Electron Capture Spectroscopy Research Articles

THEORY FOR SPIN–POLARIZED ELECTRON CAPTURE SPECTROSCOPY IN FERROMAGNETIC NICKEL

A time-dependent many-body theory is developed for the study of spin-polarized electron capture spectroscopy. As a model system we choose the head-on collision of protons with a nickel atom. For the electronic part of the Hamiltonian we use the most general on-site interaction terms allowed by atomic symmetry. The total electronic many-body states are group-theoretically classified with respect to the conserved quantum numbers L, L z and S z. The time-dependent Schrödinger equation for this system is solved exactly. The zero-, one- and two- electron capture probabilities which are treated on the same footing are monitored along the trajectories of the scattering species on the femtosecond time scale. In good agreement with experiments on surfaces we find probabilities of 22% and 0.33% for one- and two-electron capture respectively and spin polarizations between -60% and -100%. The predominant capture of minority electrons is enhanced due to electronic correlations. This implies that the probing of magnetism occurs on a significantly longer time scale than the probing of single electron properties.

Monolayer-ferromagnetism, critical behavior and anisotropies of Fe(100)/Au(100), Fe(100)/Ag(100) and Tb/Fe(100)/Ag(100) films

Using electron capture spectroscopy, we studied the temperature and magnetic field dependence of the electron spin polarization (ESP) of uncoated, ultra-thin ( d = 1–10 monolayers (ML)) Fe(100) films on Au(100) and Ag(100). Between 173 and 303 K, we find in-plane, remanent ESP and in-plane, d-dependent magnetic anisotropies. Large T-dependent, perpendicular magnetic anisotropies exist for Tb-coated Fe/Ag films. For 2 ML Fe/Au, near the surface Curie temperature T Cs, we find a critical exponent β = 0.25, in disagreement with available theoretical predictions.

Some properties of the surface magnetization of bulk magnets.

Rigorous results in the statistical mechanics of the Ising model of a magnet are used to analyze recent experimental data obtained by electron-capture spectroscopy on the surface magnetization of gadolinium as well as to test recent approximate real-space renormalization-group calculations of the surface magnetization of the Ising model. The experimental data are found to be consistent with these exact results, while the approximate renormalization-group calculations are found to be inconsistent.

Capture and emission of spin-polarized electrons to study surface magnetism and critical behavior

Using angle- and energy-resolved, spin-polarized electron emission Spectroscopy (SPEES) and electron capture Spectroscopy (ECS), the long-ranged and short-ranged surface electron spin polarization (ESP) of Fe 80B 20, Ni(110) and Tb(0001)/W(110) films is investigated. All surfaces are well characterized using AES, LEED, RHEED, STM and ion surface reflection (ISR). Nonzero ESP is found at all magnetized surfaces. For Tb(0001)/W(l10), near the surface Curie temperature T Cs, surface-enhanced long-ranged magnetic order and novel critical behavior is found. Short-ranged magnetic order exists above T Cs. The angle-resolved energy distribution of grazing-angle , ion-induced (H + and He + ions) emitted electrons (0–70 eV), which exhibits a series of pronounced “peaks” including spin-polarized and element-specific Auger electrons, is strongly different from that of electron-induced secondary electrons and contains fundamental information on the surface magnetic structure of Ni(110) and on the spin dependence of various electronic transition processes occuring at magnetic surfaces.

Ferromagnetic order and critical behavior at surfaces of ultrathin epitaxial films

The critical behavior, ferromagnetic order and magnetic anisotropies of ultrathin, epitaxial, magnetic films is studied using electron capture spectroscopy (ECS), which is capable of probing the long-ranged and short-ranged electron spin polarization (ESP) at the topmost surface layer of uncoated and coated magnetic structures. For all systems [Ni(100)/Cu(100), Ni(100)/NaCl(100), fcc Fe(111)/Cu(111), Fe(100)/Ag(100), Tb/Fe(100)/Ag(100), Fe(100)/Au(100), hcp Tb(0001)/W(110), Fe(110)/W(110), V(100)/Ag(100), Pd(100)/Ag(100), Pd/W(110)] investigated so far, ferromagnetic order is detected. It is found that the surface Curie temperatureT Cs depends on film thicknessd. ECS data obtained at the surface of various systems reveal the existence ofT- andd-dependent magnetic anisotropies. Although for V(100)/Ag(100) the measured critical exponentβ=0.128 agrees very well withβ=1/8 predicted for the two-dimensional Ising model, for other systems, such as Fe(100)/Au(100), the measuredβ value (0.25) is in disagreement with theoretical predictions. The experimental results are discussed within the framework of presently available experimental and theoretical data.

Novel magnetic surface anisotropies and critical behavior of Tb(0001)/W(110) films

Uncoated, epitaxial films of Tb(0001)/W(110) are grown, and their magnetic properties are studied using electron capture spectroscopy. The topmost layer orders ferromagnetically below a surface Curie temperature T Cs =249.96±0.02 K, which lies above both the bulk Curie and Néel temperatures T Cb =220 K and T Nb =228 K, respectively and the surface magnetization exhibits strongly non-monotomic behavior close to 245 K. Novel critical behavior is observed near T Cs, suggesting strong surface anisotropies, in accord with a recently predicted but previously unobserved surface phase transition.

Studies of novel magnetic systems by electron capture spectroscopy (ECS)

The electron spin polarization (ESP) at surfaces of bulk and artificially structured, thin film magnetic materials is studied using electron capture spectroscopy (ECS) and ion-induced emission of spin-polarized electron. For V and Tb surfaces, it is found that the temperature dependence of the surface magetic order is quite different from that of the bulk. For ultra-thin magnetic superlattices, V(100)/Ag(100) and Tb (0001)/W(110), we find novel magnetic surface anisotropies and critical behavior. At surfaces of 5 monolayers V(100) on Ag(100), the critical behavior of the surface-ESP is found to be identical to that of the two-dimensional Ising ferromagnet. At surfaces of PtMnSb and Fe80B20, we findnonzero ESP due to long-ranged or short-ranged ferromagnetic order. Thesign of the ESP at surfaces of the Heusler alloy PtMnSb is in agreement with recent band structure calculations. At surfaces of the spin glass Fe80B20, we find, in agreement with ECS data, that the ESP ofion-induced emitted spin-polarized electrons is extremely sensitive to the cleanness of thetopmost surface layer.

Ferromagnetic order at surfaces of ultrathin Fe(100)/Ag(100) and Tb/Fe(100)/Ag(100) films determined by electron capture spectroscopy

The temperature and magnetic field dependence of the electron-spin polarization (ESP) at atomically clean and flat surfaces of ultrathin (1–10 monolayers) bcc Fe(100)/Ag(100) and Tb/Fe(100)/Ag(100) films is studied as function of film thickness. For the investigation of the ESP, we use electron capture spectroscopy which enables us to study long-ranged ferromagnetic order at the topmost layer of a surface. The geometrical and chemical structure of the films is well characterized using Auger electron spectroscopy and low- and high-energy electron diffraction. Atomic flatness is studied using scanning tunneling microscopy. Nonzero ESP is found at the surface of all films. We find ambiguously that in-plane remanent ferromagnetic order exists at uncoated surfaces of 1 to 10 monolayer thick bcc Fe(100)/Ag(100) films for temperatures ranging between 173 and 303 K. It is further found that the Curie temperature of the Fe films depends on film thickness. Evidence is found for in-plane magnetic anisotropies which strongly depend on film thickness. In striking contrast to uncoated Fe films, large temperature-dependent perpendicular (out-of-plane) magnetic anisotropies exist at surfaces of Tb-coated Fe(100)/Ag(100) films.

Emission and capture of spin‐polarized electrons during ion reflection at magnetic surfaces: A new tool in surface magnetism

Using electron‐capture spectroscopy (ECS) and electron emission spectroscopy (EES), the electron‐spin polarization (ESP) at the topmost surface layer of ferromagnetic materials is investigated. In ECS, the capture of spin‐polarized electrons during grazing‐angle surface reflection of fast deuterons is used to investigate the ESP due to long‐ranged and short‐ranged ferromagnetic order. In EES, the ion‐induced emission of low‐energy, spin‐polarized electrons from magnetic surfaces is used to study long‐ranged surface ferromagnetic order. The surfaces are well characterized using Auger electron spectroscopy, low‐energy electron diffraction, reflection high‐energy electron diffraction, and scanning tunneling microscopy. Nonzero ESP is found at surfaces of atomically clean, amorphous Fe80 B20 and (110)‐surface‐oriented Ni picture‐frame single crystals. We have studied the energy‐resolved ESP of the emitted electrons at Ni(110) surfaces. The energy distribution of the ion‐induced emitted electrons is strongly different from that of electron‐induced secondary electrons and shows a marked spin dependence. The found energy and spin distributions of the electrons, originating from the topmost surface layer, bear a close resemblance to the electronic and magnetic properties of Ni(110) surfaces. The net average ESP of the emitted electrons amounts to +5.5%, which is close to the total net Ni magnetization of +5.6%.

Novel studies of spin-polarized electrons emitted during grazing incidence ion reflection at magnetic surfaces

Using a new technique, spin-polarized electron emission spectroscopy (SPEES) and electron capture spectroscopy (ECS), the electron spin polarization (ESP) at the topmost surface layer of ferromagnetic materials is investigated. All surfaces are well characterized using AES, LEED, RHEED and STM. Nonzero ESP is found at remanently magnetized surfaces of atomically clean, amorphous Fe 80B 20 and (110)-surface-oriented Ni picture-frame single crystals. The angle-resolved energy distribution of the ion-induced emitted electrons is strongly different from that of electron-induced secondary electrons and contains most valuable information on the surface magnetic structure of Ni(110) and on the spin dependence of various electronic transition processes occurring at surfaces. The ESP of electrons (energies between 9 and 12 eV) emitted during grazing-angle ion reflection amounts to + 5.5% which is close to the total net Ni-magnetization of +5.6%.

Two-dimensional ferromagnetism of ultra-thin artificial vanadium films

Artificial ultra-thin (1–7 monolayers) films of V (100)p(1 × 1) on Ag(100) are studied by electron capture spectroscopy and found to be ferromagnetic, with a surface Curie temperature T Cs depending on film thickness. The critical behavior of the magnetization of the topmost layer is found to be identical to that of the two-dimensional Ising ferromagnet.

Magnetic order at Tb surfaces determined by electron capture spectroscopy (ECS)

The magnetic order at surfaces of the 4f-rare earth metal terbium is investigated using electron capture spectroscopy (ECS). ECS probes the magnetic order of the topmost surface layer. In ECS, the capture of spin-polarized electrons during grazing-angle ion-surface reflection is used to determine the electron spin polarization (ESP) due to long-ranged and short-ranged surface magnetic order. It is found that long-ranged ferromagnetic order exists far above the bulk Curie temperature T cb of Tb. using magneto-optical Kerr effect and ferromagnetic induction, T cb is found to be at 220 K. At 140 K, the long-ranged ESP amounts to 24%. With increasing temperature, the ESP decreases monotonically up to the bulk Néel temperature T Nb = 228 K, then exhibits a pronounced maximum at T = 238 K and vanishes at the surface Curie temperature T Cs found to be located at 248 K. These novel and striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.

Electron-capture spectroscopy for identification of source ionization states and acceleration processes of SCR

It is illustrated how the radiation from electron-capture following the interaction of energetic ions, during their acceleration, with ions and atoms of the local matter is a very useful tool for diagnostics of cosmic and laboratory particle processes.

Ion-surface interaction to study ion-induced capture and emission of polarized electrons: Novel means to investigate surface magnetism

The electron spin polarization (ESP) at surfaces of bulk and thin film ferromagnetic materials has been investigated using electron capture spectroscopy (ECS), which probes the ESP of the topmost surface layer. At surfaces of PtMnSb, Tb and Fe 80B 20, we find nonzero ESP due to long-ranged or short-ranged ferromagnetic order. The sign of the ESP at surfaces of the Heusler alloy PtMnSb is in agreement with recent band structure calculations. At surfaces of the rare-earth Tb, we find that, contrary to bulk behavior, long-ranged ferromagnetic order exists far above the bulk Curie temperature ( T Cb = 220 K) up to a surface Curie temperature T Cs = 248 K. At surfaces of the spin glass Fe 80B 20, we find, in agreement with ECS data, that the ESP of ion-induced emitted polarized secondary electrons is extremely sensitive to the cleanness of the topmost surface layer.

Ferromagnetic order and critical behavior at surfaces of ultrathin V(100)p(1×1) films on Ag(100)

The magnetic order and critical behavior of well-defined (100)p(1×1) surfaces of ultrathin (1–7 monolayers) vanadium films deposited on atomically clean and flat Ag(100) substrates is studied by electron capture spectroscopy, a novel method which probes electron spin polarization (ESP) at the topmost surface. For all investigated films, the long-ranged ESP is nonzero, demonstrating the existence of long-ranged ferromagnetic order at the film surface. For a 5-monolayer-thick film, ferromagnetic order sets in at a critical surface temperature TCs=475.1 K. In the temperature range from 200 to 475.1 K, the ESP of the topmost surface layer follows the power law (TCs−T)β, with β=0.128±0.01. This result is in good agreement with the exact value β= (1)/(8) of the two-dimensional Ising model.

Ferromagnetic order at Tb surfaces above the bulk Curie temperature

The magnetic order at surfaces of the 4f rare-earth metal terbium is investigated using electron capture spectroscopy (ECS), which probes the electron spin polarization (ESP) of the topmost surface layer. In ECS, the capture of spin-polarized electrons during grazing-angle ion-surface interaction is used to determine the ESP due to long- and short-ranged surface magnetic order. It is found that long-ranged ferromagnetic order exists far above the bulk Curie temperature, measured to be TCb =220 K. At 140 K, the long-ranged ESP amounts to 24%. With increasing temperature, the ESP first decreases montonically up to the bulk Néel temperature TNb=228 K, then exhibits a pronounced maximum at T=238 K, and ultimately vanishes at the surface Curie temperature TCs =248 K. These striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.

Ferromagnetic order at surfaces of ultrathin epitaxial fcc gamma -Fe(111)p(11) films on Cu(111).

Electron-capture spectroscopy has been used to investigate long-ranged and short-ranged two-dimensional magnetic order at free (uncoated) surfaces of homogeneous (island-free), ultrathin fcc \ensuremath{\gamma}-Fe(111)(1\ifmmode\times\else\texttimes\fi{}1) films pseudomorphically deposited on Cu(111) substrate crystals. At 298 K, the electron-spin polarization at the surface of a four-monolayer thin fcc Fe film amounts to +16%, directly revealing the existence of long-ranged ferromagnetic order. There is evidence that short-ranged ferromagnetic order still exists at 400 K at free surfaces of two and one-monolayer thin fcc Fe films.

Ferromagnetic Order at (100)p(1×1)Surfaces of Bulk Paramagnetic Vanadium

Electron-capture spectroscopy has been used to investigate magnetic order at the topmost atomic layer of atomically clean and flat (100) surfaces of bulk paramagnetic vanadium. From low-energy electron-diffraction measurements, we find a $p(1\ifmmode\times\else\texttimes\fi{}1)$ surface structure. At 300 K, the long-ranged electron-spin polarization amounts to \ensuremath{\sim}34%, clearly demonstrating the existence of ferromagnetic order at the V(100) $p(1\ifmmode\times\else\texttimes\fi{}1)$ surface. With increasing temperature, the electron-spin polarization decreases almost linearly and disappears at a surface Curie temperature ${T}_{\mathrm{C}s}\ensuremath{\approx}540$ K.

Evidence for ferromagnetic order at gadolinium surfaces above the bulk Curie temperature.

The electron-spin polarization P at the topmost layer of atomically clean, magnetized gadolinium (Gd) surfaces is measured by using electron capture spectroscopy. P varies almost linearly with temperature between 160 K (P=-42%) and 315 K (P=0). It is found that the surface Curie temperature ${T}_{\mathrm{Cs}}$ is far above---lying near 310 K---the bulk Curie temperature ${T}_{\mathrm{Cb}}$=292.5 K, indicating uniaxial anisotropic magnetic exchange interactions at the topmost surface layer of Gd.

Strong temperature dependence of ferromagnetism at Ni and Tb surface determined with electron capture spectroscopy ECS

The magnetic structure of the topmost atomic layer of Ni and Tb surface has been investigated using electron capture spectroscopy (ECS). Contrary to bulk behavior, at Ni(111) surfaces, the long-range magnetic order, characterized by the long-range electron spin polarization (ESP), decreases from −43% at T= 293 K almost linearly to zero at the bulk Curie temperature T cb = 631 K. The “local” surface magnetic order (SMO) at Ni(111) surfaces is temperature independent and is detected far above T cb. At Tb surfaces, however, the “local” SMO is strongly temperature dependent and disappears above T cb. ft]+|Walter-Meissner-Institute f. Low-Temp.-Res., Bav. Acad. Sci., D8046 Garching, F.R.G.