Microwave response of single crystal YBa₂Cu₃O₇–δ films as a probe for pairing symmetry

Temperature dependences of the microwave surface impedance, Zs(T), are measured in the c-axis oriented single-crystal high-Tc superconducting cuprate YBa₂Cu₃O₇₋δ (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO₂-buffered single-crystal sapphire substrates (film thickness is d ≈ 150, 300, 480 nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at a number of several discrete frequencies within the range of 5–134 GHz. The measurements have revealed unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance Rs(T) = Re {Zs(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence Rs(T) of YBCO films under study at low temperatures obeys the exponential law: Rs(T) = Rres + R₀ exp [–∆s/T] with the small energy gap ∆s (∆s 0.5Tc at f = 5 GHz); (ii) the most perfect films reveal a distinct two-peak structure of the Rs(T) dependence with peaks positioned at 27–30 K and 48–51 K, while such peaks are not observed in less perfect films. The peaks are mostly pronounced at moderate (e.g. 34 GHz) frequencies and gradually disappear both at higher and lower frequencies, while their temperature positions remain unchanged. These features of perfect single-crystalline YBCO films are believed to reveal their intrinsic electron properties. Taking into account the possibility of a mixed (s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects (such as dislocation lines or twin planes) in Bogolyubov’s quasiparticle scattering within the most perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the Zs(T) behavior

The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films

Temperature dependences of the magnetic moment have been measured in YBa_2Cu_3O_{7-\delta} thin films over a wide magnetic field range (5 <= H <= 10^4 Oe). In these films a paramagnetic signal known as the paramagnetic Meissner effect has been observed. The experimental data in the films, which have strong pinning and high critical current densities (J_c ~ 2 \times 10^6 A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559 (1995)]. This finding indicates that the origin of the paramagnetic effect is ultimately associated with nucleation and inhomogeneous spatial redistribution of magnetic vortices in a sample which is cooled down in a magnetic field. It is also shown that the distribution of vortices is extremely sensitive to the interplay of film properties and the real experimental conditions of the measurements.Comment: RevTex, 8 figure

Magnetic and transport properties of La₀.₈Sr₀.₂MnO₃/La₀.₈Ca₀.₂MnO₃ bilayer

The effects of lattice strain on the magnetic and the transport properties of La₀.₈Sr₀.₂MnO₃ films grown on an (001) LaAlO₃ substrate and on a La₀.₈Ca₀.₂MnO₃ layer were studied. It was observed that the metal-insulator and the ferromagnetic transitions occur at higher temperatures for the film deposited on La₀.₈Ca₀.₂MnO₃ layer than on LaAlO₃. The dependence of Curie temperature on the bulk and the Jahn-Teller strains were also determined

Magnetic and transport properties controlled by structural disorder in La₀.₇Ca₀.₃MnO₃ films

The magnetic properties of an amorphous, a partially-disordered, and a lattice-strained crystalline La₀.₇Ca₀.₃MnO₃ film are investigated. It is shown that the amorphous film exhibits Curie- Weiss-type paramagnetism with the effective magnetic moment of 4.2 µB/Mn ion and a small ferromagnetic contribution governed by the formation of a quasi-two-dimensional crystalline interfacial inclusions. The crystalline film with nanocrystalline randomly-oriented inclusions demonstrates a superposition of ferromagnetic (in the crystalline matrix) and superparamagnetic (in the inclusions) nature. The fitted average size of the superparamagnetic particles in the case of a Langevin function is coincident with that of the nanocrystalline clusters reveated in high-resolution electron-microscopy images. An increase in the applied magnetic field leads to a reduction in the average magnetic moment of superparamagnetic particles, which is due to an enhancement of the ferromagnetic coupling between the individual randomly-oriented crystallites. The completely crystalline film undergoes only a ferromagnetic transition with a saturation magnetization at 5 K of 2.73 µB /Mn ion

Giant resistance switching effect in nano-scale twinned La₀.₆₅Ca₀.₃₅MnO₃ film

The magnetic and transport properties of a 20 nm-scale twinned La₀.₆₅Ca₀.₃₅ MnO₃ film are investigated in a temperature range of 77-300 K. The coexistence of ferromagnetic metallic and charge-ordered insulating phases is suggested by analyzing the temperature and current dependences of the resistance at low temperatures. It is shown that thermocycling leads to the formation of a nonequilibrium state in the ensemble of charge-ordered domains and to the appearance of a giant switching in resistance of up to 100%. The experimental results are discussed on the basis of phase separation

Observation of the strain-driven charge-ordered state in La₀.₇Ca₀.₃MnO₃₋δ thin film with oxygen deficiency

The magnetic and transport properties of La₀.₇Ca₀.₃MnO₃₋δ films with an oxygen deficiency (δ≃ 0.1) and a La₀.₉Ca₀.₁MnO₃ film with the stoichiometric oxygen content are investigated in a wide temperature range. It is shown that the charge-ordered insulating (COI) state is observed for a La₀.₇Ca₀.₃MnO₂.₉ film with thickness d ≤ 30 nm, which manifests mainly a cubic crystal structure with an anomalously small lattice parameter for this composition. An increase in the film thickness (d ≃ 60 nm) leads to a structural transition from the lattice-strained cubic to the relaxed rhombohedral phase, is accompanied by a shift of the Curie point (TC) to lower temperature and a frustration of the COI state. The magnetic and transport properties of the La₀.₇Ca₀.₃MnO₂.₉ film with d ≃ 60 nm are similar to those exhibited by the optimally oxygen-doped La₀.₉Ca₀.₁MnO₃ film. It is concluded that the formation of the COI state in the La₀.₇Ca₀.₃MnO₃₋δ compound is governed by a compression of the crystal lattice rather than accumulation of oxygen vacancies, the low doping of the substituted divalent ions, or electronic phase separation

Nonclassical magnetic dynamics and negative exchange bias in Nd₀.₅Sr₀.₅MnO₃ films

The amorphous, nanoclustered, and self-organizing bilayered Nd₀.₅Sr₀.₅MnO₃ films have been prepared by a rf-magnetron sputtering. The amorphous film turn out to be a typical paramagnet with a freely moving of the individual Mn spins, the magnetic properties of which are well described by the Curie–Weiss approximation. The nanoclustered film manifests the magnetic properties mimic to the superparamagnetic particles with a nonclassical magnetic dynamics. Taking into account the unique shape of the hysteresis loops, which have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, we suggest that each particle (nanocluster) is the closure magnetic domain (or magnetic vortex) rather than the single one. At the same time, the blocked to unblocked transition was observed with increasing temperature similar to the usual superparamagnet. The self-organizing bilayered film demonstrates a negative exchange bias, which is typical for the ferromagnet/antiferromagnet hybrid system in spite of that both layers in our case have a ferromagnetic origin. The magnetic properties of the films are discussed in detail on the base of modern theoretical models

Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films

The structural, magnetic and transport properties of La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ films deposited on LaAlO₃ (001) single-crystalline substrate by rf-magnetron sputtering using «soft» (or powder) targets are investigated. It was found that at 0.3 ≤ y ≤0.5 both rhombohedral (R c3 ) and orthorhombic (Pnma) crystal phases are coexistent at room temperature, forming a nanoclustered microstructure. The clustered films manifest the two-stage magnetic and electronic transition, which are typical for the phase-separated systems. It was shown that for 0.5 ≥ y ≥ 0 the nonlinear (almost parabolic) field-dependent magnetoresistance is typical at room temperature while for 0.65 ≤y ≤1.0 its transform to the linear behavior. The magnetotransport properties of the films are explained within the framework of a field-dependent activation energy model. The magnetic phase diagram for the La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin-film system is presented

Magnetic and transport properties driven by lattice strain in La₀.₇Ca₀.₃MnO₃/BaTiO₃ and La₀.₇Sr₀.₃MnO₃/BaTiO₃ bilayered films

The microstructure and the magnetic and transport properties of La₀.₇Ca₀.₃MnO₃ and La₀.₇Sr₀.₃MnO₃ films deposited on a BaTiO₃ layer (LCMO/BTO and LSMO/BTO) and on a LaAlO₃ (001) single crystal (LCMO/LAO and LSMO/LAO) by rf-magnetron sputtering using «soft» (or powder) targets are investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while the films grown on LAO, in contrast, manifest compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strain undergo the magnetic transition at a higher temperature than that for the biaxial compressive case. This argues that the Mn–O–Mn bond-angle variation, controlled by the lattice strain, plays a more important role in the formation of the spin ordering than the attendant modification of the Mn–O bond length. It was shown that the magnetic inhomogeneity, expressed by a significant difference between the field-cooled and zero-field-cooled temperature-dependent magnetization, has a metallurgical rather than an electronic nature, and is controlled by the crystal lattice distortion and the microstructure defects. The observed enhancement of the magnetoresistance effect in the LSMO/BTO bilayer at room temperature make this object greatly beneficial in the development of new hybrid ferromagnetic/ferroelectric devices