Strominduzierte Magnetisierungsdynamik in einkristallinen Nanosäulen

Diese Arbeit beschreibt Experimente zur strominduzierten Magnetisierungsdynamik in Nanosäulen, die einkristalline Schichten aus Eisen und Silber enthalten. Spinpolarisierte Ströme erzeugen in diesen Strukturen einerseits den Riesenmagnetowiderstandseffekt (giant magnetoresistance: GMR), der aufgrund des Stromflusses senkrecht zur Schichtebene (current perpendicular to plane: CPP) durch Spinakkumulation an den Grenzflächen spezielle Eigenschaften aufweist. Andererseits erzeugen sie durch direkten Übertrag von Spindrehimpuls an die Magnetisierungen den sogenannten Spin Transfer Torque (STT), der zu grundlegend neuer Magnetisierungsdynamik führt. Die speziellen Eigenschaften des CPP-GMR und des STT der Eisen-Silber-Grenzfläche sind in Messungen des GMR, des strominduzierten Magnetisierungsschaltens und der stromgetriebenen Hochfrequenzanregungen erkennbar. Sie folgen aus der starken Spinabhängigkeit des Grenzflächenwiderstandes. Die Messergebnisse an Nanosäulen mit 70 nm Durchmesser werden mit Modellrechnungen und Computersimulationen verglichen. Es können dabei zweischrittiges Magnetisierungsschalten und Hochfrequenzanregungen bei niedrigen Magnetfeldern nachgewiesen werden, die erst durch das Wechselspiel zwischen der Kristallanisotropie der kubisch-raumzentrierten Eisenschichten und dem STT ermöglicht werden. In Nanosäulen von 230 nm Durchmesser wird die strominduzierte Magnetisierungsdynamik in inhomogen magnetisierten Nanoelementen untersucht. Der Drehsinn des magnetischen Vortexzustandes kann bei seiner Präparation durch Ströme verschiedenen Vorzeichens eingestellt werden, da diese ein um die Säule geschlossenes Oerstedfeld erzeugen. Verschiedene resultierende Widerstandsniveaus und qualitative Unterschiede in stromgetriebenen Hochfrequenzanregungen des Vortexzustandes machen die Drehsinne unterscheidbar. Die nichtlinearen Eigenschaften der stromgetriebenen Hochfrequenzanregungen machen sogar ein Phase-locking der Vortexoszillation an ein äußeres Hochfrequenzsignal möglich

Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state

Lehndorff R, Buergler DE, Gliga S, et al. Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state. PHYSICAL REVIEW B. 2009;80(5): 054412.Current-driven magnetization dynamics in spin torque nano-oscillators (STNOs) is intensely investigated because of its high potential for high-frequency (HF) applications. We experimentally study current-driven HF excitations of STNOs for two fundamental magnetization states of the free layer, namely, vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STNO enables a direct comparison of the critical currents, agility, power, and linewidth of the HF output signals. We find that the vortex state has some superior properties, in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field

Microstructure Design for Fast Lifetime Measurements of Magnetic Tunneling Junctions

The estimation of the reliability of magnetic field sensors against failure is a critical point concerning their application for industrial purposes. Due to the physical stochastic nature of the failure events, this can only be done by means of a statistical approach which is extremely time consuming and prevents a continuous observation of the production. Here, we present a novel microstructure design for a parallel measurement of the lifetime characteristics of a sensor population. By making use of two alternative designs and the Weibull statistical distribution function, we are able to measure the lifetime characteristics of a CoFeB/MgO/CoFeB tunneling junction population. The main parameters governing the time evolution of the failure rate are estimated and discussed and the suitability of the microstructure for highly reliable sensor application is proven

Microstructure design for fast lifetime measurements of magnetic tunneling junctions

The estimation of the reliability of magnetic field sensors against failure is a critical point concerning their application for industrial purposes. Due to the physical stochastic nature of the failure events, this can only be done by means of a statistical approach which is extremely time consuming and prevents a continuous observation of the production. Here, we present a novel microstructure design for a parallel measurement of the lifetime characteristics of a sensor population. By making use of two alternative designs and the Weibull statistical distribution function, we are able to measure the lifetime characteristics of a CoFeB/MgO/CoFeB tunneling junction population. The main parameters governing the time evolution of the failure rate are estimated and discussed and the suitability of the microstructure for highly reliable sensor application is proven

Generation of imprinted strain gradients for spintronics

In this work, we propose and evaluate an inexpensive and CMOS-compatible method to locally apply strain on a Si/SiOx substrate. Due to high growth temperatures and different thermal expansion coefficients, a SiN passivation layer exerts a compressive stress when deposited on a commercial silicon wafer. Removing selected areas of the passivation layer alters the strain on the micrometer range, leading to changes in the local magnetic anisotropy of a magnetic material through magnetoelastic interactions. Using Kerr microscopy, we experimentally demonstrate how the magnetoelastic energy landscape, created by a pair of openings, in a magnetic nanowire enables the creation of pinning sites for in-plane vortex walls that propagate in a magnetic racetrack. We report substantial pinning fields up to 15 mT for device-relevant ferromagnetic materials with positive magnetostriction. We support our experimental results with finite element simulations for the induced strain, micromagnetic simulations and 1D model calculations using the realistic strain profile to identify the depinning mechanism. All the observations above are due to the magnetoelastic energy contribution in the system, which creates local energy minima for the domain wall at the desired location. By controlling domain walls with strain, we realize the prototype of a true power-on magnetic sensor that can measure discrete magnetic fields or Oersted currents. This utilizes a technology that does not require piezoelectric substrates or high-resolution lithography, thus enabling wafer-level production