Involvement of Carriers in the Size-Dependent Magnetic Exchange for Mn:CdSe Quantum Dots

Abstract

The magnetic behavior for Mn:CdSe (0.6%) quantum dots (QDs) exhibits size-dependent magnetic exchange mediated by the concentration of intrinsic carriers, which arise from surface states. High temperature paramagnetic behavior that can be fit to a Brillouin function with weak low temperature antiferromagnetic (AFM) coupling is observed for the large Mn:CdSe (5.0 and 5.8 nm) QDs. The 2.8 and 4.0 nm Mn:CdSe QDs display a size-independent blocking temperature (T(B)) at 12 K, decreasing coercivity with increasing size, and a lowering of the activation barrier for spin relaxation as the QD is increased in size. The magnetic behavior is inconsistent with classical domain theory behavior for a superparamagnet (SPM) but can be accounted for in a carrier-mediated RKKY model. Fitting the susceptibility data reveals a Pauli-paramagnetic (PPM) component that is believed to arise from the presence of carriers. The carrier density is observed to scale with the surface to volume ratio in the QDs, indicating the carriers arise from surface states that are weakly localized resulting in the onset of long-distance carrier-mediated RKKY exchange inducing overall ferrimagnetism in the Mn:CdSe QDs when the carrier concentration is above a critical threshold.

Understanding bifurcations in FC–ZFC magnetization of dilutely Fe3+ doped CdS nanoparticles

“…It could be clearly seen from the figures that irrespective of WZ or ZB phase, the bifurcation between FC and ZFC curves diminishes significantly upon pelletizing the CdS nanocrystals ($ 80% and $15% in powder and pelletized samples, respectively between 5 and 70 K temperature range). According to earlier reports [18,19,22,24] this bifurcation is not typical of that for blocked or spinglass state in particles with narrow size distribution. Magnetic measurements on various bulk and nanoparticles revealed that FC-ZFC bifurcation is direct indicative of magnetic metastability [8,[34][35][36].…”

“…(4) such that the temperature independent (TI) component of magnetic susceptibility could be extracted from that of the temperature dependent (TD) one. So, χ could be written as [22],…”

“…The blocking in SPM shows a cusp like feature in zero field-cooled (ZFC) magnetization curve as a function of temperature (T), and a bifurcation between field-cooled (FC) and ZFC magnetization curves as the temperature is lowered below the blocking temperature (T B ). The shape of FC curve at low temperature (below T B ) manifests the nature of inter-particle interactions; weak, strong or dipolar interaction in SPM nanoparticles [20][21][22][23]. Experimental investigations on various nanoparticle systems revealed that the inter-particle interactions could be modified upon dilution in nanoparticle arrangements and the blocking temperature increases (i.e.…”

Recent Advances in Magnetic Ion-Doped Semiconductor Quantum Dots

“…DMS QDs are not only known to respond to optical cues but also to charged carriers. For example, Zheng and Strouse 60 have shown carrier-mediated ferromagnetic interaction in Mn-doped CdSe QDs arising from photoexcited carriers from surface defect states of ultra small (<3 nm) QDs. Similarly, conduction band electron generated during photoexcitation is also shown to exhibit ferromagnetic exchange interactions in Mn-doped ZnO QDs under anaerobic conditions 61 54 .…”

Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organic semiconductors

“…1 that the PMMA and PS matrices with higher and lower dielectric constants correspond to negative and positive MCs, respectively, by enhancing triplet-charge reaction and dissociation in the PMMA:Ir(ppy) 3 and PS:Ir(ppy) 3 composites when the intermolecular SOC is weakened through molecular dispersion. Now we further examine the inter-molecular SOC when the phosphorescent Ir(ppy) 3 38 we can suggest that the summation of individual intermolecular SOC components can gradually increase the entire inter-molecular SOC for a single domain before reaching saturation with the domain size of $5 nm. Overall, it can be argued that changing molecular concentration can conveniently tune the inter-molecular SOC through separated and aggregated molecules in the Ir(ppy) 3 :polymer composite.…”