Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Research articles Acoustic spectroscopy of functionalized carbon nanotubes in magnetic fluid Jozef Kúdelčík a, ⁎ , Štefan Hardoň a , Peter Bury a , Milan Timko b , Peter Kopčanský b , Zuzana Mitróová b a Department of Physics, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia b Institute of Experimental Physics SAS, Watsonova 47, 040 01 Košice, Slovakia ARTICLE INFO Keywords: Carbon nanotubes Magnetic nanoparticles Acoustic attenuation Anisotropy ABSTRACT Acoustic spectroscopy is used to study the rearrangements of multi-walled carbon nanotubes (MWCNTs) func- tionalized by Fe 3 O 4 magnetic nanoparticles dissolved in transformer oil under the influence of a magnetic field at various temperatures. Three methods for the application of the magnetic field are used: a jump change, a linear increase or decrease and a constant magnetic field with a change of its orientation to the acoustic wave. The rearrangements of MWCNTs/Fe 3 O 4 to new structures (chains) by the influence of a magnetic field is confirmed by changes in the acoustic attenuation. From the measurement results, the lifetime of the chains after the switch- off of the magnetic field is less than 30 s. Such a rapid change is due to the fact that the nanotube chains are held by magnetic forces, resulting from the same direction of the magnetic moments of bound magnetic nano- particles. A temperature-dependent hysteresis effect is observed with a linear change of the magnetic field. From our experiments, it follows that the reorientation of MWCNTs by magnetic nanoparticles with the magnetic field was gradual. The effect of the anisotropy of the acoustic attenuation is observed at a magnetic flux density of 200 mT and at various temperatures. Three MWCNT/Fe 3 O 4 concentrations diluted in the transformer oil are used for the measurements and their influences on the structural changes with various developments of the magnetic field are discussed. 1. Introduction Graphene consists of a sheet of six-membered carbon rings with excellent electrical and mechanical properties. Graphene rolled into a tube form is known as a single-wall carbon nanotube (SWCNT). When more layers are added, multiwalled CNTs (MWCNTs) are formed. SWCNTs have a diameter of 0.4–2.0 nm and a length of 20–1000 nm. MWCNTs (Fig. 1a) with a diameter up to 100 nm are one of the most used types of carbon nanostructures due to their easy availability, af- fordable price, chemical inertness and ability to be easily functionalized [1–3]. Functionalisation can feature various noble metal nanoparticles, such as Au, Ag, Pt, Pd and their alloys [4–6], or magnetics nanoparticles (MNPs), such as Fe 3 O 4 (Fig. 1b) [7,8]. Up to now, various techniques, including arc discharge, evaporation, sputtering and sonochemical methods, have been reported for the fabrication of the CNT/Fe 3 O 4 hybrids. For example, Wang et al. [9] prepared polyvinyl chloride blended with functionalized MWCNTs and MWCNT/Fe 3 O 4 hybrids prepared through a hydrothermal method to change the membrane properties. Jones et al. [10] used a sonochemical technique for the in situ coating of iron oxide on the outer surface of MWCNTs, which were further used as reinforcing fillers in an epoxy-based resin. Functiona- lized carbon nanotubes with MNPs (Fig. 1b) possess magnetic proper- ties that can be studied at lower magnetic fields [11]. The experimental results in work Mitroová et al. [12] showed that nematic liquid crystals (6CHBT) in combination with SWCNT/Fe 3 O 4 were oriented along the magnetic field direction and there was also an increase of the critical voltage with the magnetic field. At the presence of a magnetic field, the magnetic moments of the nanoparticles align in parallel, and the re- sulting dipolar interactions are sufficiently large to overcome thermal motion and to reorient the magnetic CNT favoring the formation of their aligned. The connecting of the magnetic CNT in line, where touching each other in a head-to-tail fashion, creates chainlike struc- tures. Alignments of CNTs under low magnetic fields through attach- ment of MNPs (Fe 3 O 4 , γ -Fe 2 O 3 ) were observed by Correa-Duarte et al. [13] or Kumar et al. [14]. Kim et al. [15] also observed an aligned of MWCNT/Fe 2 O 3 and a change of the conductivity under the magnetic field. They and Hekmatara et al. [16] measured that at the magnetic field the conductivity increased with increasing MWCNT/Fe 2 O 3 mass fraction in the composite. They also observed that the conductivity measured parallel to the magnetic field was higher than that measured https://doi.org/10.1016/j.jmmm.2020.166538 Received 19 July 2019; Received in revised form 5 January 2020; Accepted 30 January 2020 ⁎ Corresponding author. E-mail address: kudelcik@fyzika.uniza.sk (J. Kúdelčík). Journal of Magnetism and Magnetic Materials 502 (2020) 166538 Available online 04 February 2020 0304-8853/ © 2020 Elsevier B.V. All rights reserved. T