| Description: |
This thesis reports the synthesis and characterisation of various layered mixed transition metal chalcogenide and pnictide materials. The commonality between the studied systems is that they exhibit exotic magnetic phenomena, such as high- and low-Tc superconductivity and long-range magnetic order of the 3d transition metal moments. Chapter 3 details the effect of Ni substitution in ACo2Ch2 phases, where A = K, Rb, Cs, and Ch = S, Se. All of the Co-based phases show ferromagnetism, with the sole exception of antiferromagnetic CsCo2Se2, suggesting that larger interlayer separation favours antiferromagnetic order. On the other hand, all ANi2Ch2 phases show paramagnetism with a low-Tc superconducting transition below 2 K. Upon substitution of Ni for Co, a solid solution forms, which is evident from Synchrotron PXRD, and magnetometry measurements reveal a region of antiferromagnetic ordering, before the onset of paramagnetism at high substitution levels. These drastic changes in magnetism arise from the addition of an electron to the 3d band of the metallic ACo2Ch2 phases. The antiferromagnetic members of the KCo2−xNixCh2 (Ch = S, Se) series were studied by neutron powder diffraction, revealing A-type antiferromagnetism in x = 0.5 and 1 members, while the long-range magnetic order was not detected in x = 1.5 member. The refined magnetic model is highly anisotropic and is characterised by ferromagnetic coupling between Co/Ni ions within the (Co/Ni)2Se2 sheets and an antiferromagnetic interaction between the adjacent (Co/Ni)2Se2 sheets along the stacking axis. The mechanisms responsible for the different types of magnetic coupling are direct ferromagnetic interactions between transition metal ions within the plane, while the RKKY mechanism is responsible for the between-the-plane antiferromagnetic coupling, as the distance between the planes is too large (~5 Å) for any kind of direct interaction. PND of the CsCo2−xNixSe2 solid solution also showed A-type antiferromagnetism in its x = 1 member, and an intriguing ... |