Superconductivity and magnetism are two antagonistic cooperative phenomena, and the intriguing problem of their coexistence has been studied for several decades. Recently, artificial hybrid superconductor-ferromagnet systems have been commonly used as model systems to reveal the interplay between competing superconducting and magnetic order parameters, and to verify the existence of new physical phenomena, including the predicted domain-wall superconductivity (DWS). Here we report the experimental observation of DWS in superconductor-ferromagnet hybrids using a niobium film on a BaFe(12)O(19) single crystal. We found that the critical temperature T(c) of the superconductivity nucleation in niobium increases with increasing field until it reaches the saturation field of BaFe(12)O(19). In accordance with the field-shift of the maximum value of T(c), pronounced hysteresis effects have been found in resistive transitions. We argue that the compensation of the applied field by the stray fields of the magnetic domains as well as the change in the domain structure is responsible for the appearance of the DWS and the coexistence of superconductivity and magnetism in the superconductor-ferromagnet hybrids.

The layered perovskites RCr(BO3)2 (R = Y and Ho) with magnetic triangular lattices were studied by performing ac/dc susceptibility, specific heat, elastic and inelastic neutron scattering, and dielectric constant measurements. The results show (i) both samples' Cr 3+ spins order in a canted antiferromagnetic structure with TN around 8-9 K, while the Ho 3+ ions do not order down to T = 1.5 K in HoCr(BO3)2; (ii) when a critical magnetic field HC around 2-3 T is applied below TN , the Cr 3+ spins in the Y-compound and both the Cr 3+ and Ho 3+ spins in the Ho-compound order in a ferromagnetic state; (iii) both samples exhibit dielectric constant anomalies around the transition temperature and critical field, but the Ho-compound displays a much stronger magnetodielectric response. We speculate that this is due to the magnetostriction which depends on both of the Cr 3+ and the Ho 3+ ions' ordering in the Ho-compound. Moreover, by using linear spin wave theory to simulate the inelastic neutron scattering data, we estimated the Y-compound's intralayer and interlayer exchange strengths as ferromagnetic J1 =-0.12 meV and antiferromagnetic J2 = 0.014 meV, respectively. The competition between different kinds of superexchange interactions results in the ferromagnetic intralayer interaction.

We report magnetization measurements in the Ln(Mn1−xCox)O3+λ (Ln=Eu, Nd; 0⩽x<1) and Y(Mn1−xCox)O3 (0.35⩽x⩽0.5) perovskites as a function of temperature and magnetic field. Compositions in the range of 0.35⩽x⩽0.8 (Ln=Eu), 0.4⩽x⩽0.6 (Ln=Nd), and 0.35⩽x⩽0.45 (Ln=Y) exhibit metamagnetic behavior associated with a transformation of the magnetic structure from ferrimagnetic, where the magnetic moments of Co2+ and Mn4+ ions are antiparallel, to ferromagnetic where they are parallel. A ferromagnetic state is observed for the 0.1⩽x⩽0.15 (Ln=Nd) and 0.45<x⩽0.5 (Ln=Y) compositions. Doping and H-T phase diagrams are presented, and these diagrams resemble those of Ln1−xCaxMnO3 charge-ordered manganites. Neutron diffraction studies indicate site ordering of the Co and Mn ions in the Y(Co0.5Mn0.5)O3 system, which is presumably responsible for its ferromagnetic behavior.