Magnetic resonance force microscopy: Nonlinear processes and influence of relaxation times

The phenomenon of the micromechanically detected magnetic resonance is interpreted in terms of nonlinear processes at magnetic resonance. A close analysis in the frequency domain of the irradiation and detection scheme shows that the technique corresponds to a multiple irradiation with one or more couple of frequencies separated by omegac and to the detection of the longitudinal component of magnetization oscillating at omegac. The study of longitudinal detection of magnetic resonance allows the direct measurement of the spin-lattice relaxation time of samples. Samples of Mn2+:MgO prepared in order to obtain a mixture of spin systems with very different relaxation processes were studied by electron-spin-resonance experiments with micromechanical detection: measurements evidence a very strong rejection of the system with lower longitudinal relaxation time. Direct confirmation of the theoretical interpretation is obtained; in addition the microscopy technique increases its "contrast" capability, adding the possibility of determining maps of samples based on the distribution of both concentration and longitudinal relaxation times of spin systems.