Spin freezing process in a reentrant ferromagnet studied by neutron depolarization analysis

The spin freezing process and the magnetic nature of reentrant spin-glass (RSG) and the ferromagnetic (FM) phases of a typical reentrant ferromagnet Ni₇₈Mn₂₂ were investigated based on neutron depolarization analysis, and the results were compared with the previous Mössbauer measurements [Phys. Rev. B 64, 184432 (2001)]. The wavelength-dependent polarization, under a field cooled (FC) condition, showed the damped oscillatory behavior in both the RSG and FM phases, except in the temperature region just above the RSG temperature T_RSG ∼ 60 K. At a temperature of around 80 K, however, it showed a double oscillatory behavior. The field integral I, which is proportional to the mean local magnetic induction, was deduced as a function of the temperature. Two branches of temperature-dependent field integrals were found: a low-temperature I_low-branch, which has a small value of I, stopped at a temperature below the Curie temperature T _C ∼ 160 K, and a high temperature I_high-branch, which has a large value of I, appeared just below 80 K. This means that there are two kinds of magnetic environments, and they have different values of magnetization. This is consistent with the observation of the double peak spectrum of the hyperfine field in the previous Mössbauer measurements. The present neutron data and the Mössbauer data can be interpreted along a scenario of reentrant behavior, which consists of the low-temperature spin canting state and the "melting of frustrated spins" mechanism introduced by Saslow and Parker [Phys. Rev. Lett. 56, 1074 (1986)], except for the absence of the observation of singularity in the temperature-dependent magnetization. Based on such considerations, we constructed a comprehensive picture of the spin freezing process and the magnetic nature of the RSG and FM phases in the reentrant ferromagnet.