Low Temperature Ferromagnetism in Chemically Ordered FeRh Nanocrystals

Abstract

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...

Magnetocaloric materials: From micro- to nanoscale

“…73 In contrast with thin films, stripes and larger nanoparticles, Hillion and coworkers found that small (,5 nm) nanoparticles retained the high-temperature FM state until low temperatures (down to 3 K) suggesting a rich size-dependent phase diagram for the FeRh system. 74 As numerous examples illustrate, the FeRh system is probably the magnetocaloric system that has profited the most from the size-reduction route, unveiling promising technological advantages (such as the reduction of the magnetic irreversibility) and interesting new physical states. In the near future, more detailed nanoparticles size-dependent studies should help understand what are the main mechanisms driving the magnetic ordering at this scale.…”

Higher-order exchange interactions leading to metamagnetism in FeRh

“…A limited system size restricts the maximum extent of nucleated domains of each ordering and hence the size of the interface region. Experimental observations of small FeRh nanoparticles [20] and thin films [21] have also shown a large increase in the thermal hysteresis and even the suppression of the AFM phase [22]. 3.48 .…”

Cubic chemically ordered FeRh and FeCo nanomagnets prepared by mass-selected low-energy cluster-beam deposition: a comparative study

“…While it has long been known that the bcc FeRh unit cell volume expands upon transforming to the FM order [30], we use EXAFS experiments to determine the local iron environment before and after annealing. From a quantitative FEFFIT analysis at the Fe K edge on FeRh nanoparticle, we confirmed the systematic transition upon annealing from the chemically disordered fcc (A1) phase to the ordered CsCl-type (B2) structure for 3 nm FeRh clusters assemblies embedded in a carbon matrix [13]. In the latter case, the unit cell size has been found compatible with those of B2 FeRh bulk material with a Debye–Waller (DW) factor decreasing with chemical ordering.…”

“…1), we obtained the experimental persistence of high ferromagnetic magnetization down to 3 K for size-selected chemically ordered B2-like FeRh nanocrystals up to 5 nm in diameter. In particular, magnetic measurements on annealed 3.3 nm FeRh samples, have demonstrated ferromagnetic alignment of Fe and Rh at low temperatures with respective values of 3μ B and 1μ B [13]. The ferromagnetic order increases slightly for smaller nanoparticles as confirmed from XMCD measurements on annealed 2 nm FeRh (see Fig.…”