Magnetic reversal under external field and current-driven domain wall motion in (Ga,Mn)As: influence of extrinsic pinning

Abstract

We investigate the anisotropy of magnetic reversal and current-driven domain wall motion in annealed Ga 0.95 Mn 0.05 As thin films and Hall bar devices with perpendicular magnetic anisotropy. Hall bars with current direction along the [110] and ] 0 1 1 [ crystallographic axes are studied. The [110] device shows larger coercive field than the ] 0 1 1 [ device. Strong anisotropy is observed during magnetic reversal between [110] and ] 0 1 1 [ directions.A power law dependence is found for both devices between the critical current (J C ) and the magnetization (M), with J C ∝ M 2.6±0.3 . The domain wall motion is strongly influenced by the presence of local pinning centres. Introduction(Ga,Mn)As, a model ferromagnetic semiconductor [1], has attracted much attention for fundamental physics and for its potential applications in spintronics [2,3]. Its magnetic anisotropy is dominated by magnetocrystalline effects which are dependent on carrier density and strain, in good agreement with theory [2]. (Ga,Mn)As epilayers grown on a relaxed (001) (In,Ga)As buffer layer experience a tensile strain due to the difference in lattice constant in each layer. Under these conditions the magnetic easy axis is perpendicular to the plane [4]. Stripe domain patterns in (Ga,Mn)As with perpendicular magnetic anisotropy have been observed previously using scanning Hall probe microscopy [5] and polar magnetooptical Kerr effect microscopy (PMOKM) [6,7,8,9]. The stripe domains are formed with a typical width of a few microns at low temperatures [5], and may be influenced by low temperature annealing [6,7,9].It has become clear that implementation of spintronics for memory applications requires the ability to manipulate the magnetic state of a material through the application of electric fields. Manipulation of magnetic domain walls using a spin-polarized current offers a key route to this. Currentdriven domain wall motion in both ferromagnetic metals and semiconductors has been demonstrated, but the mechanism is still under debate [10,11,12]. The critical current density for domain wall motion is predicted to be proportional to the saturation magnetization [13,14], which is typically two orders of magnitude smaller in (Ga,Mn)As than that in transition metal ferromagnets. The heating effect and the Oersted field produced by the critical current is correspondingly lower, so that (Ga,Mn)As is one of the best candidates for understanding current-driven domain wall motion.Previous studies of current-driven domain wall motion in (Ga,Mn)As obtained critical current densities of around 10 5 A/cm 2 , which is much smaller than typically reported values for metal films. These studies were performed on films of thickness around 25 nm, with either in-plane [15] or perpendicular-toplane easy magnetic axes [16]. In contrast, for studies of thicker (150 nm) films with in-plane magnetic easy axes, no evidence of current-driven domain wall motion was observed [17].In the present work, we concentrate on the domain images in magnetic reversal and current-drive...

Domain wall resistance in perpendicular (Ga,Mn)As: Dependence on pinning

“…[2] and a line defect resulting from strain-relaxation in the (In,Ga)As buffer layer). The latter has been shown to act as strong pinning potentials for domain walls, inhibiting their motion along the [1 1 0] orientation, which is perpendicular to the line defect [4,9,10]. We find that the intrinsic domain wall resistance at the etching step is much larger than that at the nonetched pinning line.…”

“…Studies of domain walls in ferromagnetic semiconductors have proved fruitful, yielding observations of much higher domain wall resistance (DWR) [1,2] and much lower critical currents for domain wall motion [3,4], compared to ferromagnetic metal films. For (Ga,Mn)As/GaAs with in-plane magnetization, the DWR was reported to be negative in sign and comparable in magnitude to the bulk resistivity [1].…”

Magneto-optical Kerr spectroscopy in ferromagnetic semiconductors: determination of the intrinsic complex magneto-optical Voigt constant

“…They can lead to strong enhancement or cancellation of the Kerr rotation or ellipticity for a specific thickness or wavelength. From equations (5) and (6) we extract the two expressions related to the Kerr rotation and ellipticity for an infinite thickness of magnetic layer as: Figure 4 shows the expected Kerr rotation and ellipticity spectra expected for an infinite thickness of samples A, B and C, obtained from equations (7) and (8). By comparing this figure with the experimental data in figure 1, one can note that the changes with respect to the infinite layer are clearly observable, especially the enhancement of the ellipticity and the change of the shape of the Kerr rotation.…”

“…This effect consists of the rotation of the polarization plane of linearly polarized light and the appearance of ellipticity upon reflection off a magnetized material. When spatially resolved, as in MOKE microscopy, or timeresolved as in laser pump-probe experiments (TR-MOKE) it proves to be a powerful and insightful tool to investigate exciting phenomena in spintronics and magnonics such as magnetic domain wall motion [5][6][7][8][9], spin waves excitation and detection [10,11] and to demonstrate novel concepts such as the optical spin transfer torque [12][13][14] and acoustic waveinduced magnetization switching [15,16]. In this respect ferromagnetic diluted semiconductors (FDS) have proved to be a test-bench material owing to their easily adjustable magnetic properties [17][18][19][20].…”

Current-driven domain wall motion across a wide temperature range in a (Ga,Mn)(As,P) device

“…[1][2][3] Theoretical studies accounting for the strong spin-orbit interaction of p-type carriers in ͑Ga,Mn͒As have pointed to a large ␤ or perpendicular spin-torque term which can substantially modify the critical current and DW velocity. While application-led research has mostly focused on current-driven DW motion in ferromagnetic transition-metal nanowires, studies of diluted ferromagnetic semiconductors such as ͑Ga,Mn͒As offer the prospect of deep physical insights and routes to minimization of the critical current J c required for DW depinning.…”