Annealing an amorphous Ni 80 B 20 alloy results in an intermediate state that is nanocrystalline with Ni 3 B crystallites surrounded by an amorphous pure nickel phase. Amorphous nickel is found to be ferromagnetic with saturation magnetization about 60% of that of crystalline nickel and a Curie temperature around 60 K lower. By means of calorimetric measurements, a difference of energy of 0.028 eV atom 21 between amorphous and crystalline nickel is reported. [S0031-9007(96)00450-4] PACS numbers: 75.50.Kj, 61.43.Dq, 61.46. + w Amorphous alloys have unique properties that have encouraged numerous experimental studies on their characterization at an atomic scale [1]. They have also conveyed much theoretical interest, particularly in regard to magnetic properties. For amorphous pure metals in the transition series, a number of calculations have been carried out, often with controversial results; for example, in the case of amorphous nickel some authors predict ferromagnetic behavior [2], whereas others predict a nonmagnetic state [3]. So far, however, the preparation of pure amorphous metals has met insurmountable barriers and, in order to stabilize amorphous structures, transition metals require the presence at significant concentrations of, at least, a second component, often a metalloid [4]. In this Letter, we report the observation and characterization of a pure nickel amorphous phase, which is found to be ferromagnetic with a magnetic moment per atom about 60% of the corresponding value in the crystalline phase and a Curie temperature about 60 K lower. The amorphous nickel is formed after annealing an amorphous alloy of nominal composition Ni 80 B 20 . Annealing results in precipitation of boron as Ni 3 B nanocrystals, whereas the excess nickel is left in the form of an amorphous nickel state. On an atomic scale, the structure of amorphous nickel resembles the so-called intercrystalline components of nanophases, which are currently the subject of much controversy [5,6]. Further annealing at a higher temperature results in a crystallization of amorphous nickel. The difference in energy per atom between the amorphous and crystalline phases has also been estimated by means of calorimetric measurements.A rapidly solidified Ni 80 B 20 (nominal composition) alloy was prepared by the melt spinning technique from a master alloy of the given composition melted in an induction furnace [1]. Samples were subsequently annealed, when necessary, in an argon atmosphere. Calorimetric measurements were carried out in a differential scanning calorimeter (DSC) operating at a fixed scanning rate. The x-ray diffractograms were obtained by using the copper l 0.1541 nm line, the effective angular resolution of the peaks being about 0.25 deg. Magnetic measurements were made in either a SQUID or a vibrating sample magnetometer. The thermal evolution of the alloy from its initial state is shown in the calorimetric curve of Fig. 1, obtained with a scanning rate of 10 K min 21 . Two very clear exothermic processes are observed, which peak, ...