Low Temperature Ferromagnetism in Chemically Ordered FeRh Nanocrystals

International audienceIn sharp contrast to previous studies on FeRh bulk, thin films, and nanoparticles, we report the persistence of ferromagnetic order down to 3 K for size-selected 3.3 nm diameter nanocrystals embedded into an amorphous carbon matrix. The annealed nanoparticles have a B2 structure with alternating atomic Fe and Rh layers. X-ray magnetic dichroism and superconducting quantum interference device measurements demonstrate ferromagnetic alignment of the Fe and Rh magnetic moments of 3 and 1 B , respectively. The ferromagnetic order is ascribed to the finite-size induced structural relaxation observed in extended x-ray absorption spectroscopy. Iron-rhodium alloys exhibit competing ferromagnetic (FM) and antiferromagnetic (AFM) phases with transition temperatures close to ambient for nearly equiatomic composition and body-centered-cubic (bcc) CsCl-like B2 structure. The competition between the two magnetic orders of FeRh holds great potential in spintronics and heat assisted magnetic recording [1,2]. Moreover, the peculiar bulk FeRh magnetic phase diagram enables its use as active material in heat pumps and refrigerators [3-5]. At ambient conditions, bulk B2 FeRh is a G-type AFM with a total magnetic moment on the iron atoms of 3:3 B and no appreciable moment on the rhodium atoms [6-8]. Above the transition temperature of 370 K, the atomic moments of Fe and Rh are ferromagnetically aligned and take on total values of 3.2 and 0:9 B , respectively [6-8]. While it has long been known that the bcc unit cell volume expands by % 1% upon transforming to FM order [9], recent experiments suggest that distortions of the bcc structure may occur [10]. Given the itinerant character of the 3d electrons, the coupling between crystallographic and magnetic order in this system is both rich and very delicate as demonstrated by the theoretical challenge to model the system [11], as well as by recent pump-probe experiments focusing on ultrafast magnetization control [12]. Finite-size systems of this alloy have received particular attention by their potential to stabilize the FM phase at room temperature and below. Strained thin films [13,14] showed traces of a FM phase down to 300 K, while ab initio calculations predicted FM down to 0 K for a Rh-terminated 9 ML FeRh(001) film [15] and for 8-atom FeRh clusters [16]. Indeed, since nanosized crystals may present significantly different interatomic distances and unit cell distortions with respect to bulk [17,18], a fundamentally modified magnetic phase diagram can be expected for FeRh nanocrystals. However, the first experiments on chemically synthesized FeRh nanopar-ticles (NPs) failed to evidence low temperature stability of the FM phase. Most notably, they raised important questions, such as partial B2 ordering, elemental segregation , and coalescence upon annealing [19-21]. In this Letter, we demonstrate the persistence of FM order down to below 3 K in size-selected FeRh nanocrys-tals with a mean diameter of 3.3 nm that are embedded into a carbon (C) matrix and thus protected from pollution and coalescence. Both structural and magnetic properties dramatically change upon annealing of the NPs. While the as-deposited ones are in a chemically disordered fcc structure , the annealed NPs are in the chemically ordered B2 phase with alternating atomic Fe and Rh planes, as evi-denced by high resolution transmission electron micros-copy (HRTEM). X-ray magnetic circular dichroism (XMCD) reveals for particles in the B2 phase FM order between Fe and Rh with magnetic moments of 3 and 1 B , respectively. Combined XMCD and superconducting quantum interference device (SQUID) magnetometry demonstrate that our 1400-atom NPs are single magnetic domain with a magnetic volume identical to the geometric one and a blocking temperature of around 12 K. The x-ray absorption spectroscopy (XAS) line shape at the Fe L 3 edge consists of a single peak, thus excluding chemical interactions with the C-matrix atoms as a possible source of the FM order. Based on extended x-ray absorption fine structure (EXAFS) analysis at the Fe K edge, we ascribe the observed FM order to finite-size induced structural relaxation in which the mean interatomic distances are PRL 110