The presence of a ferromagnetic transition in single, modulation-doped, 8 nm quantum well of Cd 0.976 Mn 0.024 Te͞Cd 0.66 Mg 0.27 Zn 0.07 Te:N is evidenced by photoluminescence magnetospectroscopy. The transition is driven by long range Ruderman-Kittel-Kasuya-Yosida interactions between Mn spins, mediated by 2 3 10 11 holes per cm 2 . It occurs at 1.8 K, in agreement with a mean-field model. [S0031-9007(97)03602-8] PACS numbers: 75.70.Cn, 75.30.Hx, 75.50.Pp, 78.55.Et Because of complementary properties of semiconductor and ferromagnetic material systems, a growing effort is directed toward studies of hybrid semiconductor-magnetic nanostructures. Such devices, in which both electric and magnetic fields are spatially modulated, have usually been fabricated by patterning a ferromagnetic metal on the top of a modulation-doped GaAs/AlGaAs heterostructure [1] or by incorporation of magnetic clusters directly into a semiconductor matrice [2].In this Letter, we show that the two-dimensional hole gas confined in modulation-doped quantum wells of Cd 12x Mn x Te produces, via the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, a ferromagnetic coupling between the Mn spins. Actually, by the direct observation of a ferromagnetic phase transition, we demonstrate that this coupling can overcompensate antiferromagnetic interactions specific to II-VI diluted magnetic semiconductors (DMS) [3]. Our results mean, therefore, that the well-established methods of modulation of the carrier concentration in semiconductor quantum structures can be applied for tailoring of the magnetic properties. The transition to the ferromagnetic phase is put into the evidence by observing colossal Zeeman splittings of interband optical transitions, probed here by means of photoluminescence (PL) and its excitation spectra (PLE), a technique equivalent to absorption spectroscopy but which can be used with a strongly absorbing substrate. A quantitative description of our findings confirms predictions of a recent model [4] on the free carrier-induced ferromagnetism in structures of doped DMS. It makes it also possible to evaluate the strength of many body effects for the case of two-dimensional hole gas. Moreover, the data provide important information on critical phenomena in the disordered magnetic systems of reduced dimensionality, illustrating, in particular, how long-range spin-spin interactions stabilize an ordered phase and make fluctuations of magnetization irrelevant.Our studies have been carried out on samples grown in a molecular beam epitaxy (MBE) chamber equipped with a home-designed electron cyclotron resonance (ECR) plasma cell as a nitrogen source. Prior to fabrication of the proper structures, doping characteristics of the barrier material Cd 12y2z Mg y Zn z Te have been determined by means of the Hall effect, capacitance-voltage profiles, cathodoluminescence, and x-ray diffraction. It has been found [5] that by lowering the growth temperature down to 220-240 ± C it becomes possible to reduce the nitrogen-induced diffusion of Mg atom...

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn 1Ϫx Mn x Te epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida ͑RKKY͒ model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.