Quantum phase transition in Pr ₂ CuO ₄ to collinear non–spin-flop state in magnetic field: A neutron diffraction study

Abstract

In the external field slightly inclined to the x-or y-axis of the frustrated tetragonal antiferromagnet Pr 2 CuO 4 , a transition is discovered from the phase with orthogonal antiferromagnetic spin subsystems along [1,0,0] and [0,1,0] to the phase with the collinear spins. This phase is shown to be due to the pseudodipolar interaction, and transforms into the spin-flop phase (S⊥H) asymptotically at very high field. The discovered phase transition holds at T = 0 and is a quantum one, with the transition field being the critical point and the angle between two subsystems being the order parameter. Rare-earth cuprates R 2 CuO 4 (R = Pr, Nd, Sm, Eu, Gd), which originally had drown attention as parent compounds for the electron-doped high-T C superconductors, now are being extensively studied as two-dimensional quantum Heisenberg antiferromagnets. These materials have a tetragonal structure with the space group I4/mmm. The CuO 2 planes give a motif of the structure, with the Cu 2+ ions being coordinated by regular squares of oxygen neighbours [1]. A very important feature of this tetragonal body-centred structure is a shift of adjacent CuO 2 planes by [1/2,1/2,1/2]. Due to very strong in-plane superexchange interaction, J = 124(3) meV [2], each Cu 2+ ion (S = 1/2) has four nearest neighbors with the opposite spins. Therefore, the mean field, produced on each copper ion by the adjacent plains is cancelled. In the absence of exchange field, the interplanar spin orientation should be very sensitive to any weak interaction that violates the symmetry. Shender [3] has shown that in a similar situation of the bcc lattice the quantum zero-point spin fluctuations stabilize a collinear orientation of the spin subsystems, which has been confirmed experimentally [4].For interpretation of the early neutron diffraction data a collinear model was used with the spins along [1,1,0] and with a propagation vector k 1 = (π/a) [1,1,0] in the same direction [5,6]. Certainly there should be another domain with the spins along [1,−1,0] and with k 2 = (π/a) [1,−1,0]. A different model with two vectors, k 1 and k 2 (2-k structure) that results in exactly the same intensities of the magnetic reflections has been also proposed [6]. In this model two antiferromagnetic subsystems are orthogonal, with the spins in positions (0,0,0) and (1/2,1/2,1/2) along [1,0,0] and [0,−1,0] directions, respectively, as shown in Fig. 1(a). Neutron diffraction experiments in the external magnetic field applied in the [1,1,0] direction [7][8][9][10][11] have definitely proved that the copper spin subsystems are orthogonal in spite of the quan-

Thermal hysteresis and anisotropy in the magnetoresistance of antiferromagneticNd2−xCexCuO4

“…The fascinating MR oscillations shown in Fig.4 has been explained to arise from the relative orientation of spins with respect to the crystal axes because the spin structure always stays collinear at high fields. The total energy does not change due to the interplane pseudo-dipolar interactions when the spin sublattices of the adjacent CuO 2 planes rotate in opposite directions [16,19,20]. Such continuous spin rotation can be induced by B because the spins gradually rotate toward a configuration perpendicular to the B at high fields.…”

Spin-Flop Transition and the Anisotropic Magnetoresistance ofPr1.3−xLa0.7CexCuO4Лавров1, Kang2, Kurita3 et al. 2004Phys. Rev. Lett.505510View full textAdd to dashboardCite

“…The three-dimensional ordering [ Fig. 1(a)] that sets in below the Néel temperature T N 250-285 K [13][14][15] is governed by weak pseudodipolar (PD) interactions, which favor a noncollinear orientation of spins in adjacent planes (alternating along the 100 and 010 directions) [13][14][15][16]. A unique feature of the interplane PD interaction is that its energy does not change if the spin sublattices of adjacent CuO 2 planes rotate in opposite directions [14 -16].…”

Quantum criticality of antiferromagnets in inclined magnetic field

“…For the nonfrustrated AF such as YB 2 Cu 3 O 6þx any deviation from the diagonal field direction leads to the first-order transition. The situation drastically changes for members of the R 2 CuO 4 family as it was shown recently for Pr 2 CuO 4 [3]. In this case the interplane pseudodipolar interaction connects the spins in the neighboring planes and they rotate coherently with increasing the field.…”

“…The flop state is attained at H-N: The stability condition N ¼ ð@E cl =@j 1 Þð@E cl =@j 2 Þ À ð@ 2 E cl =@j 1 @j 2 Þ 2 > 0 is fulfilled at all H except HðdÞ where N ¼ 0: It may be shown using results of Ref. [3] that the spin-waves are stable at N > 0 too. At the line H ¼ HðdÞ we have the gapless spin-waves as it should be for the quantum phase transition.…”

“…the quantum phase transition. Further experimental studies of the neutron scattering in Pr 2 CuO 4 demonstrated that the quantum phase transition persists in all field directions except ½1 0 0 where the transition is of the first-order one [3]. For explanation of these experimental findings one should take into account the structure of the R 2 CuO 4 compounds.…”