X-ray Magnetic Circular Dichroism Investigation Research Articles

Enhanced Sm spin projection in GdxSm1−xN

The rare-earth nitrides form a series of structurally simple $intrinsic$ ferromagnetic semiconductors, a rare class of both fundamental interest and application potential. Within the series there is a wide range of magnetic properties relating to the spin/orbit contributions to the ferromagnetic ground states. We report an x-ray magnetic circular dichroism investigation of the spin/orbit magnetic dipole alignments of Sm and Gd ions in epitaxial Gd$_x$Sm$_{1-x}$N films. The Sm spin-alignment expectation value $\langle S_{z} \rangle$ is seen to be strengthened by the Gd/Sm exchange interaction, providing guidance concerning the composition for an angular momentum compensation point (where the volume-averaged total angular momentum of a film is zero).

Large-area van der Waals epitaxy and magnetic characterization of Fe3GeTe2 films on graphene

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT)—a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM)—directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices.

X-ray magnetic circular dichroism investigation of electric field manipulation of magnetization reversal in Ta/CoFeB/(011)PMN-PT heterostructure

A tunable and nonvolatile magnetoelectric coupling effect and magnetization reversal in demagnetized Ta/CoFeB/(011)PMN-PT was obtained by applying electric fields without an external magnetic field. X-ray magnetic circular dichroism investigation under an in situ electric field indicates that the orbital magnetic moment to spin magnetic moment ratios of both Fe and Co are enhanced under the electric field. The ratio of Fe under negative electric field control is much larger than that under a positive electric field, while it remains almost unchanged for Co under different electric fields.

Structure and cation distribution of (Mn0.5Zn0.5)Fe2O4 thin films on SrTiO3(001)

A comprehensive study on growth of ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) films on single crystalline strontium titanate(001) (SrTiO3) substrates was carried out. Under the optimized conditions, a thin film with a layer thickness of 200 nm was deposited, and the structural properties were investigated. Contrary to data published in literature, no buffer layer was necessary to achieve epitaxial growth of a poorly lattice-matched layer. This was confirmed for Mn0.5Zn0.5Fe2O4(001) on SrTiO3(001) by x-ray diffraction and the adjoined phi scans, which also revealed a lattice compression of 1.2% of the manganese zinc ferrite film in the out-of-plane direction. Using x-ray photoelectron spectroscopy, the near surface stoichiometry of the film could be shown to agree with the intended one within the uncertainty of the method. X-ray absorption spectroscopy showed an electronic structure close to that published for bulk samples. Additional x-ray magnetic circular dichroism investigations were performed to answer detailed structural questions by a comparison of experimental data with the calculated ones. The calculations took into account ion sites (tetrahedral vs. octahedral coordination) as well as the charge of Fe ions (Fe2+ vs. Fe3+). Contrary to the expectation for a perfect normal spinel that only Fe3+ ions are present in octahedral sites, hints regarding the presence of additional Fe2+ in octahedral sites as well as Fe3+ ions in tetrahedral sites have been obtained. Altogether, the layer could be shown to be mostly in a normal spinel configuration.

X-ray Magnetic Circular Dichroism Investigation of the Electron Transfer Phenomena Responsible for Magnetic Switching in a Cyanide-Bridged [CoFe] Chain

The cyanide-bridged [CoFe] one-dimensional chain, [Co(II)((R)-pabn)][Fe(III)(Tp)(CN)3](BF4)·MeOH·2H2O, where (R)-pabn = (R)-N2,N(2')-bis(pyridin-2-ylmethyl)-1,1'-binaphthyl-2,2'-diamine and Tp = hydrotris(pyrazolyl)borate, exhibits magnetic and electric bistabilities originating from an electron transfer coupled spin transition between Fe-CN-Co pairs. The use of L-edge X-ray absorption spectroscopy (XAS) in combination with L-edge X-ray magnetic circular dichroism (XMCD) is explored for the investigation of the electronic structure and magnetization of Co and Fe ions separately, in both diamagnetic and paramagnetic states. It has been established from susceptibility results that the switching between diamagnetic and paramagnetic phases emanates from electron transfer between low spin Fe(II) and Co(III), resulting in low spin Fe(III) (S = 1/2) and high spin Co(II) (S = 3/2). The XAS and XMCD results are consistent with the bulk susceptibility measurements, where greater detail regarding the charge transfer process is determined. The Fe-CN-Co electron transfer pathway is highlighted by a strongly XMCD dependent transition to a cyanide back bonding orbital, giving evidence for strong hybridization with Fe(III) t2g orbitals. In addition to thermally induced and photoinduced switching, [CoFe] is found to exhibit a switching by grinding induced dehydration. Analysis of XAS shows that on grinding diamagnetic [CoFe], 75% of metal ions lock into the magnetic Co(II)Fe(III) phase. Density functional theory calculations based on the [CoFe] crystal structure in the magnetic and nonmagnetic phases aid the spectroscopic results and provide a complementary insight into the electronic configuration of the [CoFe] 3d shells, quantifying the change in ligand field around Co and Fe centers on charge transfer.

Reinvestigation of the Fe, Cu and Cr valences in (FeCu)Cr2S4 spinels

AbstractThe valence states of Fe, Cu, and Cr ions in single crystalline Fe1–x Cux Cr2S4 have been studied using X‐ray photoelectron spectroscopy (XPS), X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD). XPS measurements of compositions x = 0.2, 0.5, 0.6, and 0.9 indicate that the Cu and Cr ions remain mainly in the same valence state over the whole substitution range. XAS, XPS and XMCD investigations of Fe0.5Cu0.5Cr2S4 reveal a sample and preparation dependence of the Cu and Fe spectra. The comparison of the Cu 2p XAS spectrum with a literature measurement shows a change in the amount of Cu2+ explained by the appearance of an additional Cu2+ phase, possibly due to oxidation on the surface. XAS, XPS and XMCD spectra of Fe show a Fe3+ impurity phase present in air‐treated Fe0.5Cu0.5Cr2S4, especially in powdered samples. The lines related to trivalent Fe are found to disappear completely after cleaning the samples in‐situ and only the lines attributed to divalent Fe remain. Since many experimental methods rely on powdered samples and measure in air or in low vacuum, these findings provide an explanation for the differing results concerning the Fe valence obtained by these methods. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray Magnetic Circular Dichroism Investigations of the Origin of Room Temperature Ferromagnetism in Fe-Doped ZnO Nanoparticles

We have studied the electronic structure of Zn0.9Fe0.1O nanoparticles, which have been reported to be ferromagnetic at room temperature, by using soft X-ray absorption (XAS) and magnetic circular dichroism (XMCD). The XAS results indicate that Fe ions are predominantly in the Fe3+ state with mixture of a small amount of Fe2+. It is shown that the room temperature ferromagnetism in the Zn0.9Fe0.1O nanoparticles is primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3 nm.

XAS and XMCD Investigation of Mn12 Monolayers on Gold

The deposition of Mn(12) single molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposed approach to probe the electronic structure and magnetism of Mn(12) complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra, respectively. Partial reduction of metal ions to Mn(II) was clearly observed upon deposition on Au(111) of two different Mn(12) derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidation state, as well as the relative proportions of Mn(II), Mn(III) and Mn(IV) species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantly decreased as compared with bulk Mn(12) samples. The results highlight an utmost redox instability of Mn(12) complexes at gold surfaces, presumably accompanied by structural rearrangements, which cannot be easily revealed by standard surface analysis based on X-ray photoelectron spectroscopy and scanning tunnelling microscopy.

X-ray magnetic circular dichroism investigation of the superparamagnetic transition-metal ion-cluster r-Mn 12Bz

We present the results of an X-ray magnetic circular dichroism investigation of a reduced version of Mn 12 benzoate. At variance with the parent Mn 12 benzoate compound, which, analogously to Mn 12 acetate, has a ground-state spin equal to ten, the reduced species has a ground-state with total spin S = 19/2. The half-integer spin in the ground-state makes this compound an appealing system where to test parity effects on the efficiency of the quantum tunnelling of the magnetisation. We exploited the sensitivity of X-ray magnetic circular dichroism to the oxidation state of the absorbing metal ion to obtain information about the internal structure of the reduced Mn 12 benzoate. In particular, we performed multiplet calculations to analyse the X-ray magnetic circular dichroism spectra at the manganese L 2,3 edge and identify the contribution of the Mn II ion resulting by the reduction process.

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced Néel-cap reversal

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on Néel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of Néel-cap magnetization give clear evidence for the magnetization reversal of the Néel caps around 120mT, with a ±20mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots.

Magnetic moment of Fe in oxide-free FePt nanoparticles

We present results of x-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations at the ${L}_{3,2}$ edges of Fe in FePt nanoparticles. Plasma treated, oxide-free FePt nanoparticles with a high degree of $L{1}_{0}$ ordering and a mean diameter of about $6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were produced by gas phase condensation followed by in situ flight annealing. The presence of the $L{1}_{0}$ phase is observed by high resolution transmission electron microscopy. Compared to chemically disordered A1 FePt particles of similar size and composition, we find a fourfold enhanced orbital magnetic moment ${\ensuremath{\mu}}_{L}=0.19$ $(\ifmmode\pm\else\textpm\fi{}0.04){\ensuremath{\mu}}_{B}$, a nearly unchanged effective spin magnetic moment ${\ensuremath{\mu}}_{S}^{\mathrm{eff}}=2.21$ $(\ifmmode\pm\else\textpm\fi{}0.24){\ensuremath{\mu}}_{B}$ and a 2% increase in the white-line ratio of the ${L}_{3}$ and ${L}_{2}$ Fe edges. Nevertheless, the room temperature coercive field amounts to ${\ensuremath{\mu}}_{0}{H}_{C}=38(\ifmmode\pm\else\textpm\fi{}7)\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ only. These results are discussed in relation to possible effects of chemical order and surface anisotropy.

Plasma-assisted molecular beam epitaxy of wurtzite GaMnN displaying ferromagnetism assessed by means of X-ray magnetic circular dichroism

We report on the optimization of the growth conditions of wurtzite GaMnN grown by plasma-assisted molecular beam epitaxy in order to obtain intrinsic ferromagnetic behavior. By growing with a Ga/N ratio lower than unity and by introduction of periodic growth interruptions, we succeed in synthesizing single-phase GaMnN epilayers containing up to 6.3 at.% of Mn. The structural quality of the GaMnN epilayer and the absence of secondary phases are demonstrated by means of X-ray diffraction experiments and X-ray linear dichroism measurements performed at the Mn and Ga K-edges. The intrinsic ferromagnetism for 6.3 at.% of Mn in our GaMnN epilayer is demonstrated by means of magnetization measurements in a 5 T Quantum Design superconducting quantum interference device (SQUID) and X-ray magnetic circular dichroism investigations performed at the K-edge of Mn. The Curie temperature thus determined is equal to 8 K and a spontaneous magnetic moment of 2.4 μ B per Mn atom is found at 2 K.

Investigation of the metamagnetic transition in Y2Co7H6

The effects of hydrogen absorption on the crystalline and magnetic properties of the γ-phase Y2Co7H6 were examined by bulk magnetic measurements, neutron diffraction and X-ray absorption spectroscopy. An orthorhombic distortion of the unit cell occurs upon hydrogen insertion, accompanied by a reduction of the magnetic moment on Co and a decrease of the ordering temperature. Magnetic measurements indicate a metamagnetic transition occurring in the γ-phase hydride. The transition shows a broad hysteresis and the magnitude of the critical field is found to decrease with increasing temperature. The electronic structure of Co in Y2Co7 and Y2Co7H6 has been investigated by X-ray absorption spectroscopy. The observed decrease of the pre-peak intensities in the Co-K edge XANES spectra are related to the changes of the 4p–3d density of states upon hydrogen insertion. A comparative X-ray magnetic circular dichroism investigation at the Co-K edge has been carried out for Y2Co7 and Y2Co7H6. The observed XMCD signal enables us to propose that ferromagnetic ordering occurs in the γ-hydride.

Remnant X-ray Magnetic Circular Dichroism Investigation of the Local Magnetic Contributions to the Magnetization of a Coercive Bimetallic Molecule-Based Magnet

X-ray magnetic circular dichroism (XMCD) measurements have been performed at the Cr and Fe K-edges for the two-dimensional oxalate-based magnet {[N(C4H9)4][FeIIFeIII0.77CrIII0,23(ox)3]}n (ox = C2O42-) in its long-range ordered magnetic state. First, the XMCD spectra recorded under a 2 T applied magnetic field are used to probe the local exchange interaction between the metal ions through the relative orientation of their spins in the magnetic field. The remnant local magnetic moments on the Cr and Fe ions have been clearly evidenced, analyzed, and compared to the total remnant magnetization measured by conventional magnetometry. The respective impact of single ion anisotropy, magneto-structural anisotropy, and exchange interaction on the remnant magnetic state is then discussed.

X-ray magnetic circular dichroism investigation of magnetic contributions from Mn(III) and Mn(IV) ions in Mn12-ac

We present the results of an x-ray magnetic circular dichroism (XMCD) investigation of the molecular superparamagnet Mn12-ac. Thanks to an approach based on chemical synthesis of model compounds, the contributions of Mn(III) and Mn(IV) ions to the absorption spectrum and the dichroic signal of Mn12-ac have been separated. The two contributions have been analyzed by simulating the XMCD spectra of Mn12-ac by means of crystal-field multiplet calculations. The ferrimagnetic structure of Mn12-ac has been confirmed, and the individual magnetic moments of Mn(III) and Mn(IV) ions are in good agreement with the values obtained by means of first-principles calculations and polarized neutron diffraction experiments. The orbital and spin contributions to the magnetic moment have been evaluated for each manganese ion, providing the evidence of negligible orbital magnetic moments for both Mn(III) and Mn(IV) ions.

Soft x-ray magnetic circular dichroism investigation of the spin reorientation transition in Nd2Fe14B

A temperature dependent x-ray magnetic circular dichroism (XMCD) study at the Fe L2,3 and Nd M4,5 edges through the spin reorientation transition of a Nd2Fe14B single crystal is presented. Direct comparison of XMCD data with values of the magnetic moments derived from Mössbauer spectroscopy allows one to independently determine the evolution of the titling angle between Fe and Nd moments and the c axis. The experimental results evidence a strong noncollinearity between Fe and Nd magnetic moments in the low-temperature phase. A new result is the observed delay in the reorientation of the Fe sublattice with respect to that of the Nd sublattice by around 10 K.