Defect-induced ferromagnetism in a S=1/2 quasi-one-dimensional Heisenberg antiferromagnetic chain compound

We present evidences that defects in the spin S=1/2 Heisenberg antiferromagnetic chain (HAFC) compound can lead to ferromagnetism by studying the magnetic and thermal properties of the newly discovered quasi-one-dimensional (1D) metal-organic framework [CH 3 NH 3 ][Cu(HCOO) 3 ] (MACuF). Our findings suggest that the long-range ferromagnetic order at 3.7 K can be attributed to Cu 2+ ions from the 2D networks constructed by the endpoints of the broken chains. It is worth mentioning that the ferromagnetic saturation field is less than 5 Oe. In such a case, the intrinsic magnetism can emerge in this quasi-1D Heisenberg chain system at the background of the anti-ferromagnetism. This unusual ferromagnetism found in HAFC not only enriches magnetic features in the low-dimensional systems, but helps to understand some of the exotic magnetic phenomena in other real quasi-1D magnetic materials.