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 Ni 81 Fe 19 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 Ni 79 Fe 16 Mo 5 . 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/cm 2 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 Ni 81 Fe 19 layers 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 realiz