  Low temperature magnetization of CrI3, CrSiTe3 and CrGeTe3 single crystals
were systematically studied. Based on the temperature dependence of
extrapolated spontaneous magnetization from magnetic isotherms measured at
different temperatures, the spin stiffness constant (D) and spin excitation gap
($\Delta$) were extracted according to Bloch's law. For spin stiffness, D is
estimated to be 27${\pm}$6 meV $\r{A}^2$, 20${\pm}$3 meV $\r{A}^2$ and
38${\pm}$7 meV $\r{A}^2$ for CrI3, CrSiTe3 and CrGeTe3 respectively. Spin
excitation gaps determined via Bloch's formulation have larger error bars
yielding 0.59${\pm}$0.34 meV (CrI3), 0.37${\pm}$0.22 meV (CrSiTe3) and
0.28${\pm}$0.19 meV (CrGeTe3). Among all three studied compounds, larger spin
stiffness value leads to higher ferromagnetic transition temperature.
