Glass-Coated Cu-Mn-Ga Microwires Produced by Taylor-Ulitovsky Technique

Solid State Phenomena Vols. 152-153 (2009) pp 79-84 Online available since 2009/Apr/16 at www.scientific.net © (2009) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.152-153.79

Glass-coated Cu-Mn-Ga microwires produced by Taylor-Ulitovsky technique J.J. Suñol1,a, Ll. Escoda1, C García2, V.M. Prida3,b, V. Vega3, M.L. Sánchez3, J.L. Sánchez Llamazares3 and B. Hernando3,c 1

Universidad de Girona, Campus Montilivi, Lluís Santaló s/n, 17003 Girona, Spain Dpto Física de Materiales, Facultad de Química, UPV-EHU, 20080 San Sebastián, Spain 3 Departamento de Física, Universidad de Oviedo, Calvo Sotelo s/n, 33007 Oviedo, Spain a [email protected], [email protected], [email protected]

2

Keywords: Glass-coated microwires, Cu-Mn-Ga alloy, Heusler alloy, Crystalline phases

Abstract. Glass-coated Cu-Mn-Ga microwires were fabricated by Taylor-Ulitovsky technique. By means of energy dispersive spectroscopy microanalysis, an average alloy composition of Cu56Ga28Mn16 was determined. The temperature dependence of magnetization measured at a low magnetic field showed the coexistence of two ferromagnetic phases. The Curie temperature of one phase is 125 K and above room temperature for the other one. X-ray diffraction at room temperature and at 100 K reflects the presence of the same three crystalline phases corresponding to the cubic B2 Cu-Mn-Ga structure as a main phase and the minor phases of fcc Cu rich solid solution with Mn and Ga and the monoclinic CuO.

Introduction A growing interest is being devoted to the study of amorphous and crystalline magnetic microwires mainly owing to their technological applications in sensor of high sensitivity and rapid response. This interest is attributed to their very tiny dimensions (diameter typically between 1 and 35 m) with very attractive magnetic properties. Ferromagnetic glass-coated microwires have a number of attractive features for sensing application in high performance magnetic field, stress or temperature sensors, magnetic labels and micro machines [1-5]. Both, microwire shape and glass-coating mechanical reinforcement significantly influence the microwire magnetic behaviour [6]. Moreover, this family of materials is very promising for new solid-sate actuators as ferromagnetic shape memory alloys and as magnetocaloric materials as has been proposed [7]. Recently, attention has been drawn to investigate mechanical and magnetostructural properties and martensitic phase transformation in the Cu-Ga-Mn system due to their high ductility and potential application as ferromagnetic shape memory alloy [8,9]. In all these cases, alloys were produced as bulk polycrystalline samples. In the present study, crystalline phases, morphology and magnetic properties of glass-coated microwires of Cu-Ga-Mn alloy were investigated with the aim of finding a new ferromagnetic shape memory and magnetocaloric material candidate.

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Experimental Cu-Ga-Mn thin glass coated microwires with an inner and outer diameter of 24 µm and 33 m, respectively, were produced by the Taylor-Ulitovsky method [4]. Starting alloys of nominal composition Cu50Ga25Mn25 were prepared by arc-melting of the pure elements (>99.9%) in Ar atmosphere. Subsequently, when the metallic alloy and the Pyrex glass coating were simultaneously molten the so-formed microwire was drawn and rolled onto a rotating cylinder and quenched to room temperature. The samples thus obtained were in the form of a tiny metallic wire with the dimensions above mentioned. X-ray diffraction (XRD) analyses were performed using Cu Kα radiation in a low-temperature diffractometer (step increment 0.05º). Microstructure and elemental composition of as-cast glass-coated microwires were examined by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray microanalysis system (EDS). Magnetization measurements were performed using a physical properties measuring system platform with a vibratingsample magnetometer module. Field cooling (FC) thermomagnetic curves were recorded in a temperature range from 350 down to 50 K for applied magnetic fields of 200 Oe and 50 kOe along the microwire axis, with a cooling rate of 2 K/min. Samples were studied in ascast state. Results and discussion Typical SEM images at different magnifications of a microwire and the fracture crosssection of both metallic nucleus and Pyrex glass coating are shown in Fig. 1. It appears the inner and outer microwire diameter and dimensions can be checked. No ordered microstructure is detected, suggesting that the heat removal during the rapid solidification process do not induce any directional growth of the crystalline phase formed.

Fig. 1: SEM micrograph and EDS spectrum of as-cast glass-coated Cu56Ga28Mn16 microwires.

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+110 #111

After a systematic study by EDS microanalysis of the fractured cross-section and the cylindrical surface of an appreciable number of microwire pieces, a nearly homogeneous chemical elements distribution was shown and the average alloy composition Cu56Ga28Mn16 was determined. None appreciable contamination with silicon impurity was observed. It is believed that Mn deficiency is due to evaporation during the melting process.

CuMnGa 100 K

b)

Intensity (a.u.)

+ B2 * CuO monoclinic

+220

#311

*311 #220 * -2 2 2 +211

+200 *022

+110 #111

* -2 0 2 #200 *020 *021

* -1 1 1 *111

# Cu fcc

a)

CuMnGa RT + B2

Intensity (a.u.)

* CuO monoclinic

30

40

50

60

70

80

90

+220

#311

*311 #220 * -2 2 2 +211

+200 *022

* -2 0 2 #200 *020 *021

* -1 1 1 *111

# Cu fcc

100

110

2θ

Fig. 2: XRD patterns for as-cast glass-coated Cu56Ga28Mn16 microwires measured (a) at room temperature and (b) at 100 K. XRD pattern at room temperature (see Fig. 2a) shows the presence of several crystalline phases. The main phase correspond to the cubic B2 Cu-Mn-Ga structure with a lattice parameter a=0.2939(6) nm. The minor phases are fcc Cu rich solid solution with Mn and Ga (a=0.3592(5) nm) and an oxide, the monoclinic CuO. The last phase is due to surface oxidation. It is known the competence between several structures: A2, B2, L21 and D03 [10], in the Heusler type Cu rich ternary alloy is known. The modification of the production conditions

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or small changes in composition favours the thermal stability of different structures and, in consequence, different magneto-elastic behaviour. Furthermore, XRD pattern at 100 K, (see Fig. 2b), shows the same three crystalline phases. The lattice parameters are a=0.2926(5) nm (B2) and a=0.3584(7) nm (fcc). Thus, magnetic transformations are not associated to structural changes because at RT and 100 K the phases detected do not change and their relative intensity is similar, B2 is the main phase. Fig. 3: Field cooling M(T) measured at 200 Oe and 50 kOe for as-cast glass-coated Cu56Ga28Mn16 microwires. The low-field FC curve indicate the co-existence of two ferromagnetic phases with a Curie temperature of 125 K for the low-T phase, while that of high-T phase is above RT. M(T) behaviour should be explained by considering that some of the fluctuations of the magnetic moment would increase its projection along the applied magnetic field direction. This fact has the effect of increasing magnetization for a fixed magnetic field direction respect to the anisotropy easy axis at certain temperature intervals. However, M(T) abruptly decreases as T increases being the anisotropy effect overcome at the highest applied magnetic field. References [1] R. Varga, K.L. Garcia, M. Vázquez and P. Vojtanik: Phys. Rev. Lett. Vol. 94 (2005), p. 017201 [2] A. Zhukov: Adv. Funct. Mater. Vol. 16 (2006), p. 267. [3] K. Mohri and Y. Honkura: Sensor Lett. Vol. 5 (2007), p.5907 [4] M. Vázquez, in:Advanced Magnetic Microwires, in Handbook of Magnetism and Advanced Magnetic Materials, edited by H. Kronmuller and S. Parkin, volume 1, John Wiley & Sons (2007), p. 2193. [2] V. Zhukova, J.M. Blanco, M. Ipatov, A. Zhukov, C. García, J. González, R. Varga and A. Torcunov: Sens. Actuators A Vol. 126 (2007), p. 318. [6] A. Antonov, V. Borisov, O. Borisov, A. Prokoshin and A. Usov: J. Phys. D: Appl. Phys. Vol. 33 (2000), p. 1161. [7] M. I. Ilyn, V. Zhukova, J. D. Santos, M. L. Sánchez, V. M. Prida, B. Hernando, V. Larin, J. González, A. M. Tishin, and A. Zhukov: phys. stat. sol. (a) Vol. 205 (2008), p.1378. [8] K. Okawa, N. Koeda, Y. Sutou, T. Omori, R. Kainuma, and K. Ishida, Mater.Trans. Vol. 45 (2004), p.2780 [9] T. Kushima, K. Tsuchiya, Y. Sho, T. Yamada, Y. Todaka, and M. Umemoto, Mater. Sci. Forum. Vol.539-543 (2007), p.3157. [10] R. Kainuma, N. Satoh, X.J. Liu, I. Ohnuma, K. Ishida, Journal of Alloys and Compounds Vol.266 (1998), p.191.

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Glass-Coated Cu-Mn-Ga Microwires Produced by Taylor-Ulitovsky Technique 10.4028/www.scientific.net/SSP.152-153.79 DOI References [1] R. Varga, K.L. Garcia, M. Vázquez and P. Vojtanik: Phys. Rev. Lett. Vol. 94 (2005), p. 17201 doi:10.1111/j.1461-0248.2005.00810.x [3] K. Mohri and Y. Honkura: Sensor Lett. Vol. 5 (2007), p.5907 doi:10.1166/sl.2007.082 [6] A. Antonov, V. Borisov, O. Borisov, A. Prokoshin and A. Usov: J. Phys. D: Appl. Phys. ol. 33 (2000), p. 1161. doi:10.1088/0022-3727/33/10/305 [9] T. Kushima, K. Tsuchiya, Y. Sho, T. Yamada, Y. Todaka, and M. Umemoto, Mater. ci. Forum. Vol.539543 (2007), p.3157. doi:10.4028/www.scientific.net/MSF.539-543.3157 [2] V. Zhukova, J.M. Blanco, M. Ipatov, A. Zhukov, C. Garca, J. Gonzlez, R. Varga and A. Torcunov: Sens. Actuators A Vol. 126 (2007), p. 318. doi:10.1016/j.snb.2007.02.019 [7] M. I. Ilyn, V. Zhukova, J. D. Santos, M. L. Snchez, V. M. Prida, B. Hernando, V. Larin, J. Gonzlez, A. M. Tishin, and A. Zhukov: phys. stat. sol. (a) Vol. 205 (2008), p.1378. doi:10.1525/an.2008.49.2.4