Thick Gd Research Articles

Development of a Detector System for Transient Magnetic Field Experiments with Radioactive-Isotope Beams and a Test Experiment

For g-factor measurements of excited states of unstable nuclei, we have been developing a setup for the transient field experiment at RIKEN Accelerator Research Facility. For the preparation of ferromagnetic material, thick Gd foils have been annealed. In a test experiment using a 22Ne beam, the projectile Coulomb excitation to the first excited state has been observed.

Size effect and temperature dependence of spin conduction in Gd/SiN ultrathin film

The possibility of enhancing the resistivity while preserving magnetism is investigated by growing metal/insulator trilayers and superlattices, where the metal is Gd and the insulator is amorphous Si3N4 for the first time. A large size effect on Curie temperature (TC) is found in the susceptibility in contrast with Gd epitaxially grown on metals. TC decreases to 180 K, i.e., 70% of the bulk TC in 10 nm thick Gd film, which is attributed to an electrical isolation and surface states. On the other hand, the susceptibility χ at TC is almost unchanged as in epitaxial Gd on metal. The resistivity R doubled as the Gd layer thickness decreases from 100 to 10 nm, and the TC estimated from R agrees with that from χ. Additionally, a possible interlayer coupling or an effect of surface layers adjacent to Si3N4 was detected in superlattices.

Magnetic circular dichroism in photoelectron angular distributions from Gd(0001)

Abstract We report on the influence of photoelectron diffraction effects on magnetic circular dichroism in angular distribution as measured using core-level photoelectron spectroscopy. As an example system we study the effect by measuring azimuthal angle scans of the 4d core-level on a thick Gd(0001) film grown on W(110). In particular, for photoelectron emission in off-normal directions we observe deviations from the pure atomic picture which can be interpreted in terms of a multiple scattering theory. We also evaluate different measuring methods to see their influence on the angular behavior. Using high resolution data we can even study the state-specific contributions to the magnetic dichroism signal. The knowledge of these effects should help in interpreting absolute dichroism values on the basis of sum rules.

Combining CXMD and XSW to study magnetic and geometric properties of thin films: Gd/Fe(100)

The magnetic and geometric structure of 1–5 monolayers (ML) of Gd on Fe(100) single crystals has been investigated using synchrotron X-rays between 4 and 8 keV. Choosing a high-flux wiggler beamline made it possible to analyse the magnetic properties by studying the circular X-ray magnetic dichroism (CXMD) and, in addition, to apply the X-ray standing-wave (XSW) technique to elucidate the geometric structure of the overlayers. CXMD reveals that Gd couples antiferromagnetically to the Fe substrate and that 2 ML Gd at 243 K yield a magnetization of ∼22% with respect to the saturation value of a thick Gd foil at 123 K. A magnetic polarization of the Gd pertains even at 308 K. While LEED investigations do not show any long-range lateral order of the Gd film, XSW measurements reveal an ordering of Gd perpendicular to the (100) surface of Fe.

Morphology and magnetic properties of submonolayer Gd films

We report on scanning tunneling microscopy (STM) measurements on ultrathin Gd films in the thickness range $0.5<~{d}_{\mathrm{Gd}}<~10\mathrm{ML}$ [1 monolayer (ML)=2.89 \AA{}] grown on a Y (0001) buffer layer at substrate temperatures ${T}_{s}=473\phantom{\rule{0ex}{0ex}}\mathrm{K}$ and ${T}_{s}=573\phantom{\rule{0ex}{0ex}}\mathrm{K}$. The magnetic properties were investigated on these films capped with a 100-\AA{} Y layer with the transverse magneto-optical Kerr effect. A ferromagnetic signal was found down to the sub-ML regime ${d}_{\mathrm{Gd}}\ensuremath{\approx}0.5\mathrm{ML}$ for ${T}_{s}=473\phantom{\rule{0ex}{0ex}}\mathrm{K}$. In this regime, the STM images show a lateral growth of ML thick Gd islands of hexagonal shape. The depression of the Curie temperature ${T}_{C}$ follows a power law $\ensuremath{\Delta}{T}_{C}\ensuremath{\propto}{d}_{\mathrm{Gd}}^{\ensuremath{-}\ensuremath{\lambda}}$ with an exponent $\ensuremath{\lambda}\ensuremath{\approx}$1 up to ${d}_{\mathrm{Gd}}\ensuremath{\approx}3--5\mathrm{ML}$, where $\ensuremath{\lambda}$ changes to $\ensuremath{\lambda}=1.6$. This transition is also observed in the same thickness range for the exponent $\ensuremath{\beta}$ of the magnetization $M$.

Temperature-Dependent Spin Polarization of Magnetic Surface State at Gd(0001).

The surface electronic structure of $100\AA{}$ thick Gd(0001) films deposited on W(110) has been studied by spin-resolved inverse photoemission. At low temperature the magnetic surface state at $\overline{\ensuremath{\gamma}}$ consists of an empty minority and a partially empty majority part. The spin splitting vanishes upon approaching the bulk Curie temperature. This ``Stoner-like'' collapsing band behavior is also observed for transitions into bulklike states. No indication was found for exceptional surface magnetic properties such as enhanced Curie temperature or magnetic order different from the bulk.

Surface structure of epitaxial Gd(0001) films on W(110) studied by quantitative LEED analysis.

The surface structure of thick (400 \AA{}) Gd(0001) films, epitaxially grown on W(110), is investigated by low-energy electron-diffraction (LEED) IV measurements in combination with dynamical LEED calculations. A first-layer contraction of 2.4% and a second-layer spacing expansion of 1% is found. These findings are in good agreement with literature values determined for the (0001) surface of bulk Gd crystals. No significant difference in the LEED IV data is found between films grown at room temperature and films grown at elevated temperatures.

Magnetization behavior of thick epitaxial Gd(0001) films on W(110).

The temperature-dependent magnetic properties of thick Gd(0001)/W(110) films were investigated with respect to the annealing temperature as well as the film thickness (d=10--100 nm). After annealing to sufficiently high temperatures, thick films (d\ensuremath{\ge}40 nm) show evidence for a canted magnetization, accompanied by a strong anomaly of the coercive field just below the Curie temperature ${\mathit{T}}_{\mathit{C}}$. These effects can be attributed to the temperature-dependent magnetic anisotropy of bulk Gd. For thinner films, the magnetization behavior implies the existence of residual strain, which cannot be fully annealed by thermal treatment. The implications of our experimental results for surface magnetic order are discussed.

Magnetization of thin Gd films on W(110) near the Curie temperature

In situ temperature dependent magneto-optic Kerr effect measurements in ultrahigh vacuum are presented for 80 and 300 Å thick Gd(0001) films. Films were deposited at 300 K and stepwise annealed to 820 K. It was found that the remanent magnetization Mr(T) depends on the annealing temperature. The as-deposited films have a reduced Curie temperature Tc, e.g., Tc(80 Å)=273 K, which gradually shifts up to the bulk Tc of Gd upon annealing. The changes in Mr(T) are correlated with a sharpening of the Gd low energy electron diffraction pattern. The best magnetic homogeneity and structural coherence is found for films annealed to 620 K. At higher temperatures the films break up into large three-dimensional islands with nearly uncovered W areas in between (W Auger is observed). The magnetic behavior was simulated with a model which assumes a Gaussian distribution of Tc values. From these simulations and the known bulk pressure dependence of Tc we estimate the mean strain and the strain distribution within the films.

Angle-resolved-photoemission study of the electronic structure of Gd(0001).

The electronic structure of thick (g300 \AA{}) single-crystal Gd(0001) layers on W(110) has been studied by angle-resolved-photoelectron spectroscopy. Photoemission spectra measured with photon energies between 30 and 60 eV show intense 4f emission at 8.3-eV binding energy and 5d 6s\char21{}derived valence-band states within 4 eV from the Fermi level. No other structure is observed in the 5\char21{}11-eV binding-energy region. The valence-band spectra show a surface state at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} near the Fermi level and other higher-binding-energy features possibly related to states localized near the surface.

Anomalous behavior in the spin polarization of low-energy secondary electrons from Gd(0001).

We have measured the spin-polarization spectrum of low-energy secondary electrons from the surface of thick single-crystalline Gd(0001) films grown on W(110). Below 1.5 eV an anomalous decrease in the polarization is observed and is attributed to the presence of the 4${\mathit{f}}^{8}$ states just above the vacuum level, which leads to increased emission of spin-down electrons. Between 1.5 and 7 eV the spin polarization remains nearly constant at about 33% (at 150 K), demonstrating that the phenomenon of polarization enhancement for low-energy secondary electrons is not universal. The observed large polarizations indicate that the completely polarized 4f electrons contribute significantly to the secondary-electron yield.

Spin- and angle-resolved photoemission study of ultrathin Gd films on Fe(100).

Using spin-resolved photoemission with synchrotron radiation, we have studied the electronic structure of the Gd/Fe(100) interface. For Gd films up to one monolayer thickness, we find a high Gd 4f-electron spin polarization which is antiparallel to that of the Fe valence bands. This is the first direct observation of high Gd 4f-electron polarization. The main effect of the Gd adsorption on the Fe-derived valence bands is a depolarization. For thick Gd films (>35 A) the observed Gd 4f polarization is small even well below the Gd T/sub C/.

L-subshell X-ray Production by 100?250 keV/a.m.u. Ions

Individual L-subshell ionization cross sections have been measured for bombardment of 100-200 keY H+ ions in 10 keY steps upon a thick Gd (Z = 64) target and for bombardment of 600-1000 keY He+ ions in 100 keY steps upon thick W (Z = 74) and thick and thin Au (Z = 79) targets. Experimental results for the individual L subshells are compared with the theoretical predictions of the ECPSSR theory as developed by Brandt and Lapicki. The differences are discussed as a function of the reduCi:,d ion velocity. The occurrence of collision induced intra-shell transitions is discussed as a possible source for the discrepancy between experiment and theory.

Large transient magnetic fields for rare-earth ions recoiling in gadolinium and g-factors of high-spin states in 156, 158, 160Gd

We have determined the transient magnetic field for Coulomb-excited rare-earth nuclei recoiling with velocities in the range between 0.7 ν 0 and 6 ν 0 into ferromagnetic gadolinium cooled to a temperature T = 80 K. Measured and calculated g-factors in 169Tm have been used as calibration standards. The transient field is found at first to increase with increasing recoil velocity, and then to level off, approaching a nearly constant value of 5.5 kT at ν = 6 ν 0. At the higher velocities (3 ν 0 < ν < 6 ν 0) the transient fields for 169Tm recoils in gadolinium are a factor of 1.42 ± 0.12 larger than those in iron, whereas the densities of polarized electrons are the same in both ferromagnets. This result demonstrates that an explanation of the transient field must take into account the atomic structure of the host (and probably also that of the recoil). The transient field is too large to be described only in a statistical picture in which inner-shell ( ns) vancancies are filled by capture of polarized (4f) electrons. Possible mechanisms may involve either polarization transfer from the outer shells by spin-flip interactions, or direct vacancy polarization by diabatic molecular orbitals. The transient field calibration has been corroborated making use of known g-factors of low-spin states in 156, 158, 160Gd populated by Coulomb excitation of thick Gd single crystals. For the high-spin states in these nuclei, the g-factors are found to decrease slightly, with the ratio g(10 +) g(2 + ) reduced to 0.89±0.12, 0.83±0.11, and 0.93±0.13, respectively. Similar decreases have been observed previously for other N = 90−96 nuclei.

Surface morphologies and quality of thick liquid phase epitaxial garnet films for magneto-optic devices

Surface morphologies of thick Gd : YIG garnet films grown by liquid phase epitaxy (LPE) are found to be classified into three types: “mirror”, “striation” and “swirl”. “Mirror” appears when films are grown at low temperature or when the film thickness is small. “Striation”, which takes over the striation of the substrates, appears when the growth temperature or film thickness is medium. “Swirl” appears when the growth temperature is high or the film thickness is large. When “swirl” takes place, flux is included into the thick films. A mechanism for the morphological change with increase in film thickness is shown. An increase in half-width of the X-ray rocking curve with an increase in film thickness is found for the {111} films. This can be attributed to the generation of {110} and {211} facets which have larger lattice constants than the {111} and to crystallinity degradation due to flux inclusion. The {110} or the slightly misoriented {110} substrate is recommended to be employed so as to obtain inclusion-free thick films.

A new Faraday rotator using a thick Gd:YIG film grown by liquid-phase epitaxy and its applications to an optical isolator and optical switch

A new Faraday rotator using a thick garnet film grown by liquid-phase epitaxy (LPE) has been proposed and film growth technology for the rotator has been investigated. The new Faraday rotator had good features of very low cost and small size, due to high productivity of the LPE-grown film and low magnetic saturation field, respectively. By using the new Faraday rotator, an optical isolator and magnetooptic switch for single-mode fiber systems have been developed. The optical isolator featured 0.8-dB insertion loss and 25-dB isolation at 1.3-μm wavelength. The magneto-optic 1 × 2 switch was independent of light polarization and featured 1.3-1.7-dB insertion loss, -25-dB crosstalk, and 30-μs switching time at 1.3-μm wavelength. Minimum switching voltage was ±5 V. Magneto-optic devices using the new Faraday rotator is practical for use in 1.2-1.7-\mu m wavelength fiber-optic systems, because of good optical properties, compactness, and low cost.