JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000 Ultra Thin Films and Alloys Zbigniew Celinski, Chairman Exchange coupling through spin density wave Cr 001... using Fe whisker substrates B. Heinrich,a) J. F. Cochran, T. Monchesky, and R. Urban Physics Department, Simon Fraser University, Burnaby BC V5A 1S6, Canada Exchange coupling through spin density wave in Fe whisker/Cr/Fe 001 structures was studied by Brillouin light scattering BLS and longitudinal magneto-optical Kerr effect MOKE techniques. It will be shown that interface alloying at the Fe whisker/Cr interface profoundly affects the behavior of short wavelength oscillations. The first crossover to antiferromagnetic coupling occurs at 5 monolayers ML , the phase of short-wavelength oscillations is reversed compared to that expected for the spin density wave in Cr 001 , and the strength of coupling is significantly decreased from that obtained from first principle calculations. Using Cu and Ag atomic layers between the Cr 001 and Fe 001 films, heterogeneous interfaces showed that the exchange coupling in Cr 001 is strongly affected by electron multiple scattering. It appears that electron quantum well states in the Fe film play no important role in the strength of the exchange coupling when the Fe film is bounded on one side by Au, but they become important when the Fe film is bounded by Cr on both sides. © 2000 American Institute of Physics. S0021-8979 00 31108-2 The scanning electron microscopy with polarization H2 , the experimental measurements usually show a concave analyses SEMPA studies by the NIST group1 using Fe approach to saturation. According to micromagnetic calcula- whisker/Cr/Fe 001 samples showed that the exchange cou- tions the antiferromagnetic configuration of the Fe magnetic pling oscillates with short wavelength period 2 monolayers moments is reached via a first-order phase jump, the experi- ML . The SEMPA images revealed in a very explicit way mental measurements show a more gradual s-shaped change. that short wave and long wavelength oscillations exist in the These experimental features can be explained by an inhomo- thickness range of 5­80 ML of Cr spacer. The SFU group geneous distribution of the exchange coupling. A 10% varia- has carried out quantitative studies using Fe whisker/Cr/ tion in the exchange coupling across the measured area Fe 001 samples.2­4 It was found that the strength of the would result in hysteresis loops that are very similar to those exchange coupling through the Cr 001 spacer is extremely obtained using MOKE see Fig. 2. sensitive to small variations in growth conditions. Quantitative BLS studies have clearly exhibited short The magnetic optical Kerr effect MOKE and Brillouin wavelength oscillations in the exchange coupling, see Fig. 3. light scattering BLS measurements exhibit two critical These studies showed also that the exchange coupling fields H1 and H2 . For fields H H2 the magnetic moments through Cr 001 contains both oscillatory bilinear, J1 , and in the Fe whisker and in the ultrathin film were clearly par- positive biquadratic, J2 , exchange coupling terms. allel to the applied external field H and the sample was fully Several features are important to emphasize. First the saturated, no asymptotic approach to saturation was ob- exchange coupling crosses to the antiferromagnetic AF served, see Fig. 1 a . For H1 H H2 the magnetic moments coupling at 4 ML of Cr. Second, for a Cr spacer thickness were noncollinear, the magnetic moments deviated from the dCr 8 ML the strength of the short wavelength oscillations external field direction. For H H1 the magnetic momentor is quite weak, J1 0.1 ergs/cm2. The exchange coupling in were antiparallel. In all our samples the field dependence of this range is only AF due to the presence of an AF long- magnetization loops is consistent with the assumption that wavelength bias. This AF bias is peaked around 6­7 ML of the angular variation of the exchange coupling can be ex- Cr. It is interesting to note that the strength of the long- pressed in terms of bilinear J1 and biquadratic J2 wavelength AF bias is very close to that observed in Fe/Cr/ exchange coupling, Eexch J1 cos( ) J2 cos2( ). The Fe 001 epitaxial multilayers prepared by sputtering where strengths of J1 and J2 were determined by a comparison of the interface roughness annihilated the presence of the short the experimental data with the theory described in Ref. 5. wavelength oscillations.6 It follows that the AF bias in Fig. 3 The observed approach to saturation at the critical field is for the first 7­8 ML of Cr is most likely due to long H2 , see Fig. 1 a , and the onset of the antiferromagnetic wavelength oscillations in exchange coupling. For a Cr configuration at the critical field H1 , see Fig. 1 a , are more spacer thicker than 8 ML, d gradual than those calculated using the bilinear and biqua- Cr 8 ML, the exchange coupling is dominated by the short-wavelength oscillations. In this dratic exchange coupling terms. The calculated approach to thickness range the samples are AF coupled, J saturation clearly exhibits a well-defined kink at the field tot J1 2J2 1.0­ 1.5 ergs/cm2, for an odd number of Cr atomic layers and ferromagnetically FM coupled for an even number of a Electronic mail: bheinric@sfu.ca Cr atomic layers, Jtot 0.2 ergs/cm2. 0021-8979/2000/87(9)/5449/3/$17.00 5449 © 2000 American Institute of Physics Downloaded 20 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html 5450 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Heinrich et al. FIG. 2. Calculated MOKE signal for a 20 ML Fe 001 film exchange coupled to a bulk Fe 001 substrate and assuming an inhomogeneous distri- bution of the bilinear coupling strength J1 . The biquadratic coupling strength, J2 , has been set equal to 0.3 ergs/cm2 for all curves. The probabil- ity of a coupling strength J1 J, P(J), has been taken to be P(J) 1/( J)1/2 exp ((J J )/ J)2 with J 1 erg/cm2. Curve A J 0. Curve B J 0.2 ergs/cm2. Curve C J 0.5 ergs/cm2. FIG. 1. The magnetization along the applied external field longitudinal MOKE . The samples were grown using the same Fe whisker substrate The ARAES studies13,14 showed that the interface alloy- using a shutter during Fe deposition. a Fe whisker/11Cr/15Fe/11Cr/ ing during the growth starts already at low substrates tem- 20Au 001 . The integers represent the number of atomic layers. The satura- tion magnetization was taken to be 21.4 kOe for both the bulk Fe and the Fe peratures, Tsub 150 °C involving the top Fe atomic layer. thin film. The in-plane cubic anisotropies used were 4.76 105 ergs/cc for The interface alloying increases with an increasing substrate the bulk Fe and 3.3 105 ergs/cc for the thin film. The cubic anisotropy in temperature and at Tsub 300 °C in the top and first subsur- the Fe film was calculated using formula 1.40 in Ref. 2. The strength of the face Fe atomic layers the Fe atoms are partially replaced by bilinear and biquadratic exchange couplings J1 1.25 ergs/cm2 and J2 0.35 ergs/cm2, respectively, were calculated using a proper micromag- Cr atoms with the concentration of 30% and 20%, respec- netic treatment Ref. 5 for Fe whisker/spacer/FM film systems which in- tively. It should be noted that interface alloying is driven by cludes the rotation of the saturation magnetization inside the bulk Fe whis- the difference in binding energies and is not, in general, sym- ker domain wall . b Fe whisker/11Cr/18Fe/11Cr/20Au 001 . The cubic metric, it occurs chiefly at one interface. anisotropy in the 15 ML Fe film is 3.6 105 ergs/cc. J1 1.75 ergs/cm2 and J Recently Freyss, Stoeffler, and Dreysse16 investigated 2 0.36 ergs/cm2. The MOKE signal is not saturated due to the lack of sufficiently high external field. the phase of exchange coupling for intermixed Fe/Cr inter- faces. The calculations were carried out by d-band tight- binding Hamiltonian using a real space recursive method for two mixed layers: Fe 001 /Cr The coupling between the Fe and Cr atoms at the Fe/Cr xFe1 x /Cr1 xFex /Crn , where n represents the number of pure Cr atomic layers. This simu- interface is expected to be strongly antiferromagnetic7,8 and lates well the experimental studies which were carried out by in consequence the spin density wave in Cr is locked to the growing the first few atomic Cr layers at lower substrate orientation of the Fe magnetic moments. Since the period of short wavelength oscillations is close to 2 ML one would expect AF coupling for an even number and FM coupling for an odd number of Cr atomic layers. Surprisingly both the SEMPA9 and BLS10 measurements showed clearly that the phase of the short wavelength oscillations is exactly opposite to that expected. It is also important to note that the strength of the exchange coupling Jmax 1.0 ergs/cm2, was found to be much less than that obtained from the first principles calculations,11,12 J1 30 ergs/cm2. Our studies showed that the strength of the bilinear exchange coupling J1 is very sensitive to the initial growth conditions. The bilinear ex- change coupling can be changed by as much as a factor of 5 by varying the substrate temperature during the growth of the first few Cr atomic layers.10,13 We demonstrated by using angular resolved Auger electron spectroscopy ARAES that atomic interface alloying at the Fe whisker/Cr interface due FIG. 3. The thickness dependence of the bilinear J1 and biquadratic J2 to an interface atom exchange mechanism plays a very sig- exchange coupling. The biquadratic coupling can be measured only for nificant role and strongly affects the exchange coupling AFM coupled samples. The values of J2 for the FM coupled samples 10, 12 MLs was assumed to be equal to those for the AFM coupled samples with through the Cr spacer.3,4,13­15 9-, 11-, and 13- ML-thick Cr. Downloaded 20 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Heinrich et al. 5451 temperatures where the surface alloying is mostly confined dependence of the exchange coupling in Fe whisker/Cr/ to the two interface atomic layers. They found that the rever- Fe 001 samples where the thickness of the Fe layer was sal in the phase of coupling was predicted already for x systematically varied by using Fe wedge samples. We found 0.2. This result is in very good agreement with our studies. no evidence for an oscillatory behavior of the exchange cou- The samples with the Fe/Cr interface prepared at pling as a function of the Fe layer thickness for samples Tsub 150 °C, showing only a weak interface diffusion (x covered directly by Au 001 . However, our recent experi- 0.2),13,15 had their phase in exchange coupling already re- ments indicate that the exchange coupling is sensitive to the versed from that expected for perfect interfaces. film thickness when the Fe film is first capped with Cr pro- Heterogeneous Cr spacers were prepared for testing the ducing the structure Fe whisker/Cr/Fe/Cr/Au 001 . We pre- effect of interface composition on the exchange coupling pared two samples on the same Fe whisker substrates: a Fe strength. Fe whisker/11 Cr/1­2 Cu/Fe 001 and Fe whisker/ whisker/11Cr/15Fe/11Cr/20Au 001 and b Fe whisker/ 11Cr/1­2Ag/Fe specimens were grown, where the integers 11Cr/18Fe/11Cr/20Au 001 . For sample a the exchange represent the number of atomic layers.3,4 The behavior of coupling strength was found J1 1.25 ergs/cm2, J2 exchange coupling in Fe whisker/11Cr/1­2Cu/Fe 001 0.35 ergs/cm2, see Fig. 1 a , and for sample b J1 samples is most surprising. The strength of the exchange 1.75 ergs/cm2, J2 0.36 ergs/cm2, see Fig. 1 b . Clearly coupling in these samples was increased twofold compared changing the Fe layer thickness just by 3 ML resulted in a to that observed in samples having simple Cr interfaces. substantial change in the strength of the exchange coupling. Mirbt and Johnson presented calculations17 that were in ac- We also prepared a sample with an Fe wedge film. The Fe cord with our results. Their calculations show that the en- film thickness was in the range between 15 and 23 ML. The hanced coupling strength in Fe/Cr/Cu/Fe 001 samples is due preliminary results of BLS data show that the strength of the to an increase in the asymmetry of the spin-dependent reflec- exchange coupling has a long wavelength oscillatory period tivity of the Cr spacer electrons at the Cr/Cu/Fe interface. of 5­6 ML with maximum of the coupling around 16­17 The theoretical calculations by Mirbt and Johnson suggested and 22­23 ML, respectively. Since the exchange coupling in that a proper model for exchange coupling through spin den- Fe/Cr/Fe/Cr/Au 001 depends on the Fe film thickness, it sity waves in Cr has to include two contributions: a spin- follows that the Fe/Cr interfaces support the formation of dependent potential due to magnetic moments of antiferro- electron resonance states in Fe films. On the other hand Fe/ magnetic Cr and b spin-dependent potential at the Fe/Cr Au 001 interface tends to suppress electron resonance and Cr/Fe interfaces. states. It is appropriate at this point to raise the question whether the magnetic state of ultrathin Cr 001 layers grown or Fe whiskers is in an intrinsic spin density wave state. In 1 J. Unguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 67, 140 1991 ; fact the above MOKE and BLS results are also consistent 69, 1125 1992 . with the behavior that could be expected from a paramag- 2 B. Heinrich and J. F. Cochran, Adv. Phys. 42, 523 1993 . netic Cr. The NIST group using SEMPA and a wedge Cr 3 B. Heinrich, J. F. Cochran, T. Monchesky, and K. Myrtle, J. Appl. Phys. layer grown on Fe whisker template were able to study di- 81, 4350 1997 . 4 B. Heinrich, J. F. Cochran, T. Monchesky, and R. Urban, Phys. Rev. B 59, rectly the Cr magnetic moment with increasing Cr 14520 1999 . thickness.18 They found that the magnetic moment of surface 5 J. F. Cochran, J. Magn. Magn. Mater. 147, 101 1995 . Cr atoms exhibits short wavelength oscillations correspond- 6 E. E. Fullerton, M. J. Conover, J. E. Mattson, C. H. Sowers, and S. D. ing to transversal incommensurate spin density wave. These Bader, Phys. Rev. B 48, 15755 1993 . 7 C. Carbone and S. F. Alvarado, Phys. Rev. B 36, 2433 1987 . results were also found to be fully consistent with the behav- 8 T. G. Walker, A. W. Pang, H. Hopster, and S. F. Alvarado, Phys. Rev. ior of oscillatory exchange coupling in Fe whisker/Cr/ Lett. 69, 1121 1992 . 9 Fe 001 . It follows that the ultrathin films of Cr grown on Fe J. Unguris, D. T. Pierce, R. J. Celotta, and J. A. Stroscio, Proceedings of NATO Advanced Study Institute, edited by R. F. C. Farrow, B. Dieny, M. whiskers are indeed in the transversal spin density wave Donath, A. Fert, and B. D. Hermsmeir NATO ASI Ser., Ser. B 309, 101 state. 1993 . The spin-dependent potential in multilayer films creates 10 B. Heinrich, Z. Celinski, J. F. Cochran, and M. From, Proceedings of electron confinement and resonant states which are respon- NATO Advanced Study Institute, edited by R. F. C. Farrow, B. Dieny, M. Donath, and A. Fert NATO ASI Ser., Ser. B 309, 175 1993 . sible for the oscillatory behavior of the exchange coupling. 11 D. Stoeffler, F. Gautier, Proceedings of NATO Advanced Study Institute, According to theoretical calculations19,20 such states are not edited by R. F. C. Farrow, B. Dieny, M. Donath, A. Fert, and B. D. only restricted to nonmagnetic spacers, but are also present Hermsmeir NATO ASI Ser., Ser. B 309, 411 1993 . 12 inside the ferromagnetic layers, and the coupling cannot be M. Freyss, D. Stoeffler, and H. DreysseŽ, J. Appl. Phys. 81, 4363 1997 . 13 B. Heinrich, J. F. Cochran, D. Venus, K. Totland, C. Schneider, and K. described as the interaction which is solely localized in the Myrtle, J. Magn. Magn. Mater. 156, 215 1996 . interfaces. A strong oscillatory behavior, with a period of 14 D. Venus and B. Heinrich, Phys. Rev. B 53, R1733 1996 . 5­6 ML, of the exchange coupling with the Fe film thickness 15 B. Heinrich, J. F. Cochran, D. Venus, K. Totland, D. Atlan, S. Govorkov, in Fe/Cr/Fe multilayers was found by Okuno and Inomata.21 and K. Myrtle, J. Appl. Phys. 79, 4518 1996 . 16 M. Freyss, D. Stoeffler, and H. Dreysse, Phys. Rev. B 56, 6047 1997 . In their studies Fe/Cr/Fe 001 samples had rough interfaces 17 S. Mirbt and B. Johansson, Phys. Rev. B 56, 287 1997 . and exhibited only long wavelength oscillations with the Cr 18 J. Unguris, R. J. Cellota, and D. T. Pierce, Phys. Rev. Lett. 69, 1125 thickness. Since our interface studies showed that a signifi- 1992 . 19 cant part of the exchange coupling depends on the multiple J. Barnas, J. Magn. Magn. Mater. 128, 171 1994 . 20 P. Bruno, Europhys. Lett. 23, 615 1993 . scattering inside the Cr spacer we measured the thickness 21 S. N. Okuno and K. Inomata, Phys. Rev. Lett. 72, 1553 1994 . Downloaded 20 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html