Low temperature ferromagnetism in Rh-rich Fe-Rh granular nanowires

RIVA, J.S.; POZO-LÓPEZ, G.; CONDÓ, A.M.; FABIETTI, L.M.; URRETA, S.E.

JOURNAL OF ALLOYS AND COMPOUNDS

ELSEVIER SCIENCE SA

Año: 2018 vol. 747 p. 1008 - 1017

0925-8388

Noble/transition bimetallic nanowires of nominal composition FexRh100-x(x¼15, 25, 54) are AC electrodeposited into 20 nm diameter hexagonallyself-assembled nanopores of anodic alumina membranes.Nanowires about 18 nm in diameter and 1µm long are polycrystalline and, depending on composition, differentcrystalline phases are obtained. Iron-rich (x=54) wires are biphasic, composed by large a-Fegrains (>100 nm inlength) and clusters of small (3 nm in average) grains of fcc g-Rh(Fe)phase, with composition near (30±5) at% Fe.Rh-rich (x=15, 25) nanowires are formed by very small grains of g-Rh(Fe) phase. Grain size depends on Rh content: grains in wires with x=15 are the smallest with a mean size of (2.1±0.9) nm. The low temperature magnetic properties of these small grainednanowires exhibit new features: they are all ferromagnetic at 5 K while, atroom temperature wires with 54 at.% Fe and25 at.% Fe are ferromagnetic andthose with 15 at.% Fe are weakly superparamagnetic. These behaviors areconsistent with non-compensated and very small ferrimagnetic grains surroundedby a grain boundary disordered spin-glass-like phase that freezes below 40 K.This frozen intergranular phase favors a strong exchange coupling between theferrimagnetic grains, which undergo a cooperative, ferromagnetic-like behaviorunder an external magnetic field. Above 40 K nanowires with54 at.% Fe are ferromagnetic and those containing 25 at.% Fe exhibit aferromagnetic-like behavior arising from blocked antiferromagnetic grains.Nanowires 15 at.% Fe are weakly superparamagnetic above 40 K. Arrays containing54 at.% Fe and 25 at.% Fe exhibit a polarization reversal mechanism involvinglocalized nucleation and further expansion of inverse domains; this thermallyactivated, magnetic field assisted mechanism exhibitsan apparent activation energy between 229 kJ/mol (2.3 eV) and 298 kJ/mol (3.1eV) for nanowires 25 at.% Fe and 54 at.% Fe, respectively.