Magnetic entropy change CaBaCo4O7 compound by Al and Ni substitution

The magnetic and magnetic entropy behaviors of the CaBaCo4O7 with Al-doping CaBaCo3.96Al0.06O7 and Ni-doping CaBaCo3.96Ni0.06O7 compound are observed. When compared with the Al-doping, Ni-doping shows that a huge decrease of ferrimagnetism is produced. For Al-doping, the magnetic measurements suggest that spin-glass and ferromagnetic transitions are observed at 25 K and 45 K. For Ni-doping, besides the above two transitions, another peak is observed due to the antiferromagnetic (AFM) phase appearance at similar to 80 K. The plot of H/M vs M-2 of the isotherms in the vicinity of the Curie temperature suggests that this phase transition was first-order phase transition for all compounds. With decreasing the temperature, the system enters charge-order state and the negative entropy is observed for CaBaCo4O7 and CaBaCo3.96Al0.04O7 compound. However, unlike the nonmagnetic doping, replacement of Co2+ in the zig-zag ferromagnetic chain by Ni2+ does not truncate the chain, but it perturbs the magnetic interaction through antiferromagnetic exchange with cobalt following Goodenough-Kanamori rules. This possibly reorients the cobalt spins adjacent to the dopant in the chain which modifies ferrimagnetic ground state resulting in competing magnetic states. These are likely responsible for the change in magnetic ground state of CaBaCo4O7, this results in suppression of ferrimagnetic state with the evolution of antiferromagnetism and magnetic frustration. The result suggests that the AFM interaction between the dopant Ni2+ and Co2+ ions has a crucial role in the destabilization of the ferromagnetic structure, whereas the triangular geometry of the cobalt sublattice imposes the appearance of magnetic frustration.