Antiferromagnetic-paramagnetic Transition Research Articles

The Nd-Fe-Rh system: Phase relations determination at 600 ℃ and magnetic property measurement of related compounds

The isothermal section of the Nd-Fe-Rh system at 600 ℃ was constructed based on the phase identification and analysis results from annealed alloys using X-ray diffraction and Scanning electron microscopy equipped with Energy dispersive spectroscopy. Two previously reported binary compounds, Nd4Rh and Fe2Nd, could not be confirmed. No new ternary compounds were found in the isothermal section. In the present work, the homogeneity ranges of the Fe-Rh binary system were redefined using phase disappear method, which were determined to be about 70–100 Fe at%, 51–69 Fe at%, 0–45 Fe at% for the single-phase region α-Fe, FeRh and γ-(Rh, Fe), respectively. It was confirmed that the maximum solid solubility of Fe in NdRh2, Nd3Rh and Nd7Rh3 is about 15.6 at% Fe, 4.0 at% Fe and 3.1 at% Fe, respectively, while no appreciable solubility was observed for the other binary compounds in the Nd-Fe-Rh system. The isothermal section of the Nd-Fe-Rh ternary system at 600 ℃ consists of 13 single-phase regions, 23 two-phase regions, and 11 three-phase regions. It suggests the presence of predominant antiferromagnetic exchange interaction in the NdRh compound. For the 38Nd-8Fe-54Rh sample, two magnetic entropy change peaks originate from the combined contribution from two magnetic phase transitions, an antiferromagnetic-paramagnetic transition and a ferromagnetic-paramagnetic transition for NdRh and NdRh2 phase, respectively.

Magnetic properties and magnetocaloric effect of a metallic triangular lattice antiferromagnetic DyAl2Ge2 single crystal

We report the synthesis, magnetic susceptibility, magnetization, electrical resistivity, heat capacity, and magnetocaloric properties of the metallic triangular lattice antiferromagnetic DyAl2Ge2 single crystal. High-quality rare-earth intermetallic compound DyAl2Ge2 single crystals were synthesized by the metallic flux method. DyAl2Ge2 crystallizes in a trigonal CaAl2Si2-type structure with Dy atoms forming a triangular lattice in the ab plane. The paramagnetic-antiferromagnetic transition occurs at 8 K, as determined by temperature dependencies of magnetic susceptibility and heat capacity. No observable hysteresis was found in the field-induced antiferromagnetic to the ferromagnetic metamagnetic transition of DyAl2Ge2. The electrical resistivity shows an overall metallic behavior with an upturn at the ordering temperature. For a magnetic field change of 0–70 kOe, the maximum magnetic-entropy change is −14.2 J kg−1 K−1 at 11 K and the relative cooling power is 327 J kg−1 with the applied magnetic field along the ab plane.

Observation of Griffiths phase, spin glass behaviour and magnetocaloric effect in frustrated Ga2Mn2-xCrxO7 (x = 0, 0.1, 0.3, 0.5) pyrochlore compounds

New series of pyrochlore compounds Ga2Mn2-xCrxO7 (x = 0–0.5) has been synthesized by conventional solid-state reaction method. The X-ray diffraction analysis confirms a single-phase cubic structure of the prepared compounds. All compounds undergo a paramagnetic-antiferromagnetic transition at the Néel transition temperature, close to 42 K. Below ∼12 K temperature, magnetization measurements reveal a glassy behaviour. The value of frustration parameter indicates the presence of a spin frustration. All the compounds exhibit the Griffiths phase. The Arrott's plots confirmed a second-order magnetic phase transition. Moreover, under 5 T applied magnetic field, the maximum value of magnetic entropy change −ΔS was 1.36 J/kg.K for Ga2Mn2O7 compound, which slightly increased to 1.47 J/kg.K for the compound with x = 0.5. At 21 K, the relative cooling power value enhanced from 42.96 J/kg to 47.73 J/kg with Cr4+-ion content changing from x = 0 to x = 0.5.

Metal-Insulator Transition of Single-Crystal V2O3 through van der Waals Interface Engineering.

Strongly correlated electron materials harbor interesting materials physics, such as high-Tc superconductivity, colossal magnetoresistance, and metal-insulator transition. These physical properties can be greatly influenced by the dimensionality and geometry of the hosting materials and their interaction strengths with underlying substrates. In a classic strongly correlated oxide vanadium sesquioxide (V2O3), the coexistence of a metal-insulator and paramagnetic-antiferromagnetic transitions at ∼150 K makes this material an excellent platform for exploring basic physics and developing future devices. So far, most studies have been focused on epitaxial thin films in which the strongly coupled substrate has a pronounced effect on V2O3, leading to the observations of intriguing phenomena and physics. In this work, we unveil the kinetics of a metal-insulator transition of V2O3 single-crystal sheets at nano and micro scales. We show the presence of triangle-like alternating metal/insulator phase patterns during phase transition, which is drastically different from the epitaxial film. The observation of single-stage metal-insulator transition in V2O3/graphene compared to the multistage in V2O3/SiO2 evidence the importance of sheet-substrate coupling. Harnessing the freestanding form of the V2O3 sheet, we show that the phase transition of V2O3 sheet can generate a large dynamic strain to monolayer MoS2 and tune its optical property based on the MoS2/V2O3 hybrid structure. The demonstration of the capability in tuning phase transition kinetics and phase patterns using designed hybrid structure of varied sheet-substrate coupling strengths suggests an effective knob in the design and operation of emerging Mott devices.

Quantum phase transitions in the triangular coupled-top model

We study the coupled-top model with three large spins located on a triangle. Depending on the coupling strength, there exist three phases: disordered paramagnetic phase, ferromagnetic phase, and frustrated antiferromagnetic phase, which can be distinguished by the mean-field approach. The paramagnetic-ferromagnetic phase transition is accompanied by the breaking of the global ${Z}_{2}$ symmetry, whereas the paramagnetic-antiferromagnetic phase transition is accompanied by the breaking of both the global ${Z}_{2}$ symmetry and the translational symmetry. Exact analytical results of higher-order quantum effects beyond the mean-field contribution, such as the excitation energy, quantum fluctuation, and von Neumann entropy, can be achieved by the Holstein-Primakoff transformation and symplectic transformation in the thermodynamic limit. Near the quantum critical point, the energy gap closes, along with the divergence of the quantum fluctuation in certain quadrature and von Neumann entropy. Particular attention should be paid to the antiferromagnetic phase, where geometric frustration takes effect. The critical behaviors in the antiferromagnetic phase are quite different from those in the paramagnetic and ferromagnetic phases, which highlight the importance of geometric frustration. The triangular coupled-top model provides a simple and feasible platform to study the quantum phase transition and the novel critical behaviors induced by geometric frustration.

Strain fields and critical phenomena in manganites II: spin-lattice-energy Hamiltonians

The dynamic critical behavior at the paramagnetic-antiferromagnetic (PM-AFM) transition in manganites has recently been studied experimentally (Niermann et al 2015 Phys. Rev. Lett. 114 037204). We extend the Hamiltonian of paper I by incorporating an energy field, and study the corresponding Model C of critical dynamics. We use the dynamic renormalization group approach and calculate the dynamic critical exponents z, and the line-width exponent Δ to leading order in the small expansion parameters and . Here, d is the space dimension and σ is the long-range exponent. Using σ as an adjustable parameter, the theory gives us a good match to the experimentally available static and dynamic critical exponents at the PM-AFM transition.

Neutron diffraction study on anomalous thermal expansion of CrB2

Thermal expansion is an essential issue in the field of materials science and engineering. Investigation of anomalous thermal expansion is beneficial to controlling it and developing related functions. Here, we report disitinctly anisotropic thermal expansion of CrB2 via temperature dependence of neutron diffraction, in which positive thermal expansion is observed within basal plane whereas negative thermal expansion emerges along the c direction. Intriguingly, zero thermal expansion of unit cell volume is determined from 5 to 130 ​K with the coefficient of thermal expansion of ᾱV ​= ​0.4(1) ​× ​10−6 ​K−1. Magnetization measurement shows there is an antiferromagnetic-paramagnetic transition near 90 ​K, which may correlate to the thermal expansion anomaly. DFT calculations identify no chemical binding of Cr–Cr pair, implying such antiferromagnetic ordering originates from the double exchange interaction of Cr–B–Cr.

Control of structural and magnetic transition in GeNCr3 by doping manganese ions

As an archetypal antiperovskite nitride compound, GeNCr3 undergoes a temperature-driven first-order antiferromagnetic-paramagnetic transition at approximately 418 K, accompanied with a structural phase transition from space group P-421m to I4/mcm. In this paper, we show that the critical temperature of GeNCr3 can be tuned from 418 to 240 K by doping manganese ions. Simultaneously, the evolution of the magnetic transition is consistent with the structural transition. Using temperature-dependent X-ray diffraction and X-ray photoelectron spectroscopy, in combination with first-principles calculations, we demonstrate that doping manganese ions tend to occupy the equatorial plane of the nitrogen octahedron.

Peculiar temperature dependence of magneto-optic Kerr rotation associated with antiferromagnetic–paramagnetic transition

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr2O3/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr2O3 layer. The magneto-optic Kerr rotation angle (θ K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr2O3 layer. The temperature dependence of θ K at 2.67 eV (λ = 465 nm), at which the largest θ K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr2O3 thin films. Although the θ K spectra measured at several temperatures are generally explained by the classical interference model, θ K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ K is discussed on the basis of the anomaly in the optical parameters of the Cr2O3 layer associated with the antiferromagnetic–paramagnetic transition.

Antiferromagnetic chromium thin films as piezoresistive sensor materials

Sputter-deposited thin films of pure chromium and of chromium with small amounts of nitrogen are characterized regarding their electrical resistivity and strain dependence, i.e., piezoresistivity. They show a temperature dependent piezoresistive effect with gauge factors ranging approximately from 10 to 20. Related to this effect, they exhibit signs of a paramagnetic–antiferromagnetic transition at temperatures of 420 K and higher. For characterization, resistivity is measured at different strain levels: in a bending setup with a fixed radius and in a four-point bending system with reference strain gauges. Several parameter series of the sputter deposition of pure Cr films show that the higher gauge factor is correlated to a higher temperature coefficient of resistivity (TCR). The addition of nitrogen extends the range of TCR toward negative values, with gauge factors still in the same range as pure Cr. A Cr–N strain gauge is characterized and shows a linear, low-hysteresis strain–resistivity effect as well as a relatively large transverse sensitivity. Resistivity and gauge factor of one Cr and one Cr–N sample are measured from room temperature up to 600 K. These films have a resistivity anomaly indicating an antiferromagnetic ordering temperature TN that is much higher than in bulk Cr. The gauge factor has a maximum near TN and falls to small values at higher temperatures. The results indicate that the piezoresistivity of Cr and Cr-rich films is coupled to their spin-density wave (SDW) antiferromagnetism. Since the SDW state is known to be tunable through alloying, internal stress, and crystallinity, it appears that piezoresistivity can be influenced by these parameters as well.

Elastic modulus anomaly in a Fe-Mn-Cr-C cryogenic steel

With dynamic mechanical analysis, magnetism measurement and microstructure characterization, the elastic modulus anomaly in a Fe-Mn-Cr-C cryogenic steel was investigated to clarify the correlation of magnetic transition with lattice instability and microstructural evolution. It was found that the elastic modulus begins to decrease abnormally at temperatures higher than the critical temperature of the paramagnetic-antiferromagnetic transition and a noticeable drop of elastic modulus occurs around the Neel temperature upon cooling. These findings suggest that the elastic modulus anomaly is closely associated with lattice instability across a large temperature range and that the paramagnetic-antiferromagnetic transition worsens the lattice instability. Meticulous microstructure characterizations show that during the elastic modulus anomaly the nanodomains with the body-centered tetragonal structure emerge, providing the evidence for the lattice instability of the austenite in the Fe-Mn-Cr-C steel.

The antiferromagnetic phase transition in the layered Cu0.15Fe0.85PS3 semiconductor: experiment and DFT modelling

The experimental studies of the paramagnetic-antiferromagnetic phase transition through Mössbauer spectroscopy and measurements of temperature and field dependencies of magnetic susceptibility in the layered Cu0.15Fe0.85PS3 crystal are presented. The peculiar behavior of the magnetization - field dependence at low-temperature region gives evidence of a weak ferromagnetism in the studied alloy. By the ab initio simulation of electronic and spin subsystems, in the framework of electron density functional theory, the peculiarities of spin ordering at low temperature as well as changes in interatomic interactions in the vicinity of the Cu substitutional atoms are analyzed. The calculated components of the electric field gradient tensor and asymmetry parameter for Fe ions are close to the ones found from Mössbauer spectra values. The Mulliken populations show that the main contribution to the ferromagnetic spin density is originated from 3d-copper and 3p-sulfur orbitals. The estimated total magnetic moment of the unit cell (8.543 emu/mol) is in reasonable agreement with the measured experimental value of ∼9 emu/mol.

Synthesis and physical properties of perovskite Sm1−x Sr x NiO3 (x = 0, 0.2) and infinite-layer Sm0.8Sr0.2NiO2 nickelates

Recently, superconductivity at about 9–15 K was discovered in Nd1−x Sr x NiO2 (Nd-112, x ≈ 0.125–0.25) infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate the RNiO3 (R-113, R: rare-earth element) phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni3+ oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physical properties of polycrystalline perovskites Sm1−x Sr x NiO3 (x = 0, 0.2) in which the ionic radius of Sm3+ is smaller than that of Pr3+ and Nd3+ in related superconducting thin films. The structural and compositional analyses conducted by x-ray diffraction and energy dispersive x-ray spectrum reveal that the samples mainly contain the perovskite phase of Sm1−x Sr x NiO3 with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO3 undergoes a paramagnetic–antiferromagnetic transition at about 224 K on a global insulating background. In contrast, the Sr-doped sample Sm0.8Sr0.2NiO3 shows a metallic behavior from 300 K down to about 12 K, while below 12 K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12 K in Sm0.8Sr0.2NiO3. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm0.8Sr0.2NiO2 with square NiO2 planes. The compound shows an insulating behavior which can be described by the three-dimensional variable-range-hopping model. And superconductivity is still absent in the polycrystalline Sm0.8Sr0.2NiO2.

Electrical resistivity and magnetic susceptibility of substoichiometric CdO and In doped CdO films

Undoped and In doped substoichiometric CdO nanostructured films were prepared via vapor transport method. Mixed phases of hexagonal Cd and cubic CdO were obtained. No peaks related to In or indium oxides were observed in the In doped CdO pattern. Net-like assembles of nanowires were observed for undoped CdO sample whereas a regular morphology of equally shaped grains was observed for In doped CdO sample. The evaluated room temperature electrical resistivity values for undoped and In doped CdO films were 1.56 × 102 and 2.31 × 10–2 Ωcm, respectively. The temperature dependent resistivity measurements elucidated the semiconducting behavior for undoped CdO films, whereas a semiconductor–metal with a transition around 367 K was obtained for In doped CdO film. The magnetic susceptibility showed paramagnetic-antiferromagnetic transition with Neel temperatures of 266 and 308 for undoped and In doped CdO samples, respectively.

Influence of Thermal and Mechanical/Powder Processing on Microstructure and Dynamic Stiffness of Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The influence of thermal processing and mechanical alloying of powder particles on the dynamic behavior and microstructural changes occurring in an Fel4Mn6Si9Cr5Ni (mass%) alloy is investigated. Five batches of 66Fe-14Mn-6Si-9Cr-5Ni (mass%) powder mixtures were produced via powder metallurgy. After powder mixing, different volume fractions (from 0 to 40 vol%) were mechanically alloyed, remixed with the rest of as-blended particles, compressed, sintered and subsequently hot rolled to a thickness of 1 mm. They were then subjected to a thermal treatment (annealing) at different temperatures and subsequently quenched into water. Thus, we had 25 different specimens for experimentation. The specimens were then machined and loaded in tension and subjected to dynamic mechanical analysis (DMA) by varying the temperature and strain amplitude. In the DMA tests, the samples were cycled between the ambient temperature and 673 K keeping the amplitude constant, while varying the temperature. These results were then correlated to the thermomagnetic properties that were measured in the same range of temperatures. Based on these data, the internal friction (tan δ) maxima, correlated with storage modulus (E′) discontinuities, were associated with the multistage reverse transformations of martensite (e, hcp) to austenite, superimposed over the antiferromagnetic–paramagnetic transitions and accompanied by magnetization peaks (Neel temperature). The temperature ranges pertaining to tan δ maxima, E′ discontinuities and magnetization peaks were determined and discussed in terms of thermal processing temperature and volume fraction of mechanically alloyed powder particles. The variation of storage modulus with strain for three different cycles (with strain amplitude varying from zero to maximum) was traced for three different temperatures: (i) T = RT; (ii) T Ttanδ (max). The role of magnitude of thermal treatment temperature and volume fraction of mechanically alloyed powder particles on the storage modulus plateaus were given prominence for these temperatures (RT, below tanδmax and above tanδmax) as they influence the evolution of the microstructure.

Realization of a tunable surface Dirac gap in Sb-doped MnBi2Te4

Realization of the quantum anomalous Hall effect and axion electrodynamics in topological materials are among the paradigmatic phenomena in condensed matter physics. Recently, signatures of both phases are observed to exist in thin films of MnBi$_2$Te$_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is absent, or much smaller than previously thought, in MnBi$_2$Te$_4$. Here, we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped MnBi$_2$Te$_4$. This gap is found to be topologically nontrivial, insensitive to the bulk antiferromagnetic-paramagnetic transition, while enlarges along with increasing Sb concentration. Our work shows that Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ is a potential platform to observe the key features of the high-temperature axion insulator state, such as the topological magnetoelectric responses and half-integer quantum Hall effects.

Tuning the Magnetic Properties of Two-Dimensional MXenes by Chemical Etching.

Two-dimensional materials based on transition metal carbides have been intensively studied due to their unique properties including metallic conductivity, hydrophilicity and structural diversity and have shown a great potential in several applications, for example, energy storage, sensing and optoelectronics. While MXenes based on magnetic transition elements show interesting magnetic properties, not much is known about the magnetic properties of titanium-based MXenes. Here, we measured the magnetic properties of Ti3C2Tx MXenes synthesized by different chemical etching conditions such as etching temperature and time. Our magnetic measurements were performed in a superconducting quantum interference device (SQUID) vibrating sample. These data suggest that there is a paramagnetic-antiferromagnetic (PM-AFM) phase transition and the transition temperature depends on the synthesis procedure of MXenes. Our observation indicates that the magnetic properties of these MXenes can be tuned by the extent of chemical etching, which can be beneficial for the design of MXenes-based spintronic devices.

Are the thermodynamic properties of natural and synthetic Mg2SiO4-Fe2SiO4 olivines the same?

Abstract It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. We undertook the first heat capacity, CP, measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. Their CP(T) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The X-ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. CP (2 to 300 K) is measured by relaxation calorimetry. The CP results of the natural nearly end-member fayalite and published values for two different synthetic Fa100 samples are in excellent agreement. Even CP resulting from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn2+) in the natural crystal. The third-law entropy, S°, value is 151.6 ± 1.1 J/(mol·K). CP behavior of the natural forsterite, Fo0.904Fa0.096, and a synthetic olivine, Fo90Fa10, are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T3 behavior, but, instead, a plateauing of CP values. The S° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).

Nanowire reconstruction under external magnetic fields.

We consider the different structures that a magnetic nanowire adsorbed on a surface may adopt under the influence of external magnetic or electric fields. First, we propose a theoretical framework based on an Ising-like extension of the 1D Frenkel-Kontorova model, which is analyzed in detail using the transfer matrix formalism, determining a rich phase diagram displaying structural reconstructions at finite fields and an antiferromagnetic-paramagnetic phase transition of second order. Our conclusions are validated using ab initio calculations with density functional theory, paving the way for the search of actual materials where this complex phenomenon can be observed in the laboratory.

Surface-induced linear magnetoresistance in the antiferromagnetic topological insulator MnBi2Te4

Through a thorough magneto-transport study of antiferromagnetic topological insulator MnBi2Te4 (MBT) thick films, a positive linear magnetoresistance (LMR) with a two-dimensional (2D) character is found in high perpendicular magnetic fields and temperatures up to at least 260 K. The nonlinear Hall effect further reveals the existence of high-mobility surface states in addition to the bulk states in MBT. We ascribe the 2D LMR to the high-mobility surface states of MBT, thus unveiling a transport signature of surface states in thick MBT films. A suppression of LMR near the Neel temperature of MBT is also noticed, which might suggest the gap opening of surface states due to the paramagnetic-antiferromagnetic phase transition of MBT. Besides these, the failure of the disorder and quantum LMR model in explaining the observed LMR indicates new physics must be invoked to understand this phenomenon.