Tailoring the nature of magnetic coupling of Fe-porphyrin molecules to ferromagnetic substrates.

We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

Substrate-induced magnetic ordering and switching of iron porphyrin molecules.

To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magnetic coupling of in-situ-sublimated Fe porphyrin molecules on ferromagnetic Ni and Co films on Cu(100). Our studies involve X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments. In a combined experimental and computational study we demonstrate that owing to an indirect, superexchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferromagnetically. The Fe magnetic moment can be rotated along directions in plane as well as out of plane by a magnetization reversal of the substrate, thereby opening up an avenue for spin-dependent molecular electronics.

Spin-wave excitations: the main source of the temperature dependence of interlayer exchange coupling in nanostructures.

Quantum mechanical calculations based on an extended Heisenberg model are compared with ferromagnetic resonance experiments on prototype trilayer systems Ni(7)/Cu(n)/Co(2)/Cu(001) in order to determine and separate for the first time quantitatively the sources of the temperature dependence of interlayer exchange coupling. Magnon excitations are responsible for about 75% of the reduction of the coupling strength from zero to room temperature. The remaining 25% are due to temperature effects in the effective quantum well and the spacer-magnet interfaces.

Measuring the kernel of time-dependent density functional theory with x-ray absorption spectroscopy of transition metals.

The 2p-3d core-hole interaction in the L2.3 absorption spectra of the transition metals is treated within time-dependent density functional theory. A simple three-level model explains the origin of the strong deviations from the one-particle branching ratio and yields matrix elements of the unknown exchange-correlation kernel directly from experiment.

Direct observation of orbital magnetism in cubic solids.

We present x-ray magnetic circular dichroism determinations of the orbital/spin magnetic moment ratios of dilute 3d-series impurities in Au (and Cu) host matrices. This is the first direct measurement of considerable orbital moments in cubic symmetry for a localized impurity in a bulk metal host. It is shown that the unquenching of orbital magnetism depends on a delicate balance of hybridization effects between the local impurity with the host and the filling of the 3d states of the impurity. The results are accompanied by ab initio calculations that support our experimental findings.

Manipulation of spin reorientation transition by oxygen surfactant growth: a combined theoretical and experimental approach.

A new procedure described to manipulate the spin reorientation transition (SRT) in ultrathin ferromagnetic films, i.e., the oxygen assisted surfactant growth of Ni monolayers (ML), reduces the surface anisotropy energy. This in turn shifts the SRT down by about 5 ML. Through first principles calculations based on the full potential linearized augmented plane wave method, these characteristics are explained at the electronic structure level. The combination of experiment and theory allows us to specify the mechanism. This will be important for further engineering of new nanostructures.

T(3/2) dependence of the interlayer exchange coupling in ferromagnetic multilayers.

The temperature dependence of the interlayer exchange coupling in ferromagnetic films coupled across nonmagnetic spacers is determined via in situ ferromagnetic resonance experiments for various systems. Clear evidence for a T(3/2) law is found over a wide temperature regime.

Systematics of the induced magnetic moments in 5d layers and the violation of the third Hund's rule.

X-ray magnetic circular dichroism measurements are reported at the beginning (W) and at the end (Ir, Pt) of the 5d series of the periodic table. Considerable induced magnetic moments of about 0.2 mu(B)/atom were probed for the nonmagnetic W and Ir and compared to previous data for the Pt induced moments in multilayers. W was found to couple antiferromagnetically and Ir ferromagnetically to the 3d layers. Finally, the W spin and orbital magnetic moment couple in parallel, contrary to what is expected from the third Hund's rule. This remarkable finding shows that the induced magnetic behavior of 5d layers may be radically different than that of impurities and alloys.

Magnetic EXAFS at Gd L-edges: the spin-pair-distribution function of Gd neighbors.

The purpose of the experiment is to study the normal and the Magnetic EXAFS (MEXAFS) since EXAFS is the method of choice to investigate the local pair- and spin-pair-distribution function. We present MEXAFS and EXAFS measurements at the L-edges of a Gd single crystal in the temperature range of 10 K to 250 K. Therefore we are able to investigate the MEXAFS in a wide range of the reduced temperature t=T/Tc of 0.04 < or = t < or = 0.85 with Tc=293 K. We find a strong decrease of the nearest neighbor EXAFS which retains only about 35% of its 10 K value already at 250 K. This highlights the importance of lattice vibrations. To analyze the individual scattering contributions to the MEXAFS and the EXAFS, ab initio calculations (FEFF code) have been carried out. The comparison of the temperature-dependent damping of the normal EXAFS with the spin-dependent MEXAFS allows us to separate the influence of lattice vibrations (Debye temperature 160 K) from the magnetic ordering (Curie temperature) on the MEXAFS.

Element-specific magnetization curves and crossover in Co/Cu/Ni/Cu(001) trilayers studied by XMCD.

We present temperature-dependent measurements via the element-specific XMCD technique for Co/Cu/Ni trilayers for cases where the ordering temperature of Co is lower than the one of Ni. Our work focuses mainly on the influence of the interlayer coupling on the shape of the temperature-dependent magnetization curves of Ni. By electing 1.3 ML of Co and 4 ML of Ni we get a lower T(C)(Co) approximately 90 K and a higher T(C)(Ni) approximately 180 K. The crossing of the M(T) curves leads to a rotation of the remanent magnetization of Ni. A case where the sublayer magnetizations change gradually their angle as a function of the temperature is recorded and attributed to a competition between coupling and anisotropy mechanisms.

Shape resonances of oriented molecules: ab initio theory and experiment on hydrocarbon molecules

We report ab initio calculations of the x-ray absorption cross section for the near edge x-ray absorption fine structure of C2H6, and C2H4, and C2H2 at the C K-edge, based on a full multiple scattering formalism. The angular dependence of the electric dipole transition in the calculations as well as the angular dependent experiments for the oriented molecules give a good opportunity to compare both. The resonance can be assigned to a sigma(*) shape resonance. The multiple scattering formalism and the experiment agree well and thereby support the existence of such features in the spectra.

Layer-resolved magnetic moments in Ni/Pt multilayers

The magnetic moments in Ni/Pt multilayers are thoroughly studied by combining experimental and ab initio theoretical techniques. SQUID magnetometry probes the samples' magnetizations. X-ray magnetic circular dichroism separates the contribution of Ni and Pt and provides a layer-resolved magnetic moment profile for the whole system. The results are compared to band-structure calculations. Induced Pt magnetic moments localized mostly at the interface are revealed. No magnetically "dead" Ni layers are found. The magnetization per Ni volume is slightly enhanced compared to bulk NiPt alloys.