Characterization of the mysterious high field ordered phase around H || [111] and finding of a new phase boundary in PrFe ₄P ₁₂

We have investigated the electrical resistivity ρand Hall resistivity ρH of the filled skutterudite PrFe ₄P ₁₂ as functions of both temperature T down to 30mK and magnetic field H up to 17.9 T for the field directions near the [111] axis. At low temperatures, we observed a clear anomaly in ρ(H) and ρ(HH) within a limited range of field directions close to H || [111] at around 8 T, reflecting the onset of the high-field ordered phase (HOP) reported previously. ρ exhibits a crater like structure as a function of the field direction in the ( 110) plane with a minimum near the [111] axis. Magnetoresistance exhibits clear anomalies across the boundaries of both the low-field scalar-order phase (SOP) and the HOP. With decreasing temperature, the phase boundaries of the high-field side of the SOP and the low-field side of the HOP approach more slowly than was inferred previously. The carrier density in the HOP evaluated by the ρand ρH measurements is more than an order of magnitude higher than that in the SOP, which rules out q = (0,0,0) as the ordering wave vector for the HOP. Within the HOP, we found an additional anomaly accompanied by a clear hysteresis at around 17 T below ̃0:75 K, indicating the existence of a new phase boundary. The existence of the additional state in the HOP may indicate that almost-degenerate multiple internal degrees of freedom are involved in the HOP. This finding should provide an important clue to clarify the nature and origin of the HOP.