Mixed Magnetic Phases in(Ga,Mn)AsEpilayers

Abstract

Two different ferromagnetic-paramagnetic transitions are detected in (Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition at a higher temperature results from (Ga,Mn)As cluster phases with [110] uniaxial anisotropy and that at a lower temperature is associated with a ferromagnetic (Ga,Mn)As matrix with 100 cubic anisotropy. A change in the magnetic easy axis from [100] to [110] with increasing temperature can be explained by the reduced contribution of 100 cubic anisotropy to the magnetic properties above the transition temperature of the (Ga,Mn)As matrix.Ferromagnetism in Mn-doped p−type semiconductors can theoretically be understood by the p − d exchange interaction between hole carriers and doped Mn spins [1,2,3,4], which can be manipulated by electric-field [5] or optical-hole generation [6]. Recent studies of magnetic anisotropy in (Ga,Mn)As/GaAs(001) epilayers, on the other hand, have shown a significant change in the magnetic anisotropy with increasing temperature [7,8,9,10]; uniaxial anisotropy along [110] ([110] uniaxial anisotropy) [7,8,9,10,11,12] becomes predominant with increasing temperature [7,8,9, 10] as well as increasing hole concentration [13]. In spite of these intensive studies, the origin of [110] uniaxial anisotropy is still unclear within the current theoretical framework and an understanding of the change in the magnetic anisotropy with temperature is therefore of fundamental importance in revealing the physics of magnetic semiconductors.The aim of this study is to give a comprehensive description of the magnetic anisotropy in (Ga,Mn)As epilayers.In this Letter, two different peaks in temperature-dependent ac susceptibility that are associated with ferromagnetic-paramagnetic transitions are clearly shown. The peak profile at a higher transition temperature has a considerable dependence on frequency, indicating a blocking process in magnetic clusters, while that at a lower transition temperature shows no frequency dependence. The results provide evidence that the magnetic transitions at the lower and higher temperatures are associated with magnetic phases with cubic anisotropy and [110] uniaxial anisotropies, respectively, and the crossover of the magnetic easy axis from 100 to [110] can be interpreted by the ferromagneticparamagnetic transition of the magnetic phase with 100 cubic magnetocrystalline anisotropy accordingly. 100 nm-thick (Ga,Mn)As epilayers were grown on top of a 400-nm-thick GaAs buffer layer grown at 590• C on a semi-insulating GaAs (001) substrate using lowtemperature molecular beam epitaxy (MBE) at 190 − 235• C under an As-rich growth condition. To increase the hole carrier concentration, some epilayers were subject to post-growth annealing in an N 2 atmosphere for 60 − 240 min at 250• C [14]; hole concentration was controlled by changing the Mn content and/or lowtemperature annealing [13]. Hole carrier concentrations p (= ionized Mn acceptor concentration) were measured with electrochemical capacitance-voltage (ECV) method at room temper...

High-field magnetocrystalline anisotropic resistance effect in (Ga,Mn)As

“…In the meantime, the BMPs isolated from the FM network behave like fragmented superparamagnetic ͑SPM͒ particles. This picture of the mixed FM and SPM is supported by Monte Carlo simulations 20,21 and recent experiments; 22,23 it also successfully explains the high-field resistivity behavior in InMnSb. 11 This two-phase model makes physical sense due to the inevitable inhomogeneous Mn distribution, which leads to the relatively hole-rich FM regions coexisting with the less hole-rich SPM regions.…”

“…The size of BMPs is estimated to be about 6 nm at 50 K. A somewhat larger BMP size was reported earlier for T Ͻ T C . 23 Since the BMP size decreases rapidly above T C , we believe our result is still reasonable. For T Ͻ T C , the percolating FM phase starts to contribute to xx , which leads to a nonzero A c , as shown in Fig.…”

Magnetic anisotropy switching in(Ga,Mn)Aswith increasing hole concentration

“…[11] Thus, these bifurcations strongly support the presence of inhomogeneous magnetic phases in (Ga,Mn)As epilayers. [10,11,16,17,18,19] For our samples, since the single-domain model pre-sented by Wang et al [30] is not realistic, we hereafter discuss the cluster/matrix model in our recent reports: [10,11] (Ga,Mn)As epilayers that show two different 100 cubic anisotropy and [110] uniaxial anisotropy are composed of a matrix phase with relatively low hole concentration and highly-hole concentrated (Ga,Mn)As clusters embedded in the matrix. In this model, the distribution in p is likely due to the inhomogeneous distribution of Mn ions.…”

Out-of-plane magnetization reversal processes of (Ga,Mn)As with two different hole concentrations

“…[20] Also, the inhomogeneous distribution of Mn ions such as Mn Ga -Mn Ga and Mn Ga -Mn I -Mn Ga is proposed in theoretical studies [25,26] and direct observation using scanning tunneling microscopy. [27] Low-temperature growth of (Ga,Mn)As also suggests the presence of a cluster-like Mn distribution: recently, we have reported on a possible origin of complex in-plane magnetic anisotropy in (Ga,Mn)As on the basis of a cluster/matrix model, [28,29] where (Ga,Mn)As epilayers on GaAs(001) consist of ferromagnetic (Ga,Mn)As cluster-like regions with [110] uniaxial anisotropy and a ferromagnetic (Ga,Mn)As matrix with 100 cubic magnetocrystalline anisotropy. It was found that the Curie temperature of the clusters is higher than that of the matrix and the magnetic properties of the clusters dominate the magnetic anisotropy of (Ga,Mn)As with a high hole concentration.…”