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Magneto-optics of Nanomagnetic Structures: Watching Single Magnets Flip

Nanomagnetism is one of the largest and most rapidly evolving area of nanoscience and technology. This is due to rapidly developing nanofabrication technology and numerous potential applications ranging from high density data storage to biosensing. For example, future generations of magnetic storage devices will rely on perpendicular recording in patterned storage media [1] (Fig. 1). Information will be stored in the out-of-plane magnetization direction of magnetic elements with dimensions of ~10-100 nm. Aside from the reduced size, such nanomagnets start behaving qualitatively differently as they can only sustain a single magnetic domain [2] which leads to modified behavior in comparison to multi-domain structures, particular in magnetization dynamics.

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We are interested in observing dynamic processes of the magnetization of single-domain nanomagnets, especially in dense naomagnetic arrays where particle interactions can alter and even dominate the magnetic response.We use time-resolved MOKE spectroscopy (MOKE=magneto-optic Kerr effect, see Fig. 2) to resolve magnetization changes in nanomagnets with femtosecond resolution. We have recently reported the first observation ofmagnetization precession in SINGLE, SINGLE-DOMAIN nickel nanomagnets [3,4]. This was possible after we had developed methods to increase the weak magneto-optical signal from a single nanomagnet by using a combination of dielectric layers on both the substrate and the magnet itself [5-7] (see Fig. 3). We are also interested in the magnetic properties of nanomagnets composed of multilayer metallic films [8,9]. These structures are magnetized perpendicularly to the interfaces due to perpendicular magnetic anisotropy (PMA) and are prime candidates for patterned magnetic media elements. Being able to resolve a single magnet within a dense array is important as it allows for elimination of ensemble effects (such as averaging or interparticle interactions) and provides access to the intrinsic properties of the material. Our current goals are, therefore, to extend our nano-magneto-optical detection to dynamic measurements in dense arrays. The experimental methods we use in pursuit of this goal include e-beam lithography, metal deposition, time resolved pump-probe spectroscopy, atomic/magnetic force microscopy, and near-field microscopy.

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This work is supported by the NSF and the Molecular Foundry at Lawrence Berkeley National Laboratory. We collaborate with the patterned media group of Bruce Terris at Hitachi Global Storage Technologies, Prof. Manfred Albrecht at the University of Konstanz, Prof. Eric Fullerton at UC San Diego, and Prof. Anjan Barman at IIT New Delhi.

[1] C. Ross and H. Smith, Data Storage, 41, September 1998.

[2] A. Aharoni, "Introduction to the theory of ferromagnetism", Oxford, (2001).

[3] A. Barman, S. Wang, J.D. Maas, A.R. Hawkins, S. Kwon, A. Liddle, J. Bokor, and H. Schmidt, "Magneto-optical observation of picosecond dynamics of single nanomagnets", Nano Letters 6, 2939, (2006).

[4] A. Barman, S. Wang, J. Maas, A. R. Hawkins, S. Kwon, J. Bokor, A. Liddle, and H. Schmidt, "Size dependent damping in picosecond dynamics of single nanomagnets", Appl. Phys. Lett. 90, 202504 (2007).

[5] S. Wang, A. Barman, J.D. Maas, A.R. Hawkins, S. Kwon, B. Harteneck, S. Cabrini, J. Bokor, and H. Schmidt, "Optimization of nano-magneto-optic sensitivity using dual dielectric layer enhancement", Appl. Phys. Lett. 90, 252504 (2007).

[6] N. Qureshi, S. Wang, M. Lowther, A.R. Hawkins, S. Kwon, B. Hartleneck, and H. Schmidt, "Cavity-enhanced magneto-optical observation of magnetization reversal in individual single-domain nanomagnets", Nano Letters, 5, 1413, (2005).

[7] N. Qureshi, A.R. Hawkins and H. Schmidt, "Cavity enhancement of the magneto-optic Kerr effect for optical studies of magnetic nanostructures", Applied Physics Letters, 85, 431, (2004).

[8] A. Barman, S. Wang, O. Hellwig, A. Berger, E.E. Fullerton, and H. Schmidt, "Ultrafast magnetization dynamics in high perpendicular anisotropy [Co/Pt]n multilayers", Journal of Applied Physics, 101, 09D102 (2007).

[9] A. Barman, S.Wang, O. Hellwig, E. Dobisz, D. Kercher, E. Fullerton and H. Schmidt, "Size dependent precessional dynamics in [Co/Pd]8 patterned nanomagnet arrays", 10th Joint Intermag/MMM Conference, Baltimore, MD, Jan 7-11 (2007).