Effect of amorphous wire core diameter on the noise of an orthogonal fluxgate

Orthogonal fluxgates have been proved to provide low noise measurement of quasi-static magnetic field. Typically Co-based amorphous wires are used because they allow to reach noise as low as 2.5 pT/√Hz noise at 1 Hz for as-cast wires and 1 pT/√Hz noise for annealed wires. So far, however, only one diameter has been used for such wires, namely 125 μm. This is due to the fact that commercially available magnetic wires manufactured by Unitika
^Tm
are available only in this diameter. In this paper we investigate the possibility to reduce the noise of an orthogonal fluxgate by increasing the diameter of the magnetic wires used as core of the sensor. We manufactured (Co
_0.94
Fe
_0.06
)
_72.5
Si
_12.5
B
₁₅
in-water solidified amorphous wires with diameter in a range from 80 μm to 220 μm by changing the diameter of the ejecting hole of the crucible where the alloy was melted. For different holes diameter we adjusted the value of argon pressure inside the crucible and speed of the water wheel in order to obtain a continuos casting of the magnetic wire. We then analyzed the magnetic behaviour of the wire by measuring the hysteresis loop by means of VSM: the coercivity keeps between 80 A/m and 180 A/m for wires with diameter up to 190 μm, then it rises up to 4300 A/m for wires with diameter higher than 200 μm. This indicates that we can produce amorphous wires only with diameter up to 190 μm. For larger diameter the central part of the wire is nanocrystalline. This is cause by limited cooling rate during casting (about 10
⁶
K/s) which does not allow to cool fast enough the inner portion of the wire making possible the development of crystals.