V. Langlais

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Neel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles.

Using exchange bias to extend the temperature range of square loop behavior in [Pt∕Co] multilayers with perpendicular anisotropy

The temperature dependence of the magnetic properties of [Pt∕Co] multilayers (ML), exhibiting perpendicular anisotropy, with and without exchange biasing with an antiferromagnet (AFM) has been investigated. Upon heating, a loss of the out-of-plane anisotropy and, consequently, of the remanence to saturation ratio is observed in these systems. However, such effect occurs at higher temperatures in the [Pt∕Co] ML exchange coupled to the AFM than for the unbiased ML. This is attributed to the additional anisotropy induced to the ML by the ferromagnetic-antiferromagnetic exchange coupling.

Structure, morphology and magnetism of an ultra-thin [NiO/CoO]/PtCo bilayer with perpendicular exchange bias

Electronic and magnetic properties of nanoscale materials are closely related to the atomic arrangement at the interface shared by different chemical elements. A very precise knowledge of the surface/interface structure is then essential to properly interpret the new properties coming out. Of a particular interest is the relationship between structure, morphology and magnetic properties of exchanged-coupled interfaces in ferromagnetic (FM) and antiferromagnetic (AF) materials. The interaction at the AF/FM interface modifies the magnetic switching properties of the FM film, which turn out to be a usefull property on new magnetic devices technology. We present here an investigation of the buried exchange-coupled interface [NiO/CoO]/[PtCo] grown on a Pt(111) single crystal. The magneto-optical Kerr effect reveals a strong coupling at the interface, by an increasing coercivity, and a spin reorientation of the FM film when ordering occurs in the AF layer. The combination of grazing incidence X-ray diffraction, X-ray reflectivity and soft X-ray resonant magnetic scattering yields a comprehensive description of the system.