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We report on measurements of the sound velocity and attenuation in a single crystal of the candidate quantumspin-ice material Yb 2 Ti 2 O 7 as a function of temperature and magnetic field. The acoustic modes couple to the spins magneto-elastically and, hence, carry information about the spin correlations that sheds light on the intricate magnetic phase diagram of Yb 2 Ti 2 O 7 and the nature of spin dynamics in the material. Particularly, we find a pronounced thermal hysteresis in the acoustic data with a concomitant peak in the specific heat indicating a possible first-order phase transition at about 0.17 K. At low temperatures, the acoustic response to magnetic field saturates hinting at the development of magnetic order. Furthermore, mean-field calculations suggest that Yb 2 Ti 2 O 7 undergoes a first-order phase transition from a cooperative paramagnetic phase to a ferromagnet below T ≈ 0.17 K.

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