Chemical and magnetic order in mass-selected large FeRh nanomagnets embedded in a carbon matrix

Abstract

In this paper, we present some specific chemical and magnetic order results obtained on bimetallic FeRh nanoparticles prepared under non-equilibrium conditions using mass-selected Low Energy Cluster Beam Deposition. Clusters around 7 nm incident diameter are in-situ sandwiched between amorphous carbon films before transfer in air, with different surface coverage (from nearly isolated particles to percolating 2D films) in order to artificially increase diameters of the nanoparticles (NPs) by coalescence. On such FeRh@C samples, we observe different NPs morphologies and magnetic responses after UHV thermal treatments. By transmission electron microscopy (TEM), we show that after moderate annealing, nanoalloys evolved from a metastable structure towards the equilibrium CsCl-type (B2) chemically ordered phase with a more spherical shape. While from Superconducting Quantum Interference Device (SQUID) magnetometry measurements on 700 °C-annealed FeRh@C samples, we clearly evidence huge magnetization enhancement for such B2 nanoalloys with robust ferromagnetic (FM) signature even at very low temperature unlike their bulk counterparts.

Preserving Metamagnetism in Self-Assembled FeRh Nanomagnets

“…In particular, the decrease in the number of AF phase nucleation sites upon reducing the nanomagnet size seems to be behind the observation of the very pronounced supercooling. This aspect could also be at the origin of the complete suppression of the phase transition in FeRh at the ∼10 nm scale and below, as observed in highly ordered nanoparticles embedded in a carbon matrix, where the FM phase persists down to 2 K. 52,53 Another interesting observation is the complete suppression of phase separation in FeRh nanoislands across the phase transition. We did not observe any coexistence of AF and FM domains upon temperature cycling, indicating that the abrupt nature of the first-order phase transition is recovered within each nanoisland.…”

“…In particular, the decrease in the number of AF phase nucleation sites upon reducing the nanomagnet size seems to be behind the observation of the very pronounced supercooling. This aspect could also be at the origin of the complete suppression of the phase transition in FeRh at the ∼10 nm scale and below, as observed in highly ordered nanoparticles embedded in a carbon matrix, where the FM phase persists down to 2 K. , …”

Finite size effects on the metamagnetic phase transition in a thick B2 FeRh nanocluster film

“…In the following, we will see that the weak magnetic contribution of the FeRh A1 phase can be neglected compared to the high magnetic moment expected for the FeRh B2 majority nanoparticles. 31…”

“…The main peak of Fe 3 O 4 (311) 43 , which has a low intensity here, suggests the oxidation of iron in a minor proportion. In the following, we will see that the weak magnetic contribution of the FeRh A1 phase can be neglected compared to the high magnetic moment expected for the FeRh B2 majority nanoparticles 31 .…”

“…Some publications emphasized the strong interplay between crystal faceting, surface configuration, morphology and magnetic state in FeRh nanoparticles from DFT calculations and experimental studies [26][27][28] . By preparing FeRh nanocrystallites using low energy cluster beam deposition (LECBD) diluted in an amorphous carbon matrix, we demonstrated the persistence of the F order down to 3 K for sizeselected clusters in B2 phase less than 10 nm in diameter [29][30][31] . We proposed that significant surface relaxation affecting the unitcell parameter is responsible for the suppression of AF order even at very low temperature.…”