Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations

Abstract

We combine 57 Fe MÃűssbauer spectroscopy and 57 Fe nuclear resonant inelastic x-ray scattering (NRIXS) in nanoscale polycrystalline [bcc-57 Fe/MgO] multilayers with various Fe layer thicknesses and layer-resolved density-functional-theory (DFT) based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure to unravel the interface-related atomic vibrational properties of a multilayer system. In theory and experiment, we observe consistently enhanced hyperfine magnetic fields compared to bulk which is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic (LA) phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. arXiv:1911.05666v2 [cond-mat.mtrl-sci] 30 Jan 2020

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

“…The obtained depth profiles show interfacial enhancement of the Fe orbital and spin magnetic moments at the Fe/MgO interface. This enhancement is consistent with findings of previous theoretical studies. − By contrast, the spin magnetic moment diminished at the Fe/V interface. This decrease may balance the increase at the Fe/MgO interface and result in a nominally absent magnetic dead layer (Figure e).…”

“…It was theoretically predicted that the Fe spin and orbital magnetic moments increase when interfaced with MgO. − Experimentally, Miyokawa et al and Sicot et al observed such enhancements in a 1–2-monolayer (ML) thick Fe sandwiched between MgO and Co layers by performing X-ray magnetic circular dichroism (XMCD) measurements, , although the authors recognized that the Co underlayer most likely affected the magnetic properties of such a thin Fe layer. Jal et al showed that the magnetic moments increased by 24% in a 2–3-ML thick Fe next to an interface in an MgO/Fe (12 nm)/MgO heterostructure .…”

Anisotropic carrier dynamics in a laser-excited Fe$_{1}$/(MgO)$_{3}$(001) heterostructure from real-time time-dependent DFT

“…The Fe/MgO system was extensively investigated in the context of TMR (tunnel mag- netoresistance) and benefits from the fact that electronic, magnetic and transport properties are well established [40,41,[43][44][45][46][47][48][49][50]. Recently, also optical and lattice excitations have been subject to theoretical and experimental studies [36,42,51,52]. Bulk Fe is a ferromagnetic metal with bcc structure and a magnetic moment of 2.22 µ B /atom [53], showing a substantial density of occupied and unoccupied d-states in the vicinity of the Fermi level.…”

Phonon confinement and interface lattice dynamics of ultrathin high-k rare earth sesquioxide films: the case of Eu2O3 on YSZ(001)

“…In epitaxial thin lms and superlattices, the broken translational symmetry at the surface and interface constitutes a source of novel vibration dynamics, which relaxes towards the bulk behavior within a few atomic layers. [28][29][30][31][32][33][34][35][36][37][38] In amorphous thin lms, the structural disorder induces a reduction and a broad distribution of the mean force constants, leading to a signicant enhancement of the number of lowenergy states. [39][40][41] Fundamentally, the origin of the high-k property is directly related to the vibrational spectrum of a solid, since the static dielectric constant is a sum of the electronic and lattice contributions.…”