Research Highlights

The deliberate manipulation of magnetic anisotropy through controlled adjustments to surface and interface morphology has become important due to its potential applications in spintronics and magnetic memory devices. In the present work, oblique angle deposition (OAD) at 65° to the surface normal on a rippled substrate, keeping nanopatterns on the SiO2 substrate perpendicular to the OAD projection, has been used to induce in-plane uniaxial magnetic anisotropy (UMA) and its thermal stability in cobalt (Co) thin film. Prior to film preparation, the patterned substrate of wavelength 68 ± 4 nm and amplitude 3.4 ± 0.2 nm was prepared separately using a low-energy ion beam erosion (IBE) process. Grazing incidence small-angle X-ray scattering (GISAXS) and magneto-optical Kerr effect (MOKE) techniques have been used for film characterization. While GISAXS provided information about film structure and morphology, MOKE provided information about magnetic properties, thus making it possible to correlate the temperature-dependent evolution of morphology with that of UMA in the film. A clear anisotropy in the growth of Co film is found to result in strong UMA. Unlike previous studies, which typically observe a reduction in UMA strength with thermal annealing, the present work demonstrates a 31 % increase in UMA strength after annealing up to 200 °C, underscoring the importance of oblique angle deposition on nano-rippled substrates for achieving high UMA and ensuring its thermal stability up to moderate temperatures. The appearance of large UMA and its unusual temperature dependence is understood in terms of the shadowing effects and column coalescence, resulting in more robust shape anisotropy. This work provides insights into morphological anisotropy, elucidating the interplay of shadowing effects, shape anisotropy, and dipolar interactions within interacting column and ripple structures. This method gives rise to morphology-induced anisotropy, which can also be applied to other ferromagnetic films to enhance the magnetic anisotropy and ensure thermal stability. 

Organic spintronics has emerged as a promising field for exploring novel spin-based phenomena and devices, offering the potential for low-power, flexible, and biocompatible electronics. The interface between metallic ferromagnetic and semiconducting organic layers plays a pivotal role in spin injection, transport, and extraction processes in these devices. Therefore, achieving a comprehensive understanding of the magnetic properties at these interfaces is essential for advancing device performance and functionality. This work explores the magnetic properties at the interface between thin Fe film and the C60 layer. We employ a multi-technique approach, combining the magneto-optic Kerr effect, which provides a global assessment of magnetic properties, and depth-resolved grazing incidence nuclear resonance scattering (GINRS) under X-ray standing wave conditions, enabling us to probe magnetism with high spatial resolution within the interfacial region. GINRS measurements reveal intriguing behavior at the interface, characterized by reduced hyperfine fields in diffused 57Fe layers. This observation suggests the formation of superparamagnetic clusters, which significantly influence the magnetic properties at the interface. These findings provide valuable insights into the complex interplay between ferromagnetic materials and organic semiconductors at the nanoscale, offering potential avenues for tailoring magnetoresistance effects in organic spintronic devices and contributing to the fundamental understanding of spin-dependent phenomena in organic spintronics. 

Figure a, b XRR and NRR and their corresponding best fit (red line) to the experimental data using REFTIM, respectively. c Simulated X-ray field intensity profile inside the Mo waveguide cavity. Positions of Fe, 57Febulk, and 57Feinterface are marked by shaded bars. d Variation of intensity profile along the depth of the sample at different incident angles q1 = 0.054 Å−1 and q2 = 0.0604 Å.−1 

Magnetization reversal of soft ferromagnetic Fe layer, coupled to [Co/Pt]ML multilayer [ML] with perpendicular magnetic anisotropy (PMA), has been studied in-situ with an aim to understand the origin of exchange bias (E.B.) in orthogonal magnetic anisotropic systems. The interface remanent state of the ML is modified by magnetic field annealing, and the effect of the same on the soft Fe layer is monitored using the in-situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the centre and an unusual increase in the coercivity, similar to exchange bias phenomena, is attributed to the exchange coupling at the [Co/Pt]ML and Fe interface. 

Figure: (a) multilayer structure and formation of x0ray standing wave(b) Schematic representation of the orientation of the sample for the scattering plane and magnetic field applied during annealing. is the angle of incidence of the beam to the sample surface. (c) GI-NRS time spectra of [Co/Pt]ML/Fe multilayer after annealing at various temperatures and cooling to RT in an in-plane magnetic field (~1500 Oe). 

The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 15 Å thick Fe57 marker between [Co/Pt]ML and Fe layers. The interface signal from 57Fe is further enhanced by performing measurements under the x-ray standing wave conditions. The combined MOKE and GINRS analysis revealed the unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found responsible for the origin of E.B. with an unusual increase in coercivity. 

Interfaces in FeCoB (FCB)/MgO/FeCoB magnetic tunnel junction play a vital role in controlling their magnetic and transport properties for various applications in spintronics and magnetic recording media. This work comprehensively studies interface structures of a few nm thick FeCoB layers in FCB/MgO and MgO/FCB bilayers using x-ray standing waves (XSW) generated by depositing bilayers between Pt waveguide structures. High interface selectivity of nuclear resonance scattering (NRS) under the XSW technique allowed to measure structure and magnetism at the two interfaces, namely FeCoB-on-MgO and MgO-on-FCB, yielding an interesting result that electron density and hyperfine fields are not symmetric at both interfaces. The formation of a high-density FeCoB layer at the MgO/FCB (FCB-on-MgO) interface with an increased hyperfine field (∼34.65 T) is attributed to the increasing volume of FeCo at the interface due to boron diffusion from 57FeCoB to the MgO layer. Furthermore, it caused unusual angular-dependent magnetic properties in the MgO/FCB bilayer, whereas FCB/MgO is magnetically isotropic. In contrast to the literature, where the unusual angular dependence in the FeCoB-based system is explained in terms of in-plane magnetic anisotropy, present findings attributed the same to the interlayer exchange coupling between bulk and interface layer within the FCB layer. 

Interfaces in MgO/FeCoB/MgO trilayer have been studied using grazing incident nuclear resonance scattering (GINRS) using the x-ray standing waves (XSW) technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB-on-MgO and MgO-on-FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in-situ temperature-dependent study discovered that this UMA systematically reduces with temperature. After annealing at 250 °C, the trilayer starts following the standard Stoner–Wohlfarth (S.W.) model for in-plane UMA. The trilayer becomes isotropic at 450°C with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.