TiO2 functional applications

Accepted Manuscript Morphology- and magnetism-controlled electrodeposition of Ni nanostructures on TiO2 nanotubes for hybrid Ni/TiO2 functional applications Farzad Nasirpouri, Hamed Cheshideh, Alexey Yu Samardak, Alexey V. Ognev, Alexander A. Zubkov, Alexander S. Samardak PII:

S0272-8842(19)30501-2

DOI:

https://doi.org/10.1016/j.ceramint.2019.02.200

Reference:

CERI 20923

To appear in:

Ceramics International

Received Date: 16 December 2018 Revised Date:

22 February 2019

Accepted Date: 27 February 2019

Please cite this article as: F. Nasirpouri, H. Cheshideh, A.Y. Samardak, A.V. Ognev, A.A. Zubkov, A.S. Samardak, Morphology- and magnetism-controlled electrodeposition of Ni nanostructures on TiO2 nanotubes for hybrid Ni/TiO2 functional applications, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.02.200. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Morphology- and magnetism-controlled electrodeposition of Ni nanostructures on TiO2 nanotubes for hybrid Ni/TiO2 functional applications

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Farzad Nasirpouri1,*, Hamed Cheshideh1, Alexey Yu. Samardak2, Alexey V. Ognev2, Alexander A. Zubkov3, Alexander S. Samardak2,4,*

Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran

2

School of Natural Sciences, Far Eastern Federal University, Vladivostok, Russia

3

Technoline Co. LTD., Vladivostok, Russia

4

National Research South Ural State University, Chelyabinsk, Russia

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*- Corresponding authors. Email address: [email protected], [email protected]

Abstract

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We investigate magnetic properties of nickel nanoparticles electrodeposited on TiO2 nanotubes by different techniques including direct current (DC) and cyclic voltammetry (CV) deposition. TiO2 nanotubes were fabricated by anodic oxidation from an organic electrolyte under constant voltage on Ti substrates. Using

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scanning and transmission electron microscopies, we observe that each technique provides different morphologies from fine individual particles to large and coalesced nickel where DC electrodeposition makes

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large coalesced Ni particles mostly accumulated on top surface and mouths of the nanotubes, whereas CV deposition makes a homogenous dispersion of Ni nanoparticles across the nanotubes. The variation of coercivity and saturation magnetization values recorded is consistent with our scenario, though owing to large number of nanoparticles with size and shape varieties. The first order reversal curve (FORC) diagrams reveal the magnetic behaviour of nanoparticle ensembles in relation to their morphology and crystal structure.

Keywords: Electrodeposition, Nickel, TiO2 nanotubes, Magnetization, FORC. 1

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Graphical Abstract

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1. Introduction In the past few years, porous metal oxides such as TiO2, Al2O3 and ZrO2 with self-organized tubular structures have attracted lot attention for their desirable catalytic, electrical and optical

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properties in diverse applications such as photocatalysis [1], photoelectrochemical [2], Li-ion batteries [3] and dye-sensitized solar cells [4, 5]. Morphology and regularity of produced TiO2 nanotube arrays by anodic oxidation have important impacts on the efficiency of these devices. The

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anodic oxidation of titanium generally associated with production of rough TiO2 nanotube arrays. Hence, to achieve highly ordered TiO2 nanotubes a multistep anodizing procedure is needed [6, 7].

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Titanium oxide nanotubes as a wide band gap semiconductor are the material of choice for the applications, however, its usage is restricted by UV region optical absorption band [8] and electron transport regime in form of electron hopping between neighboring chains [9]. In order to enhance the optical/electronic/electrochemical activity properties of TiO2

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nanotubes, several works proposed including (i) doping with impurities like nitrogen, (ii) increasing the ordering of the nanotube arrangement [10], and grafting metallic nanoparticles by electrochemical deposition and other methods onto TiO2 nanotubes [11]. Among the metallic

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nanoparticles grafted by electrodeposition on TiO2 nanotubes, nickel has been much appreciated by

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its desirable bio-sensing [12], energy storage [13] and photocatalytic [14] properties. Hybrid electronic devices comprising of TiO2 as a wide-band gap or insulating material with desirable conducting or ferromagnetic materials have been developed recently for applications. One example is metal/TiO2 based memristive switch or amplifier in electronic devices [15]. Ferromagnetic materials have also been employed to make hybrid magnetic/TiO2 devices for magneto-inductive temperature sensor. This device was consisted of a soft magnetic FeCrSiBCuNb nanocrystalline wire and a capacitive TiO2 microscale element for humidity detection [16]. Another example of such magnetic/TiO2 hybrids is iron oxide/TiO2 hybrid nanoparticles which exhibited 3

ACCEPTED MANUSCRIPT high photocatalytic efficiency with recoverable potential capability for cleaning polluted water with the help of magnetic separation [17]. Other examples are N-doped TiO2/ZnFe2O4 hybrid system was also reported as an efficient and recoverable photocatalyst for degradation of aqueous organic

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pollutants [18]. Therefore, demands for fabricating nanoscale magnetic hybrid device materials with high quality and interesting properties for magnetic sensor and spintronic applications have motivated a few studies towards the electrodeposition of nickel as a ferromagnetic metal on TiO2 nanotube

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arrays. To our knowledge, there is a little literature devoted to magnetic properties of

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electrodeposited nickel nanostructures on TiO2 nanotube arrays [19-22]. In this study, we report the effect of surface preparation and anodization time to achieve highly ordered TiO2 nanotubes, as an appropriate template for electrodepositing nickel nanostructures. The influence of various morphologies of electrodeposited nickel obtained by different electrodeposition techniques like direct current and cyclic voltammetry deposition techniques onto highly ordered TiO2 nanotubes on

Methods and Materials

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magnetic properties using very careful magnetic analysis data is demonstrated.

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a. Titanium surface preparation

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Commercially accessible pure titanium sheets (99.8% purity, 700 µm thick) were cut in 1×2 cm2 pieces and used as the electrode substrate. Initially, Ti was grinded using commercial abrasive sand papers (100 to 1500 grades). The chemical polishing was performed on the grinded titanium surface by soaking them in HF + HNO3 mixture (1:3 volume ratios) for 40 s. The mechanical polishing was carried out by polishing the grinded Ti with alumina microparticles (0.3µm). After polishing, the titanium substrates were cleaned by the double distilled water. In order to eliminate any residual pollutants from Ti surface, the samples were rinsed ultrasonically in acetone and ethanol for 15 and 10 minutes, respectively and then dried. 4

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b. Fabrication of highly ordered TiO2 nanotubes (TNTs) on Ti substrate In order to produce highly ordered TiO2 nanotubes, two-step anodic oxidation technique was

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conducted in two electrode electrochemical cell. The anodization electrolyte consisted of ethylene glycol, 0.25 wt. % HF and 4 wt. % H3PO4. A polished Ti surface was achieved via two different techniques including mechanical and chemical processes. Ti substrate was used as an anode and the

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cylindrical stainless steel served as a cathode. To reach the best morphology as a template for electrodeposited magnetic nickel nanoparticles, the effect of first anodization time was investigated.

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Hence, the samples were anodized at various first anodization times of 15, 30, 60 and 120 min, respectively. The potential of 60V was applied to the anodization cell at ambient temperature via a direct current (DC) power supply (Model HY30001E, MASTECH). The electrolyte was rigorously agitated and the current was recorded versus time by a digital multimeter (ESCORT 3146A Dual

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Display Multimeter) during anodization. To elimination of prepared oxide layer in first anodization, the specimens were ultrasonically soaked in double distilled water at ambient temperature. Afterward, the second anodic oxidation was performed at identical situation as described earlier for

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20 min. Samples were rinsed with double distilled water and dried.

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c. Direct current electrodeposition of Ni The magnetic nickel particles were electrochemically deposited on modified TiO2 nanotubes. The galvanostatic conditions were conducted to graft of nickel onto TiO2 nanotubes. The modified TiO2 nanotubes and graphite served as a cathode and anode, respectively. The distance between two electrodes was adjusted 2 cm, approximately. The current density of 50mA.cm-2 was used in order to deposition of nickel for 15, 30 and 60sec (MASTECH DC Power Supply HY30001E) in a Watt bath comprising of 2.3 M nickel sulfate, 0.8 M nickel chloride and 0.3 M boric acid.

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ACCEPTED MANUSCRIPT d. Cyclic electrochemical deposition of Ni Cyclic voltammetry (CV) deposition was employed in electrodeposition bath containing 0.05 M NiSO4 + 0.32 M H3BO3 at room temperature. The electrodeposition solution was prepared by

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dissolving analytical grade chemicals in the deionized water. A typical three electrode cell including the reference electrode (saturated calomel electrode (SCE)), the counter electrode (platinum foil) and the working electrode (TiO2) was connected to a computer controlled potentiostat/ galvanostat (ZAHNER IM6). Potential was swept in a range from 0 to -1.5V in

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forward and return directions at the scan rates of 20 and 50 mV.s-1. The pH was adjusted at 2 and 5

e. Characterization

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by adding an adequate amount of H2SO4 and NaOH solutions, respectively.

Field emission scanning electron microscopy images and EDX mapping were carried out with Supra, Carl Zeiss FESEM. Atomic force microscopy (Ntegra Aura, NT-MDT) was utilized to

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investigate the surface morphology of polished substrates. Atomic force microscopy (Ntegra Aura, NT-MDT) also was employed to investigate the structure topography of surfaces studied. High resolution transmission electron microscopy accompanied by X-ray microdiffraction (HRTEM,

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Libra 200, Carl Zeiss) was used to observe the crystal structure and diffraction pictures. The

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magnetic properties (hysteresis loops, magnetization curves, first order reversal curves) were studied with a vibrating sample magnetometer (VSM 7401, LakeShore).

Results and discussion

Anodic TiO2 nanotubes/Ti substrate: substarte roughness, morphology and structure The effect of Ti substrate surface roughness and first step anodic oxidation was investigated as important factors in multi-step anodization. Former studies demonstrated multi-step anodization effective factors on the produced final morphology of TiO2 nanotubes [23, 24]. Accordingly, it is 6

ACCEPTED MANUSCRIPT essential to produce TiO2 template with a desirable tube ordering and morphology for grafting nickel as a ferromagnetic metal for hybrid ferromagnetic/semiconducting devices. The anodic oxidation was carried out on a Ti substrate with the different surface roughness and first anodic

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oxidation time. The surface quality of Ti substrates

polished

mechanical

and

was

compared.

Atomic

force

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methods

employed surface

(AFM) to

was

investigate

morphology

roughness.

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chemical

polishing

microscopy

Figure. 1 AFM images of Ti substrate prepared by (a) mechanical polishing and (b) chemical polishing. (The lower panels show height amplitude

by

Figure

and 1a,b

illustrate the AFM images of cleaned titanium polished by

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mechanical and chemical methods, respectively. It is obvious that a number of scratches are present on mechanically polished Ti surface with a rough topography (Fig 1a). After chemical polishing of

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Ti, the surface morphology has been substantially modified by eliminating the linear scratches (Fig. 1b). Line scan profiles measured from AFM images as shown in Fig. 1 were utilized to determining surface roughness defined as the average height difference between the highest point (hill) and the bottommost point (valley). Accordingly, the chemical polishing improved the surface roughness with eliminating any specific texture on the surface. This is very similar to what we have achieved by electropolishing, reported elsewhere [25], however, the chemical polishing is a quick and more convenient method. The effect of Ti surface roughness on the uniformity and regularity of TiO2

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ACCEPTED MANUSCRIPT nanotubes was recently reported [26] which confirms our strategy demonstrated here to for improving the ordering of tubular film and subsequent applications like grafting of metals. Figure 2a represents the current transient curves recorded during anodization of Ti substrates

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prepared by the two polishing methods. As is evident, three fundamental steps as marked on the curves exist which form a nanotubular structure. In Step 1, where a sudden

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current drop takes places,

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a dense barrier layer is developed as soon as the anodizing applied.

voltage

is

Alternatively,

an increase of current up to a peak. This behavior

Figure 2 (a) Current transients recorded during anodic oxidation of mechanically and chemically polished Ti, and FESEM images of TiO2 nanotubes produced by two step anodization with different first anodization time of (b) 30min on chemically polished Ti, (c) 120 min on chemically polished Ti and (d) 120 min on mechanically polished Ti substrate.

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is due to the fluorine ions

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Step 2 is accompanied by

attacking the oxide layer

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and creating pits, which form the pores under a field-assisted dissolution mechanism on the preferred zones. Finally, the growth of the nanotubular oxide film will decrease the current density and the steady state condition will be readily equalized the dissolution and growth of oxide [27, 28]. As is evident from the current transients, Step 2 is more pronounced for chemically polished Ti substrate, where the peak current is also greater than that of the mechanically polished Ti. It can be related to preferred zones. As a matter of fact, by chemical polishing, the achieved surface has a more uniform surface with less roughness, as shown by the AFM images. Hence, the surface is totally activated and the more surface will be exposed to the fluorine electrolyte. It means the 8

ACCEPTED MANUSCRIPT number of preferred nucleation sites of pores increases and consequently, the dissolution occurs on more regions, which lead to increasing current. Typical SEM images of nanotubes achieved after two-step anodization on mechanically and

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chemically polished Ti with different ordering are illustrated in Figure 2b-d. As is obvious, the ordering of nanotubes is enhanced when the first anodization time increases, as has formerly been represented [29]. The diameter of tube mouth and its distribution over the surface prove a significant increase of ordering degree with an increased first step anodization time. According to

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Fig. 2d, the top surface of tubes for mechanically polished sample is quite rough and non-uniform,

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whereas it is smoother for the tubes grown on the chemically polished Ti, Figs 2 b,c. It probably attributed to imprints that formed on barrier layer, as reported elsewhere on effect of first anodization time on the surface smoothness [29]. Indeed, the preferred zones on chemically polished barrier layer will be further distributed in two-step anodization. Undoubtedly, the achieved template by chemically polished surface is more appropriate than the mechanically polished one for

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grafting magnetic nickel particles.

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Morphology and structure of Ni nanoparticles DC electrodeposited on TiO2 nanotubes

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Nickel was galvanostatically electrodeposited on highly ordered TiO2 nanotubes grown anodically on chemically polished Ti substrates. Figure 3 indicates typical SEM micrographs of Ni/TiO2 electrodeposited under a constant current density of 50 mA.cm-2 for different times. It manifests that the morphology of metal nickel structures is significantly affected by the electrodeposition time. For short electrodeposition times (i.e. 15 s, Fig. 3a), it is seen that individually dispersed spherical Ni particles with irregular scattering have formed on TiO2 nanotubes. Transmission electron micrographs which will be shown later (in Figure 5c) indicate that no pore filling is visible as a consequence of reduction reactions across either inner wall of 9

ACCEPTED MANUSCRIPT tubes or outside the tube on top surface. We understand that the electron transport through nanotube walls takes place based on oxygen vacancies or Ti+3 defects known as electron hopping model [30]. This has made possible Ni2+ ions reaching on arbitrary preferential nucleation sites across the tube wall or on top surface of nanotublar oxide film are electrochemically reduced which further

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enlarged by subsequent growth. This is seen, when the electrodeposition was continued to longer periods up to 30 s, Fig. 3b. There are no free standing Ni particles formed any more, however, they start to coalesce forming Ni films which encompass a quasi-continuous structure with triple

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junctions and pores between granules as previously reported in [31]. For further growth of nickel (e.g. longer electrodeposition time 60 s, Fig. 3c), the entire top surface of the nanotubular surface is

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covered by nickel film having three dimensional growth. This type of electrodeposition of materials

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on TiO2 nanotubes was also reported [32] whose findings are consistent with our results.

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Figure. 3. FESEM images of Ni electrodeposited by direct current (DC) onto highly ordered TiO2 nanotubes under a constant current 50mA.cm-2 for (a) 15, (b) 30 (c) 60 and (d) 120 s. Inset to (a) shows size distribution of Ni particles before being coalesced.

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Morphology and growth mechanism of Ni nanoparticles electrodeposited on TiO2 nanotubes by cyclic voltammetry deposition

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We have recently reported on the capablity of cyclic voltammetry deposition to graft well dispersed fine Ni particles on TiO2 [33]. Typical cyclic voltammograms used to graft Ni particles at different sweep rates with corresponding SEM images are indicated in Fig. 4. The reduction of nickel starts at -0.42 V vs. SCE which is associated with the hydrogen evolution intensified on nickel itself as a strong electrocatalysts at a potential of -0.88V vs. SCE. This potential is more negative than the reduction potential of nickel meaning that high exchange current density and low hydrogen evolution overvoltage on nickel persist. In case of electrodeposited nickel at the scan rate of 20mV.s-1, the intersection between forward and return sweeps is observed, as is obvious in Fig. 11

ACCEPTED MANUSCRIPT 4a. The extracted data from the cyclic voltammograms are listed in Table 1. This shows that at higher overpotentials, the bulk deposition of nickel takes place. Owing to the drastic hydrogen evolution, this point is not visible in the voltammogram recorded at a scan rate of 100mV.s-1 (Fig. 4b). In accordance with Table 1, the oxidation of nickel occurs at shifted values to more positive

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potentials, as the scan rate rises. The increase in the currents which is the result of scan rate augmentation illustrates that the diffusion is the dominant mechanism of reaction [33].

containing 50 mM NiSO4 + 20grL-1 H3BO3 at different sweep rates.

Ep,red (V)

20 mV/s

-0.590

100 mV/s

-0.745

Ip,red

2 (mA/cm )

Ep,ox

Ip,ox

(V)

2 (mA/cm )

-0.589

-0434

0.619

-1.842

-0.417

1.615

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Scan rate

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Table 1. Electrochemical reduction data extracted from cyclic voltammograms taken from aqueous solution

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As is evident from the FESEM images (Fig.4), the nickel particles morphology is extremely dependent upon the CV deposition scan rate. Indeed, the nanoparticle size is considerably decreased, as the scan rate increases from 20mV.s-1 to 100mV.s-1 which is in consistent with our

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last publication [33]. This is clear from extracted diameter distribution histograms shown as insets to the corresponding SEM images in Figs. 4a and 4b. As a consequence, when the number of nuclei

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per surface area per time rises, the amount of nuclei or nucleation rate is determined. In fact, in spite of low nucleation rate at the scan rate of 20mV.s-1 , however, the time-accessible for the growth remains long enough. Accordingly, the achieved nanoparticles at the scan rate of 100mV.s-1 are considerably smaller than those created at the scan rate of 20mV.s-1, as denoted by arrows on the SEM image shown in Fig. 4b. Also, the stable nucleation of nickel on tube surfaces is approved by the existence of an intersection point in the cyclic voltammogram recorded at a scan rate of 20mV.s-1. Having said that, no intersection point is seen on the cyclic-voltammogram recorded at a

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ACCEPTED MANUSCRIPT scan rate of 100mV.s-1 implying of the dissolution of reduced nickel embryos with sizes less than a thermodynamically critical size in the return sweep. Obviously, no stable particle is seen in taken

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FESEM image from produced nickel at the scan rate of 100mV.s-1.

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Figure 4 - FESEM and cyclic voltammograms of CV deposited Ni on TiO2 taken at a scan rate of (a) 20 mV.s-1 and (b) 100 mV.s-1. Insets on SEM images show the diameter distribution histograms with measured average diameter. (c) A typical cyclic voltammogram taken from Nielectrolyte on TiO2 nanotunes demonstrating the definition of effective parameters used for CV deposition.

Comparison between the morphology of Ni nanoparticle growth on TiO2 nanotubes by DC and cyclic voltammetry techniques

The main difference between the structure and morphology of Ni nanoparticles electrodeposited by DC and cyclic voltammetry electrodeposition can be seen in SEM and TEM shown in Figures 3 - 5.

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Figure 5 - (a) A typical SEM image of an empty TiO2 nanotube array. HRTEM images of an empty TiO2 nanotube (b), a TiO2 nanotube with Ni electrodeposited by DC technique for 120 s (c), a TiO2 nanotube with Ni deposited by cyclic voltammetry for 5 cycles at a scan rate of 100 mV.s-1 from the tube mouth (d) and of that from the middle of the tube (e), and of a Ni film firstly deposited by cyclic voltammetry for one cycle at a scan rate of 20 mV.s-1 and then exfoliated from the TiO2’s tube wall (f). The

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inserts consist of the corresponding SAED patterns.

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HRTEM provide careful insights into the morphology and microstructure of Ni electrodeposited on TiO2 nanotubes by different electrodeposition techniques. Figure 5 demonstrates images of empty TiO2 nanotubes (a,b) and nanotubes filled by Ni using DC (c) and CV deposition (d-e). The SAED pattern shown in Figure 5(b) implies that an empty TiO2 nanotube indicates the amorphous phase. Electrodeposited nickel normally exhibits fine grain crystalline microstructure. Indeed, the SAED pattern obtained for Ni/TiO2 hybrid nanostructure fabricated by DC electrodeposition of Ni for a definite time (i.e. 120 s) shows polycrystalline structure as shown in Fig. 5(c). It is not the case any more for Ni deposited by CV on TiO2 nanotubes. Based on the SAED patterns obtained for Ni nanoparticles deposited by CV method, it is evident that at top mouth of the nanotubes, Fig. 5(d), Ni exhibits a pseudo-single fcc-crystalline microstructure with 15

ACCEPTED MANUSCRIPT a pronounced textured growth direction along <111> plane, but Ni nanoparticles grown in the middle of TiO2 nanotubes are polycrystalline without any preferred growth direction, Fig. 5(e). During the CV electrodeposition, Ni growth takes place inside the nanotubes and a solid polycrystalline Ni nano-grain film is formed Fig. 5(f). The grain size of Ni nano-grains estimated from TEM images is less than 10 nm.

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We understand that DC electrodeposition favors the granular film formation on the TiO2 porous template, while CV deposition technique forms fine

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nanoparticles forming across the tubular TiO2 whose Ni distribution exhibits kind of nanoporous films

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matching the membrane morphology with ultrafine grains. It is hard to distinguish the Ni nanoparticles deposited by CV method because they are very fine

and well distributed across the internal and external

tube walls. For CV deposited Ni/TiO2 hybrid

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material, the EDX mapping shows that Ni is

homogenously distributed in the nanotubes as

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illustrated in Figure 6.

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Fig. 6. A cross-sectional SEM image and corresponding EDX elemental mapping of nickel grown by CV deposition on TiO2 nanotubes.

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Figure 7 – Schematic explanation of the Ni film formation by DC (a-c) and CV (d-f) electrodeposition. (a) – an empty TiO2 nanoporous template, (b) – after 30 s of DC electrodeposition, (c) - after 120 s of DC electrodeposition. CV deposition at the scan rate 20 mV/s for 1 cycle (d), 2 cycles (e), 5 cycles (f).

From structural and morphological evaluation, we reach this conclusion as described here. We have

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depicted pictures of Ni nanostructures grown on TiO2 nanotubes by different methods under various controlling deposition parameters. In case of DC electrodeposition, Ni mainly forms on the top of the

TiO2 porous template. During electrodeposition, the growing particles on the template surface block

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the nanopores preventing nanowire or granule formation inside the nanotubes, Figs. 7(a-c). Thus, as

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it will be discussed later, the main contribution to the magnetic properties is arisen by the particulated film grown on the top of the matrix. In contrast, in case of CV deposition, ultrafine grains grow onto either the inner wall or outer top surfaces of TiO2 nanotubes. The nanogranular nature of Ni with a grain size of less than 10 nm does not allow the pores block which ensures the growth of a relatively continuous nickel films in form of nickel nanotubes across the tube wall which replicates the TiO2 template structures, Fig. 7 (d-f). There is an antidot film formed on the template surface. The increased number of deposition cycles forms thicker films. The increased scan rate leads to the decrease of the grain size and increased level of the film’s homogeneity.

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In order to understand the magnetic properties of Ni nanostructures electrodeposited on TiO2, we have analyzed the hysteresis loops (Fig. 8) and summarized the variation of coercivity (Hch) and squareness (Mr/Ms), where Mr and Ms are remanence and saturation magnetization, as a function of

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the electrodeposition time as shown in Table 2. The magnetic hysteresis loops were measured for Ni electrodeposited on TiO2 nanotubes for different time ranges implying the presence of magnetic

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anisotropy, which changes with the increasing deposition time. The in-plane and out-of-plane configurations were characterized, which shows that by increasing the time, the in-plane hysteresis

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loops become saturated much easier than the out-of-plane ones.

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Figure. 8. Room temperature in-plane (IP) and out-of-plane (OOP) hysteresis loops of Ni electrodeposited by direct current (DC) onto highly ordered TiO2 nanotubes under a constant current 50mA.cm-2 for (a) 15, (b) 30 (c) 60 and (d) 120 s.

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Table 2 Magnetic data extracted from the FORC-diagrams and hysteresis loops measured at room temperature for Ni

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electrodeposited on TiO2 nanotubes at 50 mA.cm-2 for different electrodeposition times (tdep).

15

tdep, s In-Plane (IP) Out-ofPlane (OOP)

30

60

HcF, Oe 50

Hch, Oe 85

Mr/Ms

0.23

∆Hu, Oe 31

0.12

50

-

160.0

HcF, Oe 70

Hch, Oe 70

Mr/Ms

45

78

HcF, Oe -

Hch, Oe 80.0

Mr/Ms

0.28

∆Hu, Oe 29

0.11

32

-

100.0

19

120 HcF, Oe 70

Hch, Oe 92

Mr/Ms

0.23

∆Hu, Oe 89

0.35

∆Hu, Oe 37

0.10

78

-

158

0.13

80

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coercivity remains almost constant. The squareness also changes as in IP it initially increases from 15 to 30 s and then decreases for 60 s deposition time. However, the OOP squareness decreases from 15 to 60 s deposition time. Such behavior of Hch and Mr/Ms is equivalent to hemispherical Ni

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nanoparticles electrodeposited on Si substrate [31]. The variation of coercivity and squareness values by the electrodeposition time is likely due to the significant change between different

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magnetic ground states. We previously reported the magnetization behavior and states of Ni electrodeposited on silicon , which seems to develops the same structural features as we observed for Ni/TiO2 here. It enables to define the surface coverage or concentration (CNi) by nanoparticles known their magnetic parameters: for tdep=15 s CNi=16%, for tdep=30 s CNi=25%, for tdep=60 s

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CNi=60% and for tdep=120 s CNi=95%.

The theoretical coercivity value for bulk nickel at room temperature is 2K1/Ms=140 Oe for

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Ni particles [34], where K1 is the anisotropy constant and Ms at RT. The deviation of coercivity values from the bulk one can be attributed to the demagnetizing factor change and possible

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magnetostatic interaction between the magnetic dots or particles or inside the patterned or partciualted film [35, 36].

To analyze the remagnetization of Ni nanoparticles and particulated films, we could distinguish different magnetization states including single domain, vortex and multi-domain states depending on material’s exchange length, magnetocrystalline and/or magnetoelastic anisotropy, and/or geometrical parameters including radius and shape of the particles. The critical particle volume of superparamagnetic behavior can be defined as [37]:

20

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/

,

(1)

where k – Boltzmann constant, T=300 K, K1=-5.6×104 erg/cm3 – uniaxial anisotropy constant for bulk Ni. For a hemispherical nanoparticle one can find the critical radius of superparamagnetic

=

) can be found as [38]:

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behavior Rs=20.64 nm. The single-domain critical diameter (

.,

(2)

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where A=8.6×10-7 erg/cm – exchange stiffness constant for Ni, Ms=484 emu/cm3. As a result, Rsd=27.0 nm. At the diameter above Rsd a vortex state or a multi-domain pattern is energetically

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more favorable in a Ni nanoparticle. Since the internal diameter of TiO2 nanotubes or pore diameter is below 80 nm, we can propose that inside the nanotubes Ni nanoparticles are either predominately superparamagnetic or single-domain. The paramagnetic contribution of these small particles can be seen due to no completely saturated tails of the hysteresis loops up to high magnetic fields

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(H>5kOe), Fig.8.

A particle size histogram on the TiO2 surface measured from SEM images of Ni electrodeposited for 15 s shows that the average Ni particles are 375 nm in diameter, Fig.3a. It

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means that most of them have to be either in a vortex or a multi-domain state. This explains the

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small value of squareness, because at R>Rsd the magnetization tends to close the magnetic flux in each nanoparticle.

Accordingly to Ref. [31], if one knows the surface concentration of Ni nanoparticles and their size, it is possible to define the magnetization reversal mechanism. Thus, samples deposited at 15 and 30 s with CNi < 53% are remagnetized through the incoherent magnetization rotation of single-domain particles (especially, inside nanotubes), which size is below 54 nm, and the 3D magnetic vortex nucleation in the larger nanoparticles. The magnitude of Hc increases for the sample deposited at 30 s, because of the rising dipolar interaction between the becoming closer 21

ACCEPTED MANUSCRIPT each other nanoparticles. In samples with CNi>53% the nanoparticles start to form agglomerates with multi-domain structure with closed magnetic flux ruled by the minimum energy law. It leads to the decrease of Hch and Mr/Ms for the sample deposited at 60 s. The sample with tdep=120 s has the practically solid Ni film with structural microdefects like pores and junctions, which pin the in-

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plane field driven domain walls. This is why the magnitude of Hch and Mr/Ms increases.

The first order reversal curve (FORC) diagram method has been carried out to understand our calculation on the magnetic properties of Ni/TiO2 obtained by DC and CV electrodeposition for

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different times. A FORC distribution is based on an algorithm, proposed in the paper [39] for an

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analysis of the Preisach model. To draw a FORC diagram for an array, we have measured a family of minor reversal curves for different magnetic fields in a range from +5 to -5 kOe. First, a sample was saturated in the field Hs=5 kOe and then the field was decreased to Hr, where the magnetization M was measured in the field H, which was increasing from Hr to Hs with the step 50 Oe. For each sample we measured 100 first order reversal curves in an external field applied parallel and

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perpendicular to the easy axis of magnetization. FORC distribution ρ(H, Hr) was defined as mixed derivatives of the second order taken from M(H, Hr): ) = − !

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,

" # $,$% ) "$"

%$&,

(3)

To improve the usability of FORC diagrams, we used a new set of coordinates (Hc, Hu), which are =

$($% )

and

=

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defined as

'

)

$*$% )

.

Figure 9 represents FORC diagrams for samples electrodeposited at different time from 15 to 120 s. The evolution of shape of the FORC diagrams allows analyzing the magnetic behavior of samples regarding to the structural modification occurred. One can mention that the hysteresis loops look near the same, but FORC diagrams are very different reflecting spatial and size distributions of Ni nanoparticles. The deposition at 15s promotes formation of Ni nanoparticles inside of nanotubes on their walls accompanied by the surface growth of larger nanoparticles, Figs. 22

ACCEPTED MANUSCRIPT 3 and 5c. The corresponding FORC diagram is symmetric regarding to the origin, Fig. 9a. The peak value of the IP coercive force taken from the FORC diagram (HcF) is the same as found from the hysteresis loop (Hch), Table 3. From the Hu-profile (Fig. 9 (a)) one can find the interaction field as ∆

)

=

$,- *$,.

, where

* )

( )

and

are values of Hu measured at the half-height of the curve in

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positive and negative sectors, respectively. For the sample electrodeposited during 15 s ∆Hu=31 Oe indicating a mean dipolar interaction between nanoparticles on the TiO2 surface. The increased deposition time to 30 s does not significantly transform the FORC diagram, Fig. 9b. However, the

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high coercive force area near the HcF peak moves towards lower values. The magnitude of the

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interaction field practically does not change if to compare with tdep=15 s: ∆Hu=29 Oe. The picture drastically changes, if tdep increases to 60 s, Fig. 9c. As seen, in this case it is impossible to define HcF, because the growth-driven structural transformation leads to the wide size distribution of nanoparticles characterized by the switching field ranging from 0 to 200 Oe indicating the simultaneous presence of superparamagnetic, single-domain and multi-domain particles. Moreover,

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the interaction field rises up to 89 Oe. This is because of the dense nanoparticle array formation with the stronger dipolar coupling between neighbors on the sample surface, Fig.3c. Further

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increase of the deposition time to 120 s promotes the particulated film formation characterized by

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the small interaction field ∆Hu=37 Oe.

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Fig. 9. IP FORC diagrams and corresponding Hc- and Hu-profiles of DC electrodeposited Ni on TiO2 nanotubes with

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different electrodeposition times (a) 15 s, (b) 30 s, (c) 60s and (d) 120 s. The insets demonstrate hysteresis loops.

Since the growing particulated film blocks the nanopores, the volume of the magnetic

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material inside of NTs is relatively small to give a significant contribution to the magnetic behavior. This fact is supported by the FORC-diagrams (Fig. 9), which have no high value peaks along Hcaxis pointing out a nanowire-like structure formation in NTs.

Magnetic properties of cyclic voltammetry deposited Ni/TiO2 nanotubes hybrid materials As discussed in section of morphological studies which were demonstrated by SEM and TEM micrographs as well as skecthed by a schematic picture shown in Figs. 3-7, CV deposited 24

ACCEPTED MANUSCRIPT Ni/TiO2 hybrids exhibit a top nano-grain layer of Ni film which replicates the tubular structure of the TiO2 template morphology. Such nanostructured systems are named as antidot films [40, 41]. Nanoholes in these films play the role of magnetic deffects, which pin domain walls resulting in the magnetic hardening relative to the granulated Ni films with Hc≈80 Oe [31]. Accordingly the

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scheme shown in Fig. 7(d-f), we have rather complex magnetic structure, which consists of the antidot Ni film on the surface and the nanowire-like Ni elements inside TiO2 NTs forming a magnetostatically coupled array.

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Figure 10 shows room temperature FORC diagrams and corresponding magnetic hysteresis

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loops measured for Ni electrodeposited by CV at a scan rate of 20 mV.s-1, but with different number of deposition cycles including 1, 2 and 5. We have extracted the main magnetic parameters for three

series of samples electrodeposited by CV deposition at three scan rates of 20, 50 and 100 mV.s-1 and summarized them in Table 3. The corresponding FORC diagrams of samples CV electrodeposited at 50 and 100 mV.s-1 are presented in the Fig. S1 and S2 of the Supplementary File. The increasing

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scan rate leads to the decreasing grain size, making Ni films more homogenious. As explained in above, particle size histograms calculated on the SEM images reveal an average particle diameter of

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38 and 8 nm for Ni electrodeposited by CV at scan rates of 20 and 100 mV.s-1, respectively. The increasing number of scan cycles brings thicker Ni films and longer NW-like elements in NTs.

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The coercive force in such complex system is driven by many sources including the DW pinning depending on the nanohole size and distance between holes, the nominal thickness of deposited Ni and the dipolar coupling between NWs in NTs. The FORC diagrams calculated for CV deposited Ni/TiO2 hybrid materials at different scan rates (Fig.10, Fig.S1 and S2 in The Supplementary File) reveal the larger values of HcF than Hch, which is due to the nanohole pinning effect of the surface film and internal interaction between the nanowire-like structure inside NTs. The average value of HcF for the scan rates 20 and 50 mV/s is about 340-370 Oe, which is attributed to the formation of NWs inside pores [42]. Moreover, if to look at the form of FORC distribution 25

ACCEPTED MANUSCRIPT for the OOP configuration (Fig. 10 (b,d,f), the characteristic wishbone-like shape is obvious, which is a fingerprint of the existence of the dipolarly coupled NW array formation [42-44]. The variation of coercivity is a sign of change of magnetization modes due to the complex structural modification depending on the CV electrodeposition parameters. There are two peaks on the Hc-profile of the IP

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FORC diagram for the Ni structure deposited for 1 cycle, Fig. 10 (a). The low-Hc peak corresponds to the coercivity of the antidot film, while the high-Hc peak is attributed to the NW array. In this field configuration the film and the NW array are decoupled. However, with the increasing number

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of cycles the both subsystems become coupled and the peak splitting is vanished as seen in Fig.10

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(c,e).

Table 3- Data of magnetic properties of Ni/TiO2 hybrid materials obtained by CV deposited of Ni at different scan rates

20 H cF

IP

73; 338

190

350 191 370 344

200 83 94 115 195

Mr/Ms

∆Hu

50 H cF

H ch

Mr/M

∆Hu

100 H cF

H ch

Mr/Ms

∆Hu

s

0.23

80

-

0.28 0.24 0.18 0.22 0.26

60 79 125 55 90

345 282 233 340

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OOP IP OOP 5 IP OOP and cycles. 2

H ch

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1

scan rate (mV/s)

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cycles

26

54

0.04

-

90

92

0.17

66

52 148 175 166 197

0.05 0.14 0.21 0.19 0.22

80 80 76 85

99 112 265 111 80

94 129 140 68 70

0.16 0.33 0.33 0.06 0.04

74 45 110 70 85

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Fig.10. FORC diagrams for CV deposited samples (scan rate 20 mV/s): 1 cycle (a,b), 2 cycles (c,d), 5 cycles (e,f) measured in IP and OOP geometries, correspondingly.

27

ACCEPTED MANUSCRIPT Conclusion In conclusion, we have shown that electrodeposition of ferromagnetic metals like nickel on oxide nonporous templates is a powerful and controllable fabrication method of ferromagnetic/oxide

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hybrid device materials. Using DC electrodeposition, a wide range of nickel particles is developed mainly on top surface of TiO2 nanotube arrays, while cyclic voltammetry deposition comprising of deposition and dissolution sequence repeated for a definite number of cycles provides fine grain nickel all over the internal and external surface of TiO2 nanotube arrays. This is much important

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when a high surface material is required for sensing or other device applications. While magnetic

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hysteresis loops provided only integral magnetic parameters of the whole nanoparticle ensembles, the FORC diagrams gave the comprehensive data on the Hc and Hu field distributions enabling to distinguish contributions from the coalesced Ni nanoparticles and from individual Ni nanoparticles, which were inside of TiO2 NTs. This approach in combination with SEM and TEM techniques allowed to reveal the NT morphology dependent grows of Ni NPs during DC and CV

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electrodeposition and understand the magnetization reversal processes driven by Ni nanogranular (as in case of DC) or antidot/nanotube (as in case of CV) structure of the samples.

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Acknowledgements

The financial support of Iranian National Nanotechnology Council is acknowledged. This work was

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supported by Far Eastern Federal University within the research program “Materials”, by the Russian Ministry of Education and Science under the state task (3.5178.2017/8.9 and 3.4956.2017), by Act 211 of the Government of the Russian Federation (contract № 02.A03.21.0011). The TEM study was done in the Far Eastern Center of Electron Microscopy, IMB FEBRAS.

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