JOURNAL OF APPLIED PHYSICS
VOLUME 87, NUMBER 9
1 MAY 2000
Magnetic moments and anisotropies in ultrathin epitaxial Fe films on ZnSe„001… E. Reiger, E. Reinwald, G. Garreau, M. Ernst, M. Zo¨lfl, F. Bensch, S. Bauer, H. Preis, and G. Bayreuthera) Institut fu¨r Experimentelle und Angewandte Physik, Universita¨t Regensburg, D-93040 Regensburg, Germany
The morphology, atomic magnetic moments, and in-plane magnetic anisotropies of ultrathin bcc Fe共001兲 films deposited by molecular beam epitaxy on ZnSe epilayers grown on GaAs共001兲 single crystal are reported. The growth mode and structure have been determined in situ by means of reflection high energy electron diffraction and Auger electron spectroscopy. The magnetic properties were characterized ex situ by an alternating gradient magnetometer, superconducting quantum interference device 共SQUID兲 magnetometry, and conversion electron Mo¨ssbauer spectroscopy 共CEMS兲. The Fe growth is epitaxial and occurs by three dimensional nucleation at the beginning. The coalescence of the islands is observed around 7 monolayers 共ML兲. In agreement with SQUID results, CEMS measurements indicate no reduction of the Fe magnetic moment compared to the bulk value even for the first Fe monolayers. Determination of the in-plane anisotropy constants as function of the Fe thickness shows a strong interface-induced uniaxial in-plane magnetic anisotropy, which leads to a continuous evolution from a pure uniaxial anisotropy with easy axis along 关110兴 direction for thickness below 10 ML to the pure bulk cubic Fe anisotropy above 40 ML. © 2000 American Institute of Physics. 关S0021-8979共00兲26808-4兴
INTRODUCTION
polarized tunnel junctions. The growth of Fe films on ZnSe共001兲 has been studied previously by Jonker et al.12,13 They succeeded in growing smooth epitaxial Fe thin films on ZnSe共001兲 epilayers at 450 K. However, they observed a significant reduction of the Fe magnetic moment in comparison to the bulk value 共about 24% for ⬃100 ML兲 which could be ascribed to some intermixing at the Fe/ZnSe interface.13 In the present work an attempt was made to avoid this intermixing by growing ultrathin Fe films at room temperature.
For about 2 decades the growth and magnetic properties of 3d ferromagnetic thin films on semiconductor surfaces have attracted a growing interest. In particular, new experimental results1 suggest that it is possible to realize a spin transistor based on such heterostructures. Another potential application are spin-polarized tunneling junctions using a semiconductor as a barrier instead of an insulator. Large tunneling magnetoresistance (TMR⬃44% at 4.2 K兲 has been observed recently in a 共GaMn兲As/AlAs/共GaMn兲As junction but only at low temperature because of the low Curie temperature (T C ⬃60 K) of the diluted magnetic semiconductor.2 Nevertheless, recent theoretical work3 suggests that heterostructures based on iron 共with a high T C 兲 and semiconductors like Ge, GaAs, or ZnSe can show a large TMR effect. Up to now, only the systems Fe/Si,4 and Fe/GaAs5–9 have been extensively studied in experiments. In contrast to Fe/Si, it is possible to obtain a sharp Fe/GaAs interface with negligible 共or no兲 interdiffusion, either for Ga-6,7 or As-8 terminated GaAs共001兲 surfaces, which prevents the formation of magnetically dead layers. Nevertheless, GaAs is probably not a suitable candidate for the growth of 3d ferromagnetic/semiconductor/3d ferromagnetic junctions because of the high deposition temperature 共above 700 K兲 required to grow high quality GaAs layers that leads to a strong interdiffusion between the metal and the semiconductor.9 Since flat and epitaxial ZnSe layers can be obtained at much lower temperature 共⬃575 K by molecular beam epitaxy10 and ⬃475 K by migration enhanced epitaxy11兲 Fe/ZnSe/Fe might be more promising for spin-
SAMPLE PREPARATION AND STRUCTURAL CHARACTERIZATION
The samples were grown under ultrahigh vacuum conditions in two different molecular beam epitaxy chambers. After chemical etching in a 5:1:1 NH3 :H2O2 :H2O solution, the GaAs共001兲 wafer was introduced into the II–VI semiconductor chamber and the oxide layer was removed by hydrogen plasma treatment at 590 K. The 100 nm thick ZnSe epilayer was deposited at 570 K by coevaporation at a rate of approximately 3.5 nm/min and finally covered with an amorphous Se layer prior to the transfer into the metal epitaxy chamber. After the thermal desorption of the Se capping layer at 450 K, the RHEED pattern showed a (2⫻1) reconstruction indicative of a Se-rich surface. The substrate temperature was then reduced to room temperature and Fe was deposited. During the evaporation, the pressure was below 3⫻10⫺10 mbar and the deposition rate was fixed between 0.7 and 1.4 Å/min. The Fe thickness measured ex situ by x-ray fluorescence spectroscopy varied from 0 to 66 ML 共one Fe monolayer corresponds to 1.433 Å兲. RHEED patterns proceeded via three phases during the Fe growth: the intensity of the short and narrow RHEED streaks of the ZnSe surface decreased continuously with in-
a兲
Author to whom correspondence should be addressed; electronic mail:
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© 2000 American Institute of Physics
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J. Appl. Phys., Vol. 87, No. 9, 1 May 2000
FIG. 1. Spontaneous magnetic moment per area of Fe films on ZnSe共001兲 as a function of the Fe thickness. The values were obtained at 10 K from SQUID measurements, m共H兲, by linear extrapolation to H⫽0.
creasing deposition and disappeared above 1.6 ML. RHEED streaks reappeared above 7 ML and remained up to the thickest layers. These patterns, which are characteristic of the Fe bcc structure, were spotty, indicative of a somewhat rough surface. A weak (2⫻2) reconstruction was observed above 10 ML resulting from segregation of Se on top of the surface. These results clearly show that Fe growth is epitaxial and three dimensional 共3D兲 at the beginning. The reappearance of RHEED spots at 7 ML probably corresponds to the coalescence of the 3D Fe islands. Note that the morphology of our films is quite different from the films deposited at 450 K which grow predominantly layer-by-layer12 and are smoother.14 In order to perform ex situ magnetic measurements, the Fe films were finally coated with a 3 nm thick Au overlayer. MAGNETIC MEASUREMENTS AND DISCUSSION
The samples were magnetically characterized at 10 K using a superconducting quantum interference device 共SQUID兲 magnetometer and at room temperature by means of an alternating gradient magnetometer 共AGM兲 and by conversion electron Mo¨ssbauer spectroscopy 共CEMS兲. In Fig. 1 the spontaneous magnetic moment per film area m S measured at 10 K is plotted as a function of the Fe thickness. m S increases linearly with the thickness. The slope of the linear fit is equal to (1750⫾40) G, which is in good agreement with the bulk Fe magnetization 共1766 G at 4.2 K兲.15 As the linear fit intercepts the thickness axis at the origin, we can rule out any significant reduction of the Fe magnetic moment compared to the bulk value for all investigated Fe layers (d Fe⭓5.4 ML). In order to obtain more detailed information about the magnetization and the structure of the Fe atoms located just at the Fe/ZnSe interface, we performed CEMS measurements on a 3 nm Au / 8 ML natural Fe / 2 ML 57Fe/100 nm ZnSe/GaAs共001兲 sample. The 8 ML of natural Fe are used to assure the coalescence of the 3D islands but give only a small contribution (⬃8%) to the Mo¨ssbauer signal. The Mo¨ssbauer spectrum and the hyperfine field distribution show that there is no paramagnetic contribution 共less than 0.03 ML兲 at the Fe/ZnSe interface and rule out the formation of an alloy at this interface.16
FIG. 2. 共a兲 AGM magnetization loop with the magnetic field applied along the 关1-10兴 direction for 5.4 ML Fe/ZnSe共001兲. The shaded area corresponds to the magnetizing energy in the 关1-10兴 direction. 共b兲, 共c兲, and 共d兲 magnetizing energy as a function of the in-plane direction for 7.5, 17, and 66 Fe monolayers deposited on ZnSe共001兲. The highest 共respectively smallest兲 magnetizing energy indicates the magnetic hard 共respectively easy兲 axis.
We analyzed the magnetic anisotropy as a function of the Fe thickness from AGM magnetization versus field loops measured at 300 K. The field was applied along different directions in the film plane. The polar plots of the magnetizing energy as determined by integrating the M–H loops 关Fig. 2共a兲兴 give a qualitative picture of the magnetic anisotropy for different Fe thicknesses 共Fig. 2兲. This anisotropy is a superposition of a uniaxial and a fourfold contribution: for a thickness above 40 ML the film presents nearly Fe bulk cubic anisotropy with magnetic easy axes along the 关100兴 and 关010兴 directions. On the contrary, films thinner than 10 ML possess a dominating uniaxial anisotropy with the 关110兴 direction as the magnetic easy axis. To determine quantitatively the uniaxial (K eff u ) and four) effective in-plane anisotropy coefficients we confold (K eff 1 sidered the in-plane anisotropic free energy per unit volume in-plane defined as 2 in⫺plane⫽⫺K eff u sin ⫺
K eff 1 4
sin2 共 2 兲
⫺H M S cos共 ␣ ⫺ 兲 ,
共1兲
where 共respectively ␣兲 is the angle between the 关1-10兴 direction and the magnetization 共respectively applied magnetic field兲 and M S the Fe bulk magnetization. We obtained an analytical expression of H as function of by putting the first derivative of Eq. 共1兲 equal to zero. A fit of the experimental magnetization loop along the magnetic hard axis 共for both uniaxial and fourfold anisotropies兲 with this expression eff gives K eff u and K 1 . This procedure is justified because of reversible magnetization rotation as indicated by the nonhys-
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Reiger et al.
J. Appl. Phys., Vol. 87, No. 9, 1 May 2000
teretic loops. The effective anisotropy coefficient K eff 共i i ⫽u,1兲 can be divided into a volume contribution and the contribution from both interfaces of the Fe layer:
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ACKNOWLEDGMENTS
This work was supported in part by the European Community under Grant No. ERBFMBICT983384 and by the German Ministry of Technology 共BMBF兲. 1
V K eff i ⫽K i ⫹
⫹K S,Fe/Au K S,Fe/ZnSe i i d Fe
.
共2兲
eff The values for both K eff u and K 1 vs Fe thickness d Fe can be fitted by Eq. 共2兲. The volume term K V1 ⫽(6.3⫾0.4) ⫻105 erg/cm3 is slightly larger than the fourth-order magnetocrystalline coefficient of bulk bcc Fe (5.2⫻105 erg/cm3). 15 Taking K S,Fe/Au from a previous work 关 (⫺2.5⫾0.2) 1 ⫽(⫺2.8⫾0.3) ⫻10⫺2 erg/cm2兴 , 17 we obtain K S,Fe/ZnSe 1 ⫻10⫺2 erg/cm2. The uniaxial anisotropy is a pure interface term (K Vu ⫽0). As the Fe/Au interface does not induce a uniaxial anisotropy,17 K eff u originates only from the Fe/ZnSe amounts to (5.9⫾0.1)⫻10⫺2 erg/cm2. interface: K S,Fe/ZnSe u To conclude, we succeeded in growing ultrathin epitaxial Fe films on ZnSe共001兲 epilayers at room temperature. These films show the full bulk atomic magnetic moment even for the first Fe monolayers. This could be of potential interest, the presence of magnetic dead layers being probably detrimental to spin-dependent transport in ferromagnetic/ semiconductor heterostructures. In addition, we found an interface-induced uniaxial anisotropy that permits us to control the effective magnetic anisotropy by a proper choice of the Fe thickness.
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