Spin Density Of Carriers Research Articles

Low field magnetization in Pb1-x-ySnyMnxTe semimagnetic semiconductors

Abstract We report and analyse the results of our calculation of low field magnetization of the degenerate dilute magnetic semiconductor Pb1-x-ySnyMnxTe for × = 0.03 and y = 0.72, for a carrier density of holes p = 1.4 × 1020 cm−3. The contribution comes from two different mechanisms namely localized magnetic impurities of Mn+2 , and band effects. For the calculation of magnetic impurity contribution we assume that the these impurities are distributed in random fashion over the host system and we use three spin cluster model which includes single, pair and triple spin clusters. Moreover, we consider two types of three spin clusters like open and closed triplets. The band effect includes lattice diamagnetism as well as spin polarization of carriers. The former has been calculated using a two band model of the host system and the effects of the impurities are incorporated through the modification of its band gap. The variation of the energy band gap as a function of Sn and Mn concentrations and temperature is considered through an empirical expression and value of the energy gap so obtained for Pb1-x-ySnyMnxTe ( × = 0.03 and y = 0.72) is about 125 meV at 5 K. The spin density of carriers is evaluated using the second order k → . π → perturbation theory. The major contribution to local moment magnetization comes from single spin clusters and then from pair and triads. The diamagnetic contribution arises from the diamagnetic susceptibility and hence it is negative. The contribution from spin density of carriers, as expected, is very small in the range of carrier density considered for present study. Good agreement between our theory with experiment is another important feature of our calculation.

Carrier induced local moment magnetization in p-type Sn1−xMnxTe

We derive a theory of carrier induced local moment magnetization of p-type Sn1−xMnxTe based on the Hubbard model, k→·π→ electronic structure method (k→ is the electronic wave vector and π→ is the relativistic momentum operator) and the statistical paramagnetic approach for the localized moments. The Hubbard model is used to derive an internal exchange magnetic field. The difference in exchange self-energy is expressed in terms of an internal exchange field that is proportional to the parameter U, the onsite Coulomb repulsion, and the spin-density of carriers. In the present theory, the k→·π→+U model is integrated with the statistical paramagnetic theory for localized spins, which is then solved in a self-consistent manner by adding the exchange field to the applied field. The technique is applied to study the magnetic properties of p-type Sn1−xMnxTe, an important material for spintronics devices. The local moment magnetization calculated using the total magnetic field self-consistently agrees with the experimental observations. Magnetization and the exchange field studied as functions of the applied field, temperature and carrier concentration yield results on expected lines. Ours is a mechanism that is different from the RKKY interaction, normally invoked for carrier induced ferromagnetism and is thus a novelty.

Identification of Sublattice Damages in Swift Heavy Ion Irradiated N-Doped 6H-SiC Polytype Studied by Solid State NMR

We have studied N-doped 6H-SiC in its pristine and Swift Heavy Ion (SHI) irradiated (150 MeV Ag(12+) ions) forms by solid state Nuclear Magnetic Resonance (NMR) at 7.01 T using (13)C and (29)Si as probe nuclei under magic angle spinning. We show that increased levels of nitrogen doping, than used before, lead to the observation of Knight shifts emanating from an increase in electron density in the conduction band, which in (13)C far exceed those in (29)Si MAS spectra. We have rationalized the differential effects in the MAS spectra and site-dependent paramagnetic shifts in terms of the nitrogen doping at the A, B, and C lattice sites. N-doping has a profound effect on (29)Si spin-lattice relaxation, and the site-dependent relaxation behavior is attributed to a difference in conduction electron properties at the different lattice sites. (29)Si T(1) measurements serve to identify the sublattice damages in SHI irradiated 6H-SiC. By determining the spin-lattice relaxation rates as a function of the SHI irradiation ion fluences, the change in relaxation behavior is correlated to the damage production mechanism. The sublattice damage leads to discernable changes in the interaction between the mobile unpaired electrons in the conduction band and the nuclear site, which profoundly influence the NMR relaxation properties. Our relaxation studies also provide evidence for site-dependent localized effects and a decrease in carrier spin density in the conduction band for the SHI irradiated 6H-SiC.

Mass modification of itinerant carriers in RKKY oscillations induced by finite-range exchange interactions

We investigate the Ruderman-Kittel-Kasuya-Yosida oscillations of the itinerant carrier spin density in a system where those oscillations appear only due to a finite distribution of a localized spin. The system represents a half-infinite one-dimensional quantum wire with a magnetic impurity located at its edge. In contrast to the conventional model of a pointlike exchange interaction the itinerant carrier spin-density oscillations in this system exist. We analytically demonstrate that when the radius of the exchange interaction is less than the wavelength of the itinerant carriers living on the Fermi surface, the long-range behavior of the oscillations is identical to the one taking place in the zero-radius limit of the same exchange interaction but for an infinite one-dimensional quantum wire where, in comparison with the original half-infinite system, the mass of the itinerant carriers is strongly modified by the exchange interaction radius. On the basis of our analysis we make a suggestion on directionality of surface Ruderman-Kittel-Kasuya-Yosida interaction shown in recent experiments: we believe that in general the anisotropy of the Ruderman-Kittel-Kasuya-Yosida interaction could result not only from the anisotropy of the Fermi surface of itinerant carriers but also from the anisotropy of the spin carrying atomic orbitals of magnetic impurities.

Theory of photo-magnetization of an interacting particle system: application toHg1−xMnxTe

We derive a theory of light-induced magnetization of an interacting particle system, whichdescribes a model diluted magnetic semiconductor. We set up a many-body Hamiltonianthat includes electron and hole kinetic energies, carrier–photon and carrier–local-momentinteractions and all the Coulomb interactions. Using Heisenberg equations of motion and amean-field approximation, we derive expressions for the carrier spin density andexcitonic amplitudes. Non-equilibrium relaxation is included phenomenologically.The interdependence of carrier spin density and the local-moment magnetizationis calculated self-consistently. We apply our theory of photo-magnetization toHg1−xMnxTe, choosing parameters of the model appropriate to this system. Our results for thephoto-magnetization versus the input laser power agree qualitatively with the observedtrends in this system. We also study the photo-magnetization as a function of temperature,manganese concentration and photon energy and the results obtained are along expectedlines.

Manipulation of Nuclear Spins in Interfaces of Diluted Magnetic Semiconductors

A way of nuclear spin manipulation using an interface of diluted magnetic semiconductors (DMSs) is proposed. The hyperfine field at an impurity site in the interface of a DMS and a base substance can be controlled by applying the external electric field. It is because that the electric field changes the spin density of carriers in the s state at the impurity site. On the basis of first-principles electronic structure calculation, we investigated the electric-field dependence of the hyperfine field at the impurity site. We show that the hyperfine field is dramatically changed by an external electric field when no clear magnetic impurity band is formed in the band gap. [DOI: 10.1380/ejssnt.2008.7]

Distribution of electron density in BaPb 1− xBi xO 3 evidenced by 207Pb and 17O NMR

The inhomogeneous magnetic broadening of the 17O and 207Pb NMR spectra resulting from a distribution of the Knight shifts was studied in the metallic phase of BaPb 1− x Bi x O 3 oxides (0⩽ x⩽0.33). It was shown that Bi atoms, which are randomly incorporated in the parent BaPbO 3, give rise to an increased spin density of carriers within the two first cation shells. The percolative overlap of these areas is expected for superconducting compositions ( x>0.1).

Theory of anisotropic diamagnetism, local moment magnetization and carrier spin-polarization in Pb1−x Eu x Te

We present theoretical analyses of anisotropic lattice diamagnetism, magnetization due to magnetic ions and carrier spin-polarization in the diluted magnetic semiconductor, Pb1-xEuxTe. The lattice diamagnetism results from orbital susceptibility due to inter band effects and spin-orbit contributions. The spin-orbit contribution is found to be dominant. However, both the contributions show pronounced anisotropy. With increase in x, the diamagnetism decreases. We consider contributions from randomly distributed isolated magnetic ions and clusters of pairs and triads for the local moment magnetization. The isolated magnetic-ion contribution is the dominant one.We calculate the magnetization for two typical magnetic ion concentrations: x=0.03 and x=0.06. Temperature dependence of the magnetization is also considered. Apart from lattice and localized magnetic ions, the carrier contribution to the spin-density is also calculated for a carrier density of p=1018 cm−3. the relative spin-density of carriers increases with increase in the magnetic field strength and magnetic ion concentration. The agreement with experiment where available is reasonably good.

Spin wave spectra in doped ferromagnetic semiconductors

We present a theory of the magnetic excitation spectrum of ferromagnetic semiconductors and semimetals. The calculations are performed up to second order in the exchange coupling between the localized spin array Si and the free carrier spin density s(ri), using the functional differentiation technique and Green functions. A high frequency spin wave mode is found, in addition to the usual low frequency spin wave branch. The damping of both modes is found to be small at low temperatures when the free carriers are spin polarized, in which case the bottleneck limit is applicable. At high temperature where 〈sz〉→0, the spin dynamics is described by the Hasegawa equations and the Korringa relaxation term which governs the width of the magnetic resonance is recovered.