Magnetic superstructures inside the ⅓ and ½ magnetization plateaux in Ba₂CoTeO₆ revealed by magnetostriction and magnetocaloric effect measurements

Ba₂CoTeO₆ is composed of two nearly decoupled magnetic spin-systems stacked as alternating 2D layers along the crystallographic c-direction in the trigonal crystal structure. Subsystem A is a S = ½ triangular Heisenberg antiferromagnet that orders at T_A = 3K. System B is a J₁-J₂ honeycomb Ising antiferromagnet with ordering temperature T_B = 12K. Both underlying spin-models are interesting due to geometrical frustration (A) and bond frustration (B) causing magnetic plateaux upon the application of magnetic fields H. We present a combination of magnetostriction and magnetocaloric effect (MCE) measurements in high pulsed and static magnetic fields to map out the phase diagram for H ll and perp. to the A and B layers. We observe distinct anomalies in the sample length ΔL/L as a function of field and temperature indicating phase transitions as well as signatures in the MCE measurement indicating a change of entropy. These anomalies are accompanied by significant hysteresis in decreasing compared to increasing magnetic fields.