The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.
Magnetism of sub-micrometer dots and wires
Nanoscale magnetic domains in mesoscopic magnets
2D arrays of magnetic dots with controlled size
Magnetization reversal and switching of sub-micrometer dots have been studied by magnetic field dependent, phase detection MFM. This approach has been tested on mono-domain Co particles embedded in a SiO2/Si substrate. The switching mode of the magnetization of a single dot can be identified and its hysteresis loop can be obtained. By varying the aspect ratio of electrochemically grown 200 nm cylindrical cobalt dots moves the easy magnetization direction from perpendicular to axial configuration.
Figure 2. MFM Images of cylinders, 200 nm in diameter in a remnant state. The magnetization is dominated by the shape anisotropy: (a)magnetization perpendicular to the substrate and (b) in the plane of the substrate.
Arrays of cobalt nanoparticles have been grown by electrodeposition into nano-cavities machined in a Si wafer by focused ion beam (FIB). By varying the dimensions of the cavities, it is possible to change the ratioheight/diameter (H/D) of the dots(Figure 2). As shown in the figure the magnetic properties of the dots are controlled by adjusting the ratio H/D: the magnetization is oriented out-of-plane when H/D=2 (Fig. 2a) and in-plane when H/D=1 (Fig. 2b). The importance of this result lays in the simplicity of the process: specific magnetic properties are achieved by controlling the electrochemical growth inside a beaker!It was shown that the same technique allows achieving lateral sizes of 30 nm. Such an arrangement of magnetic nano-cylinders within a 2D electron gas has attracted much interest for the study of the topological Hall effect.
Reversal of small particles
By exploiting the signal produced by a phase detection MFM under a variable magnetic field, the hysteresis and the direct visualization of the magnetic state of the particle is possible. The probe involved in the MFM imaging is made of a pyramidal tip coated by a thin film of CoCr. It is characterized independently and shows a square hysteresis with a coercitive field of 260 Oe. Images taken during field scan are analyzed one-by-one in order to calculate the total phase shift over the whole particle. In the case where the tip vibrates in the direction perpendicular-to-plane, we have demonstrated that this value is proportional to the z component of the particles magnetization.
Figure 3 shows hysteresis cycles obtained for cylindrical particles. MFM images are shown at various field values to illustrate the magnetic structure of the particles as a function of the applied magnetic field. The particle is single-domain from -600 Oe to 205 Oe, the reversal of the magnetization occurs by nucleation and domain propagation. One finds that the coercive field of the particle is reached for 240 Oe.The image correspondingto 250 Oepresentsthe same domain structure and the same signal intensity than the previous image for 240 Oe, except that thecontrast is inversed. This corresponds to the reversal of the magnetization of the probe. The square hysteresis loop of the probe allows us to infer directly the hysteresis loop of the particle (Fig. 3b) by a simple inversion of the MFM signal above 240 Oe. Thus we find for the particle shown in Figure 3, a remnant field equivalent to 50% of the saturation field (400 Oe). The reversal of the magnetization proceeds by nucleation, propagation and annihilation of domains.
Local magnetic Spectroscopy: ferromagnetic resonance, magnetoresistance
Reversal of strongly anisotropic samples such as a wires has been studied by means of new local detection techniques. It became possible recently to extend the resolution of magneto-resistance and ferromagnetic resonance (FMR) measurements down to the nanometer range by using a local detection sliding contact probe. In this approach (Figure 4), a special tip is used to produce a thermal wave in a sample; the change in the microwave absorption is then measured via a microwave bridge. FMR provides information on the magnetic anisotropy fields.
This so-called scanning thermally modulated FMR technique provides detailed information on local magnetic anisotropy fields. The sliding contact probe allows measuring the magnetoresistance simultaneously (at the same position).
M. Hehn, K. Ounadjela, J.P. Bucher, F. Rousseaux, D. Decanini, B, Bartenlian, and C. Chappert
Nanoscale magnetic domains in mesoscopic magnets
SCIENCE 272,1782 (1996)
M. Rastei, R. Meckenstock, J.P. Bucher, E. Devaux, T. Ebbesen,
Electrochemical growth of Co nanodots on patterned Si substrates
Appl. Phys. Lett. 85, 2050 (2004).
M. Rastei, R. Meckenstock, J.P. Bucher,
Nanoscale hysteresis loop of individual Co dots by field-dependent magnetic force microscopy
Appl. Phys. Lett. 87, 222505 (2005).
M.Rastei, S. Colis, J.P. Bucher
Growth control and homogeneous pulsed electrodeposited Co thin films on n-doped Si(111) substrates.
Chemical Physics Letters, 417, 217 (2006)
M. Rastei, S. Colis, R. Meckenstock, O. Ersen, J.P. Bucher
Pulsed electrodeposition and magnetism of two-dimensional assembly of controlled-size Co particles on Si substrates
Surf. Sci. 600, 2178 (2006).
A. Nait Abdi, J.P. Bucher,
Magnetoresistance and switching of electrochemically etched Ni wires
Appl. Phys. Lett. 82, 430 (2003)
R. Meckenstock, M. Rastei, J.P. Bucher,
Local magnetoresistance and ferromagnetic resonance measurements wit a sliding probe contact
J. Appl. Phys. 95, 6753 (2004).
D. Spoddig, R. Meckenstock, J.P. Bucher, J. Pelzl
Studies of ferromagnetic resonance line width during electrochemical deposition of Co films on Au(111)
J. Magn. Magn. Mat. 286, 286 (2005).