Insights into ultrafast demagnetization in pseudo-gap half metals

Interest in femtosecond demagnetization experiments was sparked by Bigot's discovery in 1995. These experiments unveil the elementary mechanisms coupling the electrons' temperature to their spin order. Even though first quantitative models describing ultrafast demagnetization have just been published within the past year, new calculations also suggest alternative mechanisms. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high spin polarization metals, and consequently there is broad interest in further understanding the physics of these phenomena. To gain new and relevant insights, we perform ultrafast optical pump-probe experiments to characterize the demagnetization processes of highly spin-polarized magnetic thin films on a femtosecond time scale. Previous studies have suggested shifting the Fermi energy into the center of the gap by tuning the number of electrons and thereby to study its influence on spin-flip processes. Here we show that choosing isoelectronic Heusler compounds (Co2MnSi, Co2MnGe and Co2FeAl) allows us to vary the degree of spin polarization between 60% and 86%. We explain this behavior by considering the robustness of the gap against structural disorder. Moreover, we observe that Co-Fe-based pseudo gap materials, such as partially ordered Co-Fe-Ge alloys and also the well-known Co-Fe-B alloys, can reach similar values of the spin polarization. By using the unique features of these metals we vary the number of possible spin-flip channels, which allows us to pinpoint and control the half metals electronic structure and its influence onto the elementary mechanisms of ultrafast demagnetization.Comment: 17 pages, 4 figures, plus Supplementary Informatio

Exploring the impact of the inverse Faraday effect on all-optical helicity-dependent magnetization switching

All-optical helicity-dependent magnetization switching (AO-HDS) is the quickest deterministic technique for data storage by solely using ultrashort laser pulses. Granular high data density magnetic storage media developed for heat-assisted magnetic recording (HAMR) provide an ideal playground to investigate the interplay of effects leading to magnetization switching. In the latest perception, we identify two effects, the magnetic circular dichroism (MCD) and the inverse Faraday effect (IFE), as the forces driving the switching process. During photon absorption, which leads to a rapid temperature rise and thus to magnetization quenching, the MCD ensures two distinct electron temperatures due to helicity-dependent absorption. This effect already holds a nonvanishing probability for magnetization switching. At the same time, the IFE induces a magnetic moment within the material, enhancing the switching probability. We present AO-HDS experiments using ultrashort laser pulses ($\lesssim 200\,\mathrm{fs}$) in the near-infrared range from $800\,\mathrm{nm}$ to $1500\,\mathrm{nm}$. The experiments demonstrate a strong dependence of the switching efficiency on the absorbed energy density, elevating the electron temperature in the vicinity of the Curie point, allowing for the IFE to take full effect, inducing a magnetic moment for deterministic switching in the quenched magnetization state. While we do not observe an enhanced switching due to an increased MCD, a higher induced magnetization usually improves the switching rate if the electron temperature reaches the transition temperature vicinity. Therefore, we conclude that the magnetic moment generated by the IFE is crucial for the switching efficiency and the distinct deterministic character of the switching process. Laser pulses with a higher absorption induce a higher magnetic moment and switch magnetization at lower fluences

Analysis of the adiponectin paradox in healthy older people

Background It remains unknown why adiponectin levels are associated with poor physical functioning, skeletal muscle mass and increased mortality in older populations. Methods In 190 healthy adults (59-86 years, BMI 17-37 kg/m2 , 56.8% female), whole body skeletal muscle mass (normalized by height, SMI, kg/m2 ), muscle and liver fat were determined by magnetic resonance imaging. Bone mineral content (BMC) and density (BMD) were assessed by dual X-ray absorptiometry (n = 135). Levels of insulin-like growth factor 1 (IGF-1), insulin, inflammation markers, leptin and fibroblast growth factor 21 were measured as potential determinants of the relationship between adiponectin and body composition. Results Higher adiponectin levels were associated with a lower SMI (r = -0.23, P < 0.01), BMC (r = -0.17, P < 0.05) and liver fat (r = -0.20, P < 0.05) in the total population and with higher muscle fat in women (r = 0.27, P < 0.01). By contrast, IGF-1 showed positive correlations with SMI (r = 0.33), BMD (r = 0.37) and BMC (r = 0.33) (all P < 0.01) and a negative correlation with muscle fat (r = -0.17, P < 0.05). IGF-1 was negatively associated with age (r = -0.21, P < 0.01) and with adiponectin (r = -0.15, P < 0.05). Stepwise regression analyses revealed that IGF-1, insulin and leptin explained 18% of the variance in SMI, and IGF-1, leptin and age explained 16% of the variance in BMC, whereas adiponectin did not contribute to these models. Conclusions Associations between higher adiponectin levels and lower muscle or bone mass in healthy older adults may be explained by a decrease in IGF-1 with increasing adiponectin levels