Different susceptibilities of nanosized single-domain particles derived from magnetisation measurements

We briefly recall the magnetisation curves of an ideal super-paramagnet and the fact that the susceptibility exhibits a maximum when measured versus t emperature in a constant magnetic field. This maximum is determined by the particles' magnetic moment and the magnetic held applied. In real particle systems exhibiting an anisotropy energy this susceptibility maximum (and it s dependence on the magnetic field) is determined while reaching the low-te mperature magnetic state. For such systems we show that it is possible to ' prepare' them into a quasi-paramagnetic state if the low-temperature magnet ic state is obtained during cooling in highly positive magnetic fields. The n, the theory of ideal super-paramagnetism is applicable in the limit of hi gh magnetic fields. We demonstrate these simple experimental facts by measu rements performed on samples consisting of nanosized Ni particles fixed in space by an aluminium oxide matrix. From the experiments we derive the susc eptibility from two adjacent magnetisation curves measured between the magn etic fields B and B + Delta B. (C) 2000 Elsevier Science B.V. All rights re served.