Quantum phase transition in Pr₂CuO₄ to collinear non–spin-flop state in magnetic field: A neutron diffraction study

In the external field applied to the tetragonal antiferromagnet Pr2CuO4, a transition is discovered from the phase with orthogonal antiferromagnetic spin subsystems along [1, 0, 0] and [0, 1, 0] to the phase with collinear spins, which are inclined about 45 to the field. This phase is shown to be due to the interplane pseudodipolar interaction, and transforms into the spin-flop phase (S perpendicular to H) asymptotically at very high field. The discovered phase transition is of first order, if the field is along [1, 0, 0], or [0, 1, 0], and becomes of second order if the field is slightly inclined to these axes. It holds at T = 0 and is a quantum one, with the transition field being the critical point and the angle between two subsystems below the critical point being the order parameter.