Particle size dependence of exchange-bias and coercivity in CuO nanoparticles

For CuO nanocrystals of size 6.6-37 nm, the exchange bias H_eb and coercivity H_c are measured at 5 K in zero-field-cooled (ZFC) and field-cooled (FC at 50 kOe) samples and their variations investigated as a function of particle size D. The similar 1/D variations observed for the difference coercivity ΔH_c=H_c(FC)−H_c(ZFC) and the interfacial exchange energy Δσ=H_ebM_fD are discussed in terms of the ferromagnetic magnetization M_f being produced by the uncompensated surface Cu²⁺ spins in the otherwise antiferromagnetically ordered CuO nanoparticles. This leads to the observation that the experimentally measured ΔH_c provides a good measure of Δσ in nanoparticle systems, with H_eb/ΔH_c varying as 1/M_fD.     

Particle size dependence of relaxivity for silica-coated iron oxide nanoparticles

We investigate the particle size dependence of the relaxivity of hydrogen protons in an aqueous solution of iron oxide (Fe₃O₄) nanoparticles coated in silica for biocompatibility. The T₁ and T₂ relaxation times for various concentrations of silica-coated nanoparticles were determined by a magnetic resonance scanner. We find that the relaxivity increased linearly with increasing particle size. The T₂ relaxivity (R₂) is more than 50 times larger than the T₁ relaxivity (R₁) for the nanoparticle contrast agent, which reflects the fact that the T₂ relaxation is mainly influenced by outer sphere processes. The high R₂/R₁ ratio demonstrates that silica-coated iron oxide nanoparticles may serve as a T₂ contrast agents in magnetic resonance imaging with high efficacy.     

On the coexistence of superconductivity and ferromagnetism

The magnetic scattering of electrons is studied, paying special attention to the problem of coexistence of superconductivity and ferromagnetism. The utilized model consists of two electronic bands, one of which can become superconducting, and a system of localized spins. The calculated transition temperatures of the superconduction and the magnetic subsystems show regions of coexistence for not too large exchange interactions. Generally speaking, coexistence is favoured due to spin-orbit scattering as well as due to the interaction of the superconducting band with the normal band in certain cases.     

Competition between nuclear ferromagnetism and superconductivity

A qualitative theory of nuclear magnetism against a background of superconductivity in metals is proposed. Even though the superconducting transition temperature is much higher than the nuclear ordering temperature, nuclear ferromagnetism can partially or completely destroy superconductivity. An experimental method of determining the effective electron-nuclear spin-spin interaction constant for superconductors is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 772–775 (25 May 1997)     

Coexistence of superconductivity and ferromagnetism in ferromagnetic metals

We address the question of coexistence of superconductivity and ferromagnetism. Using a field theoretical approach we study a one-fermion effective model of a ferromagnetic superconductor in which the quasiparticles responsible for the ferromagnetism form the Cooper pairs as well. For the first time we solve self-consistently the mean-field equations for the superconducting gap and the spontaneous magnetization. We discuss the physical features which are different in this model and the standard BCS model and consider their experimental consequences.     

Coexistence of ferromagnetism and superconductivity in NiBi-binary alloy

We investigate the effects of the antiferromagnetic exchange coupling between the Nickel and Bismuth2 atoms (J_int2 < 0) on the magnetizations of the NiBi-binary alloy versus temperature and external magnetic field by means of the effective field theory. We find that the magnetization of the Ni, Bi1, Bi2 and total NiBi-binary alloy has two different magnetic phase transitions for the J_int2 < 0. One of them is a first-order phase transition (FOPT) at T_t = 0.349 and the other is a second-order phase transition (SOPT) at T_c = 0.791. We also study the hysteresis behaviors and we find that the values of the coercive field points of the Bi2 are higher than those of the Ni and Bi1. Moreover, Ni, Bi1 and Bi2 components have ferromagnetic hysteresis behaviors whereas the total NiBi has type II superconducting behavior. Therefore, we suggest that ferromagnetism and superconductivity coexist in the NiBi-binary alloy that is qualitatively good agreement with the some experimental and theoretical works.     

Coexistence of superconductivity and ferromagnetism in two dimensions

Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO(3) (LAO) and SrTiO(3) (STO), a system that has been studied intensively recently. Magnetoresistance, Hall, and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field makes this a fascinating system for the study of the interplay between superconductivity and magnetism.     

Interplay between ferromagnetism and superconductivity in tunneling currents

We study tunneling currents in a model consisting of two nonunitary ferromagnetic spin-triplet superconductors separated by a thin insulating layer. We find a novel interplay between ferromagnetism and superconductivity, manifested in the Josephson effect. This offers the possibility of tuning dissipationless currents of charge and spin in a well-defined manner by adjusting the magnetization direction on either side of the junction.     

A model for coexistent superconductivity and ferromagnetism

We explore the various temperature dependences and thermodynamic quantities of a mean-field model of a ferromagnetic–superconducting system. The starting point for this model is based on an s-wave pairing scheme in the singlet channel of the superconducting state and a spontaneously broken-symmetry phase in the ferromagnetic state. We show numerically and analytically that a state of coexistence reveals itself and is favoured energetically over other possible states, and a simple phase diagram is developed. Finally, a comparison of the specific heat with experiment is shown.     

Magnetic nesting and coexistence of ferromagnetism and superconductivity

If the condition ε_σ(p)=ε_?σ(?p+nI/v_F) for magnetic nesting is fulfilled for the electron dispersion law with spin σ along a certain preferential direction n, ferromagnetism and the inhomogeneous superconducting state can coexist up to a very high magnetization I. This fact was used to explain the coexistence of ferromagnetism and superconductivity for layered cuprates of the RuSr₂GdCu₂O₈ type, which possess a finite, though rather high, critical magnetization, because the conditions for magnetic nesting are fulfilled only approximately.     

Coexistence of ferromagnetism and superconductivity in Cu-rich lanthanum Cu-oxides

We have measured the zero field and field cooled magnetization of the lightly oxygen doped Cu-rich La₂CuO _{4 + δ} in a wide temperature range (5 K to 350 K). The data together with the evolution of the magnetic hysteresis loop suggest that the ferromagnetism with Curie temperature of 280 K coexists with superconductivity below the transition temperature ∼ 34 K. The coexistence occurs in the hole-rich clusters of size ? 150 nm, which are electronic phase separated from the hole-poor antiferromagnetic background. Received 17 October 2001     

Unified theory of high-Tc superconductivity in BSCCO and YBCO

This paper shows that in YBCO and BSCCO there are five kinds of waves for the order parameter, d, pl, p2, s and extended s, and explains six important experimental facts.     

Comparison between ferromagnetism and superconductivity of intercalation compounds

A comparison between the ferromagnetism and superconductivity of graphite intercalation compounds suggests the absence of either phase transition in a two dimensional array.     

Suppression of superconductivity in YBCO/LCMO superlattices

The competition of superconductivity and magnetism in superlattices composed of alternating YBa₂Cu₃O_7−d and La_0.67Ca_0.33MnO₃ thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of super lattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symmetry and thus suppress superconductivity.     

Magnetic structures in the coexistence phase of superconductivity and weak ferromagnetism

The phases of superconductivity and weak ferromagnetism coexistence are considered. The following situations may realize below the Neel temperature depending on the weak ferromagnetic moment value: the phase with the domain-like magnetic structure when the vectors of anti-ferromagnetism 1 and weak ferromagnetism m have opposite directions in the neighbouring domains, Meissner ferromagnetic phase (1, and m are constant) which has no analogy in other magnetic superconductors and self-induced vertex phase (the directions of 1 and m are constant and their values vary smoothly in space). The new tetragonal body centered phase of ErRH₄B₄ may prove to be the first weak ferromagnetic superconductor.     

Possible phase change between itinerant ferromagnetism and superconductivity

It is pointed out theoretically that a single electron band model is able to exhibit the interchange of two phases: itenerant band ferromagnetism and superconductivity. Our theory is based on the molecular field approximation applied for a simplified electron-electron interaction. Possible phase changes are discussed in connection with two phase transitions of ferromagnetism and superconductivity in the intermetallic compound Y₄Co₃.     

Coexistence of ferromagnetism and superconductivity in Ni/Bi bilayers

In spite of a lack of superconductivity in bulk crystalline Bi, thin film Bi deposited on thin Ni underlayers are strong-coupled superconductors below approximately 4 K. We unambiguously demonstrate that by tuning the Ni thickness the competition between ferromagnetism and superconductivity in the Ni/Bi can be tailored. For a narrow range of Ni thicknesses, the coexistence of both a superconducting energy gap and conduction electron spin polarization are visible within the Ni side of the Ni/Bi bilayers, independent of any particular theoretical model. We believe that this represents one of the clearest observations of superconductivity and ferromagnetism coexisting.     

Response functions,superconductivity and ferromagnetism in the many fermion system

At the phase transition, the anomalies of thermodynamic and static response functions coincide due to the fact that the free energy, expressed in terms of response functions at imaginary frequency including zero, has its pole at zero frequency. Previous work has considered the singularities which arise from the polarisation of the medium. In this paper, a study is made of the divergences arising from the modification of the vertex function. The diamagnetic current is related to the free energy via the Ward Identity so that the specific heat anomaly inevitably gives rise to the Meissner effect. Below the transition the system adjusts to the singularity by going into the BCS condensed state. The similarity between BCS theory and ferromagnetism is shown both through a Weiss field approach and diagrammatically by including special correlations between k and -k. Its importance for an understanding of the broken symmetry problem is discussed. Finally, the parallel behaviour of the susceptibility and specific heat of a Bloch ferromagnet is demonstrated.     

On the size dependence of the melting temperature of nanoparticles

The size dependence of the melting temperature of nanocrystals has been investigated within the thermodynamic approach. A formula is obtained, which, in contrast to the classical Thomson formula, takes into account the metastable character of equilibrium between the crystal core and melt shell. Comparative investigation of the size dependence of the melting temperature, disregarding and taking into account the size dependences of the surface tension of the solid and liquid phases and the interface tension, has been performed by the example of aluminum, tin, and copper nanoparticles.