Effect of Interlayer Exchange Coupling on the Curie Temperature in Ga 1Àx Mn x As Trilayer Structures Shavkat U. YULDASHEV , Yongmin KIM, Nayoung KIM, Hyunsik IM y , Tae Won KANG, Sanghoon LEE 1; z , Yuji SASAKI 2 , Xin LIU 2 and Jacek K. FURDYNA 2 Quantum Functional Semiconductor Research Center, Dongguk University, 3-26 Pil-dong, Chung-ku, Seoul 100-715, Korea 1 Department of Physics, Korea University, Seoul 136-701, Korea 2 Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA (Received August 18, 2003; accepted November 27, 2003; published April 27, 2004) We have investigated the effect of interlayer exchange coupling on the Curie temperature of ferromagnetic (FM) layers in Ga 1x Mn x As(Be)/GaAs/Ga 1x Mn x As trilayer structures. The FM layers in the trilayers contained the same concentration of Mn (x 0:046), but the top ferromagnetic layer was additionally doped with Be. For this x the Curie temperature observed on a control single-layer sample of Be-doped Ga 0:954 Mn 0:046 As is lower than that of an undoped sample. However, in the case of trilayers, we observed that the Curie temperature of the Ga 0:954 Mn 0:046 As(Be) layer increased with decreasing thickness of the GaAs spacer separating the two FM layers. This increase in Curie temperature is due to the exchange coupling between the ferromagnetic layers mediated by the spin-polarized free carriers (holes) through the thin insulating spacer. [DOI: 10.1143/JJAP.43.2093] KEYWORDS: magnetic semiconductors, exchange coupling, magnetic trilayer structures 1. Introduction Ferromagnetic/nonmagnetic semiconductor multilayers have attracted much interest, since understanding of such geometries is of key importance for a variety of spin- injection and other spintronic devices. 1) Magnetic multi- layers on the base of ferromagnetic III-V semiconductors, such as Ga 1x Mn x As, can be easily integrated into semi- conductor circuitry and thus hold special promise in this context. 2) While most of the attention so far has been focused on the magnetoresistance effect 3) and on the value of the interlayer exchange interaction itself 4) in these semiconductor mag- netic multilayers, relatively little attention has been given to the effect of inter-magnetic-layer exchange coupling on the Curie temperature of Ga 1x Mn x As-based multilayer struc- tures. In this study we investigate the effect of the exchange magnetic coupling on the Curie temperature of FM Ga 1x Mn x As (x 0:046) layers in Ga 1x Mn x As(Be)/ GaAs/Ga 1x Mn x As trilayer structures, with different thick- nesses of nonmagnetic GaAs spacer separating the FM layers. 2. Experimental Details The Ga 1x Mn x As(Be)/GaAs/Ga 1x Mn x As trilayers were grown on semi-insulating (001) GaAs substrates in a Riber 32 MBE system. Prior to Ga 1x Mn x As deposition, a 250- nm-thick GaAs buffer layer was grown at 600 C. The substrate was then cooled to 270 C for the growth of a 2-nm- thick low-temperature (LT) GaAs layer, followed by a 30- nm-thick layer of Ga 1x Mn x As (x 0:046). Then LT-GaAs nonmagnetic spacer layers of different thicknesses were grown. The top layer of these trilayer structures was a 30- nm-thick Ga 1x Mn x As (x 0:046) epitaxial layer addition- ally doped with Be. Two single epitaxial layers of Ga 1x - Mn x As with and without Be doping were also grown under identical conditions to serve as reference samples. Mn concentration was estimated from X-ray diffraction meas- urements. A value of x 0:046 was obtained for all samples in this growth series. The single Ga 1x Mn x As (x 0:046) layer doped with Be has a Curie temperature 5) lower than that of the ferromagnetic layer without Be. This behavior is opposite the case of Ga 1x Mn x As with a lower concentration of Mn (x ¼ 0:03) where the additional Be doping increases the Curie temperature. 6) This occurs because the additional doping with Be of the Ga 1x Mn x As with a high concen- tration of Mn (x 0:04) increases the concentration of interstitial Mn and since these interstitial Mn atoms are double donor 7) the concentration of free carriers in these materials is considerably decreased. The decreasing of free- carrier concentration causes a lowering of Curie temper- ature. The result of this decreasing of free hole concentration in the thin (d ¼ 30 nm) Ga 1x Mn x As (x 0:04) layers additionally doped with Be is opposite to the results of Be doping on the concentration of free carriers in the relatively thick (d ¼ 230 nm) Be-doped Ga 1x Mn x As (x ¼ 0:05) epi- taxial layers, 8) where the free-hole concentration remains relatively constant at 5 10 20 cm 3 . This discrepancy is due to the higher concentration of Mn ions in the interstitial states in the thin layers. It was shown that the Mn interstitial site near the surface of Ga 1x Mn x As is energetically more favorable than the substitutional site. 9) Therefore, in the thin Ga 1x Mn x As layers, the concentration of interstitial Mn should be higher that in the thick epilayer. 3. Results and Discussion Figure 1 shows the temperature dependence of the resistivity at zero magnetic field for (A) undoped and (B) Be-doped Ga 1x Mn x As (x 0:046) single epitaxial layers and for the trilayer structures with different GaAs spacer thicknesses: (C) 5 ML, (D) 10 ML and (E) 30 ML. Sample A without Be shows metallic behavior, while other samples doped with Be are insulating. For sample C, the insulating behavior can be clearly seen in the inset of Fig. 1, Permanent address: Heat Physics Department, Academy of Sciences, Katartal str. 28, Tashkent 700135, Uzbekistan. E-mail address: shavkat@dongguk.edu y E-mail address: hyunsik7@dongguk.edu z E-mail address: slee3@korea.ac.kr Japanese Journal of Applied Physics Vol. 43, No. 4B, 2004, pp. 2093–2096 #2004 The Japan Society of Applied Physics 2093