Neel Temperature Research Articles

Structural and Magnetic Properties of CrN: Investigated by First-Principles Calculations, Monte Carlo Simulation, and High-Temperature Series Expansions

We have employed first-principles calculations of density functional theory (DFT) with GGA and GGA+U exchange correlation to study structural, magnetic, and electronic properties of chromium nitride (CrN) in the rock salt (NaCl) structures. The obtained data from DFT calculations is used as an input in high-temperature series expansions (HTSEs) combined with the Pade approximants (PA) method and Monte Carlo simulation (MC) for the Ising model to investigate the magnetic properties of CrN. The obtained results show that CrN is more stable in the antiferromagnetic order, and undergoes structural and magnetic transitions from an antiferromagnetic NaCl structure to a nonmagnetic Pnma phase at 201 GPa. The density of states (DOS) with GGA approach shows that CrN has a metallic behavior in both FM and AFM configurations whereas with GGA+U CrN is semiconductor in AFM and half-metallic in FM order. The Neel temperature (TN) obtained by HTSE and MC is in agreement with available experimental data.

Magnetic field effect on the dielectric properties of rare earth doped multiferroic BiFeO3

The temperature, doping concentration and magnetic field dependence of the dielectric constant in lanthanum doped barium titanate are studied using a microscopic model and the Green’s function technique. A kink in the temperature dependence of the dielectric constant is observed near the Neel temperature. The Neel temperature and the ferroelectric transition temperature decrease with increasing doping concentration. The doping dependence of the dielectric constant shows a maximum value at 0.15. For small magnetic fields the dielectric constant increases strongly, whereas for stronger ones it begins to decrease slightly, i.e. there is a critical field for the transition from spiral cycloidal to antiferromagnetic spin arrangement.

Fabrication of high performance asymmetric supercapacitors with high energy and power density based on binary metal fluoride

Commercial tetragonal phase MnF2 is exploited as novel and alternative electrode material in asymmetric supercapacitors. Temperature dependent magnetic (M-T) curve evidences the antiferromagnetic transition (AFM) at Neel temperature (68.4 K). High specific capacitances of 406 F/g at 2 A/g and 80 F/g at 1 A/g are achieved in 3 and 2 electrode system, respectively. The asymmetric supercapacitor device retained 82% of its initial capacitance after 5000 cycles even at high window potential of 1.8 V which reveals excellent electrochemical stability. Ragone plot presents very high energy density (35.99 Wh/kg) with power density (822.75 W/Kg) at cell potential of 1.8 V. Electron transport dynamics in MnF2//AC asymmetric supercapacitor is also investigated by Nyquist plot of electrochemical impedance spectroscopy. These research findings demonstrate that MnF2 can serve as efficient electrode material for the electrochemical energy storage device applications.

Structural, magnetic properties and magneto-caloric performances in the antiferromagnetic RECoSi2 (RE = Er and Tm) compounds

The ternary rare earth RECoSi2 (RE = Er and Tm) intermetallic compounds were synthesized by means of arc-melting technique and their crystal structure, magnetism and magneto-caloric effect (MCE) were studied systematically. Both ErCoSi2 and TmCoSi2 crystallize in a CeNiSi2-type structure (Cmcm space group) confirmed by FULLPROF Rietveld refinement. Magnetization data indicates the antiferromagnetic (AFM) ground state for both RECoSi2 compounds, and a magnetic field-induced ferromagnetic (FM) transition occurs below the Neel temperature TN ∼6.9 K for ErCoSi2 and ∼5.4 K for TmCoSi2. Considerable reversible MCE in ErCoSi2 and TmCoSi2 was observed. The MCE performances of RECoSi2 are evaluated by -ΔSMmax (maximal magnetic entropy change), TEC(3) (temperature averaged entropy change), and RCP (relative cooling power) with the values of 7.1 J/kg K, 6.8 J/kg K and 165.9 J/kg for ErCoSi2, as well as 8.7 J/kg K, 8.5 J/kg K and 152.9 J/kg for TmCoSi2, respectively, under ΔH (field change) of 0–50 kOe.

Stabilization of antiferromagnetism in 1T- Fe0.05TaS2

1T-TaS$_2$ is a prototypical charge-density-wave (CDW) system with a Mott insulating ground state. Usually, a Mott insulator is accompanied by an antiferromagnetic state. However, the antiferromagnetic order had never been observed in 1T-TaS$_2$. Here, we report the stabilization of the antiferromagnetic order by the intercalation of a small amount of Fe into the van der Waals gap of 1T-TaS$_2$, i.e. forming 1T-Fe$_{0.05}$TaS$_2$. Upon cooling from 300~K, the electrical resistivity increases with a decreasing temperature before reaching a maximum value at around 15~K, which is close to the Neel temperature determined from our magnetic susceptibility measurement. The antiferromagnetic state can be fully suppressed when the sample thickness is reduced, indicating that the antiferromagnetic order in Fe$_{0.05}$TaS$_2$ has a non-negligible three-dimensional character. For the bulk Fe$_{0.05}$TaS$_2$, a comparison of our high pressure electrical transport data with that of 1T-TaS$_2$ indicates that, at ambient pressure, Fe$_{0.05}$TaS$_2$ is in the nearly commensurate charge-density-wave (NCCDW) phase near the border of the Mott insulating state. The temperature-pressure phase diagram thus reveals an interesting decoupling of the antiferromagnetism from the Mott insulating state.

Chiral-anomaly induced large negative magnetoresistance and nontrivial π-Berry phase in half-Heusler compounds RPtBi (R=Tb, Ho, and Er)

We report on the observation of a large negative magnetoresistance (MR) with magnitudes of −67%, −45%, and −31% in antiferromagnetic half-Heusler compounds TbPtBi, HoPtBi, and ErPtBi, respectively. It is found that with increasing temperature, the values of the negative MR vary smoothly and persist well above their Neel temperature TN. Besides the negative MR effects, we have further observed a nontrivial Berry phase (∼π) extracted from Shubnikov–de Haas oscillation in HoPtBi. These results together with band structure calculations unambiguously give evidence of the chiral anomaly effect and are valuable for understanding the Weyl fermions in magnetic lanthanide half-Heusler compounds.

Room temperature dual ferroic behavior induced by (Bi, Ni) co-doping in nanocrystalline Nd0.7Bi0.3Fe1−xNixO3 (0 ≤ x ≤ 0.3)

Nanocrystalline samples of Nd0.7Bi0.3Fe1−xNixO3 (x = 0, 0.1, 0.2, and 0.3) are synthesized using sol-gel auto-combustion process to investigate their structural, electrical, magnetic, and thermal properties. Phase purity and crystallinity of the samples are determined through x-ray diffraction (XRD) data. XRD patterns along with Rietveld refinement reveal orthorhombic structure with Pbnm space group. Unit cell parameters, bond angles, and bond lengths for all the samples have been determined. Crystallite size calculated by Scherrer equation is found to be in the range of 15–21 nm. The characteristic bands in the FTIR spectra further confirm the formation of our samples. FE-SEM images indicate the homogeneous distribution of particles on large scale and Ni-doped samples show some wall-like nanostructure in the morphology. EDX spectra confirm the elemental composition without any impurity element. The polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) loops exhibit the multiferroic nature of the material. The sample with concentration x = 0.2 shows the maximum value of polarization (Pm) while maximum magnetization is achieved for x = 0.1 concentration. The specific heat capacity (Cp), endothermic, and exothermic peaks of the materials have been determined in the broad temperature range with the help of DTA measurements in the controlled nitrogen atmosphere. The Neel temperature of the parent system is found to be 742 K and that decreases up to 573 K on Ni doping.

Quantum Criticality of Valence Transition for the Unique Electronic State of Antiferromagnetic Compound EuCu2Ge2

The effect of pressure on the unique electronic state of the antiferromagnetic (AF) compound EuCu2Ge2 has been measured in a wide temperature range from 10 mK to 300 K by electrical resistivity measurements up to 10 GPa. The Neel temperature of TN = 15 K at ambient pressure increases monotonically with increasing pressure and becomes a maximum of TN = 27 K at 6.2 GPa but suddenly drops to zero at Pc = 6.5 GPa, suggesting the quantum critical point (QCP) of the valence transition of Eu from a nearly divalent state to that with trivalent weight. The rhomag0 and A values obtained from the low-temperature electrical resistivity based on the Fermi liquid relation of rhomag = rhomag0 + AT^2 exhibit huge and sharp peaks around Pc. The exponent n obtained from the power law dependence rhomag = rhomag0 + BT^n is clearly less than 1.5 at P = Pc = 6. 5 GPa, which is expected at the AF-QCP. These results indicate that Pc coincides with Pv, corresponding to the quantum criticality of the valence transition pressure Pv. The electronic specific heat coefficient, estimated from the generalized Kadowaki-Woods relation, is about 510 mJ/mol K^2 around Pc, suggesting the formation of a heavy-fermion state.

Magnetic phase boundary of BaVS3 clarified with high-pressure μ+SR

The magnetic nature of the quasi-one-dimensional BaVS3 has been studied as a function of temperature down to 0.25 K and pressure up to 1.97 GPa on a powder sample using the positive muon spin rotation and relaxation (mu(+) SR) technique. At ambient pressure, BaVS3 enters an incommensurate antiferromagnetic ordered state below the Neel temperature (T-N)31 K. T-N is almost constant as the pressure (p) increases from ambient pressure to 1.4 GPa, then T-N decreases rapidly for p > 1.4 GPa, and finally disappears at p similar to 1.8 GPa, above which a metallic phase is stabilized. Hence, T-N is found to be equivalent to the pressure-induced metal-insulator transition temperature (T-MI) at p > 1.4 GPa.

Strain-Modulated Slater-Mott Crossover of Pseudospin-Half Square-Lattice in (SrIrO_{3})_{1}/(SrTiO_{3})_{1} Superlattices.

We report on the epitaxial strain-driven electronic and antiferromagnetic modulations of a pseudospin-half square-lattice realized in superlattices of (SrIrO_{3})_{1}/(SrTiO_{3})_{1}. With increasing compressive strain, we find the low-temperature insulating behavior to be strongly suppressed with a corresponding systematic reduction of both the Néel temperature and the staggered moment. However, despite such a suppression, the system remains weakly insulating above the Néel transition. The emergence of metallicity is observed under large compressive strain but only at temperatures far above the Néel transition. These behaviors are characteristics of the Slater-Mott crossover regime, providing a unique experimental model system of the spin-half Hubbard Hamiltonian with a tunable intermediate coupling strength.

Large Spin Hall Angle and Spin-Mixing Conductance in the Highly Resistive Antiferromagnet Mn2Au

Antiferromagnetic (AFM) materials recently have shown interest in the research in spintronics due to its zero stray magnetic field, high anisotropy, and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn attention because it exhibits unique properties and its Neel temperature is very high. Here, we report spin pumping and inverse spin Hall effect investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance. We found large spin Hall angle {\theta}_SH = 0.22

Study of the optical, dielectric and magnetic properties of the Bi0.75La0.25FeO3 sample

Bi0.75La0.25FeO3 sample has been fabricated by a novel hydrothermal-assisted co-precipitation method. The X-Ray diffraction (XRD) and Fourier transform infrared (FTIR) studies confirmed that the sample was crystallized in orthorhombic (Pbnm) and rhombohedral (R3c) phases. Using differential thermal analysis (DTA), it is observed that the sample exhibits a Neel temperature (TN= 581 K) and Curie temperature (TC=1017K). UV-visible absorption spectrum of the sample exhibits two doubly degenerate d-d transitions and three charge transfer transitions. The temperature dependence of dielectric constant of the sample at different frequencies (0.12 to 100 kHz) exhibits two anomaly peaks at 497 and 707 K. An asymmetric shift in the coercive field of the hysteresis loop (M-H) indicates the presence of exchange bias effect in the sample.

Griffiths phase, magnetic memory and ac susceptibility of an antiferromagnetic titanate-based perovskite Er0.9Sr0.1Ti0.975Cr0.025O3 system

In this study, we investigated different physical properties of Er0.9Sr0.1Ti0.975Cr0.025O3 titanate prepared by a solid-state reaction that produces a cubic structure with Fd-3m (227) as a space group. Zero-Field Cooled and Field Cooled measurements show a second-order antiferromagnetic transition at Neel temperature TN = 23 K, and the existence of Griffiths phase at around TGP = 132 K. This soft magnetic material depicts a magnetic memory since it ‘remembers’ its thermal history. The relative cooling power of this titanate-based sample was then measured to be around 292.27 J kg−1 at 5 Tesla and 400 J kg−1 at 6 Tesla. However, these values are lower than the RCP value reported for the most magnetic refrigerant Gd, although these results are high enough compared to different perovskite systems. Therefore, Er0.9Sr0.1Ti0.975Cr0.025O3 is a very suitable, environment-friendly magnetic refrigerant.

Development of size and shape dependent model for magnetic properties from bulk to nanoscale

A simple model based on the bond energy analysis is developed. The model is used to compute the size and shape dependence of magnetic properties, viz. Curie temperature TC(D), magnetization MS(D), and Neel temperature TN(D), where D denotes the size of nanoparticles. It is found that TC(D) and MS(D) decrease with a decrease in size. TN(D) is found to increase or decrease with dropping D, depending on the interaction strength at the film/substrate interface. The model predictions have been compared with the available experimental data. There is a reasonably good agreement between model predictions and experimental results. The model may be extended to other nanomaterials in different size and shape, due to its simplicity. The results obtained for corresponding bulk material are also included for comparison purposes.

Magnetic Properties of Mn3ZnN Anti-perovskite Nanoparticles: A Monte Carlo Simulations

The magnetic properties of antiferromagnetic Mn3ZnN anti-perovskite nanoparticles have been studied using Monte Carlo simulations within the Ising model. The considered Hamiltonian includes the exchange interactions between Mn–Mn and the external magnetic field. The thermal magnetizations and magnetic susceptibilities of Mn3ZnN anti-perovskite nanoparticles are computed for different sizes. The Neel temperature is deduced. The exchange interaction between Mn and Mn atoms is obtained. Beyond the coupling value 2 K, the magnetization of system becomes independent of effect temperature. In addition, the variation of magnetizations with external magnetic field and crystal filed are also established for different temperatures. Mn3ZnN anti-perovskite nanoparticles exhibits the superparamagnetic behaviour for different temperatures value.

Cations’ ordering, magnetic properties and strongly enhanced microwave absorption properties of La2NiMn1-xRuxO6

The properties of the A2BB’O6 perovskites strongly depend on the cations' orderings at the B site. The cations' orderings of La2NiMn1-xRuxO6 are improved with the Ru doping especially in x ≥ 0.5, as illustrated in the XRD patterns. It is also supported by the FT-IR spectra. The magnetic properties show that the x = 1.0 sample is antiferromagnetic with the Neel temperature at 23 K. The antiferromagnetic transition is also observed in the other samples with the transition temperature slightly varying from 23 K to 38 K. In x = 0.1, one ferromagnetic transition is observed at 242 K, and it linearly decreases to 38 K in x = 0.8. The magnetic moments are calculated based on the superexchange model. From the comparison of the calculated and the measured one, it suggests that both ordered and disordered patches coexist in x = 0.1 and 0.3, while in x ≥ 0.5 the cations are fully ordered. The Ru doping increases the dielectric permittivity due to the enhanced dipolar polarization. The dielectric loss tangents are higher in x ≥ 0.5 than those in x = 0.1 and 0.3. In x ≥ 0.8, the effective absorption with RL < −10 dB is able to cover the full frequency range of 2.2 GHz–18 GHz. In x = 1.0, the thickness with RL < −10 dB is as thin as 1.9 mm. Triple sets of microwave absorption peaks are observed. The quarter-wavelength criteria explains the relationship between the optimum thickness and the frequency.

Room temperature ferromagnetism driven by Ca-doped BiFeO3 multiferroic functional material

Herein through the study of structural, electrical, and magnetic properties of Ca-doped bismuth ferrite Bi1−xCaxFeO3-(x/2)+δ, Rietveld analysis of the X-ray diffraction patterns shows a tendency of structural phase transition from hexagonal into tetragonal structure by Ca-doping ratio of x = 0–0.3. The transport measurement also shows the phase transitions from metallic (paramagnetic) into insulator (anti-ferromagnetic) ordered with Neel temperature from 600 to 700 K. The magnetic moment versus magnetic field measurement confirms the structural phase transitions by switching the coercivity from hard to soft ferromagnetic state at room temperature. It was concluded that insertion of Ca+2 dopants substituting Bi+3 facilitates enhancement of the charge ordering and through which the ease to switch between double exchange into direct exchange mechanism.

Antiferromagnetic and dielectric behavior in polycrystalline GdFe0.5Cr0.5O3 thin film

Single phase materials with both spontaneous electric polarization and magnetization are rare, despite remarkable efforts in developing magnetoelectric multiferroics. In this work, a single-phase polycrystalline GdFe0.5Cr0.5O3 (GFCO) thin film was spin-coated onto a platinized silicon substrate. X-ray diffraction data suggest that the film exhibits an orthorhombic perovskite structure with a Pbnm space group. No other impurity phases were detected. Magnetization measurements reveal the Néel temperature of the GFCO film to be ∼220 K and illustrate a weak ferromagnetic component at 5 K, which could be due to spin canting. Frequency dependent ferroelectric–paraelectric transition was observed around 480 K, indicating the diffuse relaxor-like behavior. The electric field dependent polarization measurements show a lossy behavior below 200 K. The electric field dependent dielectric constant (tunability) measured at 1 MHz in a wide temperature range reveals that the tunability maximizes near the observed dielectric maxima, which further confirms the ferroelectric to paraelectric transition in the present film.

Derivation of Neel Temperature and Pressure Expressions for High Temperature Superconductors

Using plasma equations beside Maxwell statistical distribution law a useful expression of the Neel temperature similar to that obtained for high temperature superconductors was derived. This expression was found by obtaining a new expression of energy from the plasma equation. The same procedures were used to find an expression of the pressure and isotope mass in terms of the critical temperature for high temperature superconductors. These expressions resembles the conventional ones.

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn_{3}P.

Antiferromagnet Mn_{3}P with Neel temperature T_{N}=30 K is composed of Mn tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at ∼1.6 GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn_{3}P, which is most likely effected by the underlying frustration effect.