Direct Observation of Magnetic Metastability in Individual Iron Nanoparticles

Abstract

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times. DOI: 10.1103/PhysRevLett.112.107201 PACS numbers: 75.30.Gw, 68.37.Yz, 75.50.Bb Metastability is a well-known phenomenon in condensed matter, where energy barriers prevent a system to relax from a higher-energy state to the ground state. Since the barrier heights usually scale with the system size, the preparation of metastable, higher-energy states becomes possible at the nanoscale [1,2]. Such states can be of profound interest when searching for materials with novel properties. For instance, much effort is currently undertaken to find magnetic nanostructures with properties that enable us to overcome the so-called "superparamagnetic limit," which will occur when further reducing the magnetic bit size for future high-density magnetic data storage devices [3][4][5]. Such applications require a magnetic energy barrier E m that is sufficiently high to prevent thermally driven switching of the magnetization at relevant temperatures (superparamagnetism). The barriers are usually provided by the magnetic anisotropy energy (MAE), which to first order scales with the particle volume. The magnetic relaxation rate v of a small magnet at a given temperature T is given by an Arrhenius law according to v ¼ v 0 expð−E m = k B TÞ with the thermal energy k B T. The attempt frequency v 0 depends on temperature, saturation magnetization, and the MAE [6][7][8]. The corresponding relaxation time is τ r ¼ 1=v.A direct experimental access to structural higher-energy states and their impact on the magnetic properties is, however, not trivial, particularly when ensemble measurements are considered. Even monodisperse nanomagnet ensembles can show considerable particle-to-particle variations in their properties either due to size effects, interparticle interactions, their individual interfaces with the environment, or surface effects [9]. Experiments with single particle sensitivity enable us to disentangle the different contributions, and thus to distinguish lower or ground state properties from higher-energy states [10,11]. Here, we study the magnetization of individual iron (Fe) nanoparticles by magnetic spectromicroscopy. The magnetocrystalline anisotropy of bulk Fe suggests superparamagnetic (SPM) behavior at room temperature (RT). Indeed, such particles have been found experimentally [12]. However, other authors reported enhanced magnetic energy barriers from ensemble measurements, which...

Synthesis and properties of ferromagnetic nanostructures embedded within a high-quality crystalline silicon matrix via ion implantation and nanocavity assisted gettering processes

“…33 Alternatively, magnetoelastic anisotropy due to strains around magnetic nanoparticles may not be ignored. 34,35 In addition to enhanced surface/interface anisotropy, the inter-particle interactions may be partially responsible for the persistence of hysteretic behavior for Ni nanoparticles at room temperature. One of the possible inter-particle interactions is the dipolar interaction between Ni nanoparticles.…”

Unusual spin correlations in a nanomagnet

“…[16][17][18][19] VWIFs differ from ordinary weak itinerant ferromagnets, as in bcc Fe, by rather a small magnetic moment. On the other hand, Co 2 Si nanowires with diameters of about 30 to 80 nm and lengths over 10 lm have shown low-temperature ferromagnetic behavior with nearly zero coercivity and exhibited negative magnetoresistance at temperatures below 50 K. [11][12][13] However, the low magnetic ordering temperatures of Co 2 Si nanowires and bulk alloys are a major impediment for room-temperature applications.…”

“…1(b) represent the standard XRD peak positions and relative intensities for the orthorhombic Co 2 Si structure of Fig. 1(a).…”

“…Co 2 Si is an intriguing but poorly explored weak magnetic material in spite the fact that it has a great potential as a material for interconnects, gates, and source electrodes in integrated devices, and for nanoelectronics and spintronics applications. [10][11][12][13] Bulk Co 2 Si, which crystallizes in the orthorhombic Co 2 Si structure (space group Pnma) 14 and whose unit cell is shown in Fig.…”

“…[10][11][12][13] Bulk Co 2 Si, which crystallizes in the orthorhombic Co 2 Si structure (space group Pnma) 14 and whose unit cell is shown in Fig. 1(a), exhibits a magnetic moment of only about 0.04 l B /Co at 4 K. 15 The small moment suggests that bulk Co 2 Si is very close to the border between exchange-enhanced Pauli-paramagnetism, as in Pt, and VWIF, as in ZrZn 2 , and is located on the VWIF side of the Stoner transition.…”