Exchange-bias phenomenon: The role of the ferromagnetic spin strucutre

The exchange bias of antiferromagnetic-ferromagnetic (AFM-FM) bilayers is found to be strongly dependent on the ferromagnetic spin configuration. The widely accepted inverse proportionality of the exchange bias field with the ferromagnetic thickness is broken in FM layers thinner than the FM correlation length. Moreover, an anomalous thermal dependence of both exchange bias field and coercivity is also found. A model based on springlike domain walls parallel to the AFM-FM interface quantitatively accounts for the experimental results and, in particular, for the deviation from the inverse proportionality law. These results reveal the active role the ferromagnetic spin structure plays in AFM-FM hybrids which leads to a new paradigm of the exchange bias phenomenon

Light induced decoupling of electronic and magnetic properties in manganites

The strongly correlated material La0.7Sr0.3MnO3 (LSMO) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. To include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide (CdS) to an LSMO layer to change its electrical conductivity. LSMO exhibits no significant optical response, however, the CdS/LSMO heterostructures show an enhanced conductivity, with ~ 37 % resistance drop, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of CdS/LSMO heterostructures are decoupled and exhibit different effects when both stimuli are applied. This unexpected behavior shows that heterostructuring strongly correlated oxides may require a new understanding of the coupling of physical properties across the transitions and provide the means to implement new functionalities

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF2, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures

Magnetism and the absence of superconductivity in the praseodymium–silicon system doped with carbon and boron

We searched for new structural, magnetic and superconductivity phases in the Pr-Si system using high-pressure high-temperature and arc melting syntheses. Both high and low Si concentration areas of the phase diagram were explored. Although a similar approach in the La-Si system produced new stable superconducting phases, in the Pr-Si system we did not find any new superconductors. At low Si concentrations, the arc-melted samples were doped with C or B. It was found that addition of C gave rise to multiple previously unknown ferromagnetic phases. Furthermore, X-ray refinement of the undoped samples confirmed the existence of the so far elusive Pr3Si 2 phase. © 2013 Elsevier B.V

Enhancements of pinning by superconducting nanoarrays

We present a comparative study of vortex pinning efficiency in superconducting (V) thin films grown on two similar triangular arrays of superconducting (Nb) and nonsuperconducting (Cu) nanodots. Resistance and magnetization anomalies at the same matching fields confirm the same pinning periodicity in both samples. However, we found two distinct features: First, the sample with superconducting dots shows stronger pinning that appears as sharper matching peaks in magnetization loops and shows higher critical current density and larger critical field at low temperatures. Second, an overall increase in the resistance of the V film with Nb nanodots is observed, while there is a crossover in the temperature dependence of the critical field and the critical current of both samples at T = 3 K. This crossover corresponds to the temperature when the superconducting coherence length of V thin film equals the edge-to- edge distance between nanodots. We argue that this change in superconducting properties is related to the change in the superconducting regime from pinning enhancement at low temperatures to a superconducting wire network at high temperatures

Magnetic field modulated microwave spectroscopy across phase transitions and the search for new superconductors

This article introduces magnetic field modulated microwave spectroscopy (MFMMS) as a unique and high-sensitivity technique for use in the search for new superconductors. MFMMS measures reflected microwave power as a function of temperature. The modulation induced by the external ac magnetic field enables the use of phase locked detection with the consequent sensitivity enhancement. The MFMMS signal across several prototypical structural, magnetic, and electronic transitions is investigated. A literature review on microwave absorption across superconducting transitions is included. We show that MFMMS can be used to detect superconducting transitions selectively with very high sensitivity

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange biased nanostructures

Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF2, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures