Coexistence of ferromagnetism and superconductivity in MWCNT/Bi2SiO5 nanocomposites

This work explores the temperature and field-dependent magnetic properties of Bi2SiO5 (BSO)/multiwall carbon nanotube (MWCNT) nanocomposites prepared by hydrothermal method using different content of magnetic MWCNT (from 1 to 20 wt%). A pure BSO prepared by the same method was also studied as a reference. Phase purity and structure of the pure BSO and CBSO nanocomposites were analyzed by employing the Rietveld refinement of an X-ray diffraction pattern. DC magnetization analysis confirms the co-existence of multiple magnetic phases in CBSO nanocomposites, where a paramagnetic (PM) to ferromagnetic (FM) transition experiences at ~33 K, and a cluster spin glass behavior at near Curie temperature (i.e., TC ~25 K). A weak superconducting transition (TSC) at around ~23 K was observed for all the CBSO nanocomposites samples (i.e., 2.5 wt% MWCNT and above samples). Moreover, a novel feature of these nanocomposites is that they exhibit a complex magnetism corresponding to the co-existence of ferromagnetism and superconductivity. This behavior can be exploited to engineer a magnetic CBSO composites system with the magnetic transition tune by compositional structure, with interesting potential applications.

Spin glass behavior and magnetic boson peak in a structural glass of a magnetic ionic liquid

Glassy magnetic behavior has been observed in a wide range of crystalline magnetic materials called spin glass. Here, we report spin glass behavior in a structural glass of a magnetic ionic liquid, C4mimFeCl4. Magnetization measurements demonstrate that an antiferromagnetic ordering occurs at TN = 2.3 K in the crystalline state, while a spin glass transition occurs at TSG = 0.4 K in the structural glass state. In addition, localized magnetic excitations were found in the spin glass state by inelastic neutron scattering, in contrast to spin-wave excitations in the ordered phase of the crystalline sample. The localized excitation was scaled by the Bose population factor below TSG and gradually disappeared above TSG. This feature is highly reminiscent of boson peaks commonly observed in structural glasses. We suggest the “magnetic” boson peak to be one of the inherent dynamics of a spin glass state.

Magnetic and structural properties of the solid solution CuAl2(1−x)Ga2xO4

CuAl2O4 is a ternary oxide spinel with Cu2+ ions ($$s=1/2$$ s = 1 / 2 ) primarily populating the A-site diamond sublattice. The compound is reported to display evidence of spin glass behavior but possess a non-frozen magnetic ground state below the transition temperature. On the other hand, the spinel CuGa2O4 displays spin glass behavior at ~ 2.5 K with Cu2+ ions more readily tending to the B-site pyrochlore sublattice. Therefore, we investigate the magnetic and structural properties of the solid solution CuAl2(1-x)Ga2xO4 examining the evolution of the magnetic behavior as Al3+ is replaced with a much larger Ga3+ ion. Our results show that the Cu2+ ions tend to migrate from tetrahedral to octahedral sites as the Ga3+ ion concentration increases, resulting in a concomitant change in the glassy magnetic properties of the solution. Results indicate glassy behavior for much of the solution with a general trend towards decreasing magnetic frustration as the Cu2+ ion shifts to the B-site. However, the $$x=0.1$$ x = 0.1 and 0.2 members of the system do not show glassy behavior down to our measurement limit (1.9 K) suggesting a delayed spin glass transition. We suggest that these two members are additional candidates for investigation to access highly frustrated exotic quantum states.

Clarification of the ordering of intercalated Fe atoms in Fe x TiS2 and its effect on the magnetic properties

Even though there has been a lot of studies on the magnetic properties of Fe x TiS2 and their corresponding atomic structures at different Fe concentrations, the dependency of the properties on the Fe atomic arrangement has not been fully clarified yet. In this study, Fe x TiS2 structures, synthesized by chemical vapor transport technique at Fe concentrations of 0.05, 0.10, 0.15, 0.20 0.25 and 0.33, were observed three-dimensionally using a transmission electron microscope and their corresponding magnetization values were measured using a superconducting quantum interference device. The results show a switch from local in-plane two-dimensional (2D) ordering of \sqrt 3 a and 2a at concentrations below 0.15 to three-dimensional (3D) ordering of 2a × 2a × 2c at x = 0.20 and 0.25, as well as \sqrt 3 a × \sqrt 3 a × 2c superstructures at x = 0.33, although it should be noted that the x = 0.20 sample only had partial ordering of Fe atoms. The type of Fe ordering present in Fe x TiS2 could be explained by the balance of cohesive energy of neighboring Fe atoms and local strain energy imposed on the host structure due to the formation of Fe clusters. It is also found that the switch from 2D to 3D Fe order coincides with the magnetic measurements, which reveal spin-glass behavior below x = 0.15 and ferromagnetic behavior above x = 0.20. This suggests that the magnetic properties of the Fe x TiS2 structure are highly influenced by the ordering of Fe atoms between planes.