Optimization of the Q factor of the magnetic $M_x$ resonance under optical pump conditions

Optimization of parameters of the $M_x$ resonance, which excited between Zeeman sublevels of a single HFS level of the ground state of an alkali metal in a vacuum cell in a circuit with a single beam for pumping and detection ($M_x$ magnetometer circuit), has been carried out. A simple model taking into account all main factors controlling the resolving power of the $M_x$ resonance (including spin-exchange broadening and absorption in a thick layer of the cell has been constructed. It is shown that the spin-resonance broadening of the resonance line is mainly determined by the requirements imposed on the optical thickness of the cell, which considerably restricts the realization of advantages of ultranarrow ($<$ 1 Hz) lines in magnetometry. The experiment confirming the efficiency of the model has been carried out.