Journal of Magnetism and Magnetic Materials 272–276 (2004) 1340–1341 Glassy magnetism in mechanically alloyed Fe 35 Cr 65 J.A. De Toro*, T. Mun˜oz, P. Mun˜iz, J.M. Riveiro Departamento de F! ısica Aplicada, Facultad de Quimicas, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Abstract A mixture of Fe and Cr powders, with the atomic concentration Fe 35 Cr 65 ; was mechanically alloyed in a high-energy planetary mill for 290 h: Upon milling, the X-ray diffraction pattern stopped evolving much earlier than the hysteresis loop. The final product (milled for 290 h) showed a strong irreversibility between the field-cooled and zero-field cooled magnetisation curves below the maximum exhibited by the former at T m E170 K; accompanied by a slightly frequency- dependent peak of the AC susceptibility. These features are explained in terms of the blocking of interacting nanoclusters. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Lk; 75.50.Tt; 75.50.Bb Keywords: Magnetic nanoparticles; Mechanical alloying; Fe–Cr; Spin-glasses Nanogranular magnetic materials have received much attention since long ago due to their rich phenomenol- ogy [1]. In the last decade, the giant magnetoresistance effect (GMR) exhibited by these systems has increased their interest from the application viewpoint [2]. From the fundamental perspective, there have been a recent series of both theoretical and experimental reports exploring the possibility of obtaining spin-glass-like (SGL) behaviour at low temperatures in a nanoparticle system with sufficiently high dipolar interaction between the particles [3–5]. Such systems could even show critical dynamics described by exponents roughly similar to those of canonical spin-glasses [6]. Mechanical alloying, a technique better known for its capability to produce extended solid solutions and nanocrystalline materials, has been shown able to synthesise bulk nanogranular materials with these SGL features [7,8]. The material presented here was obtained by milling Fe and Cr powders (in the atomic concentration Fe 35 Cr 65 ) sealed under vacuum in hardened steel vials in a high-energy planetary mill operated at 250 rpm: The ball-to-powder ratio was BRP=12. No surfactant was necessary, for most of the powder remained loose. Fig. 1 summarises the structural, as seen by X-ray diffraction (XRD), and magnetic evolution of the mixture upon milling. The XRD pattern does not evolve at all after 50 h of milling; however, the room temperature hyster- esis loop keeps diminishing during 6 times this period. This result is illustrative of the inadequacy of conven- tional X-ray diffraction (XRD) to characterise the nanostructure of disordered alloys, as has been com- mented before by Le Ca. er et al. [9]. From Fig. 1, it can be concluded that the compositional homogenisation— related to the decrease in the magnetisation—still proceeds for a long time after the nanocrystals refine- ment has apparently reached a stationary state. Since the reflexions of a-Fe and BCC Cr nearly coincide, nothing conclusive can be said about the phases responsible for the peaks appearing in the XRD patterns. The final product (alloy milled for 290 h) showed an essentially linear response of the magnetisation to the magnetic field up to 14 kOe (see inset in Fig. 1), but the slope ð3  10 4 emu=gOeÞ was too large to be attribu- table to a simple paramagnetic phase. The shape and the strong irreversibility between the field-cooled (FC) and zero-field cooled (ZFC) magneti- sation (see Fig. 2) suggest that the commented linear response is related to the initial susceptibility of very fine superparamagnetic particles. The irreversibility begins only slightly above the ZFC maximum ðT max E140 KÞ pointing to either a narrow size distribution of the particles—an improbable idea taking into account the ARTICLE IN PRESS *Corresponding author. Tel.: +34-926-295300X3760; fax: +34-926-295318. E-mail address: joseangel.toro@uclm.es (J.A. De Toro). 0304-8853/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1162