Abstract
We have investigated high-field TF-µSR and 1H-NMR on the S = 1/2 three dimensional spin dimer system NH4CuCl3, which shows a magnetic order at TN = 1.29 K under zero field and the two-stepped plateaus in high field regions HC1 = 5.0 T - HC2 = 12.8 T and HC3 = 17.9 T - HC4 = 24.7 T. Both probes showed spectra containing multiple peaks corresponding to different hyperfine fields, demonstrating the existence of magnetically inequivalent dimers in a unit cell. In the slope-state field region H < HC1, the existence of magnetic order was confirmed by the splitting in some of the NMR peaks. The whole width of NMR and µSR spectra increased with increasing field, and no drastic change in their profile was observed at HC1. This indicates that the change in the spin state is continuous on entering the plateau state, and that no liquid-solid transition of magnons takes place at the phase boundary.
Highlights
The magnetization process of quantum spin systems is attracting much interest, because some systems show the magnetization plateaus which cannot be understood in terms of the conventional classical spin vector models [1]
The 1/4 plateau appears in the field region between HC1 and HC2 = 12.8 T, and the 3/4 plateau, between HC3 = 17.9 T and HC4 = 24.7 T for the field direction H b-axis [4]
63/65Cu- and 35/37Cl-NMR [8] and μSR experiments [9] support the existence of such inequivalent dimers in the paramagnetic state below 70 K, detailed spin states in and out of the plateau-field region are still veiled
Summary
The magnetization process of quantum spin systems is attracting much interest, because some systems show the magnetization plateaus which cannot be understood in terms of the conventional classical spin vector models [1]. 63/65Cu- and 35/37Cl-NMR [8] and μSR experiments [9] support the existence of such inequivalent dimers in the paramagnetic state below 70 K, detailed spin states in and out of the plateau-field region are still veiled. 1H-NMR experiments were performed by the conventional spin-echo method at temperatures between 0.3 and 2.3 K, and in magnetic fields up to 8.3 T.
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