Proton NMR study of nickel formate Di-hydrate, Ni(HCOO)2·2H2O

In nickel formate di-hydrate, Ni(HCOO) 2 ·2H 2 O, there are two nonequivalent Ni sites forming two magnetic subsystems, A- and B-layers. Heat capacity measurements suggest that the Ni ions in the B-layer are paramagnetic even below T N and the Ni ions in the A-layer are in antiferromagnetic order. On the other hand, the magnetization suggests the weak ferrimagnetic order of Ni ions in both A- and B-layers. To solve the inconsistency above, the spin system is investigated by proton NMR. The observed resonance frequencies are compared with calculations based on various magnetic structures. The comparison shows that the ferrimagnetic spins order with canting magnetic moments in the a – b plane. The magnitude of the magnetic moment of B-ions, 0.38µ 0 , is much smaller than that of A-ions, 2.38µ 0 , where µ 0 is the Bohr magneton. It is inferred that the paramagnetic B-ions are pulled into the antiferromagnetic order by the dipolar field due to A-ions. The magnetic moments are inferred to gradually incline par...