Magnetic phase diagram copper metaborate CuB_2O_4 in magnetic field parallel c-axis: resonant, magnetic and magnetoelastic investigations

The magnetic phase diagram in a single crystal of copper metaborate CuB_2O_4 in a magnetic field parallel to a tetragonal axis $c$ has been investigated. From the resonant, magnetic and magnetostrictive data the phase diagram of CuB_2O_4 on a plane ``temperature - magnetic field'' is constructed. The magnetic incommensurate-commensurate phase transition is caused by the saturation of weak subsystem of copper ions in the strong magnetic field $H\|c$.