Flux concentrators are made up of thin strips of soft magnetic materials (High permeability, low remanence and low eddy current losses). When surrounded to current carrying conductors it increases the flux density. As the effective permeability of the material is high, it amplifies the flux density. At the same time the gap or air gap of flux concentrator aids in positioning the hall effect sensor in appropriate distance from the conductor. The distance between sensor and conductor determines the accuracy of measurement.
Factors like type of current measurement (either low or high), Frequency, Atmospheric conditions and end application determines the specific material grade to be used.