We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 ± 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales.2

Hybrid semiconductor -ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field and local proximitized magnetic exchange. In this work, we present lattice matched hybrid epitaxy of semiconductor -ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure as well as their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the bandgap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs. Induced moments in the adjacent InAs layers were not detected although our ab initio calculations indicate a small exchange field in the InAs layer. This work presents a step towards realizing high quality semiconductor -ferromagnetic insulator hybrids, which is a critical requirement for development of various quantum and spintronic applications without external magnetic fields.

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca 3 Ir 4 Sn 13 using neutron and x-ray scattering techniques. For T > T * ≈ 38 K, the x-ray diffraction data can be satisfactorily refined using the space group P m3n. Below T * , the crystal structure is modulated with a propagation vector of q = (1/2,1/2,0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T * is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T * arising from the softening of a low-energy phonon mode with an energy gap of (120 K) = 1.05 meV. Using density functional theory, the soft phonon mode is identified as a "breathing" mode of the Sn 12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single-crystal diffraction data.

Using resonant soft x-ray techniques we follow the magnetic behavior of a strained epitaxial film of CoCr 2 O 4 , a type-II multiferroic. The film is [110] oriented, such that both the ferroelectric and ferromagnetic moments can coexist in-plane. X-ray magnetic circular dichroism (XMCD) is used in scattering and in transmission modes to probe the magnetization of Co and Cr separately. The transmission measurements utilized x-ray excited optical luminescence from the substrate. Resonant soft x-ray diffraction (RXD) was used to study the magnetic order of the low temperature phase. The XMCD signals of Co and Cr appear at the same ordering temperature T C ≈ 90 K, and are always opposite in sign. The coercive field of the Co and of Cr moments is the same, and is approximately two orders of magnitude higher than in bulk. Through sum rules analysis an enlarged Co 2+ orbital moment (m L ) is found, which can explain this hardening. The RXD signal of the (q q 0) reflection appears below T S , the same ordering temperature as the conical magnetic structure in bulk, indicating that this phase remains multiferroic under strain. To describe the azimuthal dependence of this reflection, a slight modification is required to the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory for magnetic spinels.

We employ resonant soft x-ray diffraction to individually study the magnetic ordering of the Mn and the Tm sublattices in single-crystalline films of orthorhombic (o-) TmMnO 3. The same magnetic ordering wave vector of (0 q 0) with q ≈ 0.46 is found for both ionic species, suggesting that the familiar antiferromagnetic order of the Mn ions induces a magnetic order on the Tm unpaired 4f electrons. Indeed, intensity variations of magnetic reflections with temperature corroborate this scenario. Calculated magnetic fields at the Tm sites are used as a model magnetic structure for the Tm, which correctly predicts intensity variations at the Tm resonance upon azimuthal rotation of the sample. The model allows ruling out a bc-cycloid modulation of the Mn ions as the cause for the incommensurate ordering, as found in TbMnO 3. The structural distortion, which occurs in the ferroelectric phase below T C , was followed through nonresonant diffraction of structural reflections forbidden by the high-temperature crystal symmetry. The (0 q 0) magnetic reflection appears at the Mn resonance well above T C , indicating that this reflection is sensitive also to the intermediate sinusoidal magnetic phase. The model presented suggests that the Tm 4f electrons are polarized well above the ferroelectric transition and are possibly not affected by the transition at T C. The successful description of the induced order observed at the Tm resonance is a promising example for future element-selective studies in which "spectator" ions may allow access to previously unobtainable information about other constituent ions.