arXiv subjects

B. Hjörvarsson

Publications and source records attributed to B. Hjörvarsson.

4 recordsLinked to original sources

arXiv2020

Direct observation of the magnetic proximity effect in amorphous exchange-spring magnets by neutron reflectometry

In this letter we report a direct observation of a magnetic proximity effect in an amorphous thin film exchange-spring magnet by the use of neutron reflectometry. The exchange-spring magnet is a trilayer consisting of two ferromagnetic layers with high $T_c$'s separated by a ferromagnetic layer, which is engineered to have a significantly lower $T_c$ than the embedding layers. This enables us to measure magnetization depth profiles at which the low $T_c$ material is in a ferromagnetic or paramagnetic state, while the embedding layers are ferromagnetic. A clear proximity effect is observed 7 K above the $T_c$ of the embedded layer, with a range extending 50 $\unicode{xC5}$.

cond-mat.mtrl-sci↗

arXiv2019

Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction

The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically, and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, non-destructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV energy range to probe the structure of an epitaxially-grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO (001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 $\pm$ 4 angstrom, with minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and x-ray reflectivity measurements. Through the STEM measurements we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 angstrom. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few angstrom precision.

cond-mat.mtrl-sci↗

arXiv2018

Hydrogen Concentration in Photovoltaic a-Si:H Annealed at Different Temperatures Measured by Neutron Reflectometry

Amorphous hydrogenated silicon (a-Si:H) is an important material for surface defect passivation of photovoltaic silicon (Si) wafers in order to reduce their recombination losses. The material is however unstable with regards to hydrogen (H) desorption at elevated temperatures, which can be an issue during processing and device manufacturing. In this work we determine the temperature stability of a-Si:H by structural characterization of a-Si:H/Si bilayers with neutron- (NR) and X-ray reflectometry (XRR) combined with photoconductance measurements yielding the minority carrer lifetime. The neutrons are sensitive to light elements such as H, while the X-rays which are insensitive to the H-concentration, provide an independent constraint on the layer structure. It is shown that H-desorption takes place at a temperature of approximately T = $425\,^{\circ}\mathrm{C}$ and that hydrogen content and minority carrier lifetimes have a strongly correlated linear relationship, which can be interpreted as one hydrogen atom passivating one defect.

cond-mat.mtrl-sci↗

arXiv2012

Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers

The thickness dependence of the magnetic properties of amorphous Fe90Zr10 layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25 layer thickness is kept at 40 Å, while the thickness of the Fe90Zr10 layers is varied between 5 and 20 Å. The thickness of the Al75Zr25 layers is sufficiently large to suppress any significant interlayer coupling. Both the Curie temperature and the spontaneous magnetization decrease non-linearly with decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed in the multilayer with 5 Å Fe90Zr10 layers. The variation of the Curie temperature $T_c$ with the Fe90Zr10 layer thickness $t$ is fitted with a finite-size scaling formula $[1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-λ}$, yielding $λ=1.2$, and a critical thickness $t'=6.5$ Å, below which the Curie temperature is zero.

cond-mat.mtrl-sci↗