Polarized neutron reflectometry: encore presentation

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. 1st ingredient • Knowledge and control of neutron beam polarization. Spin up Spin down

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

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

6. 3rd 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 l with the time-of-flight technique.

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

9. Producing clean cold neutron beams. l>14Å qc l<14Å Principle of a Be filter l<4Å 2q l>4Å Bragg’s law satisfied when l<2d100 Principle of a frame overlap mirror

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

11. Covering an extended range in Q 4 minutes 6 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 dQ ~ 0.005 Å-1

13. A “first” PNR experiment D.J. Hughes and M.T. Burgy, Phys. Rev., 81, 498 (1951). Spin down Qc Spin up Qc 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).

14. Polarizing supermirrors. 100 nm 0.1 nm F. Mezei and P.A. Dagleish, Comm. on Phys., 2, 41 (1977). Si

15. Traditional approach An inefficient approach to simultaneously polarize multi-wavelength neutron beams. Settings for l = 5 Å. Qc(Si) = 0.01 Å-1 Qc(m=3) = 0.065 Å-1 Q l = 5 Å l = 15 Å

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

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. f Non spin-flip cross-sections yield: M|| as a function of Q .

22. A qualitative interpretation of RNSF and RSF r y

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

24. f f F = 59° <sin2f> = 0.74 f = 0.74 Domains are large compared to coherent region of the neutron beam. W.T Lee, et al., PRB, 65, 224417 (2002).

25. Measurement feature Information obtained from a sample of cm2 or so size Table of measurements and their meanings. Position of critical edge, Qc Nuclear (chemical) composition of the neutron-optically thick part of the sample, often the substrate. Intensity for Q < Qc 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 FeCo GaAs (100) 2x4 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.

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

29. Magnetization of the sample. Fe48Co52 grown on GaAs(100) 2x4 (As-rich) surface. A.F. Isakovic, et al., JAP, 89, 6674 (2001).

30. X-ray vs. polarized neutron scattering FeCo GaAs (100) 2x4 203.6±0.2 197.5±0.2 Spin  H = 1 kOe X-ray reflectivity neutron reflectivity X-Ray reflectivity Neutron reflectivity Spin  0.2 0.2 Q [Å-1] Q [Å-1] Roentgen Chadwick

31. X-rays

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

33. Fe Cr Spin down neutron scattering Contour of constant Q 2q wavelength

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

35. dQx ~ 0.003 Å-1

36. X-ray vs. polarized neutron scattering FeCo GaAs (100) 2x4 203.6±0.2 197.5±0.2 Spin  H = 1 kOe X-ray reflectivity neutron reflectivity X-Ray reflectivity Neutron reflectivity Spin  0.2 0.2 Q [Å-1] Q [Å-1] Roentgen Chadwick

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

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

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

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 Tc. • 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 LaFeO3. • 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 Tc(z). 1900 Å 5 Å 50 Å La0.7Sr0.3MnO3 Co 190-nm thick film of La0.7Sr0.3MnO3. J.-H. Park, et al., PRL, 81, 1953 (1998). • A problem tailored-made for neutron scattering: • All length scales probed with one technique on the same sample, and • Offers an opportunity to probe magnetization of a buried interface (in addition to that near the surface).

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

43. Neutron antennas Ga0.5Al0.5As Ga0.97Mn0.03As Ga0.5Al0.5As GaAs

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

45. Example #3: Small moment detection (1) Magnetic (neutron) scattering length density = 7.9x10-8Å-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 mMn = 4mB. (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? PM W.H. Meiklejohn, C.P. Bean, Phys Rev., 105, 904(1957). J. Nogués, Ivan K. Schuller, J. of Magn. Magn. Mater.,192, 203 (1999).

47. Some theoretical pictures Meiklejohn & Bean Model Problems: • HE is too large for nearly all systems. • HE often large for compensated AF planes. • Example: (110) plane of bulk FeF2 is compensated & HE~400 Oe for Fe/FeF2.

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

49. Frozen moments in the AF? q Can anything be learned at Hsat? Mshift FM AFM He He Mshift J. Nogués, et al., PRB, 61 1315 (2000).

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