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Zusammenfassung:
Spin-spin correlations and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder neutron diffraction. The present study reveals a three-dimensional long-range magnetic ordering below 16 K where the magnetic structure consists of ferromagnetic trimers that are coupled ferromagnetically along the spin-chain direction (b axis). The moment components along the a and c axes arrange antiferromagnetically. Our study establishes that the uncompensated moment components along the b axis (m(b)) result in a net magnetization per unit cell. The magnetic structure, determined in the present study, is in agreement with the results of recent first-principles calculation; however, it is in contrast to a fascinating experimental prediction of ferrimagnetic ordering based on the periodicity of the exchange interactions in CaNi3P4O14. Our study also confirms the presence of broad diffuse magnetic scattering, due to one-dimensional short-range spin-spin correlations, over a wide temperature range below similar to 50 K down to a temperature well below the T-c. Total neutron scattering analysis by the reverse Monte Carlo (RMC) method reveals that the dominating spin-spin correlation above T-c is ferromagnetic and along the b axis. The nearest-neighbor spin-spin correlations along the a and c axes are found to be weakly antiferromagnetic. The nature of the trimer spin structure of the short-range ordered state (above T-c) is similar to that of the 3D long-range ordered state (below T-c). The present investigation of microscopic nature of the magnetic ground state also explains the condition required for the 1/3 magnetization plateau to be observed in the trimer spin chains. In spite of the S = 1 trimer chain system, the present compound CaNi3P4O14 is found to be a good realization of a three-dimensional magnet below T-c = 16 K with full ordered moment values of similar to 2 mu(B)/Ni2+ (1.98 and 1.96 mu(B)/Ni2+ for two Ni sites, respectively) at 1.5 K.
