S. Langridge

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and magnetic disorder of magnetic domains in a transition metal multilayer have been studied using neutron scattering techniques. A new theoretical framework is presented connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The results unambiguously show the highly correlated nature of the antiferromagnetically coupled domain structure vertically throughout the multilayer. We are easily able to relate the neutron determined magnetic dispersion and domain correlations to magnetization and magnetotransport experiments.

Molecular weight dependent vertical composition profiles of PCDTBT:PC71BM blends for organic photovoltaics

We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic photovoltaic devices were made using a blend of the fullerene acceptor PC71BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC71BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.

Interfacial structure in semiconducting polymer devices

We discuss some recent findings relating to the structure of interfaces in semiconducting polymer devices. The structure of three different types of interface is characterized via neutron reflectivity and scanning force microscopy. In the first example we find that enrichment of dopant occurs at the surface of a doped polymeric conductor, and that this enriched layer penetrates several nanometres into the material. Secondly, we find that the interface between a semiconducting polymer and an insulating polymer is not molecularly sharp, but is rather broad with a width typically of the order of 1 nm. Finally we present some initial neutron reflectivity measurements on the interface between two different semiconducting polymers (one of which is deuterated). Again we find that this interface is diffuse, with a typical width of the order of a few nanometres.

Controlled suppression of superconductivity by the generation of polarized Cooper pairs in spin valve structures

(Received 10 April 2014; revised manuscript received 13 January 2015; published 2 February 2015) Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.

Exchange Bias in Fe@Cr Core-Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core-shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity.

Magnetic Moment in an Ultrathin Magnetite Film

We have investigated the magnetic properties of a Cu capped thin film of magnetite (Fe3O4) grown epitaxially on Pt(111). Conversion electron Mossbauer spectroscopy data show good agreement with those from bulk Fe3O4, evidencing a good degree of structural order. The data point to in-plane ferrimagnetic alignment of the magnetic moment in the Fe3O4 layer. Polarized neutron reflectivity (PNR) data determines the layer thinknesses to be 53±6 A for the magnetite film and 106±5 A for the Cu capping layer. The average magnetic moment determined by PNR for the Fe3O4 layer is 2.8±0.3 μB, smaller than the value of 4.1 μB for bulk Fe3O4. It is suggested that the reduced moment is in part a result of a reduced ordering temperature in the ultrathin film.

Remotely induced magnetism in a normal metal using a superconducting spin-valve

A switchable induced magnetic moment in a non-magnetic metal that is separated from a ferromagnet by a thick superconducting layer contradicts existing models.

Interplay between superconductivity and magnetism in Gd/La superlattices

Abstract A [Gd 30 /La 10 ] 60 superlattice has been studied using SQUID magnetometry and polarised neutron reflectivity. Zero-field cooling results in the coexistence of an antiferromagnetic alignment of the ferromagnetic Gd blocks, and 3D superconductivity at low temperature. Field cooling from room temperature results in ferromagnetic coupling between the Gd blocks, and under these conditions, the superconducting transition in the La is suppressed.

Soft x-ray resonant magnetic scattering from an imprinted magnetic domain pattern

The authors report on the use of a Co∕Pt multilayer, which exhibits strong perpendicular magnetic anisotropy, to magnetostatically imprint a domain pattern onto a 50A thick Permalloy layer. Element specific soft x-ray magnetic scattering experiments were then performed so as to be sensitive to the magnetic structure of the Permalloy only. Off-specular magnetic satellite peaks, corresponding to a periodic domain stripe width of 270nm, were observed, confirmed by magnetic force microscopy and micromagnetic modeling. Thus the authors have exploited the element specificity of soft x-ray scattering to discern the purely magnetic correlations in a structurally flat Permalloy film.

Temperature controlled motion of an antiferromagnet- ferromagnet interface within a dopant-graded FeRh epilayer

Chemically ordered B2 FeRh exhibits a remarkable antiferromagnetic-ferromagnetic phase transition that is first order. It thus shows phase coexistence, usually by proceeding though nucleation at random defect sites followed by propagation of phase boundary domain walls. The transition occurs at a temperature that can be varied by doping other metals onto the Rh site. We have taken advantage of this to yield control over the transition process by preparing an epilayer with oppositely directed doping gradients of Pd and Ir throughout its height, yielding a gradual transition that occurs between 350 K and 500 K. As the sample is heated, a horizontal antiferromagnetic-ferromagnetic phase boundary domain wall moves gradually up through the layer, its position controlled by the temperature. This mobile magnetic domain wall affects the magnetisation and resistivity of the layer in a way that can be controlled, and hence exploited, for novel device applications.