We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co$_mathrm{2}$Mn$_{beta}$Si (CMS)/MgO ($beta$=0.69, 0.99, 1.15 and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In pa
rticular, the Mn composition ($beta$) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, $m_mathrm{spin}$(Mn), derived from XMCD weakly decreased with increasing Mn composition $beta$ in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, $m_mathrm{spin}$(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of Co$_mathrm{Mn}$ antisites detrimental to the half-metallicity of CMS with increasing $beta$. This finding is consistent with the higher TMR ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.
The structural, magnetic, electrical and transport properties of FeV2O4, by doping Li and Cr ions respectively in A and B sites, have been studied. Dilution of A-site by Li doping increases the ferri-magnetic ordering temperature and decreases the fe
rroelectric transition temperature. This also decreases the V-V distances which in effect increases the A-V coupling. This increased A-V coupling dominates over the decrease in A-V coupling due to doping of non-magnetic Li. On the other hand, Cr doping increases the ferri-magnetic ordering temperature but does not alter the ferroelectric transition temperature which is due to the fact that the polarization origin to the presence of almost non-substituted regions.
We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1$_{0}$-ordered phase coated with SiO$_{2}$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L
_{rm 2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{rm orb}$) to the spin magnetic moment ($M_{rm spin}$) of Fe to be $M_{rm orb}/M_{rm spin}$ = 0.08. This $M_{rm orb}/M_{rm spin}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1$_{0}$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_{rm c}$) was found to be as large as 1.8 T at room temperature, $sim$3 times larger than the thin film value and $sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles with the $H_{rm c}$ distributed from 1 T to 5 T.
BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferro
magnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe$_{1-x}$Co$_x$O$_3$ ({it x} = 0 to 0.30) (BFCO) thin films grown on LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe$^{3+}$ and Co$^{3+}$ states. The XMCD results showed that the Fe ions show ferromagnetism while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic $gamma$-Fe$_2$O$_3$ which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.
We have performed a low temperature scanning tunneling microscopy and spectroscopy study of the iron chalcogenide superconductor FeSe_{0.4}Te_{0.6} with T_{C}~14 K. Spatially resolved measurements of the superconducting gap reveal substantial inhomog
eneity on a nanometer length scale. Analysis of the structure of the gap seen in tunneling spectra by comparison with calculated spectra for different superconducting order parameters (s-wave, d-wave, and anisotropic s-wave) yields the best agreement for an order parameter with anisotropic s-wave symmetry with an anisotropy of ~40%. The temperature dependence of the superconducting gap observed in places with large and small gap size indicates that it is indeed the superconducting transition temperature which is inhomogeneous. The temperature dependence of the gap size is substantially larger than would be expected from BCS theory. An analysis of the local gap size in relation with the local chemical composition shows almost no correlation with the local concentration of Se-/Te-atoms at the surface.
We report a detailed high-temperature powder neutron diffraction investigation of the structural behavior of the multiferroic GaFeO3 between 296 and 1368 K. Temperature dependent neutron diffraction patterns do not show any appreciable change either
in intensity or appearance/disappearance of the observed peaks upto 1368 K, ruling out any structural transition in the entire temperature range. The lattice parameters and volume exhibit normal thermal expansion behaviour, indicating the absence of any structural changes with increasing temperature. The origin of the magnetoelectric couplings and multiferroicity in GaFeO3 is known to be influenced by the site disorder from Ga/Fe atoms. Our analysis shows that this disorder remains nearly the same upon increase of temperature from 296 to 1368 K. The structural parameters as obtained from Rietveld refinement of neutron diffraction data are used to calculate the interatomic distances and distortions of the oxygen polyhedra around the Ga1, Ga2, Fe1 and Fe2 cations. Evolution of the distortion of the oxygen polyhedra around these sites suggests that the Ga1-O tetrahedron is least distorted and Fe1-O is most distorted. Structural features regarding the distortion of polyhedral units would be crucial to understand the temperature dependence of the microscopic origin of polarizations. The electric polarization has been estimated using a simple ionic model and its value is found to decrease with increasing temperature.
We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode
and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles. XAS spectra reveal that significant part of the doped Mn and Co atoms are found in the trivalent and tetravalent state in particular in the surface region while majority of Fe atoms are found in the trivalent state both in the inner core region and surface region. The XMCD spectra show that the Fe$^{3+}$ ions in the surface region give rise to the ferromagnetism while both the Co and Mn ions in the surface region show only paramagnetic behaviors. The transition-metal atoms in the inner core region do not show magnetic signals, meaning that they are antiferromagnetically coupled. The present result combined with the previous results on transition-metal-doped ZnO nano-particles and nano-wires suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles and nano-wires, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.
We have studied magnetism in anatase Ti$_{1-x}$Co$_x$O$_{2-delta}$ ({it x} = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ absorption edges. For electricall
y conducting samples, the magnetic moment estimated by XMCD was $<$ 0.3 $mu_B$/Co using the surface-sensitive total electron yield (TEY) mode, while it was 0.3-2.4 $mu_B$/Co using the bulk-sensitive total fluorescence yield (TFY) mode. The latter value is in the same range as the saturation magnetization 0.6-2.1 $mu_B$/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.
We have investigated the electronic structure of ZnO:Mn and ZnO:Mn,N thin films using x-ray magnetic circular dichroism (XMCD) and resonance-photoemission spectroscopy. From the Mn 2$p$$rightarrow3d$ XMCD results, it is shown that, while XMCD signals
only due to paramagnetic Mn$^{2+}$ ions were observed in ZnO:Mn, nonmagnetic, paramagnetic and ferromagnetic Mn$^{2+}$ ions coexist in ZnO:Mn,N. XMCD signals of ZnO:Mn,N revealed that the localized Mn$^{2+}$ ground state and Mn$^{2+}$ state hybridized with ligand hole coexisted, implying $p$-$d$ exchange coupling. In the valence-band spectra, spectral weight near the Fermi level was suppressed, suggesting that interaction between magnetic moments in ZnO:Mn,N has localized nature.
We have studied the electronic structure and the magnetism of Cu-doped ZnO nanowires, which have been reported to show ferromagnetism at room temperature [G. Z. Xing ${et}$ ${al}$., Adv. Mater. {bf 20}, 3521 (2008).], by x-ray photoemission spectrosc
opy (XPS), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). From the XPS and XAS results, we find that the Cu atoms are in the Cu$^{3+}$ state with mixture of Cu$^{2+}$ in the bulk region ($sim$ 100 nm), and that Cu$^{3+}$ ions are dominant in the surface region ($sim$ 5 nm), i.e., the surface electronic structure of the surface region differs from the bulk one. From the magnetic field and temperature dependences of the XMCD intensity, we conclude that the ferromagnetic interaction in ZnO:Cu NWs comes from the Cu$^{2+}$ and Cu$^{3+}$ states in the bulk region, and that most of the doped Cu ions are magnetically inactive probably because they are antiferromagnetically coupled with each other.
