Ac Susceptibility Studies of New and Familiar Magnetic Molecular Solids
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This compound, with a coercive field of 2.5 kemu-G/mole and remanence of 4 kG. manganese(II) octaethyltetraazaporphyrin, cx-MnOETAP, (Figure l b) is structurally and electronically similar to the known molecular ferromagnet, manganese(II) phthalocyanine, 3MnPc (Figure la).2 However, unlike for P3-MnPc, ac susceptometry on ct-MnOETAP reveals a strong dependence of the susceptibility on the frequency of the applied field suggesting the compound is a spin-glass. For comparison, we show ac susceptibility data for two molecular ferromagnets, (x-FeOETAP, which is isomorphous with its manganese analog and 13-MnPc. To explore the ubiquity of the frequency-dependent ac susceptometry result, we have examined the molecule-based solid, decamethylmanganocenium tetracyanoethenide, previously characterized by dc Mn(Cp*) 2 oTCNE, (Figure 1 c and d). This compound had been techniques to be a hard molecular ferromagnet with Tc = 8.8 K3 Our results show that it too is a spin glass.
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c d b a Figure 1. Structures of a) MnPc b) MnOETAP c) Mn(Cp*) 2 d) TCNE 471 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
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Palacio, et al. have previously made the point that dc measurements were unsufficient,4 but an examination of the literature suggests that this warning has been unheeded. For the most part, variable frequency ac measurements have only been utilized when magnetic frustration was built into the system by design and hence spin-glass properties could be anticipated. For instance, Wynn, et al. have prepared an intentionally frustrated molecule-based magnet on a5 tridentate chelate which shows spin glass properties and frequency-dependent ac susceptibility. We are unaware of any studies that suggest that the phenomenon may be more widespread among molecule-based solids. EXPERIMENT All magnetic measurements were carried out on either a 5 Tesla or 7 Tesla Quantum Design SQUID magnetometer. Full temperature scan dc susceptibility measurements were performed in 100 G applied field from 1.7 K to 350 K. Low field dc measurements were performed in 0 - 10 G or 0 - 3 G. The amplitude of the applied ac magnetic field for ac air-tight sample holder measurements was 5 G with no additional dc bias field. A flame-sealable was constructed from segments of Wilmad 507 5 mm NMR tube.6 In the glove box, the sample (- 15 mg) was placed into the holder, packed with a small piece of glass wool (-10 mg) and attached to a Teflon stopcock by means of an Ultratorr o-ring connector. The tube was evacuated in glove box by mechanical pump, sealed, brought out of the glove box, evacuated to 10-5 torr by oil diffusion pump and flame sealed. The diamagnetic correction for the complexes was estimated from Pascal's constants and from the measured magnetic susceptibility of diamagnetic analogs. The diamagnetic susceptibility of the glass wool was calculated from its measured average gram susceptibility as determined from several independent samples. The diamagnetic susceptibility of th
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