Exchange-coupled bilayers and nanocomposites based on L10-FePt

The nanoscale interaction between a soft and a hard magnetic material can be exploited to tune the magnetic properties of a composite soft/hard system, a concept which has been first proposed in 1991 to enhance the maximum energy product of permanent magnets and proposed again later, in 2005 to solve the writeability issue of perpendicular recording media with huge magnetic anisotropy, such as L10-FePt media [1-3]. More recently, the exchange-coupling between different magnetic phases in magnetic heterostructures has been studied for spintronic applications [4,5]. The long-lasting interest in exchange-coupled systems testifies the inherent attractiveness of a simple but versatile approach, which allows tailoring the loop shape and varying coercivity in a wide range. The control of the interface between hard and soft phases has been demonstrated to strongly affect the exchange- coupling and, consequently, the magnetic behaviour of the coupled system; interface quality can influence, in particular, the coercivity decrease resulting from the soft phase addition, the magnetic regime (rigid magnet or exchange-spring) and thermal stability of the composite system [5-8]. Graded interfaces, in which the magnetic anisotropy gradually varies from the hard to the soft layer, have been claimed to offer the largest possible advantage in recording media writeability, reducing switching field values without decreasing the thermal stability [9]. We have studied different exchange-coupled composite (ECC) systems based on L10-FePt as the hard phase. These are epitaxial bilayers or nanocomposites grown on MgO(100) and SrTiO3(100) substrates, in order to obtain the high anisotropy L10 phase of FePt with easy-magnetisation c-axis oriented perpendicularly to the substrate plane. The growth was realised by a radio frequency sputtering apparatus with base pressure of 3×10-8 mbar. All the samples were grown with a sputtering pressure of 1.4×10-2 mbar Ar and at growth temperatures around 400 °C. A complete set of experimental techniques was employed for samples characterisation: X-Ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS), High Resolution Transmission Electron Microscopy (HRTEM), High Angle Annular Dark Field (HAADF) and EDS in scanning mode (STEM), Atomic and Magnetic Force Microscopy (AFM, MFM), Alternating Gradient Force Magnetometry (AGFM), Anomalous Hall Effect (AHE) Magnetometry. We have realized and studied Fe/L10-FePt(10nm) ECC bilayers with increasing thickness of the Fe layer; in this system we have obtained a strong coercivity reduction compared to the hard layer and investigated the magnetic regimes as a function of soft layer thickness and soft/hard interface morphology. A coercivity reduction up to 40% was obtained for an Fe thickness of 3.5 nm. Moreover, it was found that the soft/hard coupling is optimized when FePt growth is quasi-continuous and Fe grows epitaxially on top of FePt. We have also demonstrated that low-energy ion irradiation at low doses is a convenient and useful way to realize ECC systems with graded anisotropy, starting from epitaxial L10-FePt thin films of 20 nm grown on MgO. Using low-energy (5 keV) and low-dose (5×1015 ions cm-2) Ar+ beams, we have realized different in-depth profiles of chemical disorder and, consequently, of magnetocrystalline anisotropy by changing the beam incident angle. After irradiation, the material has a reduced coercivity and shows a reversal behaviour dependent on irradiation conditions including ion energy, dose and incidence angle. More recently, we have focused on a new composite system, where the soft phase is Fe3Pt and soft/hard nanocomposite islands are grown on both MgO and SrTiO3 (STO). The reduced misfit between the soft and hard phases has allowed us to grow fully epitaxial nanocomposites with Fe3Pt growing pseudomorphic on top of L10-FePt(001) in the whole explored thickness range (tFePt fixed to 3.55 nm, tFe3Pt from 2 to 5 nm) (Figure 1). By slightly modifying the nominal composition of the soft phase, both the magnetic phase diagram and the coercivity reduction occurring with increasing soft phase thickness can be modified. The largest coercivity reduction can be obtained with the highest Fe content in the soft phase. The optimum structural quality has been obtained with the STO substrate, thus allowing the full exploitation of the soft/hard coupling. Figure 1: Cross-sectional HRTEM image of a Fe3Pt/FePt nanocomposite (a) and corresponding FFT of the nanocomposite (b) and substrate (c) References: [1]Kneller E. F. and Hawig R. H.; The exchange-spring magnet: a new material principle for permanent magnets; IEEE Trans. Magn. 27; 3588 (1991). [2]Victora R. H. and Shen X.; Composite media for perpendicular magnetic recording; IEEE Trans. Magn. 41; 537 (2005). [3]Suess D. et al.; Multilayer exchange spring media for magnetic recording; Appl. Phys. Lett. 89; 113105 (2005). [4]Anh Nguyen N. et al.; [Co/Pd]-NiFe exchange springs with tunable magnetization tilt angle; Appl. Phys. Lett. 98; 172502 (2011). [5]F. Magnus et al.; Long-range magnetic interactions and proximity effects in an amorphous exchange-spring magnet; Nat. Comm. 7; 11931 (2016). [6]Casoli F. et al.; Strong coercivity reduction in perpendicular FePt/Fe bilayers due to hard/soft coupling; Appl. Phys. Lett. 92; 142506 (2008). [7]Alexandrakis V. et al.; Magnetic properties of graded A1/L10 films obtained by heat treatment of FePt/CoPt multilayers; J. Appl. Phys. 107; 013903 (2010). [8]Goll D. et al.; Experimental realization of graded L10-FePt/Fe composite media with perpendicular magnetization; J. Appl. Phys.104; 083903 (2008). [9]Suess D.; Multilayer exchange spring media for magnetic recording; Appl. Phys. Lett. 89; 113105 (2006).