AF Phase Research Articles

Structure, vacancy, and physical properties of non-stoichiometric MnWO4 ceramics

Non-stoichiometric ceramic samples of Mn0.97WO4 and MnW0.97O4 were prepared by a solid-state reaction to study the effects of vacancies at different lattice positions on the structure and properties of MnWO4 materials caused by non-stoichiometric. Positron annihilation technology was introduced to study the vacancy characteristics of the MnWO4, Mn0.97WO4, and MnW0.97O4 ceramics. The results indicated that non-stoichiometric did not affect the monoclinic lattice structure of any of the MnWO4 samples, and had no obvious effect on the lattice parameters of the samples. The analysis of positron annihilation lifetime spectroscopy showed that W vacancies were the main defects in MnW0.97O4 samples, and Mn vacancies were favored in Mn0.97WO4 samples, the vacancies concentration for non-stoichiometric MnWO4 samples was higher than that of stoichiometric MnWO4. It was observed that the magnetization and transition temperatures of the non-stoichiometric MnWO4 system were increased, and the temperature range in which AF2 phase existed was enlarged. This study demonstrated that non-stoichiometric could enhance the magnetic properties of MnWO4, the effect of Mn vacancy on the properties of MnWO4 is higher than that of W vacancy. The intimate correlation between the vacancy defects and magnetic properties provides us strong evidence that the magnetic properties of MnWO4 can be enhanced by regulating vacancy defects.

Modulated structure and physical properties of MnWO4 ceramics via Zr4+ replacement

Ceramic samples of Mn1-xZrxWO4 (0.00 ≤ x ≤ 0.20) were prepared by a solid-state reaction to study changes in the structural, magnetic, and dielectric properties of multiferroic MnWO4 caused by replacing Mn2+ ions with Zr4+ ions. The results indicate that Zr4+ replacement caused lattice deformation but did not affect the monoclinic lattice structure of any of the MnWO4 samples. The concentration of vacancies, investigated by positron annihilation lifetime spectroscopy, increased with as the Zr4+ content x from 0.00 to 0.20. It was observed that Zr4+ replacement enhanced the dielectric constant of Mn1-xZrxWO4 but slightly increased the dielectric loss. The magnetization and transition temperatures (TAF3 and TAF2) of the Mn1-xZrxWO4 system were increased by the Zr4+ replacement, and the existing temperature range of the AF2 phase was extended upon the substitution of Zr4+ for Mn2+. This study demonstrated that the magnetic properties of Mn1-xZrxWO4 were closely related to the vacancy concentration and lattice distortion.

Impacts of sintering temperature on the structure and physical properties of MnWO4 ceramics

The effects of sintering temperature on the structure, morphology, vacancy defects, magnetic and dielectric characteristics of MnWO4 ceramics synthesized by solid-state reaction were investigated. The results revealed that the increase of sintering temperature caused lattice deformation, but had no effect on the monoclinic lattice structure of all MnWO4 samples. SEM analysis showed that all of the synthesized samples were relatively dense, and higher sintering temperature was conducive to grain growth. Positron annihilation lifetime spectroscopy revealed that the vacancy size and concentration were regulated by sintering temperature. The properties measurements showed that appropriate increase of sintering temperature increased the dielectric constant, magnetization, and expanded existing temperature range of AF2 phase for MnWO4 system. The relationship between structure and performances was discussed in this paper.

Site dependence of the magnetocaloric effect in Mn5-x Fe x Si3.

The nuclear and magnetic structures of Mn3Fe2Si3 are investigated in the temperature range from 20 to 300 K. The magnetic properties of Mn3Fe2Si3 were measured on a single crystal. The compound undergoes a paramagnetic to antiferromagnetic transition at T N2 ≃ 120 K and an antiferromagnetic to antiferromagnetic transition at T N1 ≃ 69 K. A similar sequence of magnetic phase transitions is found for the parent compound Mn5Si3 upon temperature variation, but the field-driven transition observed in Mn5Si3 is not found in Mn3Fe2Si3, resulting in a strongly reduced magnetocaloric effect. Structurally, the hexagonal symmetry found for both compounds under ambient conditions is preserved in Mn3Fe2Si3 through both magnetic transitions, indicating that the crystal structure is only weakly affected by the magnetic phase transition, in contrast to Mn5Si3 where both transitions distort the nuclear structure. Both compounds feature a collinear high-temperature magnetic phase AF2 and transfer into a non-collinear phase AF1 at low temperature. While one of the distinct crystallographic sites remains disordered in the AF2 phase in the parent compound, the magnetic structure in the AF2 phase involves all magnetic atoms in Mn3Fe2Si3. These observations imply that the distinct sites occupied by the magnetic atoms play an important role in the magnetocaloric behaviour of the family.

Magnetic transitions induced by pressure and magnetic field in a two-orbital [formula omitted]-electron model in cubic and tetragonal lattices

We investigate the onset and evolution of under the simultaneous application of pressure and magnetic field of distinct itinerant Néel states using the underscreened Anderson Lattice Model (UALM) which has been proposed to describe 5f-electron systems. The model is composed by two narrow f-bands (of either α or β character) that hybridize with a wide d-band and local 5f-electron interactions. We consider both cubic and tetragonal lattices. The Néel order parameters ϕβ and ϕα are assumed to be fixed by an Ising anisotropy. The applied magnetic field hz is parallel to the anisotropy axis. It has been assumed that the variation of the band width W is sensitive to pressure. In the absence of a magnetic field, the increase of W takes the system from the phase AF1 to another phase AF2. The phase AF1 occurs when ϕβ>ϕα>0 while in the AF2 phase the gaps satisfy ϕα>ϕβ>0. In the presence of a magnetic field hz, the phase AF2 is quickly suppressed and reappears again at intermediate values of the magnetic field while it is predominant at higher magnetic fields. The analysis of the partial density of states close to the phase transition between the phases AF1 and AF2, allows a better understanding the mechanism responsible whereby the transition is induced by an increase in the magnetic field. As an important general result, we found that the magnetic field hz favours the phase AF2 while the phase AF1 is suppressed. For the tetragonal lattice, the phase AF2 is even more favored when hz and c/a increases concomitantly, where c and a are the lattice parameters.

The Effect of Inclusions on a Medium Temperature Transformation Microstructure and Toughness of High-Strength HSLA Steel.

In this study, the evolution of high-strength HSLA steel microstructure was studied using high-temperature laser confocal microscopy and SEM, TEM, and EPMA techniques. The effect of precipitates on grain boundary migration of austenite during high-temperature heating and the effect of inclusions in undercooled austenite on AF phase transformation were studied. The effect of multiphase microstructure on impact toughness was studied by Gleeble thermal simulation at 550, 600, and 650 °C. The results show that the austenite grain is refined by TiN pinning at high temperatures, and a large number of NbC and VCN are precipitated in ferrite for precipitation strengthening. The (Ti-Mn-O) + (Al + Si + Mn-O) + MnS composite inclusions with smaller sizes have a greater promoting effect on the nucleation of acicular ferrite than single-phase MnS. With a decrease in isothermal temperature, the content of acicular ferrite increases. When the isothermal temperature is 550 °C, an increase in the maximum impact toughness of acicular ferrite with large-angle grain boundary is clearly observable.

Coexistence of Charge Order and Antiferromagnetic Order in an Extended Periodic Anderson ModelSupported by the National Natural Science Foundation of China (Grant Nos. 11974048 and 11974049), and the Beijing Science Foundation (Grant No. 1192011).

The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition, which occurs ubiquitously in heavy fermion materials. However, there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce- and Y-based compounds. We study an extended periodic Anderson model (EPAM) which includes the onsite Coulomb repulsion Ucf between the localized electrons and conduction electrons. By employing the density matrix embedding theory, we investigate the EPAM in the symmetric case at half filling. By fixing the onsite Coulomb repulsion U of the localized electrons to an intermediate value, the interplay between the RKKY interaction, the Kondo effect and the Coulomb repulsion Ucf brings rich physics. We find three different phases, the antiferromagnetic phase, the charge order phase and paramagnetic phase. When the hybridization strength V between the localized orbital and the conduction orbital is small, the Kondo effect is weak so that the AF phase and the CO phase are present. The phase transition between the two long-range ordered phase is of first order. We also find a coexistence region between the two phases. As V increases, the Kondo effect becomes stronger, and the paramagnetic phase appears between the other two phases.

Microscopic mechanisms for phase coexistence and electric polarization enhancement in Mn1−xIrxWO4 + δ

We investigate the magnetic and ferroelectric behaviors of 5d Ir4+ substituted Mn1−xIrxWO4 + δ samples. The electric polarization is enhanced to a certain extent due to the Ir substitution of Mn. Furthermore, the non-polar AF1 phase is partially suppressed by the ferroelectric AF2 phase, resulting in the coexistence of the two phases below TAF1. It is suggested that the lattice distortion, the suppressed magnetic moment, and the increased Dzyaloshinskii–Moriya interaction are the key reasons for the enhanced polarization and the stabilized ferroelectric AF2 phase.

Magnetism driven by strong electronic correlations in the heavily carrier-doped iron oxypnictide LaFeAsO0.49H0.51

The magnetism of the second antiferromagnetic phase (AF2) arising in the iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was investigated by muon spin rotation measurements under hydrostatic pressure up to 2.6 GPa. The N\'eel temperature ($T_{\rm N}$) obtained for a sample with $x=0.51$ exhibits considerably greater sensitivity to pressure than that in the pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the AF1 phase is always accompanied by the structural transition (from tetragonal to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly higher than $T_{\rm N}$, the AF2 phase prevails at higher pressures above $\sim$1.5 GPa where the structural transition is suppressed ($T_{\rm s}=0$). These features indicate that the microscopic origin of the AF2 phase is distinct from that of AF1, suggesting that electronic correlation plays important role in the former phase. We argue that the orbital-selective Mott transition is a plausible scenario to account for the observed pressure dependence of $T_{\rm N}$ and $T_{\rm s}$ in the AF2 phase.

On the limits of Reactive-Spark-Plasma Sintering to prepare magnetically enhanced nanostructured ceramics: the case of the CoFe2O4-NiO system

Magnetic materials are crucial for the efficiency of the conversion-storage-transport-reconversion energy chain, and the enhancement of their performance has an important impact on technological development. The present work explores the possibility of preparing hetero-nano-structured ceramics based on magnetic oxides, by coupling a ferrimagnetic phase (F) with an antiferromagnetic one (AF) on the nanometric scale. The field-assisted sintering technique or SPS (Spark-Plasma Sintering), adopted at this purpose, ensures the preservation of nano-sized crystals within the final solid structure. The aim is to establish how exchange bias may affect the resulting nano-consolidates and to investigate the potential of this process to increase the total magnetic anisotropy of the CoFe2O4 grains, and thus their coercive field, while keeping the saturation magnetization the same. The structure, microstructure and magnetic properties of the ceramics obtained were studied by several techniques. The results show that the sintering process, along with its typical reductive atmosphere, modifies the composition of the constituents. A new metallic phase appears as a consequence of the reciprocal diffusion of Co and Ni cations, leading to a change in the amount and structure of the AF phase. We propose a schematic representation of the atomic movements that hinder an exchange bias effect between the F and AF phases.

The Ir4+ substitution dependence of electric polarization as a probe of magnetic phase stability in multiferroic MnWO4

Hubnerite MnWO4 is a highly frustrated magnetic compound that has been known for its multiferroic properties. The intrinsic connection of ferroelectric polarization and magnetically frustrated structure allows an opportunity to probe the stability of magnetic structures against perturbations by means of measuring ferroelectric polarization. In this work, we investigate the ferroelectric polarization of Mn1 − 2xIrxWO4 to probe the stability of the low-temperature (T) collinear antiferromagnetic (AF1) phase against the Ir substitution, considering the strong spin-orbital coupling of Ir4+ that would enhance the single-ion anisotropy, on the one hand, and would favor the noncollinear spin alignment, on the other hand. Different from Mn1 − xRux/2WO4, it is suggested that the AF1 phase is only partially suppressed by the Ir substitution, allowing the emergence of the noncollinear antiferromagnetic (AF2) phase in coexistence with the collinear AF1 phase. Proper Ir substitution may promote both the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction, thus making the modulation of the magnetic structure more complicated.

Comprehensive inelastic neutron scattering study of the multiferroic .

Using high-resolution inelastic neutron scattering, we examine the spin dynamics of in the collinear AF1, the spiral AF2, and the cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions ( up to 12th nearest neighbors) and the single-ion anisotropy induced by the spin-orbit interaction. While the exchange constants are relatively unchanged, the dominant effect of doping is to change the single-ion anisotropy from easy axis in the collinear AF1 phase to easy plane in the two multiferroic phases.

Magnetic transition in the Heusler compounds Fe3-xMnxSi

It has been shown that Fe2MnSi exhibits a ferromagnetic transition at Tc and an antiferromagnetic transition at a lower temperature, TA, to a phase referred to as the AF phase. In a recent study on Fe1.3Mn1.7Si, another antiferromagnetic transition at a temperature lower than TA, defined as TA2, was found, with the phase below TA2 referred to as the AF2 phase. In this study, magnetic properties of Fe3-xMnxSi are investigated for an x range of 1.65 ≤ x ≤ 1.85 in order to study theses transitions with varying x. For x ≥ 1.75, a transition characterized by a rather rapid decrease in the temperature dependence of magnetization is observed at a temperature lower than TA at fields higher than ∼2 T. This implies the AF2-AF transition exists for x ≥ 1.75 as for x = 1.7. The magnetic field where the AF2-AF transition occurs increases with x, whereas at lower fields TA2 does not depend strongly on x. Meanwhile, at near zero field, the ferromagnetic transition and spontaneous magnetization disappears for x ≥ 1.75, in contrast to the case for x = 1.7. This implies that the transition from paramagnetism directly to the AF2 phase occurs at low fields. These results are summarized in the B-T magnetic phase diagram.

Transport properties of Heusler compounds Fe3-xMnxSi (x = 1.6, 1.7 and 1.8) under magnetic fields

Measurements of the electrical resistivity, magnetoresistance (MR), and Hall effects are performed to investigate the transport properties of Fe3-xMnxSi (x = 1.6, 1.7, and 1.8) under magnetic fields. It is known that Fe2MnSi exhibits a ferromagnetic transition at TC and an antiferromagnetic transition at a lower temperature (TA) to the phase referred to as the AF phase. A recent study on Fe1.3Mn1.7Si found another antiferromagnetic transition at a temperature below TA, defined as TA2, to a phase referred to as the AF2 phase. In this study, in MR measurements for x = 1.7 and 1.8, the change in MR with magnetic field is found to transition from positive to negative at certain magnetic fields. Comparing these results with those of magnetization measurements, these points correspond to the AF2-AF transitions. The results also indicate that these transitions occur at a higher field for x = 1.8 than for x = 1.7. The Hall effect measurements show the anomalous Hall effect for x = 1.6 and 1.7 in the temperature region where spontaneous magnetization is present, but not for x = 1.8, for which the ferromagnetic state appears to be absent. These results are consistent with the magnetization measurements.

Heisenberg Ising–Kondo necklace model with transverse field for the heavy fermion compound URu2Si2

In this work, we introduce the Heisenberg Ising–Kondo necklace with transverse field as a possible model to describe the heavy-fermion compound URu2Si2. The physics of this compound presents many open questions, like the transition to the hidden order (HO) phase at T0 = 17.5 K. Our Hamiltonian includes elements that come from crystal field processes and Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. The main idea is to allow the transverse field, that a priori acts only in the localized moments, to influence the conduction-electrons via the RKKY interaction. Our results, obtained using a spin-wave approach for a one-dimensional lattice system, reveal a re-entrance behavior on the zero temperature phase diagram. This is an interesting result since combined with the magnetization values of the canted AF phase allow to link this phase with the mixed hidden order/large moment antiferromagnetic (HO/LMAF) phase observed in URu2Si2.

Resonant x-ray scattering observation of transitional subphases during the electric-field-induced phase transition in a mixture of Se-containing chiral smectic liquid crystals.

Using resonant x-ray scattering techniques, transitional subphases during the electric-field-induced phase transition of a mixture of Se-containing chiral liquid crystals, 80% AS657 and 20% AS620, in a planar-aligned cell geometry were investigated, where the prototypical phase sequence SmC_{A}^{*}-SmC_{γ}^{*}-AF-SmC^{*} was observed; the transitional subphases were formed during the transition from the three-layer periodicity phase to the ferroelectric phase. In the lower-temperature range where the three-layer SmCγ^{*} phase appeared under the low electric field, nine- and six-layer subphases and a "streak" pattern appeared in sequence after the transition from the SmCγ^{*} phase with increasing applied electric field; the ferroelectric phase was realized. In the higher-temperature range where the four-layer AF phase appeared under a low electric field, the AF phase changed to a three-layer phase at the medium electric field. The twelve-, nine-, and six-layer subphases subsequently appeared in sequence, and finally the ferroelectric phase was generated with increasing electric field. The molecular arrangements of the field-induced subphases, especially the newly found nine-layer periodicity phase, was analyzed. The successive field-induced phase transition of the present results was compared with that of our previous results for pure Se-containing and Br-containing liquid crystals, and the relation to the three-layer ferrielectric phase was discussed.

Two spin-canting textures in the antiferromagnetic phase AF1 of MnWO4 based on the new polar atomistic model in P2

The low temperature antiferromagnetic (AF) phase of MnWO4 (the so-called AF1 phase) exhibits different spin-canting configurations at two Mn2+ sublattices of the (3 + 1)-dimensional magnetic structure. The suggested superspace group is a significant consequence of the polar space group 2 true for the nuclear structure of MnWO4. Density functional theory calculations showed that its ground state prefers this two spin-canting system. The structural difference between two independent atomic sites for Mn (Mna, Mnb) is too small to allow microscopically detectable electric polarisation. However, this hidden intrinsic polar character allows AF1 two commensurately modulated spin-canting textures. This is considered as the prerequisite onset of the improper ferroelectricity enhanced by the helical spin order in the multiferroic phase AF2 of MnWO4.

Superconducting and Antiferromagnetic Phases of Space-Time

A correspondence between the SO5 theory of high-TC superconductivity and antiferromagnetism, put forward by Zhang and collaborators, and a theory of gravity arising from symmetry breaking of a SO5 gauge field is presented. A physical correspondence between the order parameters of the unified SC/AF theory and the generators of the gravitational gauge connection is conjectured. A preliminary identification of regions of geometry, in solutions of Einstein’s equations describing charged-rotating black holes embedded in de Sitter space-time, with SC and AF phases is carried out.

Electronic, magnetic and spectroscopic properties of doped Mn(1−x)AxWO4 (A = Co, Cu, Ni and Fe) multiferroic: an experimental and DFT study

The influence of dopants (Co, Cu, Fe and Ni) on the optical, electronic and magnetic properties of multiferroic MnWO4 was studied using Raman spectroscopy, ultraviolet–visible spectroscopy (UV–Vis), magnetization measurements and density functional theory (DFT) calculations. The evolution of Raman spectra with different elemental substitutions at the Mn site was also studied, where the peak width increased with doping of higher mass elements (Co, Cu, Fe and Ni). UV–Vis diffuse reflectance spectroscopy on polycrystalline Mn(1−x)AxWO4 (A = Co, Cu, Fe and Ni) (0 ⩽ ⩽ 0. was performed. The evaluated electronic band gap decreasing with successive Co, Cu and Fe doping reflected the lower ionic radius of the substituted element, and for Ni-doped MnWO4 the band gap increased slightly compared to the parent MnWO4. Bader charge transfer and a partial density of states (PDOS) analysis from DFT simulations predict the appearance of impurity states in the band gap region (of pure MnWO4) from the d orbital of the dopant (Co, Cu and Fe) hybridized with the p orbital of the bonded O atoms due to charge transfer from O to the dopant, and reduced the band gap of Co, Cu and Fe-doped MnWO4. On the other hand, for Ni-doped MnWO4 strong W–O hybridization occurring due to large charge transfer from oxygen to tungsten leads to an increase in the band gap. The band gap, computed using the GGA + U method, is close to the experimental value. The signature of the d–d transition observed in the UV spectra is explained in terms of the crystal field stabilization energy caused by the octahedral distortion present in the lattice. Three different antiferromagnetic phases (AF1, AF2 and AF3) are identified in MnWO4 and also for the Co (18.75%)-doped sample. For Cu-doped samples, suppression of the AF1 phase and stabilization of the AF2 phase is observed up to 2 K. Successive doping of Cu leads to the diminution of magnetic frustration. A new magnetic order is identified for Ni-doped MnWO4 in the temperature range 13.7–20 K.

Phase transitions and magnetic structures in MnW1−xMoxO4 compounds (x ⩽ 0.2)

Temperature-dependent specific heat, magnetization and neutron diffraction data have been collected in zero magnetic field for polycrystalline samples of MnW1−xMoxO4 (x ⩽ 0.2) solid solution whose end-member MnWO4 exhibits a magnetoelectric multiferroic phase (AF2 phase) between T1 ≈ 8 K and T2 = 12.5 K. In MnW1−xMoxO4, diamagnetic W6+ are replaced with diamagnetic Mo6+ cations and magnetic couplings among Mn2+ (3d 5, S = 5/2) ions are modified due the doping-induced tuning of the orbital hybridization between Mn 3d and O 2p states. It was observed that magnetic phase transition temperatures which are associated with the second-order AF3-to-paramagnetic (TN) and AF2-to-AF3 (T2) transitions in pure MnWO4 slightly increase with the Mo content x. Magnetic specific heat data also indicate that the first-order AF1-to-AF2 phase transition at T1 survives a weak doping x ⩽ 0.05. This latter phase transition becomes invisible above the base temperature 2 K for higher level of doping x ⩾ 0.10. Neutron powder diffraction datasets collected above 1.5 K for a sample of MnW0.8Mo0.2O4 were analyzed using the Rietveld method. The magnetic structure below ≈ 14 K is a helical incommensurate spin order with a temperature-independent propagation vector k = (−0.217(6), 0.5, 0.466(4)). This cycloidal magnetic structure is similar to the polar AF2 structure observed in MnWO4. The AF1 up-up-down-down collinear spin arrangement observed in MnWO4 is absent in our MnW0.8Mo0.2O4 sample.