Magnetization reversal of an individual exchange biased permalloy nanotube

TL;DR

This study uses advanced magnetometry techniques to analyze how a permalloy nanotube's magnetization reverses, revealing the influence of a native oxide layer and exchange bias effects at low temperatures.

Contribution

It introduces a hybrid measurement approach combining nanoSQUID and cantilever magnetometry to study individual nanotubes and uncovers the role of native oxide in magnetization reversal.

Findings

Identification of a low blocking temperature indicating native oxide presence

Observation of exchange bias effects and training in hysteresis behavior

Reversal process nucleates at nanotube ends and propagates along its length

Abstract

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.