On the magnetostructural transition in MnCoGeB x alloy ribbons A. Quintana-Nedelcos a,b,⇑ , J.L. Sánchez Llamazares a,⇑ , H. Flores-Zuñiga a a Instituto Potosino de Investigación Científica y Tecnológica (IPICyT) A.C., Camino a la Presa San José No 2055, San Luis Potosí, 78216 S.L.P., Mexico b Marmara University, Department of Material and Metalurgy, Kadikoy 3477, Istanbul, Turkey article info Article history: Received 22 January 2015 Received in revised form 6 April 2015 Accepted 1 May 2015 Available online 8 May 2015 Keywords: MnCoGe system alloys Melt-spun ribbons Multiferroic Magneto-structural transition Ferroelastic domain walls Nucleation abstract The magnetostructural transition in the Mn 0.96 Co 1.04 GeB 0.02 ribbon alloy was investigated. Chemical, structural, microstructural, and magnetic studies were performed on the samples which were annealed at different temperatures. The resulting samples underwent a first-order phase transition in which the characteristic structural transition temperature shows a near-linear and inversely proportional depen- dence to the annealing temperature. The magnetostructural transition occurs through a simultaneous ferroelastic–magnetic transition between the ferroelastic–paramagnetic and paraelastic–ferromagnetic phases. Our results suggest that the crystal structure of the hexagonal high temperature phase allows the formation of ferroelastic domains, and the domain walls act as the natural nucleation site of the low temperature paraelastic–ferromagnetic phase. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction MnCoGe-based intermetallic alloys transform from a high-temperature Ni 2 In-type hexagonal (hex) structure to a dis- torted low-temperature TiNiSi-type orthorhombic (orth) phase [1,2]. Both phases behave as a pure ferromagnetic (FM) material. For the stoichiometric MnCoGe alloy the structural transformation (ST) occurs at T > 398 K [2], far above of the ‘‘temperature window’’ delimited by the Curie temperature of the hexagonal (T c hex ) and orthorhombic (T c orth ) phases, 276 K and 355 K respectively [2–5], in which the magnetic and structural lattices are strongly linked. It has been established that large magnetocaloric effect (MCE) can be obtained due to a coupled magnetostructural first order transformation [6]. In MnCoGe-based alloys, the FM-orth to the paramagnetic (PM)-hex phase transition leads to the giant MCE. Therefore, several efforts have been made to shift the ST of MnCoGe alloys into the T c hex to T c orth temperature window, including (a) the partial atomic substitution of one (or more) of the three main elements of the MnCoGe alloy [4,7–11]; (b) stoichiometry changes [12]; (c) the introduction of elements of small atomic radius into interstitial sites [13]; and (d) the application of physical and chemical pressure [14]. Even though those approaches have been successful in tuning ST, why/how the introduction of lattice defects (i.e. atom vacancies, atom substitutions, and deformation of the lattice structure either by the introduction of an interstitial element or by an applied pressure) affect the structural behavior of the MnCoGe system remains unclear. In the present contribution the magnetostructural phase transition of Mn 0.96 Co 1.04 GeB 0.02 rib- bon alloys is study by means of structural, microstructural and magnetic experimental analysis. 2. Experimental A 3 g bulk alloy sample with nominal composition Mn 0.96 Co 1.04 GeB 0.02 was pre- pared by arc melting from highly pure starting material (>99.98%). The bulk alloy was melted three times to ensure good homogeneity. As-spun ribbons were obtained by melt spinning under a controlled highly pure argon environment. The copper wheel’s linear speed was 20 ms 1 . Some amounts of the as-spun ribbons were encapsulated in quartz tubes under an argon atmosphere for a four-hour annealing treatment followed by air-quenching. The annealed temperature (AT) varied from 650 °C to 875 °C. Hereafter, the ribbons annealed at 875 °C, 850 °C, 825 °C, 800 °C, 750 °C, and 650 °C will be referred to as AQ-875, AQ-850, AQ-825, AQ-800, AQ-750, and AQ-650, respectively (series-AQ). The as-quenched ribbons without annealing treatment are named as AQ. Structural, microstructural and, magnetic characterizations were performed for two different sample states: (i) vir- gin sample: the sample obtained from the air-quenching after the annealing treat- ment; (ii) cycled sample: samples in which a full DSC cycle was conducted. X-ray powder diffraction (XRD) patterns were obtained with a Bruker AXS model D8 Advance diffractometer using Cu K a radiation. Phase-transition studies were carried out by differential scanning calorimetry (DSC) using a TA Instruments model Q200. DSC curves for 10 mg samples were measured at a heat- ing/cooling rate of 10 K min 1 . Microstructural characterization and chemical com- position were determined by energy-dispersive X-ray spectroscopy (EDS) using a Phillips model XL-30 scanning electron microscope (SEM). SEM images were taken at room temperature (RT). Magnetization measurements were performed from 10 to 400 K using a Quantum Design PPMS-9T platform with the vibrating sample http://dx.doi.org/10.1016/j.jallcom.2015.05.008 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding authors at: Marmara University, Department of Material and Metalurgy, Kadikoy 3477, Istanbul, Turkey (A. Quintana-Nedelcos). Tel.: +52 444 2000; fax: +52 444 7269 (J.L. Sánchez Llamazares). E-mail addresses: arisqn@gmail.com (A. Quintana-Nedelcos), jose.sanchez@ ipicyt.edu.mx (J.L. Sánchez Llamazares). Journal of Alloys and Compounds 644 (2015) 1003–1008 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom