Magnetization and NMR in pure and mixed crystal Fe[S2CN(C2H5)2] 2ClxBr1−x: 3D-Ising ferromagnetism and evidence for a strain induced ground state

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Abstract
The magnetization along easy and hard magnetic axes has been measured in three single crystal samples of Fe[S2CN(C2H5)2]2Clx Br1−x (x=1.000, 0.712, and 0.204), between 1.2 and 4.2 K and in applied fields up to 13 000 Oe. In the pure chloride an effective saturation moment of 3.28±0.16 μB per iron ion is observed along the [101] easy axis. The effective saturation magnetization observed in mixed crystals is slightly smaller than for the pure chloride and exhibits a weak dependence on the chloride–bromide ratio. This implies that the ground state of the iron ion in the bromide species is a weakly mixed component of ‖±3/2≳, which is not the case for the pure bromide. Along the hard axes, [010] and (1̄01), a linear dependence of magnetization on applied field is observed, to fields in excess of 12 000 Oe. These data are in essential agreement with previously measured zero-field susceptibilities. Numerous proton resonances appear in the NMR spectra of pure and mixed crystal samples. Four lines with nearly identical frequencies in pure and mixed crystals are observed, providing further evidence that similar ground states characterize all the iron ions in these crystals. The modulation and splitting properties of the observed resonances constitute direct evidence for a canted sublattice structure. The temperature dependence of a proton resonance in the pure chloride implies that the system is of 3D-Ising character, though an unusual value for the critical exponent is observed, β=0.265±0.01. The results are discussed in light of current theories of critical behavior in 3D-Ising systems.