Magnetocaloric effect and magnetoresistance of Ni–Fe–Ga alloys D Pal and K Mandal Magnetism Laboratory, S.N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata-700098, India Abstract Heusler alloys with nominal composition Ni 73-x Fe x Ga 27 (x = 17, 18, 19, 20, 21, 22) were prepared by arc melting and subsequent homogenization by annealing. The magnetic properties of the alloys were studied to analyse the magnetic entropy change as a function of temperature. The maximum entropy change was observed in the alloy (x = 19) where martensite–austenite structural and ferro–para magnetic transitions were very close to each other. The transport properties and magnetoresistance (MR) of the samples were also investigated up to a magnetic field of 8 T and within a temperature range 4.2–325 K. It was observed that the MR in the austenite state was higher than that of the martensite state irrespective of the composition of the sample. A large negative MR (∼9% at 8 T) was obtained for the sample Ni 54 Fe 19 Ga 27 at 300 K which was very close to the martensitic transition temperature as well as the Curie temperature. 1. Introduction The development of ferromagnetic shape memory alloys (FSMA) is currently a subject of considerable interest due to their large field induced strain which makes them a promising material for magnetic actuators [1, 2]. Some Heusler alloys such as Ni–Fe–Ga, Ni–Mn–Ga show shape memory effect as well as large magnetocaloric effect (MCE) [3–5]. Recently large negative magnetoresistance (MR) has also been reported for those alloys [6, 7]. Consequently these materials have great technological importance due to their various novel properties. Among the Heusler alloys Ni–Mn–Ga is the most studied series. However, significant disadvantages of this alloy series include brittleness and poor ductility, which limits possible applications. Due to these problems, less brittle and partly ductile intermetallic compounds near stoichiometric Ni 2 FeGa were proposed as promising alternatives. Both stoichiometric and off-stoichiometric compounds with composition close to Ni 2 FeGa undergo a first-order structural transition from tetragonal martensite to cubic austenite on heating (or the reverse process on cooling) [8, 9], which brings about fundamental differences in the magnetic properties of the low- temperature martensite and high-temperature austenite states, causing an abrupt change in magnetization. The possible martensite structures can be non-modulated (NM), five layer modulated (10 M); seven layer modulated (14 M) as well as six-layer modulated martensite [10, 11]. The structural and magnetic phase transitions in these alloys were found to be very sensitive to the composition. This opens up the possibility of variations of the transition temperatures up to their coincidence. This deliberate coupling is an enhancement criterion to obtain a large MCE which has been analysed previously on a Ni–Mn–Ga alloy system [5, 12, 13]. Most of the studies reported by different groups associated with Ni–Fe–Ga alloys are about their magnetic shape memory effect (MSME) and MCE [14–16]. In this work, we have also studied the MR of iron-rich Ni–Fe–Ga Heusler alloys in which structural and magnetic transition temperatures are wide apart as well as very close to each other. Magnetic properties of these samples have also been investigated in detail to calculate the change in magnetic entropy of these samples. We also determined a detailed phase diagram of Ni–Fe–Ga alloys for Fe substitution. 2. Experimental Polycrystalline ingots of Ni–Fe–Ga alloys were prepared by arc melting. We started with the nominal composition Ni 73-x Fe x Ga 27 (x = 17, 18, 19, 20, 21, 22). Arc-melted ingots were annealed for homogenization at 1273 K for 72 h and kept in a sealed vacuum quartz ampoule. The composition of the alloys obtained by chemical analysis (inductively