Vibration-based energy harvesting has gained increasing scientific interest in recent times [1]. This can be attributed to the growing need to interconnect smart objects looking for alternative energy sources to battery consumption. Among the various possibilities, piezoelectric resonators have achieved great success thanks to their excellent power density for a large range of voltages [2]. However, the linear systems suffer from a well-known problem: due to the mismatch between the environmental frequencies (1 - 100 Hz) and those typical of the devices (hundreds/thousands of Hz) [3] the dynamic amplification is weak and only low amount of energy can be scavenged. This issue can be managed with the introduction of intentional nonlinearities to widen the operating band. A successful solution concerns the application of permanent magnets on a seismic low-frequency mass and the harvester to achieve the so-called frequency up-conversion [4]. The seismic system vibrates at low-frequency (less than 10 Hz) and interacts through the space-dependent magnetic forces with the piezoelectric harvester. Such a fact induces an additional coupling to the problem which becomes piezo-magneto-electric. The impulsive force induced by the magnetic coupling is responsible for the frequency up-conversion. In this work, we show that by increasing the speed between the magnets through a robotic system, the impulsiveness of the magnetic force increases, and gradually inherent material nonlinearities appear. Such a fact is observed by means of shifts in the right peaks of the FFTs which represents the first bending mode of the harvester in terms of open circuit voltage. The experiments show also that by increasing the gap between the magnets, the material nonlinearity decreases since only a slight shift is recorded. The phenomenon can be also accurately simulated with a reduced order model by considering nonlinearities in the enthalpy function and in the damping.