IT/ Nano • IFF Scientific Report 2006 134 I 135
Fe/Si/Fe(001) Trilayers With Strong Antiferro- magnetic Interlayer Exchange Coupling
Prepared by Ion-Beam Sputter Epitaxy
T. Damm, M. Buchmeier, A. Steeb, D. E. Bürgler, P. Grünberg, and C. M. Schneider
CNI-Center of Nanoelectronc Systems for Information Technology (IFF-9)
We grow monocrystalline Fe/Si/Fe(001) trilayers by ion-beam sputter epitaxy on GaAs(001) and MgO(001) substrates and observe antiferromag- netic interlayer exchange coupling with coupling strengths of up to 2 mJ/m2at a Si thickness of 15 A. Although the coupling is weaker than for simi-˚ lar Fe/Si/Fe structures grown by molecular-beam epitaxy, the observed coupling is still stronger than across most metal spacers. The results confirm that Si spacers represent a very spe- cial, although not yet fully understood case of interlayer exchange coupling. From a practical point of view this work demonstrates the trans- fer of the preparation of Fe/Si/Fe trilayer with strong coupling from molecular-beam epitaxy on high-quality Ag(001) buffer layers to the more application-friendly ion-beam sputter epitaxy on standard substrates.
Antiferromagnetic interlayer exchange coupling was discovered in monocrystalline Fe/Cr/Fe structures, which were carefully prepared by molecular-beam epitaxy (MBE) under ultra-high vacuum (UHV) con- ditions [1]. Subsequently, oscillatory interlayer ex- change coupling was found for many non-magnetic metallic spacer layers and for alternative preparation methods, in particular sputtering techniques [2]. This opened the door to applications such as synthetic an- tiferromagnets in sensors and antiferromagnetically coupled storage media in hard-discs. The inter- layer exchange coupling across non-metallic spac- ers is usually negligible except for epitaxial Si spac- ers in Fe/Si/Fe structures, which mediate very strong antiferromagnetic coupling exceeding the coupling strength of most metal spacers [3]. Previously, we have measured bilinear coupling constantsJ1in ex- cess of 5 mJ/cm2for Fe/Si/Fe(001) trilayers grown by MBE [3]. Here, we report the preparation of similar structures by ion-beam sputter epitaxy, which show almost equally strong coupling [4].
Our ion-beam sputtering system is UHV compati- ble and features surface analysis tools, e.g. low- energy electron diffraction (LEED). It is equipped with a 2” RF ion gun, which is aligned towards the target (sputter gun) and a 4” electron cyclotron resonance (ECR) gun, which is aligned towards the substrate (assist gun) for pre-sputtering. The average target- to-substrate distance is 13 cm and therefore of the same order of magnitude as the mean free path for
the typical sputtering gas pressure of 10−3 to 10−4 mbar. We use an Fe and a Si target with a purity of 99.95% and 99.999%, respectively. In order to pre- vent a charging of the Si target due to the bombard- ment with positive Ar ions, a beam-switch electronics is used. It allows pulsing the ion-beam with frequen- cies between 1 and 20 kHz with varying duty cycles.
During the periods of no ion acceleration the sputter guns grid voltages can be inverted, so that electrons are extracted from the plasma. This broad electron beam hits the target and neutralizes any positive sur- face charge.
-0.3 -0.2 -0.1 0 0.1 0.2 0.3 Magnetic field (T)
1.0 0.5 0.0 -0.5 -1.0
MOKEsignal
thin top Fe layer thick bottom Fe layer Data
Fit
FIG. 1: MOKE loop of a 100 ˚A Fe/14 ˚A Si/50 ˚A Fe tri- layer on GaAs. Red dots represent data and blue circles the best fit. The coupling parameters are:J1=−0.81mJ/m2, J2 = −0.15mJ/m2. Green arrows indicate the magneti- zation directions of the thin, top and thick, bottom Fe layer, respectively, obtained from the fitting.
Furthermore, this beam-switch electronics can also be used to influence the deposition rate without changing any other sputtering parameter. The film thicknesses are monitored by a quartz crystal mi- crobalance. GaAs(001) and MgO(001) substrates of 10×10 mm2 size are mounted without any preced- ing treatment and are rotated during deposition at 5- 8 rpm. The employed sputtering parameters have been obtained by optimizing the growth of Fe lay- ers on GaAs(001) substrates [4]: deposition at 40◦C, 15 s pre-sputtering to clean the air-exposed and thus oxidized substrates, deposition angle Φ=50◦, beam energy Ubeam=0.75 kV, and 1.8×10−3 mbar Ar pressure. Series ofsubstrate/100 ˚A Fe/xA Si/50˚ A Fe trilayers with varying Si thickness˚ x are de- posited, wheresubstrate stands for GaAs(001) and MgO(001). LEED patterns of each layer confirm the
IFF Scientific Report 2006 • IT/ Nano
134 I135 monocrystalline and epitaxial growth of the whole tri-
layer stack. Ex-situperformed magneto-optical Kerr effect (MOKE) measurements clearly show the pres- ence of antiferromagnetic interlayer coupling for the trilayers on both substrate types.
Si spacer thicknessx(Å) Si spacer thicknessx(Å)
10 12 14 16 18 20
10 12 14 16 18 20
Coupling(mJ/m2)Coupling(mJ/m2)
-1.2 -0.8 0.0 -0.4 -1.2 -0.8 0.0 -0.4
J1 J2 J1+ J2 (b) MgO(001) / 100 Å Fe /xÅ Si / 50 Å Fe (a) GaAs(001) / 100 Å Fe /xÅ Si / 50 Å Fe
J1 J2 J1+ J2
FIG. 2: Spacer thickness dependence of the coupling pa- rametersJ1,J2, and total coupling strengthJ =J1+J2
derived by fitting MOKE loops of Fe/Si/Fe trilayers grown on (a) GaAs(001) and (b) MgO(001) substrates.
An example for a GaAs(001) substrate and a Si spacer thicknessx=14 ˚A is shown in Fig. 1. The pe- culiar shape of the loop with the jump in small fields can be understood by taking into account that the thinner top Fe layer contributes stronger to the MOKE signal than the thicker bottom Fe layer. The reasons are the limited penetration depth of the laser light and reflections at the Fe/Si interfaces. Additionally, the signal contains strong 2ndorder MOKE contributions due to a large angle of incidence (only about 15◦from the sample normal), which give rise to further asym- metries with respect to zero field. Taking all these ef- fects into account, we are able to well reproduce the MOKE loops (see arrows and circles in Fig. 1) and fit the bilinear and biquadratic coupling constantsJ1
andJ2 defined by the phenomenological expression for the areal free energy density of the coupling
E=−J1cos(∆Θ)−J2cos2(∆Θ),
where∆Θis the angle between the magnetizations of the two Fe layers. We take into account the in- plane 4-fold magnetocrystalline anisotropy, the Zee- man energy, and the possibly twisted magnetization state within each ferromagnetic layer [5]. The varia- tion of the coupling strengths as a function of the Si spacer thicknessxis displayed in Fig. 2 for trilayers grown on GaAs(001) as well as on MgO(001). In both cases, there is no sharp maximum for the total cou- pling strengthJ = J1+J2, but an almost constant value of|J|of the order of 1 mJ/m2. This value is
comparable to those typically found for fully metallic systems [2], but is clearly smaller than the 6 mJ/m2 observed for MBE-grown Fe/Si/Fe(001) trilayers [3].
We relate the reduced coupling strength compared to MBE samples to intermixing at the Fe/Si interface due to the higher energy of the incident particles dur- ing the sputtering process. A metallic spacer could be the reason for the increase in (bilinear) coupling strength at a Si thickness of 19 ˚A in Fig. 2a, which possibly indicates oscillatory behaviour of the cou- pling. The hypothesis that enhanced intermixing re- duces the coupling strength is confirmed in a further experiment, in which we increase the Si deposition rate by a factor of two from 0.3 to 0.6 ˚A/s in order to reduce diffusion and intermixing. Ion-beam sput- tering allows us to keep all other sputtering param- eters constant. As expected, the antiferromagnetic coupling strength increases. The maximum of the to- tal coupling strength doubles to|J| ≈2 mJ/m2at a Si thickness of 15 ˚A and exceeds the coupling strengths previously reported for sputtered Fe/Si/Fe trilayers [6- 8].
In summary, monocrystalline Fe/Si/Fe(001) trilayers are grown by ion-beam sputtering on GaAs(001) and MgO(001) substrates and show strong antiferromag- netic interlayer exchange coupling. Under optimized conditions we measure a total coupling strength of 2 mJ/m2, which is stronger than for most metal spac- ers. This work extends the observation of strong antiferromagnetic interlayer exchange coupling in monocrystalline Fe/Si/Fe(001) trilayers to standard substrates and to the ion-beam sputtering technique.
Both aspects are of relevance for the application of antiferromagnetically coupled Fe/Si/Fe trilayers in de- vices.
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