Possible routes for the synthesis of nanowires of intermetallic compounds: The case of CeIn3

In this work, we investigated the role of different parameters in the synthesis of intermetallic nanowires of CeIn3 by the metallic-flux nanonucleation (MFNN) method such as template pore diameter, crystallization temperature, heat treatment temperature, and synthesis time. Depending on the growing parameters, we obtained CeIn3 nanowires (d ∼ 350 nm) or CeAlO3 nanotubes. For the nanowires, we observed a suppression of the CeIn3 antiferromagnetic transition from the bulk TN ∼ 10 K to the nanowire system TN ∼ 3 K, which may be associated with the dimensionality affecting the interplay between magnetic exchange interactions, crystalline electrical field, and Kondo effects. We assume that the CeAlO3 nanotubes may result from a reaction with the alumina template and consequent rare-earth oxidation. Our work shows that even it is a great challenge to find the correct growth path of a particular intermetallic compound, the MFNN method can be a promising route to obtain rare-earth based nanowires.


Introduction
Correlated intermetallic electron systems exhibit emergent physical phenomena, such as unconventional superconductivity, heavy fermion behavior, complex magnetic ordering, non-Fermi-Liquid behavior, and quantum criticality [1]. These phenomena emerge from the collective behavior of interacting electrons, which is strongly dependent on the system dimensionality [2]. The study of emergent phenomena in reduced dimension systems has been previously reported in different systems [3][4][5][6]. Exploring the Fe3Ga4 system, Moura et al. reported a complex change in the phase diagram as a function of the dimensionality [3]. While bulk Fe3Ga4 shows a transition from the ferromagnetic (FM) to antiferromagnetic (AFM) state, Fe3Ga4 nanowires with diameter (d ≈ 250 nm) obtained by metallic-flux nanonucleation (MFNN) method [7] exhibit a transition from FM to ferrimagnetic or coexistence of FM and AFM. Previous studies also reported by the synthesis through the MFNN technique and further macroscopic characterization of β-Ga nanowires with a diameter of 140 nm and length around 3.8 μm [4]. In this case, it was possible to demonstrate the stabilization of a weakly coupled type II superconductor with Tc ≈ 6.2 K. Cruz et al. reported the synthesis of oxide-shell-protected Mn5Si3 nanowires with a diameter between 85 and 850 nm by the MFNN method [5]. Electrical characterization showed that nanowires are metallic at low temperatures. Rosa et al. performed a study on the dimensionality effect on the physical properties of the intermetallic compound GdIn3 [6]. Single crystals and nanowires ( d ≈ 200 nm and length l ≈ 30 m) of GdIn3 were also grown by the MFNN method. Magnetic susceptibility and specific heat measurements showed a drastic suppression of the AFM order of the bulk system ( = 45 K) for the = 3.8 K nanowire system. Since Gd 3+ is an S ion (L = 0), such reduction was associated with changes in the Ruderman-Kittel-Kasuya-Yoshida (RKKY) magnetic interaction.
Due to the flexibility of the RIn3 compound, which allows chemical substitutions in different crystallographic sites, this system represents an excellent opportunity to study the interplay between crystalline electrical field (CEF), Kondo effects, magnetic interactions, and dimensionality [2]. In particular, CeIn3 crystallizes in the simple cubic structure AuCu3 (space group Pm-3m) but has a variety of interesting properties [8]. At ambient pressure, CeIn3 orders antiferromagnetically with TN ~ 10 K. At pressures of P ~ 25 kbar, the system becomes superconducting with a = 0.2 K. This 2 unconventional superconductivity appears around a quantum critical point associated with the destruction of the magnetically ordered state at T = 0 K [9]. Here, dimensionality might be another ingredient to understanding the quantum criticality in heavy fermion systems.
In this work, we report the results of growing CeIn3 nanowires using the MFNN method. We systematically investigated the influence of the pore size and template crystallization temperature, heat treatment temperature, and synthesis time on the nanowire growth. In addition, we present the morphological and magnetic characterizations of CeIn3 nanowires and CeAlO3 nanotubes. Our work shows that the MFNN method is a possible but challenging route to obtain rare-earth based nanowires.

Experimental
Several attempts to grow CeIn3 were performed using the metallic flux nanonucleation (MFNN) method. [7]. This method is a combination of the conventional metal flux growth technique with the addition of a nanometric template for nanowire growth. In this study, we used an Al2O3 template. For the growth of CeIn3 nanowires, a stoichiometry relation of 1 Ce:10 In was used with Al2O3 membranes with pore sizes equal or smaller than (155 ± 25) nm. The crucible was covered by quartz wool and sealed inside an evacuated quartz tube. We explored the influence of the pore size, membrane crystallization temperature (Tcryst = 900 or 1150 °C), maximum heat treatment temperature (Tmax = 850, 950, 1100, 1150, 1200 °C) and synthesis time (2,8,12,24, 48 h) on the nanowire growth. After the heat treatment, the excess In flux was centrifuged. Both the membrane and the CeIn3 crystals were mechanically removed from the crucible.
The morphology and energy dispersion X-ray spectrometry (EDS) analysis of the CeAlO3 nanotubes were investigated at an FEI Nova NanoLab 200 Dual Beam system (FIB/SEM). The images of the nanowires and their EDS composition mappings were performed at an FEI Inspect F50 microscope. Xray diffraction measurements of nanotubes and templates were performed at the Brazilian Synchrotron Light Laboratory (LNLS). Magnetization measurements were performed using a commercial magnetometer.

Morphology and Composition.
We observed nanowires wherein Tcryst = 900 °C and Tmax = 1100 °C. Figure 1(a) shows the Scanning Electron Microscopy (SEM) image of an isolated nanowire with a diameter of (346 ± 8) nm. As shown in Figures 1 (b) and (c), EDS maps confirm that both Ce and In are present at In Kα (Figure 1(b)) and for Ce Lα (Figure 1(c)) energies, which indicates the formation of CeIn3 nanowires. Several growths were performed with a mean pore diameter equal to or smaller than (155 ± 25) nm. For each attempt, we varied the Tmax (850, 950, 1100, 1150, 1200 °C) or the synthesis time (2,8,24, 48 h). Regardless of the chosen parameters, all attempts performed for these templates yielded a presence of nanotubes, as evidenced by the SEM image shown in Figures 2 (a,b). The exception was a growth performed at Tmax = 850 °C with Tcryst = 900 °C, for which In nanowires were obtained inside the pores  Figure 2 (c) shows the point EDS spectrum of a nanotube. We observe the presence of only Ce, Al, and O, which is not consistent with the targeted CeIn3 phase. Figure 2d shows the X-ray diffraction patterns of an alumina template with nanotubes as well as an empty Al2O3 membrane. The orange squares and blue triangles correspond to the indexation from the Inorganic Crystal Structure Database (ICSD no. 72558 and 171679) for the CeAlO3 and In phases, respectively. Both XRD patterns shown in Fig 2d are consistent with the alumina phase. In the case of the template with nanotubes, we also identified peaks consistent with the In and with the tetragonal phase of CeAlO3 (space group: P4/mmm) with lattice parameters a = b = 3.767 Å and c = 3.797 Å [10]. Therefore, XRD measurements and EDS analyses indicate that the nanotubes present the CeAlO3 phase.

In Ce
The formation of CeAlO3 nanotubes is possibly associated with the chemical composition of the Al2O3 templates. Previous results suggest that Al2O3 templates with a "honeycomb" geometry, i.e., hexagonally ordered circular pores, have a duplex structure of pore walls in terms of chemical composition, an acid-anion contaminated outer oxide layer next to the pores with a relatively pure inner wall oxide [11,12]. In this perspective, the formation of nanotubes may occur due to the reactivity of the Ce 3+ ions with the relatively pure oxide wall. Exploring further routes by changing the chemical composition of the template could be a promising path to be explored.

Magnetic characterization.
We investigated the dimensionality role in the magnetic properties of the CeIn3 bulk and nanowire system. Figure 3 (a) shows the temperature dependence of magnetic susceptibility (T), for 1 kOe. Both curves were fitted to the Curie-Weiss law plus a temperature-independent term, ( ) = 0 + ( − ⁄ ). Table 1 shows the fitted parameters. The effective moment ( ) for the Ce 3+ ions, extracted from the Curie-Weiss constant (C), is in agreement with the theoretical value =2.54 μB [8]. Besides that, we observe a suppression of the antiferromagnetic ordering temperature from = 10 K (black arrow) to = 3 K (red arrow). This decrease is possibly associated with changes in the interplay between the RKKY exchange interaction, CEF, and Kondo effects.  Figure 3b shows the dependence of susceptibility as a function of temperature measured at 10 kOe for CeAlO3 nanotubes grown at different temperatures and synthesis time. The curves show no phase transition, exhibiting a paramagnetic behavior throughout the studied temperature range.

Conclusions
In this work, we explored the routes for synthesizing intermetallic nanowires of CeIn3 via the MFNN method. CeIn3 nanowires were successfully obtained in only one batch with an average diameter of (346 ± 8) nm. Suppression of the AFM transition from ( = 10 K) to ( = 3 K) was observed in the nanowires. This reduction is possibly associated with the dimensionality affecting the interplay between the RKKY exchange interaction, CEF, and Kondo effects. For several other attempts, the presence of CeAlO3 nanotubes was observed. The MFNN method is a promising but challenging alternative for the fabrication of nanowires of intermetallic systems.