High sensitivity magnetization measurements of nanoscale cobalt clusters

Abstract

Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization reversal of about 104 μB corresponding to a sensitivity of about 10−16 emu. The detector is a niobium micro-bridge DC superconducting quantum interference device (SQUID), fabricated using electron-beam lithography. It is operational in the temperature range of 0–7 K. Furthermore, we present a method to deposit on the SQUID loop a small number of Co clusters of about 2–5 nm in diameter. The first results obtained on these samples show that there is still a ferromagnetic coupling between the clusters and the magnetization reversal takes place by small avalanches.

Classical and Quantum Magnetization Reversal Studied in Nanometer‐Sized Particles and Clusters

“…Other low temperature techniques which may be adapted to single particle measurements are Hall probe magnetometry [26,27,28], magnetometry based on magnetoresistance [29,30,31] or spin-dependent tunneling with Coulomb blockade [32,33]. At the time of writing, the micro-SQUID technique allows the most detailed study of the magnetization reversal of nanometer-sized particles [34,35,36,37,38,39]. The following section reviews the basic ideas of the micro-SQUID technique.…”

Experimental Evidence of the Néel-Brown Model of Magnetization Reversal

“…Because of the close proximity between sample and SQUID we had a very efficient and direct flux coupling. In this configuration, we could detect magnetization reversals corresponding to 10 4 m B [15]. In order to place a nanoparticle on the SQUID detector, we dispersed the particles in ethanol by ultrasonication.…”

Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires

“…We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 m B [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping.…”