arXiv subjects

K. E. Siewierska

Publications and source records attributed to K. E. Siewierska.

4 recordsLinked to original sources

arXiv2023

Ru$_{2-x}$Mn$_{1+x}$Al thin films

The cubic Heusler alloy Ru$_{2-x}$Mn$_{1+x}$Al is grown in thin film form on MgO and MgAl$_2$O$_4$ substrates. It is a highly spin-polarised ferrimagnetic metal, with weak magnetocrystalline anisotropy. Although structurally and chemically similar to $\text{Mn}_2\text{Ru}_x\text{Ga}$, it does not exhibit ferrimagnetic compensation, or large magneto galvanic effects. The differences are attributed to a combination of atomic order and the hybridisation with the group 13 element Al or Ga. The spin polarisation is around 50 to 60 %. There is a gap in the density of states just above the Fermi level in fully ordered compounds.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic order and magneto-transport in half-metallic ferrimagnetic Mn$_y$Ru$_x$Ga thin films

The ruthenium content of half-metallic Mn$_2$Ru$_x$Ga thin films, with a biaxially-strained inverse Heusler structure, controls the ferrimagnetism that determines their magnetic and electronic properties. An extensive study of Mn$_y$Ru$_x$Ga films on MgO (100) substrates with $1.8 \leq y \leq 2.6$ and $x = 0.5$, 0.7 or 0.9, including crystallographic, magnetic order, magneto-transport and spin polarisation is undertaken to map specific composition-dependent properties in this versatile ternary system. A comparison of experimental densities obtained from X-ray reflectivity with calculated densities indicates full site occupancy for all compositions, which implies chemical disorder. All moments lie on the Slater-Pauling plot with slope 1 and all except $x = 0.5$, $y = 2.2$ exhibit magnetic compensation at \tcmp~below 500~K. The coercivity near \tcmp~exceeds 10~T. Increasing the Mn or Ru content raises \tcmp, but increasing Ru also decreases the spin polarisation determined by point contact Andreev reflection. Molecular field theory is used to model the temperature dependence of the net ferrimagnetic moment and three principal exchange coefficients are deduced. Marked differences in the shape of anomalous Hall and net magnetisation hysteresis loops are explained by substantial canting of the small net moment by up to \SI{40}{\degree} relative to the $c$-axis in zero field, which is a result of slight non-collinearity of the Mn$^{4c}$ sublattice moments due to competing intra-sublattice exchange interactions arising from antisite disorder and excess Mn in the unit cell. Consequences are reduced spin polarisation and an enhanced intrinsic contribution to the anomalous Hall effect. The systematic investigation of the physical properties as a function of $x$ and $y$ will guide the selection of compositions to meet the requirements for magnonic and spintronic MRG-based devices.

cond-mat.mtrl-sci↗

arXiv2019

Study of the Effect of Annealing on the Properties of Mn2RuxGa Thin Films

The effect of vacuum annealing thin films of the compensated ferrimagnetic half-metal Mn2RuxGa at temperatures from 250 to 400 degree Celsius is investigated. The 39.3 nm films deposited on (100) MgO substrates exhibit perpendicular magnetic anisotropy due to a small 1% tetragonal elongation induced by substrate strain. The main change on annealing is a modification in the compensation temperature, which first increases from 50 K for the as-deposited film to 185 K after annealing at 250 degree Celsius, and then falls to 140 K after annealing at 400 degree Celsius. There are minor changes in the atomic order, coercivity, resistivity and anomalous Hall effect (AHE), but the net magnetization measured by SQUID magnetometry with the field applied in-plane or perpendicular-to-the-plane changes more significantly. It saturates at 20 to 30 kA/m at room temperature, and a small soft component is seen in the perpendicular SQUID loops which is absent in the square AHE hysteresis loops. This is explained by the half-metallic nature of the compound; the AHE probes only the 4c Mn sublattice that provides the spin-polarized electrons at the Fermi level, whereas the SQUID measures the sum of the oppositely-aligned 4c and 4a sublattice magnetisations.

cond-mat.mtrl-sci↗

arXiv2018

Imaging Domains in a Zero-Moment Half Metal

We have a choice of methods for examining domains at the surface of a ferromagnet that depend on probing the stray field distribution, but these methods do not work in antiferromagnets or compensated ferrimagnets, which produce no stray field. The discovery of compensated ferrimagnetic half-metals allows for the local magnetization state to be observed directly with polarized light. The example considered here, Mn$_{2}$Ru$_{x}$Ga, has two inequivalent but oppositely-aligned Mn sublattices with equal and opposite moments, but only one of them contributes spin polarized conduction electrons at the Fermi energy. The material looks like an antiferromagnet from the outside, but from the point of view of electronic structure it resembles a spin-polarized ferromagnetic metal. The anisotropy axis is perpendicular to the film plane, which allows domains to be imaged directly by polar magneto-optic Kerr effect. The domain structure in a film with a composition of Mn$_{2}$Ru$_{0.4}$Ga has been imaged in a Kerr microscope and hysteresis loops traced. Domains have dimensions of order 20 μm with meandering domain walls and a fractal dimension D$_{f}$ = 1.85. Our results open new direct imaging possibilities for magnetically-ordered materials with no net moment.

cond-mat.mtrl-sci↗