Photovoltage detection of spin excitations of nanocrystals and nanomagnets with 2D electron systems

Abstract

Photovoltage spectroscopy is an attractive technique for investigating the dynamic properties of the spin excitations of sub-micron scale ferromagnetic elements (cobalt stripes or disks) fabricated on the surface of a GaAsAl033Ga067As heterojunction. Localised spin wave modes are detected via the photovoltage induced in a two-dimensional electron gas (2DEG) formed in the heterojunction. The two-dimensional nature of the detector separates the effects of the in-plane and normal vector components of the magnetisation. The high sensitivity of this technique arises from the high electron mobility and low electron density of 2DEGs relative to metals, given by, respectively, = 15 106cm2V 1s 1 and ns = 16 1011cm2. We report on the discrete structure of spin wave resonance as a function of the shape of the cobalt magnet, the geometry of the magnets, and the magnetic field orientation (perpendicular or parallel to the magnetisation). Spin wave resonances in a micromagnetic stripe are experimentally measured at different frequencies at low temperature and the cases of perpendicular and parallel field orientation are compared. The photovoltage spectra are modelled using micromagnetic simulation (OOMMF). The photovoltage resonances indicate the presence of Damon-Eshbach modes when Bdc is parallel to the long axis of the stripe. Bonding-antibonding spin waves are observed when Bdc is perpendicular to the long axis of the stripe. We also describe the discrete structure of photo voltage spectroscopy for individual disks. Our cobalt disk has a 40 nm radius and is magnetised perpendicular to the 2DEG by the static magnetic field. Tilting Bdc in the plane does no effect on the photovoltage for symmetry reasons. Our results show that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and versatile tool for probing the spin waves of small magnets of size 80 nm.

Date of Award	22 Jul 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Alain Nogaret (Supervisor) & Steven Davies (Supervisor)