Abstract
We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt $L{1}_{0}$ alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.
Highlights
The magnetic anisotropy of assembled nanoparticles (NPs) systems constitutes a key parameter in the understanding of their magnetic behavior, as well as in their possible technological applications [1]
We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD)
The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt L10 alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior
Summary
The magnetic anisotropy of assembled nanoparticles (NPs) systems constitutes a key parameter in the understanding of their magnetic behavior, as well as in their possible technological applications [1]. We have performed an intense research in this direction by exploring enhanced magnetic anisotropy in different cobalt granular systems [2,3,4,5,6,7]. The Co-W NPs is a system of amorphous alloyed particles, which constitute a different type with variable anisotropy, induced by alloying, irrespective of surface effects [7] Their magnetic anisotropy properties were determined as result of the shortrange order revealed in the Co-W alloy. CoPt NPs embedded in different matrices show enhanced magnetic anisotropy with respect to the Co particles [9,10,18,19] This results from the crystallization of Co-Pt alloys in the chemically ordered L10 phase, which, in bulk form, has a very high magnetocrystalline anisotropy [20]. We include angle dependent XMCD measurements, which constitute an effective method to explore the anisotropy of the orbital moment in the magnetism of Co-Pt systems with PMA [27,32]
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