V. N. Krivoruchko

Griffiths phase and the magnetic and transport properties of doped manganites

A phenomenological model is proposed to describe the magnetic and magnetoresistance properties of ferromagnetic manganites. This model is based on the methods used to describe hysteretic systems, takes into account phase separation effects, and assumes the transition of ferromagnetic manganites into the Griffiths phase at above the Curie temperature. This formalism makes it possible to describe the conducting properties of the systems in the temperature range from low temperatures to the Griffiths temperature (TG). This approach is used to qualitatively explain the experimental laws of the behavior of ferromagnetic manganites using the temperature and field dependences of the electrical resistivity and magnetization, the hysteretic properties, and the magnetoresistive effect (MRE) and to classify manganites in the magnitude of the MRE. The parameter that is responsible for the response of a system to thermal effects is the ratio of the maximum energy barrier separating various states of a system at zero temperature WA(0) to thermal fluctuation energy WCfl at TG. The WA(0)/WCfl ratio is found to determine the temperature range of the Griffiths phase. The relation between the magnitudes of the MRE and parameter WA(0)/WCfl for a certain system is revealed. The behavior of the magnetization and electrical resistivity of manganites in the Griffiths phase is discussed.

Pressure-induced variation of the magnetic structure of the surface of La0.6Sr0.4MnO3 granules

The transport properties of textured films and tunneling junctions of La0.6Sr0.4MnO3, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La0.6Sr0.4MnO3-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.

Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)

Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreev-reflection (AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained on “classic” s-wave superconductor—ferromagnetic manganites PCs, as well as related theoretical models applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected” contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as well as possibility of unconventional superconducting proximity effect, have been discussed.

Andreev spectroscopy of point contacts between a low-temperature superconductor and manganite

Andreev reflection in contacts between a low-temperature superconductor (Pb or MgB2) and the manganite La0.65Ca0.35MnO3 (LCMO) is investigated. In the majority of cases behavior typical of superconductor/half-metallic ferromagnet contacts is observed: an excess voltage Vexc and suppression of the contact conductance G(V)=dI(V)∕dV in the region of voltages e∣V∣≤ΔS, where ΔS is the energy gap of the injector. However, some of the contacts demonstrate an increase of the conductance and an excess current Iexc on the current–voltage characteristic of the contact at e∣V∣≤ΔS. The character of the dI(V)∕dV curves observed for those contacts corresponds to Andreev spectrum of a superconductor with an energy gap much larger than the injector gap ΔS. It is assumed that in contacts of the latter type, specific conditions prevail whereby the penetration of Cooper pairs into the manganite from an electrode with a singlet order parameter induces in the surface region of the manganite a superconducting state with a tripl...

Spin polarization of charge carriers and Andreev reflection in (LaCa)MnO/superconductor point contacts

Spin polarization of charge carriers in La0.65Ca0.35MnO3 (LCMO) is studied using point-contact Andreev spectroscopy. Pb and MgB2 are used to make superconducting electrodes. In all cases, the transport spin polarization obtained from the conductivity of LCMO/superconductor point contacts does not exceed 80–85%. Different models of the current flow through the superconductor-ferromagnetic metal contact and possible reasons for noncomplete spin polarization of a current in manganites are explored. The level of spin polarization observed in Sharvin contacts (contact area ∼104 Å2) is most naturally explained in terms of a model that suggests separation of the crystal into nanosized magnetic phases, only one of which is a ferromagnetic metal with full spin polarization of charge carriers.

Triple-gap superconductivity of MgB 2 -(La,Sr)MnO 3 composite

Interplay of superconductivity and magnetism in a composite prepared of the ferromagnetic half-metallic La_0.67Sr_0.33MnO (LSMO) nanoparticles and the conventional s-wave superconductor MgB_2 has been studied. A few principal effects have been found in bulk samples. With an onset of the MgB_2 superconductivity, a spectacular drop of the sample resistance has been detected and superconductivity has been observed at temperature up to 20K. Point-contact (PC) spectroscopy has been used to measure directly the superconducting energy coupling. For small voltage, an excess current and doubling of the PCs normal state conductance have been found. Conductance peaks corresponding to three energy gaps are clearly observed. Two of these gaps we identified as enhanced \Delta_{\pi} and \Delta_{\sigma} gaps originating from the MgB_2; the third gap \Delta_{tr} is more than three times larger than the largest MgB_2 gap. The experimental results provide unambiguous evidences for a new type of proximity effect which follows the phase coherency scenario of proximity induced superconductivity. Specifically, at low temperature, the p-wave spin-triplet condensate with pairing energy \Delta_{tr} is essentially sustained in LSMO but is incapable to display long-range supercurrent response because of a phase-disordering state. The proximity coupling to MgB_2 restores the long-range phase coherency of the triplet superconducting state, which, in turn, enhances superconducting state of the MgB_2.

Andreev-spectroscopy study of unconventional superconductivity in MgB2:(La,Sr)MnO3 nanocomposite

Unconventional high-temperature superconductivity in MgB2:La0:65Sr0:35MnO3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB2 in the total composites conductivity and estimated the magnitude of the electron–phonon effects originated from MgB2 in I–V characteristics of the composite at above-gap energies. It was found...

Triple-gap superconductivity of MgB2-(La,Sr)MnO3composite

Interplay of superconductivity and magnetism in a composite prepared of the ferromagnetic half-metallic La_0.67Sr_0.33MnO (LSMO) nanoparticles and the conventional s-wave superconductor MgB_2 has been studied. A few principal effects have been found in bulk samples. With an onset of the MgB_2 superconductivity, a spectacular drop of the sample resistance has been detected and superconductivity has been observed at temperature up to 20K. Point-contact (PC) spectroscopy has been used to measure directly the superconducting energy coupling. For small voltage, an excess current and doubling of the PCs normal state conductance have been found. Conductance peaks corresponding to three energy gaps are clearly observed. Two of these gaps we identified as enhanced \Delta_{\pi} and \Delta_{\sigma} gaps originating from the MgB_2; the third gap \Delta_{tr} is more than three times larger than the largest MgB_2 gap. The experimental results provide unambiguous evidences for a new type of proximity effect which follows the phase coherency scenario of proximity induced superconductivity. Specifically, at low temperature, the p-wave spin-triplet condensate with pairing energy \Delta_{tr} is essentially sustained in LSMO but is incapable to display long-range supercurrent response because of a phase-disordering state. The proximity coupling to MgB_2 restores the long-range phase coherency of the triplet superconducting state, which, in turn, enhances superconducting state of the MgB_2.

Triple-gap superconductivity of MgB2 - (La,Sr)MnO3 composite. Which of the gaps is proximity induced?

Interplay of superconductivity and magnetism in a composite prepared of the ferromagnetic half-metallic La_0.67Sr_0.33MnO (LSMO) nanoparticles and the conventional s-wave superconductor MgB_2 has been studied. A few principal effects have been found in bulk samples. With an onset of the MgB_2 superconductivity, a spectacular drop of the sample resistance has been detected and superconductivity has been observed at temperature up to 20K. Point-contact (PC) spectroscopy has been used to measure directly the superconducting energy coupling. For small voltage, an excess current and doubling of the PCs normal state conductance have been found. Conductance peaks corresponding to three energy gaps are clearly observed. Two of these gaps we identified as enhanced \Delta_{\pi} and \Delta_{\sigma} gaps originating from the MgB_2; the third gap \Delta_{tr} is more than three times larger than the largest MgB_2 gap. The experimental results provide unambiguous evidences for a new type of proximity effect which follows the phase coherency scenario of proximity induced superconductivity. Specifically, at low temperature, the p-wave spin-triplet condensate with pairing energy \Delta_{tr} is essentially sustained in LSMO but is incapable to display long-range supercurrent response because of a phase-disordering state. The proximity coupling to MgB_2 restores the long-range phase coherency of the triplet superconducting state, which, in turn, enhances superconducting state of the MgB_2.