STRUCTURAL AND MAGNETIC PROPERTIES OF Fe/Si x Fe 1! x MULTILAYERS

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1 MOLECULAR PHYSICS REPORTS 0 (00) 8-86 STRUCTURAL AND MAGNETIC PROPERTIES OF Fe/ x Fe! x MULTILAYERS P. WANDZIUK, M. KOPCEWICZ, B. SZYMAŃSKI, AND T. LUCIŃSKI Institute of Moleculr Physics, Polish Acdemy of Sciences, Poznń, Polnd Institute of Electronic Mterils Technology, Wrszw, Polnd Astrct: The Fe/ x Fe! x (x =, 6, 0.5) nd Fe/Co//Co multilyers deposited y mgnetron sputtering hve een studied. The strongest ntiferromgnetic coupling ppered in Fe/ multilyers for d =.35 nm with sturtion field H S =.5 T. Bsed on the temperture dependencies of the remnent mgnetiztion we hve estimted the semiconductor spcer lyer energy gp E g 00 mev. It suggests tht the Fe- silicide responsile for the ntiferromgnetic coupling in Fe/ multilyers is semiconducting morphous-like or nnocrystlline x Fe! x phse rich in. Anneling the multilyers destroys ntiferromgnetic interlyer exchnge coupling.. INTRODUCTION Although the Fe/ heterostructures hve een extensively studied for severl yers [-], their interlyer coupling mechnism still remins uncler. It hs not een explined how the iron-silicides formtion ffects the interlyer coupling nd mgnetic properties. The im of this report is to clrify whether the existence of the strong ntiferromgnetic (AF) coupling in the Fe/ x Fe! x multilyers (Mls) is due to the presence of intermetllic Fe- phses such s β Fe nd/or ε-fe, which my rise from the interdiffusion of Fe into sulyers.. EXPERIMENT The multilyered smples consisting of {Fe(3 nm)/ x Fe! x ( )} 5 (x =, 6 nd 0.5) nd {Fe(3 nm)/co(d Co )/(.5 nm)/co(d Co )} 5 were deposited in UHV y mgnetron sputtering t room temperture onto oxidized sustrtes. The x Fe! x spcer thickness vried etween 0.5 nd 3 nm nd the thickness of Co lyer d Co ws 0.5 nd 0.5 nm, thus the Fe/Co//Co Mls were, in fct, the Fe/ Mls with thin Co lyers inserted etween Fe nd sulyers. The interfce nd structurl properties were investigted y the conversion electron Mössuer spectroscopy (CEMS) nd X-ry diffrction, respectively. Their mgnetic properties were exmined y virting smple mgnetometer (VSM) nd extrction mgnetometer mesurements. Temperture dependencies of resistnce R(T) nd mgnetiztion were performed in the temperture rnge.-300 K. 3. RESULTS AND DISCUSSION The multilyer periodicity nd the crystlline structure were investigted y smll- nd high- ngle X-ry diffrction (SAXRD nd HAXRD), respectively. The SAXRD spectr shown in Fig. exhiit Brgg-like mxim, which confirm well defined periodic structure of the

2 Structurl nd Mgnetic Properties of Fe/ x Fe! x Multilyers 83 exmined Mls. Compring the SAXRD spectr we cn see, tht the Fe/ x Fe! x plots exhiit smller numer of peks thn the Fe/ spectrum. This cn e due to the etter contrst etween Fe nd nominlly pure sulyers, in the cse of Fe/ Ml. The HAXRD spectr shown in Fig. exhiit only single wek mximum of the cc-fe(0) phse, nd severl peks originting from the sustrte. There is lso tiny mximum t out 8º, which my e relted to iron silicide phses. The HAXRD results showed, tht the exmined Mls were morphous-like or nnocrystlline with crystllite dimension in the direction perpendiculr to the smple plne not exceeding Fe sulyer thickness. Reltive intensity Fe(3nm)/(.nm) Fe(3nm)/Fe (nm) Fe(3nm)/Fe (nm) θ [deg] Reltive intensity (00) cc Fe (0) (00) (00) cc Fe (0) (00) cc Fe (0) (00) θ [deg] (00) Fe-? Fig.. SAXRD () nd HAXRD () spectr of Fe/, Fe/ 0.33 Fe 6 nd Fe/ Mls. The sulyers thicknesses s indicted ove the plots The interfce structure of the exmined Mls ws investigted y CEMS. The CEMS spectr shown in Fig. recorded t room temperture for Fe/ Mls with d Fe = 3 nm nd d =.,. nd.3 nm consist of 3 components: the Zeemn sextet with the hyperfine field H hf 3.8 T, nd the isomer shift δ = 0 mm/s, chrcteristic of the cc-fe phse of Fe lyers, nd two spectrl components relted to Fe- system t interfces: (i) mgnetic rodened sextet with H hf 9 T nd δ +5 mm/s, originting most proly from Fe toms t vrious interfcil step-sites nd (ii) nonmgnetic component consisting of qudrupole doulet with the splitting QS 0 mm/s nd δ + mm/s. The lst component corresponds to nonmgnetic iron silicide formed t interfce nd is relted either to smll gp ε-fe semiconductor, the crystlline c-fe! x x metllic phse or to the morphous Fe phse rich in [, ]. The CEMS spectr of Fe/ 6 Fe 0.33 Mls with two different thicknesses of Fe lyers (. nd 3 nm) consist of similr 3 components, ut the spectrl contriution of the QS doulet is significntly lrger for the thicker Fe lyer (Fig. ). Bsed on the QS nd δ vlues of the nonmgnetic spectrl component it is difficult to determine the exct structure of the interfcil phse.

3 8 P. Wndziuk et l. Mgnetic properties were mesured y virting smple mgnetometer nd extrction mgnetometer. The sturtion field H S nd F AF fctor (F AF =! M R /M S, where M R nd M S denote the remnence nd sturtion mgnetiztion, respectively) versus nonmgnetic spcer lyer thickness dependence is shown in Fig. 3.. nm 3/.nm. nm Reltive intensity. nm.3 nm c Reltive intensity nm 3/3nm V elocity [m m /s] Velocity [mm/s] Fig.. CEMS spectr of Fe/ () nd Fe/ 6 Fe 0.33 () Mls with indicted spcer lyer thicknesses The existence of only single H S ( ) mximum nd exponentil decy of H S vlues ove it suggests, tht the exchnge coupling in the exmined Mls is not due to RKKY-like mechnism. It seems to correspond rther to the quntum interference model given y Bruno [5]. However, ccording to this model, in metl/insultor structures the AF coupling increses with incresing temperture, wheres our mesurements (Fig. ) reveled, tht the coupling decreses with incresing temperture. Therefore, the oserved H S ( ) ehvior suggests, tht the strong AF coupling in the Fe/ Mls hs nother origin..6.. Fe/ Fe/Fe Fe/Fe Fe/ Fe/Fe Fe/Fe H S [T] F AF =-(M R /M S ) [nm] [nm] Fig. 3. Sturtion field H S () nd F AF fctor () vs nonmgnetic spcer lyer thickness dependencies for Fe/ x Fe! x Mls

4 Structurl nd Mgnetic Properties of Fe/ x Fe! x Multilyers 85 In order to identify etter the composition of n interfcil silicide phse the energy gp of this phse ws estimted. According to Inomt et l. [3], the energy gp ws determined from the nlysis of the slope of!ln(m R /M S ) vs /T which is equl to E g /k B T (E g is the energy gp over which the crriers must e thermlly excited). Our plots of!ln(m R /M S ) vs /T for Fe/, Fe/ 6 Fe 0.33 nd Fe/ 0.50 Fe 0.50 (Fig. ) re similr to the temperture dependence of the crrier concentrtion in n impurity semiconductor. It suggests tht spcer, which is semiconducting, my induce the coupling in Fe/ x Fe! x Mls. The steep rise region of!ln(m R /M S ) ove 00 K corresponds to the intrinsic region in semiconductor. Using this nlysis we estimted tht the energy gp is out 00 mev. This gp is lrger thn tht found for ε-fe (50 mev []) nd much smller thn tht for the β-fe (80 mev []). H S [T] T [K] =.35 nm =. nm =. nm F AF -ln(m R /M S ) 3 Fe/ =.35nm Fe/ 6 Fe 0.33 =.nm Fe/ 0.50 Fe 0.50 =.nm /T [K - ] Fig.. Temperture dependencies of H S nd F AF () nd!ln(m R /M S ) vs /T dependencies () for Fe/, Fe/ 6 Fe 0.33 nd Fe/ 0.50 Fe 0.50 Mls, respectively It my suggest therefore tht the Fe- phse responsile for the AF coupling in the studied Mls is semiconducting morphous-like (or fine crystlline) x Fe! x phse rich in with E g 00meV. The existence of the morphous-like x Fe! x phse seems to e confirmed y therml nneling of our Mls ( h t 0ºC) fter which ll exmined multilyers showed the existence of the ferromgnetic coupling with strongly reduced F AF vlue up to 0.. The R(T) dependencies for ll exmined multilyered smples exhiit metllic ehvior, wheres the single thin film of x Fe! x shows semiconducting ehvior. Figure 5 displys the exmples of R(T) dependencies for Fe/ 0.50 Fe 0.50 Ml nd 0.50 Fe 0.50 thin film. It demonstrtes, tht the min contriution to the R(T) dependencies in exmined Mls rises minly from Fe sulyers. At out 5 K wek R(T) minimum ppers for Fe/ x Fe! x Mls nd persists even in the presence of mgnetic field of T. Therefore it testifies, tht the resistnce minimum is not due to the rel Kondo effect, ut my originte from the structurl disorder. According to [6], we could expect, tht thin lyer of Co inserted etween Fe nd sulyers prevented the formtion of Fe silicides. Actully, even 0.5 nm thick Co lyer set t the interfces disles the formtion of the Fe phses leding to the ferromgnetic coupling

5 86 P. Wndziuk et l. Fig. 5. R(T ) dependencies for Fe/ 0.50 Fe 0.50 ( =. nm) Ml nd 30 nm 0.50 Fe 0.50 thin film Fig. 6. Comprison of M(H ) dependencies for Fe/ nd Fe/Co//Co Mls only. The comprison of two histeresis loops of Fe/ Mls with- nd without Co sulyers is shown in Fig. 6. Acknowledgements Supported y the Stte Committee for Scientific Reserch under grnt PBZ/KBN/0/P03/00. References [] G. J. Strijkers, J. T. Kohlhepp, H. J. M. Swgten, nd W. J. M. de Jonge, Phys. Rev. B 60, 9583 (999). [] T. Luciński, M. Kopcewicz, A. Hütten, H. Brückl, S. Heitmnn, T. Hempel, nd G. Reiss, J. Appl. Phys. 93, 650 (003). [3] K. Inomt, K. Yusu, nd Y. Sito, Phys. Rev. Lett. 7, 863 (995). [] E. G. Moroni, W. Wolf, J. Hfner, nd R. Podloucky, Phys. Rev. B 59, 860 (999). [5] P. Bruno, Phys. Rev. B 5, (995). [6] T. Luciński, P. Wndziuk, F. Stoiecki, B. Andrzejewski, M. Kopcewicz, A. Hütten, G. Reiss, nd W. Szuszkiewicz, to pper in J. Mgn. Mgn. Mter.