Lattice Expansion and Melting of Spin Glass State in the Honeycomb Antiferromagnet Bi3Mn4O12(NO3)

Bi3Mn4O12(NO3) is a well-known highly frustrated honeycomb antiferromagnet, its ground state is spin glass (SG) stabilized by the intrabilayer exchange couplings. Here, we report the synthesis of nanocrystalline Bi3Mn4O12(NO3) (nano-BMNO) and the discovery of similar to 8.6% lattice expansion along the c axis which is an effective way to adjust the magnetic intrabilayer coupling. Consequently, nano-BMNO exhibits two specialized features in magnetism: (1) It has a large Weiss constant theta(CW)= -237.56 K, yet there is no long-range magnetic order or SG transition down to 2 K. (2) The field-induced long-range order transition is also absent in nano-BMNO. More interestingly, the low temperature specific heat of magnetic excitations down to 0.2 K exhibits linear temperature dependence, with a large linear coefficient gamma = 26.2 mJ/K-2 f.u. All these properties imply that nano-BMNO is more spin liquid like, instead of SG state.