Magnetization reversal of an individual exchange-biased permalloy nanotube

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 a temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature T B = 18 ± 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.

Imaging magnetic vortex configurations in ferromagnetic nanotubes

“…The magnetic shell material is covered by a few-nanometer-thick native oxide, which affects the NT magnetization only at cryogenic temperatures through antiferromagnetic exchange coupling. In Py NTs, Buchter et al observed such coupling below a blocking temperature of 18 K [32].…”

“…For the CoFeB NTs, CoFeB is then magnetron-sputtered on the NWs, producing an amorphous and homogeneous 30-nm-thick shell [31]. For the Py NTs, a 30-nm-thick polycrystalline shell of Ni 80 Fe 20 is deposited by thermal evaporation [32]. During both depositions, the wafers of upright and well-separated GaAs NWs are mounted with a 35…”

Sample Preparation for Inorganic Trace Element Analysis

“…Measurements on an individual permalloy nanotube evidenced the nucleation of magnetic vortices at the nanotube's end before propagating through its whole length, leading to the complete switching of the magnetization. Furthermore, it has been shown that a thin exchange-coupled antiferromagnetic native-oxide layer on the nanotube modifies the magnetization reversal process at low temperatures 125 . YBCO nanoSQUIDs were used for the investigation of magnetization reversal in a Fe nanowire grown inside a CNT attached on top of the SQUID 53 [ Fig.18(a)].…”

“…16(b)]. Recently, individual magnetic nanotubes, attached to an ultrasoft cantilever were brought in close vicinity to a nanoSQUID at low T 51,124,125 . This technique allowed the authors to investigate magnetization reversal of the nanotubes by combining torque and SQUID magnetometry (see section IV B).…”

“…Magnetization reversal mechanisms in single Ni and permalloy nanotubes were investigated using Nb/HfTi/Nb-based nanoSQUIDs 51,124,125 . Experiments were performed at 4.2 K with B ext = µ 0 H applied along the nanotube axis (z-axis), with the SQUID loop in the x-z plane.…”