Magnetoresistance of Multi-Layered Nanowires Studied to Optimize Measuring Devices

Researchers at the Higher School of Economics and the Federal Scientific Research Centre “Crystallography and Photonics” have developed multi-layered nanowires to analyze their magnetoresistance properties.

Enhancing this effect will enable researchers to improve the accuracy of indicators of a range of measuring instruments, such as radiation monitors and compasses. The outcomes of the research have been reported in the paper titled “Structure of Cu/Ni Nanowires Obtained by Matrix Synthesis.”

One of the distinctive features of artificial nanostructures is the enormous magnetoresistance effect in thin metal layers. This effect is utilized in different electronic devices.

The researchers developed multi-layered nickel and copper nanowires to analyze their properties, which are dependent on the composition and geometry of the layers. “We expect that the transition to multi-layered nanowires will increase the giant magnetoresistance effect considerably. Today, we are ‘choosing’ the method of nanowire synthesis, in order to get this effect,” stated Ilia Doludenko, Moscow Institute of Electronics and Mathematics (MIEM HSE) graduate and one of the authors of the study.

The scientists produced nanowires of varying lengths to determine the correlation between the crystal structure and the synthesis parameters. The length of the nanowires was determined by the number of deposition cycles; in each cycle, one copper layer and one nickel layer were deposited. A scanning electron microscope (SEM) was used to determine the size of the nanowires. It was found that the number of pairs of layers in the nanowires was 10, 20, or 50, as determined by using the number of electrodeposition cycles.

Upon comparing the length of the nanowire with the number of layers, it was discovered that the relationship between the number of layers and the length of the nanowires was nonlinear. The average lengths of the nanowires including 10, 20, and 50 pairs of layers were 1.54, 2.6, and 4.75 μm, respectively. All the developed nanowires had a grain structure with crystallites of varying sizes, from 5–20 nm to 100 nm. Large, bright reflections were predominantly caused by metals (Cu and Ni), whereas small reflections and diffuse rings are generally caused by the presence of copper oxides.

An elemental analysis confirmed the existence of alternating Cu and Ni layers in all the nanowires that were analyzed. Yet, there could be changes in the mutual arrangement of layers. Cu and Ni layers in the same nanowire could be oriented perpendicular to its axis or be at a specific angle. The thicknesses of individual units of the same nanowire might differ. The thickness of individual units in nanowires ranges from 50 to 400 nm.

The authors of the study stated that this heterogeneity is dependent on the pore parameters and reduces closer to the mouth of the pore. This results in an increase in current, improvement of deposition rate, and, consequently, an increase in the thickness of the deposited layer. Another probable reason is the variation in the diffusion mobilities of ions of different metals. This accounts for the nonlinear relationship between the number layers and the nanowire length mentioned above. The analysis of the composition of specific units revealed that copper units mainly include copper, whereas nickel is almost completely absent. In contrast, nickel units always contain a specific amount of copper. At times, this amount may be as high as 20%.

The relevance of these research outcomes is applicable to the prospective development of inexpensive and more accurate detectors of position, motion, current, speed, and other parameters. Instruments such as those could be used in the car industry, or to develop or enhance radiation monitors, medical devices, and electronic compasses.

Source: https://iq.hse.ru/en/

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