Magnetic superconductors of HoMo 6 S 8 type: The effect of magnetic field and supercurrent (original) (raw)
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We investigate the properties of the inhomogeneous domain-type magnetic structure (DS phase) produced in magnetic superconductors such as HoMo,S, in the region where superconductivity and magnetism coexist. We show that the superconducting critical current in the DS phase decreases to zero when the temperature is lowered to T $ (the supercooling temperature of the D S phase). The wave vector of the magnetic structure decreases at the same time with increasing current flowing through the sample. We investigate the behavior of the DS in a magnetic field, obtain the phase diagram in the (H,T) plane. In the region where the field penetrates, the DSphase is substantially altered: new peaks 2nQ (n is an integer) appear in the neutron scattering, and the wave vector Q decreases with increasing field.
Domain-like magnetic structure in superconductors of ErRh4 B4 and HoMo6S8 type
The problem of the coexistence of superconductivity and magnetic order is studied by taking into account the indirect exchange interaction, magnetic dipolar interaction and magnetic anisotropy. It is shown that the domain-like magnetic structure should be realized in the superconducting phases of ErRh4B4 and HoMo6S8 at the temperatures Tm= 1.4 and 0.7 K respectively. The transition from superconducting domain-like phase (DS) to the normal ferromagnetic (FN) state is described.
Inhomogeneous magnetic structure in clean magnetic superconductors
Clean magnetic superconductors are considered in which the phase transition into the ferromagnetic state takes place at a temperature 0 in the absence of superconductivity, with 0<< Tel, where Tel is the superconducting critical temperature. The exchange and electromagnetic interactions of electrons and localized magnetic moments are taken into account, as well as magnetic anisotropy. We show that below the temperature TM-~ 0 in the superconducting state the inhomogeneous magnetic structure of transverse one-dimensional domain type (DS phase) shouM occur at real values of the exchange interaction and anisotropy. In the DS phase gapless superconductivity is realized at temperatures sufficiently far from TM. Here the equilibrium direction of the magnetic structure wave vector Q can be changed by applying a supercurrent across the sample. The behavior of this DS phase in an external magnetic field is also considered.
Coexistence of Superconductivity and Ferromagnetism in a Magnetic Superconductor
2013
A model is presented utilizing a Hamiltonian with equal spin singlet and triplet pairings based on quantum field theory and green function formalism, to show the correlation between the superconducting and ferromagnetic order parameters. The model exhibits a distinct possibility of the coexistence of superconductivity and ferromagnetism, which are two usually incompatible cooperative phenomena. The work is motivated by the recent experimental evidences of a long-range magnetic order below the superconducting phase temperature in a number of ternary or pseudo-ternary compounds of the lanthanides. The theoretical results are then applied to show the coexistence of superconductivity and ferromagnetism in the lanthanide compound HoMo6S8.
Theory of magnetic structure in reentrant magnetic superconductors Ho Mo 6 S 8 and Er Rh 4 B 4
Magnetic superconductors, in which the first-and the second-order phase transition to the ferromagnetic (FN) state would occur in the absence of superconductivity, are considered. The exchange and the electromagnetic dipolar interactions of localized moments and electrons as well as magnetic anisotropy are taken into account. It is shown that one realizes the transverse domainlike magnetic structure in a superconducting state (the DS phase). Transitions S-DS-FN are considered. The proposed theory (with the secondorder transition) explains well the experimental data for HOMO& The experimental data on ErRh4B4 may be explained in the framework of the similar microscopic theory with the assumption that it is a first-order magnetic transition and that the critical-temperature (0) variations over the sample investigated by Moncton et al. [ Phys. Rev. Lett. 45, 2060 (198111 and by Sinha et al. [Phys. Rev. Lett. 48, 950 (1982)l are due to inhomogeneous stresses.
REVIEWS OF TOPICAL PROBLEMS: Magnetic superconductors
Uspekhi Fizicheskih Nauk
Superconductivity and ferromagnetism are antagonistic types of ordering, and their mutual effects give rise to several interesting phenomena which have recently been studied in rare earth compounds. A theoretical analysis shows that while a ferromagnetic superconductor is a type II superconductor near the superconducting transition point T cl , it becomes a type I superconductor near the ferromagnetic transition point T M . A new theory derived for the case T M •<7' cl predicts the formation of a transverse domain-like magnetic structure near T M . In clean superconductors the electron spectrum is gapless. A change in the behavior from type II to type I upon cooling to T M has been observed experimentally in ErRh 4 B 4 . Experimental data on ErRh 4 B 4 , HoMo 6 S g , and HoMo 6 Se g prove the existence of superconductivity and a magnetic ordering below T M .
Quantum Magnetism and Superconductivity
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The spin of the neutron allows neutron scattering to reveal the magnetic structure and dynamics of materials over nanometre length scales and picosecond timescales. Neutron scattering is particularly in demand in order to understand high-temperature superconductors, which lie close to magnetically ordered phases, and highly correlated metals with giant effective fermion masses, which lie close to magnetic order or pass through a mysterious phase of hidden order before becoming superconducting. Neutron scattering also is the probe of choice for revealing new phases of matter and new particles, as seen in the surprising behaviour of quantum spin chains and ladders where mass gaps and excited triplons replace conventional spin waves. Examples are given of quantum phenomena where neutron scattering has played a defining role that challenges current understanding of condensed matter.
Magnetic properties of two-phase superconductors
Physica C-superconductivity and Its Applications, 2008
We have recently proposed a theoretical model for superconductors endowed with two distinct superconducting phases, described by two scalar order parameters which condensate at different critical temperatures. On analyzing the magnetic behavior of such systems, we have found some observable differences with respect to the case of ordinary Ginzburg-Landau superconductors. In particular, at low temperature the London penetration length is strongly reduced and the Ginzburg-Landau parameter j becomes a function of temperature. By contrast, in the temperature region between the two-phase transitions j is constant and the system is a type-I or a type-II superconductor depending on the ratio between the critical temperatures.
Existence of superconducting domain walls in ferromagnets
A BCS model incorporating the orbital effect of the magnetic induction and the paramagnetic effect of the exchange field is used to investigate the conditions for the formation of an infinitesimal superconducting nucleus localized at domain walls in ferromagnets. It is shown that a solution of this type always exists in reentrant ferromagnetic superconductors, at least in the metastability region of the FN (ferromagnetic normal) phase. The existence of superconducting domain walls in the stable FN phase of ideal ferromagnets remains an open question, since the transition to this state is apparently of first order and the transition parameters are unknown. From the standpoint of realizing superconducting domain walls it seems more promising to consider reentrant superconductors possessing an irregular magnetic subsystem without a coexistence phase and having a small exchange contribution to the magnetic energy, i.e., compounds such as Ho, -, Y , Mo6Ss.