We show how the magnetic properties of the Pt/Co ultrathin film structure can be modified and even controlled under uniform irradiation by Ga+ ions at low fluence in the 20–100 keV range. A systematic magneto-optical study is presented for the Pt/Co(1.4 nm)/Pt(111) ultrathin-film structure. At ion fluences below D=1014 Ga+/cm2, the coercive field is steadily reduced when increasing the fluence. At large fluences, in the range D=(5–10)×1014 Ga+ ions/cm2, the magnetization of the Co layer drops rapidly and the film finally becomes paramagnetic at room temperature for D>2×1015 Ga+ ions/cm2. We demonstrate that these magnetic changes are related to the effect of ion-induced collisional intermixing of the Co/Pt interfaces, leading to the formation of stable Co–Pt alloys with varying composition across the interfaces. A simple model is derived to relate the ion beam-induced mixing to the changes in magnetic properties. The present work allows us to gain a quantitative understanding of previous experiments using a focused Ga+ ion beam to pattern magnetic properties of similar samples at a sub-100 nm scale.

Ion irradiation is an original process to pattern the structural and as a consequence the magnetic properties of ultra-thin films, down to the nanometer scale. Patterns of dots and tracks have been fabricated by focused Ga+ ion beam scanned onto a Co layer with perpendicular magnetic anisotropy. Depending on the dose, the magnetic behaviour of the nanometric irradiated lines can be tuned from the ferromagnetic with reduced coercivity to paramagnetic. The larger the fluence, the smaller is the exchange between dots or tracks. These systems enabled investigations of the competition between exchange and dipolar interactions. For arrays designed with high irradiation doses and only coupled by dipolar interactions, the magnetic relaxation proceeds by the magnetization reversal of individual dots and follows a power-law time decay. Monte Carlo simulations reproduce this time dependence.

The magnetic relaxation of two-dimensional arrays of dipolar coupled magnetic dots has been measured and simulated. Arrays (50 x 50) with perpendicular magnetized Co dots (2 mum x 2 Am) were patterned using a high resolution Cia' focused ion beam irradiation. Magnetic domain pattern and time relaxation of the dot arrays were investigated using Faraday magneto-optical microscopy. For arrays designed with high irradiation doses (greater than or equal to0.5 nC/cm), the magnetic relaxation of the array proceeds by the magnetization reversal of individual dots and follows a power-law time decay. The long-range character of the dipolar interaction is found to be responsible for magnetic relaxation with a power-law decay. Monte Carlo simulations, based on a modified Ising Hamiltonian, reproduce this time dependence, and show that the power law is not a consequence of either the finite size or the boundary of the arrays, and it is independent of the shape of dots as well.

We report the physical realization of periodic networks of micromagnetic dots, coupled by both the exchange and dipolar interactions. A series of samples with different ratios of exchange to dipolar coupling ρ, were patterned using a focused Ga+ ion beam (FIB). The parameter ρ can be controlled by varying the Ga+ ion beam fluence. Both static and dynamic studies reveal a transition between exchange and dipolar coupled states as Ga+ fluence is increased.

Linear magneto-optical Kerr effect (MOKE) microscopy and magnetization-induced second-harmonic-generation (MSHG) magnetometry and imaging were used as two complementary optical techniques to measure the magnetic hysteresis loops and to visualize the domain structure in Pt/Co/Pt thin films with a weak uniaxial in-plane magnetic anisotropy. While the MOKE is used to probe the magnetization over the full thickness of the Co layer, MSHG is used to check the Co/Pt interfaces selectively. When the magnetic field H is not applied along the easy axis a, longitudinal MOKE and MSHG hysteresis loops exhibit different shapes. The differences between the longitudinal MOKE and MSHG magnetic pattern images for H∥a are only found in domain wall regions. All of these results are interpreted by considering transverse components of the non-linear optical polarization.

Domain wall velocity measurements in ultrathin Pt/Co(0.6 nm)/Pt narrow magnetic wires with perpendicular anisotropy are reported. Tracks (0.45 and 0.9 microns wide) are defined by drawing twins of parallel etched/irradiated lines with a focused Ga+ beam (FIB). In these films, irradiation induces a large decrease in coercivity due to Co-Pt intermixing at the interfaces, as checked in areas submitted to various fluences. At the same field, the domain wall propagates at far higher velocities in a 0.45 mum track than in the virgin film. It is explained by the calculated irradiation fluence profile. Hence, irradiation can be used to design structures where the magnetic information propagates very rapidly under a small applied field.

We show how Focused Ion Beam irradiation can be used to modify the magnetic properties of a thin Co/Pt layer at a nanometric length-scale. The control of the injected ion dose enables the adjustment of the coercive field and the Curie temperature of the layer on localized parts of the sample. The mechanism induced during irradiation is identified as a collisional mixing process which incorporates some Pt into the Co layer thus lowering the Curie temperature. The ultimate resolution of this magnetic patterning technique is discussed.

He+ ion irradiation-induced interface mixing modifies the magnetic properties of Pt/Co/Pt sandwiches. A strong decrease in magnetic anisotrophy (hence of the coercivity and Curie temperature) can be controlled without significant changes in sample roughness or optical properties. This opens exciting possibilities to develop a planar technology to pattern the magnetic properties of multilayers by irradiation through a lithographically defined mask.

Summary form only given. In media without inversion symmetry, second harmonic generation is only allowed at interfaces, where the symmetry is broken. The nonlinear magnetization-induced second harmonic generation (MSHG, also called NOMOKE for NOnlinear Magneto-Optical Kerr Effect) derives its specific interface magnetization sensitivity from the magnetization (M) dependent second order /spl plusmn//spl chi//sup (2)/(/spl plusmn/M) tensor. It is therefore possible to obtain images of magnetic "domain" structures at interfaces of thin films, which is of great interest as a complement to global MSHG or bulk magnetization-sensitive MOKE measurements.

The specificities and advantages of magneto-optics (MO) for studying the magnetism of thin film structures: sensitivity, high space and time resolution, and wavelength discrimination, are ri viewed. Based on MO imaging experiments, two fundamental studies on the dynamics of the magnetization reversal in an ultrathin cobalt magnetic layer with perpendicular anisotropy and related dot arrays, or of the thermomagnetic MO recording-writing process, are presented.

We present a method that allows magnetic patterning of a continuous magnetic film without significant modification of the surface roughness or of the film's optical indices. It involves ion irradiation of Co/Pt multilayers, using either a standard ion implantation technology combined with high resolution masking, or a focussed ion beam. We fabricated arrays of lines or dots whose magnetic properties differ on a sub-100 nm scale. We describe the ion collision physics on which the techniques are based, as well as some of the observed consequences on the micromagnetic properties of the arrays and on the ultimate resolution. Possible applications to high-density information storage are briefly discussed. (C) Academie des sciences/Elsevier, Paris.

Magnetization-induced second harmonic generation is selectively sensitive to the magnetization of surfaces or buried interfaces. We have used it to obtain images covering areas up to 100 micrometers wide of the magnetic domain structures of Fe/Pd and Co/Pt buried interfaces with a micrometer resolution.

Up to now, only few results have been reported on the magnetization reversal of films with columnar structure and perpendicular easy axis under the influence of dipolar coupling 1-6. To our knowledge, only one team has ~erformed studies on regular 2D arrays of perpendicularly magnetized dots with dipolar interaction 7, • In all these cases, the lack of homogeneity and the dispersion of nucleation fields in the studied systems prevented to retrieve valuable data concerning the influence of dipolar coupling on the collective magnetization reversal of particles, and also the local studies were hampered by these drawbacks. The aim of the present contribution was to investigate into detail the effect of dipolar coupling on the local and collective magnetization reversal of a very well characterized system of perpendicularly magnetized dots. This effect will be important in future ultrahigh density magnetooptical (MO) recording applications where written bits have to be reduced in dimension and closely packed. MO Faraday effect magnetometry measurements and imaging have been used to investigate the static and dynamic magnetic properties of two-dimensional arrays of rectangular dots. The arrays were fabricated in highly homogeneous ultrathin epitaxial Pt(3.4nm)/Co(1.4nm)lPt(4nm)!Alz0 3(OOOl) sandwiches with high perpendicular anisotropy and easy domain wall propagation. Arrays were patterned directly by writing regularly spaced lattices of crossed lines using high resolution focused ion beam (FIB) etching at very low doses of Ga+ ions. In this way, the film surface stays even (there is no milling of grooves) and the created pattern is of purely magnetic nature. This causes very little optical contrast, allowing MO imaging of the same quality as in continuous films. The dots are separated from each other by narrow paramagnetic stripes

The field-induced magnetization reversal mechanism in CoNi/Pt multilayers was studied by magnetooptical magnetometry and microscopy. Nucleation or domain wall dominated types of reversal were discussed as a function of the multilayer microstructure and thickness. The preparation conditions and characteristics of the best multilayer structure for magnetooptical recording are determined.

The possibility of modifying solid state properties under ion beam irradiation is a well documented phenomenon. It stands as a very common technique in semiconductor technology. We have used it to design magnetic patterns on ultra-thin films. The local magnetization can be modified without changing such essential characteristics of the films as the perpendicular anisotropy and surface roughness. We report magnetooptical imaging experiments in such magnetically patterned samples: dipolarly coupled dot arrays obtained using a Ga+ Focused Ion Beam (FIB) and a comb-like array obtained with a non focused He+ ion beam irradiation through a mask fabricated by electron beam lithography.

Ultrathin magnetic cobalt-based Au/Co/Au trilayers have been studied by the optical second harmonic generation (SHG) and by the linear magneto-optical Kerr effect (MOKE). We show that SHG has a selective interface and surface sensitivity, whereas MOKE is bulk-sensitive. SHG was used to probe the surface plasmon resonance in Au/Co/Au films. The resonant coupling of surface plasmons with SHG, results in an enhancement and sign reversal of nonlinear magneto-optical effects. Model calculations of the observed phenomena are given on the basis of a nonlocal field theory, which permits to distinguish the different interface contributions to SHG.

Magnetization-induced second-harmonic generation with surface plasmon excitation in an ultrathin Au/Co/Au multilayer structure has been investigated. The resonant coupling of surface plasmons with the fundamental light results in drastic changes of the second-harmonic intensity and a sign reversal of nonlinear magneto-optical effects. Model analysis of the observed phenomena is given on the basis of the multiple interference of interface nonlinear contributions calculated using the Green’s functions formalism.

In high-quality Au/Co/Au ultrathin films with high perpendicular magnetic anisotropy, magnetization reversal occurs through easy domain-wall (DW) propagation following rare nucleation events, located at major structural defects. By patterning arrays of dots in such films, we block the DW propagation, and thus sample the intrinsic distribution of nucleation sites, improving precision as the dot diameter decreases. In a Au/Co (1 nm)/Au film we have fabricated large area arrays of round dots, with diameters of 1 and 2 mu m, leaving aside an unpatterned area as a witness of the magnetization reversal in a "continuous film" having undergone all patterning steps. Polar magneto-optical (MO) Kerr effect was used in both global and imaging experiments to accurately measure the hysteresis loops and aftereffect phenomena. We show that, despite limited damage induced by patterning, the expected behavior is indeed observed. A statistical mode! was developed, assuming an intrinsic distribution of nucleation sites in the initially continuous film, a uniform nucleation volume V-n, and a linear dependence of the nucleation energy barrier E-n=2M(s)V(n)(H-n-H) on both applied field H and nucleation field H-n at a given site (M-s is the saturation magnetization). Comparison between experiments and theory shows an excellent overall agreement, and allows one to obtain an approximate view of the distribution of nucleation fields. We could extract two fundamental lengths, the nucleation length xi(n), (related to V-n), and the mean distance I between nucleation sites. xi(n) was found to be equal to 26+/-1 nm, in good agreement with previous determinations on similar films, l, equal to about 430 nm, could be viewed as a measure of the typical distance between major structural defects in the "continuous film." Our method is indeed a means to characterize nanometer scale magnetic events (reversal of a nucleation volume), using micrometer scale resolution experiments (MO imaging). This is an example of how microfabrication can help us to understand magnetization reversal in a continuous ultrathin film.

We have fabricated micronic and submicronic dot arrays patterned from ultrathin films Au/Co/Au in order to study their magnetic properties using polar magnetooptical Kerr effect (PMOKE). Kerr microscopy observation of the magnetization reversal inside the dots reveals a wide distribution of nucleation energy barriers in the continuous film after processing. This is observed in the dot arrays because of the blocking of the domain wall propagation at the dot boundaries. Polar Kerr effect measurements have been performed in the specular beam and at the different diffraction orders. The Kerr rotation depends on the surface occupied by the dots, and differences on the diffracted beams are observed in the Kerr rotation between the "s" and "p" incident polarizations. Such observations have stimulated the elaboration of a theory of the magnetooptical far field Fraunhofer diffraction by a regular array of magnetic dots, which qualitatively explains the experimental results.