Magnetic anisotropy induced in NiCo granular nanostructures by ZnO nanorods deposited on a polymer substrate

Abstract

Hetero-nanostructures made of NiCo and ZnO nanostructures have been deposited on a polymer substrate through a two-step method. First a ZnO nanorod array was grown on polydimethylsiloxane (PDMS) substrate by a hydrothermal method. NiCo granular nanostructures were then deposited on the ZnO nanorod array through an electroless polyol process with precursors of Ni complex and Co complex in a mole ratio of 50 : 50. The effect of deposition time (t) on the structure and properties of the hetero-nanostructures has been studied. Ni-rich nanostructures are observed when t < 30 min. With t increasing, the atomic ratio (at.%) of Ni to Co approaches 50 : 50. In addition, a magnetic field (10 kOe) was applied on the hetero-nanostructures, which was parallel and perpendicular to the ZnO nanorods. The magnetization (M_s) of the NiCo nanostructures deposited on ZnO nanorods when t = 60 min is measured at 93.4 emu g⁻¹. The results of magnetic measurements and X-ray absorption near edge structure spectroscopy (XANES) suggest that the slightly lower magnetizations of the hetero-nanostructures may be caused by the oxidation of Co and Ni on the surface of the granular films. The values of squareness ratio (M_r/M_s) of the hetero-nanostructures with t are far below 0.5. Moreover, the coercivity (H_c) value of the hetero-nanostructures measured in the direction perpendicular to the ZnO nanorods is higher than that in the parallel direction. Therefore, the deposited NiCo granular nanostructures are perpendicular to the plane of the PDMS substrate, and show perpendicular magnetic anisotropy. These results clearly demonstrate that the rod-shaped non-magnetic nanomaterials can significantly affect the magnetic anisotropy of the hetero-nanostructures.

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Introduction

Magnetic and non-magnetic hetero-nanostructures show distinct properties, and have been applied in various devices, e.g. non-volatile magnetic random access memory based on magnetic tunnel junctions and sensors based on giant magnetoresistance. Controlling the interface of the magnetic and non-magnetic materials is a challenge to precisely obtain desired properties.

Nickel (Ni) and cobalt (Co) nanostructures have been extensively studied due to their magnetic properties. Incorporation of Ni and Co nanostructures with semiconductors has shown enhanced catalytic activity and improved corrosion resistance, and could be applied in new spintronic devices.¹ On the other hand, zinc oxide (ZnO) is a wide-band-gap semiconductor with band-gap energy of 3.37 eV and a large excitation binding energy of 60 meV. One-dimensional ZnO nanostructures including nanorods and nanowires have been considered as the most efficient potential materials for light emitting diodes, transistors, solar cells and Schottky diodes due to their electrical, optical, and piezoelectric properties.^2–5

Recently, one-dimensional ZnO nanostructures incorporating transition metals, e.g. Ni and Co, have been shown to have interesting magnetic properties depending on different processing methods, such as chemical vapor deposition, core/shell formation, sputtering, and electrochemical deposition.^6–8 Unfortunately, these methods normally require expensive deposition systems and complex processes. Compared to these vacuum-involving deposition processes, chemical solution methods may provide an alternative to develop magnetic hetero-nanostructures. For instance, Huang and co-workers developed an electrochemical method to deposit Ni-doped ZnO nanocomposites on metallic substrates.^9,10 In addition, our previous studies indicate that hybrid metallic nanostructures can be deposited on metal and ceramic substrates through an electroless polyol process, which is a reductive reagent-free, low-temperature process.^11,12

To develop magnetic and non-magnetic hetero-nanostructures on polymer substrates with more flexible mechanical properties, and to further understand the effect of the interface between magnetic and non-magnetic nanostructures on magnetic properties, a two-step process has been developed for fabricating magnetic metal/ZnO nanocomposite on a flexible polymer substrate. First an aligned ZnO nanorod (NR) array was grown on polydimethylsiloxane (PDMS) substrate. Subsequently, a NiCo granular film was deposited on the ZnO NR array by an electroless polyol method. The effect of deposition time (t) on the microstructures and magnetic properties of the NiCo–ZnO hetero-nanostructures is discussed in this paper.

Experimental details

A well-aligned ZnO NR array was grown on flexible substrate through modifying a previously reported method.^13,14 Briefly, zinc acetate dehydrate (0.01 M) was dissolved in 100 mL ethanol as a seed solution. The seed solution was dropped on a PDMS substrate repeatedly followed by a heat treatment at 100 °C for 1 hour (h). The ZnO seed-coated PDMS film was then immersed in a mixed solution of zinc nitrate hexahydrate (0.025 M) and hexamethylenetetramine (HMTA; 0.025 M) in 200 mL of distilled deionized water. After heating at 90 °C for 3 h, the ZnO nanorod array was grown on the PDMS as shown in Scheme 1. Samples were rinsed with distilled deionized water and dried at room temperature.

Scheme 1 Illustration of the two-step method for fabricating NiCo nanostructure-coated ZnO nanorod array on PDMS.

Equimolar nickel(II) acetate tetrahydrate and cobalt(II) acetate tetrahydrate were dissolved in 200 mL of ethylene glycol. The mixture was heated at 194 °C with refluxing.¹¹ Our previous studies indicate that the growth of metallic films produced by the polyol process nonlinearly depends on the deposition time (t),^11,12 e.g. the growth of film increases dramatically when t < 30 min, whereas the thickness and weight of film increase slowly when 30 min < t < 60 min, and no further growth can be observed when t > 60 min. Therefore, the produced ZnO NR array coated on PDMS was suspended into the solution. NiCo granular films were deposited on ZnO NR arrays when the deposition time (t) was 10 min, 30 min, and 60 min. Here, the deposited samples are denoted as NiCo–ZnO-10, NiCo–ZnO-30 and NiCo–ZnO-60. All substrates had the same dimensions (3 × 3 cm²). Scheme 1 shows the fabrication process of the NiCo–ZnO hetero-nanostructures.

The morphology of the aligned ZnO NR array was studied by scanning electron microscopy (SEM; Hitachi 3400S). Energy dispersive X-ray spectroscopy (EDX) was used to determine the distribution of Ni and Co on the ZnO NR array. X-ray diffraction (XRD) was carried out (Rigaku rotating-anode X-ray diffractometer with Co-Kα radiation) to study the crystal structures of the hybrid nanocomposites. In addition, magnetic properties of the NiCo–ZnO hetero-nanostructures were measured by using a vibrating sample magnetometer (VSM, LakeShore 7407, moment measure range: 10⁻⁷ to 10³ emu; field accuracy: ±0.05% full scale). All the magnetization measurements were carried out at room temperature under a maximum field of 10 kOe. The field was applied in a direction parallel (∥) or perpendicular (⊥) to the ZnO NR as shown in Scheme 2.

Scheme 2 The two directions of magnetic measurements. (Left) magnetic field parallel to ZnO nanorods. (Right) magnetic field perpendicular to ZnO nanorods.

The weights of all the films were measured by weighing the substrate before and after film deposition using a balance with ±0.0005% accuracy:

g_{NiCo film} = g_{ZnO+PDMS+NiCo film} − g_ZnO+PDMS (1)

Film growth was studied by investigating the development of morphology, structure, composition and microstructure including crystallite size, strain, and film thickness using different kinds of techniques.

To study the surface of the hybrid films, Ni and Co K-edge X-ray absorption near edge structure spectroscopy (XANES) experiments were conducted at the Soft X-ray Microcharacterization Beamline (SXRMB) of the Canadian Light Source (CLS). The samples were mounted on a copper plate in a vacuum chamber (around 10⁻⁸ Torr). XANES spectra were collected in total electron yield (TEY). TEY measured all the electrons ejected from the specimens, including photoelectrons, Auger electrons and secondary electrons. TEY mode experimentally monitored the neutralization current to ground, and is extremely surface sensitive since the electron escape depth is very short.

Results and discussion

To grow ZnO NR array on PDMS, HMTA is used to provide OH⁻ anions which help to form Zn(OH)₄²⁻;¹³ meanwhile, the ZnO NRs can be dehydrated by the intermediates. This fast and simple synthesis method also allows good control over the ZnO nanomaterial morphologies by changing the reaction conditions such as reaction temperature, pH and time duration.^14–16 SEM surface micrographs for the bare ZnO NR array prepared on PDMS substrate are shown in Fig. 1. Fig. 1a shows SEM micrographs of the vertically aligned ZnO rods grown on PDMS, and an individual ZnO NR. The average diameter of ZnO NRs is estimated at 160 ± 5 nm with a length of 2 ± 0.5 μm. Fig. 1b presents a high-magnification SEM micrograph in which an individual ZnO NR has a hexagonal-shaped structure. HRTEM image of the ZnO NR lattice (Fig. 1c) indicates the ZnO NRs are highly crystalline with a lattice spacing of 0.25 nm which corresponds to the (0002) planes in the ZnO crystal lattice.^17,18

Fig. 1 Electron micrographs of ZnO NRs. (a) SEM micrograph of ZnO NR array on PDMS. The inset shows the foldable sample. (b) SEM micrograph of ZnO NR of 200 nm in diameter. (c) High-resolution TEM micrograph of ZnO NR.

In the polyol process, ethylene glycol is used both as a solvent and as a reducing agent to produce fine metallic nanoparticles. Our previous work successfully indicated that a metallic thin film can be deposited on metal and ceramic substrates through the polyol process.^10,11 Here, bi-element NiCo nanostructures were deposited on a ZnO NR array grown on the flexible polymer substrate PDMS through the polyol process. Fig. 2 shows the cross-sectional SEM micrographs of the ZnO NR array on PDMS before and after the deposition of NiCo nanostructures when t = 15 min. Fig. 2b clearly shows the NiCo nanostructures deposited on the sidewall of ZnO NRs of 2 ± 0.5 μm in length, and the growth texture of NiCo granular film (above the ZnO NR array) prefers to be normal to the PDMS substrate, which is influenced by the ZnO NRs vertically grown on PDMS. The thickness of NiCo–ZnO increases from 6 ± 0.8 μm to 11 ± 1.5 μm when t increases from 15 min to 60 min. Fig. 3 shows the morphologies of NiCo–ZnO for different deposition times: t = 10 min, 30 min, and 60 min. Clearly, the surface of NiCo–ZnO shows granular nanostructures which are NiCo coatings. The surface compositions of the hetero-nanostructures were identified from EDX data. Only Zn, O, Co and Ni peaks can be observed in the EDX spectrum (Fig. 3d). The correlation between the deposition time and surface compositions (atomic ratio: at.%) is listed in Table 1. A Ni-rich (58.3 ± 1.25 at.%) composition can be found in NiCo–ZnO-10. The amount of Co increases from 41.7 ± 1.25 at.% to 50.2 ± 1.25 at.% when t increases from 10 min to 60 min. This indicates that it takes a longer time to deposit the reduced Co in the polyol process due to its higher reduction potential (−0.28 V) compared to Ni (−0.257 V).¹⁹

Fig. 2 Cross-sectional SEM micrographs. (a) ZnO nanorods grown on PDMS. (b) NiCo granular nanostructures deposited on ZnO nanorod arrays grown on PDMS.

Fig. 3 (a) SEM micrograph of NiCo–ZnO-10. (b) SEM micrograph of NiCo–ZnO-30 (c) SEM micrograph of NiCo–ZnO-60. (d) EDX spectrum of NiCo–ZnO-60.

Table 1 Atomic ratio of the NiCo nanostructures coated on ZnO NR array

Sample	Thickness of NiCo–ZnO (μm)	Ni (at.%)	Co (at.%)
NiCo–ZnO-10	6 ± 0.8	58.3 ± 1.25	41.7 ± 1.25
NiCo–ZnO-30	9 ± 1.0	52.6 ± 1.25	47.4 ± 1.25
NiCo–ZnO-60	11 ± 1.5	49.8 ± 1.25	50.2 ± 1.25

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In addition, XRD was carried out to further study the structure of the NiCo–ZnO hetero-nanostructures. In Fig. 4, the diffraction patterns clearly show the preferential growth along the (002) plane in the wurtzite structure of ZnO (JCPDS no. 36-1451), which indicates the highly crystalline structure with c-axis growth orientation of hexagonal structure.⁷ When t = 10 min, two new weak peaks can be observed as shown in Fig. 4, which match well with the face-centred cubic (fcc) of Ni/Co structures. It is hard to identify the fcc Ni (JCPDS no. 15-0806) and fcc Co (01-1260) peaks due to their similar crystal structures. We therefore study the long-range order of the hetero-nanostructures through XANES spectroscopy, which is discussed later. With the deposition time increasing, the intensity of the diffraction peaks of NiCo increases. There is no other characteristic peak of impurities. Meanwhile, the Scherrer equation is applied to estimate the crystallite size of the NiCo nanocomposite:^20,21

where D is the crystallite size, K is a constant depending on crystallite shape (0.89), λ is the X-ray wavelength, β is the full width at half maximum of the (111) diffraction peak, and θ is the Bragg angle. The average crystallite size of CoNi coated on ZnO NR arrays is estimated at 67.8 ± 5 nm, which is independent of t.

Fig. 4 XRD profiles of NiCo granular film coated on ZnO NR array for various t.

The magnetic hysteresis loops of the NiCo–ZnO hetero-nanostructures were measured at 300 K with an applied field of −10 kOe < H < 10 kOe. The background of PDMS substrate has been subtracted. The bare ZnO NR array shows diamagnetic behaviour while NiCo–ZnO hetero-nanostructures have strong ferromagnetic features at 300 K. In addition, the value of saturation magnetization (M_s) of NiCo–ZnO hetero-nanostructures increases from 86.36 emu g⁻¹ to 95.39 emu g⁻¹ when t increases from 10 min to 60 min as shown in Table 2. This corresponds to the increasing atomic ratio of Co in the hetero-nanostructures with t (i.e. Co increases from 41.7 ± 1.25 to 50.2 ± 1.25 at.%). Our previous studies on the polyol process indicated that the thickness and weight of thin films have non-linear relationships with the deposition time.

Table 2 Magnetic saturation (M_s), squareness ratio (M_r/M_s), and coercivity (H_c) of NiCo nanostructured granular films coated on ZnO nanorods

	Ms (emu g^−1)	Mr/Ms	Hc (Oe)
Field parallel (∥) to ZnO nanorods
NiCo–ZnO-10	85	0.15	60
NiCo–ZnO-30	92.3	0.18	86
NiCo–ZnO-60	93.4	0.18	97
Field perpendicular (⊥) to ZnO nanorods
NiCo–ZnO-10	85	0.07	116
NiCo–ZnO-30	92.3	0.08	130
NiCo–ZnO-60	93.4	0.11	138

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In most cases, both the thickness and weight increase dramatically when t < 30 min; the deposited films grow slowly when t > 30 min; there is no more growth after t = 60 min. The most interesting results are related to the magnetic anisotropy of the NiCo–ZnO hetero-nanostructures. All samples with increasing t show clear magnetic anisotropy under the field direction (parallel and perpendicular to the NR array). Fig. 5 shows the hysteresis loops of NiCo–ZnO-60 when the applied field is parallel to the ZnO NR array and perpendicular to the ZnO NR array. The values of squareness ratio (M_r/M_s) and coercivity (H_c) of the samples with the two different field directions are also listed in Table 2. The squareness ratio (M_r/M_s) is dominated by the uniaxial anisotropy, e.g. M_r/M_s = 0.5 when single domain particles are randomly oriented. Here, all samples have very small squareness ratios, which indicates the magnetic domains are oriented. Furthermore, higher H_c of samples under a magnetic field perpendicular to the ZnO NR array indicates that the deposited NiCo are oriented in the opposite direction, i.e. perpendicular to the plane of PDMS substrate. In addition, the anisotropy field (H_K) of the hetero-nanostructures decreases from 10.7 kOe to 8.6 kOe when t increases from 10 min to 60 min. For thin films, the magnetic dipolar interaction (Kv) is a major source of magnetic anisotropy. Due to its long-range character, the dipolar interaction generally results in a contribution to the anisotropy which depends on the shape of a specimen.^22,23 Consequently, the magnetic anisotropy of NiCo–ZnO hetero-nanostructures is clearly significantly influenced by the ZnO NRs.

Fig. 5 Hysteresis loops of NiCo–ZnO-60 in two directions. The inset indicates the higher coercivity of NiCo–ZnO-60 when the magnetic field is perpendicular to the nanorods.

The M_s values of bulk fcc Ni and fcc Co are 55 and 168 emu g⁻¹ at 300 K, respectively. It is noted that M_s (93.4 emu g⁻¹) of NiCo–ZnO-60 is lower than that (111.7 emu g⁻¹) of bulk Ni₅₀Co₅₀. This could be caused by the oxidation on the surface of NiCo granular film produced by the polyol process.²⁴ In order to determine the oxidation state of Co and Ni at the surface of the hetero-nanostructures, XANES spectroscopy was applied to study the surface of specimens. TEY spectra of NiCo–ZnO-60 were collected as shown in Fig. 6. The probing depth of secondary electrons is around 20 Å, which makes the TEY detection surface sensitive.²⁵ Fig. 6a shows the Ni K-edge XANES spectra of NiCo–ZnO-60 sample, bulk Ni, and bulk NiO. The main resonance located around 8343 eV is due to the electron transitions from 1s to 4p orbital,²⁶ while the shoulder peak around 8370 eV and the following broad peak around 8415 eV arise from multiple scattering. In Fig. 6a, the TEY spectrum of NiCo–ZnO-60 is very close to that of NiO. Similar to the case of Co, there are no discernible differences that can be observed between TEY and FY spectra of NiCo–ZnO-60 at the Co K-edge as well. The Co K-edge XANES spectrum is very similar to the spectrum of CoO (Fig. 6b).

Fig. 6 XANES spectra of the surface of NiCo–ZnO-60. (a) Ni K-edge XANES. (b) Co K-edge XANES.

Conclusion

We successfully developed a two-step method for coating NiCo nanostructured granular films on a well-aligned ZnO NR array which is deposited on a flexible substrate, PDMS. The ZnO NRs are highly crystalline structures of 2.5 μm in height and 200 nm in diameter. The polyol process is able to be used to deposit metallic nanostructures on ZnO NRs. The growth of the hetero-nanostructured NiCo–ZnO non-linearly depends on t. Ni-rich film is observed when t < 30 min, and the atomic ratio of Ni and Co approaches 50 : 50 when t is 60 min. The most interesting result is that the magnetic anisotropy of the hetero-nanostructured NiCo–ZnO is significantly influenced by the shape of the non-magnetic ZnO NRs. The perpendicular magnetic anisotropy of the NiCo–ZnO granular nanostructures on PDMS means they have the potential to be applied in high-density magnetic recording media, which normally require small crystallite size, and perpendicular magnetic anisotropy. Furthermore, our developed two-step method has advantages for fabricating magnetic–nonmagnetic hetero-nanostructures on polymer substrates in a simple and inexpensive manner. Polymer substrates may allow magnetic devices having low weight and flexible mechanical properties.

Acknowledgements

This work was supported by grants (to Jin Zhang) of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Innovation Fund-Leader of innovation.

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