M. Abes

Effect of ion irradiation on the structural and magnetic properties of sputtered CoPt alloy

Abstract We are interested in studying thin layers of CoPt alloy prepared by co-deposition sputtering. The CoPt layers are deposited on a MgO(001) substrate with a Pt(001) buffer layer. We obtain a layer with the L1 0 tetragonal structure, ordered in the growth direction. This equiatomic L1 0 phase is a “natural” multilayer which consists in a stacking along the (001) direction of pure Co and pure Pt monolayers. In this case, we get an easy magnetization direction perpendicular to the layer plane, which is the required configuration for the perpendicular magneto-optic recording. The order state of these thin films has been modified locally by a low energy ion irradiation. The structural and magnetic properties have changed jointly: with a 40-keV energy and a 4×10 16 -ions/cm 2 flux, the chemical order decreases (the long-range-order (LRO) parameter changed from S =0.80 to S =0.50), the coercive field diminished from 0.090 T down to 0.052 T. Such results show that it is possible to locally change the magnetization easy axis, which is promising for applications.

Magnetic patterning using ion irradiation for highly ordered CoPt alloys with perpendicular anisotropy

We used a combination of ion irradiation and e-beam lithography to magnetically pattern an ordered CoPt alloy with strong perpendicular magnetic anisotropy. Ion irradiation disorders the alloy and strongly reduces the magnetic anisotropy. Magnetic force microscopy showed a regular array of 1μm2 square dots with perpendicular anisotropy separated by 1 μm large ranges with in-plane anisotropy. This is further confirmed by magnetic measurements, which showed that arrays protected by a 200 nm Pt layer present the same coercive field and the same perpendicular anisotropy as before irradiation. This is promising for applications in magnetic recording technologies.

Magnetic switching field distribution of patterned CoPt dots

We report on the interplay between the structural and the magnetic properties of different size dots patterned in two structurally different CoPt thin films with L10 order. The significant changes observed in the magnetic domain configuration and coercive field between the samples are interpreted in the light of crystalline structure. The magnetic hysteresis curves measured for single/several dots are compared to those of the dot assembly and discussed in view of structure and edge defects. In particular, the concentration fluctuations are found to enlarge the magnetic switching field distribution despite the presence of a single variant of the L10 phase. In turn, the distribution is narrow in samples that are homogeneous in concentration and contain well defined magnetization nucleation sites, namely, the x and y variants of the L10 phase formed during annealing.

High in-plane anisotropy of epitaxial CoPt(110) alloy films prepared by cosputtering or molecular beam epitaxy on MgO

We present structural and magnetic properties of three sets of structures: as-deposited CoPt films cosputtered at 900 K on MgO(110) substrates with a Pt(110) buffer layer and CoPt films deposited by molecular beam epitaxy directly on MgO(110) substrates at 900 K, as prepared and annealed at 900 K. All layers have the L10 tetragonal structure. The chemical long-range ordering for the as-deposited CoPt films is incomplete in contrast with the annealed CoPt films, where long-range order is the highest. The structural study of these CoPt films grown on MgO(110) has pointed out that three variants of the L10 phase coexist. The proportion of x and y variants, with the concentration modulation along a vector oriented at 45° with respect to the growth direction, is higher than the proportion of the z variant with the concentration modulation within the plane. The magnetic study shows an in-plane easy magnetization axis with a large magnetic anisotropy. This is interesting for the magnetic recording media with cla...

Field-dependent behavior of a magnetic force microscopy tip probed by means of high coercive nanomagnets

Single domain uniformly magnetized CoPt dots are used to probe magnetic force microscopy (MFM) tip characteristics. Magnetic contrasts obtained in MFM experiments are exploited to get the tip hysteresis loop in the influence of the dot. Coercivity, remnant magnetization, as well as details about the magnetization reversal of the tips are obtained. Well defined dot geometries allow a quantitative determination of the stray field of the dots acting on the tip. The tip’s hysteresis loop refers to an effective tip height, involved in the imaging, which is much larger than the extension of a magnetic domain of the CoCr tip coating. When MFM tips and sample dots have widely different characteristics, it is possible to get separate magnetic information, such as hysteresis loops, on both tip and sample, depending on the magnitude of the out-of-plane external field.