Abstract

Neutron reflectivity allows to characterize surfaces and interfaces of ultra-thin film layered systems down to a nanometric scale (~2 nm). It is a powerful tool for the study of magnetic and polymer thin film structures. The neutron magnetic interaction is of the same order as the nuclear interaction and makes polarised neutron reflectivity a very sensitive tool for probing magnetic multilayers. It allows the determination of magnetic ordering and coupling in artificial magnetic multilayers (either metallic/semi-conducting or oxides). Examples of studies in the field of giant magneto resistive (GMR) sensors used in hard drive and tape read heads are given. The search for efficient spin-injection materials which could be used in spin electronics is also a growing field of activity. Several material candidates are presently evaluated. Recent studies on such materials are presented: oxide materials (Fe₂O₃–Fe₃O₄) epitaxial thin films; multilayer systems involving semiconducting materials ((Fe/Si)_n–(GaMnAs/GaAs)). Besides the high sensitivity of neutrons to magnetism, the possibility of isotopic labelling (H/D substitution) offers a way to probe polymer and protein thin film structures with great details: polymer interdiffusion or polymer grafting can be studied at the substrate–polymer or liquid–polymer interfaces. A model study of polymer grafting is presented.

Author Keywords: Magnetic films and multilayers; Polymers; Elastomers and plastics; Surfaces and interfaces; Neutron diffraction; Magnetic measurements

Article Outline

1. Introduction

2. Neutron reflectivity (NR) principles

3. Real experiments

3.1. Reflectivity spectrometers

3.2. Real systems

3.2.1. Sample sizes–measurements time

3.3. Sample environment

3.4. Analysis of experimental data

4. Example of non-polarised reflectivity

4.1. Polymers-isotopic labelling

4.2. Other possibilities

5. Polarised neutron reflectivity (PNR)

5.1. Superlattices

5.2. PNR magnetometry

6. Useful resources

7. Conclusion

References

(4K)
Fig. 1. Geometry of a specular reflectivity experiment. The scattering wave-vector Q is perpendicular to the plane of the thin film.

(15K)
Fig. 2. (a) Reflectivity on a semi infinite medium (copper substrate) and on a multilayer system Si//Cu (50 nm)/Cr (9 nm). The short range oscillations are characteristic of the total thickness of the layer (59 nm); the long range modulations are characteristic of the thin Cr cap layer (9 nm), (inset). The optical index profile as a function of the depth in the film. (b) Reflectivity on a magnetic thin film Si//Ni (40 nm). The reflectivity depends on the relative orientation of the neutron spin with respect to the magnetic field (inset) the optical index profile for the two neutron polarisations (parallel and anti-parallel).

(2K)
Fig. 3. Schematic of a time of flight reflectometer: a chopper defines neutron pulses; the neutron wavelength is defined by the travel time between the chopper and the detector. The sample position is fixed.

(24K)
Fig. 4. Reflectivity on a polymer grown using a "grafting from" method: (a) a polymer layer grafted on a silicon substrate, half of the layer is hydrogenated, the second half of the layer is deuterated; (b) optical index along the thickness of the layer assuming different interface thickness (the profile is assumed to vary as an erf function); (c) reflectivity of the system and theoretical curves for the different thickness of the interfacial layer (zone separating the H- and D-polymer layers). The best agreement is obtained for an interfacial layer of thickness 25 Å [25].

(31K)
Fig. 5. PS layer in a good solvent (toluene): (a) the measurement set-up, with the neutron beam incident on the system through the silicon substrate; (b) fit of the polymer density profile in deuterated toluene (three fitting methods of the NR data have been tested and give very similar results). The polymer density is normalised to 1 for the dry layer, (c) reflectivity measurement and numerical modelling curves [25].

(10K)
Fig. 6. Schematic of different magnetic configurations which can be observed in a magnetic super-lattice (Ferro/Spacer)_n, where the Ferromagnetic layers and the Spacer layers have thickness of the order of a few nanometres (1–4 nm). Depending on the exchange coupling between the layers, one can observe either: (a) a ferromagnetic ordering; (b) an anti-ferromagnetic ordering or (c) a magnetic structure with perpendicular magnetic moments.

(9K)
Fig. 7. PNR measurement on a super-lattice involving semi-conducting materials: GaAs//(Fe (2.4 nm)/Si (1.2 nm))₂₀ at 7 K under an in-plane magnetic field of 20 mT. The strong spin–flip (0 0 1/2) diffraction peak is indicative of a complex non-collinear magnetic arrangement. The vertical bars indicate the position of the expected super-lattice peaks. The modelling suggests that the Fe layers are arranged so that the magnetisations of alternating layers make an angle of 30° with respect to the applied magnetic field. The magnetic moment of the iron layer is, however, reduced to 1.4Fe atom because of the Si interdiffusion and of the fact that the Fe layers are very thin.

(38K)
Fig. 8. Polarised neutron magnetometry on a GMR system: (a) a typical GMR structure; (b) SQUID measurement on the system. The () curve has been shifted vertically by −0.5 T for clarity. In such low magnetic fields, only the magnetisation of the "free" layer is modified. The observed magnetisation curves are driven by the magnetic coupling of the "free" layer with the pinned layer through the Cu spacer. This coupling is responsible for the horizontal shift of the () hysteresis curve. In the other direction (•), perpendicular to the magnetisation of the pinned layer, the coupling does not induce a shift but only an anisotropy. On top of this phenomenon (which is the basis of the operation of a spin-valve), during the fabrication, an "intrinsic" anisotropy, perpendicular to the "pinned" layer direction is induced in the "free" layer. This is done to prevent the formation of magnetic domains; (c) PNR on the GMR system measured in a planar field of 1 T along the "pinned" layer magnetisation, (inset) variation of the PNR signal as a function of the applied field measured at a fixed Q = 0.25 nm⁻¹; (d) magnetic configuration of the different magnetic layers in the GMR system as a function of the field as deduced from the PNR measurement.

Table 1. The neutron, nuclear and magnetic optical indexes n=1−±_M for common materials at = 0.4 nm

References

1. G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb and G. Ostrowski. Rev. Sci. Instrum. 58 (1987), pp. 609–619. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

2. C.F. Majkrzak, J.W. Cable, J. Kwo, M. Hong, D.B. McWhan, Y. Yafet and J. Waszcak. Phys. Rev. Lett. 56 (1986), pp. 2700–2703. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

3. J. Penfold and R.K. Thomas. J. Phys. Condens. Matter. 2 (1990), pp. 1369–1412. Abstract-INSPEC   | $Order Document

4. T.P. Russel. Mat. Sci. Rep. 5 (1990), pp. 171–271.
T.P. Russel. Physica B 221 (1996), pp. 267–283.

5. L.T. Lee, D. Langevin and B. Farnoux. Phys. Rev. Lett. 67 (1991), pp. 2678–2681. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

6. J. Penfold, E.M. Lee and R.K. Thomas. Mol. Phys. 68 (1989), pp. 33–47. Abstract-INSPEC   | $Order Document

7. M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen-Van-Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas. Phys. Rev. Lett. 61 (1988), pp. 2472–2475. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

8. J. Lekner, Theory of Reflection of Electromagnetic and Particle Waves, Martinus Nijhoff, Dordrecht, 1987.

9. C. Fermon, F. Ott, A. Menelle, Neutron reflectivity, pp. 163
F. de Bergevin, Neutron reflectivity, in: J. Daillant, A. Gibaud (Eds.), X-ray, Neutron Reflectivity: Principles and Applications, Springer Lecture Notes in Physics, Berlin, 1999, pp. 3.

10. V.F. Sears. Phys. Rep. 141 (1986), p. 281. Abstract | Abstract + References | PDF (2236 K)

11. V.F. Sears, Methods of Experimental Physics, vol. 23A, Neutron Scattering, Academic Press, Orlando, 1987
V.F. Sears, Neutron News, vol. 3, 1992.

12. http://www.neutron.anl.gov.

13. G.P. Felcher. Physica B 267–268 (1999), pp. 154–161. SummaryPlus | Full Text + Links | PDF (211 K)

14. A list of all existing neutron reflectometers can be found at: http://www.studsvik.uu.se/research/NR/reflect.htm.

15. C.F. Majkrzak. Physica B 221 (1996), pp. 342–356. Abstract | PDF (1061 K)

16. T.P. Russel, A. Menelle, W.A. Hamilton, G.S. Smith, S.K. Satija and C.F. Majkrzak. Macromolecules 24 (1991), pp. 5721–5726.

17. X. Zhao, W. Zhao, X. Zheng, M.H. Rafailovich, J. Sokolov, S.A. Schwarz, M.A. Pudensi, T.P. Russell, S.K. Kumar and L.J. Fetters. Phys. Rev. Lett. 69 (1992), pp. 776–779. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

18. L.T. Lee, O. Guiselin, A. Lapp, B. Farnoux and J. Penfold. Phys. Rev. Lett. 67 (1991), pp. 2838–2841. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

19. O. Guiselin, L.T. Lee, B. Farnoux and A. Lapp. J. Chem. Phys. 95 (1991), pp. 4632–4640. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

20. L.T. Lee, B. Jean and A. Menelle. Langmuir 15 (1999), pp. 3267–3272. Abstract-Compendex   | $Order Document | Full Text via CrossRef

21. C. Marzolin, P. Auroy, M. Deruelle, J.-P. Folkers, L. Leger and A. Menelle. Macromolecules 34 25 (2001), pp. 8694–8700. Abstract-Compendex   | $Order Document | Full Text via CrossRef

22. M. Geoghegan, F. Boue, G. Bacri, A. Menelle and D.G. Bucknall. Eur. Phys. J. B 3 (1998), pp. 83–96. Abstract-Compendex | Abstract-INSPEC   | $Order Document | Full Text via CrossRef

23. M. Lbsche, J. Schmitt, G. Decher, W.G. Bouwman and K. Kjaer. Macromolecules 31 (1998), p. 8893.

24. B. Cathala, F. Cousin, http://www-llb.cea.fr/activ01–02/p122–123.pdf.

25. C. Devaux, E. Beyou, Ph. Chaumont and J.P. Chapel. Eur. Phys. J. E: Soft Matter E 7 (2002), pp. 345–352. Abstract-INSPEC   | $Order Document
C. Devaux and J.P. Chapel. Eur. Phys. J. E: Soft Matter 10 (2003), pp. 77–81. Abstract-Compendex | Abstract-INSPEC | Abstract-MEDLINE   | $Order Document | Full Text via CrossRef

26. O. Prucker and J. Rühe. Macromolecules 31 (1998), p. 592. Abstract-Compendex   | $Order Document | Full Text via CrossRef

27. V. Bertagna, A. Menelle, S. Petitdidier, D. Levy, M.-L. Saboungi, in: R.E. Sah, M.J. Deen, D. Landheer, K.B. Sundaram, W.D. Brown, D. Misra (Eds.), Proceedings of the International Symposium on Silicon Nitride and Silicon Dioxide Thin Insulating Films VII, vol. 2, 2003, pp. 525–532.

28. H. Chen-Mayer, G.P. Lamaze and S.K. Satija. AIP Conf. Proc. 550 (2001), pp. 407–411. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

29. G. Battaglin, A. Menelle, M. Montecchi, E. Nichelatti and P. Polato. Glass Technol. 43 5 (2002), pp. 203–208. Abstract-Compendex | Abstract-INSPEC   | $Order Document

30. G. Fragneto-Cusani. J. Phys.: Condens. Matter 13 (2001), pp. 4973–4989. Abstract-Compendex | Abstract-INSPEC   | $Order Document | Full Text via CrossRef

31. D.E Bürgler, P. Grünberg, S.O. Demokritov, M.T. Johnson, Interlayer exchange coupling in layered magnetic structures, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, vol. 13, Elsevier, Amsterdam, 2001, p. 1.

32. A. Schreyer, C.F. Majkrzak, T. Zeidler, T. Schmitte, P. Bodeker, K. Theis-Brohl, A. Abromeit, J. Dura and T. Watanabe. Phys. Rev. Lett. 79 (1997), pp. 4914–4917. Abstract-INSPEC   | $Order Document | APS full text | Full Text via CrossRef

33. J. Noguès and I.K. Schuller. J. Magn. Magn. Mater. 192 (1999), pp. 203–232. Abstract | PDF (383 K)

34. M.R. Fitzsimmons, P. Yashar, C. Leighton, I.K. Schuller, J. Nogues, C.F. Majkrzak and J.A. Dura. Phys. Rev. Lett. 84 (2000), pp. 3986–3989. Abstract-INSPEC   | $Order Document | APS full text | Full Text via CrossRef

35. C. Leighton, M.R. Fitzsimmons, A. Hoffmann, J. Dura, C.F. Majkrzak, M.S. Lund and I.K. Schuller. Phys. Rev. B 65 (2002), pp. 064403/1–064403/7.

36. F. Radu, M. Etzkorn, T. Schmitte, R. Siebrecht, A. Schreyer, K. Westerholt and H. Zabel. J. Magn. Magn. Mater. 240 (2002), pp. 251–253. SummaryPlus | Full Text + Links | PDF (181 K)

37. M. Gierlings, M.J. Prandolini, H. Fritzsche, M. Gruyters and D. Riegel. Phys. Rev. B 65 (2002), pp. 092407/1–092407/4.

38. S.G.E. te Velthuis, A. Berger, G.P. Felcher, B.K. Hill and E. Dan Dahlberg. J. Appl. Phys. 87 (2000), p. 5046. Abstract-INSPEC   | $Order Document | OJPS full text | Full Text via CrossRef

39. S.G.E. te Velthuis, G.P. Felcher, S. Jiang, A. Inomata, C.S. Nelson, A. Berger and S.D. Bader. Appl. Phys. Lett. 75 (1999), pp. 4174–4176. Abstract-INSPEC   | $Order Document | OJPS full text | Full Text via CrossRef

40. D. Haskel, G. Srajer, J.C. Lang, J. Pollmann, C.S. Nelson, J.S. Jiang and S.D. Bader. Phys. Rev. Lett. 87 (2001), pp. 207201/1–207201/4.

41. K.V. O’Donovan, J.A. Borchers, C.F. Majkrzak, O. Hellwig and E.E. Fullerton. Phys. Rev. Lett. 88 (2002), pp. 067201/1–067201/4.

42. E.E. Fullerton, S.D. Bader and J.L. Robertson. Phys. Rev. Lett. 77 (1996), pp. 1382–1385. Abstract-Compendex | Abstract-INSPEC   | $Order Document | APS full text | Full Text via CrossRef

43. P. Bodeker, A. Schreyer and H. Zabel. Phys. Rev. B 59 (1999), pp. 9408–9431. Abstract-INSPEC   | $Order Document | APS full text | Full Text via CrossRef

44. V. Leiner, D. Labergerie, R. Siebrecht, Ch. Sutter and H. Zabel. Physica B 283 (2000), pp. 167–170. SummaryPlus | Full Text + Links | PDF (130 K)

45. S.J. Blundell and J.A.C. Bland. Phys. Rev. B 46 (1992), pp. 3391–3400. Abstract-INSPEC   | $Order Document | Full Text via CrossRef

46. C. Fermon, C. Miramond, F. Ott and G. Saux. J. Neutron Res. 4 (1996), p. 251.

47. Z. Pleshanov. Physica B 94 (1994), pp. 233–243.

48. H. Zabel, R. Siebrecht and A. Schreyer. Physica B 276–278 (2000), pp. 17–21. SummaryPlus | Full Text + Links | PDF (226 K)

49. H. Zabel and K. Theis-Brohl. J. Phys.: Condens. Matter 15 (2003), pp. S505–S517. Abstract-Compendex | Abstract-INSPEC   | $Order Document | Full Text via CrossRef

50. G.P. Felcher. J. Appl. Phys. 87 (2000), pp. 5431–5436. Abstract-INSPEC   | $Order Document | OJPS full text | Full Text via CrossRef

51. Y.Y. Huang, G.P. Felcher and S.S.P. Parkin. J. Mag. Mag. Mater. 99 (1991), pp. 31–38.

52. A. Schreyer, J.F. Aukner, T. Zeidler, H. Zabel, C.F. Majkrzak, M. Schaefer and P. Gruenberg. Euro. Phys. Lett. 32 (1995), pp. 595–600. Abstract-INSPEC   | $Order Document

53. W. Szuszkiewicz, K. Fronc, M. Baran, R. Szymczak, F. Ott, B. Hennion, E. Dynowska, W. Paszkowicz, J. Pelka, R. Zuberek, M. Jouanne and J.F. Morhange. J. Supercond.: Incorporat. Novel Magn. 16 (2003), pp. 205–208. Abstract-Compendex | Abstract-INSPEC   | $Order Document | Full Text via CrossRef

54. W. Szuszkiewicz, E. Dynowska, F. Ott, B. Hennion, M. Jouanne, J.F. Morhange and J. Sadowski. J. Supercond.: Incorporat. Novel Magn. 16 (2003), pp. 209–212. Abstract-Compendex | Abstract-INSPEC   | $Order Document | Full Text via CrossRef

55. H. Kepa, J. Kutner-Pielaszek, A. Twardowski, C.F. Majkrzak, J. Sadowski, T. Story and T.M. Giebultowicz. Phys. Rev. B 64 (2001), pp. 121302/1–121302/4 R .

56. http://www-llb.cea.fr/prism/PRISM.html.

57. http://www.neutron.anl.gov/software.html.

58. Links towards possible industrial applications of neutron scattering: http://www-llb.cea.fr/industrie/index.html or http://www.ill.fr/index_ind.html.

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