Macrospin behavior and superparamagnetism in (Ga,Mn)As nanodots

Abstract

An array of small independent and quasimonodisperse nanodots of the dilute magnetic semiconductor ͑Ga,Mn͒As with out-of-plane easy axis has been patterned by electronic lithography and ion-beam etching from a strained epitaxial film. The thermal dependence ͑2 Ͻ T Ͻ 120 K͒ of the field-induced magnetic loops, coercivity, and anisotropy of the nanodots, have been measured by sensitive magneto-optical magnetometry. Below the superparamagnetic blocking temperature T B , all results are consistent with a quasicoherent thermally activated and field-induced spin-rotation process. In spite of the nonconventional nature of the ferromagnetism mediated by holes and of the high dilution in manganese ions, T B is well accounted for by the usual Néel-Brown model.

Low-Barrier Nanomagnets as p-Bits for Spin Logic

“…where H i is the effective field including the uniaxial and shape anisotropy terms, as well as the thermally fluctuating magnetic field due to three dimensional uncorrelated thermal noise H n having Gaussian distribution with mean H n = 0 and standard deviation H 2 n = 2αkT/|γ|M s V along each direction [34][35][36][37][38], γ is the gyromagnetic ratio and N i = M s V /µ B is the total number of Bohr magnetons comprising the magnet. Our simulations are based on the macrospin approximation, as is common in the literature [24,39,40]. This approx-FIG.…”

Electrically Defined Ferromagnetic Nanodots

“…Studies of ferromagnetic nanoparticles/dots have established fundamental understanding of many important phenomena in magnetism such as thermal fluctuation aftereffects and superparamagnetism and at the same time have laid the foundation of modern magnetic recording technology. − In addition, advancement of nanotechnology for patterning and/or formation of nanoparticles/dots, ranging from top-down electron-beam lithography to bottom-up self-assembly, has made it possible to fabricate ferromagnetic nanodots for magnetoresistive random access memories and patterned media . Nonmagnetic semiconductor nanodevices (nanowires and dots), − on the other hand, are being used to investigate quantum phenomena for semiconductor physics and applications, where an electric field E plays a significant role in realizing low dimensional structures by modulating carrier density.…”

“…Figure c shows the result of simulation. The magnetization curves are calculated by using the Néel-Brown model (see Supporting Information) for single-domain dots with two stable states (+ and − states with the magnetization directions parallel and antiparallel to H , respectively) separated by an H and T dependent activation energy E act . ,,, The magnitudes of E act needed to overcome to reach the +(−) state from −(+) state is expressed as E act +(−) = KV (1 ∓ H / H sw ) α , where K is the perpendicular uniaxial anisotropy constant, V is the volume of a dot, and H sw (=2 K / M s , where M s is the T dependent spontaneous magnetization) is the switching magnetic field. We adopt α = 2 because H is applied along the uniaxial easy axis.…”