S.K. Arora
1 , B.J. O’Dowd 1
, B. Ballesteros
2
, P. Gambardella
2,3 and I.V. Shvets
1
1 CRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland
2 Catalan Institute of Nanotechnology, UAB Campus, E-08193 Barcelona, Spain
3 ICREA and Department de Física, Universitat Autonoma de Barcelona, E-08193 Barcelona, Spain
Abstract: Planar nanowire (NW) arrays of Co grown on oxidized step-bunched Si(111) templates exhibit room temperature ferromagnetic behaviour for wire widths down to 25 nm. Temperature and thickness dependent magnetization studies on these polycrystalline NW arrays show that the magnetic anisotropy of the NW array is dominated by shape anisotropy, which keeps the magnetization in-plane with easy axis parallel to the wires. This shape related uniaxial anisotropy is preserved even at low temperatures (10 K). Thickness dependent studies reveal that the magnetization reversal is governed by the curling mode reversal for thick wires whereas thinner wires exhibit a more complex behaviour which is related to thermal effects and size distribution of the crystal grains that constitute the NWs.
One-dimensional nanostructures such as nanowires are the subject of intense interest due to their potential applications in spin-electronic devices as well as their potential to exhibit interesting physical phenomena. Many proposed applications of magnetic nanowires will require control and understanding of the magnetisation reversal behaviour in the nanowires.
Here we produce ordered two-dimensional arrays of Co nanowires by carrying out deposition at a shallow angle onto oxidised step-bunched vicinal Si (111) templates. The step array morphology of the Si templates was produced by annealing the samples, which have a miscut of 4° in the [112̅] direction, with a DC current in UHV. This results in highly ordered step-bunches with a periodicity of
110nm (terrace width 85nm and bunch width 25nm). These are subsequently oxidised using a standard thermal oxidation procedure resulting in an amorphous surface oxide layer which is approximately 100nm thick. AFM is used to confirm that this thermal annealing procedure has not had an adverse effect on the step array morphology. Co is then deposited in the ascending step direction so that deposited material lands only on the step bunches, resulting in wires with a width of
25nm, a separation of 85nm and a thickness which is determined by the deposition time. SEM analysis of the arrays show that the wires are composed of small grains which grow and coalesce as the deposited thickness is increased. Samples for magnetisation analysis are coated with an additional protective capping layer of MgO.
HRTEM results of nanowires which were grown by depositing Co in the descending step direction reveal that the wires are polycrystalline, with hcp and fcc Co structure visible in areas, as well as fcc-CoO in the vicinity of the interfaces with the oxidised Si substrate and the MgO capping layer.
The magnetic properties of Co nanowire arrays were analysed using vibrating sample magnetometry (Quantum Design Physical Property Measurements System). Four samples, A, B, C and D, were analysed, which had identical wire width (25nm) and separation (85nm) but varying thickness (3, 4.5, 6.1 and 9nm respectively). Hysteresis loops were measured in the plane of the array both parallel and perpendicular to the axis of the wires. The results of these measurements taken at
300K show an easy axis of magnetisation parallel to the axis of the wires in all cases, as expected due to the large aspect ratio of the wires. The coercivity in the parallel direction ( 𝐻
∥ 𝑐
) increases with increasing thickness for the first three samples (A – C). This is due to the increasing dominance of shape anisotropy, as the wires become more continuous and surface anisotropy becomes less influential. For the sample with greatest thickness (D), 𝐻
∥ 𝑐
is reduced. Remanence in the parallel direction ( 𝑀
∥
𝑅
) was also noted to initially increase with thickness, but was reduced for sample D.
These findings for sample D are likely due to coupling between the wires which is known to reduce coercivity and remanence, and may be brought about by the increased wire volume and/or the growth of magnetic nanoparticles on the terraces between the wires which appear as the deposited thickness increases.

Figure 1. Coercivity measured parallel (red) and perpendicular (black) to the axis of the nanowires plotted as a function of temperature for four nanowire array samples, A, B, C and D. Fitted to the data is a function which describes the behaviour of coercivity in the case of curling mode reversal.
Hysteresis measurements were taken as temperature was reduced from 300K to 10K and the values of 𝐻 𝑐
∥
and 𝐻 𝑐
⊥
were extracted and plotted as a function of temperature for each of the four samples. The results are shown in figure 1. Both 𝐻 𝑐
∥
and 𝐻 𝑐
⊥
increase with decreasing temperature for all four samples as thermal effects are gradually removed. According to previous studies the coercivity is predicted to increase with lower temperature in a manner that depends on the reversal mechanism of the magnetisation. Néel and Brown described how the energy barrier for magnetisation reversal 𝐸
𝐵
depends on coercivity as follows: where 𝐸
0
is the energy barrier at zero field and 𝐻
0
is the coercivity at 0K. The exponent 𝑚 depends on the reversal mechanism, and is equal to 1.5 for curling mode reversal and 2 for coherent reversal.
By considering the temperature dependence of 𝐸
𝐵 and 𝐻 the relation can be re-expressed as where 𝑀 𝑠
is the saturation magnetisation and additional zero subscripts for 𝐸
0
, 𝑀 𝑠
and 𝐻 𝑐
indicate those values at 0K. This equation is fitted to the data in figure 1 with 𝑚 fixed at 1.5. As can be seen, the fit is best for samples with greatest thickness, indicating that the curling reversal mode best describes the behaviour of the coercivity for thicker samples. Wires in thinner samples behave as a weakly interacting array of particles. From the fitting parameters it was possible to extract the effective volume associated with magnetisation reversal.
These results demonstrate precise control of highly regular nanowire arrays with widths as low as 25nm. The samples have an easy axis of magnetisation parallel to the length of the wires due to magnetic shape anisotropy. The effect of wire thickness on coercivity was investigated and results explained in relation to the influence of shape anisotropy and inter-wire coupling. A study of coercivity as a function of temperature reveals that the curling mode reversal model fits data best for the thickest samples, while thinner wires are more likely to rotate in a coherent mode.
This work is published in: S.K. Arora, B.J. O’Dowd, B. Ballesteros, P. Gambardella and I.V. Shvets,
Magnetic properties of planar nanowire arrays of Co fabricated on oxidized step-bunched silicon templates. Nanotechnology 23 , (2012) 235702