Novel Quaternary Dilute Magnetic Semiconductor (Ga,Mn)(Bi,As): Magnetic and Magneto-Transport Investigations

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Postgrowth annealing treatment has been shown to enhance the hole concentration and Curie temperature in the layers. Significant increase in the magnitude of magnetotransport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. Two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in nonvolatile memory devices.


Introduction
Last decades in the field of semiconductor physics were marked by tremendous improvement in the materials technologies, which might provide a basis for developing novel spintronic devices. Special attention was paid to the ternary III-V semiconductor (Ga,Mn)As, combining semiconducting properties with magnetism, which became a model among dilute magnetic semiconductors [1,2]. Homogeneous layers of Ga 1-x Mn x As containing up to above 10% of Mn atoms can be grown by a low-temperature (150-250°C) molecular-beam epitaxy (LT-MBE) [3,4]. When intentionally undoped, the layers are of p-type where Mn atoms, substituting the Ga atoms in GaAs crystal lattice, supply both mobile holes and magnetic moments. Below the Curie temperature the layers become ferromagnetic due to the holemediated ordering of Mn spins. The sensitivity of their magnetic properties, such as the Curie temperature and magnetic anisotropy, to the growth-induced strain and hole concentration allows for tuning those properties by growth peculiarities or post-growth annealing of the (Ga,Mn)As layers. Moreover, appropriate nanostructurization of thin (Ga,Mn)As layers offers the prospect of taking advantage of magnetic domain walls in novel spintronic devices. In this context, our recent studies on several types of nanostructures patterned from ferromagnetic (Ga,Mn)As layers pointed to their utility for spintronic applications. Especially, nanostructures of the three-arm [5], cross-like [6] and ring-shape [7] geometries, displaying magneto-resistive effects controlled by manipulation of magnetic domain walls in the nanostructures, could be applied in a new class of nonvolatile memory cells.
On the other hand, the replacement of a small fraction of As atoms by much heavier Bi atoms in GaAs results, due to an interaction of Bi 6p bonding orbitals with the GaAs valence band maximum, in a rapid decrease in its band-gap energy [8,9] and in a strong enhancement of spin-orbit coupling accompanied by a giant separation of the spin-split-off hole band in Ga(Bi,As) [10]. The enhanced spin-orbit coupling could be advantageous for spintronic materials as it strongly affects their magneto-transport properties. In order to explore this issue we have investigated an impact of Bi incorporation into (Ga,Mn)As layers on their structural, magnetic and magneto-transport properties. First homogeneous layers with a high structural perfection of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor, containing up to 6% Mn and 1% Bi, have been recently grown with LT-MBE under compressive misfit strain, on GaAs substrate, [11,12] as well as under tensile misfit strain, on (In,Ga)As buffer layer [13]. Their magnetic properties were similar to those of the ternary (Ga,Mn)As layers [14], with the in-plane and out-of-plane easy axis of magnetization in the layers grown under the compressive and tensile misfit strain, respectively [13]. Our results of magneto-transport characterization of those layers confirmed significant enhancement of their magnetoresistance and the planar Hall resistance as a result of Bi incorporation into (Ga,Mn)As layers [15]. In the present paper we report on magnetic and magneto-transport properties of thin epitaxial layers of (Ga,Mn)(Bi,As) quaternary compound grown under compressive misfit strain, proving their usefulness for possible spintronic applications.

Experimental
The investigated (Ga,Mn)(Bi,As) layers, of 10 nm thickness and 6% Mn and 1% Bi content, were grown on semi-insulating (001)-oriented GaAs substrate by the LT-MBE technique at a temperature of 230°C. For comparison, similar (Ga,Mn)As layers were grown under the same conditions. After the growth the layers were subjected to the annealing treatment in air at the temperature of 180°C during 50 h. Post-growth annealing of (Ga,Mn)As layers [16], as well as the (Ga,Mn)(Bi,As) ones [11], at temperatures below the growth temperature was proved to substantially improve magnetic and electrical-transport properties of the layers as a result of outdiffusion of charge-and moment-compensating Mn interstitials from the layers.
Raman spectroscopy was employed to estimate the hole densities in the p-type The measurements were performed for various orientations of the in-plane magnetic field H at the liquid-helium temperature range.

Results and discussion
The micro-Raman spectra for the (Ga,Mn)As and (Ga,Mn)(Bi,As) layers were recorded from the (001)  flow under an in-plane magnetic field or even in the absence of an applied magnetic field. In single domain model the AMR components can be described by expressions [19]: where R ⊥ and R || are the resistances for in-plane magnetization vector oriented perpendicular and parallel to the current, respectively. The longitudinal MR is described by Eq. (1) and the transverse resistance, i.e. the planar Hall resistance, is described by Eq. (2). In ferromagnetic (Ga,Mn)As, in contradiction to metallic ferromagnets, (R || −R ⊥ ) < 0, i.e. the resistance is higher when the magnetization is perpendicular to the current with respect to that when they both are parallel [20]. The same also holds for (Ga,Mn)(Bi,As), as demonstrated in our previous paper [15]. Moreover, the magnitude of PHE in (Ga,Mn)As layers can be up to four orders of magnitude greater than previously found in metallic ferromagnets, which was called as the