Anomalous stepped-hysteresis and T-induced unit-cell-volume reduction in carbon nanotubes continuously filled with faceted Fe3C nanowires

Ferromagnetically-filled carbon nanotubes have been recently considered important candidates for application into data recording quantum disk devices. Achievement of high filling rates of the ferromagnetic materials is particularly desirable for applications. Here we report the novel observation of carbon nanotubes continuously filled along the capillary with unusual μm-long faceted Fe3C nanowires. Anomalous magnetic features possibly due to strain effects of the crystal facets are reported. Magnetization measurements revealed unusual stepped magnetic hysteresis-loops at 300 K and at 2 K together with an anomalous decrease in the coercivity at low temperature. The observed unusual shape of the hysteresis is ascribed to the existence of an antiferromagnetic transition within or at the boundary of the ferromagnetic facets. The collapse in the coercivity value as the temperature decreases and the characteristic width-enhancement of the hysteresis with the field increasing appear to indicate the existence of layered antiferromagnetic phases, possibly in the strain-rich regions of the nanowire facets. Zero field cooled (ZFC) and field cooled (FC) magnetic curves evidenced presence of magnetic irreversibilities, an indicator of a possible spin-glass-like behavior induced by competing antiferromagnetic and ferromagnetic interactions. Characterization performed with low temperature XRD measurements, further revealed a slight variation in the average Fe3C unit cell parameters, suggesting the absence of additional unit-cell volume induced ferromagnetic transitions at low temperature.

reported. Observation of staircase-like hysteresis loops has been reported also in diluted magnetic semiconductors, examples of these systems include (In, Mn)As at low temperatures. In these systems formation of multiple magnetization jumps have been generally attributed to the depinning processes of magnetic domain walls [32]. Also, the formation of unusual magnetization steps, so-called Barkhausen jumps, has been reported in the presence of structural defects which strongly affect the magnetization process [33][34][35]. These effects have been recently observed in the specific cases of iron carbide/iron interfaces [35] and polycrystalline α-Fe-filled materials encapsulated within carbon nanotubes (CNTs) [33].
Possible influence of Kondo effects in such phenomena has been also excluded in recent Seebeckstudies [36].
Differently from these examples, other types of stepped hysteresis loops have been reported to arise in materials where coexistence of superparamagnetic-and ferromagnetic-single-domain-grains is present [37][38][39][40]. Existence of such magnetic phenomena has been also reported in conditions of: (I) coexistence of two magnetic components with contrasting coercivities, (II) relatively high ratios of the coercivity remanence to coercive force and (III) low coercivity components as large fraction of the total volume of the magnetic grains [37][38][39][40].
Additionally, the values of magnetic moment for certain types of ferromagnetic crystals (i.e. iron carbide Fe 3 C) have been reported to change under certain conditions of unit cell volume contraction [28,[41][42][43][44]. Interestingly, observation of temperature-driven structural transitions in CNTs filled with Fe 3 C nano-crystals was reported in a recent work by Boi et al [41]. In that study, temperature dependent x-ray diffraction (T-XRD) measurements from 12 K to 298 K and Rietveld refinement analyses revealed a cooperative reversible 2θ-shift in both the 002 peak of the graphitic CNTs-walls and the 031 and 131 peaks of the encapsulated Fe 3 C nano-crystals, evidencing a contraction in the average unit-cell volume of Fe 3 C with the decrease of the temperature [41]. Unusual variation of the magnetization with temperature and applied field in CNTs films containing a large quantity of Fe 3 C were also reported by Karmakar et al by SQUID magnetometry [43] and attributed to exchange bias effects resulting from magnetic interaction with secondary γ-Fe -phases (inside the CNTs) [43].
Appearance of butterfly shaped signal in the magnetization hysteresis has been also indicated as a hint to ferrimagnetism (see Mihalik et and Wollan et al in ferrimagnetic oxide-based materials (NdMn 1-x FexO 3+δ ) and [(1−x)La, xCa]MnO 3 systems [45,46]). Unusual hysteresis shapes were reported also by Hellwig et al in presence of an antiferromagnetic-coupling effect between ferromagnetic multilayers [47].
In this work we report the novel observation of anomalously stepped hysteresis loops exhibiting a temperature-induced collapse in the magnetic coercivity parameter, in multiwall carbon nanotubes (MWCNTs) filled with faceted long ferromagnetic Fe 3 C nanowires (diameter of 40-60 nm and length of 1-5 micrometres).
The encapsulated nanowires were found to exhibit a unit-cell with averagely large atomic-parameters a=0.510 9243 nm, b=0.676 5692 nm, c=0.454 3652 nm and an average volume of 0.157 06 nm 3 .
The observed magnetic phenomenon is ascribed to the possible existence of layered antiferromagnetic interactions at the defective grain boundaries of the Fe 3 C-facets (created by the fast cooling) within the μm-long faceted nanowires. The change in the coercivity parameters with the decrease of the temperature from 300 K to 2 K cannot be explained on the basis of previous works on Fe-filled nanotubes. The appearance of such strained faceted features (as revealed by high resolution transmission electron microscopy (HRTEM)) in the encapsulated nanowires and the observed Fe 3 C-unit cell reduction, as revealed by T-XRD and Rietveld refinements, implies instead the possible formation of ferromagnetic-antiferromagnetic interfaces at low temperature. Zero field cooled (ZFC) and field cooled (FC) magnetic curves highlighted the presence of magnetic irreversibilities, an indicator of a possible spin-glass-like behavior induced by competing antiferromagnetic and ferromagnetic interactions.
The observed small variation in the average Fe 3 C unit cell parameters, as extracted by Rietveld refinements, further suggest the absence of unit-cell volume induced magnetic moment transitions [28].

Experimental
MWCNTs filled with Fe 3 C nanowires (diameter of 40-60 nm) were produced by sublimation and pyrolysis of ferrocene and dichlorobenzene mixtures (40 mg of ferrocene were mixed with one drop of dichlorobenzene). A quartz tube reactor of 1.5 m and an Ar flow rate of 11 ml min −1 were used. The samples were cooled down with cooling times of 10-20 min by removing the furnace along a rail system (quench). Different sublimation temperatures were used (the value of sublimation temperature was measured within the area occupied by the ferrocene-containing quartz boat, within the quartz tube reactor), and the pyrolysis temperature was 990°C. The duration of each reaction was 10 min. Fe 3 C filled carbon nano-onions (CNOs) were produced for comparative purposes, following the method reported in [48]. Different average unit-cell volumes (determined via Rietveld refinement of the XRD patterns) were obtained for the MWCNTs depending on the used sublimation temperature: 0.155 28 nm 3 with 630°C-700°C; 0.156 08 nm 3 with 530°C-600°C; 0.157 06 nm 3 with 460°C-530°C and 0.157 78 nm 3 with 360°C-460°C. Note that the observed effect was then found to vanish for larger quantities of ferrocene (i.e. ∼100-200 mg), owing to the increase of the overall CNTs-diameter and systematic differences in the carbon to metal ratios within the pyrolyzed vapor. A 200 kV American FEI Tecnai G2F20 HRTEM and a Philips X'pert Pro MPD powder x-ray diffractometer (Cu K-α 1,2 , λ=0.154 18 nm) were employed for the crystal characterization. The magnetic characterization was performed at 300 K with a vibrating sample magnetometry 2.5 Tesla electromagnet East Changing 9060 by using a magnetic field of 13 000 Oe and at 2 K with a Quantum Design Superconducting Quantum Interference Device by using the magnetic field of 10 000 Oe. T-XRD measurements were performed on a PANalytical Empyrean  powder x-ray diffractometer, equipped with a primary Johansson monochromator (Cu K-α 1 , λ=0.154 06 nm), an Oxford Cryosystems PheniX cryostat operating under vacuum below 10-2 Pa, and a X'celerator linear detector. Measurements were collected from 12 K to 298 K (12 K, 20 K, 30 K, 40 K, 50 K, 60 K, 70 K, 80 K, 90 K, 100 K, 120 K, 140 K, 160 K, 180 K, 200 K, 220 K, 240 K, 260 K, 280 K and 298 K). See also [49] for comparative SQUID magnetometry measurements on Fe 3 C filled CNOs, in absence of nanowire facets.

Results and discussion
A typical example of MWCNTs completely filled with a Fe 3 C nanowire is shown in the transmission electron micrograph of figure 1(A). It is interesting to notice that a variation in the volume distribution of the Fe 3 C crystal along the MWCNT-core is present with the formation of unusually faceted atomic lattice periodicities characterized by repeated dark and bright contrasts within individual encapsulated nanowires. A typical XRD measurement confirming the presence of  length parameter of each step. The two anomalously long step-features were found at 96.6 Oe, 37 emu g −1 and at −100 Oe, −37 emu g −1 . The length of each step was found to be 58-62 emu g −1 . An extremely low coercivity of 100 Oe and a higher saturation magnetization of 120 emu g −1 were also measured at low temperature. We can immediately notice that these properties are very different with respect to those reported at 0.2 K by Wernsdorfer et al [16]. Also, the observed trend is significantly different with respect to that measured by Boi et al in non-faceted Fe 3 C filled CNOs in [49], where a progressive increase in the coercivity parameter with the decrease of the temperature was found.
Stepped hysteresis loops have been reported also in the case of single-molecule magnets [1,9], nanomagnets [25,26], FeC crystals and Fe/Sm multilayers [20][21][22][23][24][25][26][27]. However due to the large number of atomic periodicities comprised in the Fe 3 C nanowires, the temperature of observation and the unusual dynamics of coercivity decrease, the origin of the observed step features is not attributable to quantum tunnelling of magnetization effects or the possible presence of a wasp-waisted hysteresis loops. Instead, given the presence of strained regions in the encapsulated nanowires, the formation of ferromagnetic/antiferromagnetic interfacial features is possible [47]. Comparing the observed hysteresis in figure 5(B) with those observed in antiferromagnetically-coupled ferromagnetic multilayers, a similarity in the shape of the hysteresis is noticeable [47].
Additional investigation of this magnetic transition was considered by employing ZFC and FC methods to extract the variation of the magnetic moment with temperature. Figure 6 shows the ZFC and FC magnetization versus temperature signals acquired from 2 K to 300 K at the field of 300 Oe. It is important the notice the existence of a spin-glass-like behavior in both the analyzed portions of the filled CNTs in figures 6(A), (B), which may be an indicator of competing ferromagnetic and antiferromagnetic ordering within the same sample [50,51].
Further investigation of the possible existence of T-induced structural variation in the Fe 3 C unit-cell volume was considered by employing T-XRD and Rietveld refinements. These Rietveld refinements were performed on the same dataset acquired in [41] for CNTs filled partially/or continuously with Fe 3 C. Repeated Rietveld refinements were performed on a narrower 2θ region (from 39°to 50°2θ) of the XRD patterns with respect to the refinements reported in [41], allowing for a more accurate estimation of the unit-cell volume parameters.
Plots   [41]. The improved quality of the refined data allowed for a more accurate estimation of the unit cell volume variation which appears to slightly decrease with the decrease of temperature in both cases. Note however that the observed change in unit cell volume values is not comparable to that required for the observation of significant transitions in magnetic moment values, as indicated in [28,29]. 0.52% in the continuously filled CNTs case (figure 7(D)) and of 0.06% in the partially filled CNT case ( figure 7(H)). The unit cell volume appears to slightly decrease with the decrease of the temperature. Note however that the observed volume change is not comparable to that required for the observation of significant transitions in unit-cell magnetic moment values [28].
In order to further verify this interpretation, additional comparative measurements were then performed on Fe 3 C filled CNOs produced according with the method reported in [48] (see figure 8 for typical TEM images) in a comparable temperature range (see also supp. materials in [49] for comparative magnetization measurements). As shown in figures 9-11 also in this case a weak contraction in the average unit cell volume of Fe 3 C and in the graphitic c-axis of the CNOs was found with the decrease of the temperature, with a small Fe 3 C unit cell volume change of 0.27%. This observation appears to confirm the above interpretation and suggests the absence of Fe 3 C unit cell induced magnetic moment transitions at low temperature in these types of materials. Instead, a crucial role in the appearance of the observed stepped magnetization hysteresis appears to be taken by the faceted morphology of the nanowires. This observation is confirmed by direct comparison with other magnetization measurements performed in non-faceted Fe 3 C-filled CNOs, reported in [49]; indeed, in this latter CNO-case an opposite trend involving a significant increase of the coercivity parameter with the decrease of the temperature was found.

Conclusions
In conclusion in this work we have shown that MWCNTs filled with Fe 3 C nanowires can show unusual steppedlike hysteresis loops due to strain induced variation of the Fe 3 C nanowire volume. The origin of the observed stepped hysteresis was attributed to existence of antiferromagnetic-coupling in ferromagnetic facetedinterfaces. These findings open new avenues towards investigation of antiferromagnetism in faceted Fe 3 C filled CNTs systems for application in magnetic devices.   A contraction effect similar to that observed in [41] is observed, with decreasing temperature.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Conflicts of interest
There are no conflicts of interest to declare.