Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.
Intermolecular interactions of ten 2-acylamino and 2,4-bis(acylamino)pyrimidines (7 of which are previously unknown) have been investigated by X-ray structural, quantum chemical (DFT), and NMR spectral methods. Especially the concentration dependencies of the (1)H NMR chemical shifts and titrations with other molecules capable of multiple hydrogen bonding provided useful information regarding their association via triple or quadruple hydrogen bonding, which is controlled by the conformational preferences of 2-acylamino- and 2,4-bis(acylamino)pyrimidines. On comparison of the properties of 2-acylamino- and 2,4-bis(acylamino)pyrimidines with the corresponding pyridines, an additional nitrogen in the heterocyclic ring is the crucial factor in explaining the stability of various conformers and dimers of pyrimidines. Computational modeling of their dimerization (self-association) and heteroassociation supports the experimental findings. The substituent effects in 2-acylamino- and 2,4-bis(acylamino)pyrimidines are discussed via inter- and intramolecular terms. The subtle balance between several structural factors and their influence on the aggregation of studied pyrimidines was confirmed also by variable-temperature NMR and NOE experiments. X-ray structures of 2-methyl- and 2-adamantyl-CONH-pyrimidines revealed very different intermolecular interactions, showing the importance of the substituent size on the self-assembly process. As a whole NMR spectral, X-ray structural, and computational data of 2-acylamino- and 2,4-bis(acylamino)pyrimidines can be interpreted in terms of multiple intra-/intermolecular interactions.
The effect of hydrostatic pressure on the paramagnetic -ferromagnetic phase transition has been studied in (Ga,Mn)As. The variation of the Curie temperature (T C ) with pressure was monitored by two transport methods: (1) -measurement of zero field resistivity versus temperature ρ(T), (2) -dependence on temperature of the Hall voltage hysteresis loop. Two specimens of different resistivity characteristics were examined. The measured pressure-induced changes of T C were relatively small (of the order of 1K/GPa) for both samples, however they were opposite for the two.(Ga,Mn)As is one of the most intensively investigated diluted magnetic semiconductors during last decades. The understanding of physical phenomena governing its magnetic properties is crucial for increasing Curie temperature (T C ) and thus for possible application of this material in spintronic devices. The origin of ferromagnetism in (Ga,Mn)As was quantitatively explained within the p-d Zener model assuming magnetic interaction between the localized magnetic moments of Mn 2+ ions mediated by holes in the valence band [1][2][3] . This model, in the case of semiconductors, where the carrier density is smaller than the magnetic ion concentration is equivalent to the Ruderman-Kittel-Kasuya-Yosida (RKKY) approach employed in the diluted magnetic metals 4 . Within this picture the ferromagnetic ordering temperature, T C depends in particular on the local p-d exchange interaction and a free hole concentration. It was demonstrated that indeed an increase of the hole concentration in a field effect transistor structure led to an enhancement of the ferromagnetic state 5. On the other hand it was found that the exchange energy scales with the lattice constant as, N 0 β ~ a 0 -3 , 1 and therefore an external hydrostatic pressure could influence the exchange coupling. Although the studies of (In,Mn)Sb diluted magnetic semiconductor under hydrostatic pressure provided an evidence for an increase in carrier-mediated magnetic coupling 6,7 , giving rise to higher Curie temperature, the effect of hydrostatic pressure on (Ga,Mn)As semiconductor is not as clear.6 Therefore additional study was performed in order to clarify the role of external hydrostatic pressure in the paramagneticferromagnetic phase transition in (Ga,Mn)As. p-type Ga 1-x Mn x As layers were grown by molecular beam epitaxy (MBE) on (100) GaAs substrate. In our studies two different samples were used: A777 and A963. The former sample had 20 nm thick layer of (Ga,Mn)As with Mn content x = 7%. After the MBE growth this sample was capped with amorphous As and annealed in the MBE growth chamber at the temperature of 210 ºC (controlled by the IR pyrometer) for two hours (see Ref. 8 for details). The Curie temperature determined from SQUID magnetometry was close to 85 K (Fig. 1, open symbols). The latter sample had a (Ga,Mn)As layer of 50 nm and x = 6%. This sample was not annealed after the MBE growth. The Curie temperature for A963 sample was approximately 50 K (Fig. 1, solid symbols). Since the amount o...
Thin layers of transition metal dichalcogenides have been intensively studied over the last few years due to novel physical phenomena and potential applications. One of the biggest problems in laboratory...
Granular GaAs:(Mn,Ga)As films were prepared by annealing the Ga0.985Mn0.015As/GaAs layers at 500 °C or 600 °C. It is commonly accepted that this processing should result in the formation of cubic or hexagonal MnAs clusters, respectively. We demonstrate that such a priori assumption is not justified. If in the as grown sample there are not many defects with the interstitial Mn atoms, only small cubic clusters can be formed even after annealing at 600 °C. Moreover, in a sample containing solely cubic GaMnAs clusters, the Mn ions are ferromagnetically coupled at room temperature. This fact was explained by the existence of GaMnAs solid solution in the clusters, with content of Mn close to 20% (higher than ever found in the layers) as was confirmed by experiment and theory. Extended X‐ray absorption spectroscopy studies excluded the possibility of formation of the hypothetic zinc blende MnAs clusters. Not more than one Mn atom was detected in the second shell around central Mn atom. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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