Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. 1 Fine structure observation in magnetostriction near the first order phase transition temperature in Gd 5 Si 1.95 Ge 2.05 R.L.Hadimani 1, 2 , Member, IEEE, Y.Melikhov 3 , Member, IEEE, D.C.Jiles 1, 2 , Fellow, IEEE 1 Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA 2 Ames Laboratory, US Department of Energy, Iowa State University, Ames, IA 50011, USA 3 Wolfson Centre for Magnetics, Cardiff University, Cardiff CF24 3AA, UK Gd5(SixGe1-x)4 has a complex magnetic-structural phase diagram which can be divided into three distinct regions. It exhibits an unusual first order coupled magnetic-structural phase transition in the region 0.41 ≤ x ≤ 0.51. A series of magnetostrictive strain measurements were carried out as a function of magnetic field strength at different temperatures and as a function of temperature at near zero magnetic field strengths have been carried out. In this paper we report for the first time the observation of fine structure in the variation of strain with magnetic field near the first order phase transition temperature. This fine structure was observed only for the single crystalline and polycrystalline samples of Gd 5 Si 1.95 Ge 2.05 but not for Gd 5 Si 2 Ge 2 and Gd 5 Si 2.09 Ge 1.91 samples. There was a sudden increase of about 200-300 ppm in the magnetostrictive strain just prior the field induced first order phase transition. In this paper this anomaly is termed as fine structure. It was observed in measurements of both magnetostrictive strain versus magnetic field and magnetostrictive strain versus temperature. In the case of the polycrystalline Gd 5 Si 1.95 Ge 2.05 sample this anomaly was not as sharp and the sudden magnetostrictive strain change was about 40 ppm just before the field induced first order phase transition. Index Terms— First order phase transition, Gd 5 (Si x Ge 1-x ) 4 , Magnetocaloric effect, Magnetostriction. I. INTRODUCTION ARIOUS STUDIES have been conducted on Gd 5 (Si x Ge 1-x ) 4 due to its giant magnetocaloric effect near room temperature. It exhibits one of the largest room temperature giant magnetocaloric effect close to its first order phase transition temperature [1, 2]. It also exhibits a colossal magnetostriction of the order of 10,000 ppm [3-5] and a giant magnetoresistance (ΔR/R) of the order of 25% [6, 7] close to the first order phase transition temperature for x=0.5. The room temperature giant magnetocaloric effect can be utilised for energy efficient refrigeration. The energy conversion efficiency of these refrigerators can reach as high as 60% of Carnot efficiency which is twice the efficiency of normal liquid/vapor refrigerators [8]. Gd 5 (Si x Ge 1-x ) 4 has a complex phase diagram which can be divided into three main regions with distinct transition temperatures and two small regions that exhibit two-phase behavior, where the transition temperature is not distinct. The three main regions can be divided on the basis of their crystallographic structures at room temperature. The Sm 5 Ge 4 type structure occurs for the composition 0 ≤ x ≤ 0.31, Gd 5 Si 2 Ge 2 type structure occurs for the composition 0.41 ≤ x ≤ 0.51 and Gd 5 Si 4 type structure occurs for the composition 0.575 ≤ x ≤ 1 [9]. Region I with the composition 0 ≤ x ≤ 0.31 has two kinds of transitions, first order ferromagnetic orthorhombic (Gd 5 Si 4 ) to antiferromagnetic orthorhombic (Sm 5 Ge 4 ) at lower temperatures and second order antiferromagnetic to paramagnetic at higher temperatures. Region II with the composition 0.41 ≤ x ≤ 0.51 has an unusual first order phase transition from ferromagnetic orthorhombic (Gd 5 Si 4 ) to paramagnetic monoclinic. Region III with the composition 0.575 ≤ x ≤ 1 has a second order phase transition from ferromagnetic orthorhombic (Gd 5 Si 4 ) to paramagnetic orthorhombic (Gd 5 Si 4 ) [10]. Region II exhibits the largest magnetocaloric effect hence it is the most researched region in the phase diagram. In this paper we report the observation of anomaly close to the first order phase transition in magnetostriction curves for the composition Gd 5 Si 1.95 Ge 2.05 (which is in region II of the phase diagram). This anomaly near the phase transition is termed as fine structure. Magnetostrictive strain as a function of magnetic field for different temperatures and magnetostrictive strain as a function of temperature for constant magnetic field strengths were measured for single crystalline and polycrystalline samples of Gd 5 Si 1.95 Ge 2.05 (x=0.487). Similar measurements were carried out on single crystalline Gd 5 Si 2 Ge 2 (x=0.5) and polycrystalline Gd 5 Si 2.09 Ge 1.91 (x=0.522) samples but they did not show any fine structure in the measurements. II. SAMPLE PREPARATION Single crystalline samples of Gd 5 Si 1.95 Ge 2.05 and Gd 5 Si 2 Ge 2 were prepared at Ames Laboratory, US Department of Energy [11] by the Tri-arc pulling method using 99.996 % pure gadolinium (weight basis), 99.9999 % pure silicon (weight basis) and 99.999 % germanium (weight basis). The ingot was pulled at a rate of 4 mm/hour. The crystal was subsequently heat treated at 1273 K for 24 h to reduce or eliminate the amount of orthorhombic phase present. The sample was then slow cooled at a rate of 10 degrees/minute from the annealing temperature to produce a more phase-pure sample. This ingot was cut using an electric discharge machine (EDM) to limit any stress on the final cut sample. The sample was indexed using Laue X-ray diffraction technique. The polycrystalline Gd 5 Si 1.95 Ge 2.05 and the polycrystalline Gd 5 Si 2.09 Ge 1.91 samples V ————————— Manuscript received March 23, 2012. Corresponding author: R. L. Hadimani (e-mail: hadimani@iastate.edu). Digital Object Identifier inserted by IEEE