arXiv subjects

B. D. MacNeil

Publications and source records attributed to B. D. MacNeil.

3 recordsLinked to original sources

arXiv2025

Structure and magnetism of MnGe thin films grown with a nonmagnetic CrSi template

We report a method to grow B20 MnGe thin films using molecular-beam epitaxy, which employs an ultrathin CrSi template layer on Si(111). This layer is expected to be nonmagnetic, in contrast to MnSi and FeGe buffer layers that have been used previously. This template layer permits an investigation of the intrinsic properties of MnGe in the ultrathin-film limit without the influence of a neighboring magnetic layer. Single-phase MnGe(111) films were grown with thicknesses between 2 and 40 nm, which exhibited low interfacial roughnesses on the order of 0.6 nm. The films crystallized in a B20 structure with a small rhombohedral distortion. Magnetometry measurements in out-of-plane fields are consistent with a cone phase derived from helimagnetic order propagating along the film normal. However, an unexpected remanent moment develops below 35K, concomitant with features in the field dependence of the transport data. This provides indirect evidence for the presence of a low-temperature phase which has been identified by others as either a triple-Q topological spin-hedgehog lattice, or a multidomain single-Q helical state.

cond-mat.mtrl-sci↗

arXiv2023

Giant anomalous Hall effect in epitaxial Mn$_{3.2}$Ge films with a cubic kagome structure

We report on the first example of epitaxial Mn$_{3 + \delta}$Ge thin films with a cubic $L1_2$ structure. The films are found to exhibit frustrated ferromagnetism with an average magnetization corresponding to 0.98$~\pm~$0.06$~\mu_B$/Mn, far larger than the parasitic ferromagnetism in hexagonal Mn$_3$Ge and the partially compensated ferrimagnetism in tetragonal Mn$_3$Ge. The Hall conductivity is the largest reported for the kagome magnets with a low temperature value of $\sigma_{xy} = 1587~$S/cm. Density functional calculations predict that a chiral antiferromagnetic structure is lower in energy than a ferromagnetic configuration in an ordered stoichiometric crystal. However, chemical disorder driven by the excess Mn in our films explains why a frustrated 120$^\circ$ spin structure is not observed. Comparisons between the magnetization and the Hall resistivity indicate that a non-coplanar spin structure contributes the Hall signal. Anisotropic magnetoresistance and planar Hall effect with hysteresis up to 14 T provides further insights into this material.

cond-mat.mtrl-sci↗

arXiv2023

Disorder-induced ferrimagnetism in sputtered Mn$_{x}$CoGe thin films

Investigations into the magnetic properties of sputtered Mn$_{x}$CoGe films in the range $0.8 \leq x \leq 2.5$ uncovered ferrimagnetic order, unlike the ferromagnetic order reported in bulk samples. These films formed hexagonal Ni$_{2}$In-type structures when annealed at temperatures below 600$^{\circ}$C. While the Curie temperatures of the films are comparable to those of hexagonal bulk MnCoGe, there is a reduction in the magnetization of the Mn$_{x}$CoGe films relative to bulk MnCoGe, and a magnetization compensation point is observed in the $x<1$ samples. To understand the behavior, we calculated the magnetic moments of Mn-antisite defects in MnCoGe with density-function theory (DFT) calculations. Models constructed from the calculation suggest that films become ferrimagnetic due to the presence of Mn on the Co and Ge sites. In the $x<1$ samples, these defects arose from the disorder in the films, whereas for $x>1$, the excess Mn was driven onto the antisites. Mean field modeling of the temperature dependence of the magnetization provides additional evidence for ferrimagnetism. Our mean field and DFT models provide a description of how the variation in film defects with composition will transition the magnetic behavior from a compensated (V-type) to an uncompensated (Q-type) ferrimagnet.

cond-mat.mtrl-sci↗