  A sensor that integrates high sensitivity micro-Hall effect magnetometry and
high-frequency electron paramagnetic resonance spectroscopy capabilities on a
single semiconductor chip is presented. The Hall-effect magnetometer was
fabricated from a two dimensional electron gas GaAs/AlGaAs heterostructure in
the form of a cross, with a 50x50 um2 sensing area. A high-frequency microstrip
resonator is coupled with two small gaps to a transmission line with a 50 Ohms
impedance. Different resonator lengths are used to obtain quasi-TEM fundamental
resonant modes in the frequency range 10-30 GHz. The resonator is positioned on
top of the active area of the Hall-effect magnetometer, where the magnetic
field of the fundamental mode is largest, thus optimizing the conversion of
microwave power into magnetic field at the sample position. The two gaps
coupling the resonator and transmission lines are engineered differently. The
gap to the microwave source is designed to optimize the loaded quality factor
of the resonator (Q = 150) while the gap for the transmitted signal is larger.
This latter gap minimizes losses and prevents distortion of the resonance while
enabling measurement of the transmitted signal. The large filling factor of the
resonator permits sensitivities comparable to that of high-quality factor
resonant cavities. The integrated sensor enables measurement of the
magnetization response of micron scale samples upon application of microwave
fields. In particular, the combined measurement of the magnetization change and
the microwave power under cw microwave irradiation of single crystal of
molecular magnets is used to determine of the energy relaxation time of the
molecular spin states. In addition, real time measurements of the magnetization
dynamics upon application of fast microwave pulses are demonstrated
