Spin-polarized Inverse Photoemission Research Articles

Interfacial and surface magnetism in epitaxial NiCo2O4(001)/MgAl2O4 films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion.

Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces.

We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

Direct observation of a highly spin-polarized organic spinterface at room temperature

Organic semiconductors constitute promising candidates toward large-scale electronic circuits that are entirely spintronics-driven. Toward this goal, tunneling magnetoresistance values above 300% at low temperature suggested the presence of highly spin-polarized device interfaces. However, such spinterfaces have not been observed directly, let alone at room temperature. Thanks to experiments and theory on the model spinterface between phthalocyanine molecules and a Co single crystal surface, we clearly evidence a highly efficient spinterface. Spin-polarised direct and inverse photoemission experiments reveal a high degree of spin polarisation at room temperature at this interface. We measured a magnetic moment on the molecule's nitrogen π orbitals, which substantiates an ab-initio theoretical description of highly spin-polarised charge conduction across the interface due to differing spinterface formation mechanisms in each spin channel. We propose, through this example, a recipe to engineer simple organic-inorganic interfaces with remarkable spintronic properties that can endure well above room temperature.

MgO/Fe(001) andMgO/Fe(001)−p(1×1)Ointerfaces for magnetic tunnel junctions: A comparative study

The chemical, structural, and electronic properties of MgO/Fe(001) and $\text{MgO}/\text{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\text{O}$ interfaces for magnetic tunnel junctions (MTJs) have been widely investigated by means of electron spectroscopy. In particular, we present a detailed analysis of the spin-resolved electronic structure above the Fermi level, carried out by spin-polarized inverse photoemission and absorbed current spectroscopy. The MgO barrier presents good crystallinity and sharp interfaces when grown both onto Fe(001) and $\text{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\text{O}$. Moreover, we find that the exchange splitting of unoccupied bands is essentially the same for the two MgO/Fe interfaces, even though it is different for the two starting surfaces, being larger in $\text{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\text{O}$ than in Fe(001). Our findings indicate that $\text{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\text{O}$ is a good candidate for the realization of heterostructures for magnetic tunnel junctions because of its high chemical stability and reproducibility, as compared to clean Fe(001).

Spin-Polarized Tunneling Spectroscopy in Tunnel Junctions with Half-Metallic Electrodes

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La(2/3)Sr(1/3)MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La(2/3)Sr(1/3)MnO3 including the energy gap delta between the Fermi level and the bottom of the t(2g) minority-spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.

Fe thin films grown on single-crystal and virtual Ge(001) substrates

Thin Fe films have been epitaxially grown at room temperature on standard single-crystal Ge(001) substrates and virtual Ge∕Si(001) substrates. Their structural, magnetic, and electronic properties have been investigated in situ by spin-polarized inverse photoemission, x-ray photoemission spectroscopy, x-ray photoelectron diffraction, low-energy electron diffraction, and magneto-optical Kerr effect. In both cases Fe grows in a layer-by-layer fashion with very low Ge incorporation in the film (less than 3%) and the bcc local structure becomes evident for coverage larger than 5 ML. The onset of ferromagnetism appears definitively at 3 ML, while the coercive field and the spin polarization of unoccupied Fe states increase with thickness up to 30 ML. The overall behavior is very similar in the case of standard and virtual substrates, so the latter can be employed for growing high-quality Fe∕Ge interfaces.

Band structure of strained Gd(0001) films

The electronic structure of strained and unstrained Gd(0001) surfaces has been studied both theoretically and experimentally with spin-polarized photoemission spectroscopy and spin-polarized inverse photoemission spectroscopy. Good agreement between calculated surface bands and surface-induced features of the spectra provides the basis for a more detailed explanation of the origin of the spin-polarized bands than was previously possible. It has been found that observed relaxation of the expansively strained in-plane crystal lattice constant, of Gd(0001) on Mo(112), significantly affects the electronic structure of the surface.

Reduction of the Ni(110) surface spin- and orbital-magnetic moment by ac(2×2)Soverlayer

The modification of the room-temperature surface magnetic moment of Ni(110) by a $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{S}$ overlayer has been investigated using spin-polarized inverse photoemission (SPIPE) and magnetic circular dichroism (MCD) in core-level photoemission. SPIPE measurements, which are mostly sensitive to the spin contribution, indicate a reduction of the spin polarization to $(44\ifmmode\pm\else\textpm\fi{}19)%$ of the clean surface value. The MCD, which is mostly sensitive to the orbital moment, is reduced to $(25\ifmmode\pm\else\textpm\fi{}3)%.$ This apparent difference in the sensitivity to the S overlayer can in part be explained by considering the different sampling depths of the two techniques. The results provide support for theoretical predictions that the clean surface magnetic moment of Ni(110), in both the spin and orbital contributions, is enhanced compared to the bulk.

Evidence for temperature dependent moments ordering in ferromagnetic NiMnSb(100)

From magnetic circular dichroism measurements of the Heusler alloy NiMnSb, in combination with other techniques, we show that a dramatic increase in the Mn and Ni moments occurs below a ``crossover'' transition temperature of about 80--100 K. This phase transition is well below the Curie temperature of NiMnSb (\ensuremath{\sim}730 K). While the spin polarization near the Fermi level, from spin-polarized inverse photoemission, remains very high for ${k}_{\ensuremath{\parallel}}=0,$ it is unlikely that NiMnSb is a half-metallic ferromagnet near room temperature.

Layer-resolved spin polarization in Sb overlayers on NiMnSb

The magnetism of antimony overlayers on a ferromagnetic substrate is investigated by spin-polarized inverse photoemission and explained in terms of a spin-dependent envelope-function approximation (SDEFA). The atomic structure of the films, which were deposited by sputtering Sb onto a NiMnSb(001) substrate, is characterized by a unique combination of three features: (i) NiMnSb is a highly spin-polarized semi-Heusler alloy predicted to be halfmetallic, (ii) antimony is a semimetal, exhibiting a band structure reminiscent of indirect-gap semiconductors, and (iii) the small lattice mismatch ensures a well-controlled interface. Combined x-ray absorption spectroscopy and spin-polarized inverse photoemission yield a layer-resolved spin polarization decaying on a length scale of the order of 1 nm. The unusual range of the spin polarization in the paramagnetic overlayer is explained by considering the alloy–antimony interface as a spin-dependent perturbation potential and taking into account the low effective masses of the Sb conduction electrons (only about 0.1 for both electrons and holes).

A Landau-Ginzburg Description of Sb Overlayers

ABSTRACTThe spin polarization of Sb overlayers on the semi-Heusler alloy NiMnSb is investigated in terms of the Landau-Ginzburg approach. The half-metallic semi-Heusler alloy NiMnSb acts as a ferromagnetic perturbation and induces a spin polarization in the semimetallic Sb overlayer. Using a Gaussian approximation, the propagation of the spin perturbation in the overlayer is calculated. The results are compared with spin-polarized inverse photoemission spectroscopy (SPIPES) results and with recent spin-dependent envelope-function approximation (SDEFA) predictions. The Landau-Ginzburg parameters are both band-structure and temperature dependent, and it is argued that thermal spin excitations lead to an injection depth decreasing as 1//T law at high temperatures.

The polarization of Sb overlayers on NiMnSb(100)

We have investigated the induced polarization of paramagnetic Sb overlayers on the Heusler alloy NiMnSb. From combined X-ray absorption spectroscopy (XAS) and spin-polarized inverse photoemission spectroscopy (SPIPES), we can assign some of the unoccupied states of the Heusler alloy NiMnSb. With increasing thickness of the Sb overlayer, there is a decline in the density of states near the Fermi energy, as expected for a semimetal overlayer on a metallic substrate. While the Sb is polarized by the ferromagnetic NiMnSb substrate, consistent with the expectations of mean field theory, the polarization at the center of the surface/overlayer Brillouin zone cannot be easily related to the induced magnetization.

Magnetic coupling in Fe/Cr/Fe(001) by spin-resolved empty-state spectroscopies

We report on a study of empty electronic states in Cr/Fe(001) films and a trilayer as obtained by means of spin-polarized inverse photoemission and absorbed current spectroscopies: both spectroscopies are sensitive to the magnetic character of the Fe film. In relatively thick ( >7 ML) Cr films, our data do not reveal any polarization dependence; on the contrary, in Fe/Cr/Fe(001), we observe a reversal of the magnetic ordering of the Fe overlayer with respect to the buried Fe substrate as a function of the Cr spacer thickness, confirming the oscillating transition from FM to AF coupling between the Fe layers.

Observation of an exchange-split alloy surface state

The spin-resolved electronic structure at the (110) surface of the magnetic alloy ${\mathrm{FeNi}}_{3}$ has been studied with spin-polarized inverse photoemission spectroscopy in conjunction with first-principles electronic structure calculations. An unoccupied $\overline{Y}$ Schockley surface state in a bulk band gap is observed that is exchange split by 320 meV, indicating that the topmost surface layer is ferromagnetically ordered.

Strain and strain relief in Gd(0001) films on Mo(112)

The electronic structure of strained and unstrained Gd(0001) has been studied with spin-polarized photoemission spectroscopy and spin-polarized inverse photoemission spectroscopy. In this work, we observed that relaxation of the expansively strained in-plane crystal lattice constant, of Gd(0001) on Mo(112), significantly diminishes the differences in the electronic structure from that observed for Gd(0001) grown on W(110). The defects that are incorporated in the Gd films, with increasing film thickness, lead to an in-plane lattice relaxation. Such thickness dependent strain relief results a loss of net polarization for Gd(0001) grown on Mo(112) compared to the relatively unstrained Gd(0001) films grown on W(110).

Unoccupied band structure of strained gadolinium

From spin-polarized inverse photoemission, the experimental spin-resolved unoccupied band structure of gadolinium on Mo(112) has been constructed. The spin-dependent unoccupied electronic structure near the Fermi level is dominated by shallow dispersion of a spin-minority band. A spin-majority and -minority bulk-band pair also exhibits some dispersion across the Brillouin zone.

Exchange splitting of FeNi 3(1 1 0) bulk and surface bands

We have investigated the electronic structure and magnetic properties of the magnetic alloy FeNi 3 with the methods of spin-polarised inverse photoemission spectroscopy and the magneto-optic Kerr effect. We have observed bulk as well as surface states in the unoccupied electronic structure, exhibiting dispersion resembling that measured on Ni(1 1 0). However, the observed exchange splitting on FeNi 3(1 1 0) is significantly larger and the surface states are extremely sensitive to contamination. The presence of an exchange-split surface state indicates that the topmost surface layer is magnetic.

The origin of enhanced magnetization in strained gadolinium

From combined spin-resolved photoemission and spin-polarized inverse photoemission, the experimental spin-resolved band structure of gadolinium on Mo(112) has been constructed. The occupied spin dependent electronic structure near the Fermi level is dominated by shallow dispersion of a spin minority band with considerable surface weight. There is an occupied spin majority bulk band straddling the Fermi level whose spin minority counterpart remains largely on the unoccupied side of the Fermi level. This results in large spin majority weight in the occupied band structure relative to spin minority.

Wave-vector-dependent exchange splitting in a local moment system

The electronic structure of strained thin films of Gd has been studied with spin- and angle-resolved photoemission and spin-polarized inverse photoemission. The spin-dependent electronic structure is dominated by a very distinct k dependence of the exchange splitting. The surface magnetic structure is observed to be different from that of the bulk, as indicated by the different electronic structure and a much higher surface Curie temperature. The 4{percent} strain within the Gd films results in an enhanced Curie temperature. {copyright} {ital 1998} {ital The American Physical Society}

Spin asymmetries in inverse photoemission from PbS(100)

Angle-resolved spin-polarised inverse photoemission spectroscopy (ARSPIPES) has been used to study the electron-spin dependence of the photon intensity from non-magnetic samples. With the electron spin direction normal to the incidence plane and the electron wave vector k in the ΓXWK mirror plane, a spin asymmetry of up to 18% is observed from PbS(100).