Magnetic Susceptibility Essay

explanatory Essay
657 words
657 words

The magnetic susceptibility χ (=M/H) (FC and ZFC) as a function of temperature measured at low applied field (H=50 Oe) is presented in Fig.5. The molar susceptibility shows a monotonic increase upon cooling down to ~ 22 K, where a steeper increase is observed. Below this temperature a bifurcation between the ZFC and the FC curves is evident (see inset of Fig.5. On the other and above 22 K the reciprocal magnetic susceptibility (1/χ) as a function of temperature shows a linear trend (Fig. 5 right scale). In detail, above ca. 30 K, in the paramagnetic region, the Curie-Weiss law is strictly followed. By fitting the linear part of the 1/χ curve with 1/χ = (T-p)/C, in the 30-310 K temperature range, a Curie-Weiss temperature, p = -2.3 K, and the Curie constant, C = 1.30 cm3.K.mol-1, (µeff = 3.2 µB) were obtained. The small negative Curie-Weiss temperature indicates the presence weak antiferromagnetic exchangeinteraction between the Ni magnetic centres. Indeed, the χT curve (Fig. 6 left scale) shows a downward curvature, typical of systems with antiferromagnetic correlations and/or non-negligible spin-orbit coupling. The χT=1.31 cm3.K.mol-1 at 310 K undergoes a small and gradual decrease to 1.19 emu.K.mol-1 at 24.5 K. The Curie constant value, either obtained by1/χ linear fit or the χT product for T>>p is in reasonable agreement with the expected spin-only theoretical value for NiII in octahedral environment with S=1 spin state (C = 1 cm3.K.mol-1and µeff = 2.83 µB considering g = 2) for unquenched orbital moment C = 3.91 cm3.K.mol-1and µeff = 5.59 µB). Fig. 5 Fig. 6 On further cooling the χT curve shows a sudden increase to 1.23 cm3.K.mol-1 at T=21 K followed by a sharp decrease down to 0.71 cm3.K.mol-1 at 5 K. The χT maximum de... ... middle of paper ... ... magnetic orbitals on the metal and the valence orbitals of the ligands. In the present case where the nickel centres are bridged by long N-C-N hmt and N-C-N-C-N dca ligands it is reasonable to assume that only minor exchange interactions can be mediate by these bridges and, as observed here, this type of magnetic pathways generally promotes antiferromagnetic interactions [38,45-48]. Nevertheless spin canting is often observed and usually associated with single-ion magnetic anisotropy or antisymmetric exchange interaction. From structural considerations the existence of two crystallographically independent Ni(II) ions with non-collinear anisotropy axes might be at the origin of the suspected canting phenomena. To clearly ascertain for the existence of a spin canting magnetic order, as the data suggests, neutron diffractions measurements are foreseen. 4. Conclusion

In this essay, the author

  • Presents the magnetic susceptibility (=m/h) (fc and zfc) as a function of temperature measured at low applied field.
  • Explains that the t curve shows a sudden increase to 1.23 cm3.k.mol-1 at t=21 k followed by an sharp decrease down to 0.71
  • Explains that fe -1,3,5 bridged dicyanamides compounds, 3d co and ni polymers, or ni layered transition metal carboxylates, is attributed to spin-canting or ferromagnetic like interactions.
  • Explains that isothermal magnetization measurements were performed to investigate antiferromagnetic long range interactions with a possible small canting of the ni spins. no evidence for saturation at 4.5 k up to 5.5 t exists.
  • Explains that the current magnetic behavior, with a rather small magnetic interaction, should be justified within the magnetic pathways frame shown in scheme 2.
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