Temperature dependence of induced Ni2+ moment fluctuations in the Eu2BaNiO5 Haldane system.

151Eu and 153Eu Mössbauer spectroscopies have been used to study the magnetism of the Ni2+ moments in Eu2BaNiO5 by way of the exchange-induced Eu3+ moments. These independent and complementary probes indicate that at high temperatures all of the moments in the Haldane quantum spin chains are paramagnetic. On cooling below 25 K, fluctuations of ∼90% of the Ni2+ moments slow down, while the remaining 10% remain paramagnetic (indicating the presence of chain defects). Both 151Eu and 153Eu Mössbauer measurements yield the same Ni2+ moment. Furthermore, below 5 K, the 90% chain component becomes static and ordered, in excellent agreement with the ordering temperature expected from the rare-earth exchange energy. [ABSTRACT FROM AUTHOR]