T. L. Monchesky

Single-crystal magnetotunnel junctions

We have grownepitaxialsingle-crystal magnetotunnel junctions using Fe(001) substrates, MgO(001) spacers and Fe top electrodes. We have used scanning tunneling microscopy and atomic force microscopy to measure the tunneling characteristics as a function of position and demonstrated that local tunneling can be obtained such that the buried MgO can be characterized with nm resolution. Local I(V) curves revealed that most of the area had intrinsic tunneling properties corresponding to the proper MgO tunneling barrier. A small fraction of the scanned areas showed localized spikes in the tunneling current which are most likely caused by defects in the MgO.

Chiral skyrmions in cubic helimagnet films: The role of uniaxial anisotropy

This paper reports on magnetometry and magnetoresistance measurements of MnSi epilayers performed in out-of-plane magnetic fields. We present a theoretical analysis of the chiral modulations that arise in confined cubic helimagnets where the uniaxial anisotropy axis and magnetic field are both out-of-plane. In contrast to in-plane field measurements (Wilson et al., Phys. Rev. B 86, 144420 (2012)), the hard-axis uniaxial anisotropy in MnSi/Si(111) increases the energy of (111)-oriented skyrmions and in-plane helicoids relative to the cone phase, and makes the cone phase the only stable magnetic texture below the saturation field. While induced uniaxial anisotropy is important in stabilizing skyrmion lattices and helicoids in other confined cubic helimagnets, the particular anisotropy in MnSi/Si(111) entirely suppresses these states in an out-of-plane magnetic field. However, it is predicted that isolated skyrmions with enlarged sizes exist in MnSi/Si(111) epilayers in a broad range of out-of-plane magnetic fields.

The properties of isolated chiral skyrmions in thin magnetic films

Axisymmetric solitonic states (chiral skyrmions) have been predicted theoretically more than two decades ago. However, until recently they have been observed in a form of skyrmionic condensates (hexagonal lattices and other mesophases). In this paper we report experimental and theoretical investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers two years ago (Science 341 , 636 (2013)). The results of spin-polarized scanning tunneling microscopy analyzed within the continuum and discrete models provide a consistent description of isolated skyrmions in thin layers. The existence region of chiral skyrmions is restricted by strip-out instabilities at low fields and a collapse at high fields. We demonstrate that the same equations describe axisymmetric localized states in all condensed matter systems with broken mirror symmetry, and thus our findings establish basic properties of isolated skyrmions common for chiral liquid crystals, different classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics.

Surface twist instabilities and skyrmion states in chiral ferromagnets

IFW Dresden, Postfach 270016, D-01171 Dresden, Germany(Dated: May 22, 2014)In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is never reached due tothe formation of specific surface chiral modulations with the propagation direction perpendicularto the film surfaces [Wilson et al. Phys. Rev. B 88, 214420 (2013)]. In this paper we showthat the occurrence of such chiral surface twists is a general effect attributed to all bulk and con-fined magnetic crystals lacking inversion symmetry. We present experimental investigations of thisphenomenon in nanolayers of MnSi/Si(111) supported by detailed theoretical analysis within thestandard phenomenological model. In magnetic nanolayers with intrinsic or induced chirality, suchsurface induced instabilities become sizeable effects and play a crucial role in the formation ofskyrmion lattices and other nontrivial chiral modulations.

Extended elliptic skyrmion gratings in epitaxial MnSi thin films

A detailed investigation of the magnetization processes in epitaxial MnSi thin film reveals the existence of elliptically distorted skyrmion strings that lie in the plane of the film. We provide proof that the uniaxial anisotropy stabilizes this state over extended regions of the magnetic phase diagram. Theoretical analysis of an observed cascade of first-order phase transitions is based on rigorous numerical calculations of competing chiral modulations, which shows the existence of helicoids, elliptic skyrmions, and cone phases.

Discrete helicoidal states in chiral magnetic thin films

Magnetometry and magnetoresistance measurements in MnSi thin films and rigorous analytical solutions of the micromagnetic equations show that the field-induced unwinding of confined helicoids occurs via discrete steps. A comparison between the magnetometry data and theoretical results shows that finite size effects confine the wavelength and lead to a quantization of the number of turns in the helicoid. We demonstrate a prototypical spintronic device where the magnetic field can push or pull individual turns into a magnetic spring that can be read by electrical means.

The role of interfaces in the magnetoresistance of Au/Fe/Au/Fe/GaAs(001)

The electron transport and magnetoresistance (MR) were investigated in high quality crystalline epitaxial Fe(001) and Au(001) films and exchange coupled Au/Fe/Au/Fe/GaAs(001) trilayer structures. Fits to the experimental data were based on the semiclassical Boltzmann equation, which incorporates the electronic properties obtained from first-principles local density functional calculations. The fits require a surprisingly high asymmetry for the spin dependent electron lifetimes in Fe, τ↓/τ↑=10 at room temperature. Despite the large atomic terraces at the Au/vacuum and Fe/GaAs interfaces the scattering at the outer interfaces was found to be diffuse. The origin of MR in Au/Fe/Au/Fe/GaAs(001) structures is due to electron channeling in the Au spacer layer. The measured MR is consistent with the diffusivity parameters s↑=0.55, s↓=0.77 at the metal–metal interfaces.

Three-dimensional chiral skyrmions with attractive interparticle interactions

We introduce a new class of isolated three-dimensional skyrmion that can occur within the cone phase of chiral magnetic materials. These novel solitonic states consist of an axisymmetric core separated from the host phase by an asymmetric shell. These skyrmions attract one another. We derive regular solutions for isolated skyrmions arising in the cone phase of cubic helimagnets and investigate their bound states.

Giant magnetoresistance of Fe/Cu/Fe(001) trilayers grown directly on GaAs(001)

The giant magnetoresistance of crystalline Fe/Cu/Fe~001! epitaxial structures characterized byscanning tunneling microscopy are presented. Fe/Cu/Fe~001! trilayers capped with Au are growndirectly on GaAs~001! using a new procedure for producing pure Fe layers with As-free Fe surfaceson GaAs~001!. The temperature dependence of the magnetoconductance and sheet resistancemeasured from 4 to 300 K is modeled by the Boltzmann equation assuming that the mean free pathsin the crystalline epitaxial layers are equal to those in bulk materials. The results of the simple modelsuggest that the coefficient of the specular scattering at the Fe/GaAs interface isR50.45, while thescattering at the outer Au interface is diffuse. Spin asymmetry scattering at the metallic interfacesis DT5uT

Role of interfaces in the exchange coupling of Fe/Cr/Fe(001) systems

Exchange coupling has been studied in Fe whisker/Cr/Fe(001) systems that were grown in a perfect layer by layer mode. The exchange coupling through Cr was found to be very sensitive to alloying at the Fe whisker/Cr(001) interface. It will be shown that the observed reversed phase of the short wavelength oscillations compared to those predicted by ab initio calculations can be caused by alloying at the Fe whisker/Cr(001) interface. In order to test this point, we have grown samples with the Cr/Fe(001) interface intentionally alloyed by codepositing the Cr and Fe atoms during the formation of the last Cr atomic layer. The strength of the exchange coupling has also been investigated in systems fabricated with heterogeneous spacers using bcc Cu(001) and fcc Ag(001). Cu and Ag layers have been inserted between the Cr spacer and the Fe(001) film. The strength of the antiferromagnetic coupling was found to be substantially increased due to the presence of Cu at the Cr/Fe(001) interface. It will be argued that th...