Field-temperature Phase Diagram Research Articles

Temperature-field phase diagram of the two-dimensional dipolar Ising ferromagnet

We study field-induced phase transitions in the two-dimensional dipolar Ising ferromagnet with a specific ratio between the exchange and dipolar constants, $\delta=1$, which exhibits a stripe-ordered phase with the width of one lattice unit at low temperatures without magnetic field. By using a mean-field (MF) approximation and a Monte Caro (MC) method with the stochastic-cutoff algorithm, which is an $O(N)$ simulation method, we show the temperature-field phase diagrams. In the MC study the orientational order and the structure factor are evaluated. Second-order transition points are determined by a finite-size-scaling analysis and first-order transition points are identified by the analysis of the energy histogram. Although both the MF and MC phase diagrams consist of wide regions of several stripe-ordered phases and narrow regions between them characterized by complicated stripe patterns, they show qualitative and quantitative differences in possible phases and phase boundaries. In the MF phase diagram, three main stripe-ordered phases exhibit a nesting structure, while in the MC phase diagram, two main stripe-ordered phases are located separately, which causes a characteristic field-induced reentrant transition of the orientational order.

Improvements of high-field pinning properties of polycrystalline Fe(Se,Te) material by heat treatments

We present the fabrication of FeSe0.5Te0.5 polycrystalline samples by self-flux method, showing the presence of the peak-effect in the vortex lattice configuration. To improve the performances at high magnetic fields for this iron-based superconductor of the 11-family, the two-step solid-state reaction process has been successfully modified by increasing the temperature of the heat treatment and by slowing down the cooling rate. The magnetic field-temperature phase diagram has been investigated by magneto-resistance, magnetization, and heat capacity measurements in applied magnetic fields up to 18 T. The magnetization curves exhibit an enhancement of the peak-effect whose position shifts by varying the temperature, following a similar dependence than that previously reported in high-temperature superconducting materials such as YBa2Cu3O7-δ. The presence of the peak-effect can be correlated to the sample manufacture, since by tuning a proper heat treatment it becomes observable ever more in the magnetic field-temperature phase diagram. This fabrication route paves the way to a systematic increase in the critical current density thus becoming relevant for applications.

Origin of skyrmion lattice phase splitting in Zn-substitutedCu2OSeO3

We present an investigation into the structural and magnetic properties of Zn-substituted Cu$_{2}$OSeO$_{3}$, a system in which the skyrmion lattice (SkL) phase in the magnetic field-temperature phase diagram was previously seen to split as a function of increasing Zn concentration. We find that splitting of the SkL is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behaviour. No such multiphase behaviour is observed in single crystal samples.

Multiple magnetic transitions in single crystals of Ce12Fe57.5As41 and La12Fe57.5As41

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce12Fe57.5As41 and La12Fe57.5As41. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2-300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce12Fe57.5As41 and La12Fe57.5As41, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce12Fe57.5As41. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce12Fe57.5As41. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150-300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.

Double hysteresis loops in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 near the ferroelectric critical endpoint

Polarization-electric field hysteresis loops in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 have been investigated near the ferroelectric critical temperature in the bipolar field applied along the [001]c direction in the pseudo-cubic coordinate. The double hysteresis loop has been observed above the transition temperature indicating the field induced transition, and the ferroelectric critical endpoint has been found at 192 °C and 1.6 kV/cm. The temperature field phase diagram under the electric field applied along the [001]c direction has been clarified.

Magnetic phases of skyrmion-hostingGaV4S8−ySey(y=0,2,4,8)probed with muon spectroscopy

We present the results of a muon-spin spectroscopy investigation of ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8\ensuremath{-}y}{\mathrm{Se}}_{y}$ with $y=0,2,4$, and 8. Zero-field measurements suggest that ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$ and ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and 4 materials showing glassy magnetic behavior at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni).

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH3)2NH2] M(HCOO)3 ( M = Mn2+, Co2+, Ni2+) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH3)2NH2]Mn(HCOO)3 takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across TC reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH3)2NH2]Co(HCOO)3 reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co2+ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Temperature dependence of the dielectric tunability near the critical endpoint in Pb(Zn1/3Nb2/3)O3 –PbTiO3

ABSTRACTDielectric permittivities under the dc field applied along the [001]c direction (in the cubic coordinate) have been measured in Pb(Zn1/3Nb2/3)O3-7% PbTiO3 (PZN-7%PT), and the ferroelectric critical endpoint (CEP) has been found at 171 °C and 1.3 kV/cm on the temperature-field phase diagram. Near the critical tempeature, the dielectric tunability has been studied with the emphasis put on its temperature dependence. A large dielectric tunability of 92.8% in the maximum field of 10 kV/cm is obtained near 170 °C, and high-tunabilities more than 60% is found in a temperature range between 165 °C and 200 °C in PZN-7%PT. Dielectric tunability above the critical temperature has been discussed within the Landau theory.

Physical properties and field-induced metamagnetic transitions in UAu0.8Sb2

We have successfully synthesized single crystals of UAu0.8Sb2 using a flux method and present a comprehensive study of its physical properties by measuring the magnetic susceptibility, electrical resistivity and specific heat. Evidence for at least three magnetic phases is observed in the field-temperature phase diagram of UAu0.8Sb2. In zero field, the system undergoes an antiferromagnetic transition at 71 K, and upon further cooling it passes through another antiferromagnetic phase with a ferromagnetic component, before reaching a ferromagnetic ground state. A complex magnetic field-temperature phase diagram is obtained for fields along the easy c-axis, where the antiferromagnetic order eventually becomes polarized upon applying a magnetic field.

Enhancement of Energy Storage Performance by Criticality in Lead‐Free Relaxor Ferroelectrics

In modern electronic systems and energy conversion, efficient capacitors with large energy densities are needed. Relaxor ferroelectrics show the potential to achieve those requirements. The lead‐free relaxor ferroelectric 0.852(Na1/2Bi1/2TiO3)–0.028(BaTiO3)–0.12(K1/2Bi1/2TiO3) is investigated exhibiting the behavior of a wide range of Na1/2Bi1/2TiO3‐based relaxor systems. The criticality of this system is analyzed and a full electric field–temperature phase diagram is constructed. The energy storage performance is determined from polarization measurements in a wide temperature range. It is found that energy density and especially the efficiency is largely increased in the vicinity of the critical end point of the relaxor system. The concept of criticality is widely applicable to lead‐based and lead‐free relaxor ferroelectrics and therefore an important approach to increase energy storage performance in those systems.

Fulde-Ferrell-Larkin-Ovchinnikov superconductivity in the layered organic superconductor β”−(BEDT−TTF)4[(H3O)Ga(C2O4)3]C6H5NO2

Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\ensuremath{\beta}''\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{4}[({\mathrm{H}}_{3}\mathrm{O})\mathrm{Ga}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ with ${T}_{c}=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\ensuremath{\sim}21$ T, suggesting the upper critical field ${H}_{c2}\ensuremath{\gtrsim}21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to ${H}_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and ${H}_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

Electric field–temperature phase diagram of sodium bismuth titanate-based relaxor ferroelectrics

The electric field–temperature phase diagrams of three bismuth sodium titanate-based relaxor ferroelectrics are reported, namely 0.94(Na1/2Bi1/2TiO3)–0.06(BaTiO3), 0.80(Na1/2Bi1/2TiO3)–0.20(K1/2Bi1/2TiO3) and 0.75(Na1/2Bi1/2TiO3)–0.25(SrTiO3). Relaxor behavior is demonstrated by temperature-dependent dielectric permittivity measurements in the unpoled and poled states, as well as by the field-induced phase transition into a ferroelectric phase from the relaxor phase. From temperature-dependent thermometry measurements, we identified the threshold electric field to induce the ferroelectric phase and obtained the released latent heat of the phase transition. We determined the nonergodic and ergodic relaxor phase temperature range based on the absence or presence of reversibility of the relaxor to ferroelectric transition. For all three compositions, the electric field–temperature phase diagram was constructed and a critical point was identified. The constructed electric field–temperature phase diagrams are useful to find optimum operational ranges of ferroelectrics and relaxors for electromechanical and electrocaloric applications.

Field-induced uniaxial and biaxial nematic phases in the Maier–Saupe–Zwanzig (MSZ) lattice model

ABSTRACTWe report detailed statistical mechanics calculations to analyse the field-temperature phase diagram of a fully connected Maier–Saupe–Zwanzig (MSZ) lattice model for the nematic transitions in liquid-crystalline systems. If the field is applied along the uniaxial direction, with positive anisotropy, there is a first-order boundary between distinct uniaxial paranematic phases, which ends at a simple critical point. For negative values of the anisotropy, the first-order boundary becomes continuous at a tricritical point, and the transition is between field-induced uniaxial and biaxial structures. This elementary MSZ lattice model provides a way to systematically regain all of the main features of earlier phenomenological and model calculations. Due to the simplicity of the calculations, critical and tricritical points, as well as critical lines, can be located analytically, which leads to a possibility of comparisons with values for real experimental systems. Also, we formulate a generalised six-state MSZ model, which takes into account intrinsic molecular biaxiality, and perform calculations to determine the location of the critical point in the field-temperature phase diagram.

In-field X-ray and neutron diffraction studies of re-entrant charge-ordering and field induced metastability in La0.175Pr0.45Ca0.375MnO3-δ

Low-temperature high-magnetic field (2 K, 8 T) (LTHM) powder X-ray diffraction (XRD) and time of flight powder neutron diffraction (NPD), low-temperature transmission electron microscopic (TEM), and resistivity and magnetization measurements have been carried out to investigate the re-entrant charge ordering (CO), field induced structural phase transitions, and metastability in phase-separated La0.175Pr0.45Ca0.375MnO3-δ (LPCMO). Low-temperature TEM and XRD studies reveal that on cooling under zero-field, paramagnetic Pnma phase transforms to P21/m CO antiferromagnetic (AFM) insulating phase below ∼233 K. Unlike reported literature, no structural signature of CO AFM P21/m to ferromagnetic (FM) Pnma phase-transition during cooling down to 2 K under zero-field was observed. However, the CO phase was found to undergo a re-entrant transition at ∼40 K. Neutron diffraction studies revealed a pseudo CE type spin arrangement of the observed CO phase. The low-temperature resistance, while cooled under zero-field, shows insulator to metal like transition below ∼105 K with minima at ∼25 K. On application of field, the CO P21/m phase was found to undergo field-induced transition to FM Pnma phase, which shows irreversibility on field removal below ∼40 K. Zero-field warming XRD and NPD studies reveal that field-induced FM Pnma phase is a metastable phase, which arise due to the arrest of kinetics of the first-order phase transition of FM Pnma to CO-AFM P21/m phase, below 40 K. Thus, a strong magneto-structural coupling is observed for this system. A field-temperature (H-T) phase-diagram has been constructed based on the LTHM-XRD, which matches very nicely with the reported H-T phase-diagram constructed based on magnetic measurements. Due to the occurrence of gradual growth of the re-entrant CO phase and the absence of a clear structural signature of phase-separation of CO-AFM P21/m and FM Pnma phases, the H-T minima in the phase-diagram of the present LPCMO sample has been attributed to the strengthening of AFM interaction during re-entrant CO transition and not to glass like “dynamic to frozen” transition.

Upper critical and irreversibility fields in Ni- and Co-doped pnictide bulk superconductors

A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.95Ni0.05)2As2 (large grain and small grain samples), Ba(Fe0.94Ni0.06)2As2, Ba(Fe0.92Co0.08)2As2, and Ba(Fe0.92Co0.09)2As2 polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65T. The full magnetic field-temperature (H-T) phase diagrams, starting at 1.5K, were measured. The higher temperature, upper critical field Hc2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. At low temperatures, the experimental data depart from the fitted WHH curves, suggesting an emergence of a new phase that could be attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The large values of the Maki fitting parameter α indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. Table of measured physical parameters allows us to compare these pnictide superconductors for different Co- and Ni- doping levels and granularity.

Magnetic field-temperature phase diagram of multiferroic [(CH3)2NH2]Mn(HCOO)3

We combined pulsed field magnetization and first-principles spin-density calculations to reveal the magnetic field-temperature phase diagram and spin state character in multiferroic $[{({\mathrm{CH}}_{3})}_{2}{\mathrm{NH}}_{2}]\mathrm{Mn}{(\mathrm{HCOO})}_{3}$. Despite similarities with the rare earth manganites, the phase diagram is analogous to other Mn-based quantum magnets with a 0.31 T spin flop, a 15.3 T transition to the fully polarized state, and short-range correlations that persist above the ordering temperature. The experimentally accessible saturation field opens the door to exploration of the high-field phase.

Dielectric tunability in Pb(Sc1/2Ta1/2)O3 single crystals

The dc field dependence of dielectric permittivity in Pb(Sc1/2Ta1/2)O3 (PST) has been investigated near room temperature. The phase boundary on the temperature–field phase diagram in PST has been determined, where the ferroelectric critical endpoint (CEP) under the dc field applied along the [111]c direction (in the cubic coordinate) has been estimated to be between 20 and 25 °C. A high dielectric tunability of 92% in the field of 20 kV/cm along the [111]c direction has been obtained at 20 °C. The dielectric tunability above the critical temperature in the case of the first-order phase transition has been discussed on the basis of the Landau theory.

Phase diagram of Ba [formula omitted] NaOsO [formula omitted], a Mott insulator with strong spin orbit interactions

We report 23Na nuclear magnetic resonance (NMR) measurements of the Mott insulator with strong spin-orbit interaction Ba2NaOsO6 as a function of temperature in different magnetic fields ranging from 7T to 29T. The measurements, intended to concurrently probe spin and orbital/lattice degrees of freedom, are an extension of our work at lower fields reported in Lu et al. (2017) [1]. We have identified clear quantitative NMR signatures that display the appearance of a canted ferromagnetic phase, which is preceded by local point symmetry breaking. We have compiled the field temperature phase diagram extending up to 29T. We find that the broken local point symmetry phase extends over a wider temperature range as magnetic field increases.

Competing Bose-glass physics with disorder-induced Bose-Einstein condensation in the doped S=1 antiferromagnet Ni(Cl1−xBrx)2−4SC(NH2)2 at high magnetic fields

We study the interplay between disorder and interactions for emergent bosonic degrees of freedom induced by an external magnetic field in the Br-doped spin-gapped antiferromagnetic material Ni(Cl$_{1-x}$Br$_x$)$_2$-4SC(NH$_2$)$_2$ (DTNX). Building on nuclear magnetic resonance experiments at high magnetic field [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we describe the localization of isolated impurity states, providing a realistic theoretical modeling for DTNX. Going beyond single impurity localization we use quantum Monte Carlo simulations to explore many-body effects from which pairwise effective interactions lead to a (impurity-induced) BEC revival [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017)]. We further address the question of the existence of a many-body localized Bose-glass (BG) phase in DTNX, which is found to compete with a series of a new kind of BEC regimes made out of the multi-impurity states. The global magnetic field-temperature phase diagram of DTNX reveals a very rich structure for low impurity concentration, with consecutive disorder-induced BEC mini-domes separated by intervening many-body localized BG regimes. Upon increasing the impurity level, multiple mini-BEC phases start to overlap, while intermediate BG regions vanish.

Field–Temperature Phase Diagram of Intergrain Ordering in Superconducting Ceramic YBCO

The ceramic YBa2Cu4O8 superconductor composed of submicron grains is considered a random Josephson-coupled network containing the so-called π junctions and shows successive phase transitions. With decreasing temperature, first the intragrain superconductive transition occurs inside each grain at Tc1 and then the chiral-glass transition occurs among the grains at Tc2 (< Tc1). The third transition at Tc3 (< Tc2) is the intergrain superconducting transition. We measured the nonlinear susceptibility and resistivity of the ceramic YBa2Cu4O8 superconductor to determine the field dependences of the transition temperatures Tc2 and Tc3. The phase diagram of the intergrain ordering is discussed in light of the result predicted by Kawamura.