Observation of oscillatory magnetic order in the antiferromagnetic superconductor HoNi2B2C

Long-range magnetic order in hydroxide-layer-doped (Li1-x-y Fe x Mn y OD)FeSe

The (Li1-x Fe x OH)FeSe superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li1-x-y Fe x Mn y OD)FeSe system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the (Li0.876Fe0.062Mn0.062OD)FeSe superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

Magnetic order and competition with superconductivity in (Ho-Er)Ni2B2C

The rare earth magnetic order in pure and doped H o ( 1 - x ) E r x N i 2 B 2 C (x = 0, 0.25, 0.50, 0.75, 1) single crystal samples was investigated using magnetization and neutron diffraction measurements. Superconducting quaternary borocarbides, R N i 2 B 2 C where R = Ho, Er , are magnetic intermetallic superconductors with the transition temperatures~10 K in which long range magnetic order develops in the same temperature range and competes with superconductivity. Depending on the rare earth composition the coupling between superconductivity and magnetism creates several phases, ranging from a near reentrant superconductor with a mixture of commensurate and incommensurate antiferromagnetism to an incommensurate antiferromagnetic spin modulation with a weak ferromagnetic component. All of these phases coexist with superconductivity. RKKY magnetic interactions are used to describe the magnetic orders in the pure compounds. However, the doping of Er on Ho sites which have two strong magnetic moments with two different easy directions creates new and complicated magnetic modulations with possible local disorder effects. One fascinating effect is the development of an induced magnetic state resembling the pure and doped R2CuO4 cuprate with R = Nd and Pr.

Crystal growth and ambient and high pressure study of the re-entrant superconductor Tm(2)Fe(3)Si(5)

Tm(2)Fe(3)Si(5) is known to undergo a transition to the superconducting state (at ambient or applied pressure depending on the sample) at a temperature T(c1)(∼1.8 K), and at a lower temperature T(N)(≈1 K) it undergoes a transition into a long range antiferromagnetically ordered state. Superconductivity is simultaneously destroyed and the sample re-enters the normal state at T(c2) = T(N). The conditions reported in the literature for the observation of superconductivity in Tm(2)Fe(3)Si(5) are sample dependent, but it is now accepted that stoichiometric Tm(2)Fe(3)Si(5) superconducts only under pressure. Here we report single-crystal growth of stoichiometric Tm(2)Fe(3)Si(5) which does not superconduct at ambient pressure down to 100 mK. Measurements of the anisotropic static magnetic susceptibility χ(T) and isothermal magnetization M(H), ac susceptibility χ(ac)(T), electrical resistivity ρ(T) and heat capacity C(T) at ambient pressure and χ(ac)(T) at high pressure are reported. The magnetic susceptibility along the c axis, χ(c)(T), shows a curvature over the whole temperature range and does not follow the Curie-Weiss behavior, while the magnetic susceptibility along the a axis, χ(a)(T), follows a Curie-Weiss behavior between 130 and 300 K with a Weiss temperature θ and an effective magnetic moment μ(eff) which depend on the temperature range of the fit. The easy axis of magnetization is perpendicular to the c axis and χ(a)/χ(c) = 3.2 at 1.8 K. The ambient pressure χ(ac)(T) and C(T) measurements confirm bulk antiferromagnetic ordering at T(N) = 1.1 K. The sharp drop in χ(ac)(T) below the antiferromagnetic transition is suggestive of the existence of a spin gap. We observe superconductivity only under applied pressures P≥2 kbar. The temperature-pressure phase diagram showing the non-monotonic dependence of the superconducting transition temperature T(c) on pressure P is presented.

Reentrant behaviour in Y-doped Ho(0.75)Y(0.25)Ni(2)B(2)C single crystal

The transport and superconducting properties of Ho(0.75)Y(0.25)Ni(2)B(2)C single crystals were investigated to study the competing effects between superconductivity and magnetism. The superconducting transition temperature T(c) is 7.6 K, determined from the resistivity transition; meanwhile, the commensurate antiferromagnetic (AFM) transition occurs at T(N) of 3.9 K, which is lower than that of pure HoNi(2)B(2)C (T(N)≈5 K). Ho(0.75)Y(0.25)Ni(2)B(2)C reentered into the normal state at T(m) (T(N)<T(m)<T(c)) when small magnetic fields were applied along the crystallographic c-axis. In contrast to the case in HoNi(2)B(2)C, the reentrant behaviour for Ho(0.75)Y(0.25)Ni(2)B(2)C only appears when the applied field H is along the c-axis, and the reentrant peak position T(P)(H) shifts to lower temperature with increasing applied field. We suggest that the disorder of magnetic structure induced by Y doping may account for the significant difference in the reentrant behaviour between Ho(0.75)Y(0.25)Ni(2)B(2)C and HoNi(2)B(2)C. Moreover, there does not exist a deep minimum in the upper critical field H(c2)(T) line at T(N) of 3.9 K for either [Formula: see text] or [Formula: see text]. The H-T phase diagram is derived and discussed.