Direct observation of non-Gaussian distribution of local magnetic properties in ferromagnetic thin films

Abstract

We report our experimental finding that magnetic properties of Co-based multilayer films are shown to exhibit their local variations far from the Gaussian ͑or Lorentzian͒ distribution which are usually assumed. The local variations of the coercivity and the field dependence (ϭϪ‫ץ‬ln /‫ץ‬H) were determined from measurements of the hysteresis loop and the field-dependent switching time , respectively, on spatially resolved local regions 400 nm in size by means of a magneto-optical microscope magnetometer. We show that the two local magnetic properties inversely correlate with each other and a thermally activated process takes place during magnetization reversal on a submicrometer scale in ferromagnetic thin films.In general, the magnetic properties of ferromagnetic thin films are spatially inhomogeneous because of structural and/or chemical imperfections inevitably introduced in the film preparation process. [1][2][3][4] Due to the lack of available techniques capable of providing detailed information about the variations with a high spatial resolution, the variations are typically considered under an assumption of the Gaussian ͑or Lorentzian͒ distribution in most theoretical approachesfor instance, Preisach and micromagnetic modeling. 4 -6 However, there is no clear physical reason for this. The question on the variations in real films is fundamentally important for exploring a realistic model of the magnetic domain configuration and reversal dynamics, which are known to be sensitive to local magnetic properties as evidenced from recent observations-for instance, for Co films grown on Au͑111͒, 1 Pt/Co/Pt͑111͒ trilayers, 2 and Co/Pd multilayers. 3 It is also a crucial technological issue in achieving a high performance of magnetic information technology, 7-9 in which the information is stored in the form of magnetic domains.Versatile experimental techniques including magnetic force microscopy ͑MFM͒, 10 scanning electron microscopy with polarization analysis ͑SEMPA͒, 11 spin-polarized lowenergy electron microscopy ͑SPLEEM͒, 12 near-field scanning optical microscopy ͑NSOM͒, 13 and spin-polarized scanning tunneling microscopy ͑SPSTM͒ ͑Ref. 14͒ have been developed to investigate microscopic domain structures with a spatial resolution of some tens of nanometers. But despite the high spatial resolution of these techniques, quantitative measurements of the local magnetic properties relating to domain reversal behavior have not yet been developed because of limitations imposed by the impossibility of applying a magnetic field and/or slow data acquisition time. On the other hand, the magneto-optical Kerr effect ͑MOKE͒ has been utilized to characterize dynamic properties such as the activation volume and the hysteresis loop by using either a focused laser beam or an optical microscope equipped with a charge-coupled device ͑CCD͒ camera. 15-17 However, these measurement techniques basically focused on the whole area of a film or a single local area of the patterned sample.Hence, to date simultaneous local probing of the magnet...

Magnetization dynamics and Gilbert damping in ultrathin Co48Fe32B20 films with out-of-plane anisotropy

“…13 In the case of very thin films, the magnetization dynamics is known to be sensitive to local variation in the magnetic properties. Recently, Choe and Shin 14 gave undeniable proofs that for Co based thin films, local variations in their magnetic properties display a distribution far from a Gaussian. Based on those results, we implemented a square distribution of the effective perpendicular anisotropy which modifies the effective field in the LLG equation.…”

Local hysteresis loop measurements by magneto-optical scanning near-field optical microscope

“…Nowadays, local hysteresis loops ͑LHLs͒ are often measured by high-resolution magneto-optical Kerr effect ͑MOKE͒ magnetometry using either a focused laser spot 1 or from MO microscopy point by point image processing. 2 In both cases, the spatial resolution is, however, limited by the Rayleigh criterion ͑ϳ /2͒. Other authors have proposed LHLs measurements by magnetic force microscopy, 3 but unfortunately, the tip-sample and tipfield interactions can provide artifacts.…”

Metastability for the Blume-Capel model with distribution of magnetic anisotropy using different dynamics

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