No Arabic abstract
The ferromagnetic superconductor UCoGe has been investigated by high field X-ray magnetic circular dichroism (XMCD) at the U-M$_{4,5}$ and Co/Ge-K edges. The analysis of the branching ratio and XMCD at the U-M$_{4,5}$ edges reveals that the U-5$f$ electrons count is close to 3. The orbital ($sim0.70,mu_B$) and spin ($sim-0.30,mu_B$) moments of U at 2.1K and 17T (H//c) have been determined. Their ratio ($sim-2.3$) suggests a significant delocalization of the 5$f$ electron states. The similar field dependences of the local U/Co and the macroscopic magnetization indicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge-K edges reveal the presence of small Co-4$p$ and Ge-4$p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co-3$d$ moment is estimated to be at most of the order of 0.1$mu_B$ at 17T. Our results rule out the possibility of an unusual polarisability of the U and Co moments as well as their antiparallel coupling. We conclude that the magnetism which mediates the superconductivity in UCoGe is driven by U.
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 particular, 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.
We have measured X-ray magnetic circular dichroism (XMCD) spectra at the Pu $M_{4,5}$ absorption edges from a newly-prepared high-quality single crystal of the heavy fermion superconductor $^{242}$PuCoGa$_{5}$, exhibiting a critical temperature $T_{c} = 18.7~{rm K}$. The experiment probes the vortex phase below $T_{c}$ and shows that an external magnetic field induces a Pu 5$f$ magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a SQUID device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa$_{5}$ have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5$f$ electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the $5f$ magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5$f$ moment.
We have studied the local electronic structure of LaMn0.5Co0.5O3 using soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie temperature. We discovered that samples with lower Curie temperatures contain low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned. We revealed also that the Co^{2+} ions have a large orbital moment: m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the presence of a large magnetocrystalline anisotropy in the material and predicts a non-trivial temperature dependence for the magnetic susceptibility.
We report on the results of x-ray absorption (XAS), x-ray magnetic circular dichroism (XMCD), and photoemission experiments on {it n}-type Zn$_{1-x}$Co$_x$O ($x=0.05$) thin film, which shows ferromagnetism at room temperature. The XMCD spectra show a multiplet structure, characteristic of the Co$^{2+}$ ion tetrahedrally coordinated by oxygen, suggesting that the ferromagnetism comes from Co ions substituting the Zn site in ZnO. The magnetic field and temperature dependences of the XMCD spectra imply that the non-ferromagnetic Co ions are strongly coupled antiferromagnetically with each other.
X-ray magnetic circular dichroism (XMCD) at the Eu L-edge (2p->5d) in two compounds exhibiting valence fluctuation, namely EuNi2(Si0.18Ge0.82)2 and EuNi2P2, has been investigated at pulsed high magnetic fields of up to 40 T. A distinct XMCD peak corresponding to the trivalent state (Eu3+; f6), whose ground state is nonmagnetic (J=0), was observed in addition to the main XMCD peak corresponding to the magnetic (J=7/2) divalent state (Eu2+; f7). This result indicates that the 5d electrons belonging to both valence states are magnetically polarized. It was also found that the ratio P5d(3+)/P5d(2+) between the polarization of 5d electrons (P5d) in the Eu3+ state and that of Eu2+ is ~ 0.1 in EuNi2(Si0.18Ge0.82)2 and ~ 0.3 in EuNi2P2 at magnetic fields where their macroscopic magnetization values are the same. The possible origin of the XMCD of the Eu3+ state and an explanation of the dependence of P5d(3+)/P5d(2+) on the material are discussed in terms of hybridization between the conduction electrons and the f electrons.