Elio Vescovo

Robust isothermal electric control of exchange bias at room temperature

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.Roughness-insensitive and electrically controllable magnetization at the (0001) surface of antiferromagnetic chromia is observed using magnetometry and spin-resolved photoemission measurements and explained by the interplay of surface termination and magnetic ordering. Further, this surface in placed in proximity with a ferromagnetic Co/Pd multilayer film. Exchange coupling across the interface between chromia and Co/Pd induces an electrically controllable exchange bias in the Co/Pd film, which enables a reversible isothermal (at room temperature) shift of the global magnetic hysteresis loop of the Co/Pd film along the magnetic field axis between negative and positive values. These results reveal the potential of magnetoelectric chromia for spintronic applications requiring non-volatile electric control of magnetization.

Electronic Structure of the Topological Insulator Bi2Se3 Using Angle-Resolved Photoemission Spectroscopy: Evidence for a Nearly Full Surface Spin Polarization

We performed high-resolution spin- and angle-resolved photoemission spectroscopy studies of the electronic structure and the spin texture on the surface of Bi2Se3, a model TI. By tuning the photon energy, we found that the topological surface state is well separated from the bulk states in the vicinity of kz = Z plane of the bulk Brillouin zone. The spin-resolved measurements in that region indicate a very high degree of spin polarization of the surface state, ~0.75, much higher than previously reported. Our results demonstrate that the topological surface state on Bi2Se3 is highly spin polarized and that the dominant factors limiting the polarization are mainly extrinsic.

Precessional dynamics of elemental moments in a ferromagnetic alloy

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps.

Quantum size effects and the enhancement of the exchange splitting in ultrathin Co overlayers on Cu (100)

The spin dependent electronic structure of epitaxial Co overlayers on Cu (100) has been studied by spin and angle resolved photoemission with synchrotron radiation. Co layers with a thickness below 5 monolayers show an enhanced value of the exchange splitting at the center of the Brillouin zone due to quantum size effects. The experimental results are supported by FLAPW calculations for 1, 2 and 3 Co layers on Cu (100). These calculations show that the increase of the average exchange splitting is related to the enhancement of the magnetic moment in ultrathin Co layers.

Weakly coupled motion of individual layers in ferromagnetic resonance

We demonstrate a layer- and time-resolved measurement of ferromagnetic resonance (FMR) in a Ni{sub 81}Fe{sub 19}/Cu/Co{sub 93}Zr{sub 7} trilayer structure. Time-resolved x-ray magnetic circular dichroism has been developed in transmission, with resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2 deg), time-domain magnetization precession could be observed directly, and resolved to individual layers through elemental contrast at Ni, Fe, and Co edges. The phase sensitivity allowed direct measurement of relative phase lags in the precessional oscillations of individual elements and layers. A weak ferromagnetic coupling, difficult to ascertain in conventional FMR measurements, is revealed in the phase and amplitude response of individual layers across resonance.

A compact apparatus for studies of element and phase-resolved ferromagnetic resonance

We present a compact sample holder equipped with electromagnets and high frequency transmission lines; the sample holder is intended for combined x-ray magnetic circular dichroism (XMCD) and ferromagnetic resonance measurements (FMR). Time-resolved measurements of resonant x-ray detected FMR during forced precession are enabled by use of a rf excitation that is phase-locked to the storage ring bunch clock. Several applications of the combined XMCD+FMR technique are presented, demonstrating the flexibility of the experimental design.

THE OCCUPIED AND UNOCCUPIED ELECTRONIC STRUCTURE OF ADSORBED FERROCENE

Abstract We have studied both the occupied and unoccupied molecular orbitals of adsorbed ferrocene. The occupied molecular orbitals have been identified using a combination of photoemission selection rules and resonant photoemission. Ferrocene adsorbs with a strong preferential molecular orientation on Mo(112) at 150 K, like metallocenes on many other surfaces. Though the ferrocene molecular axis is largely parallel with the Mo(112) surface there is very little perturbation of the molecular orbitals relative to the gas phase.

Combined time-resolved x-ray magnetic circular dichroism and ferromagnetic resonance studies of magnetic alloys and multilayers (invited)

We present measurements of element- and time-resolved ferromagnetic resonance (FMR) in magnetic thin films at gigahertz frequencies via an implementation of time-resolved x-ray magnetic circular dichroism (TR-XMCD). By combining TR-XMCD and FMR, using a rf excitation that is phase locked to the photon bunch clock, the dynamic response of individual layers or precession of individual elements in an alloy can be measured. The technique also provides extremely accurate measurements of the precession cone angle (to 0.1°) and the phase of oscillation (to 2°, or ∼5ps at 2.3GHz). TR-XMCD combined with FMR can be used to study the origins of precessional damping by measuring the relative phase of dissimilar precessing magnetic moments. We have used the technique to measure the response of specific elements and separate layers in several alloys and structures, including a single Ni81Fe19 layer, a pseudo-spin-valve structure (Ni81Fe19∕Cu∕Co93Zr7), magnetic bilayers consisting of low damping (Co93Zr7) and high damping (Tb-doped Ni81Fe19) layers joined across a common interface, and elemental moments in Tb-doped Ni81Fe19.We present measurements of element- and time-resolved ferromagnetic resonance (FMR) in magnetic thin films at gigahertz frequencies via an implementation of time-resolved x-ray magnetic circular dichroism (TR-XMCD). By combining TR-XMCD and FMR, using a rf excitation that is phase locked to the photon bunch clock, the dynamic response of individual layers or precession of individual elements in an alloy can be measured. The technique also provides extremely accurate measurements of the precession cone angle (to 0.1°) and the phase of oscillation (to 2°, or ∼5ps at 2.3GHz). TR-XMCD combined with FMR can be used to study the origins of precessional damping by measuring the relative phase of dissimilar precessing magnetic moments. We have used the technique to measure the response of specific elements and separate layers in several alloys and structures, including a single Ni81Fe19 layer, a pseudo-spin-valve structure (Ni81Fe19∕Cu∕Co93Zr7), magnetic bilayers consisting of low damping (Co93Zr7) and high dampi...

GeOx interface layer reduction upon Al-gate deposition on a HfO2∕GeOx∕Ge(001) stack

The metallization of HfO2∕Ge by Al at room temperature was studied using photoemission and inverse photoemission. Upon deposition, Al reduces the GeOx interfacial layer between Ge and HfO2, and a thin Al2O3 layer is formed at the Al∕HfO2 interface. The band alignment across the Al∕HfO2∕Ge stacks is also addressed.

Comparison of time-resolved x-ray magnetic circular dichroism measurements in reflection and transmission for layer-specific precessional dynamics measurements

We present experimental techniques to measure magnetization precession of individual layers in a “spin-valve” trilayer. Precessional motions of individual Ni81Fe19 and Co93Zr7 layers have been separated in Ni81Fe19∕Cu∕Co93Zr7 using ±45ps time-resolved x-ray magnetic circular dichroism (tr-XMCD) at Fe and Co edges. We compare the efficacy of two experimental configurations in this paper. Pulsed-field tr-XMCD measurements in reflectivity are compared with resonant-field tr-XMCD measurements in transmission. Despite the order of magnitude larger angles of precession excited in pulsed-field reflectivity measurements, data quality is found to be superior in resonant-field transmission measurements. Relative roles of sample preparation and timing jitter in the different techniques are discussed.