Magnetic properties of exchange biased Co/CoO elongated nanoring arrays

Abstract

We present a systematic study of the magnetic properties of exchange biased Co͑25 nm͒ / CoO͑t CoO ͒ / Cu͑2 nm͒ elongated nanoring arrays. The reversal mechanism in the exchange biased nanorings is directly compared to that of unbiased nanorings of identical geometry at varying temperatures. We observe that along the field cooling direction, the exchange biased nanorings exhibit reversal via shifted vortex states with enhanced magnitudes of onion→ vortex and vortex→ reverse onion transition fields. The magnitude of exchange bias field is also found to be strongly dependent on the CoO layer thickness t CoO , and decreases monotonically with increasing t CoO . Due to the interplay between the exchange and Zeeman energies, the exchange bias field could also be varied by adjusting the field cooling strength.There has been an emerging scientific interest in the magnetization reversal mechanisms of patterned nanomagnets with precisely controlled geometry and interelement spacing 1 due to potential applications in a wide range of magnetoelectronic devices such as magnetic random access memory, 2 read head sensors, 3 programmable logic devices, 4 and ultrahigh density patterned storage media. 5 From a fundamental viewpoint, nanomagnets offer by virtue of their low dimensionality, a wide range of magnetic properties which are not observed in their continuous bulk counterparts, especially when the size becomes comparable to or smaller than certain characteristic length scales such as spin diffusion length, carrier mean free path, and magnetic domain wall width. 6 Therefore, the influence of shape and size on the magnetic properties of square, 7-9 circular, 10,11 elliptical, 12,13 diamond, 14 and triangular 15 shaped nanomagnets has been widely investigated.Recently, a substantial amount of study has also been conducted on ring shaped nanomagnets driven by its technological importance and appealing magnetic states, namely, the flux-closure or "vortex" state in which the magnetization is oriented circumferentially around the ring, either clockwise or counterclockwise, and the bidomain or "onion" state in which two halves of the ring magnetization adopt opposite circulations, with each half separated by 180°domain walls. 16 For successful implementation in any magnetoelectronic device, it is imperative to achieve a consistent and experimentally tunable switching mechanism in such ring elements. As a source of unidirectional anisotropy, it has been shown that exchange bias ͑EB͒ can be used to control the reversal mechanisms in patterned nanomagnets. 17 While the majority of work on rings has been concerned with single layer, 16,18 or pseudospin valve structures, 19-21 there is limited reported work on EB bilayer rings 22-24 especially at low temperatures.In this work, a systematic study of the magnetic properties of EB Co/CoO elongated nanoring arrays is presented as a function of temperature. We observe that the magnetization reversal in nanorings is significantly modified due to EB. The evolution in EB field H E is also inves...

Excellent microwave-absorbing properties of elliptical Fe3O4nanorings made by a rapid microwave-assisted hydrothermal approach

“…To date, many efficient synthesis strategies have been developed to synthesize different magnetic NRs (i.e. Ni 80 Fe 20 [7], Co-Sn-O [8], Co-Cu [9], CoFe 2 O 4 [10], Co [5,6,11], Ni-Co [12], Ni 80 Fe 20 /Au/ Co [13], Co/CoO [14], α-Fe 2 O 3 [15], Fe 3 O 4 [16], and γ-Fe 2 O 3 ); these strategies include lithography [5], the template-based meth-hydrothermal route [8], microemulsion [9], and the solvothermal process [10]. However, only a few complicated routes, such as modified colloidal lithography [17], electron beam lithography [18], and zone-platearray lithography and lift-off processing [5], have been developed to synthesize elliptical NRs.…”

The role of dipolar interactions for the magnetic properties of ferromagnetic nanoring

“…Recently, a great number of studies have been actively conducted on the ferromagnetic nanorings driven by its potential application for magnetic random access memory [1][2][3][4][5][6][7][8][9]. In contrast to the circular magnetic disk where the transition from the vortex state to the single-domain state can occur below a threshold value of its radius owing to the high exchange energy of the vortex core, in the magnetic nanoring the vortex state can be stably retained on a similar size scale because of the absence of vortex core.…”

Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

“…Since the discovery of EB in Co/CoO particles over 50 years ago, 9 it has been found in a variety of different systems with FM/AFM interfaces, including core-shell nanoparticles, [10][11][12] thin film systems, 3,5,13,14 lithographed nanostructures, [15][16][17][18] and (more recently) in high aspect ratio core-shell nanotubes (NTs). 19 The latter have attracted much interest due to their potential applications in nanoelectronic devices, catalysis, high-density recording media, and drug delivery.…”