Metastability of Free Cobalt and Iron Clusters: A Possible Precursor to Bulk Ferromagnetism

Abstract

Homonuclear cobalt and iron clusters Co(N) and Fe(N) measured in a cryogenic molecular beam exist in two states with distinct magnetic moments (μ), polarizabilities, and ionization potentials, indicating distinct valences. The μ is approximately quantized: μ(N)∼2Nμ(B) in the ground states and μ(N)(*)∼Nμ(B) in the excited states for Co; μ(N)∼3Nμ(B) and μ(N)(*)∼Nμ(B) for Fe. At a large size, the average μ of the two states converges to the bulk value with diminishing ionization potential differences. The experiments suggest localized ferromagnetism in the two states and that itinerant ferromagnetism emerges from their superposition.

Spin and orbital magnetic moments of size-selected iron, cobalt, and nickel clusters

“…In a size range that is similar to the one considered here, Stern-Gerlach deflection experiments yield total magnetic moments of µ J ≈ 3.0 − 5.5 µ B for neutral iron clusters of 10 to 50 atoms 7,9,11,14,20,43,167 ; µ J ≈ 2.25 − 3.9 µ B for neutral cobalt clusters of 10 to 50 atoms 8,11,14,20,44,45,139,[168][169][170] ; and µ J ≈ 0.8 − 1.3 µ B for neutral nickel clusters of 10 to 15 atoms 10,11,13,14,171 . Even though there is a large scatter in the various Stern-Gerlach data, our average XMCD results for iron (µ J ≈ 3.5 µ B ) and cobalt (µ J ≈ 3.0 µ B ) cluster ions fall well within the range spanned for neutral clusters.…”

“…Furthermore, transition elements that show antiferromagnetic order or even no magnetic order in the bulk can exhibit finite magnetic moments in small clusters [16][17][18][19] . Metastable magnetic species of iron and cobalt clusters have also been observed 20 . When deposited on surfaces, adatoms [21][22][23] , clusters [24][25][26][27] , and nanoparticles [28][29][30][31] were investigated by means of x-ray magnetic circular dichroism spectroscopy 24,25,27,[31][32][33][34][35] , spin polarized scanning tunneling microscopy 36,37 or scanning tunneling spectroscopy [38][39][40][41] , giving access to spin and orbital magnetic moments, magnetic anisotropy energies, or exchange coupling constants of these systems.…”

Structural and magnetic properties of large cobalt clusters

“…Magnetic moments per atom for clusters of Fe, Co, and Ni less than a few hundred atoms are notably enhanced when compared with bulk. 7 It has also been well established both theoretically 4,[8][9][10][11][12][13][14][15][16] and experimentally 7,[17][18][19][20][21][22][23][24][25][26][27][28] that, when presented as a function of the cluster size, the magnetic moment shows a complex non-monotonic decay from the relatively large atom-like value until converging to the bulk limit. The local maxima and minima of the moments occur at different sized clusters, i.e., clusters with "magic" numbers of atoms.…”

Multiferroic Rhodium Clusters

“…Note their temperature independence. On the other hand, if the spin is strongly coupled to an axis in the cluster (i.e., locked), then the magnetic moment distributions are significantly modified, resulting in a broad flat distribution extending from M ¼ −μN to þμN (see, for example, [20]). This "locked moment" behavior [24], first observed in rare earth clusters [25], is also clearly seen in several Rh clusters as shown in Fig.…”

“…The clusters are produced in a cryogenic pulsed laser vaporization cluster source, and they are detected in a high-resolution, position-sensitive time-of-flight mass spectrometer, where the mass and deflection of the clusters in the beam are individually measured and recorded. Specifically (for details, see, for example, [19,20]), a beam of clusters is ejected from a well-thermalized helium-filled chamber in the source, whose temperature T can be accurately adjusted from T ¼ 20 K to T ¼ 300 K. The beam is collimated to 0.3 mm. After passing through an inhomogeneous magnetic field (using a Stern-Gerlach magnet) and an inhomogeneous electric field (two-wire field), the clusters arrive at the mass spectrometer 2 m away, where they are (singly) ionized by using a pulsed excimer laser before entering the mass spectrometer.…”