a–d Isothermal elements Cij and C* ≡ (C11 − C12)/2 of the elastic tensor C as a function of magnetic field H, respectively. Although both compression (C11, C33) and shear moduli (C44, C66) display abrupt changes, C12 and C23 only exhibit a slight change in slope near Ha1 and Ha2. The accuracy of the absolute values of the Ci is determined by the quality of the fits of the resonance frequencies, which is of the order of 0.1% (see ref. 24 and table S1 in Supplementary note 1)). However, our results rely on relative shifts of the Ci, which can be determined with an accuracy of better than 0.01%, as indicated by the error bars. e Temperature (T) dependence of the resonant frequency F2419 measured for various H. Tc is the critical temperature for the paramagnetic (PM) to helimagnetic (HM) transition, and T* is the characteristic temperature below which the fluctuation-disordered (FD) regime arises. f F1654 as a function of H, measured at selected temperatures T. The accuracy of determining relative frequency shifts with our RUS setup is better than 0.01 % as denoted by the black bar in the upper left corner of the panels e, f24.