V. Chakarian

Exploring magnetic roughness in CoFe thin films

The behavior of chemical and magnetic interfaces is explored using diffuse x-ray resonant magnetic scattering (XRMS) for CoFe thin films with varying interfacial roughnesses. A comparison of the chemical versus magnetic interfaces shows distinct differences in the behavior of these two related interfaces as the chemical roughness is increased. Such changes appear to be correlated with the behavior of the magnetic hysteresis of the interface, measured by tracking the diffuse XRMS intensity as a function of applied magnetic field.

Element‐specific vector magnetometry with magnetic circular dichroism

The use of soft x‐ray magnetic circular dichroism in two‐dimensional (2D) element‐specific vector magnetometry is demonstrated by investigating a thin Fe(001) single‐crystal film grown on ZnSe(001). By measuring element‐specific magnetic hysteresis curves along directions parallel and perpendicular to an applied magnetic field, the 2D behavior of the in‐plane magnetization vector M is described as a function of the applied magnetic field. The use of this method to obtain element‐specific 3D vector magnetometry in magnetic materials that exhibit both in‐plane and out‐of‐plane magnetization is also discussed.The use of soft x‐ray magnetic circular dichroism in two‐dimensional (2D) element‐specific vector magnetometry is demonstrated by investigating a thin Fe(001) single‐crystal film grown on ZnSe(001). By measuring element‐specific magnetic hysteresis curves along directions parallel and perpendicular to an applied magnetic field, the 2D behavior of the in‐plane magnetization vector M is described as a function of the applied magnetic field. The use of this method to obtain element‐specific 3D vector magnetometry in magnetic materials that exhibit both in‐plane and out‐of‐plane magnetization is also discussed.

Identifying layer switching in magnetic multilayers with x-ray resonant magnetic scattering

The order of layer switching in a NiFe/Cu/Co spin valve is determined directly using circular polarized x-ray resonant magnetic scattering. By monitoring changes in the angular dependence of the magnetic contributions to the reflectivity near the Fe L3 and Co L3 edges as a function of applied field, the order of layer switching is directly obtained.

SOFT X-RAY RESONANT MAGNETIC REFLECTIVITY STUDY OF THIN FILMS AND MULTILAYERS

Soft x-ray resonant magnetic reflectivity measurements on thin films and multilayers in a transverse geometry using linear polarized photons are presented. Magneto-optic calculations taking into account the layer roughness allows us to reproduce all the experimental features of the angular and energy reflectivity curves as well as the asymmetry ratio in both cases. Application to FexMn1−x alloy films epitaxially grown on Ir(001) brings more insights on the magnetic transition occurring at x=0.75.

Circular polarized soft X-ray resonant magnetic scattering studies of FeCo/Mn/FeCo multilayers

We present some of our recent results of circular polarized soft X-ray resonant magnetic scattering studies, a technique which combines the power of X-ray scattering and magnetic circular dichroism. The energy, angle, and polarization dependence of the reflectivity near the L-edges of the each element provides a new means for determining multilayer magnetic ordering in a heteromagnetic multilayer in an element-specific manner. As an example, the results from a Fe25Co75/Mn/Fe25Co75 trilayer are presented.

Electron spectroscopic studies of colossal magnetoresistance material La1−xCaxMnO3

High resolution photoemission measurements have been performed for the ferromagnetictransition of La0.67Ca0.33MnO3 and for the order–disorder transition of La0.4Ca0.6MnO3. The band gap collapsed below the Curie temperature and the density of states at the Fermi level increases with cooling. Upon cooling through the order–disorder transition temperatures, T CO, the band gap increases by ∼50 meV. This change in the gap is consistent with the change in the activation energy above and below T CO estimated from conductivity data. We have also performed soft x‐ray magnetic circular dichroism measurements at Mn L 2,3 edges for ferromagnetic La0.67Ca0.33MnO3 and La0.8Ca0.2MnO3.

ROLE OF APICAL HOLES FOR SUPERCONDUCTIVITY IN P-TYPE CUPRATES : NEXAFS RESULTS

From the early days of high-temperature superconductivity on, the question of a possible role of apical sites in p-type superconductors has been controversely discussed in the community. Several recent observations made by us, using polarization-dependent O1s and Cu2p near-edge X-ray absorption spectroscopy on single crystals of infinite-layer compounds, of Ca-doped T′ phase compounds, and of Y1−xCaxBa2Cu3Oy, are all consistent with the conjecture that there is significant influence of the apical site on high-Tc superconductivity. Most notably, the Ca-dependent hole counts for the latter family show that superconductivity is absent if there are no holes on the apical site. This is independent of the hole count in the CuO2 planes.

The role of Ta and Pt in segregation within Co‐Cr‐Ta and Co‐Cr‐Pt thin film magnetic recording media

Polarization dependent extended x‐ray absorption fine structure (PD‐EXAFS) and magnetic circular dichroism (MCD) measurements of CoCrTa and CoCrPt films, sputter deposited at varying substrate temperatures, were performed to investigate the average local structure and chemistry about the Ta, Pt, and Co atoms and the average magnetic moment of the Co and Cr atoms within these films. Results from the MCD measurements indicate the average net magnetic moment of the Cr atoms is opposite in direction and five percent in amplitude relative to the Co moments. Inspection of the Fourier transforms of the XAFS data from these samples shows an increase in structural disorder around the Ta and Pt atoms with increasing substrate deposition temperature. A further comparison between the Ta and Pt edge EXAFS results show that the temperature‐dependent increase in structural disorder is greater around the Ta atoms in the CoCrTa system than it is around the Pt atoms in the CoCrPt system.

Probing local magnetic disorder by investigating spin dependent photoelectron scattering

We present here systematic temperature dependent magnetic extended x-ray absorption spectroscopy (MEXAFS) measurements. This fine structure can be explained by the spin dependent scattering of the photoelectron and yields information about the local magnetic surroundings of the absorbing atom. We find a strong temperature dependence at temperatures far below the Curie temperature. This shows that the intensity of the MEXAFS signal is not only determined by the value of the magnetic moment but also by the relative vibrations of the backscattering atom to the absorbing atom.

Element-specific vector magnetometry of buried layers

Abstract Magnetic circular dichroism is used as an element-specific vector magnetometer to determine the behavior of a buried 4.3 monolayer Mn film in a Fe 25 Co 75 /Mn/Fe 25 Co 75 trilayer, which exhibits a 90° coupling between the FeCo layers. The data indicate a net ferromagnetic Mn moment canted by 23° from the net Fe (and Co) moments.