* Corresponding author. Tel.: #81-3-3478-6811, ex. 5602; fax: #81-3-3478-0536; e-mail: oiwa@kodama.issp.u- tokyo.ac.jp.  Also at CREST, Japan Science and Technology Corpora- tion (JST). Physica B 249 — 251 (1998) 775 —779 Low-temperature conduction and giant negative magnetoresistance in III—V-based diluted magnetic semiconductor: (Ga, Mn)As/GaAs A. Oiwa*, S. Katsumoto, A. Endo, M. Hirasawa, Y. Iye, F. Matsukura, A. Shen, Y. Sugawara, H. Ohno  Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan  Research Institute of Electrical Communication, Tohoku University, Katahira, Aoba-ku, Sendai 980-77, Japan Abstract We have studied the transport and magnetic properties of Ga  Mn  As/GaAs with different Mn content (0.015(x(0.071) for temperatures 40 mK—250 K and magnetic fields 0—15 T. Ferromagnetic order was observed in the samples with x'0.02. With incresing Mn concentration, the system undergoes an insulator—metal—insulator transition. Low temperature hopping conduction on the insulator side of the metal—insulator boundaries exhibits giant negative magnetoresistance.  1998 Elsevier Science B.V. All rights reserved. Keywords: Diluted magnetic semiconductors; Ferromagnetism; Metal—insulator transition; Giant negative mag- netoresistance 1. Introduction The recent developments in the molecular beam expitaxy (MBE) technique have enabled us to grow III—V compound based diluted magnetic semicon- ductors such as In  Mn  As [1—3] and Ga  Mn  As [4—6] with high Mn content x. In the course of recent studies on In  Mn  As [1,2], Ga  Mn  As [4—6] and their related structures [3], it has been elucidated that the electrical trans- port properties are dramatically influenced by the exchange interaction between the conduction car- riers and the localized moments. In the transport studies on Mn-doped GaAs carried out in the early days [7,8], bulk samples were used and the Mn concentration was limited to x)0.005 and only an insulating behavior was observed. In this work, a series of samples of Ga  Mn  As with x ranging from 0.015 to 0.071 were grown and their magnet- ism and spin-dependent magnetotransport were investigated. The overall trend of changes as a func- tion of x, including an insulator—metal—insulator 0921-4526/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 3 1 2 - 3