Role of stacking faults in the structural and magnetic properties of ball-milled cobalt J. Sort, 1 S. Surin ˜ ach, 1 J. S. Mun ˜ oz, 1 M. D. Baro ´ , 1, * M. Wojcik, 2 E. Jedryka, 2 S. Nadolski, 2 N. Sheludko, 3 and J. Nogue ´ s 4 1 Departament de Fı ´sica, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain 2 Institute of Physics, Polish Academy of Sciences, 02 668 Warszawa, Poland 3 Faculty of Physics, University of Sofia, 1126 Sofia, Bulgaria 4 Institucio ´ Catalana de Recerca i Estudis Avanc ¸ats (ICREA) and Departament de Fı ´sica, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain Received 22 January 2003; published 16 July 2003 Stacking faults are found to play a crucial role in the evolution of the structural and magnetic properties of cobalt subjected to ball milling. This has been evidenced by using complementary techniques, i.e., magnetom- etry and torque measurements, nuclear magnetic resonance NMR and x-ray diffraction XRD. After short milling times a stacking-fault driven transformation from fcc to hcp cobalt is observed, which is accompanied by an increase of the effective magnetic anisotropy, the NMR restoring field and the coercivity. The results suggest that small amounts of stacking faults can be beneficial to enhance the coercivity in hexagonal Co. For longer milling times, both XRD and NMR results show that the hcp phase becomes heavily distorted because of the large amount of stacking faults accumulated. This induces a decrease of the magnetic anisotropy, which leads to the overall softening of the material. DOI: 10.1103/PhysRevB.68.014421 PACS numbers: 75.50.Tt, 82.56.-b, 61.72.Nn, 67.80.Jd INTRODUCTION The cobalt allotropic phase transformations between hex- agonal close packed hcp and face centered cubic fcc have been extensively studied both from the theoretical 1 and ex- perimental points of view. 2 In particular, these transforma- tions can be induced in Co by ball milling. 3,4 Recently, it has been demonstrated that the milling induced hcp-fcc transfor- mation is governed by a process of stacking fault accumulation. 4 However, the magnetic changes in Co ac- companying the structural transitions induced during the milling have not been systematically studied. Stacking faults are known to play a crucial role in the magnetic properties of Co-based hexagonal alloys e.g., longitudinal and perpen- dicular recording media and are usually considered to bring about a magnetic softening of this kind of materials. 5,6 How- ever, small amounts of defects seem to improve some of the magnetic properties of other hexagonal Co alloys, e.g., ball milled SmCo 5 . 7 Thus, a better understanding of the correla- tion between the structural and magnetic properties of Co based alloys is important for the advancement of their applications. It is well known that mechanical milling is a widespread technique for the production of nonequilibrium states. 8 In particular, ball milling has been shown to be an excellent process to introduce large amounts of defects in metals in a controlled way, by adjusting the milling conditions, e.g., the milling energy milling frequency, ball-to-powder ratio, milling atmosphere or milling time. 9 It is also noteworthy that although nuclear magnetic reso- nance NMR is particularly suited to study the structural and magnetic properties of Co and Co-based alloys, 10 it has not been used to investigate the structural and magnetic transfor- mations induced in Co by ball milling. Since NMR renders information about the local environment of the atoms it often complements the results obtained from macroscopic struc- tural or magnetic techniques such as x-ray diffraction or magnetization. In the present work we correlate the milling induced structural transformations with the evolution of the magnetic properties in Co from a microscopic i.e., NMR and macro- scopic points of views. The results indicate that the milling induced stacking faults determine the structural and magnetic properties of Co, hence the appropriate control of the stack- ing faults can lead to the improvement of the magnetic properties. EXPERIMENTAL PROCEDURE Cobalt powders 99.5%, -325 mesh, from Alfa-Aesar ® were mechanically milled in a planetary ball mill Fritsch Pulverissette 7, at 500 rpm, during times ranging from 0.1 to 20 h, using agate vials ( V =20 ml) and six agate balls with diameter  =10 mm) in a ball-to-powder weight ratio of 2:1. To avoid oxidation, the vials were previously sealed under argon atmosphere. The as-milled powders were structurally characterized by x-ray diffraction XRD, using Cu- K  ra- diation, and nuclear magnetic resonance. From the XRD pat- terns, microstructural parameters, such as crystallite sizes, microstrains or stacking faults were quantified by means of a data analysis program based on a full pattern fitting proce- dure Rietveld method. 11,12 In particular, crystallite sizes, D, and microstrains,   2  1/2 , were determined using the Delft model, 11 while stacking fault probabilities were evalu- ated according to the Warren formulas. 13 59 Co NMR spectra were recorded in a zero external dc field, at room tempera- ture, every 1 MHz, in the frequency range 205–230 MHz, by means of an automated, frequency swept spin-echo spec- trometer. Prior to NMR experiments, the as-milled powders were embedded in a polymeric resin and aligned during the resin hardening under a magnetic field ( H =8 kOe). All magnetic measurements were performed in oriented pow- ders. For each sample, several NMR spectra were taken us- PHYSICAL REVIEW B 68, 014421 2003 0163-1829/2003/681/0144217/$20.00 ©2003 The American Physical Society 68 014421-1