Evidence for a dynamical ground state in the frustrated pyrohafnate Tb₂Hf₂O₇

We report the physical properties of Tb₂Hf₂O₇ based on ac magnetic susceptibility χac(T), dc magnetic susceptibility χ(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (μSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, χac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the μSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb₂Hf₂O₇.