Visualization of the Barkhausen Effect by Magnetic Force Microscopy Alexander Schwarz, * Marcus Liebmann, † Uwe Kaiser, and Roland Wiesendanger Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany TaeWon Noh and Dong Wook Kim ‡ School of Physics & ReCOE, Seoul National University, Seoul, 151-747, South Korea (Received 18 August 2003; published 20 February 2004) By visualization of the Barkhausen effect using magnetic force microscopy we are able to provide detailed information about the physical principles that govern the magnetization reversal of a granular ferromagnetic thin film with perpendicular anisotropy. Individual Barkhausen volumes are localized and distinguished as either newly nucleated or grown by domain wall propagation. The Gaussian size distribution of nucleated Barkhausen volumes indicates an uncorrelated random process, while grown Barkhausen volumes exhibit an inverse power law distribution, which points towards a critical behavior during domain wall motion. DOI: 10.1103/PhysRevLett.92.077206 PACS numbers: 75.60.Ej, 75.60.Jk, 75.70.Ak The magnetization reversal of ferromagnets is a ra- ther complex but technological important phenomenon. Macroscopically, it is characterized by the hysteresis loop, which is composed of a series of discrete magne- tization jumps, known as the Barkhausen effect [1]. Thorough examination of such individual events provides information about fundamental mechanisms of the rever- sal processes themselves, i.e., domain nucleation or do- main wall propagation, which are related to magnetic as well as to structural properties. Detection of individual Barkhausen jumps on the nanometer scale in granular, and thereby disordered, thin films is very demanding and of particular interest due to its impact on the magnetic recording technology [2]. In such films, grain boundaries provide pinning sites, which impede domain wall motion and thus enable magnetic information storage. Since areal bit densities in today’s longitudinal media with in-plane magnetization rapidly approach the superparamagnetic limit [3], thin films with perpendicular anisotropy are currently investigated intensively, because a significantly higher maximum packing density has been predicted for them [4]. Our sample, an 80 nm thick La 0:7 Sr 0:3 MnO 3 film, was epitaxially grown on a LaAlO 3 (001) substrate by pulsed laser deposition. This material belongs to the class of mixed valence manganites, which recently attracted a lot of interest due to its colossal magneto-resistive effect [5]. Its granular nanocrystalline structure is typical for technological relevant thin films [2]. Because of a small lattice mismatch, the film is compressed in-plane and elongated in the normal direction, which induces a uni- axial perpendicular anisotropy [6,7]. Dislocation lines, 20–40 nm apart, compensate for most of the in-plane stress and are responsible for the structural disorder in the material. Above 70 nm thickness, the dislocation network develops into orthogonal shaped columns with amor- phous grain boundaries [8]. Consistent with these studies on the internal film structure are scanning force micros- copy (SFM) images of our samples, which show a granu- lar surface topography with a mean diameter around 32 nm and a root-mean-square roughness of 0.4 nm. To visualize individual Barkhausen jumps we em- ployed magnetic force microscopy (MFM) using our home-built ultrahigh vacuum low temperature force mi- croscope (Hamburg design) [9]. After saturating the sample at 5.2 K in a perpendicular magnetic field of 600 mT, the field was ramped quasistatically from rema- nence towards saturation and back again along the major hysteresis loop, while continuously recording MFM im- ages [10]. Because of our preparation technique [9], the nominally 5 nm thick iron film is evaporated only on that FIG. 1 (color). Domain structures of the La 0:7 Sr 0:3 MnO 3 thin film recorded along the decreasing branch of the major hys- teresis loop. (a) –(f) are part of a series consisting of 158 images [10], which have been acquired on the same scan area while slowly ramping the magnetic field (5 or 10 mT=image). Image parameters (see Ref. [10] for their explanation): Tip coating: 5 nm Fe, scan height h  24 nm, f 0  195 kHz, c z  47N=m, A 0 5nm, U bias 0:1V, f-range  0:7Hz. PHYSICAL REVIEW LETTERS week ending 20 FEBRUARY 2004 VOLUME 92, NUMBER 7 077206-1 0031-9007= 04=92(7)=077206(4)$22.50  2004 The American Physical Society 077206-1