At the ultimate limit of magnetic recording, suitable storage media will consist of nanometer-sized entities, each of which will carry one bit of information. Materials with a high magnetocrystalline anisotropy energy are required to guarantee thermal stability of the ferromagnetic state at realistic operating temperatures. The face-centered tetragonal (fct) L1 0 FePt alloy belongs to the promising class of materials that offer the perspective of storing one magnetic bit per nanoparticle. [1][2][3] Widespread activities have therefore arisen worldwide, targeting novel strategies for both the synthesis [1,[4][5][6][7][8][9][10][11][12][13][14] of suitable magnetic nanostructures and their organization into superlattices [4,12,[15][16][17][18] by means of parallel processes. Here, we present a new approach for the synthesis of size-selected L1 0 FePt nanoparticles based on the self-organization of spherical micelles formed by diblock copolymers, thereby significantly extending a previous technique [19][20][21] to produce large-scale arrays of elemental nanoparticles. Our approach overcomes the typical drawbacks of the current colloidal routes towards densely packed arrays of ferromagnetic FePt nanoparticles while still guaranteeing areal densities exceeding 1 Tbits inch -2 (1 inch ≈ 2.54 cm).Since the first presentation of magnetic data-storage devices five decades ago, the areal density of digital information has increased by eight orders of magnitude to reach values of about 200 Gbits inch -2 , as found in present hard disk drives.[22]A few years ago, an efficient method was developed to synthesize FePt nanoparticles on the basis of wet-chemical synthesis (hereafter referred to "colloidal"), which involves particle stabilization by an organic-ligand shell.[1] The significant advantage of this approach, allowing a simple preparation of densely packed 2D nanoparticle arrays from corresponding particle solutions, is, however, compensated by some serious drawbacks related to the thin ligand shell (1-3 nm) which serves as a spacer between the nanoparticles. As a consequence of the resulting small interparticle distance, the nanoparticles exhibit a strong tendency to aggregate during heat treatments. [23,24] Thermal annealing at 500-600°C is, however, generally required in order to transform the assynthesized, chemically disordered (Fe and Pt atoms randomly distributed over the lattice sites) face-centered cubic (fcc) structure, which results in superparamagnetic behavior, into the magnetically attractive L1 0 phase. Furthermore, undesirable collective magnetic dynamics arise at such small interparticle distances through dipolar coupling; [24,25] collective modes, however, are clearly at odds with the idea of storing magnetic data in individual nanoparticles. Finally, the heat-treated colloidal FePt nanoparticles are found to be highly oxidized and contaminated by carbon because of the thermally induced decomposition of the organic shell.[26]Recent alternative routes for the synthesis of L1 0 FePt nanoparticles include...
