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Magnetic hysteresis loops of single Co nano-islands

Guillemin Rodary - Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany


We present spin dependent transport and magnetic properties of single Co nano-islands studied by spin-dependent scanning tunneling microscopy (Spin-STM) and spectroscopy (Spin-STS). We measure magnetic hysteresis loops of well characterized individual nano-islands by Spin-STS, and we extract the islands size and temperature dependence of the switching field of the magnetization.

 

Co islands are prepared by deposition of sub-monolayer quantities of Co at 300 K onto a clean Cu(111) surface. STM reveals the growth of double-layer high Co islands, which exhibit often an equilateral triangular shape with base lengths between few nanometers and few tens of nanometers. This corresponds to islands with 500 to 20000 atoms. Spin-STS measurements are performed with a Cr-covered W-tip at the center of the Co islands at 8 K and in a magnetic field of up to 6 T, which is oriented normal to the sample surface.

 

Our measurements of the differential conductance during a magnetic field scan show a sharp drop of the differential conductance at field values which depend on the Co island size. We ascribe this drop to the magnetic switching of the Co islands, where we find switching fields between 0.6 T and 2.2 T depending of the island size. For smaller islands (less than 1200 atoms), no abrupt drop of the dI/dV signal is observed. Here, a continuous and reversible variation of the conductance with magnetic field is measured, which we ascribe to a superparamagnetic response of the spin orientation of the islands.

 

We discuss this size dependence of the switching field in view of the Stoner-Wohlfarth model of coherent magnetization switching, and extract the magnetic anisotropy of single nano-islands. We find three different regimes of magnetic behavior. We ascribe these regimes sequentially with increasing island size to a superparamagnetic state, a monodomain ferromagnetic state, and a multidomain state. Field dependent measurements of the differential conductance at different temperatures confirm this interpretation.