Soft X-ray Speckle Metrology of Magnetic Multilayer Films

Primary Collaborators:

L.B. Sorensen, M. Pierce, R. Moore, University of Washington

J.B. Kortright, Materials Sciences Division, LBNL

O.E. Hellwig and E.E. Fullerton, Hitachi

Karine Chesnel, Mark Pfeifer, Advanced Light Source, LBNL

Josh Turner, University of Oregon

An important feature of many classes of materials is that the energetic interactions that determine their thermodynamic and mechanical properties arise from weak forces that operate on the nanometer length scale. These interactions nonetheless manifest themselves on a macroscopic scale in ways that lead to unusual and useful properties. These statements apply as well to superconductors and ferromagnets as they do to complex fluids and biological materials. Despite the many spectacular advances made in developing new microscopy, spectroscopy, and scattering techniques, a mechanistic understanding of this microscopic-macroscopic connection has not been achieved in many cases. Part of the reason for this is that most techniques do not provide simultaneous spatial and dynamical information on key length and time scales. Diverse phenomena that involve, for example, thermal activation or exotic phase separation, can only be partially studied at present because the important microscopic modes are characterized by nanometer length scales and microsecond time scales - a regime that is not well-covered by existing experimental techniques.

In a growing collaboration, we are developing a technique to cover this spatio-temporal regime based upon the scattering of transversely coherent beams of soft x-rays.[1-3] Similar efforts are underway at other facilities around the world.[4-8] Conceptually, these techniques constitute a blend of small angle neutron or x-ray scattering, which probe static density fluctuations on a nanometer length scale, with dynamic laser light scattering, which probes temporal fluctuations on a microsecond time scale albeit at a length scale comparable to the wavelength of visible light. Thus, the goal is to produce a beam of soft x-rays that have some laser-like properties, and to utilize this to do dynamic scattering. We utilize the high optical brightness of the Advanced Light Source in Berkeley to produce a transversely coherent beam by spatial filtering. This means, essentially, that we aperture the beam in such a way that the product of our transverse spatial and momentum acceptance is comparable to Planck’s constant.

Fig. 1: Speckle-diffraction pattern of a Pt:Co multilayer, collected at a wavelength of 1.6 nm (very close to the Co L-edge). Scattering contrast is provided by the huge magneto-optical variation near the edge. The black stripe and center are the shadow a blocker that eliminates the direct beam. The pattern is collected in transmission.

Recently, we have turned our attention to measuring spatial and temporal fluctuations in magnetic thin films, in collaboration with Jeff Kortright at LBNL and Eric Fullerton at IBM Almaden Laboratory. By operating at the relevant 3d transition metal L-absorption edge, it is possible to achieve marked magnetic contrast.[9] We have completed extensive measurements of the static speckle patterns produced by magnetic domain structures of Co:Pt multilayers, an example of which is shown in Fig. 1 above. This material system exhibits perpendicular anisotropy and, therefore, is being intensively studied as a candidate for next-generation recording material. Speckle patterns such as these have been used to test the notion of microscopic return point memory. Specifically, we have measured the degree to which the microscopic magnetic domain structure reproduces itself after traversing major and minor magnetization loops. We find for this system that the degree of microscopic return point memory upon traversing a major loop depends critically on the level of microscopic inhomogeneity of the film.

A new beamline and scattering apparatus has recently been commissioned and is being used to study a variety of complex materials and thin film structures.

Acknowledgement:

This work was carried out in part at the Advanced Light Source at Lawrence Berkeley National Laboratory which is supported by the U.S. Department of Energy. Financial support from the USDOE under grant DE-FG06-86ER45275.

References

1. Pierce, M.S., et al., Quasistatic X-Ray Speckle Metrology of Microscopic Magnetic Return-Point Memory. Phys. Rev. Lett., 2003. 90: p. 175502.

2. Price, A.C., et al., Soft X-Ray Dynamic Light Scattering from Smectic A Films. Phys. Rev. Lett., 1999. 82: p. 755.

3. Hu, B., et al., Coherent Soft X-ray Magnetic Scattering. Synch. Rad. News, 2001. 14(2): p. 11-19.

4. Sutton, M., et al., Observation of speckle by diffraction with coherent X-rays. Nature, 1991. 352: p. 608.

5. Cai, Z.H., et al., Observation of X-ray speckle by coherent scattering at grazing incidence. Phys. Rev. Lett., 1994. 73: p. 82.

6. Brauer, S., et al., X-ray intensity fluctuation spectroscopy observation of critical dynamics in Fe₃Al. Phys. Rev. Lett., 1995. 74(11): p. 2010-13.

7. Dierker, S.B., et al., X-ray Photon Correlation Spectroscopy Study of Brownian Motion of Gold Colloids in Glycerol. Phys. Rev. Lett., 1995. 75: p. 449.

8. Mochrie, S.G.J., et al., Dynamics of block copolymer micelles revealed by X-ray intensity fluctuation spectroscopy. Phys. Rev. Lett., 1997. 78(7): p. 1275-8.

9. Kortright, J.B. and S.-K. Kim, Resonant magneto-optical properties of Fe near its 2p levels: Measurement and applications. Phys. Rev. B, 2000. 62(18): p. 12216-28.