Vortex state and effect of anisotropy in sub-100-nm magnetic nanodots

Abstract

Magnetic properties of Fe nanodots are simulated using a scaling technique and Monte Carlo method, in good agreement with experimental results. For the 20-nm-thick dots with diameters larger than 60 nm, the magnetization reversal via vortex state is observed. The role of magnetic interaction between dots in arrays in the reversal process is studied as a function of nanometric center-to-center distance. When this distance is more than twice the dot diameter, the interaction can be neglected and the magnetic properties of the entire array are determined by the magnetic configuration of the individual dots. The effect of crystalline anisotropy on the vortex state is investigated. For arrays of noninteracting dots, the anisotropy strongly affects the vortex nucleation field and coercivity, and only slightly affects the vortex annihilation field.

Development of vortex state in circular magnetic nanodots: Theory and experiment

“…For dot diameters around 65 nm, this ratio is greatly reduced. This is typical for reversal via a vortex state as also observed with FORC measurements, 23 confirmed by simulations 9 and discussed in detail in the theoretical section below. The virgin curve measured from the as-grown state ͑never exposed to magnetic field͒ or after demagnetization by field cycling about minor hysteresis loops is almost linear in a large field range.…”

“…This very unusual behavior is in a good agreement with the results of Monte Carlo simulations. 9 Below, we will focus on an array of Fe dots of average diameter of 65Ϯ 7 nm with spacing of 110Ϯ 12 nm and thickness of 20 nm covering a ϳ1.8 cm 2 ͑Fig. 3͒.…”

“…In response to the changing in-plane external field, the vortex propagates across the nanostructure, annihilating at a critical, "annihilation" field. 9,10 Thus, after in-plane saturation, as the field is decreased the spins are able to orient out of plane, overcoming the in-plane shape anisotropy. The return to the in-plane configuration in the reversed in-plane saturated state may also exhibit very interesting physics.…”

Temperature induced single domain–vortex state transition in sub-100nm Fe nanodots

“…6 Observation of the VS at remanence using magnetic microscopy 5 has indeed become a common practice. However, ultrasmall nanodots or nanorings may exhibit a SD remanent state, due to the more prominent role of boundaries, even though the magnetization reversal involves a VS. 4,11 Similarly, when a strong anisotropy is present, such as in elliptical or rectangular dots with a well defined shape anisotropy, the minimal demagnetization field along the magnetic easy axis may shift the vortex nucleation field to negative values, leading to a SD state at remanence. 12,13 The hysteresis loops, particularly those of a collection of nanomagnets, no longer bear the characteristic highly "pinched" shape with a zero remanence.…”

“…Because the nanodot center-to-center spacing is roughly twice its diameter, dipolar interactions are negligible. 11 Magnetic properties have been measured using a Princeton Measurements Corp. vibrating sample magnetometer with a liquid helium flow cryostat. The applied field is in the plane of the nanodots.…”