The low-field magnetoresistance properties of the re-entrant spin-glass system (Fe0.65Ni0.35)1-xMnx

The magnetoresistance (MR) of two re-entrant spin-glass (SG) alloys in (Fe 0.65 Ni 0.35 ) 1-x Mn x with x=9.1 at.% and 11.36 at.% have been measured over the temperature range 4.2-300 K using a new ultra-sensitive MR apparatus enabling operation in magnetic fields comparable with that of the Earth. In this temperature range, the MR exhibits three distinct features: (i) a hysteresis effect over the whole so-called ferromagnetic (FM) region which generally saturates in only a few Gauss followed by reversible behaviour at higher fields; (ii) predominantly positive MR under low field conditions indicative of FM-type coupling between moments; but (iii) in the x=11.36 at.% sample near the FM-paramagnetic (PM) and FM-SG boundaries, the MR changes sign in higher fields and becomes negative. The results are discussed in terms of a recently proposed magnetic phase diagram for this system. The results support the existence of a mixed FM-like and SG phase but it is proposed that the 'FM' phase is mixed also with little or no long-range ferromagnetism. Instead, a system of FM-coupled clusters and a system of weakly coupled moments is indicated by the results, the latter behaving paramagnetically near the so-called Curie temperature but freezing into an SG state at lower temperatures.