogen ( both n and n2 ) on the top of the graphene sheet and two graphitic and pyridinic 
 nitrogen atoms in the graphene lattice ( see figure 5 ) . top view of graphene doped with graphitic ( in para configurations ) , trimerized pyridinic and chemisorbed 
 ( a ) n and 
 ( b ) n2 marked respectively by red , green , and magenta , 
 with positive ( negative ) spin densities plotted in blue ( red ) for 
 isosurfaces at 1  10 ( panel ( a ) ) and 
 2.5  10e ( panel ( b ) ) .
( c , d ) the corresponding dos plot for 
 configuration shown in panel ( a ) and panel ( b ) , respectively .
the computational data fully supported 
 the experimental results . 
 in particular , the energetically most stable structures , as presented 
 in figure 5a ,
b , exhibited 
 fm ordering with 1.3 and 0.3 b magnetic moment per 
 supercell for n and n2 , respectively , chemisorbed on the 
 graphene surface . the density of state ( dos ) plots presented in figure 5c , d indicated exchange 
 coupling mediated by the conduction electrons .
moreover , the strong 
 spin - polarized electron features at the fermi level in the dos structures 
 showed a predominant contribution from the graphitic n atoms , which 
 highlights their important role in developing stoner - like magnetism 
 in n - doped graphenes .
it should be noted that depending 
 on the position of nitrogen on the graphene surface with respect to 
 the graphitic and pyridinic nitrogen atoms , a large number of possible 
 magnetic configurations were found with a varying effective magnetic 
 moment per cell and exhibiting both fm and antiferromagnetic behavior 
 or even disappearance of magnetic order .
this may explain the complexity 
 of the magnetic measurements discussed above for the gn0.051 sample , as the experimental response represented an average over 
 a large number of structures with different net magnetic moments .
thus , to understand the role and contribution of individual nitrogen 
 motifs in triggering magnetism in n - doped graphenes , we carried out 
 an extensive set of additional calculations with a smaller cell containing 
 48 atoms .
the magnetic properties of graphene doped with graphitic 
 nitrogen exhibited a strong dependence on both the nitrogen concentration 
 and configurations .
% of graphitic nitrogen ( the percentages used result 
 from the size of the cell employed in the calculations ) are shown 
 in figure s6 in supporting information . 

the magnetism of graphene doped with graphitic nitrogen has been attributed 
 to delocalized electrons occupying narrow peaks at the fermi level 
 ( ef ) .
% of nitrogen was predicted to be nonmagnetic , in accord 
 with dft calculations by wang et al . , and 

the narrow electron donor states near ef were absent in the partial density of states ( pdos ) . at 4.2 at . 

% of nitrogen , the computational screening identified a magnetic structure 
 with a magnetic moment of 0.2 b per supercell 
 in which n atoms substituted c atoms at para positions 
 ( see figure 6 ) .
the 
 same motif was responsible for triggering ferromagnetism in the larger 
 96-atom cell , which highlights the importance of the graphitic nitrogen 
 motif in imprinting fm order in n - doped graphene .
n - doping generating 
 the fm state gave rise to a strong pz electron peak at the fermi level in the electronic 
 structure according to pdos , similar to recent reports for graphene 
 doped with 4.2 at .
the magnetic 
 polarization was confined to a narrow part of the cell , in zigzag 
 directions between the doping atoms , as can be seen in the inset in figure 6 presenting isosurfaces 
 of spin densities .
importantly , formation of zigzag edges by cutting 
 graphene along a certain crystallographic direction has been shown 
 to give rise to peculiar edge localized states near the fermi level 
 responsible for the spontaneous formation of magnetic ordering in 
 graphene nanoribbons .
partial densities of 
 states calculated for graphene doped with 
 nitrogen embedded in the lattice at para positions 
 at a concentration of 4.2 at .
the supercell is shown in the inset , 
 where an isosurface of spin - density plotted at 5  10e is also 
 displayed .
% of graphitic 
 nitrogen , a greater number of magnetic configurations with the magnetic 
 moment varying between 0.1 and 0.8 b / supercell ( see figure 7 and figure s7 in supporting information ) were produced .
these 
 magnetic structures contained two n atoms in the meta configuration embedded in the graphene lattice .
the magnetic behavior 
 of this system could be tuned by changing the position of the third 
 n atom in the host lattice as it stayed on the zigzag paths between 
 the other two n atoms . finally , because of the similar atomic sizes 
 of nitrogen and carbon atoms , incorporation of nitrogen into the honeycomb 
 network of graphene did not lead to any significant distortion of 
 the host lattice . in - plane
atomic displacements caused by nitrogen 
 were below 2% of the c  c distance of pristine graphene and 
 the system remained planar .
top view of graphene doped with nitrogen at a concentration 
 of 
 6.25 at .
positive ( negative ) spin densities are plotted in blue 
 ( red ) for isosurfaces at 5  10e : ( a ) 0.1 , ( b ) 0.4 , ( c ) 0.7 , 
 ( d ) 0.8 , ( e ) 0.4 , and ( f ) 0.6 b per supercell .
we also considered the effect 
 of nitrogen atoms substituting carbons 
 in the graphene lattice ( employing a 48-atom graphene cell ) on pyrrolic 
 bonding sites .
a single pyrrolic nitrogen in graphene occupied a site 
 inside the pentagonal ring in the vicinity of single vacancy ( sv ) 
 or divacancy ( dv ) defect ( see figure s8a , b in supporting information ) .
substitution by monomeric pyrrolic 
 nitrogen did not result in any long - range magnetic order .
c 
 bond with an n atom led to opening of the 5-membered ring and s = 1/2 paramagnetism due to 
 the carbon dangling bond .
such a bonding configuration transformed 
 the initial pyrrolic nitrogen into pyridinic nitrogen ( see figure 
 s8c , d in supporting information ) .
we also 
 considered the effect of adding pyrrolic nitrogen inside the octagonal 
 ring of the dv defect ( figure s8e , f in supporting information ) , as proposed in the work of li et al .
the present calculations indicated that the 
 nitrogen atom moved to the neighboring 5-membered ring , in accord 
 with the study by lin et al .
, forming 
 a 6-membered ring upon relaxation in which nitrogen ( transformed into 
 pyridinic ) was nonmagnetic .
next , we considered the role of 
 pyridinic nitrogen , which was also 
 identified as an important motif in the experimental samples .
besides 
 monomeric pyridinic nitrogen ( s = 0 or s = 1/2 due to carbon dangling bonds ) , we also considered dimerized 
 ( with s = 1/2 magnetic moment due to carbon dangling 
 bond ) and trimerized pyridinic nitrogen ( with a magnetic moment of 
 0.3 b per supercell ) . note that the calculated ground 
 state arrangements of multiple pyridinic nitrogen in the graphene 
 lattice were in agreement with annular dark - field imaging reported 
 in the work by lin et al . however , according 
 to dos of the trimerized pyridinic nitrogen ( see figure s9 in supporting information ) , the electronic structure 
 indicated that indirect exchange mediated by the conduction electrons 
 was strongly compromised compared to the exchange coupling developed 
 for graphene doped with graphitic nitrogen ( compare figure 6 and figure s9 in supporting information ) .
this is in line with 
 the suppressed magnetic moment of trimerized pyridinic nitrogen in 
 comparison to that of trimerized graphitic nitrogen ( see the structure 
 presented in figure 7f and dos plot in figure s7f in supporting information ) .
it is also important to mention that half - metallic properties 
 were recently predicted for the graphene - based c4n3 polymer , i.e. , a 2d radical 
 polymer containing many trimerized pyridinic nitrogens .
in contrast , 
 the dos plot presented in figure s9 in supporting information showed finite ( nonzero ) density of states above ef in both spin - up and spin - down channel , which 
 is most likely due to a significantly lower concentration of trimerized 
 pyridinic nitrogen in the present study .
finally , we investigated 
 whether magnetism in graphene can also 
 be induced by nitrogen adatoms / molecule , which were identified in 
 the xps patterns and tga / ega measurements .
thus , structures resulting 
 from both nitrogen additions on a pristine graphene layer and simultaneous 
 single nitrogen atom addition and graphitic substitution were analyzed 
 ( see figure s10 and figure s11 in supporting information ) .
a single n - adatom carried a magnetic moment of 0.5 b , which can be understood based on an electron counting argument : 
 two valence electrons formed covalent bonds with neighboring c atoms , 
 two formed a lone - pair , and the fifth valence electron gave rise to 
 the magnetic moment .
another nitrogen atom contributed 0.5 b , 
 but no magnetic moments were induced on the c atoms . by placing another 
 nitrogen atom in a close proximity to the preadsorbed adatom ,
an n2 dimer spontaneously formed , which was adsorbed over the surface 
 and had zero magnetic moment .
% of graphitic nitrogen ( see figure s11 in supporting information ) also did not induce a magnetic response 
 in the system .
to conclude , the computational study allowed 
 elucidation of the 
 synergistic effect of nitrogen atoms in various bonding configurations , 
 as evidenced from high - resolution xps data . to follow closely the 
 total and relative content of nitrogen in the experimentally prepared 
 samples
, we considered graphitic nitrogen in the para configuration , which turned out to promote formation of the motif 
 most important for imprinting magnetism in graphene , followed by trimerized 
 pyridinic nitrogen and chemisorbed n adatoms . in the ground state , 

the structure resulting from the combined effects of all these species 
 is strongly fm .
however , it should be noted that coupling between 
 pyridinic nitrogens is much less effective in maintaining the fm structure , 
 and chemisorbed nitrogen adatoms can only generate paramagnetism . 

finally , pyrrolic nitrogen has no effect on magnetism in graphene , 
 in line with the work by ito et al . , who 
 reported a decrease in the magnetization values with increasing concentration 
 of pyrrolic nitrogen in the graphene lattice .
in 
 summary , on the basis of electronic - structure calculations and 
 magnetization measurements , we have provided new insights into the 
 role of nitrogen as a highly electronegative n - type 
 dopant for imprinting the magnetic properties to graphene .
the magnetic 
 features of n - doped graphene depend on both the nitrogen concentration 
 and the configuration in the host lattice , with a complex interplay 
 between graphitic , pyridinic , and chemisorbed nitrogen . among these 
 structural motifs ,
% of nitrogen , graphene behaves dominantly as a diamagnet ; paramagnetic 
 centers are induced upon doping ; however , they do not produce magnetically 
 active motifs .
if the doping concentration is increased above the 
 threshold doping value , magnetic interactions mediated by the conduction 
 electron system emerge between the substitution - generated paramagnetic 
 centers .
% shows a transition to an fm state at the curie 
 temperature of 69 k and saturation magnetization reaching 
 1.09 emu / g .
such a high value of the saturation magnetization ranks 
 n - doped graphenes among the magnetically strongest graphene - based 
 systems developed so far for which the magnetic properties are imprinted 
 by defects . as n - doping
is also expected to maintain or even improve 
 the electric ( i.e. , conduction ) features of graphene , the present 
 work opens possibilities for further optimization of n - doped graphenes 
 ( e.g. , exclusive presence of graphitic nitrogen ) to produce new kinds 
 of spintronic materials .
 nitrogen doping opens 
 possibilities for tailoring the electronic 
 properties and band gap of graphene toward its applications , e.g. , 
 in spintronics and optoelectronics . 
 one major obstacle is development 
 of magnetically active n - doped graphene with spin - polarized conductive 
 behavior . 
 however , the effect of nitrogen on the magnetic properties 
 of graphene has so far only been addressed theoretically , and triggering 
 of magnetism through n - doping has not yet been proved experimentally , 
 except for systems containing a high amount of oxygen and thus decreased 
 conductivity . here , we report the first example of ferromagnetic graphene 
 achieved by controlled doping with graphitic , pyridinic , and chemisorbed 
 nitrogen . 
 the magnetic properties were found to depend strongly on 
 both the nitrogen concentration and type of structural n - motifs generated 
 in the host lattice . 
 graphenes doped below 5 at . 
 % of nitrogen were 
 nonmagnetic ; however , once doped at 5.1 at . 
 % of nitrogen , n - doped 
 graphene exhibited transition to a ferromagnetic state at 69 
 k and displayed a saturation magnetization reaching 1.09 emu / g . 
 theoretical 
 calculations were used to elucidate the effects of individual chemical 
 forms of nitrogen on magnetic properties . 
 results showed that magnetic 
 effects were triggered by graphitic nitrogen , whereas pyridinic and 
 chemisorbed nitrogen contributed much less to the overall ferromagnetic 
 ground state . 
 calculations further proved the existence of exchange 
 coupling among the paramagnetic centers mediated by the conduction 
 electrons . 