Structural and magnetic changes in hydrogenated TiFe 1  x Ni x alloys A. Kocjan a,b,n , A. Gradiˇ sek a,c , N. Daneu a,b , T. Apih a,c , P.J. McGuiness a,b , S. Kobe a a ‘‘Joz ˇef Stefan’’ Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia b NAMASTE Centre of Excellence, Jamova cesta 39, SI-1000 Ljubljana, Slovenia c EN-FIST Centre of Excellence, Dunajska 156, SI-1000 Ljubljana, Slovenia article info Article history: Received 18 November 2011 Received in revised form 2 February 2012 Available online 22 February 2012 Keywords: Melt-spinning VSM 1 H NMR TEM Mass-spectrometry Hydrogenation Thermal desorption abstract A series of crystalline B2 and amorphous TiFe 1 x Ni x samples with different compositions and hydrogen loading ratios was characterized by XRD, VSM, TG, mass-spectrometry, and 1 H NMR in order to investigate possible correlations between the sample magnetization and hydrogen content. In crystal- line samples, a reversible second-order phase transition from a-phase (B2 with soluble hydrogen) through a mixture of a- and ß-phase (B19 TiFe 1x Ni x H y ) into a pure ß-phase was found. An increase of magnetization upon hydrogen loading was observed, depending on the concentration of iron in the samples. No unambiguous correlation between the hydrogen content and the saturation magnetization was found. The saturation magnetization of as-prepared amorphous samples was found to be signifi- cantly higher than in as-prepared crystalline ones. For both types of samples, magnetization remained high after dehydrogenation as well. Thermal desorption mass-spectra was found to strongly depend on composition, whereas structure, charging pressure, and H-content did not have tremendous effect. By 1 H NMR, enhanced relaxation due to strong magnetic moments was observed; however, by TEM the existence of Fe-clusters in hydrogenated samples could not be verified. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Ever since the hydrogen storage characteristics of TiFe struc- ture were discovered by Reilly and Wiswall in 1968 [1], there has been a lot of work conducted on the TiFe-related alloys. Substitu- tion of other transition metals for Fe has been made in order to improve thermodynamic properties, and the hydrogenation properties, crystal and electronic structures have been measured and modeled. However, up to date no systematic investigation of hydrogen-induced modification of magnetic properties had been made. Hempelmann and Wicke [2] found that hydrogenation of TiFe causes an irreversible change of the magnetic properties due to the formation of iron clusters. As reported by Jankowska and Jurczyk [3], TiFe charged with hydrogen is not stable in body centered cubic structure (B2 phase), but Ti- and Fe-clusters are formed during the phase transition from B2 cubic to B19 orthor- hombic hydride phase under the influence of the distortions of the lattice, brought about by the dissolved hydrogen. In addition, Abe et al. [4] have determined such phase transition by M¨ ossbauer spectroscopy, whereas Z ¨ uchner and Kirch [5] found that iron clusters on the surface serve as catalytically active centers for hydrogen uptake. Lee et al. [6] calculated the influence of adsorbed hydrogen on the magnetic moment of the surface atoms of TiFe alloy. They found that hydrogen significantly influences the magnetic moment of the Fe atoms in both the surface and subsurface layers. Furthermore, significant change of density of states (DOS) was observed in the hydrogen-containing surfaces, which was primarily related with the formation of strongly hybridized H s-states with the Fe or Ti s- and d-bonding states. Authors also found a small induced magnetic moment of 0.23 m B for the surface Ti atoms upon hydrogenation. In addition, during hydrogen charging DOS at the Fermi level (E F ) increased, which causes a weakening of inter-atomic bonds and phase instability. As Nikolowski et al. [7] have reported a martensitic transfor- mation of a B2 to a B19 structure is well known for titanium- based shape memory alloys. They found that such transition occurs in the Ti 45 Zr 38 Al 17 alloy by plastic deformation, where the P6 3 /mmc phase is formed as an intermediate. Similar mechanism was observed by Potapov et al. [8] in TiNi–Cu and TiNi–Pd systems. In this study we tried to systematically probe irreversible changes of magnetization during reversible B2/B19 phase trans- formations in TiFe 1 x Ni x system by means of vibrating sample magnetometer (VSM). As complementary methods, we used 1 H NMR and TEM in an attempt to provide information about Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.007 n Corresponding author at: ‘‘Joz ˇef Stefan’’ Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia. Tel.: þ386 1 477 3947; fax: þ386 1 477 3221. E-mail address: andraz.kocjan@ijs.si (A. Kocjan). Journal of Magnetism and Magnetic Materials 324 (2012) 2043–2050