David Pappas | University of Colorado, Boulder (original) (raw)
Papers by David Pappas
We have measured the optical spectra of light-emitting tunnel junctions with two different roughn... more We have measured the optical spectra of light-emitting tunnel junctions with two different roughnesses and measured the power spectrum of the roughness with scanning tunneling microscopy (STM). Our experiments show that most of the information on the roughness relevant to light emission can be inferred from STM measurements. We discuss the aspects of the STM data that are unique to the determination of long-wavelength roughness relevant to optical studies. The earliest studyl of light emission from metal-insulator metal tunnel junctions shows that surface roughness of the films is resporisible for the emission of light. Subsequent theoretical work 2 provides the following description of the emission process. Although nominally biased with a dc tun neling current, there are current fluctuations that extend to a frequency ofUJ = eV Iii, where V is the dc bias voltage and e and fz have their usual meaning. The current fluctuations have a broad distribution of Fourier components in kll' the wave vector parallel to the films. The current fluctuations excite electric fields in the junction. An alternative mecha nism for the excitation of the electric fields has been suggest ed by Kirtley et al. 3 who attribute the excitation to hot elec trons. There are resonances in the electric field strengths for particular values of kll and UJ. The resonant values of kll and UJ lie along trajectories in the (k ll ,UJ) plane that are called surface plasmon dispersion curves. The dispersion curves and the strengths of the resonances along them are deter mined by the dielectric functions and thicknesses ofthe films in the junction. Thus the current fluctuations or hot elec trons create a broad spectrum in both UJ and kll of electric field strengths at the surface of the junction. For the hypothetical case ofperfectly flat junctions, not all of the Fourier components of the electric field at the surface can radiate light. Light emitted from the structure can be thought of as a plane wave traveling away from the junction with frequency UJ and wave vector k = UJIc at an angle B to the junction normal. The fields at the surface of the flat junc tion must obey the usual electromagnetic boundary condi tions. These require, among other things, that the parallel components of the wave vectors of the fields on both sides of the surface be equal. Thus the electric field just inside the surface of the junction with frequency UJ and wave vector kll can only couple to a plane wave above the junction if kll = (UJlc)sin B for some value of B between 0° and 90°. Otherwise the field above the junction will die exponentially and will not represent the emission of light. Interestingly none of the resonantly enhanced fields along the surface plasmon dispersion curves can radiate in flat junctions be 7.
Magnetism in ultrathin (l-10 ML) Fe films grown on Cu(100) has been studied by spin-polarized sec... more Magnetism in ultrathin (l-10 ML) Fe films grown on Cu(100) has been studied by spin-polarized secondary electron emission spectroscopy. The variation of the magnetization with temperature and oxygen adsorption was investigated for various film thicknesses. The orientation of the magnetization for films between 5 and 6 ML thick switches reversibly between perpendicular (at low temperature) to in-plane (at high temperature). The switching transition temperature decreases with increasing film thickness, and is accompanied by a loss of long-range order over a range of 20-30 K. The transition is attributed to the temperature dependence of the perpendicular anisotropy. The effect of oxygen adsorption onto films with perpendicular remanence is to first suddenly turn the magnetization into the plane at a critical coverage, and then to kill the magnetization gradually' with continued exposure. This indicates that the uniaxial surface anisotropy at the Fe-vacuum interface plays a major role in the magnetization of the film. Magnetic thin films are an exciting field for the study of the magnetic properties of condensed matter in novel systems and at interfaces.* The two-dimensional nature of the films indicates that the magnetism is driven by an-isotropies,2*3 which are dominated by the surface anisot-ropy K, and dipole demagnetizing energy.4 For thin fee Fe overlayers on Cu(loo), K,, is perpendicular to the film,5P6m while the dipole demagnetizing energy always favors in-plane magnetization. In this study we have observed the competition between these two anisotropies in Fe/Cu(100) by measuring the direction of the remanent magnetization for different film thicknesses, temperatures, and oxygen coverages. The Fe films were grown at low temperature (z 125 *K) in order to minimize Fe/& intermixing,' and then annealed to 300 K before data were taken. The base pressure of the
Rapid Communications are intended for the accelerated publication of important new results and ar... more Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial ogive and in production A. Rapid Communication in Physical Review B should be no longer than 4 printed pages and must be accompanied by an abstract Pag. e proofs are sent to authors A study of the remanent magnetization M, in ultrathin Fe films grown at l25 K on Cu(100) and Ag(100) substrates was conducted using spin-polarized secondary-electron spectroscopy. For bcc-Fe/Ag(100) and fcc-Fe/Cu(100) films, perpendicular M, was observed for film thickness less than 7 and I I A, respectively. In-plane M, was observed for thicker films. For thickness below the critical thickness, both systems showed a reversible switching transition of the magnetization orientation from perpendicular to in plane as the temperature was increased. For the Fe/Cu(100) films the switching could be observed for many temperature cycles providing that the film was not annealed above 350 K, while the Fe/Ag(100) films switching was stable for only 1-2 annealing cycles to 300 K. A loss of M, during the switching transition is observed, and is attributed to a change in the magnetic nature of the film. The details of the loss of magnetization observed in Fe/Ag(100) and Fe/Cu(100) are significantly different.
Surface magnetic properties of epitaxial Gd(OOO1) films grown on W(110) were studied with spin-po... more Surface magnetic properties of epitaxial Gd(OOO1) films grown on W(110) were studied with spin-polarized resonant 4f photoemission. Films grown at room temperature and subsequently annealed to 550 % show 66% polarization at 150 K, whereas films grown at 400 " C show only 46% polarization. Both types of films exhibit surface enhanced magnetic order, with the highest observed surface critical temperature exceeding the bulk value by as much as 60 K. For the first time, a rich variety of novel surface magnetic phenomena, i.e., sizeable perpendicular polarization component and unusual temperature hysteresis in the spin polarization, are observed, demonstrating that surface magnetic reconstruction is present in Gd(OOO1) films.
Circularly polarized x rays from a synchrotron light source were used in photoemission from thin ... more Circularly polarized x rays from a synchrotron light source were used in photoemission from thin fee Fe films on Cu(100). The measured exchange splitting for the 3s and 2p levels in these films is reduced from that measured in bulk bee Fe, For both the 2~s,~ and the 2pli2 levels the splittings are about half that of bulk bee Fe. This trend is in line with measurements of the 3s exchange splitting from fee Fe compared to bee Fe as well. A 0.25 eV exchange splitting is observed in the 3p core level. The difference in photoemission spectra for left versus right circularly polarized radiation (i.e., circular dichro-ism) can allow for a direct identification of the spin character of core-level states in magnetic samples. l-3 Baumgar-ten et ul. ' have recently exploited this advantage of magnetic circular dichroism in photoemission (MCDPE) to deconvolute the exchange split 2p2p3,2 and 2pv2 core levels in bulk single-crystal bee Fe samples. This experiment confirmed that the unusually broad L, and L3 lines do indeed contain contributions from exchange split lines. " While it is still not possible to identify the direct cause of the line splitting (i.e., as a ground-state property of the system versus an exchange interaction of the photoelectron with the photohole) it is clear that this MCDPE gives similar information as, for example, spin-resolved photo-emission (SRPE). However, with MCDPE only a factor of 10 loss of intensity is given up to obtain highly circularly polarized light,5 while SRPE entails the use of an electron spin detector, with typical efficiencies of 10-4.6 In addition, the inherent surface sensitivity and elemental specificity of photoemission makes the study of the core-level splittings of very thin Fe films (2-4 atomic layers, AL) possible. The magnetism and structure of thin Fe films grown on Cu(100) is interesting from a more general perspective. The lattice constant of the high-temperature, antiferromag-net.ic, fee phase of Fe (y-Fe) matches closely to that of Cu (3.59 vs 3.61 A at room temperature) 17 and it has been shown that fee Fe does grow on the various Cu crystalline faces up to s 10 AL. However, thin fee Fe films grown on Cu(100) show distinct reconstructions as observed by low-energy electron diffraction (LEED),* and also e'xhibit fer-romagnetism with a strong surface anisotr0py.s We therefore chose hlCDPE in order to compare the core-level splittings of the metastable, fee Fe with those of bee Fe, In order to preserve the integrity of the Fe-Q interface , the Fe layers were grown on a Cu(100) substrate at 150 K. " A subsequent anneal to 300 K was then used to order the film. The LEED pattern sharpened significantly after the anneal, with the 5 x 1 reconstruction that is characteristic of thin fee Fe layers grown at room temperature.
Large magnetic circular dichroism using circularly polarized synchrotron radiation has been obser... more Large magnetic circular dichroism using circularly polarized synchrotron radiation has been observed at the I!& absorption edges of thin (1-12 monolayers) fee Fe films grown on Cu(00 1). Dramatic changes in the 2p branching ratio are observed when the orientation of photon helicity and sample magnetizat.ion are varied from parallel to antiparallel. The temperature and film thickness dependence of the perpendicular anisotropy in these films could be monitored by variations in the 2p branching ratio. Finally, our results are described in a simple theoretical framework that allows a determination between the predicted low-and high-spin phases for fee Fe. Our data suggest a high-spin phase with a moment of2.0-2.5 ~Jatom. Interestingly, thicker films with remanant magnetic. moments in the film plane present smaller branching ratio variations consistent with either a reduced moment or with domain closure for these films. The surface sensitivity and elemental specificity of this technique make it particularly attractive for the study of surface and thin film magnetism.
With spin polarized electron spectroscopies, we have investigated ordered Gd(OOO I) films deposit... more With spin polarized electron spectroscopies, we have investigated ordered Gd(OOO I) films deposited on W(llO). The photoemission features of the gadolinium 5d snrfac~ state, the 4£ levels, and the background exhibit considerable spin polarization along the same direction in the plane of the film, indicative of ferromagnetic coupling between the surface and the bulk. The 4f spin polarized photoemission data. provides strong evidence that the surface 4£ polarization differs from the bulk 4f polarization for Gd(OOO I). Onr temperature dependent measurements with spin polarized secondary electron spectroscopy conclusively establishes that the surface of clean Gd(OOOl) possesses a perpendicuiRr polarization component which persists to an enhanced surface Curie temperature. Small amounts of contamination at the surface result in the disappearance of the pcrpcudicular component and, therefore, a more perfect ferromagnetic coupling between the surface and the bulk.
We have measured the optical spectra of light-emitting tunnel junctions with two different roughn... more We have measured the optical spectra of light-emitting tunnel junctions with two different roughnesses and measured the power spectrum of the roughness with scanning tunneling microscopy (STM). Our experiments show that most of the information on the roughness relevant to light emission can be inferred from STM measurements. We discuss the aspects of the STM data that are unique to the determination of long-wavelength roughness relevant to optical studies. The earliest studyl of light emission from metal-insulator metal tunnel junctions shows that surface roughness of the films is resporisible for the emission of light. Subsequent theoretical work 2 provides the following description of the emission process. Although nominally biased with a dc tun neling current, there are current fluctuations that extend to a frequency ofUJ = eV Iii, where V is the dc bias voltage and e and fz have their usual meaning. The current fluctuations have a broad distribution of Fourier components in kll' the wave vector parallel to the films. The current fluctuations excite electric fields in the junction. An alternative mecha nism for the excitation of the electric fields has been suggest ed by Kirtley et al. 3 who attribute the excitation to hot elec trons. There are resonances in the electric field strengths for particular values of kll and UJ. The resonant values of kll and UJ lie along trajectories in the (k ll ,UJ) plane that are called surface plasmon dispersion curves. The dispersion curves and the strengths of the resonances along them are deter mined by the dielectric functions and thicknesses ofthe films in the junction. Thus the current fluctuations or hot elec trons create a broad spectrum in both UJ and kll of electric field strengths at the surface of the junction. For the hypothetical case ofperfectly flat junctions, not all of the Fourier components of the electric field at the surface can radiate light. Light emitted from the structure can be thought of as a plane wave traveling away from the junction with frequency UJ and wave vector k = UJIc at an angle B to the junction normal. The fields at the surface of the flat junc tion must obey the usual electromagnetic boundary condi tions. These require, among other things, that the parallel components of the wave vectors of the fields on both sides of the surface be equal. Thus the electric field just inside the surface of the junction with frequency UJ and wave vector kll can only couple to a plane wave above the junction if kll = (UJlc)sin B for some value of B between 0° and 90°. Otherwise the field above the junction will die exponentially and will not represent the emission of light. Interestingly none of the resonantly enhanced fields along the surface plasmon dispersion curves can radiate in flat junctions be 7.
Magnetism in ultrathin (l-10 ML) Fe films grown on Cu(100) has been studied by spin-polarized sec... more Magnetism in ultrathin (l-10 ML) Fe films grown on Cu(100) has been studied by spin-polarized secondary electron emission spectroscopy. The variation of the magnetization with temperature and oxygen adsorption was investigated for various film thicknesses. The orientation of the magnetization for films between 5 and 6 ML thick switches reversibly between perpendicular (at low temperature) to in-plane (at high temperature). The switching transition temperature decreases with increasing film thickness, and is accompanied by a loss of long-range order over a range of 20-30 K. The transition is attributed to the temperature dependence of the perpendicular anisotropy. The effect of oxygen adsorption onto films with perpendicular remanence is to first suddenly turn the magnetization into the plane at a critical coverage, and then to kill the magnetization gradually' with continued exposure. This indicates that the uniaxial surface anisotropy at the Fe-vacuum interface plays a major role in the magnetization of the film. Magnetic thin films are an exciting field for the study of the magnetic properties of condensed matter in novel systems and at interfaces.* The two-dimensional nature of the films indicates that the magnetism is driven by an-isotropies,2*3 which are dominated by the surface anisot-ropy K, and dipole demagnetizing energy.4 For thin fee Fe overlayers on Cu(loo), K,, is perpendicular to the film,5P6m while the dipole demagnetizing energy always favors in-plane magnetization. In this study we have observed the competition between these two anisotropies in Fe/Cu(100) by measuring the direction of the remanent magnetization for different film thicknesses, temperatures, and oxygen coverages. The Fe films were grown at low temperature (z 125 *K) in order to minimize Fe/& intermixing,' and then annealed to 300 K before data were taken. The base pressure of the
Rapid Communications are intended for the accelerated publication of important new results and ar... more Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial ogive and in production A. Rapid Communication in Physical Review B should be no longer than 4 printed pages and must be accompanied by an abstract Pag. e proofs are sent to authors A study of the remanent magnetization M, in ultrathin Fe films grown at l25 K on Cu(100) and Ag(100) substrates was conducted using spin-polarized secondary-electron spectroscopy. For bcc-Fe/Ag(100) and fcc-Fe/Cu(100) films, perpendicular M, was observed for film thickness less than 7 and I I A, respectively. In-plane M, was observed for thicker films. For thickness below the critical thickness, both systems showed a reversible switching transition of the magnetization orientation from perpendicular to in plane as the temperature was increased. For the Fe/Cu(100) films the switching could be observed for many temperature cycles providing that the film was not annealed above 350 K, while the Fe/Ag(100) films switching was stable for only 1-2 annealing cycles to 300 K. A loss of M, during the switching transition is observed, and is attributed to a change in the magnetic nature of the film. The details of the loss of magnetization observed in Fe/Ag(100) and Fe/Cu(100) are significantly different.
Surface magnetic properties of epitaxial Gd(OOO1) films grown on W(110) were studied with spin-po... more Surface magnetic properties of epitaxial Gd(OOO1) films grown on W(110) were studied with spin-polarized resonant 4f photoemission. Films grown at room temperature and subsequently annealed to 550 % show 66% polarization at 150 K, whereas films grown at 400 " C show only 46% polarization. Both types of films exhibit surface enhanced magnetic order, with the highest observed surface critical temperature exceeding the bulk value by as much as 60 K. For the first time, a rich variety of novel surface magnetic phenomena, i.e., sizeable perpendicular polarization component and unusual temperature hysteresis in the spin polarization, are observed, demonstrating that surface magnetic reconstruction is present in Gd(OOO1) films.
Circularly polarized x rays from a synchrotron light source were used in photoemission from thin ... more Circularly polarized x rays from a synchrotron light source were used in photoemission from thin fee Fe films on Cu(100). The measured exchange splitting for the 3s and 2p levels in these films is reduced from that measured in bulk bee Fe, For both the 2~s,~ and the 2pli2 levels the splittings are about half that of bulk bee Fe. This trend is in line with measurements of the 3s exchange splitting from fee Fe compared to bee Fe as well. A 0.25 eV exchange splitting is observed in the 3p core level. The difference in photoemission spectra for left versus right circularly polarized radiation (i.e., circular dichro-ism) can allow for a direct identification of the spin character of core-level states in magnetic samples. l-3 Baumgar-ten et ul. ' have recently exploited this advantage of magnetic circular dichroism in photoemission (MCDPE) to deconvolute the exchange split 2p2p3,2 and 2pv2 core levels in bulk single-crystal bee Fe samples. This experiment confirmed that the unusually broad L, and L3 lines do indeed contain contributions from exchange split lines. " While it is still not possible to identify the direct cause of the line splitting (i.e., as a ground-state property of the system versus an exchange interaction of the photoelectron with the photohole) it is clear that this MCDPE gives similar information as, for example, spin-resolved photo-emission (SRPE). However, with MCDPE only a factor of 10 loss of intensity is given up to obtain highly circularly polarized light,5 while SRPE entails the use of an electron spin detector, with typical efficiencies of 10-4.6 In addition, the inherent surface sensitivity and elemental specificity of photoemission makes the study of the core-level splittings of very thin Fe films (2-4 atomic layers, AL) possible. The magnetism and structure of thin Fe films grown on Cu(100) is interesting from a more general perspective. The lattice constant of the high-temperature, antiferromag-net.ic, fee phase of Fe (y-Fe) matches closely to that of Cu (3.59 vs 3.61 A at room temperature) 17 and it has been shown that fee Fe does grow on the various Cu crystalline faces up to s 10 AL. However, thin fee Fe films grown on Cu(100) show distinct reconstructions as observed by low-energy electron diffraction (LEED),* and also e'xhibit fer-romagnetism with a strong surface anisotr0py.s We therefore chose hlCDPE in order to compare the core-level splittings of the metastable, fee Fe with those of bee Fe, In order to preserve the integrity of the Fe-Q interface , the Fe layers were grown on a Cu(100) substrate at 150 K. " A subsequent anneal to 300 K was then used to order the film. The LEED pattern sharpened significantly after the anneal, with the 5 x 1 reconstruction that is characteristic of thin fee Fe layers grown at room temperature.
Large magnetic circular dichroism using circularly polarized synchrotron radiation has been obser... more Large magnetic circular dichroism using circularly polarized synchrotron radiation has been observed at the I!& absorption edges of thin (1-12 monolayers) fee Fe films grown on Cu(00 1). Dramatic changes in the 2p branching ratio are observed when the orientation of photon helicity and sample magnetizat.ion are varied from parallel to antiparallel. The temperature and film thickness dependence of the perpendicular anisotropy in these films could be monitored by variations in the 2p branching ratio. Finally, our results are described in a simple theoretical framework that allows a determination between the predicted low-and high-spin phases for fee Fe. Our data suggest a high-spin phase with a moment of2.0-2.5 ~Jatom. Interestingly, thicker films with remanant magnetic. moments in the film plane present smaller branching ratio variations consistent with either a reduced moment or with domain closure for these films. The surface sensitivity and elemental specificity of this technique make it particularly attractive for the study of surface and thin film magnetism.
With spin polarized electron spectroscopies, we have investigated ordered Gd(OOO I) films deposit... more With spin polarized electron spectroscopies, we have investigated ordered Gd(OOO I) films deposited on W(llO). The photoemission features of the gadolinium 5d snrfac~ state, the 4£ levels, and the background exhibit considerable spin polarization along the same direction in the plane of the film, indicative of ferromagnetic coupling between the surface and the bulk. The 4f spin polarized photoemission data. provides strong evidence that the surface 4£ polarization differs from the bulk 4f polarization for Gd(OOO I). Onr temperature dependent measurements with spin polarized secondary electron spectroscopy conclusively establishes that the surface of clean Gd(OOOl) possesses a perpendicuiRr polarization component which persists to an enhanced surface Curie temperature. Small amounts of contamination at the surface result in the disappearance of the pcrpcudicular component and, therefore, a more perfect ferromagnetic coupling between the surface and the bulk.