Very low noise fluxgate: 1 pT/√Hz at 1 Hz

low noise fluxgate

Here we are. We finally published a very important paper for fluxgate sensors.
In this paper we present a new method to anneal magnetic wires which later are used as core for orthogonal fluxgate in fundamental mode.
This method allows us to strongly reduce the offset in the fluxgate.

That was the primary purpose of our research. Previously it was found out that annealing the microwire while a dc current was flowing through it created a field induced circular anisotropy in the wire. This helped in reducing the noise because in fundamental mode orthogonal fluxgate we want to have the magnetization always well saturated in circular direction.
However, from time to time, we observed some large offset appearing in the final sensor. What was the cause of that offset? we wondered.
After investigating the possible causes we understood the reason of such causes was a minor axial field accidentally present during the annealing process. Such field slightly deviates the magnetization from circular direction, so that the resulting anisotropy was not exactly circular.
Therefore, we developed a method to identify if such axial field is present and to remove it before starting the annealing.
In this way we suppress the offset and… we also reduce the noise.
As a matter of fact, we invented this method to suppress the offset which appeared after annealing but we also got an improvement of the noise.
In the paper we describe how the noise is dependent on the offset, and prove it by artificially induce an offset (moving the working loin of the sensor) and observing an increment of the noise.
Finally, for wires properly annealed using the proposed method we achieve a remarkable result of 1 pT/√Hz at 1 Hz.
Unfortunately this method also decreases the sensitivity of the sensor, therefore the noise has noise floor already at frequency larger than 20 Hz, because of the noise of the electronics which becomes dominant over magnetic noise.
Improvement of the electronic should lead to further decrease the noise at larger frequency.
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Link to the paper
Method for Offset Suppression in Orthogonal Fluxgate with Annealed Wire Core
M. Butta and I. Sasada
Sensor Lett. 12, 1295-1298 (2014)