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Magnetic property of neptunyl (+2) complex NpO$$_{2}$$(C$$_{6}$$H$$_{7}$$O$$_{2}$$)$$_{2}$$(C$$_{5}$$H$$_{5}$$N)

Nakamura, Akio; Nakada, Masami  ; Nakamoto, Tadahiro*; Kitazawa, Takafumi*; Takeda, Masuo*

Magnetic property of a nitrogen (N)-coordinated neptunyl (+2) (Np(VI)) complex. 1 NpO$$_{2}$$(acac)$$_{2}$$(py) (acac=C$$_{6}$$H$$_{7}$$O$$_{2}$$, py=C$$_{5}$$H$$_{5}$$N) was studied by DC magnetization (M) measurements. While previous $$^{237}$$Np M$"o$ssbauer data of 1 exhibited para magnetic-relaxation spectra similar to other oxo-neptunyls (+1,+2), the DC-$$M$$ data have revealed many strikingly different features of 1 from the latter: first, its reciprocal susceptibility ($$chi$$$$^{-1}$$) vs. $$T$$(K) plots showed a peculiar strongly field ($$H$$)-dependent nonparamagnetic behavior, which suggests the presence of small ferromagnetic component with $$T$$$$_{C}$$$$>$$300 K. In addition, its low-temperature isothermal $$M$$ exhibited anomalous relaxation behavior involving initial over-responsive $$M$$ change and subs equent drift back to its "equilibrium" value ($$M$$$$_{rm eq}$$). Moreover, such $$M$$$$_{rm eq}$$ showed strong perturbation effect from the SQUID-$$M$$ measurement itself. Such novel magnetic features suggest the presence of a largely metastable fluctuating magnetic (spin plus orbital) state in 1 as a collective body of individual Ising-type neptunyl (O=Np=O)$$^{+2}$$ molecular magnets.



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