Laurens W. Molenkamp

A frequency-controlled magnetic vortex memory

Using the ultra low damping NiMnSb half-Heusler alloy patterned into vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile memory controlled by the frequency. A perpendicular bias magnetic field is used to split the frequency of the vortex core gyrotropic rotation into two distinct frequencies, depending on the sign of the vortex core polarity

Bistability of vortex core dynamics in a single perpendicularly magnetized nanodisk.

p=pm1

Detection of electrical spin injection by light-emitting diodes in top- and side-emission configurations

inside the dot. A magnetic resonance force microscope and microwave pulses applied at one of these two resonant frequencies allow for local and deterministic addressing of binary information (core polarity).

Optimal control of vortex-core polarity by resonant microwave pulses

Microwave spectroscopy of individual vortex-state magnetic nanodisks in a perpendicular bias magnetic field H is performed using a magnetic resonance force microscope. It reveals the splitting induced by H on the gyrotropic frequency of the vortex core rotation related to the existence of the two stable polarities of the core. This splitting enables spectroscopic detection of the core polarity. The bistability extends up to a large negative (antiparallel to the core) value of the bias magnetic field Hr, at which the core polarity is reversed. The difference between the frequencies of the two stable rotational modes corresponding to each core polarity is proportional to H and to the ratio of the disk thickness to its radius. Simple analytic theory in combination with micromagnetic simulations give a quantitative description of the observed bistable dynamics.

Giant magnetoresistance in an epitaxial NiMnSb∕Cu∕CoFe multilayer

Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode (LED) fitted with a spin injecting contact (a spin-LED) allows the direct determination of the spin polarization of the injected carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs fitted with a (Zn,Be,Mn)Se spin injector in top- and side-emission configurations. In contrast to top emission, we cannot detect the electrical spin injection in side emission by analyzing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in side emission, we have analyzed structures with mesa sizes as small as 1 μm.

Molecular-beam epitaxy of (Zn,Mn)Se on Si(100)

In magnetic nanostructures, the core of a vortex points either up or down, and the polarity can be reversed by alternating-field pulses. An experiment now demonstrates deterministic and coherent control of vortex-core polarity using sequences of resonant microwave pulses and highlights routes to optimizing the technique, which might find application in magnetic-storage devices.

Application of Microcontact Printing and Nanoimprint Lithography

We have fabricated current-in-plane giant magnetoresistive (GMR) devices based on multilayers of epitaxial NiMnSb and sputtered Cu and CoFe. The devices show a magnetoresistance of up to 3.5% at room temperature. The amplitude of the current-in-plane GMR signal and the nature of the coupling between the two magnetic layers depend on the thickness of the Cu layer. For a 1.5nm thick Cu layer, the device exhibits antiferromagnetic coupling, whereas a parallel alignment is observed for 2.2 or 3nm thick Cu layers at low field.

Spin injection in the nonlinear regime: band bending effects.

We have investigated the growth by molecular-beam epitaxy of the II–VI diluted magnetic semiconductor (Zn,Mn)Se on As-passivated Si(100) substrates. The growth start has been optimized by using low-temperature epitaxy. Surface properties were assessed by Nomarski and scanning electron microscopy. Optical properties of (Zn,Mn)Se have been studied by photoluminescence and a giant Zeeman splitting of up to 30meV has been observed. Our observations indicate a high crystalline quality of the epitaxial films.

Electrical spin-injection into semiconductors-from cobalt contacts to semiconductor spin aligners

Microcontact printing (μCP) and Nanoimprint lithography (NIL) have both proven to be high resolution lithography processes suitable for large area parallel lithography.1–7 This property makes them both promising for application in device fabrication, where high throughput is an issue. Especially NIL is used by numerous groups due to its high resolution and potential for applications.8–10