Monte Carlo study of the exchange bias effect in Co/CoO core–shell nanowires

Abstract

We study the magnetic properties of cylindrical ferromagnetic core-antiferromagnetic shell nanowires using Monte Carlo simulations and a classical Heisenberg Hamiltonian in order to elucidate the impact of the oxidized shell on the magnetic properties and the magnetization reversal mechanism. We find that the coupling to the antiferromagnetic shell leads to suppression of the coercivity and emergence of a weak exchange bias effect. Comparison of the magnetization reversal mechanism in the bare and the surface-oxidized nanowire reveals that the domain wall propagation and annihilation remains the dominant reversal mechanism in surface oxidized nanowires as in their ferromagnetic counterparts. However, the interface exchange coupling introduces a secondary reversal mechanism activated in the central part of the wire with characteristics of coherent rotation, which acts in synergy to wall propagation leading to enhancement of the wall mobility. This effect is more pronounced in nanowires with large exchange bias values and is attributed to the uncompensated interface moments that act as nucleation centers for magnetization reversal. Our results are in good agreement with recent measurements in Co and Co/CoO nanowires.

Exchange bias effect in cylindrical nanowires with ferromagnetic core and polycrystalline antiferromagnetic shell

“…1 we use dimensionless energy parameters scaled by J F M , which is arbitrarily taken as J F M = 10, and J AF /J F M = -0.5, J int /J F M = -0.5, K F M /J F M = 0.1, K AF /J F M = 1.0 and g = 0.05/J F M . These parameters capture the main features of the Co/CoO exchange coupled system as previous studies of oxide-coated cobalt nanoparticles 14,24 and nanowires 21 have shown. Temperature T and the magnetic field strength H are measured in units of J F M .…”

“…The third term in Eq. 1 is the Zeeman energy due to the applied field H and the fourth term is the dipolar energy with strength g. For computational efficiency, the dipolar energy term is treated in an embedded cluster approximation with a cluster radius r 0 = 3a 21 .…”

Magnetic properties of nanowires with ferromagnetic core and antiferromagnetic shell

“…Mag- Table 1. netization reversal in FM nanowires proceeds by nucleation of a pair of domain walls at the free ends of the nanowire, their propagation with opposite velocities and eventually their merge at the center of the wire [6,8,25]. The coupling to an AF shell (S2) modifies the reversal mechanism, as previous experimental [19] and numerical [19,23] works have demonstrated. In an ideal FM/AF nanowire (S2), as it was mentioned above, the unsatisfied bonds at the interface act as nucleation centers of a secondary magnetization reversal mechanism.…”

“…The third term in Eq. 1 is the Zeeman energy due to the applied field along the cylinder axis and the fourth term is the dipolar energy with strength g. For computational efficiency, we decompose the local dipolar field H d−d i ≡ g j D ij · S j into a near-field part that extends up to 3rd nearest neighbor cells and a far-field part that is treated in a mean field approximation [23]. We use micromagnetic parameters typical of cobalt A = 1.3 · 10 −11 J/m, M s = 1.4 · 10 6 A/m, and K u = 3 · 10 5 J/m 3 .…”

“…We numerically simulate the field-cooling process ( Fig. 1) and the isothermal hysteresis loop implementing the Metropolis Monte Carlo algorithm with single spin updates, as previously described [23].…”