Interfacial Fe Atoms Research Articles

Different oxygen migration behaviors at CoFe/MgO and CoFe/HfO2 interfaces and their effects on the magnetic anisotropy

We have studied annealing-induced oxygen migration at CoFe/MgO and CoFe/HfO2 interfaces and its effect on the magnetic anisotropy of Ta/CoFe/MgO (HfO2) films. Through x-ray photoelectron spectroscopy, we found that the Fe–O bonds exist at both CoFe/MgO and CoFe/HfO2 interfaces at the as-deposited state due to the oxidation of interfacial Fe atoms during the deposition of the MgO and HfO2 layers. After annealing, the amount of the Fe–O bonds at the CoFe/MgO interface decreases, whereas at the CoFe/HfO2 interface, it increases, indicating that the oxygen atoms migrate from Fe–O bonds to MgO layers at the CoFe/MgO interface, whereas they migrate from the HfO2 layer to Fe–O bonds at the CoFe/HfO2 interface. Correspondingly, the magnetic anisotropy energy decreases in the Ta/CoFe/MgO film but increases in the Ta/CoFe/HfO2 film after annealing. We attributed these results to interfacial Fe 3d–O 2p orbital hybridization modulated by different oxygen migration behaviors. Our results may improve the understanding of the oxygen migration effect on magnetic anisotropy and anomalous Hall sensitivity in ferromagnet/oxide heterostructures.

Ta cap-induced stabilization of interfacial ferromagnetism and enhanced magnetoelectricity in ultrathin FeRh films

We report ab initio electronic structure calculations to study the effect of Ta cap on the magnetic properties and the magnetoelectric response of Ta/FeRh/MgO nanojunctions. The calculations reveal that the Ta cap reverses the magnetic phase stability in ultrathin FeRh films leading to the stabilization of the ferromagnetic phase and interfacial reconstructed ferromagnetism. We demonstrate that the Ta cap induces a large charge transfer which in turn reduces dramatically the magnetic moments of the interfacial Fe atoms regardless the magnetic configuration and enhances the in-plane (out-of-plane) magnetization orientation of the antiferromagnetic (AFM) (ferromagnetic (FM)) phase compared to the uncapped bilayer. The Ta cap modifies substantially and enhances the magnetoelectric response under an external electric field, where the voltage controlled magnetic anisotropy (VCMA) changes from ∨-shape in the AFM to linear behavior in the FM phase with large VCMA efficiency. These findings demonstrate the manipulation of magnetic ordering of FeRh films with heavy metal capping and electric field which can promote the application of FeRh alloy in MeRAM.

Origin of the large interfacial perpendicular magnetic anisotropy in MgO/Co2FeAl

Interfacial perpendicular magnetic anisotropy in the $\mathrm{MgO}/{\mathrm{Co}}_{2}\mathrm{FeAl}$ heterostructure is desired for technological applications, while the origin of the large interfacial anisotropy constant ${K}_{i}$ remains controversial. Here we show that, by modeling four types of interface models for the $\mathrm{MgO}/{\mathrm{Co}}_{2}\mathrm{FeAl}$ system using first-principles calculations, the $\mathrm{MgO}/{\mathrm{Co}}_{2}$ interface is energetically more favorable than the MgO/FeAl interface and the interfacial Co atoms at the former interface produce out-of-plane ${K}_{i}$, while the interfacial Fe atoms at the latter interface produce in-plane ${K}_{i}$. The origin of these different behaviors can be explained by the atomic-resolved and orbital-resolved ${K}_{i}$ along with the perturbation theory energy analysis. In addition, we also studied the influence of 26 capping layers on the interfacial magnetic anisotropy of $\mathrm{MgO}/{\mathrm{Co}}_{2}\mathrm{FeAl}$ and found that Fe and W capping can significantly enhance ${K}_{i}$ in the $\mathrm{MgO}/{\mathrm{Co}}_{2}\mathrm{FeAl}$ with a particularly large ${K}_{i}$ of $4.90\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$ in the W-capped model. This work clarifies the atomistic origin of the interfacial perpendicular magnetic anisotropy and provides guidance to further enhance interfacial ${K}_{i}$ by adding capping layers in the $\mathrm{MgO}/{\mathrm{Co}}_{2}\mathrm{FeAl}$.

First-principles study of interfacial magnetoelectric coupling in Fe3Ga/BaTiO3/Fe3Ga heterostructure

Electrically controlled magnetism is highly desirable for future magnetic memories and spintronic devices. In this work, magnetoelectric (ME) coupling at the interfaces in the Fe3Ga/BaTiO3/Fe3Ga heterostructure is investigated by using first-principles calculations. It is found that the ME coupling in the Fe3Ga/BaTiO3/Fe3Ga heterostructure originates from interfacial bonding, which results in a changed magnetization of interfacial atoms when the ferroelectric layer reverses its polarization direction. The results of electronic density of states indicate that the induced magnetization arises from the changed hybridization state among the d-orbitals of interfacial Fe and Ti atoms. The ME coefficient of the Fe3Ga/BaTiO3 bilayer structure is estimated to be 4.13 × 10−7 μB m/V from the change of magnetic moment induced by polarization reversal, providing a theoretical guidance on further development of multiferroic heterostructures for potential multifunctional device applications.

Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

Isotope-selective $^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering (NRIXS) measurements and atomic-layer resolved density functional theory (DFT) calculations were used to investigate the effect of interfaces on the vibrational (phonon) density of states (VDOS) of (001)-oriented nanoscale Fe/Ag and Fe/Cr multilayers. The multilayers in the experiment contained isotopically enriched $^{57}\mathrm{Fe}$ monolayers as probe layers located either at the Fe/Ag or Fe/Cr interfaces or in the center of the Fe films. This allows probing of the vibrational dynamics of Fe sites either at the buried interfaces or in the center of the Fe films. For Fe/Ag multilayers, distinct differences were observed experimentally between the Fe-partial VDOS at the interface and in the center. At the Fe/Ag interface, the high-energy longitudinal-acoustic (LA) phonon peak of Fe near $\ensuremath{\sim}35$ meV is suppressed and slightly shifted to lower energy, and the low-energy part of the VDOS below $\ensuremath{\sim}20$ meV is drastically enhanced, as compared to the Fe-specific VDOS in the center Fe layers or in bulk Fe. Similar phenomena are found to a less degree in the Fe/Cr multilayers. The measured Fe-partial VDOS was used to determine the Fe site-selective vibrational thermodynamic properties of the multilayers. Our theoretical findings for the layer-dependent VDOS of the multilayers are in qualitative agreement with the experimental results obtained by NRIXS. For Fe/Ag multilayers, which are characterized by a large atomic mass ratio, the experimental and theoretical results demonstrate phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces due to the energy mismatch between Ag and Fe LA phonons. These phenomena are reduced or suppressed in the Fe/Cr multilayers with their about equal atomic masses. Moreover, direction-projected Fe VDOS along the (nearly in-plane) incident x-ray beam was computed in order to address the intrinsic vibrational anisotropy of the Fe/Ag multilayer. We have also performed spin-resolved electronic band structure (DFT) calculations, predicting an enhanced magnetic moment $({\ensuremath{\mu}}_{\mathrm{Fe}}=2.8\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}})$ of the interfacial Fe atoms and a high electron spin polarization (79%) at the Fermi energy for the Fe/Ag interface, as compared to the case of Fe center layers. This is a result of charge transfer from Fe to Ag at the interface. On the contrary, Cr tends to donate electrons to Fe, thus reducing the interfacial Fe moment $({\ensuremath{\mu}}_{\mathrm{Fe}}=1.9\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}})$. This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.

DFT calculations of strain and interface effects on electronic structures and magnetic properties of L1-FePt/Ag heterojunction of GMR applications

In this work, density functional theory (DFT) was employed to investigate the effect of strain and interface on electronic structures and magnetic properties of L10-FePt/Ag heterojunction. Two possible interface structures of L10-FePt(001)/Ag(001), that is, interface between Fe and Ag layers (Fe/Ag) and between Pt and Ag layers (Pt/Ag), were inspected. It was found that Pt/Ag interface is more stable than Fe/Ag interface due to its lower formation energy. Further, under the lattice mismatch induced tensile strain, the enhancement of magnetism for both Fe/Ag and Pt/Ag interface structures has been found to have progressed, though the magnetic moments of “interfacial” Fe and Pt atoms have been found to have decreased. To explain this further, the local density of states (LDOS) analysis suggests that interaction between Fe (Pt) and Ag near Fe/Ag (Pt/Ag) interface leads to spin symmetry breaking of the Ag atom and hence induces magnetism magnitude. In contrast, the magnetic moments of interfacial Fe and Pt atoms reduce because of the increase in the electronic states near the Fermi level of the minority-spin electrons. In addition, the significant enhancements of the LDOS near the Fermi levels of the minority-spin electrons signify the boosting of the transport properties of the minority-spin electrons and hence the spin-dependent electron transport at this ferromagnet/metal interface. From this work, it is expected that this clarification of the interfacial magnetism may inspire new innovation on how to improve spin-dependent electron transport for enhancing the giant magnetoresistance (GMR) ratio of potential GMR-based spintronic devices.

Enhanced magneto-optical Kerr effect at Fe/insulator interfaces

Using density functional theory calculations, we have found an enhanced magneto-optical Kerr effect in Fe/insulator interfaces. The results of our study indicate that interfacial Fe atoms in the Fe films have a low-dimensional nature, which causes the following two effects: (i) The diagonal component $\sigma_{xx}$ of the optical conductivity decreases dramatically because the hopping integral for electrons between Fe atoms is suppressed by the low dimensionality. (ii) The off-diagonal component $\sigma_{xy}$ of the optical conductivity does not change at low photon energies, but it is enhanced at photon energies around 2 eV, where we obtain enhanced orbital magnetic moments and spin-orbit correlations for the interfacial Fe atoms. A large Kerr angle develops in proportion to the ratio $\sigma_{xy}/\sigma_{xx}$. Our findings indicate an efficient way to enhance the effect of spin-orbit coupling at metal/insulator interfaces without using heavy elements.

Simulating the Effect of Contact Atomic Structure on the Spin-Dependent Transport Properties of Gold Nanowires

Some experimental research works report on the superb magnetoresistance properties of magnetically contacted gold nanowires. With the intention of trying to understand the spin-dependent transport mechanism of these structures, in this work we have used first-principles density functional theory methods to investigate effects of interface structure on the spintronic characteristics of Au nanowires. Monatomic chains of gold are sandwiched between two ferromagnetic electrodes of Fe and by substituting the interfacial Fe atoms with some other transition metal elements (including Cr, Mn, Co and Ni) the occurrence of possible enhancement in the electronic conductance and magnetoresistance characteristics of the device are investigated. It is observed that replacing the interfacial atoms with Ni raises the junction’s magnetoresistance ratio to as high as 2000%.

Giant interfacial perpendicular magnetic anisotropy in Fe/CuIn1−xGaxSe2 beyond Fe/MgO

We study interfacial magnetocrystalline anisotropies in various Fe/semiconductor heterostructures by means of first-principles calculations. We find that many of those systems show perpendicular magnetic anisotropy (PMA) with a positive value of the interfacial anisotropy constant $K_{\rm i}$. In particular, the Fe/CuInSe$_2$ interface has a large $K_{\rm i}$ of $\sim 2.3\,{\rm mJ/m^2}$, which is about 1.6 times larger than that of Fe/MgO known as a typical system with relatively large PMA. We also find that the values of $K_{\rm i}$ in almost all the systems studied in this work follow the well-known Bruno's relation, which indicates that minority-spin states around the Fermi level provide dominant contributions to the interfacial magnetocrystalline anisotropies. Detailed analyses of the local density of states and wave-vector-resolved anisotropy energy clarify that the large $K_{\rm i}$ in Fe/CuInSe$_2$ is attributed to the preferable $3d$-orbital configurations around the Fermi level in the minority-spin states of the interfacial Fe atoms. Moreover, we have shown that the locations of interfacial Se atoms are the key for such orbital configurations of the interfacial Fe atoms.

Titanium magnetic polarization at the Fe/BaTiO3 interfaces: An effect of ferroelectric polarization discontinuity

The exotic magnetic phenomena and the associated functionalities have attracted extensive scientific interest in fundamental physics and cater to the purpose of the novel material search. In this article, with a combination of the electron energy-loss spectroscopy and the X-ray absorption spectroscopy, we have investigated the interfacial Fe atoms and the induced ferromagnetic moment of Ti atoms in Fe/BaTiO3 (BTO) heterostructures. The samples were grown with two different BTO thicknesses, thus resulting in two different states of distorted oxygen environments or different electrostatic potentials. We demonstrate that in these systems, the electronic and magnetic proximity effects remain coupled as the ferroelectric polar discontinuity is held responsible for an induced transfer of the interface electrons. These electrons migrate from the Fe2+ layers to the Ti(4+)–δ layers with the hybridization via O-2p oxide orbitals into Ti orbitals to screen the ferroelectric polarization. These findings, in charge neutral BaO–TiO2 and FeO layers or nonpolar/nopolar interface, essentially underline the central role of the covalent bonding in defining the spin-electronic properties.

Interface-Confined FeOx Adlayers Induced by Metal Support Interaction in Pt/FeOx Catalysts.

Active oxide nanolayers can be stabilized on noble metal surfaces through interface confinement effect in oxide/metal inverse catalysts. Here, using normal metal/oxide catalysts we show that Fe oxide nanolayers can be confined on Pt nanoparticles (NPs) when treating a Pt/FeOx catalyst in Ar or H2/O2 atmospheres at elevated temperatures. Pt NPs partially covered with Fe oxide nanopatches are more active in CO oxidation than bare Pt NPs, while those with fully encapsulated Fe oxide shells in strong metal-support interaction (SMSI) state show much lower activity. Characterization results indicate that three steps play an important role in the formation of Fe oxide overlayers: Pt-aided reduction of interfacial Fe oxide, Pt alloying of interfacial Fe atoms, and surface segregation of alloyed Fe atoms onto surface of Pt NPs. Active surface oxides in so-called support-metal interface confinement (SMIC) state and fully encapsulated oxide layers in the SMSI state can be sequentially produced which depend on the treatment conditions.

Are insulating LiF barriers relevant for spin-polarized tunnelling applications? Insights from first-principles calculations

Structural, electronic and magnetic properties of Fe/LiF (0 0 1) interfaces and spin-polarized transport properties of Fe/LiF/Fe (0 0 1) heterostructures are studied by means of self-consistent atomistic first-principles calculations. Total energy calculations performed for various interfacial geometries show that the Fe/LiF (0 0 1) interface with Fe atoms located above anionic sites is the most stable. F–Fe and Li–Fe substitutional intermixings at Fe/LiF (0 0 1) interfaces are not energetically favourable, but F accumulation in the interstitial void spaces at Fe/LiF (0 0 1) interfaces is possible. The magnetism of interfacial Fe atoms is robust. The majority-spin ferromagnetic state conductances decay rapidly with respect to the barrier thickness, while the minority-spin ones have very large contributions at specific hot spots and play a major role in the transport properties. Depending on the interfacial geometry, tunnelling magnetoresistance ratios ranging from 460%–2400% are evidenced. Interfacial interdiffusion can affect the transport properties. The exchange coupling between Fe electrodes through LiF barriers is negligible.

A first-principles study on magnetocrystalline anisotropy at interfaces of Fe with non-magnetic metals

Magnetocrystalline anisotropy (MCA) of Fe(001) interfaces with various non-magnetic metals (Hf, Zr, Ti, Ta, Nb, V, Ir, Rh, Pt, Pd, Au, Ag, Cu, and Zn) was investigated by first-principles calculations. We found that Fe interfaces with non-magnetic metals with fully occupied d states tend to show perpendicular MCA. The spin-orbit coupling in interfacial Fe atoms plays an important role in perpendicular MCA. Conversely, Fe interfaces with non-magnetic metals with partially occupied d states exhibit in-plane MCA. The Hf/Fe(001) interface shows an exceptionally large perpendicular MCA energy of 1.5 mJ/m2, which corresponds to that of the MgO/Fe(001) interface. In these cases, contributions from interfacial Fe atoms to MCA are relatively small, and the large spin-orbit coupling of non-magnetic atoms is the primary contribution to MCA. We conclude that formation of Hf/Fe(001) interfaces will enhance the perpendicular magnetization of MgO/CoFeB-based magnetic tunnel junctions.

Enhancement of magnetoelectric effect by combining different interfacial coupling mechanisms

First-principles calculations were used to investigate the interfacial electronic structure and magnetoelectric effect in the Fe/PbTiO3 heterointerface. We demonstrate that the large magnetoelectric effect in this system is determined by the combination of different magnetoelectric coupling mechanisms, i.e., the conjunction of interface bonding mechanism and the electrostatic screening of the spin-polarized carriers. The change of induced magnetic moments on interfacial Ti atoms is due to the variation of interface bonding when the ferroelectric polarization reverses, while the contribution to magnetoelectric coupling from interfacial Fe atoms is determined by the electrostatic screening of spin-polarized carriers. The combination of different interfacial magnetoelectric coupling mechanisms enhances the magnetoelectric coefficient at the Fe/PbTiO3 heterointerface to be several times larger in magnitude than that of individual magnetoelectric coupling mechanism. Our results indicate a new strategy to design multiferroic materials with large magnetoelectric effects.

Electronic States and Magnetic Coupling in Fe/Fe3O4 Junctions

ABSTRACTExchange coupling observed recently in Fe/ Fe3O4 (001) junctions shows comparable intensity to that in Co/Ru/Co trilayers, and has potential applicability to spintronics devices. To clarify the mechanism of the exchange coupling, electronic and magnetic states of Fe/ Fe3O4 junctions are calculated in the first principles method by assuming four junction structures of bcc Fe and Fe3O4 layers. It is shown that the local moments of bcc Fe atoms at the interface increase, but those of Fe ions at the interface of Fe3O4 layer decrease. The total energy of the junctions is plotted as a function of distance between Fe and Fe3O4 layers. Calculated results of the coupling energy between Fe and Fe3O4 layers, however, are larger than experimental ones by two orders of magnitude, and they correspond to inter-atomic exchange interactions at the interface. In order to explain the experimental results, we propose a mechanism of exchange coupling mediated by impurity-like states of interfacial Fe atoms which possess reversed magnetic moments in bcc Fe layer. Frustration effects in exchange coupling between Fe and Fe3O4 layers are also discussed.

Stabilizing Monomeric Iron Species in a Porous Silica/Mo(112) Film

The stabilization of single Fe atoms in the nanopores of an ultrathin silica film grown on Mo(112) is demonstrated with scanning tunneling microscopy (STM) and density functional theory (DFT). The Fe atoms are able to penetrate the openings in the oxide surface and adsorb in two different binding configurations at the metal-oxide interface. In the energetically preferred site, the Fe stays monomeric even at temperatures above 300 K. In the second configuration that is adopted in 10% of the cases, surface atoms can be attached to the subsurface species, resulting in the formation of Fe surface clusters. The interfacial Fe atoms preserve their magnetic moment, as shown by a distinct Kondo-like response in STM conductance spectra and DFT calculations.

Theory of tunneling magnetoresistance in a disorderedFe∕MgO∕Fe(001)junction

Calculation of the tunneling magnetoresistance (TMR) of an Fe/MgO/Fe(001) junction with a disordered Fe/MgO interface is reported. It is shown that intermixing of Fe and Mg atoms at the interface decreases the TMR ratio rapidly and when about 16% of interfacial Fe atoms are substituted by Mg the calculated TMR saturates with increasing MgO thickness in good agreement with experiment. It is demonstrated that the saturation of TMR occurs because interfacial scattering leads to a redistribution of conductance channels, which opens up the perpendicular tunneling channel in the antiferromagnetic configuration that is forbidden for a perfect epitaxial junction.

Distribution of magnetic moments and hyperfine fields for Fe/Cr multilayers with different interface roughness

Distribution of d-electron magnetic moments has been calculated within the Periodic Anderson Model (PAM) for Fe/Cr superlattices with various interface roughness. Special random algorithms were developed for the modelling of stepped interfaces with different average size of the steps as well as for the modelling of interface alloying. Self-consistent calculation of magnetic moment distribution for alloyed interfaces show strong correlation with hyperfine fields (hff) on Fe nuclei, measured by the Mössbauer spectroscopy. It allows one to correlate the hff with specific environment of interfacial Fe atoms and leads to the essential correction of the empirical approach for the interpretation of Mössbauer spectra. New criterion for the testing of smoothness of the interface using Mössbauer data is suggested.

NONMAGNETIC IRON LAYERS AT THE Fe/Ru INTERFACE

Magnetic dead layers near the Fe/Ru interface in iron films grown on Ru(0001) and in Fe/Ru multilayers have been reported previously. In this paper the CEMS analysis is applied to explain why these interfacial Fe atoms are nonmagnetic. The CEMS spectra of the uncoated bcc-Fe surface clearly show the ferromagnetic order. After covering with ruthenium, the characteristic changes in the conversion electron Mössbauer spectrum were observed, which indicate existence of a nonferromagnetic phase near the Fe/Ru interface. The measured value of the isomer shift of the interface component suggests that the bcc phase persists after covering of the Fe film with Ru.