We report our NMR study of the spin excitations in the quasi-one-dimensional $S=\frac{1}{2}$ quantum magnet ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Cu}{\left(\mathrm{HCOO}\right)}_3$ lying in the one-dimensional–three-dimensional crossover regime with Dzyaloshinskii-Moriya (DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a power-law temperature dependence gradually, with the index increasing from zero to $\sim 3$, and finally $\sim 5$, which are the respective typical characteristics for the dominating two-magnon Raman process and three-magnon scattering contributions to the nuclear relaxation in a conventional three-dimensional magnet. A possible physical mechanism for this magnetic field tuning of quantum spin excitations related with the enhanced effective staggered field created by the DM interactions under magnetic field is discussed.

• Received 15 December 2018
• Revised 29 May 2019

DOI:https://doi.org/10.1103/PhysRevB.100.125126