Optical control of 4f orbital state in rare-earth metals

Information technology demands continuous increase of data-storage density. In high-density magnetic recording media, the large magneto-crystalline anisotropy (MCA) stabilizes the stored information against decay through thermal fluctuations. In the latest generation storage media, MCA is so large that magnetic order needs to be transiently destroyed by heat to enable bit writing. Here we show an alternative approach to control high-anisotropy magnets: With ultrashort laser pulses the anisotropy itself can be manipulated via electronic state excitations. In rare-earth materials like terbium metal, magnetic moment and high MCA both originate from the 4f electronic state. Following infrared laser excitation 5d-4f electron-electron scattering processes lead to selective orbital excitations that change the 4f orbital occupation and significantly alter the MCA. Besides these excitations within the 4f multiplet, 5d-4f electron transfer causes a transient change of the 4f occupation number, which, too, strongly alters the MCA. Such MCA change cannot be achieved by heating: The material would rather be damaged than the 4f configuration modified. Our results show a way to overcome this limitation for a new type of efficient magnetic storage medium. Besides potential technological relevance, the observation of MCA-changing excitations also has implications for a general understanding of magnetic dynamics processes on ultrashort time scales, where the 4f electronic state affects the angular momentum transfer between spin system and lattice.Comment: Manuscript (14 pages, 3 figures) and Supplementary Information (22 pages, 9 figures

Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L 3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole ( E 1 , 2 p → 5 d ) or quadrupole ( E 2 , 2 p → 4 f ) transition allows us to selectively and independently study the spin dynamics of the itinerant 5 d and localized 4 f electronic subsystems via the suppression of the magnetic (2 1 3 − τ ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4 f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4 f − 5 d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak

Strain analysis from M-edge resonant inelastic X-ray scattering of nickel oxide films

Electronic structure modifications due to strain are an effective method for tailoring nano-scale functional materials. Demonstrated on nickel oxide (NiO) thin films, Resonant Inelastic X-ray Scattering (RIXS) at the transition-metal M2,3-edge is shown to be a powerful tool for measuring the electronic structure modification due to strain in the near-surface region. Analyses from the M2,3-edge RIXS in comparison with dedicated crystal field multiplet calculations show distortions in 40 nm NiO grown on a magnesium oxide (MgO) substrate (NiO/MgO) similar to those caused by surface relaxation of bulk NiO. The films of 20 and 10 nm NiO/MgO show slightly larger differences from bulk NiO. Quantitatively, the NiO/MgO samples all are distorted from perfect octahedral (Oh) symmetry with a tetragonal parameter Ds of about −0.1 eV, very close to the Ds distortion from octahedral (Oh) symmetry parameter of −0.11 eV obtained for the surface-near region from a bulk NiO crystal. Comparing the spectra of a 20 nm film of NiO grown on a 20 nm magnetite (Fe3O4) film on a MgO substrate (NiO/Fe3O4/MgO) with the calculated multiplet analyses, the distortion parameter Ds appears to be closer to zero, showing that the surface-near region of this templated film is less distorted from Oh symmetry than the surface-near region in bulk NiO. Finally, the potential of M2,3-edge RIXS for other investigations of strain on electronic structure is discussed

Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L 3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole ( E 1 , 2 p → 5 d ) or quadrupole ( E 2 , 2 p → 4 f ) transition allows us to selectively and independently study the spin dynamics of the itinerant 5 d and localized 4 f electronic subsystems via the suppression of the magnetic (2 1 3 − τ ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4 f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4 f − 5 d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.This article is published as Rettig, Laurenz, Christian Dornes, N. Thielemann-Kühn, N. Pontius, Hartmut Zabel, D. L. Schlagel, T. A. Lograsso et al. "Itinerant and localized magnetization dynamics in antiferromagnetic Ho." Physical Review Letters 116, no. 25 (2016): 257202. DOI: 10.1103/PhysRevLett.116.257202. Posted with permission.</p