Temperature dependence of magnetic anisotropy in ferromagnetic (Ga,Mn)As films: Investigation by the planar Hall effect D. Y. Shin, S. J. Chung, and Sanghoon Lee* Physics Department, Korea University, Seoul 136-701, Korea X. Liu and J. K. Furdyna Physics Department, University of Notre Dame, Notre Dame, Indiana 46556, USA Received 6 February 2007; revised manuscript received 25 April 2007; published 23 July 2007 We carried out systematic planar Hall effect PHE measurements of GaMnAs ferromagnetic semiconductor film as a function of temperature. The two-step switching of the PHE occurring in the magnetization-reversal process was observed to change significantly as the temperature was increased. To investigate the mechanism responsible for such behavior, the temperature dependence of the PHE was continuously measured with and without an external magnetic field after the sample was first magnetized along one of the easy axes to produce an initial single-domain state at 3 K. A detailed temperature dependence of the magnetization direction was then obtained by taking the ratio of the planar Hall resistance measured with and without a magnetic field. As the temperature was increased, the direction of the easy axis of magnetization was observed to change from the 010 crystallographic direction to 110. This reorientation of the easy axis direction can be understood in terms of the temperature dependence of the relative strengths of the magnetic anisotropy constants i.e., of the ratio of uniaxial-to-cubic anisotropy of the GaMnAs film. DOI: 10.1103/PhysRevB.76.035327 PACS numbers: 75.50.Pp, 75.70.i, 75.60.d, 75.47.m I. INTRODUCTION The discovery of the magnetic properties of Ga,MnAs has opened up a new research field of carrier-mediated ferromagnetism, 1 and a great deal of experimental and theo- retical work has already been devoted to the exploration of the fundamental properties of this and related III-Mn-V materials. 2 For example, the ability to electrically control ferromagnetism, 3 enhanced planar Hall effect, 4 and efficient spin injection into semiconductor nanostructures 5 have al- ready been demonstrated using ferromagnetic GaMnAs- based geometries. The enhancement of spin phenomena ob- served in this material provides a strong indication that spintronic device applications based on ferromagnetic semi- conductors are indeed a strong possibility. Among the many interesting properties of GaMnAs, phe- nomena arising from magnetic anisotropy are the most prominent—and probably the most important—from the viewpoint of practical spin memory device applications. 6,7 A detailed investigation of the magnetic anisotropy in this ma- terial is therefore essential not only for the purpose of gain- ing a better understanding of the underlying physics, but also for practical reasons. It is known from earlier studies that the magnetic anisotropy either in-plane or out-of-plane of a GaMnAs film strongly depends on the strain within the film. 8–10 Specifically, in a GaMnAs film under compressive strain the in-plane magnetic anisotropy is dominant; i.e., the system prefers to be magnetized within the plane. The actual orientation of the easy axis of magnetization within the plane will then depend on the relative values of the cubic and uniaxial anisotropy constants. 4 Interestingly, the magnetic anisotropy constants of GaM- nAs film are known to depend on temperature via changes in the carrier concentration. 11,12 Magnetization measurements using a superconducting quantum interference device 13 SQUID as well as magnetic imaging by high-resolution magneto-optical techniques 14 have demonstrated a switching of the easy axes of magnetization between in-plane crystal- lographic directions e.g., from 100 to 110 and vice versa as the temperature was increased. The temperature depen- dence of the cubic and uniaxial anisotropy constants of GaMnAs was also obtained in those experiments. In the present paper we have undertaken a detailed study of the magnetic anisotropy of GaMnAs by electrical measurements, which are considerably simpler to carry out and thus provide a straightforward opportunity to obtain a very detailed map- ping of the magnetic anisotropy parameters as a function of temperature. Recently Tang et al. 4 reported the observation of the giant planar Hall effect PHE in GaMnAs films and related the observed behavior directly to the magnetic anisotropy con- stants of the system. Even though the PHE measurement was shown to be very sensitive to the direction of magnetization, this feature was not fully utilized for investigating the tem- perature dependence of the magnetic anisotropy. In this study, we have adapted a technique of PHE measurement specifically for determining the temperature-dependent prop- erties of the magnetic anisotropy in GaMnAs films. Specifi- cally, by measuring the temperature-dependent planar Hall resistance PHR with and without an external magnetic field, we are able to obtain a detailed profile of the orienta- tion of magnetization, from which the temperature- dependent changes of magnetic anisotropy parameters can be directly deduced. II. SAMPLE FABRICATION AND EXPERIMENTAL PROCEDURE The GaMnAs films studied in this investigation were pre- pared by molecular beam epitaxy MBE in a Riber 32 R&D PHYSICAL REVIEW B 76, 035327 2007 1098-0121/2007/763/0353276 ©2007 The American Physical Society 035327-1