Since the discovery of the Giant Magnetocaloric Effect (MCE) on Gd5Si2Ge2 by Pecharsky and Gschneidner [1], the Gd5(Si1-xGex)4 family compounds have been intensively studied for applications in magnetic refrigeration (MR) at room temperature. The vast investigation at the macroscale on these alloys up to this time revealed many other important properties, such as giant magnetoresistance and colossal magnetostriction (CMS) [2]. Such behaviors arise from a strong magneto-volume coupling extreme sensitivity to variation of external (e.g., temperature, magnetic field, and pressure) and internal (e. g., stoichiometry, dimensionality, and doping) parameters. For this reason, this family compound presents a rich phase diagram that, for example, gives a wide range of temperatures for device operations. However, the low mechanical stability and oxidations issues are some drawbacks for long periods of appliance operation. Reducing it to the micro and nanoscales can improve the alloys' properties and also find novel properties. In the last years, some experimental approaches have been used to produce micro/nanostructures of Gd5(Si,Ge)4 [3]. It is possible to mention high-energy ball milling, RF magnetron sputtering, and laser ablation techniques, being the most promising. Using a femtosecond pulsed laser deposition, our group successfully obtained Gd5Si1.3Ge2.7 nanogranular film composed of 80 nm particles presenting a magnetostructural transition [4] and a negative thermal expansion [5]. Although this evidence revealed the great potential of ultrashort laser ablation, a more conventional and low-cost technique should be pursued aiming practical applications of these compounds. For example, Tarasenka et al. were able to produce colloidal suspensions of Gd-Si-Ge nanoparticles in liquid with a more accessible Nd:YaG nanosecond pulsed laser [6]. Although the NPs presented fine crystalline features, the formation of silicon carbide and particle segregation after an additional laser ablation occurs. Given this, in addition to finding the most appropriate experimental technique, reasonable control of the intrinsic structures is essential to optimize the material production on a large scale. Here, we demonstrate that a more simple gas-phase synthesis using a Nd:YaG and KrF Excimer nanosecond pulsed lasers can be used to produce Gd5(Si1-xGex)4 nanostructures with x = 0, 0.45 and 0.60. The stoichiometries were selected to cover all the possible structures adopted by this family: orthorhombic-II [O(II)] for x = 0; monoclinic (M) of x = 0.45; and orthorhombic-I [O(I)] for x = 0.60. It is worth mentioning that x = 0.45 and 0.60 are in the vicinity of the phase-mixed region to show the feasibility of this method along with the family phase-diagram. In particular, the KrF Excimer was chosen due to the laser wavelength that presents higher penetration depth and reduced reflectance in metals. The nanostructures were obtained by ablating the polished surface of ingots produced by tri-arc melting with 10 Hz of frequency and maximum energy of 550 mJ and 200 mJ, for Nd:YaG and KrF Excimer lasers, respectively. The chamber was pumped several times down to a base pressure of 10-6 Torr to prevent oxidation. During the deposition, Argon inert gas was used in a continuous flux in constant pressure of 1 Torr inside the chamber. From this process, Gd5(Si,Ge)4 grains with an average particle size between 10-30 nm were achieved with good crystalline features, as can be seen in Fig.1(a). Synchrotron X-ray Diffraction confirmed the great level of the structures crystallinity and revealed a change in symmetry for x = 0 and 0.45 respectively from O(II) and M to O(I). The structural shrinkage was found to be around 2% for the Si-rich compositions and less than 1% for Gd5Ge4 (x=0). Given the lower unit cell volume for the O(I) structure, a shortening between the Si(Ge)-Si(Ge) distances is responsible for a change in the transition order from a first to a second one for x = 0.45. Also, the magnetic transition occurs at 290 K for this 0.45 composition nanostructure, see Fig.1(b), confirming the XRD analysis. Distinctly, for x = 0, the reduction to the nanometric scale leads to a superparamagnetic-like behavior that leads to a maximum of 8 J/kgK at 5 K that drastically reduces to null values as the temperature increases. Furthermore, the reduction of the unit cell volume was found to be directly related to the bulk target volumetric compressibility that is due to intrinsic surface tension (st) rising from the nanostructuring process. Thereby, we can conclude that the nanosecond PLD successfully produces dispersed crystalline nanoparticles of Gd5(Si1-xGex)4 family compounds. This technique allows the formation of NPs with particles size under 30 nm without loss on the stoichiometry of the targets, as observed through Synchrotron XRD [7]. Notwithstanding, the direct relation between bulk compressibility and nanoparticle size due to an intrinsic surface pressure might be the path to understand the particles' growth from pulsed laser deposition techniques under inert gas in the chamber.

**References:**

[1] V. K. Pecharsky and K. A: Gschneidner Jr., Phys. Rev. Lett., vol. 78, p. 4494 (1997); [2] L. Morellon, et al., Phys. Rev. B, vol. 58, p. R14721(R) (1998); [3] J. H. Belo, et al., J. of Mater. Res., vol. 34, p. 134 (2019); [4] R. L. Hadimani, et. al., Appl. Phys. Lett., v. 106, p. 032402 (2015); [5] J. H. Belo, et al., PRB, vol. 100, p. 134303 (2019); [6] N. Tarasenka, et al., J. Nanosci. Nanotechnol., vol. 16, p. 7451 (2016); [7] V. M. Andrade, et al., J. Alloys Compd., vol. 849, p. 156384 (2020[AP1] ).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/afe5422997f395a3795a6766687f1f04.jpg)
Figure 1: (a) TEM images for x = 0.45 nanoparticles with 27 nm of the mean size where the SAED reveals the good crystalline features of the structures. (b) Magnetic entropy change curves for all produced samples for an applied field of 5 T.

 Unit cell volume reduction of Gd5(Si,Ge)4 nanoparticles controlled by bulk compressibility

The study of 1D-nanostructures has been intensified in the last few decades due to the development of fabrication methods of metallic, semiconductors and insulator materials aiming applications in several areas [1]. In particular, multi-segmented magnetic nanowire (NW) arrays are prospective candidates for an extensive range of applications such as racetrack memory devices, spintronic, and drug delivery [1, 2, 3]. These possibilities are due to the number of degrees of freedom related to the intrinsic shape anisotropy combined with the potential to introduce different materials along the NW’s length. Concerning practical applications, understanding the role of layers with distinct structural and magnetic properties can be a path to avoid some issues such as system overheating and Walker breakdown in racetrack devices. For instance, micromagnetic simulations revealed that the combination of Co segments with different structures, bcc and hcp, promotes a faster way of writing information in 3D racetrack memory devices [2]. This observation results from the high magnetocrystalline anisotropy along the wire length induced by the Co-hcp structure. Although these evaluations seem promising, thorough experimental research of multi-segmented NWs with low and high anisotropy components still needs to be performed. Here, we present the FeCo layer properties’ evaluation onto multi-segmented NW arrays produced through DC electrodeposition into ordered porous alumina templates with 40 nm of pore diameter and 60 µm of length. For each layer’s deposition, a Pt electrode was used as the counter electrode, Ag/AgCl as the reference electrode, and an Au layer as the working electrode [5]. Two samples were prepared: 1) Au [200 nm] / Fe33Co67 [24 µm] / Au [10 nm] and 2) Au [200 nm] / Ni90Cu10 [1 µm] / (Fe33Co67 [13 µm] / Au [10 nm])x3, where the indicated length was obtained from scanning electron microscopy (SEM) images and the alloy’s stoichiometry from energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) measurements at room temperature revealed the formation of bcc structure of FeCo with the (110) preferential orientation for both samples and an additional hcp phase with the (110) preferential orientation for FeCo tri-segmented NWs (Figure 1). The magnetic hysteresis loops acquired at room temperature for the Au/FeCo/Au arrays present the easy magnetization axis along the parallel direction, evidencing a typical strong shape anisotropy [5]. Distinctly, when three FeCo segments of 13 µm are deposited along the NW’s length, the arrays present an isotropic magnetic behavior with an increase of the coercive field when applying the magnetic field perpendicular to the nanowires’ axis (Figure 2). This change in magnetic behavior for the segmented NWs might be related to the presence of an hcp phase that induces a perpendicular magnetocrystalline anisotropy. However, further research should be performed to better comprehend the interplay between the NiCu and FeCo layers into the NW arrays. In summary, multi-segmented NWs of FeCo alloys were successfully achieved through the electrodeposition process into porous templates. The growth of FeCo segments leads to the formation of hcp structures and the observation of an isotropic magnetic behavior. This can be related not only to the structural properties but also to the interplay between FeCo segments along the NW’s length. A more complete evaluation of the magnetic properties of the produced samples should be performed to fully understand the role of the different layers in the magnetocrystalline anisotropy of the arrays. This work was financially supported by projects POCI-01-0145-FEDER-028676 and UIDB/04968/2020 from Portuguese FCT and COMPETE 2020 (FEDER) and IF/01159/2015, POCI-01-0145-FEDER-031302/PTDC/FIS-MAC/31302/2017. .

**References:**

References:: [1] C. T. Sousa et al., Appl. Phys. Rev., vol.1 , p. 031102 (2014); [2] J. Rial and M. P. Proenca, Nanomaterials, vol. 10, p. 2403 (2020); [3] D. Wang et al., Materials, vol. 12, p. 3908 (2019); [4] C. Bran et al., J. Phys. D: Appl. Phys., vol. 48, p. 145304 (2015); [5] E. M. Palmero et al., Nano Research, vol. 12, p. 1547 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/419fc130f5d8af612615b50ad0e953e0.jpg)
Figure 1: XRD patterns for Au/FeCo/Au and Au/NiCu/[FeCo/Au]3 segmented nanowires with the Bragg peak positions for bcc (green) and hcp (red) structures.

![](https://assets.underline.io/uploads/markdown_image/1/image/59bc97224bca21ea1066106778e1c21d.jpg)
Figure 2: Hysteresis loops obtained by applying the magnetic field perpendicular to the NWs’ length

 Isotropic magnetic behavior of multi-segmented FeCo nanowire arrays

*Session Chair: Radhika Barua, Virginia Commonwealth University*

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Unit cell volume reduction of Gd5(Si,Ge)4 nanoparticles controlled by bulk compressibility

Isotropic magnetic behavior of multi-segmented FeCo nanowire arrays

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