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Register a new userEnhancement in magnetic anisotropy in Co$_{40}$Fe$_{40}$B$_{20}$/Fullerene bilayers
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Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel properties due to the formation of the spinterface at the interface. Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the interface when it is placed next to a ferromagnet (FM) such as Fe or Co. Formation of spinterface occurs due to the orbital hybridization and spin polarized charge transfer at the interface. In this work, we have demonstrated that one can tune the magnetic anisotropy of the low Gilbert damping alloy CoFeB by introducing a C60 layer. We have shown that anisotropy is enhanced by increasing the thickness of C60 which might be a result of the formation of spinterface. However, the magnetic domain structure remains same in the bilayer samples as compared to the reference CoFeB film.
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Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT-MRAM), which is non-volatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density functional theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising Fe(n)Co(m)Fe(n)/MgO storage layers with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m2, leveraging the interfacial perpendicular anisotropy of Fe/MgO along with a stress-induced bulk PMA discovered within bcc Co. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure Fe/MgO case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the Fe(3ML)Co(4ML)Fe(3ML) as a storage layer for MgO-based STT-MRAM cells. The large PMA in strained bcc Co is explained in the framework of Brunos model by the MgO-imposed strain and consequent changes in the energies of dyz and dz2 minority-spin bands.
Transition metal dichalcogenides (TMD) possess novel properties which makes them potential candidates for various spintronic applications. Heterostructures of TMD with magnetic thin film have been extensively considered for spin-orbital torque, enhancement of perpendicular magnetic anisotropy etc. However, the effect of TMD on magnetic anisotropy in heterostructures of in-plane magnetization has not been studied so far. Further the effect of the TMD on the domain structure and magnetization reversal of the ferromagnetic system is another important aspect to be understood. In this context we study the effect of MoS2, a well-studied TMD material, on magnetic properties of CoFeB in MoS2/CoFeB heterostructures. The reference CoFeB film possess a weak in-plane anisotropy. However, when the CoFeB is deposited on MoS2 the in-plane anisotropy is enhanced as observed from magneto optic Kerr effect (MOKE) microscopy as well as ferromagnetic resonance (FMR). Magnetic domain structure and magnetization reversal have also been significantly modified for the MoS2/CoFeB bilayer as compared to the reference CoFeB layer. Frequency and angle dependent FMR measurement show that the magnetic anisotropy of CoFeB increases with increase in thickness of MoS2 in the MoS2/CoFeB heterostructures.
The depth profile of the intrinsic magnetic properties in an Fe/Sm-Co bilayer fabricated under nearly optimal spring-magnet conditions was determined by complementary studies of polarized neutron reflectometry and micromagnetic simulations. We found that at the Fe/Sm-Co interface the magnetic properties change gradually at the length scale of 8 nm. In this intermixed interfacial region, the saturation magnetization and magnetic anisotropy are lower and the exchange stiffness is higher than values estimated from the model based on a mixture of Fe and Sm-Co phases. Therefore, the intermixed interface yields superior exchange coupling between the Fe and Sm-Co layers, but at the cost of average magnetization.
Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high surface PMA. Moreover, the effective magnetic anisotropy has a strong tendency to go from in-plane to out-of-plane character as a function of Cr and V concentration favoring out-of-plane magnetization direction for ~1.5 nm thick Fe layers at impurity concentrations above 20 %. At the same time, spin polarization is not affected and even enhanced in most situations favoring an increase of tunnel magnetoresistance (TMR) values.
Thin highly epitaxial BiFeO$_3$ films were prepared on SrTiO$_3$ (100) substrates by reactive magnetron co-sputtering. Detailed MOKE measurements on BiFeO$_3$/Co-Fe bilayers were performed to investigate the exchange bias as a function of the films thicknesses and Co-Fe stoichiometries. We found a maximum exchange bias of H$_{mathrm{eb}}$=92 Oe and a coercive field of H$_{mathrm{c}}$=89 Oe for a 12.5 nm thick BiFeO$_3$ film with a 2 nm thick Co layer. The unidirectional anisotropy is clearly visible in in-plane rotational MOKE measurements. AMR measurements reveal a strongly increasing coercivity with decreasing temperature, but no significant change in the exchange bias field.
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