In 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. DOI: 10.1103/PhysRevLett.110.087207 PACS numbers: 75.30.Kz, 75.75.Cd, 81.07.Bc 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][4][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][7][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][7][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 nanoparticles (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][20][21].In this Letter, we demonstrate the persistence of FM order down to below 3 K in size-selected FeRh nanocrystals with a mean d...

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated in situ under H₂ and O₂ atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with ex situ transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined Fe-Me nanoalloys with an initial size of 5 nm was achieved by using the mass-selected low energy cluster beam deposition (LECBD) technique. The spectroscopic methods permit the direct observation of the surface segregation and composition under different gas atmospheres and annealing temperatures. The ambient conditions were found to have a significant influence on the mixing pattern and oxidation state of the nanoparticles. In an oxidative atmosphere, iron oxidizes and segregates to the surface, leading to the formation of core–shell nanoparticles. This structure persists upon mild reduction conditions, while phase separation and formation of heterostructured bimetallic particles is observed upon H₂ annealing at a higher temperature (400 °C). Depending on the noble metal core, the iron oxide shell might be partially distorted from its bulk structure, while the reduction in H₂ is also significantly influenced. These insights can be of a great importance in understanding the activity and stability of Fe-based bimetallic nanoparticles under reactive environments

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