Investigation of Electrodeposited Nifemo for Application as Flux Concentrator in Magnetic Field Sensors

Introduction

To enhance a magnetic field sensor’s sensitivity, flux concentrators made of soft-magnetic materials featuring high permeability and low coercivity are used. By electroplating, these materials can be completely integrated into the sensor design. Whereas Ni₈₁Fe₁₉ is already characterized by high permeabilities of 1,500 to 3,000 [1] and a low coercivity of 16 A/m [2], NiFeMo layers promise even more favorable magnetic characteristics (permeabilities of 2,200 to 20,000 and coercivities in the range of 8-28 A/m [3]). The preferred composition is Ni₇₉Fe₁₆Mo₅. The presented investigation deals with the influence of plating conditions and post-treatment on the NiFeMo characteristics.

Experiments

Based on the plating bath published in [3], the influence of plating parameters like current density, the relationship between forward and reverse current (pulse current relationship), the relationship between the time of forward and reverse current (pulse time relationship) and frequency on the composition of the plated NiFeMo and its magnetic properties are analyzed. Moreover the influence of thermal annealing below and above Curie temperature of NiFeMo on its magnetic characteristics is investigated.

The NiFeMo is electroplated into micromolds made of photoresist. The micromold has the geometry of flux concentrators as applied in magnetic field sensors. Fig. 1 shows the deposited structure.

The plating parameters are varied in the following ranges:

-  Current density: 15 – 30 mA/cm²

-  Pulse current relationship (I_forward/I_reverse): -10 – +10

-  Pulse time relationship (t_forward/t_reverse): 1 – 9

-  Frequency (1/(t_forward+t_reverse)): 50 – 150 Hz

The deposition time is adapted according to the aspired deposition height of 2 µm. The magnetic properties are measured by a Vibrating Sample Magnetometer (VSM) and the surface quality is investigated by Atomic Force Microscopy (AFM) and Light Optical Microscopy. For comparison Ni₈₁Fe₁₉layers are plated and thermally treated and the measurement results are finally opposed to the NiFeMo characteristics.

Results

Main influence on the composition of the electroplated NiFeMo is taken by the current density. The magnetic properties are moreover determined by the pulse time relationship, whereas the composition is not changed significantly.

With a higher current density the Fe-content in the layers increases while the Ni and Mo concentration decrease. The highest permeability and lowest coercivity are realized at the targeted composition of Ni₇₉Fe₁₆Mo₅. The sample with a composition of Ni₆₂Fe₃₅Mo₃ shows the highest saturation flux density. The influence of the pulse time relationship shows that a long time of forward current in relation to the reverse current leads to an increase in permeability. These layers are characterized by a reduced roughness and potentially smaller grains.

Annealing the NiFeMo-films above their Curie temperature in a magnetic field for 2 h increases their coercivity.

Compared to the plated NiFe-layers, a higher permeability of the plated NiFeMo layers can be achieved, whereas the coercivity is similar.

Conclusion

NiFeMo is a quite promising material for magnetic flux concentrators in sensor application. Its high permeability enhances the flux concentrator’s ability to focus the magnetic field into the sensitive area of the device and hence increase the sensor’s sensitivity. Its fabrication by electroplating demands an adequate selection of parameters. By choosing an appropriate current density and pulse time relationship, high permeable layers with low coercivity can be deposited.

 

References

[1] Feng J, Thompson D (1977) Permeability of narrow permalloy stripes. IEEE Trans. Magn 13(5): 1521–1523

[2] Taylor WP (1999) A NiFeMo Electroplating Bath for Micromachined Structures. Electrochem. Solid-State Lett 2(12): 624

[3] Theis M, Ediger S, Schmitt MT, Hoffmann J, Saumer M (2013) Nanocrystalline electroplated NiFe-based alloys for integrated magnetic microsensors. Phys. Status Solidi A 210(5): 853–858