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Register a new userLocal Magnetic States of the Weakly Ferromagnetic Iron-Based Superconductor Sr$_2$VFeAsO$_{3-delta}$ Studied by X-ray Magnetic Circular Dichroism
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We have performed x-ray magnetic circular dichroism (XMCD) measurements on the iron-based superconductor Sr$_2$VFeAsO$_{3-delta}$ to study the origin of weak ferromagnetism (WFM) reported for this compound. While Fe 3$d$ electrons show a magnetic response similar to the other iron pnictides, signals from V 3$d$ electrons remain finite at zero magnetic field and may be responsible for the WFM.
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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.
Element-specific x-ray resonant magnetic scattering investigations were performed to determine the magnetic structure of Eu in EuRh2As2. In the temperature range from 46 K down to 6 K, an incommensurate antiferromagnetic (ICM)structure with a temperature dependent propagation vector (0 0 0.9) coexists with a commensurate antiferromagnetic (CM) structure. Angular-dependent measurements of the magnetic intensity indicate that the magnetic moments lie in the tetragonal basal plane and are ferromagnetically aligned within the a-b plane for both magnetic structures. The ICM structure is a spiral-like magnetic structure with a turn angle of 162 deg between adjacent Eu planes. In the CM structure, this angle is 180 deg. These results are consistent with band-structure calculations which indicate a strong sensitivity of the magnetic configuration on the Eu valence.
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 electrically 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.
In addition to unconventional high-Tc superconductivity, the iron arsenides exhibit strong magnetoelastic coupling and a notable electronic anisotropy within the a-b plane. We relate these properties by studying underdoped Ba(Fe{1-x}Co{x})2As2 by x-ray diffraction in pulsed magnetic fields up to 27.5 Tesla. We exploit magnetic detwinning effects to demonstrate anisotropy in the in-plane susceptibility, which develops at the structural phase transition despite the absence of magnetic order. The degree of detwinning increases smoothly with decreasing temperature, and a single- domain condition is realized over a range of field and temperature. At low temperatures we observe an activated behavior, with a large hysteretic remnant effect. Detwinning was not observed within the superconducting phase for accessible magnetic fields.
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.
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