Magneto-optical Kerr effect measurements of epitaxial AFM/FM bilayers and FM/AFM/FM trilayers on Cu 3 Au(001), where "AFM" stands for a Ni 25 Mn 75 antiferromagnetic layer, and "FM" for ferromagnetic layers that are either Ni or Ni/Co with out-of-plane or in-plane easy axis of magnetization, show that trilayers with collinear magnetization directions of both FM layers exhibit always a much lower exchange bias field H eb at a fixed temperature compared to bilayers of the same Ni 25 Mn 75 thickness. At the same time, the blocking temperature for exchange bias T b is strongly reduced. In trilayers with orthogonal easy axes of the two FM layers (in-plane and out-of-plane), in contrast, both H eb and T b are nearly identical to that of the corresponding bilayers. Such a behavior can be explained by pinned magnetic moments inside the bulk of the AFM layer that coexist independently for orthogonal spin directions, but have to be equally shared between both interfaces in the case of collinear spin directions. This result thus also confirms a 3Q-like noncollinear spin structure of Ni 25 Mn 75 .
temperature (T = 40 ~ 300 K) dependence of Hall-effect analysis on the dual Si-δ-doped AlGaAs/ InGaAs/AlGaAs quantum-well (QW) structures with various space layer thicknesses (t S = 5, 10 and 15 nm) was performed. An interesting hysteresis behavior of electron sheet concentration [n 2D (T)] was observed for t S = 10 and 15 nm but not for t S = 5 nm. A model involving two different activation barriers encountered respectively by electrons in the active QW and by electrons in the δ-doped layers is proposed to account for the hysteresis behavior. However, for small enough t S (= 5 nm ≤ 2.5 s, where s = 2.0 nm is the standard deviation of the Gaussian fit to the Si-δ-doped profile), the distribution of Si dopants near active QW acted as a specific form of "modulation doping" and can not be regarded as an ideal δ-doping. These Si dopants nearby the active QW effectively increase the magnitude of n 2D , and hence no hysteresis curve was observed. Finally, effects from t S on the T-dependence of electron mobility in active QW channel are also discussed.
Zero-dimensional molybdenum disulfide (MoS 2 ) quantum dots (QDs) have attracted remarkable interest due to their peculiar properties such as the quantum-confinement effect and high surface area. Exploring recombination dynamics in MoS 2 QDs is not only expected to gain a deeper insight into their fundamental physics, it is also important for potential applications in optoelectronics and energy-conversion technology. This study synthesized p-type MoS 2 QDs doped with diethylenetriamine (DETA) using pulsed laser ablation method. A hole concentration as high as 2.08 × 10 12 cm −2 has been demonstrated by gatedependent conductance measurements. A 110-fold enhancement of photoluminescence in the p-type MoS 2 QDs has been found after the introduction of DETA, and the dependence of the radiative and nonradiative recombination of MoS 2 QDs on carrier densities were studied. As the carrier density was increased, a decrease of the radiative lifetime was found, which is similar to the behavior of the radiative lifetime in monolayer MoS 2 . The Shockley-Read-Hall (SRH) and Auger recombination dominates the nonradiative recombination at low and high carrier densities, respectively. The SRH lifetime of MoS 2 QDs increases with the increased carrier density, suggesting that the recombination mechanism at the low carrier density is dominated by the SRH recombination. This study found that as the carrier densities exceeded 0.53 × 10 12 cm −2 , the Auger recombination was responsible for the reduction of PL. Furthermore, MoS 2 QDs was used as a fluorescent sensor for the detection of ammonium hydroxide (NH 4 OH). The PL intensity of MoS 2 QDs demonstrates a gradual decrease with increasing NH 4 OH concentration. By investigating the time-resolved PL (TRPL), the mechanism that leads to the decrease of PL in MoS 2 QDs is addressed. This investigation is expected to demonstrate a promising development of an effective and low-cost MoS 2 QDsbased fluorescent sensor with superior sensitivity for the rapid detection of ammonia in aqueous media.
Magnetic proximity effects in single-crystalline NixMn100-x/Ni(/Co) bilayers on Cu3Au(001) are investigated for in-plane (IP) and out-of-plane (OoP) magnetization by means of the longitudinal and polar magneto-optical Kerr effect. Attention is paid to the influence on concentration- and thickness-dependent antiferromagnetic ordering (TAFM) and blocking (Tb) temperatures as well as the exchange bias field (Heb). For all the NixMn100-x films under study in contact with IP Ni, increasing TAFM is observed with decreasing Ni concentration from ∼50 to ∼20%, whereas only a slight change in TAFM is observed for the OoP case. Between ∼28% and ∼35% Ni concentration, a crossover temperature exists below which TAFM for the IP samples is higher than for the OoP samples and vice versa. Tb is higher for the IP case than for OoP, except for an equi-atomic NiMn film, while Heb increases significantly for both magnetization directions with decreasing x. These results are attributed to: (i) a rotation of the non-collinear 3Q-like spin structure of NixMn100-x from the more-OoP to the more-IP direction for decreasing Ni concentration x, along with an associated increased magnetic anisotropy, and (ii) a smaller domain wall width within the NixMn100-x films at smaller x, leading to a smaller thickness required to establish exchange bias at a fixed temperature.
The surface of expanded face-centered tetragonal (e-fct) antiferromagnetic Mn films of a few atomic monolayers thickness grown epitaxially on Co/Cu(001) was investigated at room-temperature by scanning tunneling microscopy and scanning tunneling spectroscopy using a ferromagnetic ring-shaped bulk iron probe. We show that the main contribution to the contrast modulation observed as a function of Mn thickness in differential conductance maps is not due to spin-polarized tunneling from a layer-wise antiferromagnetic spin alignment. Instead, it is mainly of electronic origin resulting from layer-dependent electronic properties of the Mn film, probably related to different levels of intermixing with Co atoms. On the atomic scale, the Mn surface demonstrates a geometrical reconstruction with a (12×2) periodicity in two orthogonal domains on the four-fold symmetric substrate with an apparent surface corrugation of up to 0.3 Å. Simultaneously recorded differential conductance maps show different textures in the two orthogonal domains, providing evidence for non-collinearity in the Mn surface spin structure.
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