Magnetic phase diagram and crystal fields of superconducting ErNi2B2C

We have measured the specific heatc _p in magnetic fields and the resistance of the antiferromagnetically ordered borocarbide superconductor ErNi₂B₂C. From our data we show that the superconductivity at 10.6 K coexists with antiferromagnetic order beginning at 5.8 K, and we determine the magneticB-T phase diagram. The specific heat in zero field gives additional information about the crystalline electric field (CEF) splitting. We show that the low-temperature specific heat is dominated by several low-lying CEF doublets with the first excited level at about 10 K, while the splitting of the next two levels is about 50 to 100 K.     

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi2B2C

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.     

Local superconducting density of States of ErNi2B2C

We present local tunneling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C . The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.     

Magnetic study of the superconducting phase YNi2B2C

Il Nuovo Cimento D - We report the synthesis procedure and the chemical-physical characterization of the YNi2B2C superconductor. From magnetic measurements we show that this material is a type-II...     

Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn₂Ge₂ are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn₂Ge₂ in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.     

Magnetic phase diagrams based on static and dynamic magnetic behaviour in Ru-based superconducting ferromagnets

In this work, we present magnetic phase diagrams of a RuSr(2)Eu(1.5)Ce(0.5)Cu(2)O(10-δ) (Ru-1222) superconducting ferromagnet derived from its static and dynamic magnetic responses, measured by temperature and field dependences of dc magnetization and nonlinear ac susceptibility in both low and high magnetic fields. Comparison of magnetic phase diagrams of phase pure and impure samples singles out the intrinsic and extrinsic magnetic features, naturally proposing a unified model of Ru-1222 magnetic behaviour. The results considered within the proposed interpretation indicate full agreement between static and dynamic properties which, if measured in combination, effectively complement each other, uncovering existing ambiguities.     

Direct observation of spontaneous weak ferromagnetism in the superconductor ErNi2B2C

Neutron measurements show that superconducting ErNi2B2C (T(C) = 11 K) develops antiferromagnetic spin density wave magnetic order (T(N) = 6 K), which squares up with decreasing temperature yielding a series of higher-order magnetic Bragg peaks with odd harmonics. Below T(WFM) = 2.3 K where magnetization indicates a net moment develops, even-order Bragg peaks develop which low field (approximately 3 Oe) polarized beam measurements show are magnetic in origin. The data directly demonstrate the existence of a net magnetization with a periodicity of 20a, confirming the microscopic coexistence of spontaneous weak ferromagnetism with superconductivity.     

Magnetic phase shifter for superconducting qubits

We have designed and investigated a contactless magnetic phase shifter for flux-based superconducting qubits. The phase shifter is realized by placing a perpendicularly magnetized dot at the center of a superconducting loop. The flux generated by this magnetic dot gives rise to an additional shielding current in the loop and induces a phase shift. By modifying the parameters of the dot an arbitrary phase shift can be generated in the loop. This magnetic phase shifter can, therefore, be used as an external current source in superconducting circuits, as well as a suitable tool to study fractional Josephson vortices.     

Magnetic phase diagrams of axial third-nearest-neighbour Ising model

The magnetic phase diagrams of axial third-nearest-neighbour Ising (A3NNI) model under an external field have been studied by means of the molecular field approximation. The phase boundaries among paramagnetic, (anti)ferromagnetic and various modulated phases are determined by analysing the frequency-dependent susceptibility, or by solving the coupled equations for magnetisations for spins up to 17, iteratively. The resultant phase diagram is found to be very consistent with the exact results for the ground state spin ordering and to be far improved than existing finite temperature phase diagrams for the same model.     

Pauli paramagnetic effects on vortices in superconducting TmNi2B2C

The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.     

Magnetic phase diagrams of the CrB- and FeB-type HoSi compounds

The temperature magnetic phase diagrams of the dimorphic HoSi compound were studied by neutron diffraction. The sample comprises 35.5% CrB- (Cmcm) and 64.5% FeB-type (Pnma) of structure. Both phases order antiferromagnetically below T_N=25 K and undergo first-order magnetic transitions at T_ic=16.5 K. Their T-phase diagrams comprise a low temperature (LT) 2.7 K−T_ic and a high temperature (HT) range T_ic−T_N with distinct wave vectors.The LT magnetic ordering of the CrB-type HoSi with the wave vector q₁=(1/2, 0, 1/2) corresponds to a uniaxial magnetic structure, with the Ho moments along the shortest axis c. At 2.7 K the ordered moment value is 8.6(2) μ_B/Ho atom. The HT ordering, described by the wave vector q₂=(q_2x, 0, q_2z) with a T-variable length, corresponds to an amplitude modulated structure.The magnetic ordering of the FeB-type HoSi requires two symmetry independent vectors q₃=(0, q_3y, q_3z) for the LT- and q₄=(q_4x, q_4y, 0) for the HT range. Both vectors correspond to sine wave modulated structures with the Ho magnetic moments confined along the shortest axis b. The q₃ vector has an almost invariable length vs. T close to ≈(0, 9/17, 1/11). At 2.7 K the amplitude of the wave is 10.9(1) μ_B/Ho atom. At T_icq₃ jumps to the wave vector q₄=(q_4x, q_4y, 0) with a T-variable length. At 17 K q₄=(0.092(1), 0.538(3), 0). Around T_ic there is a narrow coexistence range of the q₃ and q₄ competing phases. Various models are discussed and compared with the isomorphic RSi (R=rare earth) compounds counterparts of HoSi, a comparison that has led us to briefly review the magnetic structures available in the literature for this interesting class of compounds.     

Magnetic phase diagrams of manganites in the electron doping region

The paper reviews the physical properties of the R_1?xA_xMnO₃ manganites (R=La, Pr, Nd, Sm, etc., A=Ca, Sr, Ba) in the region of their electron doping where the divalent atom concentration x in the compound lies in the interval 0.5<x<1.0. Experimental magnetic phase diagrams of the most well-studied compounds and the results of theoretical calculations of these diagrams made in the tight-binding approximation within the degenerate double-exchange model for T=0 are presented. The experimental section of the review deals primarily with neutron diffraction studies of the magnetic and crystal structures of the manganites, and the theoretical part, with the relation between their magnetic and orbital structures. The review describes, in considerable detail, the method of calculation of the energy spectrum ?(k) and of the total carrier energy for all possible magnetic and orbital configurations of the system corresponding to the translation symmetry of the lattice. The theoretical analysis is carried out separately for two models of the crystal structure, with two and four manganese atoms in the unit cell. All equilibrium magnetic and orbital configurations of the four-sublattice manganite model were determined by minimizing the total energy of the system with respect to the directions of the local manganese magnetic moments and orbital states of the e_g electrons. It is shown that, by using the effective Hamiltonian of the degenerate double-exchange model for the e_g electrons, which takes into account the e_g level splitting, and the Heisenberg Hamiltonian of the localized t_2g electrons, one can describe the diversity of the magnetic phases, the sequence of their alternation with increasing x, and the correlation between the spin and orbital degrees of freedom, which are observed in most electron-doped manganites.     

Magnetic phase diagrams of Fe–Mn–Al alloy on the Bethe lattice

The magnetic behaviors of the Fe–Mn–Al alloy are simulated on the Bethe lattice by using a trimodal random bilinear exchange interaction(J) distribution in the Blume–Capel(BC) model. Ferromagnetic(J 0) or antiferromagnetic(J 0)bonds or dilution of the bonds(J = 0) are assumed between the atoms with some probabilities. It is found that the secondor the first-order phase boundaries separate the ferromagnetic(F), antiferromagnetic(AF), paramagnetic(P), or spin-glass(SG) phases from the possible other one. In addition to the tricritical points, the special points at which the second- and the first-order and the spin-glass phase lines meet are also found. Very rich phase diagrams in agreement with the literature are obtained.     

Magnetic phase diagrams of ferrimagnets having two magnetically unstable subsystems

The molecular field approximation is used to construct magnetic phase diagrams along with magnetization curves for ferrimagnets having two magnetically unstable subsystems. Diverse sequences of phase transitions of various kinds—first order (metamagnetic) and second order (via a noncollinear phase)—are realized in the course of magnetizing such ferrimagnets. Analysis reveals how the magnetization curves depend on the relative values of parameters of the two subsystems, e.g., sublattice magnetizations in weak and strong states, critical fields, and values of the exchange interaction between sublattices. Fiz. Tverd. Tela (St. Petersburg) 40, 280–284 (February 1998)     

de Haas-van Alphen effect in the superconducting state of YNi2B2C

To examine the Fermi surfaces of the recently discovered quaternery compounds RENi₂B₂C measurements of the de Haas-van Alphen (dHvA) effect were made on YNi₂B₂C single crystals in magnetic fields up to 12 T. ForBc we observe two dHvA frequenciesF ₁=0.499 kT andF ₂=6.933 kT which corresponds to approximately 1,5% and 21% of the Brillouin zone cross section area. Both frequencies could be observed deep in the vortex state of the type-II superconductor, the lower dHvA frequency down to 2 T corresponding to roughly 1/5 of the upper critical fieldB _c2 which was found to be 10.6 T in resistivity measurements. The field dependent quasiparticle damping in the superconducting state is very weak, since the amplitude of the dHvA oscillations seems to be unaffected by the phase transition atB _c2.