Mossbauer and magnetic susceptibility studies of the alloy series FeAl_1-xCu_x

Abstract

Magnetic susceptibility and Mossbauer spectroscopy studies were carried out on the alloy series FeAl1-xCux (0<x<or=0.4). The variation of susceptibility with temperature showed transitions from a paramagnetic to a spin glass phase for copper contents 0.15<or=x<or=0.25. For x=0.325 transitions from a paramagnetic to a ferromagnetic and from a ferromagnetic to a magnetic glass phase were observed as the temperature was decreased. Mossbauer spectra gave information on the microscopic conditions at the iron sites. The electron density at the iron nuclei increased linearly with copper concentration x. Distributions of hyperfine fields were observed for all alloys at 4.2 K and related to iron cluster structure. Comparison between phase transition temperatures obtained by susceptibility and Mossbauer observations gave information on the relaxation of clusters. The size of these clusters as a function of x was studied via a simulation calculation and used to account for the occurrence of a ferromagnetic phase at a critical value of x.

Mossbauer and magnetic susceptibility studies on the alloy series FeAl1-xCrx

“…Complexities in fitting the magnetic spectrum for the x = 0.60 sample made its value insufficiently precise to be included in this trend. The overall decrease in 8 with x is similar to that observed in FeAll-xCux [5] and is, we believe, attributable to the same cause whereby with increasing x the population of the 3d electrons around the Fe atom decreases reducing the shielding of the s shell electrons. The increased s electron density at the Fe nucleus as x increases produces the decrease in the isomer shift value 8.…”

“…The ferromagnetic phase arises for a Cr content of 0.4 < x < 0.5. Similar studies on the series FeAl, [5] determine that the ferromagnetic phase occurs for a Cu concentration of 0.25 < x < 0.325 while in FeA1, -,Fe, a ferromagnetic phase is observed forx = 0.10 [ 11 setting the limit at x < 0.1 for this series. Thus in order to produce a ferromagnetic phase progressively larger amounts of transition metal T are required as T changes from Fe to Cu to Cr.…”

“…The temperature of the transition Tf corresponds to the fluctuations of the iron [ 2 ] . At the lowest temperatures a decrease in susceptibility is seen which, by analogy with similar results for FeAl,_,Cu, [5], may herald a phase transition to the spin glass phase at temperatures Tf < 6 K. The graph of the x = 0.60 sample is interpreted as being in the ferromagnetic phase throughout, thus setting limits of T, > 300 K and possibly Tf < 5 K.…”

“…In the case of the FeA1, -$U, series of alloys a simulation calculation showed [5] that it was likely that a percolating cluster of nearest neighbour iron atoms was formed at a copper concentration of x -0.24, close to the observed critical concentration. Similar calculations are more problematic in the case of the Cr series since the assumption of random substitution on Fe and A1 sublattices cannot be made but we believe that the qualitative argument above shows that the largest factor in the establishment of a ferromagnetic phase is the efficiency with which the substituting transition metal T forms iron atom clusters.…”

“…This investigation on FeAl,-,Cr, forms part of a set of studies on the magnetism of ternary alloys formed by progressively replacing A1 atoms of the ordered alloy FeAl by a transition metal. Former studies dealt with the series FeAl,-,Fe, [l, 2,3] and FeAl, -,CU, [4,5]. The parent alloy Fe-A1 crystallizes in the ordered B2 (CsC1) structure and does not show magnetic ordering [ 11.…”