Remanent Zero Field Spin Splitting of Self-Assembled Quantum Dots in a Paramagnetic Host

Abstract

A key element in the emergence of a full spintronics technology is the development of voltage controlled spin filters to selectively inject carriers of desired spin into semiconductors. We previously demonstrated a prototype of such a device using a II-VI dilute-magnetic semiconductor quantum well which, however, still required an external magnetic field to generate the level splitting. Recent theory suggests that spin selection may be achievable in II-VI paramagnetic semiconductors without external magnetic field through local carrier mediated ferromagnetic interactions. We present the first experimental observation of such an effect using non-magnetic CdSe self-assembled quantum dots in a paramagnetic (Zn,Be,Mn)Se barrier.PACS numbers: 72.25. Dc, 85.75.Mm Nanomagnetics has over the past few years produced a series of fascinating and often unanticipated phenomena. To name a few, molecular magnets exhibit quantum tunnelling of the magnetization[1], magnetic atoms on a surface exhibit giant magnetic anisotropies [2], and magnetic domain walls are being harnessed as data carriers [3]. Here, we report on another remarkable phenomenon: self-assembled quantum dots, fabricated from II-VI dilute magnetic semiconductors (DMS) that macroscopically exhibit paramagnetism, possess a remanent magnetization at zero external field. This allows us to operate the dots as voltage controlled spin filters, capable of spin-selective carrier injection and detection in semiconductors. Such spin filter devices could provide a key element in the emergence of a full spintronics technology [4]. We present the first experimental observation of such a device using an approach based on the incorporation of non-magnetic CdSe self assembled quantum dots (SADs) in paramagnetic (Zn,Be,Mn)SeWe previously demonstrated a prototype of such a spin filter using a II-VI DMS-based resonant tunnelling diode [5]. However, while that device was tuned by a bias voltage, the spin filtering mechanism still required an external magnetic field. Moreover, ferromagnetic III-V semiconductors like (Ga,Mn)As are not suitable for resonant tunnelling devices due to the short mean free path of holes [6]. Recent theoretical works [7,8,9] have suggested that spin selection may be achievable in II-VI DMS without any external magnetic field by creating localized carriers that might mediate a local ferromagnetic interaction between nearby Mn atoms.Our sample is an MBE-grown all-II-VI resonant tunnelling diode (RTD) structure consisting of a single 9 nm thick semi-magnetic Zn 0.64 Be 0.3 Mn 0.06 Se tunnel barrier, sandwiched between gradient doped Zn 0.97 Be 0.03 Se injector and collector. Embedded within the barrier are 1.3 monolayers of CdSe. The lattice mismatch between the CdSe and the Zn 0.64 Be 0.3 Mn 0.06 Se induces a strain in the CdSe material, which is relaxed by the formation of isolated CdSe dots [10]. The full layer stack is given in Fig. 1. Standard optical lithography techniques were used to pattern the structure into 100 µm square pillars, and contacts wer...

Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

“…[1][2][3][4] Compared to their bulk counterparts, [5][6][7][8][9] magnetically doped semiconductor QDs could provide control of the magnetic ordering, [10][11][12][13][14][15][16] with the onset of magnetization at substantially higher temperatures. [17][18][19][20][21] Experiments typically focus on Mn-doped II-VI and III-V QDs, in which it is possible to include both single [22][23][24][25] and several magnetic impurities, [17][18][19][20][21][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] having similarities with nuclear spins. 41,42 In the first case (single magnetic ion), such systems could be considered as potential quantum bits, quantum memories, or probes to detect an unconventional orbital ordering.…”

Tailoring Magnetism in Quantum Dots

“…Unlike in the bulk structures, adding a single carrier in a magnetic QD can have important ramifications. An extra carrier can both strongly change the total carrier spin and the temperature of the onset of magnetization which we show can be further controlled by modifying the quantum confinement and the strength of Coulomb interactions.We study the magnetic ordering of carrier spin and magnetic impurities in (II,Mn)VI QDs identified as a versatile system to demonstrate interplay of quantum confinement and magnetism [4,5,6,15,16,17,18]. Because Mn is isoelectronic with group-II elements it does not change the number of carriers which in QDs are controlled by either chemical doping or by external electrostatic potential applied to the metallic gates.…”

“…We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations. Magnetic doping of semiconductor quantum dots (QDs) provides an interesting interplay of interaction effects in confined geometries [1,2,3,4,5,6,7,8] and potential spintronic applications [9]. In the bulk-like dilute magnetic semiconductors the carrier-mediated ferromagnetism can be photoinduced [10,11] and electrically controlled by gate electrodes [12], suggesting possible nonvolatile devices with tunable optical, electrical, and magnetic properties [9].…”

Magnetic field control of hysteretic switching in Co/Al2O3 multilayers by carrier injection

“…It would be interesting to mention that only a small part of the structure may be contributing to the switching process as has been reported in similar systems where electrons flow between nanodots separated by insulating barriers. 17,19,20 The effect of magnetic field on the switching behavior is studied by applying the magnetic field parallel to the direction of the average current in the film plane. It is clear from Fig.…”