Nonmagnetic Phase Research Articles

Effect of Nb doping on microstructure and magnetic properties of hot-deformed Nd-Fe-B magnets with Nd-Cu eutectic diffusion

To restrict grain growth in coarse grain regions caused by the diffusion of Nd-Cu eutectic alloys, the Nb element was introduced into the precursor alloy to regulate the microstructure of melt-spun powder and die-upset magnets. The magnetic properties and thermal stability of die-upset magnets were appreciably improved through the addition of Nb. For the Nb-doped diffusion die-upset magnet, the grains inside the ribbons were refined and the coarse non-oriented surface crystallite got suppressed on the interface of ribbons during the hot-deformation process to form the anisotropic magnet. Moreover, Nd gathers at the intergranular phases, which is considered to enforce domain wall pinning force. The Nb-modified microstructure is advantageous to thermal stability and coercivity enhancement. High-resolution transmission electron microscopy images revealed that the Nb element gathered on the grain boundary and triple grain boundary to form c-Nb and h-NbFeB to hinder the grain growth during the hot-deformation process, which led to direct enhancement in the coercivity. Furthermore, the c-Nb and h-NbFeB are nonmagnetic phases that strengthened the magnetic isolation. However, the h-NbFeB precipitated from the hard magnetic phase and formed crystal defects which led to remanence deterioration.

Structural, mechanical, electronic, and thermoelectric properties of new semiconducting d0 quaternary Heusler compounds CaKNaZ (Z =Si, Ge, Sn). A density functional theory study

Due to the increasing demand for energy, the development of new and good thermoelectric (TE) materials is very vital. In this study, with ab initio calculations, based on the density functional theory (DFT) using the self-consistent full potential linearized augmented plane wave (FPLAPW) method were performed to explore the structural, mechanical, electronic and thermoelectric properties of quaternary alloys CaKNaZ (Z = Si, Ge, Sn) with quaternary Heusler structure. optimization confirmed the most stable structure for CaKNaZ (Z = Si, Ge, Sn) compounds is Y1-type in the non-magnetic phase. All of the compounds have been shown to behave like semiconductors, with indirect band gaps of 0.82 and 0.69 for CaKNaSi, CaKNaSn respectively, and direct band gap of 0.46 for CaKNaGe. The theoretical study of thermoelectric properties for CaKNaZ (Z = Si, Ge, Sn) was carried out by Boltzmann theory as implemented in BoltzTraP code. we have obtained a high of figure of merit at moderate temperatures. This indicates that the studied alloys can be used in thermoelectric applications.

Spinons and damped phonons in the spin- 12 quantum liquid Ba4Ir3O10 observed by Raman scattering

In spin-$\frac{1}{2}$ Mott insulators, nonmagnetic quantum liquid phases are often argued to arise when the system shows no magnetic ordering, but identifying positive signatures of these phases or related spinon quasiparticles can be elusive. Here, we use Raman scattering to provide three signatures for spinons in a possible spin-orbit quantum liquid material $\mathrm{Ba}{}_{4}\mathrm{Ir}{}_{3}\mathrm{O}{}_{10}$: (1) a broad hump, which we show can arise from Luttinger liquid spinons in Raman with parallel photon polarizations normal to one-dimensional chains; (2) strong phonon damping from phonon-spin coupling via the spin-orbit interaction; and (3) the absence of (1) and (2) in the magnetically ordered phase that is produced when 2% of Ba is substituted by Sr [($\mathrm{Ba}{}_{0.98}\mathrm{Sr}{}_{0.02}){}_{4}\mathrm{Ir}{}_{3}\mathrm{O}{}_{10}$]. The phonon damping via itinerant spinons seen in this quantum liquid insulator suggests a mechanism for enhancing thermoelectricity in strongly correlated conductors, through a neutral quantum liquid that need not affect electronic transport.

Investigation of the obtaining process copper ferrite by a modified coprecipitation method

Copper ferrite has been synthesized by modified co-precipitation method. A comparative analysis of the pH-static and pH-dynamic modes of ferrite copper production has been carried out. The kinetics of the processes has been investigated. The use of plasma technology for the production of copper ferrites is shown. It was found that the use of the pH-static method leads to the formation of non-magnetic phases. The use of the pH-dynamics mode at pH = 11 leads to the production of magnetic copper ferrite with a saturation magnetization of 103.3 Emu/g and a coercive force of 25 Oe.

Nonequlibrium magnetic features in a mixed spin (2, 5/2) Ising system driven by the external oscillating magnetic field by path probability method

Utilizing the path probability method, we investigated nonequilibrium magnetic features in a mixed spin (2, 5/2) Ising model Hamiltonian composed of bilinear and crystal-field interactions in the presence of an external oscillating magnetic field. We numerically solved the time dependence of average magnetizations to find the phases in the system. We examined the dynamic magnetizations to obtain dynamic phase transition (DPT) temperatures, the nature of the DPTs, and phases in the system. The dynamic phase diagrams (DPDs) were constructed in reduced temperature and the amplitude of oscillating magnetic field plane for various interaction parameters. We observed that the system gives very rich and interesting topological behaviors of DPDs, such as up to two dynamic tricritical points, six critical end points, a double critical end points, a zero-temperature critical point, one inverse critical end point and a quadruple point depending on interaction parameters. The system also exhibits paramagnetic, six distinct ferrimagnetic and three different nonmagnetic phases as well as up to eleven different mixed or hybrid phases. In addition, the system exhibits the reentrant behavior.

Temperature induced phase transformation in Co

Temperature dependent phase transformation behavior in cobalt from hexagonal close-packed (hcp) to face centered cubic (fcc) has been found to be contradictory to that reported earlier. It is found that hcp phase stabilizes at both low and high temperature (sim 873 K) while fcc phase is stabilized at sim 500 K. At 298 K, hcp Co has been found to be predominant (sim 70%) where hcp magnetic phase is sim 60%. At 973 K, hcp phase is again predominant (sim 73%), but it is mainly the non-magnetic phase (sim 67%). Contrary to present results, it was found earlier that fcc phase was stabilized at high temperature and hcp to fcc transformation occured at sim 700 K. Present results from perturbed angular correlation measurements, therefore, requires a new theoretical interpretation for Co phase transformation. From present measurements, hyperfine magnetic fields in Co at room temperature for the hcp and fcc phases have been found to be 18.7(6) and 12.8(3) T, much lower than earlier reported results. The hyperfine magnetic fields at ^{181}Ta impurity atom have been calculated by density functional theory (DFT) employing the full potential (linearized) augmented plane wave method (FP-LAPW). Present calculated results for both hcp and fcc phases corroborate our experimental results.

Electronic structure, magnetic properties, and pairing tendencies of the copper-based honeycomb lattice Na2Cu2TeO6

Spin-$1/2$ chains with alternating antiferromagnetic and ferromagnetic couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density matrix renormalization group methods, we have systematically studied the dimerized chain system Na$_2$Cu$_2$TeO$_6$. Near the Fermi level, the dominant states are mainly contributed by the Cu $3d_{x^2-y^2}$ orbitals highly hybridized with the O $2p$ orbitals in the nonmagnetic phase, leading to an "effective" single-orbital low-energy model. Furthermore, the bandwidth of the Cu $3d_{x^2-y^2}$ states is small ($\sim 0.8$ eV), suggesting that electronic correlations will strongly affect this system. By introducing such electronic correlations, we found this system is a Mott insulator. Moreover, by calculating the magnetic exchange interactions ($J_1$, $J_2$ and $J_3$), we explained the size and sign of the exchange interactions in Na$_2$Cu$_2$TeO$_6$, in agreement with neutron experiments. In addition, we constructed a single-orbital Hubbard model for this dimerized chain system, where the quantum fluctuations are taken into account. Both AFM and FM coupling ($\uparrow$-$\downarrow$-$\downarrow$-$\uparrow$) along the chain were found in our DMRG and Lanczos calculations, in agreement with DFT and neutron results. We also calculated the hole pairing binding energy $\Delta E$ which becomes negative at Hubbard $U \sim 11$ eV, indicating incipient pairing tendencies. Finally, we also looked at various cases of hole doping that always exhibit tight pairs. Thus, we believe our results for Na$_2$Cu$_2$TeO$_6$ could provide guidance to experimentalists and theorists working on this dimerized chain system, such as short-range magnetic coupling, doping effects, and possible pairing tendencies.

Analysis of Mg-based spinels MgGd2X4 (X = S, Se) for spintronic and thermoelectric device applications: Ab-initio calculations

The spin-dependent magnetic, thermoelectric, and electronic characteristics of Mg-based spinels MgGd2X4 (X = S, Se) are investigated using density functional theory (DFT) calculations. The modified Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol-GGA) is utilized to compute the structural and elastic characteristics. In addition, the magnetic, thermoelectric, and electronic characteristics are explored employing the modified-Becke and Johnson (mBJ) potential. After the energy difference calculations among the ferromagnetic (FM) and nonmagnetic (NM) phases, the stability of the structures is confirmed in the FM phase. Further, thermodynamic stability is determined by calculating formation energies and phonon dispersion spectra. The Charpin method is used to analyze elastic characteristics in detail. The total magnetic moments are produced by 2p-states of chalcogenides and f-states of Gd atoms, which result in substantial hybridization at the Fermi level. Finally, electronic-thermal coefficients, such as the power factor, Seebeck coefficient, and thermal and electrical conductivity, are investigated, concluding that investigated spinels can be used in thermoelectric devices.

Magnetically tightened multifunctional phase change materials

As a non-contact, real-time, accurate, and flexible control technique, the magnetic field has been widely utilized in a variety of applications, including industrial grab, robot drives, medical assistance, etc. However, non-magnetic phase change materials (PCMs) are difficult to manipulate with magnetic fields. Recently in <i>Nature Communications</i>, He et al. innovatively proposed magnetically tightened form-stable PCMs with the integrated multi-functions of being leakage-proof, having highly thermal/electrical conductivities, robust mechanical strength, dynamic assembly, and shape reconfiguration by introducing hard magnetic particles. Additionally, they developed free-standing temperature control and high-performance thermoelectric conversion systems based on magnetically tightened form-stable PCMs.

Local multiplet formation around a single vacancy in graphene: An effective Anderson model analysis based on the block-Lanczos DMRG method

To better understand the electronic structure of a single vacancy in graphene, we study the ground state property of an effective Anderson model, consisting of three dangling $sp^2$ orbitals of the surrounding carbon atoms around the vacancy and the $\pi$ orbitals of carbon atoms that form the honeycomb lattice with a single vacancy. Employing the block-Lanczos density-matrix renormalization group method, we show that there are two phases in the relevant parameter space, i.e., a nonmagnetic phase in the weak coupling region and a free magnetic moment phase in the realistic parameter region. The systematic analysis finds that, in the free magnetic moment phase, local multiplets of the doubly degenerate irreducible representation of the $\mathcal{C}_3$ point group with spin 1 become dominant in the ground state, and approximately half of this local spin 1 is screened by electrons in the surrounding $\pi$ orbitals, indicating the emergence of the residual spin-1/2 free magnetic moment. Furthermore, we find that the emergence of the free magnetic moment is robust against carrier doping, which is in sharp contrast to the case of graphene with an adatom, thus explaining the qualitative difference observed experimentally in these two classes of systems. We also find the enhancement of the spin correlation function between $\pi$ electrons around the vacancy and those in the conduction band away from the vacancy in the undoped case, as compared to that in the doped case, while the spin correlation function between the $\sigma$ electrons in the $sp^2$ dangling orbitals around the vacancy and the $\pi$ electrons in the conduction band remains large in both undoped and doped cases. Our calculations thus support qualitatively the previous experiment that suggests the emergence of free magnetic moment with two distinct origins.

Obtaining a dual-phase magnetic material based on electrical silicon steel sheet

Dual-phase magnetic materials are materials that have different magnetic permeability in selected areas. This feature of dual-phase magnetic materials opens up great prospects for new design solutions for some topologies of electrical machines. This article presents the results of a study on obtaining a dual-phase magnetic material from sheets of electrical silicon steel with a Si content of 2.4–3.8% during ion-plasma nitriding in a glow discharge. Based on the optimal parameters of the vacuum environment, samples of silicon steel sheets were processed with different holding times. Then, the structure of each of the samples was studied using an X-ray diffractometer with the determination of the composition of the surface layer of the treated samples and the identification of the non-magnetic phase. As a result of the study, the possibility of obtaining a non-magnetic austenite phase during ion-plasma nitriding was shown, however, the formation of a significant amount of the nitride phase was noted. An experiment on transformers shows a decrease in magnetic permeability and magnetization for a dual-phase magnetic material. Strength tests show satisfactory characteristics for the non-magnetic phase of the obtained dual-phase magnetic material, as well as a strong decrease in strength characteristics for the magnetic phase of the dual-phase magnetic material.

Anderson localization in the Anderson–Hubbard model with site-dependent interactions

We consider Anderson localization in the half-filled Anderson–Hubbard model in the presence of either random on-site interactions or spatially alternating interactions in the lattice. By using dynamical mean field theory with the equation of motion method as an impurity solver, we calculate the arithmetically and geometrically averaged local density of states and derive the equations determining the critical value for the phase transition between metallic, Anderson and Mott insulating phases. The nonmagnetic ground state phase diagrams are constructed numerically. We figure out that the presence of Coulomb disorder drives the system toward the Anderson localized phase that can occur even in the absence of Anderson structural disorder. For the spatially alternating interactions, we find that the metallic region is reduced and the Anderson insulator one is enlarged with increasing interaction modulation. Our obtained results are relevant to current research in ultracold atoms in disordered optical lattices where metal–insulator transition can be observed experimentally by using ultracold atom techniques.

Novel design of self-compensated thermally stable Ce magnets without critical elements

A novel design concept was proposed for optimizing the grain boundaries from the theoretical aspect, which could play the self-compensation effect on the thermal stability of the coercivity. The Pr-Nd-Al Ce magnet without critical elements was investigated that experimentally verified the self-compensation effect of grain boundaries structure on the thermal stability of the coercivity. As a result, the Pr-Nd-Al Ce magnet with 3 wt% (Pr0.25Nd0.75)80Al20 as the additive inside the grain boundaries had better thermal stability than that of Nd-Fe-B magnet. The coercivity temperature coefficient β of Pr-Nd-Al Ce magnet was improved by 5.7%, and the irreversible loss of magnetic flux hirr was enhanced by 58% at the temperature of 100 ℃ compared with the Nd-Fe-B magnet. The improvement of thermal stability was attributed to the special sandwich-like structure consisting of two amorphous nonmagnetic phases sandwiching a layer of Re6(Fe, TM)11Al3 tetragonal phase, that compensated the coercivity of Ce magnets without critical elements.

Gapless quantum spin liquid and global phase diagram of the spin-1/2 [formula omitted] square antiferromagnetic Heisenberg model

The nature of the zero-temperature phase diagram of the spin-1/2J1-J2 Heisenberg model on a square lattice has been debated in the past three decades, and it remains one of the fundamental problems unsettled in the study of quantum many-body theory. By using the state-of-the-art tensor network method, specifically, the finite projected entangled pair state (PEPS) algorithm, to simulate the global phase diagram of the J1-J2 Heisenberg model up to 24×24 sites, we provide very solid evidences to show that the nature of the intermediate nonmagnetic phase is a gapless quantum spin liquid (QSL), whose spin-spin and dimer-dimer correlations both decay with a power law behavior. There also exists a valence-bond solid (VBS) phase in a very narrow region 0.56≲J2/J1≤0.61 before the system enters the well known collinear antiferromagnetic phase. We stress that we make the first detailed comparison between the results of PEPS and the well-established density matrix renormalization group (DMRG) method through one-to-one direct benchmark for small system sizes, and thus give rise to a very solid PEPS calculation beyond DMRG. Our numerical evidences explicitly demonstrate the huge power of PEPS for highly frustrated spin systems. Finally, an effective field theory is also proposed to understand the physical nature of the discovered gapless QSL and its relation to deconfined quantum critical point (DQCP).

Tuning of Curie temperature in Mn5Ge3 films

We report a change in the structural and magnetic properties of epitaxial Mn5Ge3 on a Ge-on-Si (111) substrate by applying strain engineering through ms-range flash lamp annealing (FLA). X-ray diffraction results demonstrate that during FLA for 20 ms, the formation of nonmagnetic MnxGey secondary phases is suppressed, while the in-plane expansion of the lattice increases with increasing annealing temperature. Temperature-dependent magnetization results indicate that the Curie temperature of Mn5Ge3 rises from 287 K in the as-prepared sample to above 400 K after FLA, making Mn5Ge3 an attractive material for spintronics. Experimental results together with theoretical Monte Carlo simulations allow us to conclude that the expansion of the in-plane lattice causes the increase of the Curie temperature due to enhancement of the ferromagnetic interaction between Mn atoms.

High-coercivity SmFe10V2 powder with Sm-rich layers prepared by a reduction-diffusion process

The remarkable intrinsic hard magnetic properties of SmFe12-based powders are difficult to transform into extrinsic properties for practical applications; in particular, the coercivity is insufficient because the conventional magnetic isolation structure, which relies on nonmagnetic intergranular phases and is critical for inducing high coercivity in commercial Nd2Fe14B systems, has not yet been perfected. In this study, we present a new approach for achieving magnetic isolation with a reduction-diffusion process that endows SmFe10V2 powder with nonmagnetic Sm-rich layers. The optimal sample of this Sm-rich layer surrounding SmFe10V2 powder has a high coercivity of 744.9 kA/m and a moderate maximum magnetization of 61.7 Am2/kg. Based on phase characterization and microstructure observation, the principal SmFe10V2 magnetic phase is well maintained, and the surrounding Sm-rich layers separate the magnetic phases, weakening the ferromagnetic interaction of neighboring magnetic phases and contributing to the high coercivity. Our study presents a new method for developing high-coercivity magnetic powders by introducing nonmagnetic layers.

The role of microstructural evolution during spark plasma sintering on the soft magnetic and electronic properties of a CoFe\u2013Al2O3 soft magnetic composite

For transformers and inductors to meet the world’s growing demand for electrical power, more efficient soft magnetic materials with high saturation magnetic polarization and high electrical resistivity are needed. This work aimed at the development of a soft magnetic composite synthesized via spark plasma sintering with both high saturation magnetic polarization and high electrical resistivity for efficient soft magnetic cores. CoFe powder particles coated with an insulating layer of Al2O3 were used as feedstock material to improve the electrical resistivity while retaining high saturation magnetic polarization. By maintaining a continuous non-magnetic Al2O3 phase throughout the material, both a high saturation magnetic polarization, above 1.5 T, and high electrical resistivity, above 100 μΩ·m, were achieved. Through microstructural characterization of samples consolidated at various temperatures, the role of microstructural evolution on the magnetic and electronic properties of the composite was elucidated. Upon consolidation at relatively high temperature, the CoFe was to found plastically deform and flow into the Al2O3 phase at the particle boundaries and this phenomenon was attributed to low resistivity in the composite. In contrast, at lower consolidation temperatures, perforation of the Al2O3 phase was not observed and a high electrical resistivity was achieved, while maintaining a high magnetic polarization, ideal for more efficient soft magnetic materials for transformers and inductors.

Magnetic terahertz resonances above the Néel temperature in the frustrated kagome antiferromagnet averievite

Time-domain magneto-THz spectroscopy is utilized to study the frustrated magnet averievite ${\mathrm{Cu}}_{5\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{10}$(CsCl). Pronounced THz resonances are observed in unsubstituted samples ($x=0$) when cooling below the onset of short-range magnetic correlations. The influence of external magnetic effects confirms the magnetic origin of these resonances. Increasing Zn substitution suppresses the resonances, as frustration effects dominate, reflecting the nonmagnetic phases for $x&gt;0.25$ compounds. The temperature evolution of the THz spectra is complemented with electron spin resonance spectroscopy. This comparison allows a direct probe of the different contributions from magnetic order, frustration, and structural properties in the phase diagram of averievite. Our results illustrate the effect of magnetic interactions in THz spectra of frustrated magnets.

Magnetic permeability stability of composite material with nominal composition Ni0.6Fe2.4O4

A composite material with a nominal composition of Ni0.6Fe2.4O4 was prepared by the solid-phase reaction method. X-ray diffraction analysis shows that the product is composed of α-Fe2O3 and a solid solution of Fe3O4 and NiFe2O4. Although α-Fe2O3 non-magnetic phase reduces the saturation magnetization, it greatly improves the temperature and frequency stability of magnetic permeability. Based on the measurement results of the amplitude permeability in the range of −60 °C to 100 °C, the temperature coefficient αμ and specific temperature coefficient β were calculated. αμ and β of the composite material are 7.15 ×10-5/°C and 8.6 ×10-6/°C, respectively, which are much smaller than αμ (1.26×10-3/°C) and β (9.5×10-5/°C) of NiFe2O4. At the same time, the magnetic spectrum curves show that the real part of the complex permeability μ' basically remains unchanged within the measurement frequency range (f < 106 Hz). This improvement in temperature and frequency stability is attributed to the effect of the α-Fe2O3 phase.

Pressure-induced dimerization and collapse of antiferromagnetism in the Kitaev material α−Li2IrO3

We present magnetization measurements carried out on polycrystalline and single-crystalline samples of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ under hydrostatic pressures up to 2 GPa and establish the temperature-pressure phase diagram of this material. The N\'eel temperature (${T}_{\mathrm{N}}$) of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is slightly enhanced upon compression with $d{T}_{\mathrm{N}}/dp$ = 1.5 K/GPa. Above 1.2 GPa, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ undergoes a first-order phase transition toward a nonmagnetic dimerized phase, with no traces of the magnetic phase observed above 1.8 GPa at low temperatures. The critical pressure of the structural dimerization is strongly temperature dependent. This temperature dependence is well reproduced on the ab initio level by taking into account lower phonon entropy in the nonmagnetic phase. We further show that the initial increase in ${T}_{\mathrm{N}}$ of the magnetic phase is due to a weakening of the Kitaev interaction $K$ along with the enhancement of the Heisenberg term $J$ and off-diagonal anisotropy $\mathrm{\ensuremath{\Gamma}}$. Our study reveals a common thread in the interplay of magnetism and dimerization in pressured Kitaev materials.