Journal of Magnetism and Magnetic Materials 198}199 (1999) 303}305 Temperature dependence of the interlayer exchange coupling in MBE-grown Fe/Cr/Fe sandwiches J. Dekoster *, J. Meersschaut , S. Hogg , S. Mangin , E. NordstroKm , A. Vantomme , G. Langouche Institut voor Kern en Stralingsfysica, Celestijnenlaan 200D, B-3001 Leuven, Belgium Laboratorium voor Vaste-Stowysica en Magnetisme, Celestijnenlaan 200D, B-3001 Leuven, Belgium Abstract The structure in MBE-grown Fe/Cr/Fe (d$ "40 As, 20 As (d!(110 As) has been investigated with ion beam channeling. The temperature dependence of the interlayer exchange coupling is strongly dependent on the growth temperature of the layers. For the layers grown at 573 K the interlayer exchange coupling is suppressed well below room temperature. This e!ect is not related to the strain in the Cr layers, which is found to be largely relaxed and only moderately dependent on the growth temperature. 1999 Elsevier Science B.V. All rights reserved. Keywords: Interlayer exchange coupling; Multilayers The subject of the magnetism of Cr in the vicinity of Fe C-SDW they observed the suppression of the non-col- layers has been studied extensively over the last years. linear coupling, a transition which was predicted Recently it was pointed out that the interlayer exchange previously by Shi et al. [3]. Here we report on the coupling is highly sensitive to the magnetic properties of temperature dependence of the interlayer exchange coup- the Cr layers and is therefore an excellent testing ground ling in Fe/Cr/Fe sandwiches grown epitaxially with for the various phase transitions which can be observed MBE. The suppression of the coupling above a critical when varying the Cr thickness or the temperature in temperature is found to be strongly dependent on the Fe/Cr/Fe structures. Fullerton et al. have observed a sup- growth temperature of the layers. For Cr-spacer thick- pression of the biquadratic coupling below the NeHel ness larger than 60 As and grown at high temperatures transition of Cr [1]. With neutron di!raction they probe a narrow temperature range exists where coupling is the Cr layers to have an incommensurate spin density observed. This suppression of the coupling with increas- wave magnetism above a critical thickness of 42 As Cr. ing temperature is not observed in the Fe/Cr/Fe sand- Below this thickness their results are consistent with wiches grown at lower temperatures where the interlayer a commensurate Cr phase. Schreyer et al. have observed exchange coupling is present above room temperature. non-collinear coupling in Fe/Cr superlattices and ob- We show that this e!ect is not likely to be related to the served with neutron di!raction a transition from the strain in the Cr layers. I-SDW to the C-SDW with increasing temperature for Epitaxial Fe/Cr/Fe sandwiches with constant Fe layer thick Cr layers while for thin Cr layers the C-SDW thickness (40 As) and Cr layer thickness varying between structure is found [2]. Above the NeHel temperature of the 20 and 110 As were grown with MBE (base pressure 2;10\ Torr) on MgO (0 0 1). To enhance the mass resolution for the ion beam channeling measurements Fe was evaporated from a Knudsen cell at a rate of * Corresponding author. fax: #32-16-327985; e-mail: 0.1 As/s. Due to the higher mass and the isotopic purity of johan.dekoster@fys.kuleuven.ac.be. the Fe with respect to natural Fe, the backscattering 0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 1 0 - X 304 J. Dekoster et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 303}305 signals of the Fe and Cr layers in such sandwich struc- axis. The strain measurement for layers grown at 423 K tures can be easily resolved. Cr was evaporated from an are shown for various Cr spacer thickness in Fig. 2. One electron beam gun stabilized with a quadrupole masspec- observes a deviation from the cubic [1 1 1] axis (54.573) trometer set-up at a rate of 0.3 As/s. Two series of samples for both the Fe and Cr angular scans. The Fe [1 1 1] axis were grown at di!erent growth temperatures, 423 and is at a slightly larger angle from the surface normal which 573 K, respectively. RHEED measurements con"rm the means that the strain is compressive as is expected from epitaxial growth of (0 0 1)Fe,Cr on (0 0 1)MgO with the lattice mismatch between the MgO and the Fe. A re- the epitaxial relation [1 1 0]Fe,Cr//[0 1 0]MgO. Fig. 1 sidual tetragonal distortion is present. The [1 1 1] Fe axis shows the random and aligned energy spectrum of is observed at an angle of 55.063 with respect to the 1.97 MeV He> ions backscattered from an Fe/Cr/Fe normal [0 0 1] direction while for complete pseudomor- layer on MgO. The Fe bu!er and epilayer can be re- phic growth of the Fe on MgO(0 0 1), assuming a Poisson solved due to the thick Cr spacer layer. The Cr contribu- ratio of 0.29, the [1 1 1] axis is expected at 56.663. The Cr tion is separated from the Fe contribution, which allows [1 1 1] axis is found at a smaller angle than a [1 1 1] us to obtain structural information on both Fe and Cr cubic axis which is consistent with a tensile strain. For layers. The amount of tetragonal distortion of the indi- the layers grown at 423 K the tetragonal distortion de- vidual layers was determined directly from the angular creases with the Cr layer thickness. For a 102 As thick Cr yield pro"les of the Fe and Cr signals along the [1 1 1] layer grown at 573 K the tensile strain is larger than for a similar layer grown at 423 K and comparable to the value found for the thin Cr layers grown at 423 K. This is probably due to a di!erence in tetragonal distortion of the "rst Fe layer grown at 573 K with respect to 423 K. The Fe layer grown at 573 K is more relaxed. This might be due to a di!erent thermal expansion of the MgO and Fe which for di!erent growth temperatures gives rise to a di!erent strain value. The interlayer exchange coupling in these Fe/Cr/Fe sandwich structures was investigated with magnetization measurements. Experiments were carried out in an Ox- ford Instruments vibrating sample magnetometer equip- ped with a continuous #ow cryostat. In Fig. 3 is shown a hysteresis loop measured along the [0 1 0] in-plane axis at 240 K of an Fe(40 As)/Cr(80 As)/Fe(40 As) grown at 423 K. One notices clear jumps in the magnetization Fig. 1. Random (circles) and aligned (dots) backscattering of an when increasing the external "eld. When lowering the Fe(40 As/Cr(102 As)/Fe(40 As) sandwich epitaxially grown on external "eld from the saturation value the magnetiz- MgO(0 0 1). ation again shows a discontinuous jump. The loops can be qualitatively explained by a 903 orientation of the Fe layers magnetization. We attribute the typical loop shapes to the presence of interlayer exchange coupling Fig. 2. Angular yield pro"le for Cr and Fe in Fe/Cr/Fe in the (1 0 0) plane through the [1 1 1] axis. The solid vertical line indicates the position of the cubic [1 1 1] axis. Fig. 3. M(H) loop of an Fe(40 As)/Cr(80 As)/Fe(40 As). J. Dekoster et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 303}305 305 in the Fe/Cr system is suppressed above the NeHel temper- ature for thick Cr spacer layers. Although we have no direct measure of the NeHel temperature of the Cr in these spacer layers, our results seem to indicate that the NeHel temperature is strongly reduced below room temperature for the layers grown at 573 K. It is well known that the NeHel temperature of C-SDW Cr can be reduced to similar values by introducing a low concentration of Fe impu- rities [4]. Interface mixture in Fe/Cr has been observed by Heinrich et al. [5] and is probably the origin for the strong decrease of the coupling with temperature in these Fe/Cr/Fe sandwiches. In conclusion we have observed a strong variation in the temperature dependence of the interlayer coupling in Fe/Cr/Fe sandwiches grown at various temperatures. Fig. 4. Temperature dependence of the saturation "eld for Higher growth temperatures result in a smaller tetrag- Fe/Cr/Fe sandwich grown at 423 (open aquares) and 573 K onal distortion of the Cr layer so that the strong reduc- (solid circles). tion of the critical temperature for coupling is probably not due to the strain in Cr. and have used the position of the #ip "eld as a measure of This work was supported by the Belgian Fund or the strength of the interlayer exchange coupling. Fig. Scienti"que Research, Flanders (FWO), Concerted Ac- 4 shows the temperature dependence of the #ip "eld for tion (GOA) and the Inter-University Attraction Pole two sandwiches grown at 423 K (80 As Cr) and 573 K (IUAP P4/10). J. Dekoster and A. Vantomme are Post- (102 As). One readily notices that the interlayer coupling doctoral Researcher FWO. is only present above a certain temperature. The sup- pression of the coupling at this temperature has been observed "rst by Fullerton et al. and attributed to the NeHel transition in the Cr spacer layer in sputtered Fe/Cr References superlattices [1]. The strength of the coupling decreases much more rapidly with temperature for the layers [1] E.E. Fullerton et al., Phys. Rev. Lett. 75 (1995) 330. grown at 573 K than for the layers grown at 423 K. This [2] A. Schreyer et al., Phys. Rev. Lett. 79 (1997) 4914. [3] Zhu-Pei Shi, R.S. Fishman, Phys. Rev. Lett. 78 (1997) seems to indicate that the magnetic behaviour of the Cr 1351. spacer is very di!erent in both systems. Theoretical re- [4] A. Arrot, S.A. Werner, H. Kendrick, Phys. Rev. 153 (1967) sults by Shi et al. [3] and subsequent experimental evid- 153. ence by Schreyer et al. [3] have shown that the coupling [5] B. Heinrich et al., J. Appl. Phys. 79 (1996) 4518.