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In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2nd order temperature behavior.

We report on x-ray and neutron scattering studies that reveal the structure of interfaces of C60 layers with adjacent transition metal layers, in this instance, Cu. Such interfaces produce room-temperature long-range spin order that is not described by conventional theories of metallic magnetism. We use a combination of hard x-ray reflectivity and neutron scattering to investigate the interfacial structure of two C60/Cu layered samples: a superlattice with multiple C60/Cu repeats and a simpler tri-layer structure. For both structures, we develop a consistent structural model for the two scattering techniques, which details the critical interfacial roughness between the layers. We find that while x-ray reflectivity provides a strong contrast between the C60 and Cu layers, the similar neutron scattering length density of the two materials severely reduces the neutron scattering contrast. Our results can be used to design material systems that permit studies of the magnetism of the C60/transition metal interfaces with spin-sensitive scattering probes such as polarized neutron reflectometry.

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically &lt, 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.

International audience; Temperature- and coverage-dependent studies of the Au(1 1 1)-supported spin crossover Fe(II) complex (SCO) of the type [Fe(H2B(pz)2)2(bipy)] with a suite of surface-sensitive spectroscopy and microscopy tools show that the substrate inhibits thermally induced transitions of the molecular spin state, so that both high-spin and low-spin states are preserved far beyond the spin transition temperature of free molecules. Scanning tunneling microscopy confirms that [Fe(H2B(pz)2)2(bipy)] grows as ordered, molecular bilayer islands at sub-monolayer coverage and as disordered film at higher coverage. The temperature dependence of the electronic structure suggest that the SCO films exhibit a mixture of spin states at room temperature, but upon cooling below the spin crossover transition the film spin state is best described as a mix of high-spin and low-spin state molecules of a ratio that is constant. This locking of the spin state is most likely the result of a substrate-induced conformational change of the interfacial molecules, but it is estimated that also the intra-atomic electron–electron Coulomb correlation energy, or Hubbard correlation energy U, could be an additional contributing factor.

Ferromagnetic nanoparticles in the 10-14 nm size range are examined for their size and interaction dependent magnetic properties. From X-ray magnetic circular dichroism the orbital-to-spin magnetic moment ratio is determined and found to decrease significantly with particle size. This is in accordance with previous complementary studies on smaller particles and highlights the difficulty of fitting to a simple core-shell model. Vibrating sample magnetometry experiments on samples with more than 1000 particles per square micron show a wide distribution of blocking temperatures from 50 to greater than 650 K. This is attributed to the dipole-dipole magnetic coupling forces between particles. The blocking temperatures show an unexpected negative correlation with increasing particle density.

We demonstrate a layer- and time-resolved measurement of ferromagnetic resonance (FMR) in a Ni81Fe19 / Cu / Co93Zr7 trilayer structure. Time-resolved x-ray magnetic circular dichroism has been developed in transmission, with resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2 degree), time-domain magnetization precession could be observed directly, and resolved to individual layers through elemental contrast at Ni, Fe, and Co edges. The phase sensitivity allowed direct measurement of relative phase lags in the precession oscillations of individual elements and layers. A weak ferromagnetic coupling, difficult to ascertain in conventional FMR measurements, is revealed in the phase and amplitude response of individual layers across resonance., 22 pages, 6 figures submitted to Physical Review B

International audience; We have carried out resonant magnetic x-ray scattering from Co/ Ru multilayers with weak antiferromagnetic coupling. We have measured hysteresis loops at different points in reciprocal space ͑specular or diffuse, integer or half-order peaks͒, which reveal a rich variety of different shapes. These arise from different degrees of mixing of the scattering arising from the field dependence of the order parameters describing the degree of ferromagnetic, antiferromagnetic correlations in the sample, with the off-specular measurements giving information about their lateral extent. We make a comparison with macroscopic measurements that give information about the degree of different forms of magnetic order averaged over the whole sample in real space.

We report the results of soft x-ray resonant magnetic scattering experiments conducted on Co∕Ru magnetic multilayers patterned into a reflection grating. Sample magnetization dependent scans of the in-plane wave vector transfer were performed to probe the relationship between the lateral physical and magnetic structures. Different Fourier components were found to dominate the sum and difference of these spectra, corresponding to different lateral wave forms for the structure-structure and structure-magnetism correlations.

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps., see http://magnet.ap.columbia.edu/pubs for related work