Mixed magnetic phase in Co-rich metallic glasses

Abstract

ac susceptibility (χ) and magnetoresistance (MR) have been measured in Co-rich amorphous ferromagnets [Fe5Co50Ni17−xCrx(BSi)28 (x=0,5,10,15), Fe5Co50Mn17(BSi)28, and Fe7.8Co31.2Ni24Mn15(BSi)22] to understand the changes in magnetic phase brought by the addition of Cr/Mn. The Tc decreases monotonically with Cr/Mn addition. Samples with x=0–15 remain ferromagnetic down to 16 K. But for the Mn substituted samples below 80 K, χ decreases drastically and at 16 K it is only 5% of its value at 80 K indicating spin freezing. dc magnetization data show a peak around 30 K as well as a time dependence in the Mn17 sample. Presence of significant ferromagnetic anisotropy of resistance (FAR) even at the lowest temperature for both Mn-containing samples indicates that ferromagnetism persists even when transverse spin freezing is present. With lowering of temperature FAR increases, and below 30 K it should have decreased considerably if the lowest temperature phase was purely spin-glass-like but here it remains almost constant. The high-field slopes for T < Tc in both orientations are negative. In particular, for Mn15 sample the slope has a broad maximum decreasing on both sides as T → Tc and T<80 K. This behavior is interpreted in terms of a transverse spin freezing. We conclude that the lowest temperature (<30 K) phase in samples containing Mn is a mixture of spin-glass and long-range ferromagnetism. Samples containing Cr show a large positive slope for T < Tc and a very low FAR. The ratio Δρ∥/Δρ⊥ (≂0) indicates that the domain magnetization lies in the ribbon plane and is parallel to the ribbon axis.