High-field Phase Research Articles

Superconducting subphase in the layered perovskite ruthenateSr2RuO4in a parallel magnetic field

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ down to 40 mK. For high-quality single crystals with the transition temperature $({T}_{\mathrm{c}})$ of 1.48--1.49 K, an abrupt jump of the torque signal is found near 1.5 T in field parallel to the conducting ${\mathrm{RuO}}_{2}$ planes below $\ensuremath{\sim}0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The jump corresponds to the first order transition recently revealed by magnetocaloric and magnetization measurements [Yonezawa et al., Phys. Rev. Lett. 110, 077003 (2013); Kittaka et al., Phys. Rev. B 90, 220502(R) (2014)]. Furthermore, weak diamagnetic and irreversible signals are found to persist above the first order transition up to 1.85 T. The result indicates the presence of a subphase boundary separating low- and high-field phases in the superconducting phase. The high-field subphase disappears when the field is tilted from the conducting planes only by a few degrees. Quantum oscillation measurements are also reported to clarify the strong sample-quality dependence of the high-field subphase.

Tuning of cu doping on phase transition and high-field phase diagram of Nd0.5Sr0.5Mn1−xCuxO3

Pulsed high magnetic fields up to 52T have been used in the systematic investigation of the magnetic properties of manganites Nd0.5Sr0.5Mn1−xCuxO3 (0≤x≤0.15). The Cu-doping dependent first-order metamagnetic transitions are observed below the charge ordering temperature, which is ascribed to both Cu-doping and field-induced collapse of the charge ordering with antiferromagnetic phase. Based on the magnetization and electrical transport measurements, a three-dimensional phase diagram with coordinate axis of temperature, magnetic field, and doping level has been obtained, in which the critical fields of the metamagnetic transitions increase with the increase in Cu content and decrease with increasing temperature. The experimental results confirm that Mn-site substitution with Cu destroys the Mn3+–O2−–Mn4+ bridges and weakens the double exchange interaction between Mn3+ and Mn4+ ions, which shows an obvious tuning effect on the metamagnetic transition under the external magnetic field.

Spin-Flop Transition and a Tilted Canted Spin Structure in a Coupled Antiferromagnet

We study a uniaxial coupled Heisenberg antiferromagnet that consists of two subsystems of classical spins with small and large lengths and spin-flop transitions in a magnetic field parallel to the magnetic easy axis. It is proved that the anisotropy of inter-subsystem coupling stabilizes an asymmetric canted antiferromagnetic phase with a tilted direction of antiferromagnetism that is not perpendicular to the magnetic field. In contrast to the conventional first-order spin-flop transition, the spin-flop transition from the Neel phase to such a tilted canted antiferromagnetic (TCAF) phase is of the second order in the absence of simple anisotropic energies in the subsystems. The transition from the TCAF phase to the high-field saturated spin phase is of the second order in the strong coupling limit of the exchange interactions J1 between the small spins, whereas when J1 is finite, it becomes first-order. Therefore, in the former case, the TCAF phase converts the Neel phase continuously into the saturated p...

Magnetoelectric effect in antiferromagnetic LiCoPO4in pulsed magnetic fields

The magnetoelectric effect in single-crystal LiCoPO4 (TN = 21.8 K) is studied in strong pulsed magnetic fields which destroy the antiferromagnetic structure of the crystal spin ordering. The electric polarization along the crystallographic a axis induced by a magnetic field H||b is measured. New features of the electric polarization in a magnetic field are found, including a spike in the polarization near the field for the first magnetic transition at H1 = 123 kOe, a recovery of the electric polarization at the second magnetic transition H2 = 210 kOe, and a gradual reduction to zero on approaching the third transition at H3 = 263 kOe. Various possible magnetic structures in the high field phases are examined which are consistent with their magnetization and electric polarization. The observed linear dependence of the polarization on field strength in the initial antiferromagnetic phase and the vanishing of the polarization in the first high-field phase are in good agreement with previous studies of the ma...

Magnetic phase diagram of the helimagnetic spinel compound ZnCr2Se4 revisited by small-angle neutron scattering

We performed small-angle neutron scattering (SANS) measurements on the helimagnetic spinel compound ZnCr2Se4. The ground state of this material is a multi-domain spin-spiral phase, which undergoes domain selection in a magnetic field and reportedly exhibits a transition to a proposed spin-nematic phase at higher fields. We observed a continuous change in the magnetic structure as a function of field and temperature, as well as a weak discontinuous jump in the spiral pitch across the domain-selection transition upon increasing field. From our SANS results we have established the absence of any long-range magnetic order in the high-field (spin-nematic) phase. We also found that all the observed phase transitions are surprisingly isotropic with respect to the field direction.

Coupled k-space structure ofd-wave superconducting and magnetic orders induced by paramagnetic pair-breaking effect

We theoretically investigate k-space structures of $d_{x^2-y^2}$-wave superconducting (SC) and spin-density-wave (SDW) orders in their coexistent phase induced by a paramagnetic pair-breaking (PPB) effect in relation to the high field and low temperature (HFLT) SC phase in CeCoIn$_5$. It is shown that, in k-space, the SDW order develops near the gap nodes where the SC order is suppressed by PPB, and the nesting condition for the SDW ordering is satisfied. By comparing the results in the $d_{x^2-y^2}$-wave SC model and those in an artificial model with no sign change of the gap function in k-space with each other, it is shown that the $d_{x^2-y^2}$-wave SC and SDW orders are enhanced altogether in k-space due to the sign change of the $d_{x^2-y^2}$-wave gap function there, and that this mutual enhancement largely stabilizes the coexistence of these orders in real space. It is also discussed that the field dependence of a SDW moment can be affected by the k-space structure of these orders, which is dependent on the curvature of the Fermi surface.

Nonuniform superconducting state in an antiferromagnetic superconductor

We consider the superconducting state in a clean crystal with antiferromagnetic (AF) structure of localized magnetic moments taking into account the exchange interaction between magnetic moments and conduction electrons. We assume that the localized moments order at the Neel temperature ${T}_{N}$ due to the RKKY interaction predominantly. In such crystals, the periodic exchange field acting on conducting electrons results in the formation of an insulating gap on the Fermi surface for electrons moving in directions that depend on the orientation of the wave vector of AF ordering. We assume a scenario in which the Cooper pairing occurs in the open parts of the Fermi surface at the temperature ${T}_{c}$. We show that at high amplitudes of exchange field ${h}_{e}g{T}_{c}/{\ensuremath{\mu}}_{B}$, the structure of superconducting state just below the temperature ${T}_{c}$ depends on the relation between ${T}_{c}$ and ${T}_{N}$. At low ratio ${T}_{c}/{T}_{N}$, a nonuniform superconducting state, like the Fulde-Farrell-Larkin-Ovchinnikov high-field phase, should exist, while at bigger ratio superconducting order parameter is uniform. The nonuniform structure of superconducting state may be probed by tunneling measurements.

Al3+ doping effects and high-field phase diagram of La0.5Sr0.5Mn1−xAlxO3

Magnetization measurements of La0.5Sr0.5Mn1−xAlxO3 (0 ⩽ x ⩽ 0.25) under pulsed high magnetic fields up to 50 T have been carried out, in which the Al3+ ions doping and magnetic field effects on the charge-ordering/antiferromagnetic to ferromagnetic phase transitions have been discussed. A triple-phase diagram with the critical field, doping level and temperature has been determined, in which the antiferromagntic and ferromagnetic phase boundaries were clearly defined. The change from long-range charge-ordered/antiferromagnetic phases to the robust short-range ones upon the Al3+-doping was observed. According to the experimental results, we assume that Al3+ ion doping at the Mn sites dilutes the Mn3+-O-Mn4+ network, weakens the double-exchange interaction and further suppresses the FM phase (metallic conduction), which leads to the critical magnetic fields destroying the antiferromagnetic order increase with the increase of the doping level.

Micromagnetic analysis of spin-reorientation transitions. The role of magnetic domain structure

A method for calculating micromagnetic state of ferro- or ferrimagnetic single-crystals based on the Néel's method of phases is proposed. The standard Néel technique requires different approaches to calculation of micromagnetic state of samples with different anisotropy types. Furthermore, this technique cannot be used to calculate magnetization curves of materials with a complex anisotropy type, in which the first-order magnetization process (FOMP) occurs. On the contrary, the technique proposed in the present work makes it possible to calculate micromagnetic state of a sample within one unified approach. This technique has no limitations in terms of the anisotropy type as well. In case of the FOMP, the simulation methods that we used show results different from conventional calculation methods. The reason is that the conventional methods imply coherent rotation of magnetization in single domain particle (so-called Stoner–Wohlfarth model). We explain this discrepancy by the fact that a magnetic domain structure appears in the region of the FOMP. In the present work we show that magnetization processes do not occur in a jump under the FOMP but gradually pass though nucleation and new high-field phase growing, which substitutes for the low-field phase.

Phase boundary ofθphase of solid oxygen in ultrahigh magnetic fields

The phase diagram of solid oxygen in the magnetic-field-temperature ($B\ensuremath{-}T$) plane is revealed by magnetization and magnetotransmission measurements. The high-field phase of solid oxygen, which we term the $\ensuremath{\theta}$ phase, is induced at the temperature below 42 K. The transition fields at the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\theta}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\theta}$ transitions are almost temperature independent, with the phase boundary being very steep. This phase boundary indicates that the entropy change is very small at the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\theta}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\theta}$ transitions, in contrast to the entropy-driven $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ transition at zero field. We argue that the $\ensuremath{\theta}$ phase differs from the high-temperature $\ensuremath{\gamma}$ phase in terms of entropy; the molecular rotation is inhibited in the $\ensuremath{\theta}$ phase, whereas permitted in the $\ensuremath{\gamma}$ phase. This finding consolidates the novelty of the $\ensuremath{\theta}$ phase.

Field-induced phase transitions and magnetoferroelectricity in the perfect triangular lattice antiferromagnet RbFe(MoO4)2 in a vertical magnetic field

We performed electric polarization P, dielectric constant ϵ and magnetization M measurements on the perfect triangular-lattice antiferromagnet RbFe(MoO4)2 in high magnetic fields B applied along the c-axis. Whereas M(B) at T=1.5K monotonically increases with B up to the saturation at Bs=21T, P(B) disappears about 14T, far below the saturation point. In a field range 13≤B≤14T, the temperature variation of ϵ shows a double peak structure below 2.8K, indicating two successive transitions. These results strongly suggest the presence of a new high-field phase; the magnetic structure changes from a low-field conical state having chirality to a high-field coplanar non-collinear structure without chirality. The obtained phase diagram is in agreement with a theoretical prediction on XY-like Heisenberg spin triangular lattice antiferromagnets.

Advanced magnetic resonance spectroscopy and imaging techniques applied to brain development and animal models of perinatal injury

Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are widely used in the field of brain development and perinatal brain injury. Due to technical progress the magnetic field strength (B0) of MR systems has continuously increased, favoring 1H-MRS with quantification of up to 18 metabolites in the brain and short echo time (TE) MRI sequences including phase and susceptibility imaging. For longer TE techniques including diffusion imaging modalities, the benefits of higher B0 have not been clearly established. Nevertheless, progress has also been made in new advanced diffusion models that have been developed to enhance the accuracy and specificity of the derived diffusion parameters. In this review, we will describe the latest developments in MRS and MRI techniques, including high-field 1H-MRS, phase and susceptibility imaging, and diffusion imaging, and discuss their application in the study of cerebral development and perinatal brain injury.

Field and temperature dependence of electromagnon scattering inTbMnO3studied by inelastic neutron scattering

Inelastic neutron scattering techniques have been used to study the field-induced multiferroic transition and the temperature dependence of magnetic excitations in ${\text{TbMnO}}_{3}$. The significant changes in the spin-wave spectra across the field-induced transition perfectly agree with a rotation of the cycloidal spiral plane and with efficient pinning in the commensurate high-field phase. Further analysis of the $\mathbf{Q}$ dependence allows the identification of an electromagnon in the multiferroic high-field phase whose energy and polarization precisely matches previous infrared data. This and the zero-field temperature dependence of a zone-center magnon, which exactly agrees with that of an optically detected excitation, document that the inverse Dzyaloshinskii-Moriya interaction induces an electromagnon hybrid excitation in ${\text{TbMnO}}_{3}$.

High-magnetic-field phase transitions and H–T phase diagram of the Kagome-staircase compound Ni3V2O8

High-field magnetization and specific heat of the Kagome staircase compound Ni3V2O8 have been measured in magnetic fields H applied along the b and the c axes. The magnetization data shows magnetic transitions above 10T for H//b; while no additional transition is probed for H//c. The resulting H–T phase diagram for H//b is extended to magnetic fields higher than previously reported and explores a different high-field phase. The experimental data of the magnetization and the specific heat also suggest the existence of multiple phases inside this high-field phase. These results indicate that this compound is a more complex spin system than previously studied.

Microscopic model calculations for the magnetization process of layered triangular-lattice quantum antiferromagnets.

Magnetization processes of spin-1/2 layered triangular-lattice antiferromagnets (TLAFs) under a magnetic field H are studied by means of a numerical cluster mean-field method with a scaling scheme. We find that small antiferromagnetic couplings between the layers give rise to several types of extra quantum phase transitions among different high-field coplanar phases. Especially, a field-induced first-order transition is found to occur at H≈0.7H_{s}, where H_{s} is the saturation field, as another common quantum effect of ideal TLAFs in addition to the well-established one-third plateau. Our microscopic model calculation with appropriate parameters shows excellent agreement with experiments on Ba_{3}CoSb_{2}O_{9} [T. Susuki etal., Phys. Rev. Lett. 110, 267201 (2013)]. Given this fact, we suggest that the Co^{2+}-based compounds may allow for quantum simulations of intriguing properties of this simple frustrated model, such as quantum criticality and supersolid states.

Electron–Hole Pairing in Presence of a Strong Magnetic Field in Graphite

The collective electronic states induced by a strong magnetic field in graphite are believed to emerge because of the formation of electron–hole pairs. Researchers in Tokyo uncover new features in the high-field phase diagram and put forward a new scenario.

Large field-induced irreversibility in Ni-Mn based Heusler shape-memory alloys: A pulsed magnetic field study

We present a pulsed magnetic field study on the magnetic and magnetostriction properties of Ni-Mn-$Z$ $(Z=\mathrm{In}$, Sn, and Sb) based Heusler shape-memory alloys. These materials generally display a field-induced magnetostructural transition that could lead to an irreversible phase transition, when measured near the martensitic transition temperature. Here, we show that independently of the transition temperature, the critical field for the phase transition sensitively depends on the main-group element in the sample. Irrespective of their compositions, all samples display a magnetization of around $2{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ in the martensite phase and about $6{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ in the cubic austenite phase. Our magnetic and magnetostriction measurements at low temperatures exhibit a partial or complete arrest of the high-field austenite phase below the reverse martensitic transition. This results in a large irreversibility with a hysteresis width as high as 24 T. We introduce a theoretical model to discuss the experimental results.

Field desorption microscopy of graphene on iridium in intercalation with alkali metals

Field-desorption and field-emission electron microscopy methods have been used to study the effect of a strong electric field on the processes of potassium and cesium desorption from the graphene surface on an iridium field emitter. It was found that two phases of field desorption may exist for these alkali metals. In the low-field phase, alkali metal atoms adsorbed on the surface are desorbed. The existence of the high-field phase is attributed to the egress from under the graphene film and ionization of intercalated alkali metal atoms under the action of the electric field.

Localized and propagating excitations in gapped phases of spin systems with bond disorder

Using the conventional $T$-matrix approach, we discuss gapped phases in one-, two-, and three-dimensional (3D) spin systems (both with and without a long-range magnetic order) with bond disorder and with weakly interacting bosonic elementary excitations. This work is motivated by recent experimental and theoretical activity in spin-liquid-like systems with disorder and in the disordered interacting boson problem. In particular, we apply our theory to both paramagnetic low-field and fully polarized high-field phases in dimerized spin-$\frac{1}{2}$ systems and in integer-spin magnets with large single-ion easy-plane anisotropy $\mathcal{D}$ with disorder in exchange coupling constants (and/or $\mathcal{D}$). The elementary excitation spectrum and the density of states are calculated in the first order in defects concentration $c\ensuremath{\ll}1$. In 2D and 3D systems, the scattering on defects leads to a finite damping of all propagating excitations in the band except for states lying near its edges. We demonstrate that the analytical approach is inapplicable for states near the band edges and our numerical calculations reveal their localized nature. We find that the damping of propagating excitations can be much more pronounced in considered systems than in magnetically ordered gapless magnets with impurities. In 1D systems, the disorder leads to localization of all states in the band, while those lying far from the band edges (short-wavelength excitations) can look like conventional wave packets.

Magnetic ordering in the frustratedJ1−J2Ising chain candidateBaNd2O4

The AR$_2$O$_4$ family (R = rare earth) have recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the $c$-axis. We have investigated polycrystalline BaNd$_2$O$_4$ with a combination of magnetization, heat capacity, and neutron powder diffraction (NPD) measurements. Magnetic Bragg peaks are observed below $T_N$ $=$ 1.7 K, and they can be indexed with a propagation vector of $\vec{k}$ $=$ (0 1/2 1/2). The signal from magnetic diffraction is well described by long-range ordering from only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations. Furthermore, low temperature magnetization and heat capacity measurements reveal two field-induced spin transitions at 2.5 T and 4 T for $T$ $=$ 0.46 K. The high field phase is paramagnetic, while the intermediate field state may arise from a spin transition of the long-range ordered Nd chains, resulting in an up-up-down intrachain spin configuration. The proposed intermediate field state is consistent with the magnetic structure determined in zero field for these chains by NPD, as both phases are predicted for the classical Ising chain model with nearest neighbor and next nearest neighbor interactions.