# Magnetic Nanowires

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

## 3. Results

#### 3.1. Magnetization Reversal Processes in Magnetic Nanowires

#### 3.2. Tunable Magnetic Anisotropy of Magnetic Nanowire Arrays

#### 3.3. Spin Transport in Multilayered Nanowires

#### 3.4. Interplay between the Magnetic and Magneto-Transport Properties in Interconnected Nanowire Networks

#### 3.5. Spin Caloritronics in Nanowire Networks

## 4. Discussion

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**SEM images of (

**a**) track-etch polycarbonate membrane and (

**b**) porous alumina; (

**c**) photograph and (

**d**) schematic of the electrodeposition cell used to grow nanowire arrays.

**Figure 3.**(

**a**,

**b**) SEM images of metal nanowires; (

**c**) schematic of Co/Cu nanowires embedded in a polymer template; (

**d**,

**e**) TEM image of Ni80Fe20/Cu and Co/Cu multilayered nanowires; (

**f**) SEM image of Ni nanotubes.

**Figure 4.**(

**a**) schematics of 3D nanoporous polymer template, (

**b**) crossed nanowire and (

**c**) crossed nanotube networks, and (

**d**) crossed multilayered nanowire network; (

**e**–

**h**) SEM images of self-supported interconnected nanowire and nanotube networks; (

**e**,

**f**) Ni NW network film with 230 nm diameter; (

**g**) Ni NW network film with 40 nm diameter; (

**h**) Ni crossed nanotube network with 230 nm diameter.

**Figure 5.**(

**a**) AMR characteristics at 300 K of a single 75 nm Ni nanowire for 2 angles between the wire axis and magnetic field direction. Inset: zoom of R(T) at low fields (from [17]); (

**b**) nucleation field ${H}_{\mathrm{n}}$ at 300 K vs. the angle between wire axis and field direction for a single 75 nm Ni wire (from [17]). Dashed line: expected nucleation field for an infinite cylinder 75 nm in diameter (curling reversal mode). Solid line: expected nucleation field for a volume of aspect ratio 4.3:1 (curling reversal mode). Figure adapted from ref. [17] with permission of the author.

**Figure 6.**(

**a**) microwave absorption spectra as a function of the applied field parallel to the wires of an array of Ni nanowires of diameter 100 nm and packing density of 27%. Continuous lines correspond to measurements made with the applied field value indicated by the numbers; (

**b**–

**d**) dispersion relations and the corresponding hysteresis loops measured in arrays of Ni nanowires with the field applied parallel (closed symbols) and perpendicular (open symbols) to the wires. The wire diameter and packing density are (

**b**) 180 nm, 4%, (

**c**) 100 nm, 27%, and (

**d**) 115 nm, >35% (from [19]).

**Figure 7.**(

**a**) effective field as a function of the pH for 30 nm Co nanowires electrodeposited at current densities of 5 mA/cm${}^{2}$ (squares) and 50 mA/cm${}^{2}$ (circles). The dashed line shows the expected value of ${H}_{\mathrm{EF}}$ for an array of Co nanowires ($P\approx $ 3.5%) assuming no contribution from magnetocrystalline anisotropy; (

**b**,

**c**) TEM images and corresponding diffraction patterns of two 30 nm Co nanowires electrodeposited at pH 5.2 at current densities of (

**b**) 5 mA/cm${}^{2}$ and (

**c**) 50 mA/cm${}^{2}$ (from [29]). Figure adapted from ref. [29] with permission of the author.

**Figure 8.**Hysteresis loops measured at 300 K in arrays of Co${}_{47}$Pt${}_{53}$ nanowires with the field applied parallel (solid line) and perpendicular (dashed line) to the wires. (

**a**) As-deposited sample and (

**b**) annealed sample at 700 ¼C. Note the different magnetic field scales in (

**a**) and (

**b**) (from [28]). Figure adapted from ref. [28] with permission of the author.

**Figure 9.**(

**a**) CPP-GMR versus applied field parallel to the layers at 77 K for Py/Cu (full line) and Co/Cu (dashed line) multilayered nanowires; (

**b**,

**c**) GMR versus ${t}_{\mathrm{FM}}$ at $T=$ 77 K (

**b**) for Co(${t}_{\mathrm{Co}}$)/Cu(8 nm) multilayered nanowires and (

**c**) for nanowires composed of [Py(${t}_{\mathrm{Py}}$)/Cu(10 nm)/Py(${t}_{\mathrm{Py}}$)] trilayers separated by 100 nm of Cu (open circles) or 500 nm of Cu (filled squares). The solid lines in (

**b**) and (

**c**) represent a variation as $1/{t}_{\mathrm{FM}}$ (adapted from [5]). figure adapted from ref. [5] with permission of the author.

**Figure 11.**(

**a**) illustration of the spin transfer torque device with spin valves stacked in each nanowire electrodeposited in a nanoporous alumina template; (

**b**) differential resistance as a function of bias direct current at $H=-$50 Oe for a Py(30 nm)/Cu(10 nm)/Py(6 nm) spin valve nanowire with the magnetic field in the plane of the layer. Arrows mark the scan direction; (

**c**) evolution of the emitted signal frequency by injecting a positive dc current of 6.0 mA as a function of the perpendicular applied magnetic field for the two magnetization configurations ${V}_{\mathrm{down}}/{V}_{\mathrm{up}}$ (in blue) and ${V}_{\mathrm{up}}/{V}_{\mathrm{down}}$ (in red) (adapted from [30,41]). Figure adapted from ref. [41] with permission of the author.

**Figure 12.**(

**a**) schematic representation of an electrode design for electrical measurement of CNW network embedded in 3D porous PC membrane; (

**b**,

**c**) magnetoresistance curves for (

**b**) Ni${}_{75}$Co${}_{25}$ and (

**c**) Ni${}_{32}$Co${}_{68}$ CNW networks measured at room temperature by applying the external field in the OOP (continuous lines) and IP (dashed lines) directions; (

**d**) variation of the magnetocrystalline anisotropy field ${H}_{\mathrm{MC}}$ with respect to the Ni content (x) for the Ni${}_{\mathrm{x}}$Co${}_{1-\mathrm{x}}$ CNW networks; (

**e**) variation of the AMR ratio with respect to the alloy composition at $T=$ 20 K, 150 K, and 290 K.

**Figure 13.**(

**a**) calculated thermopower for Co/Cu multilayers in the layer parallel (dash-dotted line) and perpendicular (solid line) directions vs thickness ratio $\lambda =$ t${}_{\mathrm{FM}}$/t${}_{\mathrm{Cu}}$ using Equations (7) and (8), and bulk values for ${S}_{\mathrm{Co}}$, ${\rho}_{\mathrm{Co}}$, ${S}_{\mathrm{Cu}}$ and ${\rho}_{\mathrm{Cu}}$. The grey dashed line shows the values for $\lambda =$ 1. The inset shows a FM/Cu multilayer stack; (

**b**) experimental set-up for Seebeck coefficient and the magneto-thermoelectric effect measurements. The color represents the generated temperature profile in the NW networks; (

**c**,

**d**) Electrical resistance (

**c**) and Seebeck coefficient (

**d**) of a Co/Cu NW network (80 nm in diameter and 3% packing density) at room temperature. The curves in (

**c**,

**d**) were obtained with the applied field in-plane (IP—in blue) and out-of-plane (OOP—in red) of the NW network film (adapted from [65]). Figure adapted from ref. [65] with permission of the author.

**Figure 14.**(

**a**) the two-current model for the resistivity and the thermopower considering both parallel (P) and antiparallel (AP) magnetic configurations; (

**b**) measured Seebeck coefficients at zero applied field ${S}_{\mathrm{AP}}$ (blue circles) and at saturating magnetic field ${S}_{\mathrm{P}}$ (red circles) of a CoNi/Cu NW network 80 nm in diameter and 3% packing density, along with the corresponding calculated ${S}_{\uparrow}$ (orange circles) and ${S}_{\downarrow}$ (violet circles) from Equations (11) and (12) (from [64]).

**Figure 15.**(

**a**) anisotropic magnetoresistance and Seebeck coefficient of NiCr and CoCr CNW vs. Cr content at different temperatures. (

**a**,

**b**) MR ratio vs. the Cr content for NiCr (

**a**) and CoCr (

**b**) samples at RT (blue) and $T=$ 100 K (red). The insets in (

**a**) and (

**b**) show the MR ratio of the Ni${}_{96}$Cr${}_{4}$ and Co${}_{99}$Cr${}_{1}$ samples in function of temperature, respectively. The grey areas in (

**a**,

**b**) indicate negative AMR. (

**c**,

**d**) Seebeck coefficient vs. Cr content for NiCr (

**c**) and CoCr (

**d**) samples at RT (blue), $T=$ 200 K (yellow) and $T=$ 100 K (red) (from [66]). Figure adapted from ref. [66] with permission of the author.

**Figure 16.**(

**a**) schematics of a set-up for direct observation of Peltier and magneto-Peltier effects; a Cernox thermometer is used to monitor the temperature changes at the junction between the NWs and the electrode; (

**b**) measured temperature changes $\Delta {T}_{H}$ at the Peltier junction in the Co/Cu CNWs (105 nm in diameter and packing density of 22%) during the magnetic field sweep for a DC current of −50 mA and +50 mA. The magneto-Peltier effect leads to heating and cooling at the saturation field depending on current flow direction (from [65]). figure adapted from ref. [65] with permission of the author.

**Figure 17.**(

**a**) sketch of a microwave circulator based on an unbiased ferromagnetic nanowires array embedded in a porous anodic alumina membrane; (

**b**) measured (red) and predicted (blue squares) results for a circulator based on a ferromagnetic NiFe NW array (adapted from [51]); (

**c**) photograph of a substrate integrated waveguide (SIW) resonance isolator device using ferromagnetic NW grown in nanoporous alumina; (

**d**) isolation, defined as the ratio expressed in dB between forward |S21|and reverse |S12| transmissions of signal ($I=20{\mathrm{log}}_{10}\left(\right|{S}_{21}|/|{S}_{12}\left|\right))$, predicted by Comsol simulator (dashed line) and observed experimentally (solid line) are shown; the positive value of isolation confirms that transmission is favored from port 1 to port 2 (adapted from [52]). Figure adapted from refs [51,52] with permission of the author.

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Piraux, L.
Magnetic Nanowires. *Appl. Sci.* **2020**, *10*, 1832.
https://doi.org/10.3390/app10051832

**AMA Style**

Piraux L.
Magnetic Nanowires. *Applied Sciences*. 2020; 10(5):1832.
https://doi.org/10.3390/app10051832

**Chicago/Turabian Style**

Piraux, Luc.
2020. "Magnetic Nanowires" *Applied Sciences* 10, no. 5: 1832.
https://doi.org/10.3390/app10051832