IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 23 (2012) 225601 (14pp) doi:10.1088/0957-4484/23/22/225601 Control of crystallite orientation and size in Fe and FeCo nanoneedles Raquel Mendoza-Res´ endez 1,2 , Carlos Luna 2,3 , Enrique Diaz Barriga-Castro 3 , Pierre Bonville 4 and Carlos J Serna 5 1 Facultad de Ingenier´ ıa Mec´ anica y El´ ectrica, Universidad Aut´ onoma de Nuevo Le´ on, San Nicol´ as de los Garza, Nuevo Le´ on, 66450, Mexico 2 Centro de Innovaci´ on, Investigaci´ on y Desarrollo en Ingenier´ ıa y Tecnolog´ ıa, Universidad Aut´ onoma de Nuevo Le´ on, Apodaca, Nuevo Le´ on, 6440, Mexico 3 Centro de Investigaci´ on en Ciencias F´ ısico Matem´ aticas/Facultad de Ciencias F´ ısico-Matem´ aticas, Universidad Aut´ onoma de Nuevo Le´ on, San Nicol´ as de los Garza, Nuevo Le´ on, 66450, Mexico 4 CEA, CE Saclay, Service de Physique de l’ ´ Etat Condens´ e, F-91191 Gif-sur-Yvette, France 5 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049, Madrid, Spain E-mail: raquel.mendozars@uanl.edu.mx and carlos.lunacd@uanl.edu.mx Received 23 February 2012, in final form 12 April 2012 Published 10 May 2012 Online at stacks.iop.org/Nano/23/225601 Abstract Uniform magnetic nanoneedles have been prepared by hydrogen reduction of elongated nanoarchitectures. These precursors are as-prepared or cobalt-coated aggregates of highly oriented haematite nanocrystals (⇠5 nm). The final materials are flattened nanoneedles formed by chains of assembled Fe or FeCo single-domain nanocrystals. The microstructural properties of such nanoneedles were tailored using renewed and improved synthetic strategies. In this fashion, the needle elongation and composition, the crystallite size (from 15 up to 30 nm), the nanocrystal orientation (with the h110i or h001i directions roughly along the long axis of the nanoneedle) and their type of arrangement (single chains, frustrated double chains and double chains) were controlled by modifying the reduction time, the axial ratio of the precursor haematite and the presence of additional coatings of aluminum or yttrium compounds. The values of the coercivity H C found for these nanoneedles are compared with the values predicted by the chain of spheres model assuming a symmetric fanning mechanism for magnetization reversal. S Online supplementary data available from stacks.iop.org/Nano/23/225601/mmedia (Some figures may appear in colour only in the online journal) 1. Introduction Nanostructured materials with elongated morphologies have attracted great interest due to their anisotropic and size- dependent behaviours. In fact they are of crucial importance for the study of quantum confinement [1], the efficient transport of electrons and excitons in nanodevices [2] and the design of hard magnetic systems [3]. Nevertheless, until recently, investigations of these nanostructures have progressed slowly in comparison with those of other low-dimensional materials (quantum dots and thin films) owing mainly to limitations of the preparation techniques used. Recently, a large amount of work has been carried out to develop new and more efficient routes of synthesis, and as a consequence nanosized wires [4], tubes [5], tubes in tubes [6], ribbons [7], needles [8] and hollow needles [9] have been successfully produced. However, it is worth pointing out that the synthesis of tailored linear chains of nanocrystals remains a huge challenge [10]. This fact gives rise to several fundamental and applied problems concerning nanochain systems. In this context, chains of single-domain magnetic nanoparticle deserve special attention because they are critical for the understanding of the magnetization reversal processes in nanoparticles with shape anisotropy [11]. In fact, the chain of spheres model introduced by Jacobs and Bean [12], 1 0957-4484/12/225601+14$33.00 c  2012 IOP Publishing Ltd Printed in the UK & the USA