Shape induced magnetic vortex state in hexagonal ordered cofe nanodot arrays using ultrathin alumina shadow mask

Journal of Magnetism and Magnetic Materials 451 (2018) 51–56

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Shape induced magnetic vortex state in hexagonal ordered cofe nanodot arrays using ultrathin alumina shadow mask B. Sellarajan a,b,c, P. Saravanan c, S.K. Ghosh d, H.S. Nagaraja b, Harish C. Barshilia a, P. Chowdhury a,⇑ a

Nanomaterials Research Laboratory, Surface Engineering Division, Council of Scientific and Industrial Research-National Aerospace Laboratories, Bangalore 560017, India Department of Physics, National Institute of Technology Karnataka, Surathkal 575025, India c Defence Metallurgical Research Laboratory, Hyderabad 500058, India d Materials Processing Division, Bhabha Atomic Research Centre, Mumbai 400085, India b

a r t i c l e

i n f o

Article history: Received 23 August 2017 Received in revised form 4 October 2017 Accepted 27 October 2017 Available online 28 October 2017

a b s t r a c t The magnetization reversal process of hexagonal ordered CoFe nanodot arrays was investigated as a function of nanodot thickness ðt d Þ varying from 10 to 30 nm with fixed diameter. For this purpose, ordered CoFe nanodots with a diameter of 80 ± 4 nm were grown by sputtering using ultra-thin alumina mask. The vortex annihilation and the dynamic spin configuration in the ordered CoFe nanodots were analyzed by means of magnetic hysteresis loops in complement with the micromagnetic simulation studies. A highly pinched hysteresis loop observed at 20 nm thickness suggests the occurrence of vortex state in these nanodots. With increase in dot thickness from 10 to 30 nm, the estimated coercivity values tend to increase from 80 to 175 Oe, indicating irreversible change in the nucleation/annihilation field of vortex state. The measured magnetic properties were then corroborated with the change in the shape of the nanodots from disk to hemisphere through micromagnetic simulation. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Arrays of discrete patterned ferromagnetic entities in the form of nanopillars [1–5], nanorings [6,7] and nanodots [8–14] have been recently proposed as a potential candidate for ultra-high density patterned magnetic storage media. These ordered magnetic nanodots also provide model systems for studying magnetic interactions and switching behavior, towards realizing the upcoming spin-based electronic devices. In such nanodot arrays, the remanent magnetization state depends on geometry of the nanostructures such as diameter (D), thickness (t) or length (L) and aspect ratio (D=t or D=L) [10–13], while their magnetization reversal process is driven by the intrinsic material properties, viz., exchange length, saturation magnetization and crystallographic orientation, as well as by the magnetostatic interaction [1–6]. In general, the magnetic configurations of nanodots exist in the form of single domain and flux closure magnetic domains. The characteristic behavior of single domain state is discerned by the coherent or uniform spin rotation configured with a large magnetic remanence. Whereas in the case of flux closure magnetic domains (vortex state), in-plane rotation of the spins (chirality) around the ⇑ Corresponding author. E-mail address: [email protected] (P. Chowdhury). https://doi.org/10.1016/j.jmmm.2017.10.115 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.

center core of magnetization with spins either up or down located at the centre of the individual nanostructure (polarity) are observed. This spin configuration contributes a very low stray field with negligible static interaction in the magnetic nanodot arrays. Nevertheless, both single domain and vortex state are geometrically limited by the critical diameter (Dc ), i.e., when D < Dc , the magnetic spins are aligned uniformly either in-plane or out-ofplane directions with respect to L=D. In contrast, arrays with D > Dc , a stable magnetization state is reported with vortex or multi-domain states. Until now, understanding of magnetic vortices in various nanostructures, viz., Fe [15–17], Co [18,19], Ni [20], NiFe [12,21,22], and FePd [23] was obtained based on the fact that the magnetostatic interaction is mainly due to the inter-dot distance. It should be noted that the variation in L is also an additional factor, which can influence the static interaction in the nanodot arrays [1–6]. In fact, such studies assume greater importance to elucidate the vortex behavior of nanodot arrays towards exploring them as a free active layer in spin torque microwave nanooscillators. Along these lines, we herein study the magnetization reversal processes of vortex state in CoFe nanodot arrays as a function of dot thickness (td ) keeping nanodots diameter (Dd ) and inter-nanodots distance (Sd ) constant. The main reason behind in choosing CoFe nanodot arrays in this study is due to the fact that it exhibits zero magnetostriction, good thermal stability and

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corrosion resistance, which enables them as a most suitable candidate for sensor and spintronic applications. Over the past couple of decades, complement to lithography techniques, porous anodic ultrathin alumina membrane (UTAM) with tunable pore parameters (i.e., pore diameter, interpore distances, etc.) were significantly exploited for the fabrication of ordered arrays of magnetic nanostructures and were further considered as a model system for understanding magnetization reversal mechanisms [2–5]. It is well known that the physical properties of the nanostructures were greatly influenced by the size and the shape of the nanostructures. In particular, though, the controlled variation in size and shape of the nanodots were reported previously for the fabrication of nanodot arrays using the UTAM as a physical mask during sputtering or evaporation; the issue of shadowing effect is often unavoidable [24–26]. We herein present the reversal magnetization mechanisms of ordered magnetic CoFe nanodot arrays with assistance of UTAM and their by experimental and micromagnetic simulation results. In this report, the ordered magnetic CoFe nanodot arrays were grown by sputtering using the UTAM with varying t d from 10 to 30 nm while keeping the Dd as 80 ± 4 nm and Sd as 105 ± 3 nm. The changes in the magnetic hysteresis loop shape measurements were analyzed with a special emphasize given on reversal magnetization process of shape-controlled growth of magnetic CoFe nanodot arrays by using the micromagnetic simulation studies.

2. Experimental details 2.1. Fabrication of ultrathin alumina membrane Higher order hexagonal symmetry in the pores of porous anodic alumina were prepared by two-step anodization process of Al (99.99%) in C2H2O4 electrolyte, as described previously [3,4,27,28]. The thicknesses of the alumina membrane were predominantly controlled by the anodization time during the second step anodization of Al. This leads to grow porous alumina membrane with thickness of 6300 nm, herein after named as UTAM. The fabrication details of UTAM can be found elsewhere [29]. In this study, the thickness of the alumina membrane was kept constant 100 nm to avoid the shadowing effect while sputtering; the pore size and the inter-pore distance of the porous membrane were 80 ± 4 and 110 ± 3 nm, (see inset of Fig. 1(a)) respectively.

2.2. Growth of ordered arrangement of high density magnetic nanodot arrays Ordered CoFe nanodots were fabricated on Si substrate by liftoff process by sputtering through the pores of UTAM. High purity CoFe target with atomic composition of Co: 90% and Fe: 10% was used for film deposition and t d was varied from 10 to 30 nm by adjusting the sputtering time with estimated deposition rate of 1.6 AA/s. the UTAM was removed from Si surface after deposition by ultrasonication. 2.3. Characterization The surface morphology of the nanodot arrays was analyzed by a field emission scanning electron microscopy (FESEM, ZEISS Supra 40VP) and atomic force microscopy (AFM). The magnetic properties of the patterned nanodot arrays were investigated by vibrating sample magnetometer (VSM). Micromagnetic simulations of magnetic nanodot arrays at different td were performed to complement the magnetic hysteresis loop shapes obtained from VSM. The magnetization reversal mechanism was studied considering pinching effect in the coercivity (Hc ) using 3-D object oriented micromagnetic framework (OOMMF) package [30]. 3. Nanodot arrays morphology In Fig. 1(a)–(b), we show the well-ordered hexagonal arrangement of CoFe nanodots with an average diameter of 80 ± 4 nm and inter-dot distance of 105 ± 3 nm (i.e., separation between dots is 25 nm) for two different film thicknesses: t d ¼ 20 and 30 nm, respectively. The cross-sectional AFM profile of CoFe nanodots with t d ¼ 10 and 30 nm is shown in Figs. 2(a)-(b), which clearly shows a gradual change in the shape of nanodots from flat-like feature to vertically elongated structure when the film thickness is increased. This is attributed to the shadowing effect of UTAM membrane while sputtering [24–26], thus leads to change in the shape of the observed nanodots from disc to hemisphere with increasing film thickness. However, the inter-dot separation from bottom to top of the dots is sustained irrespective of the film thickness. The estimated diameters of the nanodots were 80 ± 4 nm, i.e., directly calculated from the AFM line profile by using two cursors at the edge of the individual nanodots as shown in Fig. 2(i)–(ii). These estimated nanodot diameters were in good agreement with the pore diameters of the UTAM (see inset Fig. 1(a)).

Fig. 1. The morphology of CoFe nanodot arrays with two different thicknesses: (a) t d = 20 nm and (b) 30 nm, respectively. Inset shows the highly ordered porous ultrathin alumina membrane. The scale bar used is 200 nm.

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Fig. 2. AFM analysis of CoFe nanodot arrays for two different thicknesses (a) 10 nm and (b) 30 nm. (i) and (ii) represents a 2-D view and cross-section using a line profile in the corresponding CoFe nanodots, respectively. In the section analysis, the dimension scale were used same for both the horizontal and vertical coordinates to accurately reflect the shape of the nanodots.

4. Magnetic properties 4.1. Experimental results The room temperature magnetic hysteresis loops of CoFe nanodot arrays with different thickness are shown in Fig. 3. The

MðHÞ loops were performed while applying the external magnetic fields (H) in both parallel and perpendicular to the substrate (i.e., along the dot axis). However, for t d = 30 nm (see inset in Fig. 3 (c)) shows the easy axis of magnetization along the dot axis, and the non-saturated magnetization (H = 12 kOe) (i.e., hard magnetization axis) along the perpendicular direction with remanent

Fig. 3. The experimental data of MðHÞ loops for different CoFe thickness (td ), (a) 10 nm, (b) 20 nm, (c) 30 nm and (d) pinched neck characteristic loop width, d (=W M¼0 =W M¼0:5Ms ) obtained from measured magnetization curves shown as a function of thickness in (a)-(c) and the micromagnetic simulated MðHÞ for two different shapes: disk (half-filled square) and hemisphere (half-filled circle). The symbols used here are open square for experimental data and disk and hemisphere for micromagnetic simulated data. Inset in (c) is for parallel and perpendicular MðHÞ loops of CoFe nanodots at td = 30 nm.

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magnetization (M r = 0.06) and Hc = 200 Oe. For this reason, the inplane magnetization curves of CoFe nanodot arrays are considered for further discussion and analysis. Irrespective of the dot thickness, Fig. 3(a)–(c) show the easy axis of magnetization lies along the dot axis for different thicknesses from 10 to 30 nm. An increasing trend in the M r ; Hc and saturation magnetization (M s ) values could be noticed with increase in the film thickness. For td = 20 nm, the hysteresis loop appeared with a sharp pinching near zero field with Hc of 80 Oe. With further increase in dot thickness (t d = 30 nm), this pinching behavior changed slightly in its appearance with increased Hc (175 Oe). This unique shape of pinched neck or the narrow hysteresis loop near zero field can be considered as a vortex-like curling spin arrangement, as previously demonstrated by several researchers in micron or sub-100 nm dots [15– 18,22,23]. The characteristic behavior of pinch neck hysteresis loop (shapes) was estimated by measuring the loop width d (¼ W M¼0 =W M¼0:5Ms ) where W M¼0 is the width of magnetization curve at M = 0 (zero magnetization) and W M¼0:5Ms is the width of magnetization curve at M = 0.5Ms , [17]. Fig. 3(d) represents the variation of d as a function of td . It is evident that when t d = 20 nm, d was estimated to be 0.3, which is relatively low in comparison to a normal ferromagnetic material where d P 1:0 is the most possible scenario. However, d becomes close to 1.0 at t d = 10 nm, which signifies existence of a single domain state [15–18,22,23]. For t d = 30 nm, the remanent magnetization was found to be less pinched neck with higher Hc than at td = 20 nm (see Fig. 3(c)). This is our first identification that in circular nanodots, the well-known shape anisotropy could not change the magnetic phase from vortex to single domain state while increasing the film thicknesses. However, the critical thickness for the phase transition from single to vortex domain has been reported almost four times larger than the circular nanodots in oblate spheroid [10]. In this view, it is suggested that the increase in d (i.e., 0.6) for td = 30 nm may be attributed to the possible variation in the shape of the nanodots, i.e., due to unavoidable shadowing effect [24–26].

4.2. Micromagnetic simulations To further understand the vortex magnetization states, micromagnetics simulation has been carried out for hexagonally ordered arrays of 7 (i.e., six nanodots are positioned in a hexagonal order and one at the middle) as well as (7*7) number (not shown here) of nanodots using 3D-OOMMF package [30]. The parameters used for the simulations are: A (exchange constant) = 13  1012 J/m, M s (saturation magnetization) = 1460 kA/m, and K 1 (cubic magnetocrystalline anisotropy) = 6.7  104 J/m3. In these calculations, the cell size was selected to be 2  2  2 nm3, which is much smalpffiffiffiffiffiffiffiffiffiffiffi ler than the exchange length Lex ¼ A=K 1 (14 nm for Co). For simulation experiments, the in-plane magnetization direction along the nanodot axis is considered and the results are shown in Fig. 4 for MðHÞ loops and Fig. 5 for 3-D view of the x-component magnetization distribution at the remanence state (H = 0 Oe) for arrays of seven nanodots. The demagnetization process was simulated for ordered hexagonal array of CoFe nanodots with two different shapes such as disk and hemisphere. It should be mentioned that independent of the dot shape and thickness, the reversal process was found to be highly progressive due to the formation and annihilation of vortex state. However, a drastic variation in the vortex nucleation/annihilation fields for all the thickness is discerned with respect to the shape of nanodots (Fig. 4). The detailed reversal processes of two different shapes (disc and hemisphere) are described below for thicknesses 10–30 nm, when the field, H, decreases from +ve saturation to ve saturation.

Fig. 4. Micromagnetic simulated magnetization curves as a function of thickness for two different shapes: disk (half-filled square) and hemisphere(half-filled circle): (a) 10 nm, (b) 20 nm and (c) 30 nm, respectively.

For t d = 10 nm, the simulated MðHÞ loops show a sharp transition at a nucleation field, Hn = 210 and 760 Oe for disk and hemisphere nanodots (see Fig. 4(a)). This is attributed to the spin flipping mechanism which leads to abrupt increase in magnetization due to small increment in field towards () ve saturation. The sharp spin flipping at -Hn could be dominated by the shape anisotropy of circular dots for td  D, i.e., easy axis should lie along the nanodot axis [10,13]. This transition results in very high M r with two different Hc values for the disk and hemisphere nanodots, as shown in Figs. 5(a) and (d) with single domain magnetic structure. The differences in Hc values are due to the variation in the physical geometry of nanodisk and nano-hemisphere. In the case of nanodisk, the diameter of dots and the gap between neighboring dots are considered to be uniform throughout the sample surface. In contrast, a gradual increase in the gap from bottom to top surface of the dots with increasing td is quite obvious in the case of hemisphere dots (i.e., see inset of Fig. 5, a schematic view of nanodots for disk and hemisphere). In this view, a shrinkage effect develops along the radial direction and it exists even when the dot size smaller than Dc . Accordingly, combination of both small and radially elongated dots arrangement show complicated reversal processes with the variation of td in nano-hemisphere. For similar reasons, arrays of nano-hemisphere show higher Hc

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Fig. 5. 3-D view of x-component magnetic distribution at H = 0 Oe for hexagonal ordered seven CoFe nanodots at two different shapes: (a)–(c) for disk and (d)–(f) for hemisphere as a function of td : (a) and (d) for 10 nm, (b) and (e) for 20 nm, and (c) and (f) for 30 nm, respectively. The xy planes at different z of the dots is shown here for better clarity of disk and hemisphere and their vortex state. The cone schematically indicate the magnetization direction and the colors indicate the longitudinal magnetization, M x . The external field applied here was parallel to the dot axis.

value than the nanodisk arrays irrespective of their t d , as shown in Fig. 4. However, with td = 20 nm as shown in Fig. 4(b), two different magnetization behaviours can be observed as a function of the shape in circular nanodots. For nanodisk arrays, a sharp fall in the magnetization value by 50% is observed due to the formation of a vortex state within a nanodot in the x-y plane at Hn = 300 Oe. Fig. 5(b) shows the mixed magnetic structure of both single domain (i.e., magnetization parallel to the applied field direction in few dots (middle row)) and presenting the vortex configuration in the xy-plane. Whereas for nano-hemispheres, magnetization reversal via vortex state is appeared after high remanent magnetization at zero field (see Fig. 5(e)), and nucleated with 50% reduction at Hn = 300 Oe. Irrespective of the shapes, the reduction in magnetization values is responsible for the presence of both inplane single domain and vortex states at Hn in the nanodot arrays. Therefore, the vortex nucleation field is highly dependent on the nanodots shape even for similar dimensions (Dd ; Sd , and t d ). Further increasing the applied field towards the negative saturation field value, Hs , a process of vortex annihilation is depicted by a long tail as observed in MðHÞ loop for td = 20 nm in Fig. 4(b) as shown by the arrow mark Hn for vortex nucleation and Ha for annihilation field. For nano-hemisphere with td = 30 nm, a vortex nucleation is observed with 50% reduction in the magnetization value at Hn = 350 Oe in the MðHÞ loop behavior similar to that of those observed for nanodisks at t d = 20 nm (see in Figs. 4(c) and 5(f)). However, for nanodisks with t d > 20 nm, the simulated MðHÞ loops showed a sharp transition at Hn followed by a linear region where the magnetization spin vectors are reversible with H, i.e., the reversible region is marked as DH in Fig. 4(c). This leads to zero M r and Hc at H = 0 in the MðHÞ loop, which signifies the propagation of vortex magnetic state (see in Fig. 5(c)) in the field region DH in the nanodot arrays. In fact, this reversible field region increases with thickness due to shift in the Hn value towards (+) ve saturation field. This signifies that the effect of shape anisotropy might hinder

the spins to lie along the hard magnetization direction (plane of the dot) while increasing the td , i.e., for td D. With consideration of shape of a nanodots (disk and hemisphere) and their magnetostatic interaction among the nanodots in arrays, the simulated MðHÞ behaviors of CoFe nanodots for td  D predict the possibility of in-plane magnetization behaviour which corroborates the experimental findings of in-plane easy axes with pinched loop at H = 0. It also showed that the magnetization reversals are dominated by the creation and annihilation of vortex structure in the nanodots. It is interesting to see in Fig. 3(d) that the simulated curve matches quite consistent with the experimental results with respect to two different shapes: for thicknesses td 620 nm with nano-disk and td P 20 nm with nanohemisphere in the nanodot arrays. However, the structural irregularity and geometrical nonuniformity in the high density nanodot arrays show a small discrepancy in the simulated data. This can be minimized by considering the following factors [3,4]: (i) increasing the number of nanodots (i.e., 108 for an area of 55 mm2 as in real case) in an array while carrying out the simulation, (ii) physical shape variations among the nanodots and (iii) presence of imperfection within nanodots, especially for the disordered nanodots domain in arrays. Nevertheless, the present study brings out the magnetization reversal behavior by considering the shape of the nanodots as disk and hemisphere along with their geometry (Dd ; Sd and t d ) for the CoFe nanodot arrays. The results thus obtained with the micromagnetic simulations are found to be in good agreement with the experimental observations.

5. Conclusion In summary, we have investigated the reversal magnetization process of ordered CoFe nanodot arrays as a function of t d through UTAM shadow mask. The magnetic vortex state was demonstrated

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by the measured hysteresis loop shapes with pinched-neck behavior at t d =20 nm. However, unexpected increases in the hysteresis loop width at t d =30 nm was elucidated by change in the shape of the nanodots from disk to hemisphere by using micromagnetic simulations, in which it corroborates the experimental results. Acknowledgments Authors would like to thank the Director, NAL for giving the support toward the development of nano-dimensional materials. B. Sellarajan is grateful for the financial support provided by Council of Scientific and Industrial Research (CSIR) File No. 31/3 (41)/2012-EMR-I, India. Authors are grateful for the financial support given by BRNS (sanction Ref. No.:34/14/03/2015 dated 23.06.2015). References [1] T.J. Bromwich, T. Kasama, R.K.K. Chong, R.E.D. Borkowski, A.K.P. Long, O.G. Heinonen, C.A. Ross, Nanotechnology 17 (2006) 4367. [2] M.P. Proenca, C.T. Sousa, J. Escrig, J. Ventura, M. Vazquez, J.P. Araujo, J. Appl. Phys. 113 (9) (2013) 093907. [3] B. Sellarajan, P.D. Kulkarni, M. Krishnan, Harish C. Barshilia, P. Chowdhury, Appl. Phys. Lett. 102 (2013) 122401. [4] P.D. Kulkarni, B. Sellarajan, M. Krishnan, Harish C. Barshilia, P. Chowdhury, J. Appl. Phys. 114 (2013) 173905. [5] B. Sellarajan, H.S. Nagaraja, Harish C. Barshilia, P. Chowdhury, J. Magn. Magn. Mater. 404 (2016) 197. [6] S.P. Li, W.S. Lew, J.A.C. Bland, M. Natali, A. Lebib, Y. Chen, J. Appl. Phys. 92 (12) (2002) 7397. [7] Z. Guo, L. Pan, H. Qiu, M.Y. Rafique, S. Zeng, Adv. Mat. Res. 710 (2013) 80. [8] I.L. Prejbeanu, M. Natali, L.D. Buda, U. Ebels, A. Lebib, Y. Chen, K. Ounadjela, J. Appl. Phys. 91 (10) (2002) 7343. [9] D. Kumar, S. Barman, A. Barman, Sci. Rep. 4 (2014) 4108. [10] H. Hoffmann, F. Steinbauer, J. Appl. Phys. 92 (9) (2002) 5463.

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