Nature of the magnetic ground state in Fe-Ni Invar alloys

Abstract

Systematic low-temperature $^{57}\mathrm{Fe}$ M\"ossbauer effect (ME) and ac susceptibility (\ensuremath{\chi}) studies in the temperature range 70--1.7 K on the classical ferromagnetic (FM) ${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$ Invar alloy are reported. We unexpectedly observe a relative decrease in the average effective hyperfine (hf) field, B${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{eff}}$ of about 6% on lowering the temperature from 30 to 4.2 K. The analysis of the experimental results (ME and \ensuremath{\chi}) denies the existence of a reentrant spin-glass ground state as recently suggested. It is concluded that the observed low-temperature hf field anomaly is caused by a few percent of inhomogeneously distributed spin-flipped Fe atoms which exhibit FM order above 30 K.