Antiferromagnetic Interlayer Coupling in Ferromagnetic ...

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Europhysi s Letters

Antiferromagneti interlayer oupling in ferromagneti semi ondu tor H. Kpa

EuS/PbS(001)

1,6 ∗

( ),

superlatti es 1

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J. Kutner-Pielaszek , J. Blinowski , A. Twardowski , C.F.

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Majkrzak , T. Story , P. Ka man , R.R. Gaª¡zka , K. Ha , H.J.M. Swagten ,

and T.M. Giebultowi z Department of Physi s, Warsaw University  ul. Ho»a 69, 00-681 Warszawa, Poland 2 National Institute of Standards and Te hnology  Gaithersburg, MD 20899, USA 3 Institute of Physi s, Polish A ademy of S ien es  Al. Lotników 32/46, 02-668 Warszawa, Poland 4 Department of Applied Physi s and COBRA  Eindhoven University of Te hnology, 5600 MB Eindhoven, The Netherlands 5 Kharkov State Polyte hni al University  21 Frunze St., 310002 Kharkov, Ukraine 6 Physi s Department, Oregon State University  Corvallis, OR 97331, USA 4

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arXiv:cond-mat/0102194v2 17 Jul 2001

W.J.M. de Jonge , A.Yu. Sipatov , V. Volobuev

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PACS. 61.12Ha  Neutron ree tometry. PACS. 68.65Cd  Superlatti es. PACS. 75.50Pp  Magneti semi ondu tors.

Abstra t.  Antiferromagneti oupling between ferromagneti layers has been observed for the rst time in an all-semi ondu tor superlatti e stru ture EuS/PbS(001), by neutron s attering and magnetization measurements. Spin-dependent superlatti e band stru ture ee ts are invoked to explain the possible origin and the strength of the observed oupling.

Sin e the dis overy of antiferromagneti (AFM) oupling between metalli ferromagneti (FM) layers separated by a nonmagneti metal in late 1980s, the subje t has been an a tive resear h area in fundamental magnetism. The AFM oupling plays a key role in many te hnologi al appli ations su h as magnetoresistive sensors and magnetoopti al devi es [1℄. In metalli systems, the interlayer oupling results from the quantum interferen e of ondu tion ele tron waves in the spin dependent potential of magneti multilayer or, equivalently, via the spin polarization of ondu ting arriers (an analog of the well-known RKKY intera tion [2,3℄). An important question arises as to how interlayer oupling an exist in non-metalli systems. Re ently, eorts have been made to study the interlayer oupling in hybrid systems omposed of metalli ferromagnets separated by semi ondu tor layers. In parti ular, Fe/Si trilayers and multilayers had attra ted mu h attention, but the oupling turned out to pro eed via metalli iron sili ide, and not via the sili on, due to intermixing between iron and sili on o

urring even at room temperature [4℄. This is one illustration of the severe limitations brought about by the te hnologi al in ompatibility of FM metals and semi ondu tors. There is no su h limitation in the ase of the magneti /nonmagneti multilayers omposed of properly hosen (∗ )

E-mail: Henryk.Kepafuw.edu.pl

EDP S ien es

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all-semi ondu tor materials. By means of neutron dira tion, an interlayer ex hange intera tion was observed in semi ondu tor antiferromagneti /nonmagneti EuTe/PbTe, MnTe/ZnTe and MnTe/CdTe superlatti es (SLs) [57℄. The ferromagneti /nonmagneti semi ondu tor GaMnAs/AlGaAs system [8℄ was also studied where a ferromagneti interlayer ex hange was dete ted in magneti hysteresis measurements. Here the oupling is believed to be mediated, as in metals, by ondu ting arriers (holes) in the nonmagneti layer, though the very origin of ferromagnetism in GaMnAs [9, 10℄ is an on-going debate. Re ently developed models of interlayer oupling spe i to the FM semi ondu tor multilayers, onsidered in [1113℄, lead also to the FM orrelations. In this Letter, we present the rst on lusive eviden e for the existen e of AFM interlayer

oupling in all-semi ondu tor ferromagnet/diamagnet EuS/PbS(001) multilayers, based on neutron dira tion, neutron ree tivity, and magnetization measurements. AFM orrelations in EuS/PbS multilayers are seen for PbS thi kness up to 90 Å. Sin e AFM interlayer oupling is a ru ial element for the operation of many important magneti thin lm stru tures, su h as spin valves, the present nding is of potential te hnologi al as well as fundamental s ienti interest. Although many samples with dierent EuS and PbS thi knesses (30Å 80Å and 4Å 90Å, respe tively) have been examined for this study, we will fo us on the results of three representative samples, whi h we shall label as samples I, II, and III. The ompositions of the samples are [30Å EuS/4.5Å PbS℄×20, [35Å EuS/10Å PbS℄×10, and [60Å EuS/23Å PbS℄×15 respe tively. EuS and PbS are semi ondu ting materials whi h rystallize in the ro ksalt stru ture with a latti e mismat h of about 0.5%. EuS is a well-known nonmetalli (semi-insulating) Heisenberg ferromagnet. PbS is a diamagnet with a narrow band gap; its arrier on entration is typi ally of the order of 1017 to 1018 m−3 . The multilayers were grown epitaxially on mono rystalline KCl(001) substrate and several hundred angstroms thi k PbS buer using ele tron beam for EuS evaporation and standard resistive heating for PbS evaporation. The quality of the SLs was examined by X-ray and neutron dira tion. Figure 1(a) shows X-ray ree tivity proles for the samples III and I. Four order of SL Bragg peaks are visible for the sample III, and two order are shown for the sample I, the latter one having mu h smaller SL periodi ity. In Figure 1(b) high angle dira tion spe tra for the sample II and again the sample I are presented. Large number of SL sattelites visible in both ases learly indi ates the high degree of the superlatti e stru tural perfe tion. Detailed studies of the growth and magneti properties of EuS/PbS multilayers with thi k PbS spa ers (magneti ally de oupled

ase) has been reported elsewhere [14℄. Neutron dira tion and ree tivity measurements were arried out at the NIST Center for Neutron Resear h (NCNR) in Gaithersburg (USA). A triple-axis spe trometer (neutron wavelength λ = 2.35 Å) was set to the elasti mode of operation for the dira tometry experiments. Fig. 2(a) shows a dira tion s an with Qz parallel to the SL [001℄ growth dire tion for sample III in zero magneti eld. Measurements above and below the Curie temperature TC = 18.5± 0.5 K (22 K and 4.3 K, respe tively) were taken in order to separate the magneti ontribution from the stru tural part. The entral peak (Qz = 0) orresponds to the ommon (EuS and PbS) nu lear in-plane (020) Bragg ree tion of the whole epitaxial stru ture grown on the KCl substrate. The smaller satellites visible only in the spe trum taken at 4.3 K are a signature of the AFM ordering between adja ent EuS layers. These peaks o

ur at Qz = ± 0.038 Å−1 , whi h is about ±2π/2d where d is the bilayer repeat thi kness of the SL (d = 83 Å for this sample). The expe ted positions of the rst-order stru tural (nu lear) SL satellites are at Qz = ± 0.076 Å−1 . They are not visible in the s ans due to the ombined ee t of the superlatti e hemi al stru ture fa tor, whi h has minima in the vi inity of Qz ≈ ±0.07 Å−1 , and onsiderably smaller nu lear s attering length density

H. Kpa

et al.: Antiferromagneti interlayer oupling et .

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(SLD) ontrast between EuS and PbS than the magneti SLD ontrast. Similar arguments

an be applied for the ree tometry measurements des ribed below. The pure magneti

ontribution to the s attering as shown in Fig. 2(b) is obtained by taking the dieren e of the two spe tra in Fig. 2(a). The absen e of any magneti s attering at the entral peak position and the presen e of the satellites exa tly halfway between the entral peak and the expe ted rst-order nu lear peak are lear eviden e of AFM interlayer orrelations in this EuS/PbS system. The on lusion from the dira tion experiments is further orroborated by the results of the ree tometry studies. A ree tometer operating on the NCNR NG-1 old neutron guide was employed to get the neutron ree tivity spe tra. Due to the longer neutron wavelength (λ = 4.75 Å), the Q−resolution in the ree tivity measurements was onsiderably better than in the dira tion experiments. Fig. 3 shows the ree tivity proles obtained for samples I and III at 4.3 K. In zero applied magneti eld, pronoun ed peaks at the Qz position orresponding to twi e the hemi al SL periodi ity were observed in both samples. These maxima are again

lear indi ation of the AFM alignement of the magnetizations in su

essive EuS layers. To

onrm the magneti origin of these peaks, ree tivity spe tra were also taken with an inplane magneti eld. Appli ation of a su iently strong, external magneti eld results in full parallel alignment of the ferromagneti EuS layers; thus the AFM peak disappears, while the intensity of the FM peak at the stru tural position in reases (Fig. 3). Details of the AFM peak de ay with applied magneti eld are dis ussed below. Fig. 4 shows the eld dependen e of the magnetization taken by a SQUID magnetometer with the in-plane eld applied along the rystallographi [100] dire tion at 5K. The magneti hysteresis loop for sample I shows a onsiderably harder magneti behaviour than for sample III, indi ating the interlayer oupling is stronger with the thinner PbS spa er. The lower remanen e of Sample I  7% ompared to 62% of Sample III  learly demonstrates that the oupling is antiferromagneti . For sample II with intermediate spa er thi knesses, a step-like behavior is observed in the magnetization loop, whi h is shown in the lower-right inset of Fig. 4. The step-like behavior is a manifestation of the abrupt transition from one stable magneti arrangement of the EuS layers to another. The upper-left inset shows an anomalous temperature dependen e of the same sample (dPbS = 10Å). The de rease of the net magnetization with de reasing temperatures, observed for elds below 100 G, indi ates that AFM oupling be omes stronger at low temperatures. For elds higher than 100 G, the temperature behavior of the magnetization starts resembling that typi ally seen in ferromagneti systems. This is due to the oupling being too weak to over ome the applied eld strength for all temperatures down to 5K. The measurements taken along the [110] rystallographi dire tion show essentially the same eld and temperature dependen e for all the samples used in this study. The results of the eld dependen e studies of the AFM peak in the neutron ree tivity spe trum are shown in Fig. 5 for all three samples. The measurement pro edure starts with the samples in zero magneti eld. The eld is gradually in reased until the AFM peak disappears, and then is de reased ba k to zero. All samples show initially a strong AFM peak before the appli ation of the magneti eld. To erase entirely the AFM peak, a onsiderably larger elds are needed for sample I (∼ 700 G) and sample II (∼ 200 G) than for sample III (∼ 75 G). The AFM peak for sample I and sample II is re overable. The removal of the external eld leads to the restoration of the AFM oupled state. The AFM peak of sample III is not re overable. The sample remains in the ferromagneti onguration after the removal of the external eld. The irreversible destru tive ee t of the magneti eld on the AFM ree tivity peak is onsistent with the fa t that there are no tra es of AFM oupling in the magneti loop measurements for sample III. This suggests that the AFM oupling strength in sample

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III, with the thi kest spa er, is mu h weaker than in the sample I or II. It is also weaker than the anisotropy (domain pinning) elds in sample III that lo k the FM spin onguration after removal of the external magneti eld. Similar behavior of the AFM peak in external magneti eld has been previously observed in Fe/Nb multilayers [15℄. As noted, AFM oupling in all-semi ondu tor SL stru tures has not been predi ted by previous theoreti al models. In the ase of layers exhibiting a domain stru ture with small enough average domain size (1µm or less), the dipolar oupling, proposed in [16℄ for metalli stru tures, may be ome sizable. A hara teristi feature of this oupling is the relatively weak dependen e of its strength on the spa er thi kness. The mean domain size an be evaluated from the width of transverse s ans (waveve tor transfer omponent parallel to the layers surfa e) through the magneti superlatti e Bragg peaks. In these s ans performed for the sample I, a narrow, spe ular peak is observed, its width and shape being almost identi al as that seen in the orresponding X-ray transverse s an. Thus the existing broadening beyond the instrumental resolution may be attributed to interfa e roughness and other stru tural imperfe tions and not to the magneti domain s attering. Even if we attributed the total FWHM of this peak to domain s attering, it would set a lower limit on domain size of ∼ 3µm. The dipolar oupling strength resulting from domains of this size with a 30 Å EuS layer thi kness is lower than 80 G (see formula (9) in [16℄). Hen e, the me hanism might only

ontribute for spa er thi knesses larger than in the presented samples. Tra es of the interlayer

oupling, slowly de reasing with the spa er thi kness, indeed observed in the neutronographi spe tra for SLs up to dPbS = 90 Å, an be perhaps attributed to the dipolar me hanism. However, the dipolar for es annot explain the behavior for dPbS < 25 Å, where the data show a rapid in rease of the oupling strength with de reasing PbS thi kness (∼ 700 G for sample I), learly suggesting that another stronger AFM intera tion takes over. In sear h for a me hanism that ould explain that stronger interlayer intera tion, we analyzed the sensitivity of the total energy of valen e ele trons to the magneti stru ture of a perfe t (EuS)m /(PbS)n (001) SL, where m and n are the numbers of EuS and PbS monolayers respe tively (the monolayer thi kness of both onstituents is ≈ 3 Å). Spe i ally, we al ulated the dieren e between the total ele tron energies of SLs with parallel and antiparallel spin dire tions in onse utive EuS layers. This dieren e is a measure of the magneti interlayer

oupling. We used an empiri al tight-binding method, whi h, although being a one-ele tron method, should still des ribe adequately the small spin-dependent hanges in the total energy. The values of most tight-binding parameters are hosen so as to reprodu e the essential features of the band stru tures of bulk EuS [17℄ and PbS [18℄, the remaining being estimated by Harrison s aling. Our al ulations indi ate that the AFM spin alignment in su

essive EuS layers leads to a lower energy of the SL system, in a

ord with experimental observations. The

oupling energy results primarily from the Eu ions at the magneti /non-magneti interfa es. The al ulated oupling strength de reases with the thi kness of the non-magneti spa er roughly like 2−n . This is in a very good agreement with the data presented in Fig. 5, whi h allow to estimate the strength of the oupling, from the value of magneti eld erasing the AFM neutron ree tivity peak (saturation eld). A rough estimate of the magnitude of

oupling strength an be made using the formula J1 = Ms Bt/4 where Ms is the saturation magnetization, and t the thi kness of an EuS layer [15℄. As an be seen from the Fig. 5 these elds are ∼700 G, ∼350 G and ∼200 G for n=1, 2 and 3 respe tively. The rst two values were dedu ed from the two slopes for the sample I visible in Fig. 5. As 4.5 Å spa er is obout 1.5 monolayers thi k, the sample I may be onsidered as a mixture of two superlatti es with n = 1 and 2. Theoreti ally al ulated values of J1 for the perfe t SL with n=1, 2 and 3  0.77, 0.33, and 0.18 mJ/m2 , respe tively  although follow the same PbS spa er thi kness dependen e are about an order of magnitude higher than the experimental values estimated from the

et al.: Antiferromagneti interlayer oupling et .

H. Kpa

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saturation elds using the above formula (0.063, 0.031, and 0.019 mJ/m2 respe tively). One of the possible reasons for the latter dis repan y may be interfa ial roughness and interdiusion. Substan ial redu tion of the oupling strength in metalli multilayers due to the alloying ee ts in the interfa ial regions have previously been reported [19℄. The full, detailed des ription of the above theoreti al al ulations an be found in [20℄. In summary, we have presented the rst on lusive experimental eviden e, based on neutron dira tion, neutron ree tivity, and magnetization measurements, for the existen e of antiferromagneti interlayer ex hange oupling in a purely semi ondu tor, ferromagneti /nonmagneti epitaxial system with negligible arrier on entration. We have also shown that the experimentally observed sign, strength and range of the intera tion may be well a

ounted for by band stru ture ee ts sensitive to the magneti onguration of the multilayer. ∗∗∗

A knowledgments: Work supported by proje ts: NATO PST.CLG 975228, NSF DMR9972586, KBN 2 P03B 007 16, and KBN 2 P03B 154 18. REFERENCES [1℄ [2℄ [3℄ [4℄ [5℄

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sample III sample I

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Fig. 1  (a) X-ray ree tivity proles taken for the samples III and I. Total ree tion regions and a number of superlatti e Bragg peaks (marked by arrows) are learly visible. (b) Wide angle dira tion spe tra taken about (002) re ipro al latti e point for the samples II and I. Apart from strong (002) Bragg ree tions from the KCl substrate and PbS buer layer, large number of smaller SL sattelites is also present proving the high stru tural quality of the investigated samples. Fig. 2  (a) Neutron dira tion s an with Qz parallel to the SL [001] growth dire tion about the re ipro al latti e point (0, 2.14, Qz ) above and below TC for sample III. The arrows in (a) shows the expe ted positions of the rst order nu lear SL satellites. (b) The purely magneti ontribution is found by taking the dieren e between the two spe tra in (a).

Intensity [arb. units]

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et al.: Antiferromagneti interlayer oupling et .

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Fig. 3  Neutron ree tivity spe tra taken at 4.3 K in zero eld and in 250 G (below the saturation value) for sample I (a), and in zero and 185 G (well above the saturation value) for sample III (b). For sample I the applied intermediary eld strength 250 G redu es the AFM peak intensity only by half, emerging FM peak is also visible. Data taken at 35 and 30 K (above TC ) show the nonmagneti

ontributions to the s attering. The ree tivity urves measured in magneti elds are shifted one order of magnitude up for greater larity.

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Fig. 4  Field dependen e of the magnetization of sample I and III along the rystallographi [100] dire tion at 5 K is studied using a ryogeni super ondu ting quantum interferen e devi e (SQUID) magnetometer. The lower-right and upper-left insets show the magnetization behaviour versus eld and temperature respe tively of the sample II. Fig. 5  Magneti eld dependen e of the AFM ree tivity peak intensity for samples I, II, and III. Closed and open symbols represent the data taken with in reasing and de reasing elds respe tively. The two slopes visible for the sample I, indi ated by the dotted lines, show the behavior of two regions in the spe imen with the 1 and 2 monolayer thi k PbS spa er (see text).

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