Observation of a Ferromagnetic Transition Induced by Two-Dimensional Hole Gas in Modulation-Doped CdMnTe Quantum Wells

Abstract

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...

Hole–hole correlation effects on magnetic properties of Mn III1−V diluted magnetic semiconductors

“…(10), the critical temperature increases with the subband wavefunction, ϕ(z), and magnetic impurities overlap, is proportional to J 2 pd and m * . Similar T c -equation was derived [7] using the RKKY theory and its predictions are in agreement with the measured carrier-induced ferromagnetism in II-VI quantum wells [7,8]. Here we emphasize the role of hole-hole interactions and Mn doping profile on magnetic properties of biased III-V DMS quantum wells.…”

Diluted Magnetic III-V Semiconductors

“…We note that a similar situation was observed for Mn-based IV-VI DMS, which show FM or paramagnetic behavior depending on the hole concentration [7]. Also for quantum wells fabricated of Mn-based II-VI DMS (CdMnTe or ZnMnTe), FM interaction arises if the wells are filled with holes [8]. There were essentially two microscopic mechanisms proposed to explain FM in III-V DMS.…”

“…This d-d exchange may produce a spinglass phase in a certain temperature and composition range [1][2][3], as well as a truly long ranged AFM order [5,6]. For the systems *e-mail: andrzej.twardowski@fuw.edu.pl (203) with high free holes concentration carrier induced ferromagnetic (FM) coupling occurs [7,8]. This coupling may override AFM superexchange and yield FM ordering of magnetic moments system.…”

Theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors

“…Our theory could be also applied to model ferromagnetism observed in a quantum well of p-doped Cd 1Ϫx Te x Mn. 37 In Sec. V we have presented our calculations for the double-layer system together with the experimental results for multilayers.…”