Concentration Of Magnetic Impurities Research Articles

High temperature ferrimagnetic semiconductors by spin-dependent doping in high temperature antiferromagnets

To realize room temperature ferromagnetic (FM) semiconductors is still a challenge in spintronics. Many antiferromagnetic (AFM) insulators and semiconductors with high Neel temperature TN are obtained in experiments, such as LaFeO3, BiFeO3, etc. High concentrations of magnetic impurities can be doped into these AFM materials, but AFM state with very tiny net magnetic moments was obtained in experiments because the magnetic impurities were equally doped into the spin up and down sublattices of the AFM materials. Here, we propose that the effective magnetic field provided by a FM substrate could guarantee the spin-dependent doping in AFM materials, where the doped magnetic impurities prefer one sublattice of spins, and the ferrimagnetic (FIM) materials are obtained. To demonstrate this proposal, we study the Mn-doped AFM insulator LaFeO3 with FM substrate of Fe metal by the density functional theory (DFT) calculations. It is shown that the doped magnetic Mn impurities prefer to occupy one sublattice of the AFM insulator and introduce large magnetic moments in La(Fe, Mn)O3. For the AFM insulator LaFeO3 with high TN = 740 K, several FIM semiconductors with high Curie temperature TC > 300 K and the band gap less than 2 eV are obtained by DFT calculations when 1/8 or 1/4 Fe atoms in LaFeO3 are replaced by the other 3d, 4d transition metal elements. The large magneto-optical Kerr effect (MOKE) is obtained in these LaFeO3-based FIM semiconductors. In addition, the FIM semiconductors with high TC are also obtained by spin-dependent doping in some other AFM materials with high TN, including BiFeO3, SrTcO3, CaTcO3, etc. Our theoretical results propose a way to obtain high TC FIM semiconductors by spin-dependent doping in high TN AFM insulators and semiconductors.

Impurity and hybridization effects on the symmetry classification and magnetic response function of a two-band superconductor with interband pairing order

Abstract The effects of hybridization and impurity (magnetic and nonmagnetic) potentials on the pairing symmetries and magnetic response of a two-band superconductor with an equal-time s-wave interband pairing order parameter in the framework of Green’s function technique are investigated theoretically. First, the effects of spin-independent and spin-dependent hybridization on the generation of even- or odd-frequency Cooper pairs which determines the symmetry classification and the response of the superconductor are studied. Next, the impurity effect on creating different symmetry classes and the kernel response function of a two-band superconductor are discussed. By separating the contributions of even- and odd-frequency pairing to the Meissner kernel, it is shown that the competition between these two terms determines the total Meissner effect of the superconductor. For a two-band spin-singlet superconductor, nonmagnetic impurity scatterings do not change transition temperature according to Anderson’s theorem, while both intra- and interband magnetic impurity scattering cause superconducting transition temperature suppression with the rate following the Abrikosov–Gor’kov theory. For spin-triplet pairing, interband magnetic scattering has no impact on pair breaking, whereas intraband magnetic scattering acts as a pair breaker and suppresses the transition temperature in the Born limit. In this case, the odd-frequency superconducting pairs can be induced in the simultaneous presence of both intra- and interband magnetic impurities. Thus, by controlling the concentration of magnetic impurities, it is possible to engineer triplet-pairing odd-frequency superconductors with a total diamagnetic Meissner response which stabilizes the superconducting state. This technique opens up an avenue for designing stable odd-frequency superconductors.

Optimizing thermoelectric performance of carbon-doped h-BN monolayers through tuning carrier concentrations and magnetic field

The thermoelectric properties of carbon-doped monolayer hexagonal boron nitride (h-BN) are studied using a tight-binding model employing Green function approach and the Kubo formalism. Accurate tight-binding parameters are obtained to achieve excellent fitting with Density Functional Theory results for doped h-BN structures with impurity type and concentration. The influence of carbon doping on the electronic properties, electrical conductivity, and heat capacity of h-BN is studied, especially under an applied magnetic field. Electronic properties are significantly altered by doping type, concentration, and magnetic field due to subband splitting, merging of adjacent subbands, and band gap reduction. These modifications influence the number, location, and magnitude of DOS peaks, generating extra peaks inside the band gap region. Heat capacity displays pronounced dependence on both magnetic field and impurity concentration, exhibiting higher intensity at lower dopant levels. Electrical conductivity is increased by double carbon doping compared to single doping, but is reduced at high magnetic fields because of high carrier scattering. The electronic figure of merit ZT increases with lower impurity concentration and is higher for CB versus CN doping at a given field strength. The power factor can be improved by increasing magnetic field and decreasing doping concentration. In summary, controlling doping and magnetic field demonstrates the ability to effectively engineer the thermoelectric properties of monolayer h-BN.

The Study of Transport Properties of (III−Mn) V Diluted Magnetic Semiconductors

We investigated the transport properties of diluted magnetic semiconductors (DMSs) theoretically by using the Heisenberg and Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interaction models by considering both spin and charge disorder. The formalism is applied to the specific case of Ga 1 − x Mn x As . Using the Heisenberg model and the Green function formalism the total thermal excitation of the magnon is calculated. The magnetization and Curie temperature of Mn -doped GaAs is calculated. The theoretical calculation of T C of Ga 1 − x Mn x As at x = 0.08 has a good agreement with the experimental calculation at x = 0.08 (i.e., 162 k). The exchange interaction constant and spin-dependent relaxation time is calculated by using RKKY interaction. The electrical conductivity and hole mobility are calculated by using the Boltzmann transport equation and the spin-dependent relaxation time. The electrical conductivity of Mn -doped III–V DMS is exponentially increased with temperature and magnetic impurity concentration. Hole mobility of Mn -doped III–V diluted magnetic semiconductor is increased with the magnetic impurity concentration.

Electronic and magnetic properties of Lu and LuH2

Clarifying the electronic and magnetic properties of lutetium, lutetium dihydride, and lutetium oxide is very helpful to understand the emergent phenomena in lutetium-based compounds (such as superconductivity-like transitions near room temperature). However, this kind of study is still scarce at present. Here, we report on the electronic and magnetic properties of lutetium metals, lutetium dihydride powders, and lutetium oxide powders. Crystal structures and chemical compositions of these samples were characterized by x-ray diffraction and x-ray photoemission spectroscopy, respectively. Electrical transport measurements show that the resistance of lutetium has a linear behavior depending on temperature, whereas the resistance of lutetium dihydride powders changes little with decreasing the temperature. More interestingly, paramagnetism-ferromagnetism-spin glass transitions were observed at near 240 and 200 K, respectively, in lutetium metals. To understand their origins, the measurement of inductively coupled plasma optical emission spectroscopy was performed, revealing that the concentrations of dilute magnetic impurities in lutetium and lutetium dihydride are around 0.01%, which can lead to the presence of spin glassy behavior. Our work uncovers the complex magnetic properties of lutetium and lutetium dihydride and suggests their close connections to the electronic and magnetic transitions of Lutetium-based compounds (such as Lu–H–N).

Large suppression of spin-relaxation rate in graphene nanoribbons in the presence of magnetic impurities

Achievement of low spin-relaxation rate is an important goal for spintronics development. We study the spin-relaxation rate arising from a low concentration of magnetic impurities in armchair graphene nanoribbons (AGNR). Large suppression in the spin-relaxation rate, exhibited as dip structures, is found when the Fermi energy approaches a subband band edge. This suppression originated from the quasi-one-dimensional density of states and is manifested via the singular features in the AGNR same-site Green's function ${G}_{\mathcal{II}}$, where $\mathcal{I}$ denotes site locations of magnetic impurities. Analytic analysis of the spin-relaxation rate in the close vicinity of a subband band edge is performed to further reveal the physical nature of the suppression. The robustness of the suppression feature in the spin-relaxation rate is demonstrated by systematically increasing the number of magnetic impurities involved in a coherent multiple scattering with the electrons. Major peaks in the spin-relaxation rate are analyzed in light of their connection with spin-flipped resonances. Competition between magnetic impurities with similar resonance energies is found to lead to suppression in the spin-relaxation rate. Our calculations have taken into account the hydrogen-passivation effects at the AGNR edges when the hopping constant between edge carbons is modified.

Theoretical study on various contributions to the magnetization of Pb1-xEuxSe

The results of the calculation on magnetization (M) for diluted magnetic semiconductor Pb1-xEuxSe are reported. Our calculation includes two distinct contributions namely contribution from the localized magnetic impurity (Mi) and contribution from the band effect (Mb). Mi has been calculated using a three-spin cluster model which comprises the clusters consisting of one-spin (M1), two-spins (M2) and three-spins (M3). Moreover, M3 includes both open triplets (Mot) and closed triplets (Mct). The statistical distribution of the different clusters has been considered using random distribution approximation in the dilute limit. The contributions from these clusters to the magnetization have been evaluated using a modified Heisenberg's exchange Hamiltonian in the nearest approximation. The magnetization has been calculated as a function of magnetic field and impurity concentration. The band effect includes the contribution from lattice diamagnetism(χlatt). This mechanism has been considered in the effective mass formalism using a two-band k→.π→model. Our calculated values of M compare very well with experimental data where available.

Spin relaxation in diluted magnetic semiconductors: GaMnAs as example

The paper deals with a study of the magnetic impurities spin relaxation in the diluted magnetic semiconductors above the Curie temperature. Systems with a high concentration of magnetic impurities where magnetic correlations take place were studied. The proposed theory assumes the main channel of the spin relaxation being the mobile carriers, which provide the indirect interactions of the magnetic impurities. This theoretical model is supported by the experimental measurements of the manganese spin relaxation time in the GaMnAs by means of spin-flip Raman scattering. As has been found with a temperature increase the spin relaxation rate of the ferromagnetic samples grows, tending to that measured in a paramagnetic sample.

Effects of magnetic impurity concentration and hole carriers on ferromagnetic stability of Li(ZnCr)As and Li(ZnCrCu)As

First-principles calculations were performed to study electronic structure and magnetic properties of Li(ZnCr)As and Li(ZnCrCu)As. The ground state of Li(ZnCr)As was ferromagnetism. To enhance the ferromagnetic stability of the doping system, two methods were proposed and the exchange mechanism was analyzed. More Cr impurities increased the energy gain and decreased p-d hybridization, contributing to the double-exchange interaction and thus promoting ferromagnetic stability of Li(ZnCr)As. By substituting Zn(3d104s2) with Cu(3d104s1), additional hole carriers were introduced and the ferromagnetism was more stable than that in the single Cr doping case. The ferromagnetism of Li(ZnCrCu)As system was further stabilized with the increase of hole carrier concentration. The Li32(Zn22Cr8Cu2)As32 configuration showed the strongest ferromagnetic stability. Therefore, it was possible to fabricate a new type of dilute magnetic semiconductor material with high ferromagnetic stability by adjusting the concentration of Cr impurity and additional hole carriers.

The Magnetic Domain Structure Properties in Diluted Magnetic Semiconductors

We present a comprehensive analysis of the domain structure formation in the ferromagneticphase of diluted magnetic semiconductors (DMS) of the p-type. Our analysis is carried outon the base of the effective magnetic free energy of DMS calculated by us earlier. This freeenergy, substituting DMS (a disordered magnet) by an effective ordered substance, permits usto apply the standard phenomenological approach to the domain structure calculation. Usingthe coupled system of Maxwell equations with those obtained by the minimization of the freeenergy functional, we show the existence of the critical ratio vcr of concentration of chargecarriers and magnetic ions such that the sample critical thickness Lcr (such that the sampleis monodomain at L < Lcr) diverges as v → vcr. At v > vcr, the sample is monodomain. Thisfeature makes DMS different from conventional ordered magnets, as it gives a possibility tocontrol the sample critical thickness and the emerging domain structure period by a variationof v. As the concentration of magnetic impurities grows, vcr → ∞, restoring a conventionalbehavior of ordered magnets. Above facts have been revealed by the examination of the tem-perature of the transition to an inhomogeneous magnetic state (stripe domain structure) inthe slab of a p-type DMS with finite thickness L. Our theory can be easily generalized for anarbitrary disordered magnet.

Intrinsic Bulk Quantum Oscillations in a Bulk Unconventional Insulator SmB6.

SummaryThe finding of bulk quantum oscillations in the Kondo insulator SmB6 proved a considerable surprise. Subsequent measurements of bulk quantum oscillations in other correlated insulators including YbB12 lent support to our discovery of a class of bulk unconventional insulators that host bulk quantum oscillations. Here we perform a series of experiments to examine evidence for the intrinsic character of bulk quantum oscillations in floating zone-grown single crystals of SmB6 that have been the subject of our quantum oscillation studies. We present results of thermodynamic, transport, and composition analysis experiments on pristine floating zone-grown single crystals of SmB6 and compare quantum oscillations with metallic LaB6 and elemental aluminum. These results establish the intrinsic origin of quantum oscillations from the insulating bulk of floating zone-grown SmB6. The similarity of the Fermi surface in insulating SmB6 with the conduction-electron Fermi surface in metallic hexaborides is at the heart of a theoretical mystery.

Phase diagrams of Kondo alloys

We analyze the ground state phase diagrams of the Kondo alloy model (KAM) as a function of Kondo coupling (JK), electronic band filling (nc) and magnetic impurity concentration (x). We use DMFT/CPA method to treat the disorder and the inter-site electronic exchange correlations along with mean-field (MF) approximations to decouple the Kondo interaction. Here, we generalize “Doniach-like” phase diagrams of the periodic Kondo lattice (x=1) to Kondo alloys. We consider the competition between the paramagnetic Kondo phase (K) and purely ferromagnetic (F) or staggered antiferromagnetic (AF) ordered phases. Our phase diagrams are consistent with various experimentally obtained phase diagrams for alloys like CexLa1-xPt2Si2, CexLa1-xNi2Ge2, CexLa1-xRu2Si2, CexLa1-xPd2Si2, or CexLa1-xCu2Ge2.

Observation of an antiferromagnetic quantum critical point in high-purity LaNiO3

Amongst the rare-earth perovskite nickelates, LaNiO3 (LNO) is an exception. While the former have insulating and antiferromagnetic ground states, LNO remains metallic and non-magnetic down to the lowest temperatures. It is believed that LNO is a strange metal, on the verge of an antiferromagnetic instability. Our work suggests that LNO is a quantum critical metal, close to an antiferromagnetic quantum critical point (QCP). The QCP behavior in LNO is manifested in epitaxial thin films with unprecedented high purities. We find that the temperature and magnetic field dependences of the resistivity of LNO at low temperatures are consistent with scatterings of charge carriers from weak disorder and quantum fluctuations of an antiferromagnetic nature. Furthermore, we find that the introduction of a small concentration of magnetic impurities qualitatively changes the magnetotransport properties of LNO, resembling that found in some heavy-fermion Kondo lattice systems in the vicinity of an antiferromagnetic QCP.

Magnetic impurities as the origin of the variability in spin relaxation rates in Cu-based spin transport devices

The Elliott-Yafet spin relaxation mechanism posits linear proportionality between spin and momentum lifetimes in low spin-orbit coupling nonmagnetic metals, and is widely accepted in spintronics. Accurate experimental determination of the Elliott-Yafet proportionality constants (${\ensuremath{\beta}}_{i}$) between the spin and momentum relaxation times for individual scattering sources is challenging, however. This is apparent from the literature on nonlocal spin transport in Cu, for example, where reported phonon (${\ensuremath{\beta}}_{\mathrm{ph}}$) and defect (${\ensuremath{\beta}}_{\mathrm{def}\phantom{\rule{4pt}{0ex}}}$) Elliott-Yafet constants vary by an order of magnitude. In recent work we discovered that even part-per-million-level magnetic impurity concentrations can substantially influence spin relaxation in Cu, via a spin transport analog of the Kondo effect. To clarify whether this could explain the reported variability in ${\ensuremath{\beta}}_{i}$, here we report on a comprehensive study of spin transport in Cu-based lateral nonlocal spin valves, varying the ferromagnetic contact material, interface structure, Cu thickness, and post-fabrication annealing conditions, resulting in widely varied microstructures and magnetic impurity concentrations. Quantifying the effects of magnetic impurities on charge and spin transport we demonstrate the dramatic, even dominant, effect these can have on spin relaxation rates, and thus extracted ${\ensuremath{\beta}}_{i}$. Minimization of magnetic impurity effects is achieved via Al interlayer insertion or moderate annealing, restoring the expected temperature dependence for phonon-mediated spin relaxation, and enabling more reliable determination of ${\ensuremath{\beta}}_{i}$ for phonons (740 \ifmmode\pm\else\textpm\fi{} 200), and nonmagnetic defects (240 \ifmmode\pm\else\textpm\fi{} 50). The latter contribution is shown to be dominated by grain boundaries in these polycrystalline Cu films. Cross-sectional transmission electron microscopy measurement of grain sizes in actual nonlocal spin valve devices then establishes a useful empirical relationship between average grain size and spin diffusion length. These measurements highlight the importance of magnetic impurities in metallic spin transport, explain the wide variability in reported ${\ensuremath{\beta}}_{\mathrm{ph}}$ and ${\ensuremath{\beta}}_{\mathrm{def}}$ in Cu, and elucidate the relationship between metallic spin transport and microstructure.

Anomalous Hall effect in magnetic topological insulators: Semiclassical framework

The anomalous Hall effect (AHE) is studied on the surface of a 3D magnetic topological insulator. By applying a modified semiclassical framework, all three contributions to the AHE, the Berry curvature effect, the side-jump effect and the skew scattering effects are systematically treated, and analytical expressions for the conductivities are obtained in terms of the Fermi level, the spatial orientation of the surface magnetization and the concentration of magnetic and nonmagnetic impurities. We demonstrate that the AHE can change sign by altering the orientation of the surface magnetization, the concentration of the impurities and also the position of the Fermi level, in agreement with recent experimental observations. We show how each contribution to the AHE, or even the whole AHE, can be turned off by properly adjusting the given parameters. For example, one can turn off the anomalous hall conductivity in a system with in-plane magnetization by pushing the system into the fully metallic regime.

Breakdown of coherence in Kondo alloys: crucial role of concentration versus band filling

We study the low energy states of the Kondo alloy model (KAM) as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling , that may be separated by a transition at x = nc. Here, we analyze the KAM for finite on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination , we show that the real part of the local potential scattering may be located outside the conduction electron band, revealing a possible breakdown of Luttinger theorem for intermediate values of impurity concentration x. Unusual physical signatures are expected, e.g. in ARPES experiments. In order to take into account fluctuations associated with finite dimensionality, i.e. finite Z, we extend this analysis by studying the KAM with an adaptation of the statistical-DMFT method that was developped elsewhere. We review the distributions of local potential scattering and their evolution with model parameters: concentration, strength of Kondo coupling, coordination number, local site neighborhood, connection with percolation issue. Relevence for Kondo alloys material with f -electrons is also discussed.

Effect of magnon-exciton coupling on magnetic phase transition of diluted magnetic semiconductors

This article reports effects of magnon-exciton interaction on magnetic ordering in diluted magnetic semiconductors (DMS). Quantum field theory is employed using the double time temperature dependent Green function technique to obtain dispersion. It is understood that interaction of the two quasi particles take place in the exciton cloud and consequently the spontaneously ordered localized electrons might be partly trapped and subjected to different angular precision resulting in increase ofthe number of magnons. According to our analysis the exciton-magnon coupling phenomena may be the reason for the attenuation of spontaneous magnetization and ferromagnetic transition temperature T c . Further observations indicate that there is a significant departure of magnetic impurity concentration, x m , vs. T c relation from the linearity as suggested by electronic calculations and experimental estimations near absolute zero temperature.

Anomalous Nernst effect on a magnetically doped topological insulator surface: A Green's function approach

By generalizing the Kubo-Streda formula for calculating electrical conductivities to the thermoelectric coefficients, we theoretically study the anomalous Nernst effect (ANE) on the surface of a topological insulator induced by a finite concentration of magnetic impurities. The ANE is found to be modulated by the impurity scattering and thermal fluctuations, simultaneously, and so exhibits rich structures in the energy space. While the anomalous Hall conductivity is half-integer quantized with the Fermi level across the magnetic-impurity-induced gap, the anomalous Nernst signal (ANS) is fully suppressed and the thermopower is linear-dependent on the Fermi energy. Around the magnetic-impurity-induced localized levels, the ANS and thermopower are resonant enhanced. The suppression and enhancement of the thermoelectric coefficients will compete with each other as the magnetic impurity potential increases continually. More interestingly, when a finite charge potential is included, the resonant peaks of the ANS and thermopower will be renormalized, making the signs of the ANS and thermopower tunable by the strength of the charge potential.

Quasiparticle scattering in 3 MeV proton irradiated BaFe2(As0.67P0.33)2

Effects of 3 MeV ${\mathrm{H}}^{+}$ (proton) irradiation on superconductivity in ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33})}_{2}$ was investigated through in situ resistivity measurements and magnetotransport measurements. The suppression of ${T}_{\mathrm{c}}$ by ${\mathrm{H}}^{+}$ irradiation is more moderate in comparison with electron irradiation, yet more effective than that of irradiation of heavier ${\mathrm{He}}^{+}$ ions. Surprisingly, the $B$-linear magnetoresistance (MR) in the pristine crystal crosses over to an anomalous negative MR after the irradiation. We discuss that the negative MR is most likely to emanate from sparse magnetic impurities. The appearance of a paramagnetic signal in magnetization measurements supports the existence of magnetic impurities in the sample. Although the estimated concentration of local magnetic moments suggests that the concentration of magnetic impurities is dilute, thereby signifying that irradiation-induced defects are dominantly nonmagnetic, the role of magnetic scattering cannot be completely neglected. Hence, ${\mathrm{H}}^{+}$ irradiation in this system contains magnetic-scattering centers, which are pair breakers for both the ${s}_{++}$- and the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-gap structures, indicating that the suppression of ${T}_{\mathrm{c}}$ is a consequence of both magnetic and nonmagnetic scatterings. This result opens up the possibility for the scenario of the slow but steady suppression of ${T}_{\mathrm{c}}$ emanating from the ${s}_{++}$-gap symmetry with pair breaking due to dilute magnetic scatterers.

Orientational Instability and Hysteresis Phenomena in a Ferronematic Liquid Crystal in a Magnetic Field

A magnetic-field-induced orientational structure in a ferronematic (FN) liquid crystal (LC) layer is studied within the continuum theory. The rotation angles of the director and the magnetization and the concentration of magnetic impurity corresponding to a supertwisted orientational structure of the suspension are calculated. It is shown that the deviation angle of the director from the direction of the external field has the hysteresis region in which the orientational structure of the FN changes stepwise from a state with a positive twist of the director to a state with a negative twist. A value of the magnetic field strength is found above which orientational bistability regions arise. It is shown that orientational instability under the rotation of the field most clearly manifests itself in FNs with strong anchoring of particles to the LC matrix. It is established that the effect of magnetic segregation responsible for the redistribution of magnetic particles in the layer leads to the expansion of the hysteresis region and to a decrease in the field at which orientational instability arises. It is shown that, in FNs with soft anchoring between magnetic and LC subsystems, there exist several response modes to a quasistatic rotation of the magnetic field.