1. Polarized neutron reflectometry: encore presentation M.R. Fitzsimmons Los Alamos National Lab

2. Outline Description of a polarized neutron reflectometer. –Ingredients of a polarized neutron reflectometer. –Measurement of wavelength with the time-of-flight technique. –How are polarized neutron beams made? –How are spins flipped? An example worked in detail. –What is the specific question? –Formulate experimental procedure. –Collect data. –Fit/interpret data. –Publish! So, why use neutron scattering?

3. 1 st ingredient Knowledge and control of neutron beam polarization. Spin upSpin down

4. A need for well polarized neutron beams. Fe Si Strongly magnetic materials best served by well polarized neutron beams.

5. 2 nd ingredient Capability to measure the intensity and polarization of the neutron beam reflected by a sample.

6. 3 rd ingredient Ability to measure intensity and polarization of the scattered beam as a function of wave vector transfer parallel and perpendicular to the sample surface.

7. Measuring with the time-of-flight technique.

8. Neutron guides A neutron will stay inside the guide provided: Glass 58 Ni  For Ni:

9. Producing clean cold neutron beams. 22 <4Å >4Å Bragg’s law satisfied when <2d 100 Principle of a Be filter cc >14Å <14Å Principle of a frame overlap mirror

10. Reflectometry a good match. (100) Be edge (110) Be edge

11. Covering an extended range in Q 6 minutes 4 minutes 24 minutes 160 minutes 600 minutes

12. How are polarized neutron beams made? Answer: any magnetic film will polarize a neutron beam to some degree. Fe Si  Q ~ 0.005 Å -1

13. A “first” PNR experiment D.J. Hughes and M.T. Burgy, Phys. Rev., 81, 498 (1951). Results supported Schwinger’s model of neutron moments as current loops and the predicted dependence on B not H (in contrast to Bloch’s model). Spin down Q c Spin up Q c

14. Polarizing supermirrors. Si 100 nm0.1 nm F. Mezei and P.A. Dagleish, Comm. on Phys., 2, 41 (1977).

15. Traditional approach [Å] Q min [Å -1 ] Q max [Å -1 ] 50.010.065 150.003  0.01 0.018 Settings for = 5 Å. An inefficient approach to simultaneously polarize multi-wavelength neutron beams. Q Q c (Si) = 0.01 Å -1 Q c (m=3) = 0.065 Å -1 = 5 Å = 15 Å

16. Mezei polarization cavity 1.Efficient polarization of the neutron beam for > min = 4Å. 2.Maintains divergence of the neutron guide. 3.Polarization of large beams, e.g., 25 mm by 130 mm. 4.No deflection of beam line. Ewald’s sphere small big

17. The Asterix polarization cavity

18. Adiabatic rotation of neutron spins

19. Radio frequency gradient field spin flipper I. Rabi, N.F. Ramsey, J. Schwinger, Rev. of Mod. Phys., 26, 167 (1954).

20. Pictures of the RF gradient flipper

21. Polarized neutron reflectometry z Spin-flip cross-sections yield M  as a function of Q. Non spin-flip cross-sections yield: M || as a function of Q. 

22. A qualitative interpretation of R NSF and R SF  y

23. Why neutron reflectometry?  Domains are large compared to coherent region of the neutron beam. Sinha discusses the case of small domains this p.m.

24.  f  = 0.74  ff W.T Lee, et al., PRB, 65, 224417 (2002). = 0.74 magnetometryreflectometry Quantity || to H R + -R -  Quantity  to H R SF  Domains are large compared to coherent region of the neutron beam.

25. Table of measurements and their meanings. Measurement featureInformation obtained from a sample of cm 2 or so size Position of critical edge, Q c Nuclear (chemical) composition of the neutron-optically thick part of the sample, often the substrate. Intensity for Q < Q c Unit reflectivity provides a means of normalization to an absolute scale. Periodicity of the fringes Provides measurement of layer thickness. Thickness measurement with uncertainty of 3% is routinely achieved. Thickness measurement to less than 1 nm can be achieved. Amplitude of the fringes Nuclear (chemical) contrast across an interface. Attenuation of the reflectivity Roughness of an interface(s) or diffusion across an interface(s). Attenuation of the reflectivity provide usually establishes a lower limit (typically of order 1-2 nm) of the sensitivity of reflectometry to detect thin layers.

26. FeCo on GaAs: an example worked in detail. What is the specific question to be answered? Reality test: simulate possible answers. Formulate experimental protocol. Write proposal. Collect data. Interpret data. Write experiment report, publish results.

27. Magnetic vs. chemical thickness How does the magnetization of the FeCo/GaAs interface affect the polarization of spin current passing through the interface? (1) A conducting and magnetically dead interface is a source of unpolarized spins. (2) Spins passing through the interface may suffer spin flip scattering. We need to understand the magnetic structure of the as-prepared buried interface. FeCo GaAs (100) 2x4

28. Collaborators LANL: S. Park UMN: X. Dong B.D. Schultz C.J. Palmstrøm

29. Magnetization of the sample. Fe 48 Co 52 grown on GaAs(100) 2x4 (As-rich) surface. A.F. Isakovic, et al., JAP, 89, 6674 (2001).

30. X-ray vs. polarized neutron scattering Roentgen Chadwick X-Ray reflectivity Neutron reflectivity Spin  H = 1 kOe Spin  X-ray reflectivity neutron reflectivity Q [Å -1 ] 0.2 FeCo GaAs (100) 2x4 203.6±0.2197.5±0.2

31. X-rays

32. True and perceived specular reflectivity Homework: Quantify the influence on .

33. wavelength  Contour of constant Q  Fe Cr Spin down neutron scattering

34. Importance of diffuse scattering illustrated. Fe Cr AF Bragg reflection Specular component Diffuse scattering Scatters specularly Scatters diffusively

35.  Q x ~ 0.003 Å -1

36. X-ray vs. polarized neutron scattering Roentgen Chadwick X-Ray reflectivity Neutron reflectivity Spin  H = 1 kOe Spin  X-ray reflectivity neutron reflectivity Q [Å -1 ] 0.2 FeCo GaAs (100) 2x4 203.6±0.2197.5±0.2

37. Uniaxial anisotropy offers a resolution. 1 saturate reduce 2 Rotate 90º 3 H=9 Oe M M

38. Chemical thickness  magnetic thickness  Spin   Spin   Spin flip

39. Magnetic vs. chemical thickness Distance from surface [Å] The FeCo/GaAs(100) 2x4 interface is not ferromagnetic at 300 K (for this sample). Al-oxide FeCo GaAs   

40. So, why use neutron scattering? Profiling non-uniformity in magnetic thin films. –Example #1: Magnetic vs. chemical thickness of FeCo on GaAs. –Example #2: Measuring depth dependence of T c. –Also, lateral non-uniformity (off-specular and diffuse scattering, Sinha). “Small” moment detection and discrimination. –Example #3: Small moments in (Ga, Mn)As on GaAs. –Example #4: Small moments in the presence of big moments, Co on LaFeO 3. A different kind of vector magnetometer. –Example #5: Asymmetric magnetization reversal and exchange bias (Schuller). Magnetic structure determination of anti-ferromagnets.

41. Example #2: Measuring T c (z). 190-nm thick film of La 0.7 Sr 0.3 MnO 3. J.-H. Park, et al., PRL, 81, 1953 (1998). 5 Å 50 Å 1900 Å Co La 0.7 Sr 0.3 MnO 3 A problem tailored-made for neutron scattering: (1)All length scales probed with one technique on the same sample, and (2)Offers an opportunity to probe magnetization of a buried interface (in addition to that near the surface).

42. Collaborators LANL: S. Park J.D. Thompson Uni-Wuerzburg: L. Molenkamp G. Schott C. Gould

43. Neutron antennas Ga 0.5 Al 0.5 As Ga 0.97 Mn 0.03 As GaAs

44. Magnetic signature most apparent. Model reproduces data using a uniform distribution of magnetization.

45. Example #3: Small moment detection (1) Magnetic (neutron) scattering length density = 7.9x10 -8 Å -2 (±10%) (2) Number density of (Ga, Mn)As formula units = 0.025 Å -3 (to  1%). (3) Mn concentration = 3%. (4) Using 1-3, we calculate  Mn = 4  B. (5) The measured magnetic moment is 2x10 -4 emu. (6) Magnetic vs. chemical thickness is 394 Å vs. 397 Å.

46. What is exchange bias? W.H. Meiklejohn, C.P. Bean, Phys Rev., 105, 904(1957). J. Nogués, Ivan K. Schuller, J. of Magn. Magn. Mater., 192, 203 (1999). PM

47. Some theoretical pictures Meiklejohn & Bean Model Problems: H E is too large for nearly all systems. H E often large for compensated AF planes. Example: (110) plane of bulk FeF 2 is compensated & H E ~400 Oe for Fe/FeF 2.

48. Random-field, domain state, etc., models H CF +’ve H E Frustrated super exchange (AF-coupling) +1 -’ve H E Super exchange (AF-coupling) H CF 10nm U. Nowak et al., JMMM, 240, 243 (2002). A.P. Malozemoff, JAP, 63, 3874 (1988).

49. Frozen moments in the AF? FM AFM q Can anything be learned at H sat ? J. Nogués, et al., PRB, 61 1315 (2000). M shift HeHe HeHe

50. Collaborators LANL: A. Hoffmann (now at ANL) IBM, Zürich: J.W. Seo H. Siegwart J. Fompeyrine J.P. Locquet NIST: J. Dura C. Majkrzak

51. Magnetization depth profiling 10 Å Pt 25 Å Co (FM) 350 Å LaFeO 3 (AFM) (100) SrTiO 3 substrate The “Nature” Sample F. Nolting, et al., Nature, 405, 767 (2000).

52. Saturation at 300K, when H e =0. - 7500 Oe+ 7500 Oe No difference for +/- saturation Results well reproducible FM AFM

53. Saturation at 18K, when H E = -20 Oe. - 7500 Oe + 7500 Oe Field cooled in +7500 Oe Asymmetry for + and - saturation FM AFM

54. Example #4: Detecting small moments near large moments. Magnetization profiles are not the same for +’ve and -’ve saturation. –Effect correlated with magnitude of H E, and –direction of cooling field. Greatest change of the profile observed for H sat parallel to the cooling field. –Implies frozen magnetization in the AF aligned anti-parallel to the cooling field. –Result consistent with observed negative exchange bias. A. Hoffmann et al., PRB, 6, 406 (2002).

55. Influence of cooling field on the magnetic order parameter of Zn 0.2 Fe 0.8 F 2 11-T superconducting magnet 250 nm thick film sample (100) AF Bragg reflection D. Belanger (UCSD), D. Lederman (WVU)