Magneto-Optical Observation of Picosecond Dynamics of Single Nanomagnets Anjan Barman, ² Suqin Wang, ² Jeffrey D. Maas, ‡ Aaron R. Hawkins, ‡ Sunghoon Kwon, §,| Alexander Liddle, § Jeffrey Bokor, § and Holger Schmidt* ,² School of Engineering, UniVersity of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, ECEn Department, Brigham Young UniVersity, 459 Clyde Building, ProVo, Utah 84602, and Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received October 5, 2006; Revised Manuscript Received November 2, 2006 ABSTRACT We report measurements of picosecond dynamics of individual nickel nanomagnets as a function of magnet dimension, aspect ratio, and magnetic environment. Spatial sensitivity to nanomagnet diameters as small as 125 nm is achieved by use of cavity enhancement of the magneto-optic Kerr effect (CE-MOKE). The importance of single-particle measurements without ensemble effects for extracting the size dependence of the intrinsic nanomagnet material properties is demonstrated. In recent years, nanomagnet research has evolved into a diverse field with increasingly multidisciplinary applications. Initially, much work focused on obvious areas such as pat- terned magnetic media for data storage, 1 magnetic memory (MRAM), 2 and logic. 3 More recently, exciting biomedical applications of nanomagnets including MRI enhancement 4 and “magnetic virus” sensors 5 have emerged. More applica- tions are certain to follow, for example, genetically engi- neered nanomagnet arrays 6 or localized magnetic fields on a chip. A thorough understanding of the material properties is essential for utilization of nanomagnets in any application. This is particularly exciting as new effects such as transitions to the single magnetic domain and superparamagnetic 7 regimes come into play as magnet size approaches the deep nano- meter scale. Consequently, new experimental techniques need to be developed that reveal the material properties of indi- vidual magnetic nanoparticles without ensemble effects that may mask intrinsic behavior in nanomagnet arrays. These include particle inhomogeneities, signal reduction due to oppos- ing magnetization directions in unsaturated arrays, averaging over phase and kinetic pathways in dynamic processes, magnetostatic interactions, and others. An understanding of dynamic properties is necessary for virtually all applications. Several techniques have been applied to studying single magnetic particles. At one extreme are inherently slow methods with high spatial resolution such as magnetic force microscopy, 8 spin-polarized scanning tunneling microscopy, 9 Lorentz force microscopy, 10 and spin-polarized low-energy electron microscopy. 11 At the other extreme, sub-nanosecond temporal resolution on the microscale has been demonstrated with Brillouin light scattering, 12,13 ferromagnetic reso- nance, 14,15 time-resolved Kerr microscopy, 16-21 and inductive techniques. 22 The best spatiotemporal sensitivities were ob- tained with magnetoresistive methods, 23,24 X-ray microscopy, 25 and time-resolved magneto-optic Kerr effect (MOKE). 21 Recently, precessional dynamics from nanomagnet ensembles has been explored using Brillouin light scattering 26 and time- resolved Kerr microscopy 27,28 with superior temporal resolu- tion (∼100 fs). However, recording such processes from individual magnets with simultaneous sub-picosecond resolu- tion and nanometer sensitivity remains challenging, and no studies of magnetization dynamics in individual nanomagnets in the single-domain regime with sub-picosecond resolution have been reported yet. Here, we report the first measurements of ultrafast magnetization dynamics of individual nickel nanomagnets. Recent work on quasi-static magnetization switching has built on the well-known increase in the MOKE signal from magnetic surfaces coated by a suitable dielectric layer. 29 A substantial increase in areal sensitivity down to individual single-domain nanomagnets using cavity-enhanced MOKE (CE-MOKE) was observed. 30 We demonstrate that the additional fabrication steps required for cavity enhancement do not affect the dynamic material properties, making CE- * Corresponding author. E-mail: hschmidt@soe.ucsc.edu. ² School of Engineering, University of California Santa Cruz. ‡ ECEn Department, Brigham Young University. § Molecular Foundry, Lawrence Berkeley National Laboratory. | Present address: School of Electrical Engineering, Seoul National University. NANO LETTERS 2006 Vol. 6, No. 12 2939-2944 10.1021/nl0623457 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2006