Dilute Magnetic Systems Research Articles

Contribution of magnons created in volume and within the surface in low-dimensional dilute magnetic semiconductor: eTBA Calculation for [formula omitted

The magnetic properties in low-dimensional dilute magnetic semiconductor system (Sn1−xCoxO2) are studied in the framework of extended Tight Binding Approximation (eTBA). According to the parameters of the solid, we established the Schrödinger equation to a single spin-wave state constructed as a sum of Bloch of atomic-type individual states of creations of spin deviations on a given site. We developed a tight binding interatomic potential that essentially represents the combined effect of direct exchange J and the magneto-crystalline anisotropy Δ. We demonstrated the existence of two kinds of magnons created in volume and in the surface of the Sn1−xCoxO2/SiO2 multilayers. We calculated the excitation spectrum of these two kinds of magnons. The magnetization per spin is also obtained. The calculated values are in very satisfactory agreement with the experimental measurements.

Recent progress of MnBi2Te4 epitaxial thin films as a platform for realising the quantum anomalous Hall effect.

Since the first realisation of the quantum anomalous Hall effect (QAHE) in a dilute magnetic-doped topological insulator thin film in 2013, the quantisation temperature has been limited to less than 1 K due to magnetic disorder in dilute magnetic systems. With magnetic moments ordered into the crystal lattice, the intrinsic magnetic topological insulator MnBi2Te4 has the potential to eliminate or significantly reduce magnetic disorder and improve the quantisation temperature. Surprisingly, to date, the QAHE has yet to be observed in molecular beam epitaxy (MBE)-grown MnBi2Te4 thin films at zero magnetic field, and what leads to the difficulty in quantisation is still an active research area. Although bulk MnBi2Te4 and exfoliated flakes have been well studied, revealing both the QAHE and axion insulator phases, experimental progress on MBE thin films has been slower. Understanding how the breakdown of the QAHE occurs in MnBi2Te4 thin films and finding solutions that will enable mass-produced millimetre-size QAHE devices operating at elevated temperatures are required. In this mini-review, we will summarise recent studies on the electronic and magnetic properties of MBE MnBi2Te4 thin films and discuss mechanisms that could explain the failure of the QAHE from the aspects of defects, electronic structure, magnetic order, and consequences of their delicate interplay. Finally, we propose several strategies for realising the QAHE at elevated temperatures in MnBi2Te4 thin films.

Влияние немагнитного беспорядка на фазовые переходы первого рода

Using the Monte Carlo method, weakly dilute magnetic systems described by two-dimensional five-component Potts model, which in the pure state undergoes a phase transition first order. Numerical studies have revealed that an insignificant quenched disorder, realized in the form of non-magnetic impurities, can change the order of phase transition in magnetic systems described by the two-dimensional five-component Potts model on a square lattice, for which a phase transition of the first order is observed in a homogeneous state. It is shown that such a phase change the transition is due to the fact that non-magnetic disorder prevents the coexistence of local phases characteristic of a phase transition of the first order at T = Tl .

Influence of polarization, multi-valences and multi-coordination of cobalt: Third-order nonlinear, dielectric and Faraday properties of Bi0.5Co1.5S3/high optical basicity glass

In this study, we have successfully grown Co doped Bi0.5Co1.5S3 nanocrystals inside heavy metal oxide diamagnetic 42PbO-48TeO2-10B2O3 glass whose morphology, structure and properties were characterized using various techniques. The optimum parameters for Bi0.5Co1.5S3 nanocrystals formation is 380°C for 10hours. Glasses having different amounts of Bi0.5Co1.5S3presented colors from light yellow to green and pure blue due to the different optical absorption of Co2+ ions in octahedral and tetrahedral coordination units. X-ray diffraction pattern revealed the formation of orthorhombic Bi2S3 nanocrystals in glass and Raman spectra demonstrated modification on structure such as TeO4 → TeO3 and BO → NBO conversion. The red shift in transmission cutoff edge of glasses showed experimental evidence of quantum confinement effect. Due to the high optical basicity system, X-ray photoelectron spectra studies confirmed the co-existence of Co2+, Co3+ and Co4+ which presented characteristic absorption in ultraviolet-visible spectra. The introduction of Bi0.5Co1.5S3 greatly increased the polarizability of glass, resulting in significant improvement on third-order nonlinearity, AC conductivity and dielectric constant. 1% Bi0.5Co1.5S3 doped glass presents giant Verdet constant of 62 rad/T.m at 633nm, promising nonlinear refractive index (5.662 × 10−11 esu) and large dielectric constant (78). This result proved that the synthesis of multivalence states of Co doped Bi2S3 in glass is an efficient method to improve the optical, dielectric and magneto optical performance of glass. Such dilute magnetic semiconductor nanocrystals-glass system has great potential applications in nonlinear, dielectric and magneto optical current sensors and isolators devices.

Effect of Fe Substitution on Dielectric, Electrical and Photocatalytic Behavior of ZnO Nanoparticles

Aims: To develop a simple and cost effective synthetic strategy for the preparation of Fe substituted ZnO nanoparticles. Background: The optoelectronic, electrical, dielectric, optical and magnetic properties of nanocrystalline transition metal substituted ZnO are being explored worldwide for a variety of applications in optoelectronic devices, solar cells, transparent thin film transistors, ultraviolet photodetector, piezoelectric devices, light emitting diodes as well as in the biomedical field. Fe substituted ZnO nanoparticles are being looked upon as promising material in dilute magnetic semiconductor system. Objective: To establish chemical identity and purity in order to ensure the complete substitution of Fe3+ in ZnO lattice and study the effect of Fe substitution on dielectric, electrical and photocatalytic behavior of ZnO nanoparticles. Methods: The nearly spherical ZnO and Fe substituted ZnO nanoparticles were synthesized at a low temperature via solution combustion synthesis employing metal nitrate and sucrose. Results: The powder X-ray diffraction measurement has revealed the monophasic character and complete substitution of Fe in the wurtzitic ZnO lattice. The lattice constants and aspect ratio of Fe substituted ZnO were nearly constant and comparable to that of pristine ZnO. The average crystallite size was found to decrease with increasing Fe substitution. SEM images revealed porous spongy network like morphology. TEM measurements revealed a nearly spherical particle with narrow size distribution between 10 nm - 25 nm. Conclusion: The dielectric constant and dielectric loss decrease upto x = 0.04 and increases with further increase in Fe concentration. The lower value of dielectric loss in the higher frequency region indicates the less lossy nature of Fe substituted samples. AC conductivity behaviour suggests small polaron hopping type of conduction mechanism. The RT DC resistivity was found to decrease with increasing Fe substitution. Pristine ZnO displayed very high degradation efficiency for photodegradation of MB dye. The photodegradation efficiency was found to decrease considerably with increasing Fe substitution.

Defect induced room-temperature ferromagnetism and enhanced photocatalytic activity in Ni-doped ZnO synthesized by electrodeposition**Project supported by the UGC-DAE, Consortium for Scientific Research, Indore through its CRS project bearing No. CSR-IC/MSRSR-12/CRS-220/2017-18/1301.

Zn0.90Ni0.10O nanoparticles have been synthesized by single-bath two-electrode electrodeposition at constant voltage. X-ray diffraction, UV vis and photoluminescence studies reveal that a single-phase polycrystalline hcp wurtzite crystal structure of ZnO is evolved. The material consists of a large number of defects such as oxygen vacancy (Ov) and zinc interstitial (Zi). The magnetization study reveals that the sample exhibits room-temperature global ferromagnetism and the ferromagnetic ordering seems to be defect induced via bound magnetic polaron mechanism, and double exchange is also expected to have played role. Interesting optoelectronic properties have been found in the synthesized sample and the material seems to be a potential candidate to be used as a UV sensor. Such a transition metal doped ZnO based dilute magnetic semiconducting system exhibiting room-temperature ferromagnetism is likely to be first of its kind in the sense that such materials have not yet been reported to be synthesized by the simple method of electrodeposition to the best of our knowledge on the basis of ample literature review.

Two-dimensional ordering and collective magnetic excitations in the dilute ferromagnetic topological insulator (Bi0.95Mn0.05)2Te3

Employing elastic and inelastic neutron scattering (INS) techniques, we report on detailed microscopic properties of the ferromagnetism in he magnetic topological insulator (Bi$_{0.95}$Mn$_{0.05}$)$_{2}$Te$_{3}$. Neutron diffraction of polycrystalline samples show the ferromagnetic (FM) ordering is long-range within the basal plane, and mainly 2D in character with short-range correlations between layers below $T_{\mathrm{C}} \approx 13$ K. Despite the random distribution of the dliute Mn atoms, we find that the 2D-like magnetic peaks are commensurate with the chemical structure, and the absence of (00L) magnetic peaks denote that the Mn$^{2+}$ magnetic moments are normal to the basal planes. Surprisingly, we observed collective magnetic excitations, in this dilute magnetic system. Despite the dilute nature, the excitations are typical of quasi-2D FM systems, albeit are severely broadened at short wavelengths, likely due to the random spatial distribution of Mn atoms in the Bi planes. Detailed analysis of the INS provide energy scales of the exchange couplings and the single ion anisotropy.

Thermoelectric and magnetic properties of rare earth borides: Boron cluster and layered compounds

Several interesting physical properties which are closely related to the interesting crystal structures of rare earth borides will be reviewed. Experimentally it has been indicated that the B12 icosahedral cluster enhances magnetic interaction and mediates it in dilute magnetic systems. There is interest in the potential of borides as high-temperature thermoelectric materials for use in topping cycles for power plants and other applications. Boron icosahedral compounds generally exhibit large Seebeck coefficients. They also tend to possess relatively small thermal conductivities despite the typically strong covalent bonding and high sound velocities of the materials. The paper reviews possible mechanisms proposed to lower the lattice thermal conductivity in borides, p–n controllable compounds, and several borides recently found with enhanced thermoelectric properties.

Theoretical Investigation of the Electronic Structure and Magnetic Properties in Ferromagnetic Rock-Salt and Zinc Blende Structures of 3d (V)-Doped MgS

In this work, we studied the properties of vanadium doped binary MgS compound Mg1−xVxS (x = 0.125, 0.25, 0.50 and 0.75) in both ferromagnetic rock-salt and zinc-blende structures. The studied properties are structural, electronic and magnetic. The objective of this study is to explore the new dilute magnetic semiconductor systems. We have used two approximations to simulate these properties: Wu–Cohen generalized gradient approximation for structural properties, and the modified Becke–Johnson potential combined with the local density approximation for electronic and magnetic properties. The general context in which the calculations are done is based on the formalism of the spin-polarized density functional theory and the full-potential linear-augmented plane wave method. Among the various compounds studied, we have identified a ferromagnetic semiconducting behavior in the rock-salt structure. In the zinc-blende structure, we have one metallic candidate consisting of 75% V-doping of the Mg-sublattice, and all other compounds have a half-metallic ferromagnetic behavior. We conclude that the Mg1−xVxS compounds, for x = 0.125, 0.25 and 0.50, in the zinc-blende structure are a diluted magnetic semi-conductors materials. To see the effects of the exchange splitting process, the exchange constants N0α and N0β are calculated. For each concentration x, we have found the values of the total magnetic moment equal to 3 μβ excepted for the case of Mg0.25 V0.75S compound in the zinc-blende structure. The value of the total magnetic moment is due principally to V magnetic atom and to other both nonmagnetic atoms Mg and S, which show small local magnetic moments localized on their sites. The presence of ferromagnetic order in this type of compound can be explained by the p-d hybridization phenomena.

Applications of low temperature calorimetry in material research

Low temperature calorimetry is an experimental method of heat capacity measurements, and heat capacity is one of the most important and fundamental thermodynamic properties of substances. The heat capacity can provide an average evaluation of the thermal property of a sample since it is a bulk property of substances. In the other hand, the condensed states of substances could be mainly controlled by the molecular motions, intermolecular interactions, and interplay among molecular structures. The physical property reflected in a material may be closely related to the energy changes in these three factors, which can be directly observed in a heat capacity curve. Therefore, low temperature calorimetry has been used not only to obtain heat capacity, entropy, enthalpy and Gibbs free energy, but also to investigate and understand lattice vibrations, metals, superconductivity, electronic and nuclear magnetism, dilute magnetic systems and structural transitions. In this review, we have presented the concept of low temperature calorimetry and its applications in the related field of material researches, such as nano-materials, magnetic materials, ferroelectric materials, phase change materials and other materials.

Low temperature calorimetry and its application in material research

Low temperature calorimetry is an experimental method of specific-heat measurement of condensed matters, and the specific heat is one of the most important and fundamental thermodynamic properties. There are mainly four methods of specific heat measurement, including adiabatic, relaxation, AC and continuing heating calorimetry. The adiabatic calorimetry is the most accurate and reliable in the specific heat measurement, since it is a static method that the sample can reach a thermal equilibrium during the measurement. However, there is no commercial adiabatic calorimeter yet because of complicated instrumentation, time consuming experimental procedures, and large sample amount requirements. The other three are dynamic specific heat measurement method, in which the sample is measured in a heating manner, and consequently a good thermal conductivity is required for a sample to obtain a reliable measurement. The specific heat is a bulk property of substance, and it can provide an average measure of the thermal properties of a sample. It is well known that the condensed states of matters are mainly controlled by the molecular motions, intermolecular interactions, and interplay among molecular structures. The physical property reflected in a material may be closely related to the energy changes in these three factors, which can be directly observed in a specific heat curve. Therefore, specific-heat measurements have been used not only to obtain entropy, enthalpy and Gibbs free energy, but also to investigate and understand lattice vibrations, metals, superconductivity, electronic and nuclear magnetism, dilute magnetic systems and structural transitions.

Dilute ferrimagnetism of ilmenites Mn3FeTiSbO9 and Mn4FeTi2SbO12

Metastable solid solutions (SS) Mn3FeTiSbO9 and Mn4FeTi2SbO12 with the ilmenite structure (space group R\(\bar 3\)) have been prepared by quenching at normal conditions. The compositions of the compounds have been justified using EDX spectroscopy and X-ray diffraction. The magnetic properties of SSs have been analyzed by comparison with ferrimagnetic ilmenite Mn2FeSbO6 (TN = 269 K) as a natural mineral and ceramics obtained at high pressure and high temperature. The solid solutions have been characterized as dilute magnetic systems formed as a result of substitution of nonmagnetic cations Ti4+ for a part of Fe3+ and Sb5+ cations. Mn3FeTiSbO9 is considered as a ferromagnetic with TN = 171 K and Mn4FeTi2SbO12 as a magnetic with the concentration of magnetic clusters below the percolation threshold.

Spin jam induced by quantum fluctuations in a frustrated magnet

Since the discovery of spin glasses in dilute magnetic systems, their study has been largely focused on understanding randomness and defects as the driving mechanism. The same paradigm has also been applied to explain glassy states found in dense frustrated systems. Recently, however, it has been theoretically suggested that different mechanisms, such as quantum fluctuations and topological features, may induce glassy states in defect-free spin systems, far from the conventional dilute limit. Here we report experimental evidence for existence of a glassy state, which we call a spin jam, in the vicinity of the clean limit of a frustrated magnet, which is insensitive to a low concentration of defects. We have studied the effect of impurities on SrCr9pGa12-9pO19 [SCGO(p)], a highly frustrated magnet, in which the magnetic Cr(3+) (s = 3/2) ions form a quasi-2D triangular system of bipyramids. Our experimental data show that as the nonmagnetic Ga(3+) impurity concentration is changed, there are two distinct phases of glassiness: an exotic glassy state, which we call a spin jam, for the high magnetic concentration region (p > 0.8) and a cluster spin glass for lower magnetic concentration (p < 0.8). This observation indicates that a spin jam is a unique vantage point from which the class of glassy states of dense frustrated magnets can be understood.

Quantification of the magnetic exchange via element-selective high-field magnetometry: Co-doped ZnO epitaxial films

We have measured the element specific $M(H)$ curves for 5$%$, 10$%$, and 15$%$ Co-doped ZnO epitaxial films with high crystalline perfection using x-ray magnetic circular dichroism (XMCD). The XMCD$(H)$ curves do not saturate up to 17 T directly evidencing antiferromagnetic exchange between neighboring Co dopant atoms. Angular-dependent XMCD$(H)$ measurements in combination with theoretical calculations based on a well-established model Hamiltonian allow to determine both the next-neighbor exchange $J$ as well as the single ion anisotropy $D$ quantitatively. While for 5$%$ and 10$%$ Co doping, $J=15$ K and $D=3$ K, for 15$%$ Co doping, they are reduced to 10 and 2 K, respectively. High-field, element selective magnetometry based on XMCD is thus shown to be a suitable technique to determine microscopic magnetic parameters even in thin epitaxial films of dilute magnetic systems.

IrSr2Sm1.15Ce0.85Cu2.175O10: A reentrant spin-glass material

A new iridium containing layered cuprate material, IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10, has been synthesized by conventional ambient-pressure solid-state techniques. The material's structure has been fully characterized by Rietveld refinement of high resolution synchrotron X-ray diffraction data; tilts and rotations of the IrO_6 octahedra are observed as a result of a bond mismatch between in-plane Ir-O and Cu-O bond lengths. DC-susceptibility measurements evidence a complex set of magnetic transitions upon cooling that are characteristic of a reentrant spin-glass ground-state. The glassy character of the lowest temperature, Tg=10 K, transition is further confirmed by AC-susceptibility measurements, showing a characteristic frequency dependence that can be well fitted by the Vogel-Fulcher law and yields a value of \Delta_(T_f)/[T_f \Delta log({\omega})] =0.015(1), typical of dilute magnetic systems. Electronic transport measurements show the material to be semiconducting at all temperatures with no transition to a superconducting state. Negative magnetoresistance is observed when the material is cooled below 25 K, and the magnitude of this magnetoresistance is seen to increase upon cooling to a value of MR = -9 % at 8 K.

Spin-glass-like behavior of CaNi1−xMnxGe

We report on the structural evolution and the magnetic properties of a CaNi${}_{1\ensuremath{-}x}$Mn${}_{x}$Ge system that combines a Pauli paramagnetic matrix CaNiGe with a magnetic substituent Mn. The results indicate that the system crystallizes into a single-phase CeFeSi-type structure for $x$ \ensuremath{\le} 0.1 and $x$ \ensuremath{\ge} 0.95. For 0.05 \ensuremath{\le} $x$ \ensuremath{\le} 0.1, DC and AC magnetization measurements confirm that the system undergoes a ferromagnetic ordering at around 130 K followed by a magnetic glass transition at the susceptibility maximum near 80 K. AC measurements for the sample $x$ $=$ 0.05 reveal a frequency (\ensuremath{\nu}) dependence factor \ensuremath{\Delta}${T}_{\mathrm{f}}$/[${T}_{\mathrm{f}}$(\ensuremath{\Delta} log\ensuremath{\nu})] of 0.006 and a field dependence of ${T}_{\mathrm{f}}$ $=$ $a$ $+$ $b{H}^{1.28}$ for the glass-transition temperature ${T}_{\mathrm{f}}$. The observed spin-glass-like magnetic behavior for this dilute magnetic system of Mn-doped CaNiGe is interpreted in terms of the coexistence of the magnetic glass state and ferromagnetic clusters.

Defect-induced magnetism: Test of dilute magnetism in Fe-doped hexagonal BaTiO3single crystals

Single crystalline Fe-doped hexagonal BaTiO${}_{3}$ samples with varying oxygen content are created by specifically intended post-growth annealing treatments, in order to check the influence of defects on the unusual high temperature ferromagnetism observed in this system. The various defects have been shown to play a crucial role in dilute magnetic systems and therefore, it is important to carry out this check for the Fe-doped BaTiO${}_{3}$ system also, in which unusual ferromagnetism was reported even in its bulk single crystalline form. The x-ray diffraction and dielectric studies carried out here have confirmed that the Fe doping of Ti is intrinsic, while the high resolution transmission electron microscopy (HRTEM) and x-ray photoemission spectroscopy (XPS) studies proved the absence of unwanted magnetic metal clusters in the sample. The transport studies show that the oxygen concentrations could be varied substantially by the thermal treatments. Finally, magnetization measurements on the samples demonstrated that ferromagnetism is stronger in samples with higher oxygen deficiency, which could interestingly be retreated under high oxygen atmosphere and reversibly be taken back to a lower magnetic state. The vacancy-induced ferromagnetism is further confirmed by EPR measurements, which is consistent with earlier studies and, consequently, put the doped BaTiO${}_{3}$ in the list of true dilute magnetic oxide (DMO) systems.

Analysis of Ce‐ and Yb‐Doped TAGS‐85 Materials with Enhanced Thermoelectric Figure of Merit

AbstractDoping of TAGS‐85 with 1 at% Ce or Yb forms a dilute magnetic semiconductor system with non‐interacting localized magnetic moments that obey the Curie law. X‐ray diffraction patterns and slight broadening in125Te NMR, attributed to paramagnetic effects, suggest that Ce and Yb atoms are incorporated into the lattice.125Te NMR spin‐lattice relaxation and Hall effect show similar hole concentrations of ≈1021cm−3. At 700 K, the electric conductivity of the Ce‐ and Yb‐doped samples is similar to that of neat TAGS‐85, while the thermal conductivity and the Seebeck coefficient are larger by 6% and 16%, respectively. Possible mechanisms responsible for the observed increase in thermopower may include i) formation of resonance states near the Fermi level and ii) carrier scattering by lattice distortions and/or by paramagnetic ions. Due to the increase in the Seebeck coefficient up to 205 μV K−1, the thermoelectric power factor of Ce‐ and Yb‐doped samples reaches 36 μW cm−1K−2, which is larger than that measured for neat TAGS‐85, 27 μW cm−1K−2. The increase in the Seebeck coefficient overcomes the increase in the thermal conductivity, resulting in a total increase of the figure of merit by ≈25% at 700 K compared to that observed for neat TAGS‐85.

Magnetic Field Enhancement of the Hall Effect in Dilute Magnetic System La1 - xCexB6(x ≤ 0.1)

Magnetic Field Enhancement of the Hall Effect in Dilute Magnetic System La_{1 - x}Ce_xB₆(x ≤ 0.1)

Investigation of annealing-induced oxygen vacancies in the Co-doped ZnO system by CoK-edge XANES spectroscopy

To clarify the mechanism of the observed room-temperature ferromagnetism (RTF), many studies have been focused on dilute magnetic semiconductor systems. Several investigations have demonstrated that oxygen vacancies play a significant role in mediating the RTF behavior so that much effort has been devoted to confirm their presence. In this investigation, X-ray absorption spectroscopy was combined with ab initio calculations of the electronic structure of Co and Zn in the Zn(0.9)Co(0.1)O system before and after annealing, which has been recognized as an effective method of originating oxygen vacancies. A feature at about 20 eV after the rising edge of the Co K-edge XANES that disappears after annealing has been associated with the presence of an oxygen vacancy located in the second shell surrounding the Co atom. Moreover, Zn K-edge XANES spectra point out that this oxygen vacancy affects the electronic structure near the Fermi level, in agreement with density functional theory calculations.