Muon Spin Relaxation Study on The Magnetic Ordering and Spin Fluctuation of Nd2Pt2O7

A pyrochlore oxide, A2B2O7, has a cubic structure with interpenetrating corner-sharing sublattices of the A and B sites. The interplay between magnetic frustration, crystal electric field (CEF), exchange interaction, and dipolar interaction in the pyrochlore oxides induced various exotic physical properties such as spin-ice, spin-glass, and spin liquid states [1]. Nd pyrochlores have been studied in the last decade. The Nd3+ ions in the Nd pyrochlore were known to have a Kramers doublet ground state and were expected to have a dipolar-octupolar characteristic [2].
Previous studies showed a magnetic ordering of Nd spins accompanied by the remaining spin fluctuation below the transition temperature of Nd2B2O7 with non-magnetic B ions (B = Zr, Hf, and Sn) [3-6]. The origin of the persistent spin fluctuation in the ordered state of Nd pyrochlores is still debatable. Petit et al. suggested that such phenomenon in Nd2Zr2O7 is to be due to the magnetic fragmentation of Nd moment [6]. On the other hand, a theorist successfully explained the persistent spin fluctuation on the basis of the spin-wave excitation on the all-in-all-out (AIAO) magnetic ground state of Nd moment [7]. The investigation on other Nd pyrochlores needs to be done to see the universality of the magnetic ground state in Nd pyrochlores.
The Pt4+ ion in Nd2Pt2O7 is in the low spin state (t2g6 eg0), behaving as a non-magnetic ion. Even though there are some reports on the magnetic properties of other Nd pyrochlores with non-magnetic B ions, there is no information on the magnetic properties of Nd2Pt2O7. Accordingly, the muon spin relaxation (SR) measurement was carried out accompanied by the bulk measurements such as the DC susceptibility, the AC susceptibility, and the heat capacity to investigate the magnetic properties of Nd2Pt2O7 in the current study. A magnetic transition was observed at TN = 1.4 K in the temperature dependence of DC susceptibility, AC susceptibility, and heat capacity. It corresponds to the magnetic transition of Nd spins from the paramagnetic state to the ordered state, eliminating the possibility of the spin-glass state due to the frequency-independent of TN observed in the AC susceptibility. The negative Curie-Weiss temperature from the Curie-Weiss fitting and the field-dependent of TN from the DC susceptibility measurement indicates that the effective exchange interaction in Nd2Pt2O7 is antiferromagnetic (AFM) interaction.
The SR measurements were carried out at Paul Scherrer Institute (PSI) in Switzerland to confirm the appearance of magnetic ordering. Neither oscillation in the zero-field (ZF)-SR time spectra nor decreasing of initial asymmetry at t = 0 was observed down to 0.3 K. Instead, the time spectra below 1.5 K showed a Gaussian behavior at early time spectra accompanied by the Lorentzian relaxation at a longer time. Accordingly, the time spectra were fitted to the summation of Gaussian and Lorentzian exponential functions. Below 1.5 K, the Lorentzian fraction sharply decreased in concomitant with the growth of the Gaussian fraction. A sharp peak in the temperature dependence of muon spin relaxation rate, (T), was observed at 1.5 K. It corresponds to the critical slowing down behavior of Nd spins represent the appearance of Nd ordering. The  does not vanish below TN denoted the persistent spin fluctuation in the ordered state. The longitudinal field (LF)-SR time spectra also showed a muon spin relaxation under applied magnetic field indicates the remaining spin fluctuation at 0.3 K.