Magnetic Field Phase Diagram Research Articles

Magnetic-field-induced gapless state in multiband superconductors

We investigate theoretically the properties of $s$-wave multiband superconductors in the weak-coupling (BCS) limit in the presence of pair-breaking effects of magnetic field. It is shown that a gapless superconducting state must appear in quasi-two-dimensional superconductors in magnetic fields parallel to the plane, corresponding to a Sarma state induced on one of the Fermi surfaces. The emergence of the state in $s$-wave multiband superconductors in the absence of anisotropy or spin-orbit interaction is usually accompanied by a zero-temperature first-order metamagnetic phase transition. For anisotropic or non-$s$-wave multiband superconductors the order of the zero-temperature metamagnetic transition depends on model parameters, and it may take the form of a smooth crossover. The details of the temperature--magnetic-field phase diagram for multiband superconductors are investigated analytically at zero temperature and numerically at a finite temperature. It is shown the zero-temperatures first-order phase transition gives rise to a critical region on the $B\text{\ensuremath{-}}T$ phase diagram. We suggest possible experiments to detect the gapless state.

Real space renormalization group study of the two-dimensional super-antiferromagnetic Ising model

The phase diagram of the two-dimensional super-antiferromagnetic (SAF) Ising model in the presence of a magnetic field is investigated within the framework of a real-space renormalization-group (RSRG) approximation. We consider nearest neighbor ferromagnetic interactions along the x direction and antiferromagnetic interactions in the y direction. The main results are depicted in the magnetic field versus temperature phase diagram. A second-order transition line separates a SAF phase from a field induced ferromagnetic (FIF) phase. Our study reveals that the magnetic field induces a phase transition at a single temperature value, thus, we did not find any evidence of reentrant behavior as claimed by some authors.

Thermodynamics of the Spin Luttinger Liquid in a Model Ladder Material

The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modeling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the entire phase diagram.

Fulde–Ferrell–Larkin–Ovchinnikov states in quasi-two-dimensional d + s-wave superconductors: Enhancement of the upper critical field

We investigate the spatially nonuniform Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting state in a quasi-two-dimensional (Q2D) d+s-wave superconductor. In the d+s-wave superconductor, the shape of the superconducting gap function and the nature of FFLO state depend on the degree of s-wave (or d-wave) admixture. The magnetic field versus temperature phase diagrams for the d+s-wave states with different shape of gap function are studied systematically. We find in particular that, a slight deviation of gap function from that of d-wave state may lead to significant change of FFLO state and remarkable enhancement of the upper critical magnetic field. The superconducting density of states (DOS) for d+s-wave superconductor is also calculated and found to be strikingly different from that for pure s-wave and pure d-wave superconductors.

Field-Induced Antiferromagnetism and Upper Critical Field in Pressure-Induced Superconductor CeRhIn5

An antiferromagnet CeRhIn 5 is well known to be changed into a superconductor by applying pressure P . Its electronic state is also clarified to be changed at the critical pressure P c ≃2.5 GPa under magnetic fields. We measured the low-temperature electrical resistivity of CeRhIn 5 in magnetic fields up to 17 T and at pressures up to 2.45 GPa, and constructed phase diagrams. At P = 2.45 GPa, which is close to P c ≃2.5 GPa, there are four kinds of phases, namely, the paramagnetic, superconducting, superconducting and antiferromagnetic, and antiferromagnetic phases in the temperature vs magnetic field phase diagram. The antiferromagnetic phase is found to be induced in the superconducting phase under magnetic fields and expanded in magnetic fields higher than the upper critical field in superconductivity. Interestingly, the anisotropy of the upper critical field in CeRhIn 5 is found to be different from that in a similar non-magnetic heavy-fermion superconductor CeCoIn 5 , revealing the anisotropic paramag...

Multicritical End Point of the First-Order Ferromagnetic Transition in Colossal Magnetoresistive Manganites

We have studied the bandwidth-temperature-magnetic-field phase diagram of RE0.55Sr0.45MnO3 colossal magnetoresistance manganites with ferromagnetic metal (FM) ground state. The bandwidth was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p;{*} where the character of the zero-field FM transition changes from first to second order. Below p;{*} the first-order FM transition extends up to a critical magnetic field. It approaches zero on the larger bandwidth side where the surface of the first-order FM phase boundary is terminated by a multicritical end point. The change in the character of the transition and the decrease of the colossal magnetoresistance effect is attributed to the reduced charge-order and orbital-order fluctuations.

Comparison between Ce and Yb heavy fermion compounds: [formula omitted] versus [formula omitted

The similarities and differences of the pressure and magnetic field phase diagrams of the heavy fermion systems CeRh 2 Si 2 and YbRh 2 Si 2 are reviewed. In CeRh 2 Si 2 the transition from antiferromagnetic order to a paramagnetic state seems to be first order with a clear volume discontinuity. Drastic differences appear not only in the pressure dependence of their Néel temperature but also in their magnetic field response as a comparison of the CeRu 2 Si 2 case with YbRh 2 Si 2 shows which reflects the position of their quantum critical field with respect to the magnetic field Kondo scale. However, a new generation of experiments in extreme conditions is required: very high magnetic field for CeRh 2 Si 2 , very low temperature for YbRh 2 Si 2 .

Estimation of spin transfer torque effect and thermal activation effect on magnetization reversal in CoFeB∕MgO∕CoFeB magnetoresistive tunneling junctions

In the CoFeB∕MgO∕CoFeB magnetoresistive tunneling junctions, the bias-dependent magnetic switching curves are investigated and the spin transfer torque effect and the thermal activation effect on the magnetic switching are estimated. The different switching behaviors at the positive and negative biases were observed, which are the asymmetric changes of the coercive force and the shift field. In order to investigate the asymmetric reduction of the coercive force according to the bias direction, the magnetic excitation effect due to the hot electron was introduced. Consequently, from the analysis by using the current versus magnetic field phase diagram, the asymmetric reduction of the coercive force at the positive and negative biases was explained quantitatively by the relationship between the thermal activation effect due to the Joule heating and the magnetic excitation effect due to the hot electron.

The phase diagrams of Ni–Mn–Ga alloys in the magnetic field

With the help of the Ginzburg–Landau theory the temperature–magnetic field phase diagrams of Heusler alloys Ni–Mn–Ga are theoretically investigated. The influence of the parameters of the magnetoelastic constants and elastic moduli on the phase diagrams are discussed. It is shown that with the determined combination of the phenomenological parameters, the critical magnetic field for the phase transition appears. The theoretically predicted value of the critical magnetic field is possible to be achieved in the experiment with the help of modern magnetic field sources.

Evolution of Ferroelectric and Antiferromagnetic Phases of TbMn2O5 Under High Magnetic Field up to 45 T

We determine high magnetic field (B//a) vs temperature phase diagram of TbMn2O5 by measurements of dielectric constant (ϵ//b) and magnetization under static and pulsed B up to 45 T, and pyroelectric/magnetoelectric currents up to 9 T. Our results reveal that a ferroelectric (FE) transition temperature at T C = 38 K at B = 0 T changes little under B up to 33 T, while incommensurate antiferromagnetic phase, characterized as increased ϵ and negative polarization below 25 K at B = 0 T, becomes unstable above B = ∼20 T at low temperatures. Furthermore, a positive FE polarization component, coined with a Tb f-spin ordering below ∼15 K at B = 0 T, abruptly disappears with Tb f-spin reorientation under B ≤ ∼2 T. Determined phase diagram shows that both magnitude of FE polarization and ϵ value are sensitively dependent on the evolution of magnetic order parameters of both Mn-d and Tb-f spins tuned by high magnetic fields.

Phase diagrams of spin-1 Ising model with bilinear and quadrupolar interactions under magnetic field. Two-particle cluster approximation

The spin-1 Ising model with bilinear and quadrupolar short-range interactions under magnetic field is investigated within the two-particle cluster approximation. It is shown that for those values of the quadrupolar interaction when at zero magnetic field the system undergoes a temperature phase transition between quadrupolar and paramagnetic phases, a triple point may exist in the temperature vs magnetic field phase diagrams, necessarily along with a critical point. It is also shown that the critical points in the temperature vs magnetic field phase diagrams of the investigated model can be of three types.

Emergent phases near the metamagnetic quantum critical point in [formula omitted

This paper presents a brief summary of recent electrical transport, magnetization and specific heat measurements on U ( Ru 1 - x Rh x ) 2 Si 2 ( x = 0 , 2 , 2.5 , 3 , and 4%) in magnetic fields up to 45 T. The results uncover the formation of multiple phases near their putative metamagnetic quantum critical points at low temperatures. From the established magnetic field versus temperature phase diagram in our series of samples, we find a clear correlation between the field-dependence of the phases and that of the putative quantum critical point. This provides an archetypal example that the field-tuned metamagnetic quantum criticality is intimately related to the formation of thermodynamic phases.

Distinct order of Gd4fand Fe3dmoments coexisting inGdFe4Al8

Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by thermodynamic, transport, and x-ray resonant magnetic scattering measurements. In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at TN with an incommensurate propagation vector (tau,tau,0) with tau varying between 0.06 and 0.14 as a function of temperature, and maintain this order over the entire T<TN range. The Gd 4f moments order below T2 with a ferromagnetic component mainly out of plane. Below T1, the ferromagnetic components are confined to the crystallographic plane. Remarkably, at low temperatures the Fe moments maintain the same modulation as at high temperatures, but the Gd 4f moments apparently do not follow this modulation. The magnetic phase diagrams for fields applied in [110] and [001] direction are presented and possible magnetic structures are discussed.

The Magnetic Property of a Uniaxial Spin–Ladder Material Na2Fe2(C2O4)3(H2O)2

Magnetic susceptibility, magnetization and heat capacity measurements are made on a single crystal sample of a spin–ladder material, Na 2 Fe 2 (C 2 O 4 ) 3 (H 2 O) 2 . The susceptibilities show a broad maximum at a temperature of about 25 K and 20 K parallel (χ || ) and perpendicular (χ ⊥ ) to the ladder direction, respectively. The maximum value of χ || is about three times larger than that of χ ⊥ . Both χ || and χ ⊥ decrease steeply at low temperatures down to about 5 K, at which χ || shows an upturn. We find a broad peak in the temperature dependence of the heat capacity. Susceptibility, magnetization and heat capacity data are used to construct the magnetic field versus temperature phase diagram of this compound. Quantum fluctuations are not strong enough to form a singlet ground-state, probably due to the presence of Ising anisotropy. The actual ground state is expected to be close to the quantum critical-point between the spin-gapped and a Néel ordered phases with the help of the inter-ladder interaction.

Dimensionality of superconductivity and vortex dynamics in the infinite-layer cuprateSr0.9M0.1CuO2(M=La,Gd)

The high magnetic field phase diagram of the electron-doped infinite layer high-temperature superconducting (high-T_c) compound Sr_{0.9}La_{0.1}CuO_2 was probed by means of penetration depth and magnetization measurements in pulsed fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical fields with H parallel (H_{c2}^{ab}) and perpendicular (H_{c2}^c) to the CuO_2 planes, with H_{c2}^{ab} extrapolating to near the Pauli paramagnetic limit of 160 T. The longer superconducting coherence length than the lattice constant along the c-axis indicates that the orbital degrees of freedom of the pairing wavefunction are three dimensional. By contrast, low-field magnetization and specific heat measurements of Sr_{0.9}Gd_{0.1}CuO_2 indicate a coexistence of bulk s-wave superconductivity with large moment Gd paramagnetism close to the CuO_2 planes, suggesting a strong confinement of the spin degrees of freedom of the Cooper pair to the CuO_2 planes. The region between H_{c2}^{ab} and the irreversibility line in the magnetization, H_{irr}^{ab}, is anomalously large for an electron-doped high-T_c cuprate, suggesting the existence of additional quantum fluctuations perhaps due to a competing spin-density wave order.

Link between magnetic field-induced quantum criticality and phase formation in [formula omitted

The magnetization and resistivity studies in magnetic fields up to 45 T were used to establish magnetic field versus temperature phase diagram and quantum criticality in U ( Ru 1 - x Rh x ) 2 Si 2 . For x = 4 % , the hidden order is completely destroyed, stabilizing a single field-induced phase II. A correlation between the field dependence of this phase and that of the quantum critical point, combined with the suppression of the T 2 coefficient of the resistivity within it, shows that the field-tuned quantum criticality is intimately related to the phase formation.

Specific Heat Study of the Impurity- and Field-Induced Antiferromagnetic Ordering in TlCu1-xMgxCl3

Specific heat measurements have been performed in the doped spin gap system TlCu1ixMgxCl3 with x ≤ 0.3 in order to investigate the impurity- and field-induced antiferromagnetic phase transitions. The parent compound TlCuCl3 is a three-dimensional coupled spin dimer system, which has a singlet ground state with an excitation gap ∆/kB=7.5 K. Well-defined phase transitions were observed in TlCu1ixMgxCl3 for H ≤ 9 T. The magnetic field vs temperature phase diagram was obtained. It was found that there is no boundary separating impurity- and field-induced antiferromagnetic phases, i.e., these two phases are identical. The result is discussed in connection with the recent theory by Mikeska et al. [Phys. Rev. Lett. 93 (2004), 217204].

High magnetic field phase diagram of PrOs4Sb12.

The magnetic phase diagram of PrOs4Sb12 has been investigated by specific heat measurements between 8 and 32 T. A new Schottky anomaly, due to excitations between two lowest crystalline-electric-field (CEF) singlets, has been found for both H parallel (100) and H parallel (110) above the field where the field-induced ordered phase (FIOP) is suppressed. The constructed H-T phase diagram shows weak magnetic anisotropy and implies a crossing of the two CEF levels at about 8-9 T for both field directions. These results provide an unambiguous evidence for the Gamma(1) singlet being the CEF ground state and suggest the level crossing (involving lowest CEF levels) as the driving mechanism of FIOP.

Vortex states in a two-dimensional superconductor at high magnetic field in a periodic pinning potential

The effect of a periodic pinning array on the vortex state in a two-dimensional superconductor at low temperatures is studied within the framework of the Ginzburg-Landau approach. It is shown that an attractive interaction of vortex cores to a commensurate pin lattice stabilizes vortex solid phases with long-range positional order against violent shear fluctuations. Exploiting a simple analytical method, based on the Landau orbital description, we derive a rather detailed picture of the low-temperature vortex-state phase diagram for the first matching magnetic field. It is predicted that for sufficiently clean samples application of an artificial periodic pinning array would enable one to directly detect the intrinsic shear stiffness anisotropy characterizing the ideal vortex lattice.

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H c. Above H c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty. From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C 5H 14N 2) 2N 3(PF 6), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.