AMPERE CONGRESS AMPERE on Magnetic Resonance and Related Phenomena. Under the auspices of The GROUPEMENT AMPERE

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1 AMPERE 2000 th 30 CONGRESS AMPERE on Mgnetic Resonnce nd Relted Phenomen Lison, Portugl, 23-2 July 2000 Under the uspices of The GROUPEMENT AMPERE Edited y: A.F. MARTINS, A.G. FEIO nd J.G. MOURA Sponsoring orgniztions: ISBN: Legl Deposit: FCT-UNL,2001

2 CONDUCTION ELECTRON SPIN RESONANCE IN GRAPHITE AND ITS INTERCALATION COMPOUNDS: SURFACE AND INTERFACE SPIN RELAXATION EFFECTS A.M. Zitdinov Institute of Chemistry, Fr Estern Brnch of the Russin Acdemy of Sciences. 159, Prosp. 100-letiy, Vldivostok, RUSSIA. E-mil: Astrct: In ll previous investigtions of conduction ESR (CESR) phenomenon in grphite nd grphite intercltion compounds (GICs) t the nlysis of the resonnce line shpe the surfce nd interfce spin relxtion effects were neglected. We hve studied the dependences of line shpe, line width nd intensity of CESR signl in grphite nd GICs on smple dimensions nd experimentl conditions nd hve shown the presence of the strong surfce nd interfce spin relxtion effects in smples investigted. Introduction The method of conduction ESR (CESR) hs een ctively used in studies of grphite nd grphite intercltion compounds (GICs) for determining the kinetic prmeters of the spin crriers from n nlysis of the CESR line shpe [1-11]. For long time the nlysis of the CESR line shpe for the grphite itself [1,2,11-14] nd its intercltion compounds [3-10] ws crried out using the well-known theory of Dyson [15] nd Kpln [1] not including the surfce spin relxtion of current crriers y the stndrd procedures of Feher nd Kip [17], Koder [1], nd Pifer nd Mgno [19]. However, in strict sense, Dyson s theory [15] of the CESR is pplicle only for infinite metl pltes of ritrry thickness with isotropic conductivity nd single crrier type. Although experiments hve shown the vlidity of using this theory for nlyzing the CESR line shpe in metl pltes with finite dimensions, its pplicility to the cse of grphite nd GICs with lrge nisotropy of skin depths, s well s nisotropy of crrier diffusion, is not ovious. First, it ws pointed out y Müller et l. [20]. Sint Jen et l. [] nd Blinowski et l. [21] hve studied this prolem mthemticlly strictly using the Mxwell equtions. To otin the correct CESR line shpe nlysis in the cse of nisotropic conductors, they hve extended the Dyson theory [15] y tking into ccount the nisotropy of conductivity nd diffusion. Herewith, uthors, s well s ll preceding reserchers, implied tht in GICs it is possile to neglect the surfce spin relxtion effects. In this pper, we present the experimentl results for the dependence of CESR signl prmeters in highly oriented pyrolitic grphite (HOPG) nd in GICs with nitric cid on smple dimensions nd experimentl conditions. The nlysis of this results unequivoclly points to the presence of the strong surfce nd interfce spin relxtion effects in smples investigted. Experimentl The CESR mesurements were crried out t room temperture using n X-nd E-line spectrometer in rectngulr cvity with TE 102 mode. The constnt mgnetic field (H 0 ) modultion frequency nd mplitude were 2.5 khz nd ~0.1 mt, respectively. All pltes for the experiments were cut from single HOPG smple with the conductivity long (σ а ) nd

3 perpendiculr (σ с ) to the sl plne eing equl to (1.2±0.2) 10 4 S/cm nd 7.7 S/cm t 300 K, respectively. They were in the shpe of rectngulr prllelepipeds with the dimensions: width (l) height (h) thickness (d)=l cm 3, where h l is the re of the sl plne. Synthesis of GICs C 10 HNO 3 ws crried out in liquid nitric cid with density ρ 1.55 g/cm 3. The stge of GICs ws determined y X-ry diffrctometer. The intercltion of nrrow HOPG plte y HNO 3 ws crried out on the smple with the dimensions: 2δ c cm 3 (δ c is the skin-depth governed y the σ с conductivity) which ws situted in qurtz tue connected vi vlve to the reservoir with liquid nitric cid. During the mesurements H 0 (the mgnetic component of the microwve field, H rf ) ws prllel (perpendiculr) to the c-xis of the pltes. Results Grphite. For HOPG pltes studied the CESR spectrum consists of single line with the principl vlues of g tensor determined y Feher-Kip [17] nomogrms eing equl to: A/B 4 2 (A) T=300 K, H 0 c; g=200 cm -1, R = l, 10-1 cm l, 10-1 cm Fig. 1. The experimentl (dots) nd the theoreticl (solid lines) vlues of CESR line shpe symmetry prmeter, A/B (A), nd line width, H (B), in grphite vs. smple width l. In (B), curve () corresponds to the vlue of Dyson [15] surfce spin relxtion prmeter g=200 (0) cm -1. g =2.0474± (H 0 с) nd g =2.0029± (H 0 с). For the thick pltes (d>0.045 cm) the dependence of symmetry prmeter, A/B, of the first derivtive of CESR sorption line, which is equl to the rtio of the pek intensity of the more intense wing, A, to tht of the less intense wing, B, vs. l hs three-pek form (Fig. 1A). In the intervl l 1m < l < l 2m, where l 1m (l 2m ) is the coordinte of the first (second) pek (in the direction of l increse) the line hs n inverted line-shpe phse the A pek is locted t higher mgnetic field thn the B pek. At l 1m nd l 2m the line is symmetricl out the A pek, nd the vlue of A/B is mximum. The third, wek mximum is not ssocited with the chnge of phse of the line shpe. At l 0 the experimentl vlues of CESR line width tends to the infinity (Fig. 1B). For ll orienttions of H 0 reltive to the c-xis the H(T) dependence hs mximum ner 20 K (Fig. 2A). H, 10-4 T (B) T=300 K, H 0 c

4 Grphite intercltion compounds: C 10 HNO 3. For ll pltes of GICs C 10 HNO 3 studied, the CESR spectrum, s in grphite, consists of single line with the xil ngulr dependence reltive to the c- xis. The principl vlues of g tensor re equl to g =2.0023± nd g =2.002± The vlue of A/B does not depend on d nd h. In qusi-liquid phse of the interclte susystem (T>T c 250 K) this dependence hs qulittively the sme form s the corresponding dependence in grphite, except for the smll extremum t l* 0.0 cm. This extremum is oserved s well in solid phse of the interclte (T<T c ), where t l>l* the A/B(l) dependence hs n one-pek shpe. Intercltion of grphite y HNO 3. With the intercltion of HNO 3 into grphite the line width (the intensity), H (I=(A+B) H 2 ), of the grphite CESR signl increses (decreses) monotonously (Fig. 2A). At the eginning of the rection, the A/B rtio of signl increses, ut it is still norml reching mximum vlue of A/B~13. The principl vlues of g-tensor for grphite (interclted grphite) signl do not chnge up to its disppernce (up to the end of rection). H, 10-4 T Fig. 2. The experimentl (dots) nd theoreticl (lines) vlues of CESR line width, H, in HOPG pltes vs. temperture (A) nd thickness,, of the non-interclted prt of plte (for two different experiments of grphite intercltion y HNO 3 ) (B). In (A), the theoreticl curve () ws clculted with constnt vlue of intrinsic conduction electron spin relxtion time (using the Elliot [] lw for tempertures much less thn the Deye temperture) nd Dyson [15] surfce spin relxtion prmeter g=200 cm -1. Discussion 10 (A) 20 - H T, K - H c 0,0 0,1 0,2 0,3 0,4 =l-2d*, 10-1 cm The dependence of CESR line shpe symmetry prmeter nd line width on grphite plte width (Fig. 1A nd 1B) differs from the known theoreticl curves, clculted from the Dyson [15] CESR line shpe expression without tking into ccount the effects of surfce spin relxtion. First, the presence of l vlues, for which the CESR line shpe hs n inverted phse is chrcteristic property of the theoreticl curves A/B(l) for the rtio R =(T D /T 2 ) 1/2 (where T D is the time of spin diffusion cross the skin-depth δ c governed y the σ с conductivity, nd T 2 is the intrinsic spin-relxtion time) eing less thn 0, [11,1], wheres the experimentl vlues of A/B for l >> δ c re consistent with the theoreticl vlues of this prmeter for R >0. [11,1] (Fig. 1A). Second, the vlues of A/B in the extrem of the experimentl A/B(l) dependence differ considerly from those for the theoreticl curves [11,1]. Third, t l 0 the experimentl vlues of CESR line width tends to the infinity (Fig. Η, 10-1 mt 10 H 0 c c (B) c - 10 g, cm-1

5 1B), wheres the corresponding Dyson [15] theoreticl curve tends to the finite vlue, which differs from tht for pltes with l >> δ c y 10% only (Fig. 1B). Additionlly, the H(T) dependence hs pek ner 20 K (Fig. 2A), which cn not e explined within the frmework of existing theories of CESR line width in grphite. In Figs. 1A, 1B nd 2A, the results of theoreticl clcultions, respectively, of A/B(l), H(l) nd H(T) dependences in the frmeworks of the Dyson [15] theory including surfce spin relxtion effect of current crriers re presented. The A/B(l) curve ws clculted tking into ccount the sorption of microwve field through ll lterl surfces oth prllel nd perpendiculr to the с- xis nd with the uniform distriution of microwve field. From Fig. 1A nd 1B it cn e seen tht the theoreticl curves with the vlue of Dyson [15] surfce spin relxtion prmeter g=(3ε/4λ )=200 cm -1 (ε is proility of spin reorienttion during the collision of current crriers with the surfce nd Λ is men free pth of current crriers in sl plne) descries the experimentl A/B(l) nd H(l) dt well. The nlysis hve shown tht the experimentl curve H(T) my e lso explined well (Fig. 2.A) under the simultneous presence of the following three fctors: 1) surfce spin relxtion of current crriers, 2) smll mount of the loclized spins (~1% of the current crrier concentrtion or ner one loclized spin per 10 cron toms) with the vlue of g-fctor eing nerly equl to tht for conduction electrons nd 3) complete verging of g-fctors of the conduction electrons nd loclized spins. The nlyzis of origin of the dditionl wek extremum t l* 0.0 cm in A/B(l) dependence for C 10 HNO 3 hs shown tht the similr pek in corresponding theoreticl curve ppers only t the simultneous presence of two fctors in systems investigted: 1) smll mount of the loclized spins nd 2) surfce spin relxtion effect of current crriers. But in this cse the g-fctors of loclized (ner the smple surfce) spins nd deloclized spins re not verged. The invriility of g -vlues for grphite (interclted grphite) up to the disppernce of grphite CESR signl (up to the end of rection), indictes tht the oundry, which seprtes the interclted nd s-yet the non-interclted prts of smple my e considered s non-conductive. We suppose tht the reson for the significnt rodening of the grphite CESR signl is the collisions of current crriers with this non-conductive oundry. Using the reltion d * =(2D int τ) 1/2 (where d * is the thickness of the interclted lyer, D int is interclte two-dimensionl diffusion constnt) the experimentl dependence H(τ) (Fig. 2B) cn e esily trnsformed into the dependence H(), where = l-2d * is the thickness of the noninterclted prt of HOPG plte (Fig. 2B). The ltter dependence cn e clculted theoreticlly s well, using the Dyson theory for the CESR in metls, including the effects of surfce relxtion. It is shown in Fig. 2B, where the results of such nlysis re presented, tht the theoreticl dependence of the grphite CESR line width, with Dyson [15] prmeter g=1 cm -1 (curve ) nd g=10 cm -1 (curve c) descries the experimentl dt well. Acknowledgments This work ws prtilly supported y the Russin Foundtion for Bsic Reserch (grnts No nd No ).

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