Magnetic investigations of phase transitions, exchange interactions, and magnetic ground state in nanosheets of β-Co(OH).

Detailed investigations of the magnetic properties of the layered system β-Co(OH) are presented. X-ray diffraction and scanning electron microscopy of the sample show it to consist of hexagonal nanosheets with thickness  ≈30 nm and width ~100 nm-200 nm. Analysis of its measured magnetization (M) as a function of temperature (T  =  2 K to 300 K) and magnetic field (H up to 90 kOe) yields a Neel temperature T   =  9.2 K. This lower T   =  9.2 K, compared to T   =  11.6 K reported for bulk β-Co(OH), is due to finite-size effects. Analysis of the data for T  >  T shows that the M versus T data does not quite fit the Curie-Weiss law since both the Curie constant C and Weiss temperature θ have noticeable temperature dependence. This temperature dependence is interpreted to be due to the effect of spin-orbit coupling, yielding a low-temperature effective spin S  =  1/2 ground state with magnetic moment µ  =  4.745 µ and g  =  5.479. For T  <  T , M versus H data show two transitions, first at H   ≃  15 kOe and second at H   ≃  32 kOe. The transition at H is a spin-flop transition and H is due to forced alignment of the spins yielding saturation magnetization M   =  160 emu g at 2 K, in agreement with the calculated M   =  163 emu g for the complete alignment of the spins at T  =  0 K for the spin S  =  1/2 ground state with g  =  5.479. The fitting of the M versus T data for T  >  T to the high temperature series for S  =  1/2 XY model yields the in-plane ferromagnetic exchange constant J /k   =  (1.8  ±  0.2) K for Co ions, with the interplane exchange constant J /k   ≃  -0.2 K determined from the magnitude of T . The temperature dependence of H and H is presented and discussed.