Lifshitz-point behaviour of ferromagnetic superconductors

The critical behaviour of the electromagnetically coupled superconductor magnet system is investigated by means of a generalized mean field theory and a renormalization group analysis. We show that in the presence of a genuine anisotropy in systems with an additional pressure-like parameter (like concentration in pseudo-ternary ferromagnetic superconductors (FMS), e.g. Er_1?x Ho _x Rh₄B₄) the indirect coupling between superconducting and magnetic order parameters (i.e. gauge coupling) can lead to a peculiar kind of critical behaviour characterized by Lifshitz points (LP). These points (quite generally) occur as merging points of three phases: a (magnetically) disordered phase, a homogeneously ordered phase and a modulated phase. In FMS the latter phase may result from exchange screening by supercurrents. This unusual critical behaviour is found in two varieties:

1. a regular LP which may occur on the lower transition line of a reentrant FMS
2. a similar but slightly different critical point which we term modified Lifshitz point (MLP), and which is to be expected at the merging point of the upper and lower superconducting transition lines with the magnetic order disorder transition lines in the (x, T) phase diagrams of FMS's.

     

Magnetic and structural transition of Ni50+xMn25−x/2Ga25−x/2 (x=2–5) alloys

Magnetic and structural transitions of non-stoichiometric Ni_50+xMn_25−x/2Ga_25−x/2 (x=2–5) alloys are systematically investigated. Differential scanning calorimetry and modified thermogravimetry (TG) are used to measure magnetic and structural transitions simultaneously. The structural transition temperatures increase monotonically with increasing Ni substitution for Mn and Ga. Different magnetic transition sequences on heating were observed from ferromagnetic martensite to ferromagnetic autensite, then to paramagnetic autensite, from ferromagnetic martensite to paramagnetic austensite or from ferromagnetic martensite to paramagnetic martensite, respectively. Three kinds of NiMnGa alloys were obtained according to the sequence of the structural and magnetic transition, whose structural transition temperatures are lower, equal to or higher than the magnetic transition temperatures.     

Resonance enhancement of electron-phonon interaction

A superconducting state near the phase transition curve from paramagnetic phase to superconducting phase for perovskites La_{2 ? x} Sr _x CuO₄ and YBa₂Cu₃O₆ is considered. Expressions for effective parameters of electron-spin-phonon interaction are obtained. It is shown that the critical temperature of the phase transition from paramagnetic phase to superconducting phase T _c is determined by the enhancement of electron-phonon interaction by spin fluctuations of exchange type.     

Re-entrant superconductivity and magnetic ordering in the pseudoternary system (Er1−xHox)Rh4B4

The boundaries between the normal paramagnetic, superconducting, and normal magnetically-ordered phases in the tetragonal pseudo-ternary rare earth system (Er_1?xHo_x)Rh₄B₄ have been established by means of ac susceptibility measurements to temperatures as low as 0.07 K.     

Electron spin resonance study of the local properties of the microscopic phase separation in La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> single-crystal films

Electron spin resonance is studied in films of La_1?x Ca _x MnO₃ manganites with compositions in the vicinity of the calcium content x = 0.5, in which the phase separation is most clearly pronounced. It is found that the La_0.5Ca_0.5MnO₃ manganite undergoes different types of phase separation: (i) at temperatures above the Curie point, ferromagnetic regions exist in the paramagnetic phase; and (ii) at temperatures below the Néel point, antiferromagnetic microregions coexist with ferromagnetic microregions. Two types of ferromagnetic domains with different magnetization orientations are revealed In the temperature range between the Curie and Néel points.     

Magnetic phase diagram of MnP: Field parallel to the hard direction

The phase diagram of MnP, for magnetic field parallel to the a axis, was determined. Three ordered phases were found. They are tentatively identified as ferromagnetic, fan, and cone phases. The triple point where the ferro, fan, and paramagnetic phases meet has features which are expected for a Lifshitz point.     

Effect of allied and alien ions on the EPR spectrum of Mn-containing lithium-manganese spinel oxides

Electron paramagnetic resonance spectroscopy was used for studying the effect of allied and alien ions on the EPR spectrum of Mn⁴⁺-containing lithium-manganese spinel oxides. Manganese spinel oxides with paramagnetic Mn⁴⁺ and diamagnetic substituents in the 16d spinel sites were studied: Li[Mg_0.5Mn_1.5]O₄, Li[Mg_0.5−xCo_2xMn_1.5−x]O₄, 0<x≤0.5, and Li[Li_1/3Mn_5/3]O₄. Ni²⁺-ions with integer-spin-ground state (S=1) were selected as alien ions: Li[Mg_0.5−xNi_xMn_1.5]O₄ (0≤x≤0.5), Li[Li_(1−2x)/3Ni_xMn_(5−x)/3]O₄ (0≤x≤0.5), and Li[Ni_0.5Mn_1.5−yTi_y]O₄ (0≤y≤1.0). It was shown that in Ni-substituted oxides the low temperature EPR response comes from magnetically correlated Ni-Mn spins, while at high registration temperature Mn⁴⁺ ions give rise to the EPR profile. Analysis of the EPR line width allows differentiating between the contributions of the density of paramagnetic species and the strength of the exchange interactions in magnetically concentrated systems. The density of allied and alien paramagnetic species has no effect on the EPR line width in cases when the strengths of antiferro- and ferromagnetic interactions on an atomic site are close. On the contrary, when antiferro- or ferromagnetic interactions on an atomic site are dominant, the EPR line width increases with the density of paramagnetic species.     

Coexistence of magnetism and superconductivity in the hole doped FeAs-based superconducting compound

The magnetic and superconducting properties of the Sm-doped FeAs-based superconducting compound were investigated under wide ranges of temperature and magnetic field. After the systematical magnetic ion substitution, the superconducting transition temperature decreases with increasing magnetic moment. The hysteresis loop of the La_0.87?xSm_xSr_0.13FeAsO sample shows a superconducting hysteresis and a paramagnetic background signal. The paramagnetic signal is mainly attributed to the Sm moments. The experiment demonstrates that the coexistence of magnetism and superconductivity in the hole doped FeAs-based superconducting compounds is possible. Unlike the electron doped FeAs-based superconducting compounds SmFeAsOF, the hole doped superconductivity is degraded by the substitution of La by Sm. The hole-doped and electron-doped sides are not symmetric.     

The magnetic and superconducting phase diagram of Gd-doped lanthanumsulfides

The system La_3-x( )_xS₄ [where ( ) denotes a vacancy] containing up to 25 at.% Gd is shown to be a model system in which magnetic interactions in metals can be studied. By measuring the temperature dependence of the low field a.c. susceptibility, the superconducting, spin-glass and ferromagnetic transitions can be determined as a function of the Gd concentration. The depression of the superconducting transition temperature mainly follows the theory of Abrikosov and Gor'kov, with a possibility for coexistence of superconductivity and spin-glass magnetic order near a critical concentration of 3 at.% Gd. The transition from spin-glass to ferromagnet is well-defined with the percolation limit for the long range ferromagnetic order at 14 at.% Gd.     

Influence of magnetic impurities on superconducting property of polycrystalline iron-based NdFeAsO0.88F0.12

Polycrystalline iron-based superconducting NdFeAsO_0.88F_0.12 was prepared via solid-state reaction in ambient pressure. Small amounts of ferromagnetic impurities were detected by Scanning Electron Microscopy (SEM) analysis. To study the influence of magnetism on superconducting properties of the sample, magnetization M(T, H) measurements were performed in fields up to 1.5 T and in the temperature range of 5-60 K. The abnormal behavior was observed in the χ(T) curves, and the magnetization hysteresis loops (MHLs) can be described by a sum of the contributions of superconductive hysteresis, ferromagnetic hysteresis of the impurities and the paramagnetic background of the isolated Nd³⁺ ions. The influence of the ferromagnetic impurities and the paramagnetic background were investigated and discussed. After the corrections of the magnetism contributions, the intrinsic superconductive MHLs as well as the critical current density were obtained.     

Magnetism of Cu2(Mn1−xNix)Sn Heusler alloys

New mixed Heusler alloys of the system Cu₂(Mn_1?xNi_x)Sn were prepared with x = 0–1. Magnetic measurements in the ferromagnetic region were undertaken. For manganese, the only atom displaying a magnetic moment in this row of Heusler alloys, 4μ_B was found, deviations were due to the degree of order. Measurements in the paramagnetic region were not possible because of phase transitions on heating leading to polyphase samples. The critical concentration of magnetic ions was estimated to 13 at.% and compared with models given by Duff and Cannella [6]. With the same plot the Curie point for Cu₂MnSn could be determined (630 K).     

Magnetic properties of low-doped bulk Sr(Ti, Co)O3−δ perovskites

Magnetic properties of bulk SrTi_1−xCo_xO_3−δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO₃ matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.     

Heat capacity of La1−x SrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La_1?x Sr_xMnO₃ (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La_0.7Sr_0.3MnO₃ sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.     

I–V characteristics and critical currents in superconducting/ferromagnetic bilayers

Current–voltage (I–V) characteristics and critical current density, J_c, for the onset of vortex motion were measured at different magnetic fields, H, and temperatures, T, in a superconducting (S)/ferromagnetic (F) bilayer and in a single Nb film. We choose Nb as a superconductor and a weak ferromagnetic alloy, Pd_1−xNi_x with x = 16, as F. We found that J_c was smaller for the S/F bilayer with respect to the single Nb film. The result was related to the reduced value of the superconducting order parameter in the bilayer.     

Coexistence of superconductivity and magnetism in the La0.87−xLnxSr0.13FeAsO (Ln=Sm, Gd, Dy) system

The interplay between the superconducting phase and spin density wave order phase was studied. We report the magnetic and superconducting properties of the hole-doped FeAs-based superconducting compound La_0.87−xLn_xSr_0.13FeAsO (Ln=Sm, Gd, Dy; 0≤x≤0.06). Both resistivity and magnetic susceptibility measurements show that the superconducting transition temperature decreases with increase in composition of magnetic ions. The hysteresis loop of the La_0.87−xLn_xSr_0.13FeAsO sample shows a superconducting hysteresis in addition to a paramagnetic background. The experiment demonstrates that the magnetism and superconductivity coexist in hole-doped FeAs-based superconducting compounds. Among these three magnetic rare-earth elements, the influence of Dy³⁺ doping on superconductivity is more evident than that of Gd³⁺ doping, while the influence of Sm³⁺ doping is the weakest. The trend is consistent with the variation of the lattice parameter along c-axis.     

Vanadium-induced decrease in the magnetization of a VxCo1−x alloy film on the W(110) surface

The magnetic and structural properties of a series of V_xCo_1?x alloy films prepared in a variety of compositions on a W(110) surface were investigated using spin-resolved photoemission spectroscopy (SRPES) and low-energy electron diffraction (LEED) measurements. The epitaxial structures of the films were preserved over all V concentrations, despite the observation of a structural phase transition (SPT: hcp/bcc at x = 0.38), which was apparent from the LEED images. A magnetic phase transition (MPT) from ferromagnetic to paramagnetic occurred in the Co-rich region at x = 0.38. The SPT (from hcp and bcc) and MPT (from ferromagnetic to paramagnetic) were clearly correlated at x = 0.38, possibly due to changes in the electronic and magnetic structures of the valence band, as monitored by SRPES and LEED. The magnetization in the V_xCo_1?x alloy films on a W(110) surface varied systematically with the V concentration, and correlations between the structural changes and magnetic properties were revealed using SRPES and LEED.     

Structural and magnetic properties of Fe–Ni mecanosynthesized alloys

Fe_100???x Ni _x samples with x?=?22.5, 30.0 and 40.0 at.% Ni were prepared by mechanical alloying (MA) with milling times of 10, 24, 48 and 72 h, a ball mass to powder mass (BM/PM) ratio of 20:1 and rotation velocity of 280 rev/min. Then the samples were sintered at 1,000°C and characterized by X-ray diffraction (XRD) and transmission Mössbauer spectrometry (TMS). From the refinement of the X ray patterns we found in this composition range two crystalline phases, one body centered cubic (BCC), one face centered cubic (FCC) and some samples show FeO and Fe₃O₄ phases. The obtained grain size of the samples shows their nanostructured character. Mössbauer spectra were fitted using a model with two hyperfine magnetic field distributions (HMFDs), and a narrow singlet. One hyperfine field distribution corresponds to the ferromagnetic BCC grains, the other to the ferromagnetic FCC grains (Taenite), and the narrow singlet to the paramagnetic FCC grains (antitaenite). Some samples shows a paramagnetic doublet which corresponds to FeO and two sextets corresponding to the ferrimagnetic Fe₃O₄ phase. In this fit model we used a texture correction in order to take into account the interaction between the particles with flake shape and the Mössbauer $\upgamma$ -rays.     

An electron paramagnetic resonance study of phase segregation in Nd0.5Sr0.5MnO3

We present results of an electron paramagnetic resonance (EPR) study of Nd_1−xSr_xMnO₃ with x=0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T_c) and the antiferromagnetic, charge ordering transition (T_N=T_co) at 150 K. The results are compared with those on Nd_0.45Sr_0.55MnO₃ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T_N=230 K and on La_0.77Ca_0.23MnO₃ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x=0.5, the EPR signals below T_c consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x=0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La_0.77Ca_0.23MnO₃. The comparison of behaviour of A-type AFM signal observed in both Nd_0.5Sr_0.5MnO₃ and Nd_0.45Sr_0.55MnO₃ with the two FM phases of La_0.77Ca_0.23MnO₃, vis-à-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd_0.5Sr_0.5MnO₃ undergoes phase separation into A-type AFM and FM phases.     

Magnetovolume effects in Fe80-xNixCr20 alloys

Temperature and magnetic field dependences of the thermal expansion between 4 and 300 K and in fields up to 6 T were made on Fe_80-xNi_xCr₂₀ for 14⩽x⩽49 at%. This concentration range covered the regions in which the samples were antiferromagnetic, paramagnetic and ferromagnetic as well as spin glass and reentrant spin glass at low temperatures. We develop a method of determining the lattice contribution to the thermal expansion for such systems showing mixed magnetic behavior and analyze the present data accordingly. We find in ferromagnetic samples large magnetic contributions to the thermal expansion even at temperatures much higher than the Curie temperature. The field dependence of the lenght change shows behavior which is characteristic of the magnetic state of the system.     

Magnetic phase evolution in Fe substituted GdBaCo2O5.5

Magnetization and magneto-resistance experiments have been carried out on well characterized samples of the GdBaCo_2−xFe_xO_5.5 series. Zero field cooled magnetization measurements in the low concentration Fe samples suggest, that the low temperature anti-ferromagnetic phase transforms sequentially to several ferromagnetic phases, before transforming to a paramagnetic state with increase in temperature. The anti-ferromagnetic to the first ferromagnetic phase transition is associated with a large negative magneto-resistance for Fe fractions upto x=0.075. Isothermal magnetization measurements in the ferromagnetic like region of the samples, suggests the presence of mixtures of two ferromagnetic phases. Similar measurements performed at low temperatures where anti-ferromagnetic-like phase is stabilised suggest the presence of a mixture of anti-ferromagnetic and ferromagnetic phases. Magnetization and magneto-resistance are seen to collapse for Fe fractions, x>0.1. Based on these studies a plausible scenario of the evolution of magnetism with Fe substitution in GdBaCo₂O_5.5, is suggested.