Field-temperature Phase Diagram Research Articles

Magnetization and magnetic phase diagrams of a spin-1/2 ferrimagnetic diamond chain at low temperature* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11374215 and 11704262), the Scientific Study Project from Education Department of Liaoning Province of China (Grant No. LJ2019004), and the Natural Science Foundation Guidance Project of Liaoning Province of China (Grant No. 2019-ZD-0070).

We used the Jordan–Wigner transform and the invariant eigenoperator method to study the magnetic phase diagram and the magnetization curve of the spin-1/2 alternating ferrimagnetic diamond chain in an external magnetic field at finite temperature. The magnetization versus external magnetic field curve exhibits a 1/3 magnetization plateau at absolute zero and finite temperatures, and the width of the 1/3 magnetization plateau was modulated by tuning the temperature and the exchange interactions. Three critical magnetic field intensities H CB, H CE and H CS were obtained, in which the H CB and H CE correspond to the appearance and disappearance of the 1/3 magnetization plateau, respectively, and the higher H CS correspond to the appearance of fully polarized magnetization plateau of the system. The energies of elementary excitation ℏ ω σ,k (σ = 1, 2, 3) present the extrema of zero at the three critical magnetic fields at 0 K, i.e., [ℏ ω 3,k (H CB)]min = 0, [ℏ ω 2,k (H CE)]max = 0 and [ℏ ω 2,k (H CS)]min = 0, and the magnetic phase diagram of magnetic field versus different exchange interactions at 0 K was established by the above relationships. According to the relationships between the system’s magnetization curve at finite temperatures and the critical magnetic field intensities, the magnetic field-temperature phase diagram was drawn. It was observed that if the magnetic phase diagram shows a three-phase critical point, which is intersected by the ferrimagnetic phase, the ferrimagnetic plateau phase, and the Luttinger liquid phase, the disappearance of the 1/3 magnetization plateau would inevitably occur. However, the 1/3 magnetization plateau would not disappear without the three-phase critical point. The appearance of the 1/3 magnetization plateau in the low temperature region is the macroscopic manifestations of quantum effect.

Thermal and magnetoelastic properties of α−RuCl3 in the field-induced low-temperature states

We discuss the implications that new magnetocaloric, thermal expansion and magnetostriction data in $\alpha$-RuCl$_{3}$ single crystals have on its temperature-field phase diagram and uncover the magnetic-field dependence of an apparent energy gap structure $\Delta (H)$ that evolves when the low temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data is modeled, $\Delta (H)$ can show a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model hosting itinerant Majorana fermions and localized $\mathbb{Z}_{2}$ fluxes. Our magnetocaloric effect data provides, below $1\,\mathrm{K}$, unambiguous evidence for dissipative phenomena at $H_{\mathrm{c}}$, smoking gun for a first order phase transition. Our results, on the other hand, show little support for a phase transition from a QSL to a polarized paramagnetic state above $H_{\mathrm{c}}$.

Enhanced spin correlations in the Bose-Einstein condensate compound Sr3Cr2O8

Combined experimental and modeling studies of the magnetocaloric effect, ultrasound, and magnetostriction were performed on single-crystal samples of the spin-dimer system Sr$_3$Cr$_2$O$_8$ in large magnetic fields, to probe the spin-correlated regime in the proximity of the field-induced XY-type antiferromagnetic order also referred to as a Bose-Einstein condensate of magnons. The magnetocaloric effect, measured under adiabatic conditions, reveals details of the field-temperature ($H,T$) phase diagram, a dome characterized by critical magnetic fields $H_{c1}$ = 30.4 T, $H_{c2}$ = 62 T, and a single maximum ordering temperature $T_{{\rm max}}(45~$T$)\simeq$8 K. The sample temperature was observed to drop significantly as the magnetic field is increased, even for initial temperatures above $T_{{\rm max}}$, indicating a significant magnetic entropy associated to the field-induced closure of the spin gap. The ultrasound and magnetostriction experiments probe the coupling between the lattice degrees of freedom and the magnetism in Sr$_3$Cr$_2$O$_8$. Our experimental results are qualitatively reproduced by a minimalistic phenomenological model of the exchange-striction by which sound waves renormalize the effective exchange couplings.

Extremely large magnetoresistance, anisotropic Hall effect, and Fermi surface topology in single-crystalline WSi2

We report on the observation of a nonsaturating, extremely large magnetoresistance (XMR) and the Fermi surface topology of a high quality $\mathrm{W}{\mathrm{Si}}_{2}$ single crystal grown by the Czochralski method. The magnetoresistance at $T=2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ reaches a value $\ensuremath{\approx}{10}^{5}%$ in 14 T magnetic field with no sign of saturation. The Hall resistivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ was found to be highly anisotropic. The analysis of magnetoconductivity data of $\mathrm{W}{\mathrm{Si}}_{2}$ revealed a near compensation of charge carrier with relatively low carrier density as compared to that of a normal metal. The observed anisotropic Hall resistivity in $\mathrm{W}{\mathrm{Si}}_{2}$ is due to the presence of multiple bands and Fermi pockets responsible for the transport phenomena in it. The extremely large carrier mobility and near compensation of charge carriers are responsible for the nonsaturating XMR behavior in $\mathrm{W}{\mathrm{Si}}_{2}$ crystal. The band structure calculation and de Haas-van Alphen effect measurement depict a cylindrical Fermi surface from which the associated quantum parameters have been obtained. The magnetotransport data of $\mathrm{W}{\mathrm{Si}}_{2}$ along both the crystallographic directions follows the universal temperature-field triangular phase diagram as observed in other materials exhibiting XMR behavior.

Magnetism ofNd2O3single crystals near the Néel temperature

Single crystals of Nd$_2$O$_3$ were grown and characterized using neutron scattering and thermodynamic measurements. Nd$_2$O$_3$ has long-range antiferromagnetic order below $T_{\rm N}$ = 0.55 K and specific heat measurements have demonstrated that a significant amount of the magnetic entropy is released above $T_{\rm N}$. Inelastic neutron scattering experiments reveal a magnetic mode(s) with little dispersion peaked at $\approx$ 0.37 meV that is of greatest intensity below $T_{\rm N}$ but persists above 2$T_{\rm N}$. This persistence of dynamic correlations is likely related to frustrated interactions associated with the nearly-ideal stacked triangular lattice geometry of $J_{\textrm{eff}}$ = 1/2 spins on Nd$^{3+}$ ions. The magnetization is observed to be strongly anisotropic at all temperatures due to crystal field effects, with easy-plane anisotropy observed. A non-compensated magnetic structure is inferred from the temperature-dependence of the magnetization when a magnetic field of sufficient strength is applied within the basal plane near $T_{\rm N}$, and the evolution of the long-range order is summarized in a temperature-field phase diagram.

A comparative study on magnetic order and field-induced magnetic transition in double perovskite iridates: RE2ZnIrO6 and RE2MgIrO6 (RE = Pr, Nd, Sm, Eu, Gd)

We perform a comparative magnetic study on two series of rare-earth (RE) based double perovskite iridates RE2BIrO6 (RE = Pr, Nd, Sm–Gd; B = Zn, Mg), which show Mott insulating state with tunable charge energy gap from ∼330 meV to ∼560 meV by changing RE cations. For nonmagnetic RE = Eu cations, Eu2MgIrO6 shows antiferromagnetic (AFM) order and field-induced spin-flop transitions below Néel temperature (TN) in comparison with the ferromagnetic (FM)-like behaviors of Eu2ZnIrO6 at low temperatures. For magnetic-moment-containing RE ions, Gd2BIrO6 show contrasting magnetic behaviors with FM-like transition (B = Zn) and AFM order (B = Mg), respectively. While, for RE = Pr, Nd and Sm ions, all members show AFM ground state and field-induced spin-flop transitions below TN irrespective of B = Zn or Mg cations. Moreover, two successive field-induced metamagnetic transitions are observed for RE2ZnIrO6 (RE = Pr, Nd) in high field up to 56 T, the resultant field temperature (H–T) phase diagrams are constructed. The diverse magnetic behaviors in RE2BIrO6 reveal that the 4f-Ir exchange interactions between the RE and Ir sublattices can mediate their magnetism.

Magnetic impurities in thin films and 2D Ising superconductors

In the theory of dilute magnetic impurities in superconductors, the effect of all impurity spin-components is expressed via a single magnetic scattering rate Γm. In a more realistic setting, magnetic impurities are anisotropic. In this case, the spatial randomness of three spin-components of impurities gives rise to generally different scattering rates Γi (i=1,2,3). We explore the effects of anisotropic magnetic impurities on the in-plane critical field in 2D superconductors. We discuss singlet, triplet, and parity-mixed order parameters allowed in systems without the inversion center. Also, the addition of a small amount of magnetic impurities may cause singlet to triplet crossovers. In all cases, different components of impurity spin affect the magnetic field—temperature phase diagram differently. We show that anisotropy of the magnetic impurities can serve as a probe of unconventional triplet or parity-mixed superconductivity.

Statistics of geometric clusters in Potts model: statistical mechanics approach.

The percolation of Potts spins with equal values in Potts model on graphs (networks) is considered. The general method for finding the Potts clusters' size distributions is developed. It allows full description of percolation transition when a giant cluster of equal-valued Potts spins appears. The method is applied to the short-ranged q-state ferromagnetic Potts model on the Bethe lattices with the arbitrary coordination number z. The analytical results for the field-temperature percolation phase diagram of geometric spin clusters and their size distribution are obtained. The last appears to be proportional to that of the classical non-correlated bond percolation with the bond probability, which depends on temperature and Potts model parameters.

Temperature–field phase diagram and spontaneous polarization in Pb(Sc1/2Ta1/2)O3 single crystal

The temperature–field phase diagram and the temperature dependence of the spontaneous polarization in Pb(Sc1/2Ta1/2)O3 (PST) have been investigated by measuring the permittivity in the dc biasing field and observing the p–E hysteresis loop, respectively. X-ray diffraction studies to clarify the B-site cation ordering have also been performed. We have confirmed the PST single crystal used in our experiment to be a disordered PST. We have also determined that the ferroelectric critical endpoint (CEP) on the temperature–field phase diagram in the dc biasing field applied along the [111]c direction (in the cubic coordinate) was at 19.3 °C and 4.85 kV cm−1. Our experimental results of the ferroelectric CEP and the spontaneous polarization in disordered PST are discussed in this paper on the basis of the Landau-type phenomenological theory.

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4 is slightly smaller than that in Ba0.6K0.4Fe2As2, which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of Jc0(0) ∼ 1.0 × 108 and 2.2 × 108 A cm−2 for CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors, respectively, which suggest potential applications.

Magnetoelastic properties and behaviour of 4C pyrrhotite, Fe7S8, through the Besnus transition

Pyrrhotite, Fe7S8, is a commonly occurring carrier of magnetic remanence and has a low temperature transition, the Besnus transition, involving a change in spin state. Variations of the thermodynamic, magnetic and elastic properties through this transition at ∼33 K in a natural sample of 4C pyrrhotite have been tested against a group theoretical model for coupling between order parameters relating to Fe/vacancy ordering (irrep U1(1/2,0,1/4)) and magnetic ordering (irreps m and m). Magnetoelastic coupling is weak but the pre-existing microstructure of ferroelastic and magnetic domains, that develop as a consequence of Fe/vacancy and ferrimagnetic ordering during slow cooling in nature (P63/mmc → C2′/c′), causes subtle changes in the low temperature transition (C2′/c′ → P). The Besnus transition involves a rotation of magnetic moments out of the a–c plane of the monoclinic structure, but it appears that the transition temperature might vary locally according to whether it is taking place within the pre-existing domain walls or in the domains that they separate. Evidence of metamagnetic transitions suggests that the magnetic field–temperature phase diagram will display some interesting diversity. Low temperature magnetic transitions in minerals of importance to the palaeomagnetism community have been used to identify the presence of magnetite and haematite in rocks and the Besnus transition is diagnostic of the existence of pyrrhotite, Fe7S8.

Orientation dependence of the magnetic phase diagram of Yb2Ti2O7

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\vec{B}\,\parallel\,\langle 100\rangle$, while field parallel to $\langle 110 \rangle$ and $\langle 111 \rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

Relation between dielectric permittivity and electrocaloric effect under high electric fields in the Pb(Mg1/3Nb2/3)O3-based ceramics

The correlation between dielectric permittivity and electrocaloric (EC) temperature change (ΔTEC) has been investigated in (1 − x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN–100xPT, with x = 0, 0.05, and 0.10) relaxor ferroelectric ceramics. At a given electric field, both peak temperatures, including the temperature of the permittivity peak (Tm) and the temperature of the maximum of the ΔTEC (TEC max) increase with increasing PT content. The peak of the dielectric permittivity is, regardless of the applied electric field, always at a higher temperature than is the TEC max, and the temperature gap between both maxima progressively increases with increasing applied DC bias. This is particularly true above the threshold field, which induces the long-range ordered ferroelectric state. The results, which are explained in terms of the electric field–temperature phase diagram of relaxor systems, thus reveal that Tm can only roughly mark the temperature of the upper boundary of the temperature–electric field window, where the EC responsivity (ΔTEC/ΔE) is the highest.

Magnetic-glass-like transition induced by kinetic arrest in ferrimagnetic Mn1.975Cr0.025Sb alloy

The magnetic glass state has been proposed and studied in the ferromagnetic (FM) compounds undergoing the first-order FM-antiferromagnetic (AFM) transition. In this work, we have unequivocally demonstrated the occurrence of magnetic-glass-like transition during the field cooling process in the ferrimagnetic (FIM) Mn1.975Cr0.025Sb alloy showing a first-order FIM-AFM transition. The magnetic, resistance and relaxation studies characterize the formation of a nonergodic magnetic glass-like state at low temperature arising from the coexistence of AFM and kinetically arrested FIM phases above a critical applied field. The magnetic field-temperature (μ0H-T) phase diagram based on magnetic and resistance measurements further evidences the generality of onset of glassy behavior in the first-order magnetic transition materials, in which the transition temperature shifts strongly with increasing applied field.

Magnon bound states versus anyonic Majorana excitations in the Kitaev honeycomb magnet \u03b1-RuCl3

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as α-RuCl3. Currently, two competing scenarios exist for the intermediate field phase of this compound (B = 7 − 10 T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here, we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

Magnetotransport and electronic structure of the antiferromagnetic semimetal YbAs

A number of rare-earth monopnictides have topologically non-trivial band structures together with magnetism and strong electronic correlations. In order to examine whether the antiferromagnetic (AFM) semimetal YbAs ($T\rm_N$ = 0.5 K) exhibits such a scenario, we have grown high-quality single crystals using a flux method, and characterized the magnetic properties and electronic structure using specific heat, magnetotransport and angle-resolved photoemission spectroscopy (ARPES) measurements, together with density functional theory (DFT) calculations. Both ARPES and DFT calculations find no evidence for band inversions in YbAs, indicating a topologically trivial electronic structure. From low-temperature magnetotransport measurements, we map the field-temperature phase diagram, where we find the presence of a field stabilized phase distinct from the AFM phase at low temperatures. An extremely large magnetoresistance (XMR) for both YbAs and the nonmagnetic counterpart LuAs, is also observed, which can consistently be accounted for by the presence of electron-hole compensation. Moreover, an angle-dependent study of the Shubnikov-de Haas effect oscillations reveals very similar Fermi surfaces between YbAs and LuAs, with light effective masses down to at least 0.5 K, indicating that the Yb-$4f$ electrons are well localized, and do not contribute to the Fermi surface. However, the influence of the localized Yb-$4f$ electrons on the magnetotransport of YbAs can be discerned from the distinct temperature dependence of the XMR compared to that of LuAs, which we attribute to the influence of short-ranged spin correlations that appear well above $T\rm_N$.

Temperature-field phase diagrams of one-way quantum work deficit in two-qubit XXZ spin systems

The spin-1/2 XXZ chain in a uniform magnetic field at thermal equilibrium is considered. For this model, we give a complete classification of all qualitatively different phase diagrams for the one-way quantum work (information) deficit. The diagrams can contain regions (phases, fractions) with both stationary and variable (state-dependent) angles of optimal measurement. We found cases of phase diagrams in which the sizes of regions with the variable optimal measurement angle are large and perhaps such regions can be detected experimentally. We also established a relationship between the behavior of optimal measurement angles near the boundaries separated different regions and Landau's theory of phase transitions of the second and first kind.

Spontaneous relaxor to ferroelectric transition in lead-free relaxor piezoceramics and the role of point defects

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na1/2Bi1/2)TiO3-BaTiO3 with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.

Structural and magnetic properties of antiferromagneticCe2IrGa12

We report a study of the structural and magnetic properties of single crystals of Ce$_2$IrGa$_{12}$. Ce$_2$IrGa$_{12}$ crystallizes in a layered tetragonal structure, and undergoes an antiferromagnetic transition below 3.1 K. We characterize the temperature-field phase diagrams of Ce$_2$IrGa$_{12}$ for fields both within the $ab$-plane and along the $c$-axis, where the presence of a field-induced magnetic phase is found for in-plane fields. The ordering temperature is moderately enhanced upon the application of pressures up to 2.3~GPa, suggesting that Ce$_2$IrGa$_{12}$ corresponds to the well localized region of the Doniach phase diagram.

Robust Perpendicular Skyrmions and Their Surface Confinement.

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic X-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field, we find that a state of perpendicularly ordered skyrmions forms, in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined to a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface guiding makes the robust state particular interesting for racetracklike devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.