The nuclear specific heat of the hyperfine-enhanced Van Vleck paramagnet PrNi 5 has been measured from 0.2 to 100 mK in magnetic fields up to 6 T. The 141 Pr nuclei order ferromagnetically at 0.40±0.02 mK. Other results reported are hyperfine enhancement factor 1 + K= 12.2 ±0.5, enhanced nuclear magnetic moment ju = (0.027± 0.004)ju B , internal field B 0 (T = 0) = 66± 10 mT 9 and nuclear exchange parameter £,-Jij N /k B = 0.20± 0,04 mK. PACS numbers: 75.30.Et, 71.70.Gm, 75.10.Jm, 75.40.Fa In this Letter we report on the first investigation of the magnetic field and temperature dependence of the nuclear specific heat of a metal through its nuclear magnetic ordering transition. The investigated compound, PrNi 5 , as well as other Van Vleck paramagnets were investigated above their nuclear ordering temperatures by Andres and co-workers, 1 ' 2 while Babcock et at. recently observed a nuclear ordering transition in PrCu 6 at 2.5 mK. 3 We have cooled the nuclei and electrons of PrNi 5 by adiabatic nuclear demagnetization to 0.2 mK, 4 the lowest temperature obtained for such a material, and have investigated the nuclear magnetic transition, discovered at 0.40±0.02 mK. By applying a magnetic field, , we could study the gradual change from spontaneous nuclear ordering to Schottky-like behavior" Analysis of our data has yielded the first detailed information on nuclear ordering in a material where indirect exchange interactions dominate. We have obtained the hyperfine enhancement factor, the internal magnetic fields in the paramagnetic and ferromagnetic range, the saturation magnetization, the enhanced nuclear moment of 141 Pr in PrNi 5 , the Van Vleck susceptibility, and the exchange and the molecular field constants. In addition, we demonstrate purely from thermodynamic data that the order is ferromagnetic 0 For rare-earth ions in Van Vleck compounds in an external field B eJ the interactions can be summarized by the Hamiltonian, 5
#=#cf ~^M B B e •£ J< -i£^*J*' V(1)The crystal field term # cf alone produces an electronic singlet ground state. Together with the electronic Zeeman term it gives a Van Vleck susceptibility which is temperature independent for T<2 K in PrNI 5C 2 ' 6 The electronic moments are coupled to each other by indirect exchange interaction and to the nuclei by the hyperfine interaction (with A/k B = 52.5 mK for 7 Pr 3 + ), represented by the third and fourth terms. The last two terms are the nuclear Zeeman and the nuclear electric quadrupole interactions,, A model nuclear-spin Hamiltonian can be written as 5i J where 1 +K is the hyperfine enhancement factor due to the Van Vleck susceptibility x and K= \A/ The exchange parameter 1 J {i "=K*J { Ag K VL H /g,H B ) adescribes the indirect exchange interaction of nuclear moments through electronic exchange and hyperfine interactions. They are believed to cause the magnetic ordering. In Ref c 6 it was shown, from NMR data taken at T>1 K, that the nuclear electric quadrupole term H Q and the pseudoquadrupole term H PQ , which is a second-order magnetic hy...
