Element specific determination of the magnetic properties of two macrocyclic tetranuclear 3d-4f complexes with a Cu3Tb core by means of X-ray magnetic circular dichroism (XMCD).

We apply X-ray magnetic circular dichroism to study the internal magnetic structure of two very promising star shaped macrocyclic complexes with a CuII3TbIII core. These complexes are rare examples prepared with a macrocyclic ligand that show indications of SMM (Single Molecule Magnet) behavior, and they differ only in ring size: one has a propylene linked macrocycle, [CuII3TbIII(LPr)(NO3)2(MeOH)(H2O)2](NO3)·3H2O (nickname: Cu3Tb(LPr)), and the other has the butylene linked analogue, [CuII3TbIII(LBu)(NO3)2(MeOH)(H2O)](NO3)·3H2O (nickname: Cu3Tb(LBu)). We analyze the orbital and spin contributions to the Cu and Tb ions quantitatively by applying the spin and orbital sum rules concerning the L2 (M4)/L3 (M5) edges. In combination with appropriate ligand field simulations, we demonstrate that the Tb(iii) ions contribute with high orbital magnetic moments to the magnetic anisotropy, whereas the ligand field determines the easy axis of magnetization. Furthermore, we confirm that the Cu(ii) ions in both molecules are in a divalent valence state, the magnetic moments of the three Cu ions appear to be canted due to 3d-3d intramolecular magnetic interactions. For Cu3Tb(LPr), the corresponding element specific magnetization loops reflect that the Cu(ii) contribution to the overall magnetic picture becomes more important as the temperature is lowered. This implies a low value for the 3d-4f coupling.

The x-ray magnetic circular dichroism spin sum rule for 3d4 systems: Mn3+ ions in colossal magnetoresistance manganites.

The colossal magnetoresistance manganites La(0.87±0.02)Sr(0.12±0.02)MnO(3+δ), La(0.78±0.02)Sr(0.17±0.02)MnO(3+δ), and La(0.66±0.02)Sr(0.36±0.02)MnO(3+δ) (δ close to 0) were investigated by using soft x-ray magnetic circular dichroism (XMCD) and magnetometry. Very good agreement between the values for the average Mn magnetic moments determined with these two methods was achieved by correcting the XMCD spin sum rule results by means of charge transfer multiplet calculations, which also suggest a charge transfer of ~50% for Mn(4+) and approximately equal to 30% for Mn(3+). The magnetic moment was found to be localized at the Mn ions for x = 0.17 and 0.36 at 80 K and for x = 0.12 in the temperature range from 80 to 300 K. We discuss our findings in the light of previously published data, confirming the validity of our approach.

Monodispersed NiO nanoflowers with anomalous magnetic behavior.

Nickel oxide (NiO) nanoflowers, prepared by thermal decomposition, exhibit anomalous magnetic properties far below the blocking temperature, i.e., a cusp in both the zero-field-cooled and field-cooled curves at about 21 K. Detailed characterization discloses that the individual NiO nanoflower consists of porous crystals with holes (1.0-1.5 nm in size) inside. We believe that the low temperature magnetic feature observed here could be a new kind of spin transition for the uncompensated spins around the holes and will trigger more studies in other nanostructured antiferromagnetic materials.

Magnetization dynamics of Landau structures: tuning the response of mesoscopic magnetic objects using defects.

Magnetic vortex cores are interacting with and can even be annihilated by artificial defects, such as holes. These defects have been fabricated by focused ion beam milling (FIB) into the magnetic domains, domain walls and the center of square-shaped vortices, known as Landau structures. We report the imaging of the magnetization dynamics of Landau structures containing holes by means of x-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM). Due to the high lateral and temporal resolution of this method, the magnetic excitation spectrum, which is characteristic for the vortex-hole interaction, is investigated in detail. We find that the vortex core as well as domain walls can be trapped by small holes. With the help of micromagnetic simulations we show that the vortex gyrotropic motion frequency is enhanced, whereas the amplitude is significantly reduced in the case of non-centric holes in domain walls.

Dynamic vortex-antivortex interaction in a single cross-tie wall.

A fascinating property of micromagnetism comes from the possibility to control the domain and vortex configuration through the sample shape and size. For instance, in a rectangular platelet a configuration containing a stable combination of vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. Here we report on its magnetization dynamics including the vortex-antivortex interactions. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. We show that the vortex dynamics can be used to identify the core configuration, which is not directly accessible to x-ray microscopy because of its limited spatial resolution.

Electronic structure study by means of x-ray spectroscopy and theoretical calculations of the "ferric star" single molecule magnet.

The electronic structure of the single molecule magnet system {M[Fe(L(1))(2)](3)}4CHCl(3) [M=Fe,Cr;L(1)=CH(3)N(CH(2)CH(2)O)(2) (2-)] has been studied using x-ray photoelectron spectroscopy, x-ray-absorption spectroscopy, soft-x-ray emission spectroscopy, as well as theoretical density-functional-based methods. There is a good agreement between theoretical calculations and experimental data. The valence band mainly consists of three bands between 2 and 30 eV. Both theory and experiments show that the top of the valence band is dominated by the hybridization between Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe in the molecule is most likely in the 2+ charge state. Its neighboring atoms (O,N) exhibit a magnetic polarization yielding effective spin S=52 per iron atom, giving a high-spin state molecule with a total S=5 effective spin for the case of M=Fe.

Electronic structure of A- and B-site doped lanthanum manganites: a combined X-ray spectroscopic study.

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.

Direct investigation of orbital ordering in a colossal magnetoresistance manganite by means of X-ray linear dichroism at the Mn L edge.

We have investigated for the first time the orbital ordering in a three-dimensional colossal magnetoresistance manganite, namely La(7/8)Sr(1/8)MnO3, by applying soft X-ray linear dichroism (XLD) to the Mn L edge. We found that the cooperative Jahn-Teller distorted orthorhombic phase, which is present at a temperature of 240 K, is probably accompanied by a predominantly cross type (x2 - z2)/(y2 - z2) orbital ordering. This result is discussed in the light of different exchange interaction models.