Grain-size dependent high-temperature ferromagnetism of polycrystalline Mn x Si 1-x (x~0.5) films

Transcription

1 Grin-size dependent high-temperture ferromgnetism of polycrystlline Mn x Si 1-x (x~0.5) films S.N. Nikolev 1, A.S. Semislov 2,3, V.V. Rylkov 1,4(), V.V. Tugushev 1,5(b), A.V. Zenkevich 6,7, A.L. Vsiliev 1, E.M. Pshev 1, K.Yu. Chernoglzov 1, Yu.M. Chesnokov 1, I.A. Likhchev 1, N.S. Perov 3, Yu.A. Mtveyev 6,7, O.A. Novodvorskii 8, E.T. Kultov 5, A.S. Bugev 4,6, Y.Wng 2, S. Zhou 2 1 Ntionl Reserch Centre Kurchtov Institute, Moscow, Russi 2 elmholtz-zentrum Dresden-Rossendorf, Institute of Ion Bem Physics nd Mterils Reserch, Butzner Lndstrsse 400, Dresden, Germny 3 Fculty of Physics, Lomonosov Moscow Stte University, Moscow, Russi 4 Kotel nikov Institute of Rdio Engineering nd Electronics RAS, Fryzino, Moscow Region, Russi 5 Prokhorov Generl Physics Institute RAS, Moscow, Russi 6 Moscow Institute of Physics nd Technology, Dolgoprudny, Moscow Region, Russi 7 Ntionl Reserch Nucler University MEPhI, Moscow, Russi 8 Institute on Lser nd Informtion Technologies RAS, Shtur, Moscow Region, Russi E-mils: () vvrylkov@mil.ru; (b) tuvictor@mil.ru Abstrct We present the results of comprehensive study of mgnetic, mgneto-trnsport nd structurl properties of nonstoichiometric Mn x Si 1-x (x ) films grown by the Pulsed Lser Deposition (PLD) technique onto Al 2 O 3 (0001) single crystl substrtes t T = 340 C. A highlight of our PLD method is the using of non-conventionl ( shdow ) geometry with Kr s scttering gs during the smple growth. It is found tht studied films exhibit high-temperture (T) ferromgnetism (FM) with the Curie temperture T C ~ 370 K ccompnied by positive sign nomlous ll effect (AE); they lso revel the lyered polycrystlline structure with self-orgnizing grin size distribution. The T FM order is originted from the bottom interfcil nnocrystlline lyer, while the upper lyer possesses the low temperture (LT) type of FM order with Т С 46 K, gives essentil contribution to the mgnetiztion t T 50 K nd is homogeneous on the nnometer size scle. Under these conditions, AE chnges its sign from positive to negtive t T 30K. We ttribute observed properties to the synergy of self-orgnizing distribution of Mn x Si 1-x crystllites in size nd peculirities of defect-induced FM order in PLD grown polycrystlline Mn x Si 1-x (x~0.5) films. 1. Introduction Mn x Si 1-x (x 0.5) lloyed films with composition close to the mngnese monosilicide MnSi re mterils with exceptionl combintion of mgnetic nd trnsport properties; t the sme time they re promising for spintronic pplictions [1-7]. The perfect single crystl -MnSi with B20-type of structure possesses t low tempertures ( 30 K) intriguing mgnetic nd trnsport phenomen cused by formtion of new mgnetic qusiprticles skyrmions (see [6] nd references therein). On the other hnd, the Mn x Si 1-x (x 0.5) thin lyers grown on Si(001) or Al 2 O 3 (0001) substrtes demonstrte the high-temperture (T) ferromgnetism (FM) with the Curie temperture T c of the order of room temperture [2-4]. This fct is in contrst to the cse of

2 bulk -MnSi single crystl, where only the low-temperture (LT) FM ws reported with T C 30 К [8, 9]. The T FM order t х (tht just corresponds to single crystl -MnSi belonging to berthollides [9, 10]) ws observed in the Mn x Si 1-x /Si(001) structures but t enough smll Mn x Si 1-x film thickness less thn one -MnSi monolyer [2, 11]. This order is explined by the formtion of c-mnsi phse with B2-like (CsCl) crystl structure stbilized with tetrgonl distortion due to fvorble lttice mismtch between the film nd substrte [1]. Recently we reported the T FM ppernce with T C 330 K in 70 nm thick Mn x Si 1-x (х ) films grown on the Al 2 O 3 (0001) substrtes by pulsed lser deposition (PLD) technique [3,4]. We rgued tht the observed T FM hs defect-induced nture: it is due to formtion of locl mgnetic moments on the Si vcncies inside the MnSi mtrix nd the strong exchnge coupling between these moments medited by spin fluctutions of itinernt crriers [12]. The Mn x Si 1-x films in [3, 4] were deposited t reltively slow deposition rte (~2 nm/min) using PLD method in conventionl direct geometry (DG) when the surfce of Al 2 O 3 (0001) substrte is exposed to the Mn-Si lser plume. Accordingly to tomic-force microscopy (AFM) mesurements, the structure of thus grown films is mosic, with the crystllite size ~0.5-1 m. In this work we present pioneering results of comprehensive study of mgnetic, mgnetotrnsport nd structurl properties of the Mn x Si 1-x (х 0.52) polycrystlline films grown by PLD technique employing unconventionl shdow geometry (SG) with Kr s buffer gs. As compred to the conventionl direct geometry (DG) of Ref. [3, 4], in the SG method the effective scttering of blted prticles in the buffer gs results in the lower energy of the depositing toms s well s very high deposition rte [13]. We found tht SG grown Mn x Si 1-x (x 0.5) films hve two mgnetic phses: T FM phse with T C 370 K nd LT FM phse with T C 46 K. At the sme time, the nomlous ll effect (AE) chnges its sign from the positive to negtive one t the temperture below 30 K. We explin obtined experimentl results by the interply of two effects: 1) self-orgniztion of polycrystlline film leding to the formtion of two lyers with strongly differing sizes of crystllites; 2) peculirities of defect-induced FM ordering in such system. 2. Smples nd experimentl detils The SG grown Mn x Si 1-x thin films were deposited in Kr tmosphere (~10-2 mbr) onto the Al 2 O 3 (0001) substrtes 10х15 mm 2 in size using the single crystl MnSi trget [13]. The substrte temperture during the deposition (340 C) ws the sme s for previous DG deposited films, while the deposition rte ws higher ( 7 nm/min). The Rutherford bckscttering spectrometry (RBS) ws used to determine the film composition nd thickness [13]. The film thickness d depends on the distnce L to the trget; the vlue d decreses from 270 to 70 nm with 2

3 the increse of L t the length L 15 mm. The Mn content t the sme deposited re increses from up to When the film thickness decreses from 160 to 70 nm ( L 10 mm), the film composition chnges only slightly with L (х ). To investigte the effect of the film composition nd deposition rte on the mgnetic nd mgneto-trnsport properties, the s grown smple ws cut into seven 2х10 mm 2 stripes with different thicknesses. ere we present the results for the most distnt from the trget smples with slightly chnging Mn content х nd different film thickness nm. The structurl properties of Mn-Si smples were investigted by X-ry diffrction (XRD) mesurements using Rigku SmrtLb diffrctometer. To elucidte the microscopic structure of Mn-Si films, they were further investigted by scnning trnsmission electron microscopy (STEM) using TITAN TEM/STEM instrument (FEI, US) operting t n ccelerting voltge of U=300 kv, equipped with Cs-probe corrector, high-ngle nnulr drk-field detector (AADF) (Fischione, US) nd energy dispersive X-ry (EDX) micronlysis spectrometer (EDAX, US). Cross-section trnsmission electron microscopy (TEM) specimens were prepred by the mechnicl polishing of sndwiched pieces followed by Ar + ion-bem milling until perfortion in Gtn PIPS (Gtn, US). 3. Mgnetic nd mgneto-trnsport mesurements The temperture dependence of sturtion mgnetiztion M s (T) of three Mn x Si 1-x smples 1-3 (х 0.517, nd 0.514) with the thicknesses d 70, 90 nd 160 nm, respectively, is presented in Fig. 1. The pplied field ws 0 = 1 T. The obtined dt of Fig.1 reveled presence of two ferromgnetic phses: T phse with T C 370 K nd LT phse with T C 46 K. The reltive contribution of the LT FM phse clerly increses with the increse of the film thickness. Such behvior is in contrst to tht of DG grown Mn x Si 1-x films (Fig. 2). When х 0.52, the decrese of M s (T) in the temperture rnge T = К does not exceed 6% nd fits well to the Bloch lw [13]. Moreover, the M s (T) vlue does not increse significntly with lowering T even in cse of T FM degrdtion, s observed in DG films with the Mn content x 0.53 (Fig. 2, see lso [3]). Fig. 3 shows the mgnetiztion vs. mgnetic field M() dependence for the smple 1 (х 0.517, d 70 nm) t T = 5, 100 nd 300 К. The hysteresis loop opens t temperture below 100 K (see inset in Fig. 3), which is not observed in bulk -MnSi single crystl. The mgnetiztion sturtes in the mgnetic field T t low temperture (T = 5 K) nd then linerly increses like in the cse of -MnSi single crystl [9, 14]. 3

4 It is curiously to note tht t T > 46 K the surfce density of mgnetic moment J m /A of the T FM phse (i.e. the totl mgnetic moment J m normlized to the film surfce A; see inset to Fig. 1) does not depend on the film thickness. This fct clerly indictes tht the T FM phse (T C 370 К) is formed only in the interfcil lyer directly deposited on the substrte with fixed thickness, while the LT FM phse (T C 46 К) is formed in the upper lyer with vrible thickness. So, the mgnetometry dt llude to presence of two FM lyers with different thicknesses, mgnetic moments nd Curie tempertures in our films. The ll effect provides rich informtion on the correltion between mgnetic nd trnsport properties of Mn-Si system under investigtion. Let us recll tht in ordinry FM mteril, the ll resistnce R contins two components following the expression [15]: R d where is the totl ll resistivity, n n, R B, M, (1) n nd 0 R s re the norml nd nomlous components of the ll resistivity, respectively, d is the thickness of FM mteril, R 0 is the norml ll effect constnt relted to the Lorentz force, B is mgnetic induction, R s ( xx ) α is the nomlous ll effect (AE) constnt relted to the spin-orbit interction in FM mteril, M is the mgnetiztion. For skew-scttering driven mechnism of AE, α = 1, nd for intrinsic nd side-jump mechnisms of AE, index α = 2 [15]. Usully, t the temperture T T C nd for the mgnetic field corresponding to the sturtion mgnetiztion, the second term in Eq.(1) domintes, i.e.. Note tht in the cse of -MnSi single crystl, the third term my lso n pper in Eq.(1) due to skyrmions formtion [16] (so-clled component of the topologicl ll effect), but in our system we presume tht skyrmions re destroyed due to the scttering on the structurl nd mgnetic disorder in the Mn x Si 1-x lloy. Fig. 4 demonstrtes the mgnetic field dependence of (B), s mesured for the smple 1 (d 70 nm, х 0.517) t the temperture rnge T = K. One cn see tht the nomlous component in the sturtion regime (t B 1 T) decreses up to 10 times s the temperture decreses from 200 K to 5 K. One cn notice tht in cse of the DG grown film the vlue of in the sme temperture rnge is either nerly constnt (for х 0.52) or increses s the temperture lowers (up to 2 times for х 0.55, see [3]). The unusul behvior of (B) in the SG grown film cn be explined s prtil compenstion of the positive ll emf from the bottom T FM lyer nd the negtive ll emf from the upper LT FM lyer (see inset to Fig. 4). To justify this explntion we hve to suggest tht in the upper lyer, the effect of LT FM order on the ll trnsport is similr to the cse of bulk -MnSi, where AE hs the negtive sign [14, 16]. At the sme time, we hve to postulte tht in the bottom lyer, the effect of the T FM 4

5 order on the ll trnsport is similr to the cse of DG films [3], where the AE of positive sign ws reported [3] (the AE of positive sign is observed lso in morphous Mn x Si 1-x lloys [7, 17]). Evidently, in the two-lyer SG grown film prtil compenstion of negtive nd positive contributions (B) should be more pronounced t tempertures below the Curie temperture of the LT FM lyer (T C 46 К); this compenstion becomes more efficient with the film thickness incresing. The temperture dependence of (T) of the thicker smple 2 (d 90 nm, х 0.516) mesured t В = 1.2 T is presented in Fig. 5. One cn see tht below T 50 К the (T) function flls down nd then chnges its sign to the opposite below T 30 К. In the temperture rnge T 30 К, the hysteresis loop (B) cquires unusul shpe (Fig. 5b). Obviously, this is the result of superposition of two AE components: the first one is hysteretic nd provided by T FM lyer, 0, while the second one is non-hysteretic nd provided by the LT FM lyer, Notice, tht due to the lrger vlues of thickness nd conductivity of the LT FM lyer, its contribution to the ll resistnce is lrger in bsolute vlue thn tht from the T FM lyer (see Eq. 4 below). The positive sign of 1 component is not surprising nd testifies to similrity of structurl, mgnetic nd trnsport properties of the bottom T FM lyer nd DG grown Mn x Si 1-x films. The negtive sign of 2 my be ttributed to similrity of the properties of the upper LT FM lyer nd -MnSi, where is negtive [14, 16]. It is lso importnt to note tht norml ll effect in -MnSi is positive [14, 16]; therefore, the liner behvior of the (B) dependence in fields the B 0.7 T corresponds to the hole type of conductivity (see Fig. 5b). In order to nlyze better the results of ll effect mesurements in two-lyer system, we hve lso studied the temperture dependence of longitudinl resistivity (T) for grown SG films. In Fig.6, the normlized temperture dependences (T) = SG (T) nd (T)= DG (T) (tken from [3]) re shown, respectively, for SG nd DG grown Mn x Si 1-x films (d 70 nm; x 0.52), in comprison with (T) = SC (T) for -MnSi single crystl (tken from [18]). Note the similrity between SG (T) nd SC (T) nd its drstic difference from DG (T). 4. Structure mesurements The results of XRD mesurements of s grown Mn x Si 1-x /Al 2 O 3 (0001) smples 10x15 mm 2 in size (before cutting) re shown in Fig. 7. The ngulr rnge 2θ = contins severl peks, which re ll ttributed to -MnSi phse with B20 structure. An dditionl intense diffrction pek observed t 2θ = 64.5 does not belong to -MnSi nd could point t (200) plne 5

6 diffrction of c-mnsi phse (similrly to c-fesi phse in Ref. [19]). owever, further nlysis of XRD rocking curve revels tht this pek is due to qusi-forbidden reflection (0009) from the Al 2 O 3 substrte nd ppers s result of multiple reflections (so-clled multi-wve diffrction, see insert to Fig. 7). The results of the TEM nlysis, prticulrly, low mgnifiction bright field TEM imge of Mn x Si 1-x /Al 2 O 3 (0001) is shown in Fig. 8. Mn x Si 1-x film hs columnr microstructure with the lterl grin sizes of bout 50 nm. The electron diffrction (ED) study nd Fourier nlysis of lttice imges (not shown) pointed to the B20 type of crystl structure of the MnSi film consistent with XRD dt. Drk-field high resolution STEM imges of the Mn x Si 1-x /Al 2 O 3 (0001) interfce, shown in Fig 8b, reveled the presence of nnometer size crystllite lyer ner the interfce with the thickness of ~10 nm. The grins exhibit equixed morphology with the size of ~5 nm. The Fourier nlysis of igh Resolution TEM imges (Fig. 8 c-e) evidences tht these crystllites dopt B20 crystl structure of -MnSi single crystl. 5. Discussion The results of TEM investigtion clerly indicte two-lyer structure in the studied Mn x Si 1-x (x ) films, pprently due to the peculirity of the SG growth process. The importnt difference between the structures of ech lyer is their grin sizes which re dopted during the growth. The bottom interfcil T FM lyer directly deposited on the substrte is composed of the nnometer size crystllites (~5 nm) nd hs the fixed thickness (~10 nm), while the upper LT FM lyer is prcticlly homogeneous on the nnometer scle nd chnges its thickness from ~ 60 nm to ~ 150 nm in studied SG grown films. On the bsis of this two-lyer picture, let us nlyze the dt of mgnetic nd trnsport mesurements of Mn x Si 1-x films (x ). First of ll, we hve to estimte the vlue of effective mgnetic moment on Mn tom in both lyers, suggesting tht the density of Mn x Si 1-x (x ) lloy is equl to tht of the bulk -MnSi single crystl, i.e g/cm 3 [20]. The T FM nd LT FM phse contributions to the totl mgnetiztion of the film cn be found using the simplified Brillouin function fit for M s (T): n M ( T ) M (0)[1 ( T / T ) ]. (2) s s In our cse, n = 1.5 leds to the best fit of experimentl M s (T) dt (Fig. 1). Using Eq. (2), we hve found for the smples with x ( ) the effective mgnetic moments m = ( ) B /Mn nd ( ) B /Mn for for T nd LT FM phse, respectively. C 6

7 The effective mgnetic moment of the bottom T FM lyer in Mn x Si 1-x films (x ) grown in the SG significntly exceeds the mgnetic moment of MnSi single crystl, m 0.4 B /Mn [9]. It is lso higher s compred to the cse Mn x Si 1-x film grown in DG (x 0.52, T C 330K), where the effective mgnetic moment is m 1.1 B /Mn [3]. These fcts do not leve doubt bout existence of defect-induced locl mgnetic moments in the T FM phse, which re formed due to the sme mechnism s in DG grown nonstoichiometric Mn x Si 1-x (x 0.52) lloys. The origin of this mechnism, following Ref. [3], consists in the vrition of coordintion number of Mn tom ner the Si vcncy. Due to strong hybridiztion between 3d-electron sttes of Mn nd 3(s,p)-electron sttes of Si this vrition leds to the corresponding locl redistribution of chrge nd spin densities ner the Si vcncy, which is thereby responsible for the formtion of complex defect with locl mgnetic moment ~( ) B /Mn nd effective (verge) mgnetic moment ~( ) B /Mn. The effective mgnetic moment of the upper LT FM lyer is in good greement with the mgnetic moment of single crystlline -MnSi; this fct my be nturlly interpreted s n bsence of locl mgnetic moments in the upper LT FM lyer. At first glnce, this conclusion is surprising, since ccording to the results of TEM studies nd Rutherford bckscttering nlysis [13] the composition of the film is homogeneous cross the film thickness, i.e. LT FM phse contins the sme excess mount of Mn toms s in T FM phse. Therefore, we hve to suggest tht most prt of Mn contining defects in the upper LT FM lyer is in wek-mgnetic or non-mgnetic ( mgneticlly ded ) configurtion. Following Ref. [3, 4], s n exmple of such the configurtion we cn imgine n interstitil Mn tom introduced into the MnSi mtrix. The clculted mgnetic moment on this Mn tom is extremely smll (~0.09 B /Mn) nd the effective (verge) mgnetic moment is ~0.34 B /Mn for Mn x Si 1-x (x 0.52) film. To explin mgnetic dt we suppose tht due to the specificity of the SG method the Si vcncies minly rise in the lower lyer of the film. The nnocrystllite boundries in this lyer form vst network; they eventully cn work s the gettering regions for Si vcncies nd, consequently, for locl mgnetic moments on these vcncies. So, following our supposl, nnocrystllite boundries ply the key role in the mgnetic properties of T FM lyer, cting s mgnetic envelope of the nnometer scle non-mgnetic crystllite. Erly in Ref. [12] in the frme of the spin-fluctution model of FM ordering, we hve nlyzed the role of dimension effects in grnulr dilute Si-Mn lloys. We considered the precipitte nnoprticles of SiMn 1.7 type in the Si mtrix nd estimted vrition of the Curie temperture s function of the shpe nd size of these precipittes. Similr nlysis cn be effectuted for the cse of Mn x Si 1-x (x ~ 0.5) lloys. For sphericl crystllite of wek itinernt FM with the smll rdius r 0 << ζ, where ζ is FM correltion length, encircled by n envelope with defect induced locl mgnetic 7

8 moments S hving the surfce density roughly estimte the Curie temperture T C s k 0 2,where is the lttice prmeter, we cn 1/ 2 1/ 2 2 1/ 2 BTC ~ JS( W / vfqsf ) ( / r0 ) ( 0 ). (3) ere J is exchnge interction potentil between the locl moment on the defect nd itinernt electron spin, W is itinernt electron bndwidth, v F is the Fermi velocity, Q SF is spin-fluctution cutoff wve vector. At JS ~ 0.1 ev, W v Q F ~ 10, /r 0 ~ 10-1, / SF K tht is not fr from bove obtined experimentl results. 2 0 ~ we hve T C ~ Let us now consider the trnsport dt. As the temperture decreses from 300 to 5 K, the DG (T) curve in Fig.6 demonstrtes reltively slow (bout 1.3 times) temperture decresing. It ws lso shown in Ref. [3] tht, contrry to the cse of -MnSi single crystl, for the DG films the crrier mobility strongly increses (bout fifteen times t 60 K), but the crrier concentrtion drsticlly decreses (bout twenty five times t 100 K). Thus, DG (T) behvior is driven by n interply of these two effects nd s result, the vlue DG (T) for Mn x Si 1-x (x 0.52) film below ~ 40 K significntly exceeds SC (T) for -MnSi, where SC (T) flls down drmticlly [18]. The physicl origin of this remrkble phenomenon of simultneous increse of crrier mobility nd decrese of crrier concentrtion t the doping of single crystl -MnSi with dditionl Mn toms is not yet cler. A possible (but certinly open to discussions) reson qulittively explining experimentl dt hs been proposed in Ref. [3]. It presumes tht: 1) the Mn doping induces the crrier locliztion on the defect center (e.g., the bove discussed Si vcncy) in the MnSi mtrix; 2) this doping lso destroys collective (Kondo or spin-polron type) resonnce, probbly existing in -MnSi single crystl. The combintion of these two effects obviously leds to the simultneous decrese of crrier concentrtion nd the increse of crrier mobility, if we suggest tht the dditionl crrier mobility decrese due to the crrier scttering on the defects is smll compred to the crrier scttering on the collective resonnce. The temperture resistivity dependence SG (T) in the high temperture region (bove T ~250 K) hs lmost the sme chrcter s DG (T), but differs from it t low tempertures (see Fig. 6). Between T = 250K nd 40K, the SG (T) function decreses lmost 1.6 times more thn DG (T); below T 40 K, the SG (T) function flls down similr SC (T) in the cse of -MnSi single crystl (Fig. 6), lthough not so drsticlly. Obviously, tht extrction of serious physicl informtion from the direct comprison of SG (T) nd DG (T) is hmpered, since the SG film hs two-lyer structure, but the DG film is homogeneous. The problem is to estimte correctly the contribution of ech lyer to SG (T). Assuming tht conductivities of both lyers re of the sme order, we cn roughly suggest tht for thick films (d ~ d 2 >> d 1 ) the function SG (T), 8

9 which hs lmost the sme chrcter s for -MnSi single crystl (Fig. 6), minly corresponds to the temperture dependence of the resistivity of the upper LT FM lyer. Thus, t lest for qulittive purposes we my fncy the upper LT FM lyer s the -MnSi single crystl with nonmgnetic electro-neutrl defect centers nd not completely destroyed collective resonnce. Unfortuntely, it is difficult to conclude definitely bout the internl structure of the lower T FM lyer if one tkes into ccount only SG (T) dt. We cn only speculte tht the mteril of this lyer is similr to the one of DG film. We re ble to obtin dditionl informtion on the physicl properties of the LT FM nd T FM phses nlyzing the mgneto-trnsport dt for the SG film. If we present this film s two prllel conducting lyers (see inset to the Fig. 4) the effective ll resistivity cn be written s R d d , (4) 2 ( 1d1 2d2 ) where the indices 1 nd 2 correspond to the lower (T FM) nd upper (LT FM) lyer, respectively. From Eq. (4) it is seen tht in thick films (d ~ d 2 >> d 1 ) the chnge of the ll effect sign is possible when temperture decreses below upper lyer Curie temperture (T C2 46 К) nd the negtive nomlous component of the ll effect ( 0 ) in this lyer strts to ply 2 dominnt role due to its similrity to the cse of bulk -MnSi [14, 16]. The rtio between the conductivities of two lyers 2 / 1 cn be found using following ssumptions: 1) the AE resistivity of lower lyer t T < 200 K is the sme s for DG film [3], i.e cm; 2) the AE resistivity of upper lyer t Т = (25-40) К is the sme s for -MnSi single crystl [14, 16], i.e. 2 -( ) 10-6 cm; 3) the sign of the ll effect chnges to the opposite t the thickness d = d 2 + d 1 = (70-90) nm (Figs. 4 nd 5). Substituting these dt in Eq.(4) we obtin the rtio 2 / 1 2. In other words, in spite of significnt decrese of the nno-crystllites size in the bottom lyer compred to tht in the upper lyer, the conductivity of bottom lyer t low temperture does not significntly decrese. Probbly, we observe here the effect of prtil compenstion of two effects (crrier concentrtion decrese nd crrier mobility increse) hving the sme physicl origin s in bove discussed cse of DG thin film [3]. 6. Conclusions In this work, we present for the first time the results of comprtive study of mgnetic nd trnsport properties of nonstoichiometric Mn x Si 1-x (x ) films grown by the PLD technique onto the single crystl Al 2 O 3 (0001) substrtes t T = 340 C using SG nd DG method. 9

10 The key point of SG pproch is the using Kr s scttering gs which results in the lower energy of deposited toms. At the sme time, the verge deposition rte in SG is much higher ( 7 nm/min) thn in DG. The SG grown Mn x Si 1 x films on the rectngulr substrte 10x15 mm 2 in size possess slightly vrying composition (x= ) nd lrge vrition in thickness (d= nm) depending on the distnce from the Mn-Si trget. X-ry diffrction nlysis revels tht textured -MnSi-like phse with the B20-type crystl structure domintes in both SG nd DG type of films. While the -MnSi single crystl hs the Curie temperture T C 30 K [8, 9], the studied Mn x Si 1-x films t x 0.52 exhibit T FM with T C > 300 K ccompnied by the mnifesttion of the positive sign of AE. For SG grown Mn x Si 1-x films, it is found tht t low temperture the essentil contribution to the mgnetiztion is given by LT FM phse with Т С 46 K; t the sme time, AE chnges the sign from the positive to negtive t T 30K nd film thickness d 90 nm. We explin these results s the mnifesttion of self-orgnizing effect in the SG polycrystlline Mn x Si 1-x film, i.e. the formtion of two lyers with significntly different thickness nd grin size, leding to the opposite sign contributions in to AE. The bottom interfce lyer djcent to Al 2 O 3 (0001) substrte is ~10 nm in thickness with T C 370 K nd consists of smll (~ 5 nm) rounded grins. The top lyer ~ nm in thickness with columnr grin structure ~50 nm in lterl size represents LT phse, which exhibits negtive AE similr to tht in the -MnSi single crystl [14, 16]. Finlly, we discuss obtined experimentl results in terms of the model of defect-induced FM order with effective exchnge coupling strongly ffected by spin fluctutions [12] tking into ccount the structure peculirities of studied films. We rgue tht the observed T FM of nonstoichiometric Mn x Si 1-x lloys strongly depends on the type of defects ( mgneticlly ctive Si vcncies vs. mgneticlly ded interstitil Mn tom) s well s on the size of crystl grins which interfces cting s the gettering regions for Si vcncies. Acknowledgements The work ws prtly supported by the RFBR (grnt Nos , , , , , , ), NBICS Center of the Kurchtov Institute nd MIPT Center of Collective Usge with finncil support from the Ministry of Eduction nd Science of the Russin Federtion (Grnt No. RFMEFI59414X0009). The work t ZDR is finncilly supported by DFG (Z 225/6-1). A.S.S. cknowledges the finncil support of DAAD-MSU progrm "Vldimir Verndsky". 10

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13 Figure cptions Fig. 1. Temperture dependence of sturtion mgnetiztion M s for Mn х Si 1-х films with different thickness nd close Mn content (x 0.516) grown in shdow geometry. The insert shows the temperture dependence of mgnetic moment J m normlized by film squre A. (For smple with d = 70 nm the J m (T)/A curve prcticlly coincides with one for smple with d = 90 nm nd is not shown on the insert). Solid lines re fitting dependencies of M s (T) with using eqution (2). Fig. 2. Temperture dependence of sturtion mgnetiztion M s for Mn х Si 1-х films with x 0.52 и 0.53 (d 70 nm) grown t direct geometry by PLD. Solid line is clculted dependence of M s (T) from [3]. Fig. 3. Mgnetiztion versus mgnetic field for SG grown smple 1 (d 70 nm; x 0.517) t different tempertures. The insert shows M() dependences in n enlrged scle. Fig. 4. Resistivity of the ll effect versus mgnetic field for SG grown smple 1 (d 70 nm; x 0.517) t different tempertures. The inset shows the cross-section of Mn x Si 1-x /Al 2 O 3 structure. Fig. 5. () Temperture dependence of the ll resistivity for SG grown smple 2 (d 90 nm; x 0.516) mesured t B = 1.2 T. (b) Resistivity of the ll effect versus mgnetic field for smple 2 t T = 9K. Fig. 6. Normlized temperture dependence of the longitudinl resistivity (T) for DG (curve 1) nd SG (curve 2) grown Mn x Si 1-x films (d 70 nm; x 0.52) in comprison with (T) for -MnSi (tken from [18]). Fig. 7. The results of X-ry diffrctometry for SG Si 1-x Mn x /Al 2 O 3 (0001) structure. Insert shows qusi-forbidden reflection (0009) from Al 2 O 3 substrte. Fig. 8. The cross-section imges nd the study of crystl structure of SG Mn x Si 1-x /Al 2 O 3 (0001) smple: ()-BF imge of the film. (b)- AADF STEM imge of the Mn x Si 1-x /Al 2 O 3 (0001) interfce. (c)- RTEM imge of the Mn x Si 1-x /Al 2 O 3 (0001) interfce re. Severl grins studied by Fourier nlysis re mrked by red rectngles. (d)-enlrged REM imge of one grin. (e)-fourier spectr of tht grin which mtches to B20 MnSi crystl structure in [102] zone xis. 13

14 M s (emu cm -3 ) (J m /A) (memu cm -2 ) d=70nm; x=0.517 d=90nm; x=0.516 d=160nm; x= T, K T C =370 K T, K Fig M s (emu cm -3 ) x~0.53 Si 1-x Mn x x~0.52 T C =330 K T, K Fig

15 400 5 K M (emu cm -3 ) M (emu cm -3 ) K 300 K 5 K (T) (T) Fig LT FM Mn xsi 1-x (x ~ 0.5) lyer; AE < 0 1 T FM Mn xsi 1-x (x ~ 0.5) lyer; AE > 0 Al 2O 3 substrte 0.2 ( cm) K 164K 53K 5K Mn x Si 1-x, x=0.517 d=70nm B (T) Fig

16 ( cm) Mn x Si 1-x, x=0.516 d=90 nm 30 K ll effect t 1.2 T () T (K) ( cm) T=9 K (b) B (T) Fig

17 (T)/ (290K) DG Mn x Si 1-x film (x~0.52) 2 - SG Mn x Si 1-x film (x~0.52) MnSi T, K Fig. 6. Al 2 O 3, K Al 2 O 3, K 1,2 Intensity (cps) 10-4 Al 2 O 3 (0009) Intensity (cps) 2x10 4 MnSi (210) (deg) MnSi (211) MnSi (311) 0 -MnSi (B20 structure) (deg) Fig

18 () ~50 nm (b) MnSi MnSi ~10 nm (c) MnSi (d) (e) Fig.8. 18