Formation Of Magnetic Polarons Research Articles

Ab initio calculations on magnetic and optical characteristics of pure ZnO and Mn(II)-doped ZnO monolayers with and without neutral charged vacancies defects

At high Curie temperatures, diluted magnetic semiconductors exhibit induced ferromagnetism and spin-dependent interactions, making them useful in magneto-electronic devices. In spite of the fact that ferromagnetism and optical emission dynamics are characterized by intricate microstructural and compositional features, the origins of these phenomena are not yet completely understood. Here, we used first principles calculations to study how vacancy defects affect the electrical, optical, and magnetic properties of pure ZnO and Mn@ZnO monolayers. In the pristine monolayer, the Zn vacancy produces magnetism with a total magnetic moment of 2 μB, but the oxygen vacancy does not. The magnetic semiconducting characteristic of the Mn substitution at the Zn site is shown by a total magnetization of 5 μB. The magnetic ground state of the ZnO monolayer doped with Mn is significantly influenced by the presence of native defects. More specifically, Zn vacancies are essential for stabilizing the FM states due to the bound magnetic polarons (BMPs) formation, but O vacancies have no influence on the magnetic ground state of the Mn(II)@ZnO monolayer. Based on the optical study, we discovered that Zn and O vacancies in the ZnO ML produce optical absorption peaks at 1.92 eV and 2.90 eV, respectively. The substitution of Mn(II) at Zn site not only increases the band gap of the ZnO monolayer but also produces d-d transition bands at lower energy side of fundamental energy gap peak. The Zn vacancies in the Mn(II)@ZnO ML produce an infrared absorption band around 1.13 eV, which could be due to the formation of the BMP, and enhance the optical absorption efficiency. The ZnO ML’s energy gap and Mn ion’s d-d transition bands exhibit a blueshift in AFM systems and a redshift in the FM systems. In far configuration, the Mn(II)@ZnO monolayer show no shift in the fundamental bandgap and intra-band transition of the Mn(II) spins due to the Mn(II) ion’s paramagnetic nature.

Valence instability and collapse of ferromagnetism in EuB[formula omitted] at high pressures

Despite the simplicity of their cubic crystal lattice, rare-earth hexaborides display complex physical properties including a (long debated) onset of metallization via magnetic polaron formation at Tc1≈ 15 K preceding ferromagnetic ordering at Tc2≈ 12 K. In this work, we used applied pressure to tune the interplay between electronic structure and magnetism in EuB6. We probed the magnetism, valence, and structure of EuB6 under quasi-hydrostatic pressures up to 30 GPa using X-ray techniques. Our findings show evidence for collapse of ferromagnetism above 20 GPa following a monotonic increase of mean Eu valence. While X-ray diffraction measurements in the paramagnetic state at room temperature show that the lattice retains cubic symmetry, a measurable quadrupole interaction seen by time-domain synchrotron Mössbauer spectroscopy suggests a lowering of symmetry associated with magnetic ordering, becoming more prominent across the magnetic transition. The interplay between conduction band electron count and magnetism observed under applied pressure in EuB6 opens possibilities for fine-tuning metallization and magnetic properties of similar Eu-based semi-metal systems.

Magnetic field-enhanced oxygen reduction reaction for electrochemical hydrogen peroxide production with different cerium oxide nanostructures

We investigated cerium oxide nanoparticles of various morphologies (nanosheets, nanocubes, and nanoparticles) supported on carbon Vulcan XC-72 for the two-electron oxygen reduction reaction (ORR). A continuous magnetic field (2000 Oe) was used for the first time in the literature. The best results were for 5 % (w/w) CeO2 for all three different morphologies, more than doubling the ring current, enhancing the hydrogen peroxide selectivity from 51 % (Vulcan XC-72) to 84–89 %, and modifying the onset potential to lesser negative values. The presence of the magnetic field led to even higher ring currents with 5 % (w/w) CeO2, H2O2 selectivity from 54 % (Vulcan XC-72) to 88–96 % and changing even more the onset potential. Those results were correlated with the Zeeman effect, the Lorentz force, generating magnetohydrodynamic effects, the Kelvin force, and the formation of Bound Magnetic Polarons. This pioneering research introduces an innovative approach, highlighting the potential of an external continuous magnetic field.

Exploring kinetically induced bound states in triangular lattices with ultracold atoms: Spectroscopic approach

Quantum simulations with ultracold fermions in triangular optical lattices have recently emerged as a new platform for studying magnetism in frustrated systems. Experimental realizations of the Fermi Hubbard model revealed striking contrast between magnetism in bipartite and triangular lattices. In bipartite lattices magnetism is strongest at half filling, and doped charge carriers tend to suppress magnetic correlations. In triangular-type lattices for large U/t and t>0, antiferromagnetism (ferromagnetism) gets enhanced by doping away from n=1 with holes (doublons) because kinetic energy of dopants can be lowered through developing magnetic correlations, corresponding to formation of magnetic polarons. Snapshots of many-body states obtained with quantum gas microscopes demonstrated existence of magnetic polarons by revealing the magnetic correlations around dopants at temperatures that considerably exceed superexchange energy scale. In this paper we discuss theoretically that additional insight into properties of magnetic polarons in triangular lattices can be achieved using spectroscopic experiments with ultracold atoms. We consider starting from a spin polarized state with small hole doping and applying a two-photon Raman photoexcitation, which transfers atoms into a different spin state. We show that such magnon injection spectra exhibit a separate peak corresponding to formation of a bound state between a hole and a magnon. This polaron peak is separated from the simple magnon spectrum by energy proportional to single particle tunneling and can be easily resolved with currently available experimental techniques. For some momentum transfer there is an additional peak corresponding to photoexciting a bound state between two holes and a magnon. We point out that in two component Bose mixtures in triangular lattices one can also create dynamical magnetic polarons, with one hole and one magnon forming a repulsive bound state.

Enhanced room-temperature magnetic field effect on spin-polarized excitons of cesium lead bromine-magnetite nanocomposites

The generation and manipulation of spin-polarized electrons are the basis of spintronic applications. Spin injection from a ferromagnetic surface or heterojunction to a semiconductor adjacent layer is often used to create spin-polarized carriers; however, for halide perovskites, due to the mismatch of lattice parameters with most ferromagnets at room temperature, this spin injection is challenging. In this study, we synthesize all-inorganic perovskite CsPbBr3 surrounded by Fe3O4 magnetic nanoparticles. It is found that this nanocomposite shows both magnetization and magnetic field-induced circularly polarized photoluminescence at room temperature. Specifically, with the attachment of Fe3O4 nanoparticles closely on the surface of CsPbBr3 nanocrystals, both the degree of circular polarization and g-factor enhanced by 3.5 times compared with that of pure CsPbBr3. The phenomenon should be due to the formation of exciton magnetic polaron through the coupling of the magnetic aligned nanoparticles with the excitonic state of the host semiconductor in the external magnetic field. The effective spin injection provides a method of controlling the excitonic spin polarization of all-inorganic halide perovskites for spintronic applications.

Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets

Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing. Their magnetisation dynamics are determined by the interplay between electronic and vibrational degrees of freedom, which can couple coherently, leading to complex vibronic dynamics. Building on an ab initio description of the electronic and vibrational Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy magnetic degrees of freedom in monometallic single-molecule magnets. Describing their low-temperature magnetism in terms of magnetic polarons, we are able to quantify the vibronic contribution to the quantum tunnelling of the magnetisation, a process that is commonly assumed to be independent of spin-phonon coupling. We find that the formation of magnetic polarons lowers the tunnelling probability in both amorphous and crystalline systems by stabilising the low-lying spin states. This work, thus, shows that spin-phonon coupling subtly influences magnetic relaxation in single-molecule magnets even at extremely low temperatures where no vibrational excitations are present.

Nonequilibrium Hole Dynamics in Antiferromagnets: Damped Strings and Polarons

We develop a nonperturbative theory for hole dynamics in antiferromagnetic spin lattices, as described by the t-J model. This is achieved by generalizing the self-consistent Born approximation to nonequilibrium systems, making it possible to calculate the full time-dependent many-body wave function. Our approach reveals three distinct dynamical regimes, ultimately leading to the formation of magnetic polarons. Following the initial ballistic stage of the hole dynamics, coherent formation of string excitations gives rise to characteristic oscillations in the hole density. Their damping eventually leaves behind magnetic polarons that undergo ballistic motion with a greatly reduced velocity. The developed theory provides a rigorous framework for understanding nonequilibrium physics of defects in quantum magnets and quantitatively explains recent observations from cold-atom quantum simulations in the strong coupling regime.

Energy and spin relaxation of excitons in CdMnTe/CdMgTe quantum wells

The energy and spin relaxation of excitons and trions is studied in the (Cd, Mn)Te/(Cd,Mg)Te quantum wells containing hole magnetic polarons by means of polarized photoluminescence technique. It was found that under circularly polarized photoexcitation with an energy higher than the ground state energy of the exciton, photoluminescence is also polarized. This fact points to the presence of the spin memory of the excitons in these systems. It is suggested that the long spin memory is due to the magnetic polaron formation stabilizing the spin of the exciton. Under quasiresonant excitation of the trion states an inverted sign of circular polarization degree is observed in the radiation spectrum. Such inversion of the polarization degree requires the special mechanism of the spin relaxation, associated with the spin flip of an electron in the trion magnetic polaron state. Application of the external magnetic field in the Voigt geometry leads to the depolarization of the excitonic radiation. The cause of this depolarization is the destabilization of the magnetic polaron and the shortening of the spin relaxation time of the exciton.

Inducing spin-order with an impurity: phase diagram of the magnetic Bose polaron

We investigate the formation of magnetic Bose polaron, an impurity atom dressed by spin-wave excitations, in a one-dimensional spinor Bose gas. Within an effective potential model, the impurity is strongly confined by the host excitations which can even overcome the impurity-medium repulsion leading to a self-localized quasi-particle state. The phase diagram of the attractive and self-bound repulsive magnetic polaron, repulsive non-magnetic (Fröhlich-type) polaron and impurity-medium phase-separation regimes is explored with respect to the Rabi-coupling between the spin components, spin–spin interactions and impurity-medium coupling. The residue of such magnetic polarons decreases substantially in both strong attractive and repulsive branches with strong impurity-spin interactions, illustrating significant dressing of the impurity. The impurity can be used to probe and maneuver the spin polarization of the magnetic medium while suppressing ferromagnetic spin–spin correlations. It is shown that mean-field theory fails as the spinor gas approaches immiscibility since the generated spin-wave excitations are prominent. Our findings illustrate that impurities can be utilized to generate controllable spin–spin correlations and magnetic polaron states which can be realized with current cold atom setups.

Microscopic probe of magnetic polarons in antiferromagnetic Eu5In2Sb6

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence for the formation of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provide strong evidence that magnetic polarons grow in size below T' and start influencing the macroscopic properties of the system.

Semimetallic nature of and magnetic polarons in EuB$$_6$$ studied using angle-resolved photoemission spectroscopy

Temperature-dependent angle-resolved photoemission spectroscopy (ARPES) was carried out on single-crystalline EuB6 samples. By measuring ARPES spectra in an extended Brillouin zone, a B 2p hole pocket centered at the X point is clearly observed, thus proving the semimetallic nature of EuB6. Below the Curie temperature TC, ARPES spectra show two B 2p bands of which separation is due to an exchange interaction between local Eu 4f and itinerant B 2p electrons. The exchange splitting becomes smaller as the temperature increases and disappears well above TC. Additionally, a diffuse structure near the Fermi level survives just above TC. Such behavior is well described by Monte Carlo simulations of a Kondo lattice model, thus supporting the formation of magnetic polarons in EuB6, which accounts for the resistivity upturn near above TC when lowering the temperature.

Quantum many-body simulations of the two-dimensional Fermi-Hubbard model in ultracold optical lattices

Understanding quantum many-body states of correlated electrons is one main theme in modern condensed matter physics. Given that the Fermi-Hubbard model, the prototype of correlated electrons, has been recently realized in ultracold optical lattices, it is highly desirable to have controlled numerical methodology to provide precise finite-temperature results upon doping, to directly compare with experiments. Here, we demonstrate the exponential tensor renormalization group (XTRG) algorithm [Phys. Rev. X 8, 031082 (2018)], complemented with independent determinant quantum Monte Carlo (DQMC) offer a powerful combination of tools for this purpose. XTRG provides full and accurate access to the density matrix and thus various spin and charge correlations, down to unprecedented low temperature of few percents of the fermion tunneling energy scale. We observe excellent agreement with ultracold fermion measurements at both half-filling and finite-doping, including the sign-reversal behavior in spin correlations due to formation of magnetic polarons, and the attractive hole-doublon and repulsive hole-hole pairs that are responsible for the peculiar bunching and antibunching behavior of the antimoments.

Tuning the dynamics of Bound Magnetic Polaron in asymmetric Cd1-x1Mnx1Te/Cd1-x2Mnx2Te Semimagnetic Concentric Double Quantum Rings: Through magnetic ion concentration and magnetic field

Rigorous analysis on the formation of Bound Magnetic Polaron (BMP) in Cd1-x1Mnx1Te/Cd1-x2Mnx2Te Semimagnetic Concentric Double Quantum Ring (SCDQR) for various magnetic field and ring dimensions has been made by estimating Spin Polaronic Shift (SPS) using variational technique. The concentration of Mn2+ has been varied in both the ring and barrier material to estimate the shift in the BMP formation from the inner ring to outer ring with respect to the distribution of Mn2+ ions and impurity location. It is found that the SPS of Inner Ring Centered Donor (IRD) and Outer Ring Centered Donor (ORD) are opposite to each other at lower concentrations. Strengthening the magnetic field and increasing the concentration of magnetic ions increases the SPS because of the large magnetic ordering. Thus the concentration of magnetic ions and magnetic field is of paramount importance in determining the BMP formation and its usage in devices.

Substitutional site effects of Cr(II) ions on optical and magnetic properties of 1D CdS semiconductor nanoneedles for optoelectronic and spintronic applications

Spin controlled diluted magnetic semiconductors (DMSs) could bring revolution in the processing speed as well as in enhancing the storage capacity. To achieve its industrial application, several impurities have been doped from the 3d/4f into semiconductors, however, Cr has not yet been doped into CdS 1D nanostructures. Here, in the present study, we report on the Cr-doped CdS 1D nanoneedles synthesized by CVD and studied its optical and magnetic properties. The successful incorporation of Cr-ions into CdS confirmed by EDX and XRD. Along with the increase in Cr-ions concentration, first PL-peak shifted from 514.1 to 558.3 nm associated with the formation of excitonic magnetic polarons (EMPs) and the second peak centered at 993.8 nm may correspond to the aggregation of Cr-ions. For the first time, we tuned CdS emission up to ~ 482 nm covering visible spectral region up to near-infrared (NIR) region. With increasing the Cr-ions concentration up to 6.2%, saturation magnetization (Ms) was significantly enhanced from 0.614 × 10−3 Am2/kg to 0.987 × 10−3 Am2/kg. These findings will promote their future applications in the optoelectronic and spintronic devices.

Defects and dopant alliance towards bound magnetic polarons formation and mixed magnetic characteristics in Fe doped ZnO nanoparticles

Fe doped ZnO powders, Zn1-xFexO (x=0–0.03) confirmed to be single phased using X-ray diffraction (XRD) measurement (step size of 0.01°) revealed a change in oxidation state of Fe with increased fraction of Zinc interstitials, Zni and Oxygen vacancy, Vo defects through Photoelectron spectroscopy and Photoluminescence (PL) studies. The Raman and PL spectra have registered the inclusion of Vo-Zni complex defects also. An ideal position of E2(Low) Raman mode justifies the effectual cationic substitution of Fe species. Inclusion of paramagnetic (PM) component is successfully substantiated through Bound Magnetic Polaron (BMP) model fitting. For x=0.01, a residual PM component is ensured through temperature dependent magnetization, M(T) characteristics as well. A close fit of the Field cooled (FC) branch by the 3-D spin wave model (T=99−300K) points towards the intrinsic nature of ferromagnetism mediated by the effectual substitution of Fe2+ ions and the inclusion of BMPs. At lower temperature (T<99K), a good fit to the Curie-Weiss model (C–W) quantifies an estimated cluster size of the order of 10−3, subtle enough to restrict the introduction of spin glass state, thereby justifying the suitability of these nanoparticles for spintronics applications. Excellent PM properties are achieved for x=0.03 at T=300K.

Photo-Induced Magneto-Optical Kerr Effect in Europium Sulfide EuS

The influence of light exposure on the magneto-optical Kerr effect in the ferromagnetic semiconductor europium sulfide EuS has been studied. It has been experimentally established that when a sample is exposed to the light with a photon energy greater than the band gap, photoinduced magnetization occurs, which is associated with the formation of magnetic polarons with a large magnetic moment of about 3000 μB. The polarons are excited in a narrow temperature range of 12–18 K and form a superparamagnetic ensemble with an average polaron lifetime of 13 μs. A method for observing photo-induced magnetization using optical pump-probe technique has been described.

Magneto-optical effect of (Sm, Co) co-doping in ZnO semiconductor

In this paper, the effect of (Sm, Co) co-doping on the structure, optical and magnetic properties of ZnO nanoparticles was studied. Pure ZnO and (Sm, Co) co-doped nanoparticles, Zn1-2xSmxCoxO with x = 0.00, 0.005, 0.01, 0.02, 0.04 and 0.06, were synthesized by co-precipitation technique. The X-rays diffraction (XRD) scans were performed with 2θ varies from 20° to 80°.The XRD patterns of all samples confirmed the formation of wurtzite-type structure of ZnO with the formation of Sm2O3 and Co3O4 as minor secondary phases with x ≥ 0.005. The spherical morphology of pure ZnO is transformed to nanorods as the concentration of Sm3+ and Co2+ increases. The diameter and length of the nanorods were nearly 35.1–66.5 nm and 136.2–596 nm, respectively. UV–vis spectra were used for the calculation of the energy band gap (Eg) by different methods. Eg is found to increase with increasing (Sm, Co) doping up to x = 0.02. Energy band gap and Urbach energy are observed to have a reverse trend. Magnetic properties were investigated by the vibrating sample magnetometer. M − H hysteresis analysis reveals that for small doping, the samples have a ferromagnetic signal combined with diamagnetic contribution. The enhancement in ferromagnetism behaviour for 0.01 < x < 0.02 was justified by the formation of Bound Magnetic Polarons (BMPs) as it significantly enhances with the doping of Sm and Co by stabilizing Zn vacancy and other magnetic defects. Then the ferromagnetism is reduced for the doped samples up to x = 0.06, and an antiferromagnetic alignment is displayed.

Increased bound magnetic polaron formation in the dilute magnetic semiconductor Zn1-xNixO

Ni doped ZnO nanoparticles (NP’s) was fabricated by hydrothermal method. XRD result investigated that all peaks could be indexed to hexagonal wurtzite structure of ZnO NP’s with no impurity phases. The values of saturation magnetization when expressed in terms of emu/g increases as the doping level increases but decreases when MS is expressed in terms of µB/Ni ion. PL spectra exhibit an increased blue shift as more Ni doping. The blue shifts of the peak in the visible light spectrum of the PL emissions of the Ni doped ZnO NP’s also indicate that more singly ionized oxygen vacancies are being created. The combination of their being more polarized d electrons available and more singly ionized oxygen vacancies around which the d electrons of the Ni2+ ions can localize to form the BMP’s would lead to an increase in the antiferromagnetic contribution to the net exchange interaction between Ni ions to increase.

Circular Polarization Dynamics during Magnetic Polaron Formation in Type-II Magnetic Quantum Dots

We used time-resolved photoluminescence spectroscopy to study the circular polarization dynamics of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots in the presence of an external magnetic ...

Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10

Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.