Small Spin Research Articles

Enhanced magnetocaloric effect in manganite nanodisks

Manganites are an important class of materials that exhibit a large magnetocaloric effect (MCE). The large MCE is often accompanied by complex magnetic phase transitions associated with electronic phase separation (EPS) in manganites. In previous studies, the effect of EPS on MCE has been examined in chemically synthesized manganite nanoparticles, in which the EPS vanishes when the particle size is smaller than a few tens of nanometers. However, the surface area to volume ratio is high in these small nanoparticles and surface spin disordering can lead to controversial results for MCE measurements. In this study, we used $\mathrm{L}{\mathrm{a}}_{0.425}\mathrm{P}{\mathrm{r}}_{0.2}\mathrm{C}{\mathrm{a}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ nanodisk arrays fabricated from epitaxially grown thin films as model systems to investigate how MCE changes during a clean transition from the EPS state in thin film and 2-\ensuremath{\mu}m-sized disk arrays to a single ferromagnetic state in 500-nm-sized disk arrays. The MCE of the 500-nm-sized disk array is almost twice that of its parent film. Our results present an approach to improve the MCE in manganite systems for future magnetic refrigeration applications.

Electronic and Magnetic Properties of Fe2−xCoxTiSi (x = 0, 0.5, 1, 1.5, 2) Heusler Alloys

The first principles calculations are carried out to understand the half metallic and ferromagnetic properties of Fe2−xCoxTiSi (x = 0, 0.5, 1, 1.5, 2) full Heusler alloys. The stable structure is predicted as L21 structure (Cu2MnAl phase). The lattice constant value of Fe2−xCoxTiSi Heusler compounds increases as the value of x increases. These alloys exhibit half metallic ferromagnetism (except Fe2TiSi) with small spin flip gap at the minority spin state. At high pressure, semiconductor to metallic phase transition is observed in Fe2TiSi while half metallic to metallic phase transition is observed in Fe1.5Co0.5TiSi, Fe1.0 Co1.0TiSi, Fe0.5Co1.5TiSi and Co2TiSi. The spin polarization of these compounds (except Fe2TiSi) is 100%. Hence, these compounds are suitable for spin injection devices in spintronics.

Electrocaloric effect in the two spin-1/2 XXZ Heisenberg edge-shared tetrahedra and spin-1/2 XXZ Heisenberg octahedron with Dzyaloshinskii–Moriya interaction

In the present paper, we consider two species of small spin clusters known as; two spin-1/2 Heisenberg edge-shared tetrahedra and spin-1/2 Heisenberg octahedron with the corresponding Dzyaloshinskii–Moriya terms in a longitudinal magnetic field, then we examine magnetization process and electric polarization of the models as functions of magnetic and electric fields at Low temperature using exact numerical diagonalization. Our exact results are in a good agreement with recent analysis carried out by Strečka and Karľová (2018 AIP Adv. 8 101403); Karľová et al (2017 J. Phys.: Condens. Matter 29 125802). It is demonstrated that the polarization behavior coincides the magnetization curves (including sequential intermediate plateaus), also it reflects the respective stepwise changes of ground-state phase transitions. We find that the polarization has significant effects on the magnetic field dependencies of the magnetization. Furthermore, we investigate other isothermal strategies such as cooling rate, magnetocaloric effect (MCE), as well as, electrocaloric effect (ECE) for both models. Since, new electrocaloric materials with high performance is of great interest and importance in condensed matter physics, here, we report on the ECE of the both aforedescribed spin-1/2 Heisenberg small clusters. We conclude that two spin-1/2 Heisenberg edge-shared tetrahedra is a material with significant reversible temperature change capability under an external electric field compared with the spin-1/2 Heisenberg octahedron, and can be used for cooling/heating process. On the other hand, our exact results obtained by studying the MCE and ECE of the spin-1/2 Heisenberg octahedron prove that this model displays a rich ECE close to the relevant critical points, thus can be considered as a material with rich electrocaloric(magnetocaloric) performance even in the presence of an external electric(magnetic) field. Generally, by comparing both models with each other, we point out that the two spin-1/2 Heisenberg edge-shared tetrahedra is a high performance electrocaloric material rather than spin-1/2 Heisenberg octahedron, while the spin-1/2 Heisenberg octahedron demonstrates greater MCE.

Structural and electronic phase transitions of Co2Te3O8 spiroffite under high pressure

The structural and electronic phase transitions of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{T}{\mathrm{e}}_{3}{\mathrm{O}}_{8}$ spiroffite have been studied with a suite of in situ high-pressure characterization techniques including synchrotron x-ray diffraction, Raman, x-ray emission spectroscopy, UV-vis absorption, and electrical transport measurement. Two pressure-induced phase transitions were observed at about 6.9 and 14.4 GPa. The first transition is attributed to a small spin transition of Co along with discontinuity in unit-cell volume change, while the second one represents a first-order phase transition with a volume collapse of 4.5%. The latter transition is accompanied by the relaxation of distortion in $\mathrm{Co}{\mathrm{O}}_{6}$ octahedron, which enhances the crystal-field strength inhibiting the occurrence of spin transition. What is more, the competition between contributions of electrons and oxygen ion to the overall conductivity is observed and affected by the phase transition under high pressure. This demonstration provides insights into the relationship between the lattice-structural and spin degrees of freedom, and highlights the impact of pressure on the control of structural and electronic states of a given material for optimized functionalities.

Effect of pressure and Hubbard potential on the electronic and magnetic properties of thorium monopnictides ThPn (Pn = N, P, As, Sb, Bi) in respect of crystal field splitting, charge transfer and spin flipping of magnetic moments

Thorium pnictides, besides their fundamental implications, are potential candidates for the emerging clean nuclear energy technology. Hereby, we investigate from first-principles the structural, electronic and magnetic properties of thorium monopnictides ThPn (Pn = N, P, As, Sb, Bi) subjected to high pressure with the Hubbard correction. The total energy is calculated by employing the full-potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method. The structural properties of the compounds subjected to elevated pressure are computed by treating the exchange-correlation energy terms in Kohn-Sham equation (KSE) within Perdew-Burke-Ernzerhof (PBE) corrected generalized gradient approximation (GGA). However, in order to investigate the electronic band structure, density of states and spin magnetic moments of the monopnictides, we employed Hubbard DFT model (LDA + U) at various pressures. In conformation to the earlier studies, the monopnictides show nonmagnetic metallic character having small spin magnetic moments. The external pressure and Hubbard potential play a key role in mechanism of charge transfer, crystal field splitting as well as spin flipping of magnetic moments.

Enhancement of spin current generation in epitaxial α -Ta/CoFeB bilayer

We report the enhancement of the spin Hall effect in a (111)-oriented epitaxial $\ensuremath{\alpha}$-Ta/CoFeB bilayer system, which was previously believed to show the small spin Hall angle (SHA). The SHA of the epitaxial $\ensuremath{\alpha}$-Ta/CoFeB structure estimated by spin Hall magnetoresistance measurement is 0.15, a value that is 1.5 times larger than the conventional amorphous-Ta/CoFeB structure. Effective SHA estimated by harmonic Hall measurement qualitatively supports the SMR result, whereas the strength of the field-like field of epitaxial $\ensuremath{\alpha}$-Ta samples is similar to that of amorphous Ta samples. According to the resistivity dependence on the SHA, the origin of the enhanced SHA in epitaxial $\ensuremath{\alpha}$-Ta is due to the extrinsic contribution to the SHA.

Fourth-order spin correlation function in the extended central spin model

Spin-noise spectroscopy has developed into a very powerful tool to access the electron spin dynamics. While the spin-noise power spectrum in an ensemble of quantum dots in a magnetic field is essentially understood, we argue that the investigation of the higher-order cumulants promises to provide additional information not accessible by the conventional power-noise spectrum. We present a quantum-mechanical approach to the correlation function of the spin-noise power operators at two different frequencies for small spin bath sizes and compare the results with a simulation obtained from the classical spin dynamics for large number of nuclear spins. This bispectrum is defined as a two-dimensional frequency cut in the parameter space of the fourth-order spin correlation function. It reveals information on the influence of the nuclear-electric quadrupolar interactions on the long-time electron spin dynamics dominated by a magnetic field. For large bath sizes and spin lengths the quantum-mechanical spectra converge to those of the classical simulations. The broadening of the bispectrum across the diagonal in the frequency space is a direct measure of the quadrupolar interaction strength. A narrowing is found with increasing magnetic field indicating a suppression of the influence of quadrupolar interactions in favor of the nuclear Zeeman effect.

Small body shapes and spins reveal a prevailing state of maximum topographic stability

Over the past thirty years, spacecraft missions, Earth-based radar experiments, and telescopic observations have revolutionized our knowledge of Main Belt asteroids, near-Earth asteroids, and comet nuclei. As a result of this effort, we now possess high resolution shape models and well-constrained spin rates, pole orientations, and basic surface properties for a few dozen such small bodies. Here we present the results of a geomorphological examination of 32 such small body shape models along with their associated spin properties, and show that small body shape, gravity, and spin combine to gradually drive the surface towards a condition of maximum topographic stability; that is, a state of low topographic variation, low slopes, and low surface erosion (mass-wasting) rates. Of the 32 bodies investigated, 15 (47%) reside within this ‘zone of maximum topographic stability’, and when asteroid lightcurve-derived rotation rate and body elongation estimates are included, 1941 (70%) of 2791 well-observed asteroids reside within this zone. This finding indicates that given a mobile surface layer and sufficient time, small body surfaces naturally tend to erode, and their spin states gradually evolve, towards a state of maximum topographic stability, which also corresponds to a state of lowest internal stress. This erosional effect is most prominent on bodies several kilometers and larger in diameter, where YORP induced spin-state changes are small.

Investigating the general condition of compensation temperature appearance in ferrimagnetic system

In this paper, we use the double-time Green’s function method to study the magnetic properties of the mixed spin-1/2 and spin-1 Heisenberg ferrimagnets with magnetic fields on a simple cubic lattice. We derive the equations of motion of the Green’s function by a standard procedure. In the course of this, the higher order Green’s functions have to be decoupled. A Tyablikov or random phase approximation decoupling is used to decouple the higher order Green’s functions. Based on the above procedures, the effects of the magnetic fields on the critical and compensation temperatures are investigated. The cause of compensation temperature appearance of a spin-1/2 and spin-1 ferrimagnetic system is discussed in detail. Our results show that the magnetizations of the two sublattices approach to zero at the phase transition point when h a= h b=0. When h a=0, the dropping speed of the sublattice magnetization | m b| with the increase of temperature is slower than that of sublattice magnetization | m a| of small spin when the value of h b increases. Under general condition, S b is larger than S a so that | m a| is always smaller than | m b|. It means that the relation of m a=– m b≠0 cannot be satisfied. It indicates that when h a=0, no matter what value of other parameters in the Hamiltonian is, the compensation temperature cannot appear. On the contrary, in the case of h b=0, as the value of h a increases, the dropping speed of | m b| with the increase of temperature is faster than that of | m a|. It indicates that the difference between | m a| and | m b| decreases with the increasing of h a below the critical temperature. As h a increases to a certain value h amin, we obtain m a=– m b≠0 below the critical temperature. It means that the compensation point appears. The compensation temperature increases with increasing h a. And the value of h amin depends on h b=0. Therefore the condition for T com to appear is that h a/ h b should be beyond some value that depends on h b. By comparing and analyzing the previous research, we obtain the general condition for the appearance of compensation temperature of the ferrimagnetic system, that is, there is at least one additional force. This additional force causes that the decreasing speed of the sublattice magnetization of the large spin with increasing temperature is greater than that of the sublattice magnetization of the small spin. Only in this way, the relation m a=– m b≠0 can be satisfied, i.e., the compensation temperature appears. Otherwise the compensation point will not occur. Meanwhile, it should be noted that our Hamiltonian will recover an ordinary three-dimensional Heisenberg antiferromagnetic model with the nearest-neighbor interactions at S a= S b. Our results agree with the high temperature series, the linked-cluster series approach and ratio method results.

Electronic and magnetic properties of Fe2−xCoxTiGe (x = 0, 0.5, 1, 1.5, 2) full Heusler alloys

Half metallic and magnetic properties of Fe2TiGe and Co2TiGe full Heusler alloys are studied. The structural stability between the two possible structures, namely, L21 (Cu2MnAl phase) and XA (Hg2CuTi phase) structures is analyzed. It is found that L21 phase is the stable phase for these alloys. The density of states predicted that Fe2TiGe is a semiconductor and Co2TiGe is half-metallic. When cobalt is substituted for Fe, it is observed that Fe1.5Co0.5TiGe, Fe1.0Co1.0TiGe, Fe0.5Co1.5TiGe alloys exhibit half metallic ferromagnetism with small spin flip gap at the minority spin state. This is due to the presence of d7 electronic configuration within Co2+ in Fe2+ ions site. Half metallic to metallic phase transition is observed at high pressure. Our work paves the way for designing novel Heusler materials at normal and high pressure. Furthermore, it is observed that Co- doping is an effective strategy to improvise the ferromagnetic performance of Fe2TiGe full Heusler compounds.

Theoretical Aspects of the Cross Effect Enhancement of Nuclear Polarization under Static Dynamic Nuclear Polarization Conditions.

In this study, we perform quantum calculations of the spin dynamics of a small spin system that includes nine coupled electrons and one nucleus placed in an external magnetic field and exposed to microwave irradiation. This is an extension of a previous work in which we have demonstrated on a system of 11 coupled electron spins the dynamics of the electron polarizations composing the electron paramagnetic resonance (EPR) line during static dynamic nuclear polarization (DNP) experiments. There we have shown that the electron polarizations are determined by a spectral diffusion process, induced by the dipolar interaction and cross-relaxation. Additionally, we showed that a distinction had to be made between strong and weak dipolar-coupled systems relative to the inhomogeneity of the EPR line with only the first behaving according to the thermal mixing DNP (with two electron spin temperatures) description. The EPR spectra in the weak and strong dipolar interaction cases show different types of spectral features. In the extended spin system, we again make a distinction between weak and strong electron-electron interactions and show that the DNP spectra for the two cases are different in nature but that the DNP spectra can be derived in all cases from the EPR line shapes using the indirect cross effect.

Manipulating quantum spins by spin-polarized current: an approach based upon -symmetric quantum mechanics

We propose a quantum processor based upon single-molecule magnets and spin transfer torque described by -symmetric quantum mechanics. In recent years -symmetric Hamiltonians have been used to obtain stability thresholds of various systems out of equilibrium. One such problem is the magnetization reversal due to the spin transfer torque generated by a spin-polarized current. So far the studies of this problem have mostly focused on a classical limit of a large spin. In this work we are discussing spin tunneling and quantum dynamics of a small spin induced by a spin polarized current within a -symmetric theory. This description can be used for manipulating spin qubits by electric currents.

Numerical study of barriers and valleys in the free-energy landscape of spin glasses

We study the problem of glassy relaxations in the presence of an external field in the highly controlled context of a spin-glass simulation. We consider a small spin glass in three dimensions (specifically, a lattice of size L = 8, small enough to be equilibrated through a parallel tempering simulations at low temperatures, deep in the spin glass phase). After equilibrating the sample, an external field is switched on, and the subsequent dynamics is studied. The field turns out to reduce the relaxation time, but huge statistical fluctuations are found when different samples are compared. After taking care of these fluctuations we find that the expected linear regime is very narrow. Nevertheless, when regarded as a purely numerical method, we find that the external field is extremely effective in reducing the relaxation times.

Spin valve effect induced by spin-orbit torque switching

The spin valve effect driven by the magnetic field in sandwich structures stands out as a seminal phenomenon in the emerging field of spintronics, but it has not been realized in low-power-dissipation devices operated via electrical means. Here, we investigate spin-orbit torque switching in a perpendicularly magnetized CoFeB/Mo/CoFeB sandwich, where the spin current generated in the Mo layer flows upward and downward to the ferromagnetically coupled CoFeB layers. When scanning the current, two CoFeB layers can be switched from one antiparallel state to a parallel state and then to the other antiparallel state, producing an unprecedentedly current-dependent spin valve effect. Such a spin valve effect is sensitive to the additional magnetic field direction due to the small spin torque efficiency and efficiency difference of the two CoFeB layers. The experimental observations are supported by the dynamics simulation based on the Landau-Lifshitz-Gilbert equation. Besides the fundamental interest, our finding would add a different dimension to energy efficient memory devices and sensors.

Long spin diffusion lengths in doped conjugated polymers due to enhanced exchange coupling

Carbon-based semiconductors such as conjugated organic polymers are of potential use in the development of spintronic devices and spin-based information processing. In particular, these materials offer a low spin–orbit coupling strength due to their relatively light constituent chemical elements, which should, in principle, favour long spin diffusion lengths. However, organic polymers are relatively disordered materials and typically have a carrier mobility that is orders of magnitude lower than crystalline inorganic materials. As a result, small spin diffusion lengths of around 50 nm have typically been measured using vertical organic spin valves. Here, we report measuring spin diffusion lengths in doped conjugated polymers using a lateral spin transport device architecture, which is based on spin pumping injection and inverse spin Hall detection. The approach suggests that long spin diffusion lengths of more than 1 μm and fast spin transit times of around 10 ns are possible in conjugated polymer systems when they have a sufficiently high spin density (around 1020 cm−3). We explain these results in terms of an exchange-based spin diffusion regime in which the exchange interactions decouple spin and charge transport. Measurements with a lateral spin pumping device architecture suggest that long spin diffusion lengths of more than 1 μm are possible in conjugated polymer systems that have a sufficiently high spin density.

Singlet ground state in the alternating spin- 12 chain compound NaVOAsO4

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO$_4$ by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and $^{75}$As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift could be described well using an alternating spin-$1/2$ chain model with the exchange coupling $J/k_{\rm B}\simeq 52$ K and an alternation parameter $\alpha \simeq 0.65$. From the high-field magnetic isotherm measured at $T=1.5 $ K, the critical field of the gap closing is found to be $ H_{\rm c}\simeq 16$ T, which corresponds to the zero-field spin gap of $\Delta_0/k_{\rm B}\simeq 21.4$ K. Both NMR shift and spin-lattice relaxation rate show an activated behavior at low temperatures, further confirming the singlet ground state. The spin chains do not coincide with the structural chains, whereas the couplings between the spin chains are frustrated. Because of a relatively small spin gap, NaVOAsO$_4$ is a promising compound for further experimental studies under high magnetic fields.

Thermally Assisted Magneto Resistance and Spin-Filtration in Single Layer MoS2

We investigate the dependence of spin polarized transport properties on temperature in single-layer ferromagnetic molybdenum disulfide (MoS 2 ). The density functional theory alongside the nonequilibrium Green's function formulation is utilized for spin polarized transport calculations. The spin dependent transmission spectrum and current are computed to clarify the impact of temperature on spin filtration and spin Seebeck effect. A high estimation of spin polarization efficiency of nearly ~100% is accomplished at small range of values of temperature difference. Large temperature distinction between left electrode (T L ) and right electrode (T R ) results in small spin polarization efficiency. The results suggest a thermal giant magneto-resistance (MR) of ~2.16 × 10 3 %, which advances with advancement in temperature gradient/difference (ΔT) among left and right electrode, MR additionally relies upon T L . The magnificent attributes, for example, high magnetoresistance and near perfect spin polarization suggests the potential of MoS 2 material in applications based on spin caloritronics.

First-principles dynamics of collisional intersystem crossing: resonance enhanced quenching of C(1D) by N2.

Intersystem crossing is a common and important nonadiabatic process in molecular systems, and its first-principles characterization requires accurate descriptions of both the electronic structure and nuclear dynamics. Here, we report an accurate full-dimensional quantum dynamical investigation of collisional quenching of the excited state C(1D) atom to its ground state C(3P) counterpart by N2, which is an important process in both combustion and interstellar media, using full-dimensional ab initio potential energy surfaces and spin-orbit couplings. Satisfactory agreement with experimental rate coefficients is obtained. Despite relatively small spin-orbit couplings, it is shown that intersystem crossing is efficient because of multiple passages via long-lived collisional resonances.

Thermodynamics and spin mapping of quantum excitations in a Heisenberg spin heptamer

In this study, we examine the thermodynamics and spin dynamics of spin-1/2 and spin-3/2 heptamers. Through an exact diagonalization of the isotropic Heisenberg Hamiltonian, we find the closed-form, analytical representations for thermodynamic properties, spin excitations, and neutron scattering structure factors. Furthermore, we investigate the {cluster-like excitations of quantum spin heptamer} in the three-dimensional pyrochlore lattice material MgCr$_2$O$_4$. Using a spin mapping of the spin-1/2 heptamer excitations, the calculated structure factors of the spin-3/2 heptamer are be determined, which provides clarification for the spin excitations in MgCr$_2$O$_4$. Overall, this study demonstrates the ability to use the spin mapping of structure factors for small spin systems to analyze more complex structures.

Synthesis and Magnetic Properties of (Pyrrolidin-1-oxyl)-(Nitronyl Nitroxide)/(Iminonitroxide)-Dyads.

Unlike extensively studied diradicals linked by π-conjugated systems, only a few studies have investigated weakly coupled diradicals linked by an sp3 carbon atom. Herein, we prepared pyrrolidin-1-oxyl-(nitronyl nitroxide)-dyad 5 and pyrrolidin-1-oxyl-iminonitroxide-dyad 6. From the observed temperature dependence of the magnetic susceptibility, 5 and 6 were determined to be in singlet ground states with 2Jintra /kB =-35.2 K and -13.6 K, respectively. From these results and theoretical calculations of related diradicals, the spin-polarization model counting the small spin density of the sp3 carbon atom could be used as a spin-prediction model.