Controlling Magnetic Anisotropy of Permalloy/Cu by Electrochemical Deposition Electrochemical Deposition

3.2.1

Introduction

Chemically synthesized nano crystals are the focus of increasing research interest, because these materials can behave as artificial atoms or quantum dots and because they have unique self-assembly properties that can be exploited in the formation of two-dimensional (2D) and three-dimensional (3D) superlattices [1-6]. Nanowires are a focus of research interest because they can be used in one-dimensional nanosystems and potentially have exciting applications in nanotechnology, ranging from nanoelectronic devices, through cell separation, to magnetic labeling in biomedicine [7,8]. Numerous methods for preparing nanowires with novel optical, electrical, catalytic and magnetic properties from magnetic [9], semiconductor [10], inorganic [11], organic [12], polymer [13], metallic [14] and dielectric materials [15]. Magnetic multilayers are of particular interest because of their unique magnetic properties and their potential use in magneto-optic recording [16-18]. Currently, interest in the effects of spin transfer torque (STT) that is generated by a spin-polarized current in a nanosized ferromagnet is great. The main advantages of multilayer nanowires are that they exhibit zero-field microwave absorption that can be easily tuned over a large range of frequencies [19], and they are both low-cost and can be reproduced rapidly over large areas, unlike standard ferrite devices. A microwave circulator has been demonstrated and discussed in the systems of nanowires embedded in a porous polymer membrane. The device thus fabricated also has shortcomings, the polymer

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membranes have large dielectric losses [20,21], however alumina has been known to exhibit low dielectric losses [22]. Early experimental studies of Ni3Fe1/Cu nanowires with a high magnetoresistance ratio at 4.2 K of 78% [23], have indicated that the ratio depends on the magnetic anisotropy, the thickness of the nonmagnetic Cu layer and the state of spin alignment between pairs of two ferromagnetic layers [24,25]. The magnetoresistance ratio can be increased by the highly ordered alignment of spins in the ferromagnetic segments. The most effective method of so doing is to modulate the shape of the magnetic segments to increase the energy of perpendicular anisotropy and the spin diffusion length of Ni₃Fe₁ , calculated using the Valet-Fert model, is between 3.3 and 5.3 nm [26]. Multilayer nanowire systems of single element of ferromagnetic segment, as example, nickel, cobalt and iron, have been examined. Cho et al. [27] and Chen et al. [28] have developed the simple and fast method to study the

relationship between the shape morphology and nano-scale magnetic properties. They demonstrate that the nano-scale magnetic properties is strongly dependent on the aspect ratio as well as on dipolar coupling and enhancement in the anisotropy due to modulate the shape morphology. The dependencies of coercivity and remanence of the multilayer nanowires on the diameter and aspect ratio were demonstrated by both experimental data and micromagnetic simulations. However the role of dipolar interactions in ferromagnetic alloy multilayer nanowire arrays, such as Ni₃Fe₁/Cu, is not yet fully understood. This work explores a simple method that is based on anodic aluminum oxide (AAO) template synthesis for preparing Ni₃Fe₁ (permalloy)/Cu magnetic nanowires with controllable segment lengths and tunable perpendicular magnetic anisotropy. The magnetic dipolar interactions can be incorporated by a factor that is applied to shape anisotropy energy. The improvement in perpendicular magnetic anisotropy matches theory, and the results herein were also demonstrated by

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simulation of the nano-scale magnetism using a phenomenological model that was recently developed elsewhere [29,30]. Multilayer nanowires that are synthesized using an AAO template are suited to use in microwave devices with a large saturation magnetization and high resonance frequencies. for 16 h to prepare an AAO film. The as-prepared AAO film was dipped into 5 wt%

phosphoric acid for three hours to eliminate the obstructing film, and then a 500nm-thick silver film was evaporated onto one side of the template to act as a conductive contact.

Electrodeposition was performed at room temperature, using a three-electrode potentiostatic control with an Ag/AgCl reference electrode and a platinum wire as a counter electrode. The Ag-coated AAO template was used as the working electrode in an electrochemical cell, and with the porous side) exposed to the electrolyte. Solution was freshly prepared from NiSO₄·6H₂O, FeSO₄·7H₂O, CuSO₄.5H₂O and boric acid.

Ni3Fe1/Cu multilayered nanowires were grown in an electrolyte that contained 0.057 M Ni²⁺ ion, 0.008 M Fe²⁺ ion, 0.001 M Cu²⁺ ion and 0.1 M H₃BO₃. Alternative constant potentials of -1.3 V to deposit Ni3Fe1 segments and -0.3 V to deposit Cu segments were applied.

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The morphology of AAO templates was characterized by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The Ni3Fe1/Cu multilayer nanowires that were embedded in AAO were observed by transmission electron microscopy (TEM, Hitachi H-7100). X-ray powder diffraction (XRD, BL-01C2 NSRRC, Taiwan) was performed to elucidate the morphology and crystallographic structures. The deposition rates of permalloy and copper were measured using an electrochemical quartz crystal microbalance (EQCM, D3100 Digital Instrument). The magnetic hysteresis loops were measured using a commercial vibrating sample magnetometer (VSM) with fields of up to 4.5 T at room temperature. Measurements were made on a silica micro-slide that was held perpendicular and parallel to the magnetic field.

3.2.3

Results and discussion

Nanoscale templates have an important role in fabricating arrays of nanowires because the templates have porous with ordered hexagonal honeycomb structures.

Figure 7 shows an FE-SEM image of the porous template that is used in this work.

The pore diameter and interpore distance of the homemade template are 50 nm and 10 nm, respectively. The nano-scale pores were formed under the electrical field in the acid electrolyte, the pore diameters can be controlled by applied voltage and the length of the template is determined by the process time. The morphology of the homemade template is rough and tendency of the pore shape to be spherical rather than hexagonal, caused by etching of the barrier layer by the alkali solution. To modulate the magnetic anisotropy, the nanoscale template was fabricated with the

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diameter fixed at 50 nm, the thickness fixed at 35m and the pores density of 1.2 × 10¹⁰ pores/cm². Overall, the template exhibits an order two-dimensional array pattern and the aspect ratio (height/diameter) of the pores is maintained at approximately 700.

Figure 8 plots a typical curve of the corresponding EQCM responses of Ni3Fe1

and the Cu layer. A quartz crystal with a nominal frequency equal of 5 MHz, covered on both sides with an Au film, was used as the working electrode. The geometric area of the electrode was 1.37 cm². The EQCM signal f was recorded as a function of the change in mass at the working electrode. The experimental frequency change is expressed as

f = -Cf m Eq. (1)

where Δm is the change of mass per unit area in g cm^-2; Δf denotes the shift in the resonant frequency in Hz; Cf is the sensitivity factor of the crystal in Hz ng^-1cm² and the Cf m is called the Sauerbrey term. Two deposition potentials (–1.3V and –0.3V versus Ag/AgCl) were applied to deposit Ni3Fe1/Cu multilayer nanowires. The deposition rates of Ni3Fe1 and Cu, calculated from the curve of mass as a function of time, for unit area and average length in each segments of the nanowires, was confirmed from the TEM image. The deposition rates of Ni3Fe1 and Cu were 1.3 nm/s and 0.3 nm/s, respectively.

The crystal structure of the Ni3Fe1/Cu multilayer nanowires was eroded using aqueous solutions of NaOH and H3NO3 to remove the anodic alumina template and the silver electrode as the working electrode. It was characterized by X-ray powder

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diffraction (XRD) with a wavelength in 0.77 nm. Figure 9 presents the XRD pattern of Ni3Fe1/Cu multilayer nanowires in the 2θ region between 15° and 45° and compares it with the standard diffraction peaks of the corresponding constituent materials (JCPDS nos. 65-3244 and 85-1326). Reflection peaks are observed at 21.78°, 25.20°, 35.94° and 42.42°, corresponding to the (111), (200), (220) and (311) facets of Ni3Fe1, respectively, and at 21.40°, 24.76°, 35.29° and 41.65°, corresponding to the (111), (200), (220) and (311) facets of Cu, respectively. These results reveal that the crystal structures of Ni3Fe1 and Cu are both simple cubic. Due to the same crystal structure, the crystallinity of Ni₃Fe₁ segments can be grown well on the Cu segments. The crystallinity of the Cu layer exceeds that of the Ni3Fe1 layer, because the process to form the structure of the alloy requires more energy, but in this study, increasing the temperature of electrodeposition increases the diffusion of Cu impurities in the Ni₃Fe₁ layer, destroying the crystal structure. To prevent this problem, the electrodeposition process was performed at room temperature and the concentration of the Ni₃Fe₁ is controlled larger 57 times than Cu.

Figure 10 presents a typical TEM image of composite Ni3Fe1/Cu nanowires.

Alternating Ni₃Fe₁ layers and Cu layers are clearly observed. The average diameter of the nanowires is measured to be around 50 nm, consistent with the size of the pores in the homemade AAO template. The darker sections correspond to segments of Ni₃Fe₁ that are around 50 nm in length, and the brighter sections correspond to Cu segments of approximately 4 nm in length. The others different lengths of Ni₃Fe and Cu ssegments are shown in Fig. 10(b), 10(c) and 10(d) and the length of each segment was random to choose to calculate the deposition rates. The deposition rates calculated are also demonstrated with the EQCM results. The Ni3Fe1 layers can easily be distinguished from the Cu layers and the diameters of the Ni₃Fe₁ segments exceed

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those of the Cu segments, because the concentrations of Ni²⁺ and Fe²⁺ ions exceed that of Cu²⁺ by a factor of about 50. The concentration of Cu²⁺ ions is too low to resupply in the nano-scale channel and the diameter of Cu is less than that of Ni₃Fe₁. The selected area electron diffraction pattern (SAED) in Fig. 10 confirms the presence of the cubic phase Ni₃Fe and Cu and the result is matched with the XRD pattern. The nanowires that are obtained by double-pulsed deposition have a unique bamboo-like structure. The different layers are seen to have uniform thickness. The thickness of the alternating layers can be easily controlled by varying the deposition time of each layer.

The ferromagnetism of multilayer nanowires is effectively modulated by changing the aspect ratio of the length of the Ni₃Fe₁ segments to their diameter of the wire. Figure 11 plots M–H curves of the Ni3Fe1/Cu multilayer nanowires whose ferromagnetic Ni₃Fe₁ segments have aspect ratios of 3.0 (rod-shaped), 1.0 (intermediate), and 0.1 (disk-shaped) and the total length of multilayer nanowires are all the same equal to about 1 m. Table 1 presents the values of coercivity (Hc), saturation magnetization (Ms), residual magnetization (Mr), and squareness (Mr/Ms) of the hysteresis loops. An external magnetic field was applied parallel (∥) and perpendicular (⊥) to the axes of the nanowires. Figures 11(a) and (b) plot the magnetization curves in an applied parallel and perpendicular field for multilayer nanowires with diameters of 50 nm [Ni3Fe1(150 nm)/Cu(4 nm) ; rod-shaped], [Ni₃Fe₁(50 nm)/Cu(4 nm) ; intermediate] and [Ni₃Fe₁(5 nm)/Cu(4 nm) ; disk-shaped].

Figure 11(a) plots the magnetization curves obtained on the multilayer nanowires in an applied parallel field. The coercivity reveals the change in the shape of the Ni₃Fe₁ segments from disk-shaped to rod-shaped and the change in the easy axis from

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perpendicular to the wire axis to parallel to it (from 102 Oe to 252 Oe). Figure 11(b) plots the magnetization curves obtained of the multilayer nanowires in an applied perpendicular field. Similarly, the easy axis of the multilayer nanowires tends to become parallel to the wire axis, as revealed by the decline in coercivity (from 119 Oe to 80 Oe) as the aspect ratio of the Ni₃Fe₁ segments increases. The results reveal that the magnetic easy axis tends rapidly to become parallel to the axis of the nanowires as the aspect ratio increases, and the magnetic perpendicular anisotropy can be enhanced by modulating the length of ferromagnetic segments.

Figure 12 plots the coercivity and squarness (Mr/Ms) of hysteresis loops of the nanowires in applied parallel and perpendicular magnetic fields. It reveals typical transition behavior between the hard and easy axes of the hysteresis loop. As the aspect ratio of magnetic segments increases, the coercivity associated in a parallel magnetic field increases and the coercivity associated in a perpendicular magnetic field declines; the squareness varies similarly. The magnetic anisotropy moves from perpendicular to parallel to the wire axis and the easy axis of the system is parallel to the wires. This arrangement is characteristic of polycrystalline wires where the shape anisotropy dominates the intrinsic magnetocrystalline anisotropy, determining the magnetic behavior of the system.

Figure 13 plots the magnetoresistive hysteresis loop of Ni₃Fe₁(150 nm)/Cu(4 nm) multilayer nanowires that were electrodeposited into AAO templates and the top electrode, an Au pad, was evaporated on the AAO template before electrodepositon.

The magnetic field was applied parallel to the nanowires and the current through the nanowires from one side of Ni₃Fe to another side is shown in Fig. 13. The measurement conditions were as follows; the source current was 1 μA; the

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temperature was 298 K, and the magnet field was varied 4000 Oe to -4000 Oe. The resistance of the multilayer nanowires slowly increased as the magnetic field declined from a positive saturation field, and the antiparallel spin state was reached. Beyond the zero field, the resistance decreased as the magnetic field increased, until the spin parallel state was reached. The MR ratio was 26% when the magnetic field was parallel to the nanowires; the difference between the resistance in the antiparallel and parallel states is around 15 .

3.2.4

Conclusions

Anodized alumina templates have a network of locally hexagonally arranged channels with a diameter of 50 nm which were obtained in a two-step anodization process. Ni3F1 segments of various lengths and multilayer nanowires of various lengths were electrochemically grown using AAO as templates. The easy axis of the multilayer nanowires moved to the wire axis as the aspect ratio increasing, owing the large magnetic shape anisotropy energy. The perpendicular magnetic anisotropy was modulated by controlling the aspect ratio of the ferromagnetic segments. The magnetic/non-magnetic multilayer nanowires have potential applications as key components in the next generation of nanoscale perpendicular magneto-electronic devices, including perpendicular magnetoresistive random access memory devices.

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3.3 Spin-Valve Like Perpendicular Magnetic Tunnel Junctions