Easy axis reorientation and magneto-crystalline anisotropic resistance of tensile strained (Ga,Mn)As films

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Abstract

We present a study of magnetic anisotropy by using magneto-transport and direct magnetization measurements on tensile strained (Ga,Mn)As films. The magnetic easy axis of the films is in-plane at low temperatures, while the easy axis flips to out-of-plane when temperature is raised or hole concentration is increased. This easy axis reorientation is explained qualitatively in a simple physical picture by Zener’s p–d model. In addition, the magneto-crystalline anisotropic resistance was also investigated experimentally and theoretically based on the single magnetic domain model. The dependence of sheet resistance on the angle between the magnetic field and [1 0 0] direction was measured. It is found that the magnetization vector M in the single-domain state deviates from the external magnetic field H direction at low magnetic field, while for high magnetic field, M continuously moves following the field direction, which leads to different resistivity function behaviors.

Introduction

The discovery of diluted magnetic semiconductor (DMS) (Ga,Mn)As [1], [2], [3] has attracted great attention for both basic and applied research, paving the way for the development of semiconductor spintronics [4], [5]. Although this material is still limited to the laboratory use because the Curie temperature (TC) is still well below room temperature [6], [7], [8], [9], it has been proved to be a model material in the family of DMS.

The most widely accepted theoretical method to describe the ferromagnetism of (Ga,Mn)As is the p–d mean field Zener’s model of valence band holes mediated ferromagnetism [10], which has been used to explain a number of experiments, including magnetic anisotropy [11], [12], [13]. The ability to manipulate anisotropy has great implications for fundamental research and is of vital significance in potential applications in magnetic recording technologies. A lot of experimental results [14], [15], [16], [17], [18], [19] and theoretical predictions [11], [12], [13] have shown that magnetic anisotropy of (Ga,Mn)As is determined by a combination of hole density, strain, and temperature. Depending on lattice constant of the substrate, (Ga,Mn)As film can be compressively strained if it is grown on a substrate with a smaller lattice constant (e.g., on GaAs buffer), or tensile strained in the opposite case (e.g., on (In,Ga)As buffer). Magnetic anisotropy of compressive biaxial strained (Ga,Mn)As films has been carefully investigated [14], [15], [16], [17], [18], [19]. The easy axis is out-of-plane at low temperature when the hole density is low. On the contrary, the easy axis will fall into the plane either when the hole density is high. However, the number of works focusing on tensile strained (Ga,Mn)As films [19], [20], [21] is limited. An early study reported that (Ga,Mn)As under tensile strain shows strong out-of-plane easy axis [22]. Later theories of magnetic anisotropy [11], [12], [13] predicted that tensile strained (Ga,Mn)As films should show rich physical phenomena. However, to confirm these predictions, particular experimental data are needed.

In this paper, we have studied the magnetic anisotropy of tensile strained (Ga,Mn)As films by measuring magneto-transport behaviors and magnetization characters. Our results show that (Ga,Mn)As films under tensile strain do exhibit an in-plane easy axis at low temperature. But when temperature is raised or hole concentration is increased by low-temperature annealing, the opposite tendency occurs. We explained this easy axis reorientation qualitatively by Zener’s model. We have also investigated angle dependence of longitudinal resistivity of the tensile strained (Ga,Mn)As films, where easy axis lies in-plane or out-of-plane. It is found that the magnetization vector M in the single-domain state deviates from the external magnetic field H direction at low magnetic field, while for high magnetic field, M continuously moves following the field direction, which leads to different resistivity function behaviors.

Section snippets

Experiment

The (Ga,Mn)As films were grown on semi-insulating GaAs (0 0 1) substrates by molecular-beam epitaxy (MBE). The surface was monitored in situ by a reflection high-energy electron diffraction (RHEED) system. Standard effusion cells supplied fluxes of Ga, Mn, In, and As4. In the first step, a GaAs buffer of a thickness of 100 nm was grown at 560 °C. Then the substrate temperature was lowered to 500 °C, and a 500 nm strain-relaxed (In,Ga)As buffer layer was grown. At last the substrate was cooled down to

Temperature and carrier induced the easy axis reorientation of tensile strained (Ga,Mn)As

The Hall resistivity [25] of ferromagnetic materials can be expressed as ρxy=RoH+RsM, where Ro is the ordinary Hall coefficient, Rs the anomalous Hall coefficient arising from spin–orbit interaction, which breaks the symmetry of the scattering mechanism of spin-up and spin-down carriers and H and M are the magnetic field and magnetization perpendicular to the sample surface, respectively. The anomalous Hall effect usually exceeds the ordinary Hall effect by several orders of magnitude; thus the

Summary

We investigated the magnetic anisotropy of tensile strained (Ga,Mn)As films. The anomalous Hall effect reveals that the easy axis lies in-plane at low temperature and out-of-plane when the temperature is increased. The same process happens when the hole concentration is increased by low-temperature annealing. This easy axis reversal behavior has also been confirmed by direct magnatization measurements, and can be understood qualitatively by the p–d Zener’s model. At the same time, to

Acknowledgements

The authors acknowledge Y. Zhang and H.Y. Zhang for SQUID measurements, and H.Z. Zheng and Y.T. Wang for valuable discussion. This work was supported partly by the National Natural Science Foundation of China under Grant nos. 60836002, and the special funds for the Major State Basic Research Contract no. 2007CB924903 of China, and the Knowledge Innovation Program Project of Chinese Academy of Sciences no. KJCX2.YW.W09-1.

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