arXiv subjects

Alexander Gerber

Publications and source records attributed to Alexander Gerber.

8 recordsLinked to original sources

arXiv2025

Conductance, continuity, and ferromagnetic percolation thresholds in thin films

Classical percolation models predict the metal-insulator transition and the onset of the long-range ferromagnetic order at the same topological continuity threshold. We tested this prediction in thin films of ferromagnetic CoPd and found a dramatic difference between the conductance and magnetic thresholds. While the long-range ferromagnetic phase develops at or very close to the continuity threshold, the transition from the metal-like to insulator-like conductance develops in films several times thinner. We argue that atomically narrow low resistance gaps intersecting the fractal network of metallic clusters provide a consistent explanation of the effect. We identify the conduction threshold as the point in discontinuous films at which the resistance of intergranular junctions exceeds the quantum resistance mark.

cond-mat.mes-hall↗

arXiv2022

Tuning the perpendicular anisotropy of ferromagnetic films by thickness, width, and profile

Perpendicular magnetic anisotropy was found to be highly sensitive to the nominal thickness and morphology of laterally heterogeneous CoPd films in the vicinity of the metal insulator transition. We used the effect to tune the anisotropy by the width of lithographically patterned stripes with non-uniform cross-sectional thickness profiles. The phenomenon and the method can be suitable to control the local anisotropy of spintronic logic circuitry elements by their shape and lateral dimensions.

cond-mat.mtrl-sci↗

arXiv2021

Non-reciprocal magnetoresistance, directional inhomogeneity and mixed symmetry Hall devices

Phenomenology similar to the nonreciprocal charge transport violating Onsagers reciprocity relations can develop in directionally inhomogeneous conducting films with nonuniform Hall coefficient along the current trajectory. The effect is demonstrated in ferromagnetic CoPd films and analyzed in comparison with the unidirectional magnetoresistance phenomena. We suggest to use an engineered inhomogeneity for spintronics applications and present the concept of mixed symmetry Hall devices in which transverse to current Hall signal is measured in a longitudinal contacts arrangement. Magnetization reversal and memory detection is demonstrated in the three terminal and the partitioned normal metal-ferromagnet (NM - FM) device designs. Multi-bit memory is realized in the partitioned FM-NM-FM structure. The relative amplitude of the antisymmetric signal in the engineered ferromagnetic devices is few percent which is 10 to 1000 times higher than in their unidirectional magnetoresistance analogues.

cond-mat.mes-hall↗

arXiv2020

Positive versus negative resistance response to hydrogenation in palladium and its alloys

Resistive solid state sensors are widely used in multiple applications, including molecular and gas detection. Absorption or intercalation of the target species varies the lattice parameters and an effective thickness of thin films, which is usually neglected in analyzing their transport properties in general and the sensor response in particular. Here, we explore the case of palladium-based thin films absorbing hydrogen and demonstrate that expansion of thickness is an important mechanism determining the magnitude and the very polarity of the resistance response to hydrogenation in high resistivity films. The model of the resistance response that takes into account modifications of thickness was tested and confirmed in three Pd-based systems with variable resistivity: thin Pd films above and below the percolation threshold, thick Pd-SiO2 granular composite films with different content of silica, and Pd-rich CoPd alloys where resistivity depends on Co concentration. Superposition of the bulk resistivity increase due to hydride formation and decrease of film resistance due to thickness expansion provides a consistent explanation of the hydrogenation response in both continuous and discontinuous films with different structures and compositions.

cond-mat.mtrl-sci↗

arXiv2020

High mobility conducting channel at semi-insulating GaAs -- metal oxide interfaces

Absence of an efficient technology of GaAs passivation limits the use of III-V semiconductors in modern electronics. The effect reported here can possibly lead to a solution of this long standing problem. We found that an electrically conducting interfacial channel is formed when insulating metal oxide dielectrics are deposited on untreated semi-insulating GaAs wafers by reactive RF sputtering in argon/oxygen plasma. The conducting channel is n-type with the surface charge density of 10^7 to 10^10 cm^-2 and Hall mobility as high as 6000 cm^2/Vsec, depending on the RF plasma excitation power and oxygen content during the deposition. The conducting channel is formed by depositing any of the tested metal oxide dielectrics: MgO, SiO2, Al2O3 and HfO2.

physics.app-ph↗

arXiv2018

Interpretation of experimental evidence of the topological Hall effect

The topological Hall effect in magnetic materials is considered the ultimate trademark of the skyrmion phase. The phenomenon is identified by distinct non-monotonic features in the Hall effect signal presumed to be the evidence of the topological origin. It is demonstrated here that similar features, unrelated to the skyrmion physics, arise in heterogeneous ferromagnets when components of the material exhibit the extraordinary Hall effect with opposite polarities. Relevance of this mechanism to the published data is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Longitudinal and transverse magnetoresistance in films with tilted out-of-plane magnetic anisotropy

Tilted off-plane magnetic anisotropy induces two unusual characteristic magnetotransport phenomena: extraordinary Hall effect in the presence of an in-plane magnetic field, and non-monotonic anisotropic magnetoresistance in the presence of a field normal to the sample plane. We show experimentally that these effects are generic, appearing in multiple ferromagnetic systems with tilted anisotropy introduced either by oblique deposition from a single source or in binary systems co-deposited from separate sources. We present a theoretical model demonstrating that these observations are natural results of the standard extraordinary Hall effect and anisotropic magnetoresistance, when the titled anisotropy is properly accounted for. Such a scenario may help explaining various previous intriguing measurements by other groups.

cond-mat.mtrl-sci↗

arXiv2004

Determination of the critical current density in the d-wave superconductor YBCO under applied magnetic fields by nodal tunneling

We have studied nodal tunneling into YBa2Cu3O7-x (YBCO) films under magnetic fields. The films' orientation was such that the CuO2 planes were perpendicular to the surface with the a and b axis at 450 form the normal. The magnetic field was applied parallel to the surface and perpendicular to the CuO2 planes. The Zero Bias Conductance Peak (ZBCP) characteristic of nodal tunneling splits under the effect of surface currents produced by the applied fields. Measuring this splitting under different field conditions, zero field cooled and field cooled, reveals that these currents have different origins. By comparing the field cooled ZBCP splitting to that taken in decreasing fields we deduce a value of the Bean critical current superfluid velocity, and calculate a Bean critical current density of up to 3*10^7 A/cm2 at low temperatures. This tunneling method for the determination of critical currents under magnetic fields has serious advantages over the conventional one, as it avoids having to make high current contacts to the sample.

cond-mat.supr-con↗