This PhD work has two main topics; one on neutron instrumentations, and one on correlated electron systems. There have been a total of ten different subprojects. Common to all the projects is the neutron scattering technique that is presented in the first chapters of the thesis. Neutrons are a unique probe for studying the atomic and molecular structure and dynamics of materials. Even though neutrons are very expensive to produce, the advantages neutrons provide overshadow the price. As neutrons interact weakly with materials compared to many other probes, e.g. electrons or photons, it is possible to make a neutron scattering experiment through sample environment equipment like cryostats or pressure cells. Another advantage of neutron experiments is that the wavelength and energy of the neutron match the inter-atomic distances and basic excitations of solid materials. The scattering cross section varies through the periodic table in a seemingly random fashion. Neutron scattering offers a unique possibility to study light elements that have relatively high cross sections. The first main topic is on neutron instrumentation for the European Spallation Source (ESS). ESS is currently under construction in Lund, Sweden, and is going to be the first high powered long pulsed spallation source. The long source pulse calls for long instruments, and hence new guide designs are being developed. The new instrument designs are tested using the Monte Carlo ray tracing tool McStas. Two papers describing the impact of the time structure (pulse length and repetition frequency) choice for ESS are appended. McStas simulations of a low resolution cold powder diffractometer and high resolution thermal powder diffractometer with wavelength frame multiplication have been carried out for 20 different settings of the time structure. The instrument designs were changed to fit each setting with pulse lengths between 1 ms and 2 ms and repetition frequencies between 10 Hz and 25 Hz. The cold powder diffractometer was found to perform well with all the different source settings. The thermal powder diffractometer, however, has a performance five times better for 1 ms pulse length and 10 Hz than for 2 ms and 25 Hz. For the full suite of 15 generic neutron instruments a discussion of the figure-of-merit for a full facility is given. In addition, it is noted that for the simple guide systems used in the simulations, most neutrons reaching the sample originate from the central spot on the moderator with a radius of 3 cm to 5 cm. The simulation work on the "guide-split" concept is appended as part of the thesis. With this concept it is possible to feed several cold neutron instruments with low divergence on the sample, from the same beam port without notably compromising the neutron flux at any of the sample positions. Three guide split set-ups have been tested via computer xiv simulations in McStas; with two, four, and eight secondary guides with a total length of 150 m. During the optimization, the parameter space was found to have several plateaus with high brilliance transfer (>90%). A smooth divergence distribution at the sample position can be obtained by choosing parameters that further this. A description of the multipurpose machine, HEIMDAL, accepted for construction at ESS, is given in full length in the appended published paper. A short summary is given in the main text of the thesis. HEIMDAL will be a multi length scale neutron scattering instrument for the study of structures covering almost nine orders of magnitude from 0.01 nm to 50 mm. The instrument features a variable resolution thermal neutron powder diffractometer, combined with small angle neutron scattering and neutron imaging. The instrument uses a novel combination of a cold and a thermal guide to fulfill the diverse requirements for diffraction and small angle neutron scattering. With an instrument length of 170 m, HEIMDAL will take utilize the high neutron flux of the long pulse at ESS, whilst maintaining a high q-resolution due to the long flight path. Full virtual experiments for the powder diffraction and small angle scattering have been performed to assess the performance of the instrument design. The second main topic is on correlated electron systems. Here the magnetism of six different compounds have been studied with neutron scattering, including three different hole-doped cuprate high-temperature superconductors (HTSC), an electron-doped iron pnictide HTSC, a mineral with small clusters of geometrically frustrated magnetism, and a multiferroic manganite. A study of the inter-planar correlations in the HTSC La1:88Sr0:12CuO4 (Tc = 27 K) is appended as a published paper. The correlations are studied both in a magnetic field applied perpendicularly to the CuO2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field increases the magnetic scattering signal at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L 0 is probed. A paper draft submitted for publication describing the results of elastic and inelastic neutron scattering experiments performed on the oxygen-doped La2CuO4+y HTSC is appended (Tc 40 K). The work is also included as a section in the thesis where the magnetic incommensurate stripe phase and fluctuations are described. A neutron scattering study with high flux and high resolution reveals that the dynamic stripes in a cuprate are not the Goldstone modes of the static stripes, but rather that the signal originates from different phases in the samples. As a consequence, future models of the stripes in these materials will need to include phase separation. The work on the co-doped HTSC La1:94Sr0:06CuO4+0:035 (Tc = 37 K) is described in a chapter 7 of the thesis. Neutron scattering and muon spin rotation experiments show that xv the system has no static magnetism in zero applied field. A magnetic volume fraction can be field induced and has a linear dependence with no onset field. This behavior indicates that the system is at a quantum critical point. This co-doped sample with a Sr content of x = 0:06 has a balance between the quenched disorder from the Sr impurities and annealed order of the excess oxygen where no pinning of the magnetic order is present. With competing order parameters of the superconducting and magnetic phases, one would expect a boost of Tc in this sample without static magnetism. This is in contrast to what is observed as the critical temperature is slightly lower for this system compared to other co-doped systems, suggesting that the magnetic and superconducting phases co-exist. A published manuscript describes the study of magnetic and superconducting properties of Ba(Fe1􀀀xCox)2As2. A combined neutron scattering and muon spin rotation study of the system as a function of temperature and external magnetic fields was performed. Below the superconducting transition temperature, the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. A microscopic modeling of the data using a five-band Hamiltonian relevant to iron pnictides was performed. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple, they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments. Appended is a study of the magnetic frustration in the mineral boleite. The crystal structure of boleite contains antiferromagnetically coupled Cu2+ S = 1=2 ions forming truncated 24-spin cube clusters of linked triangles. The clusters in boleite afford a situation intermediate between molecular and bulk magnetism, accessible to both experiment and numerical theory, in which a spin liquid can be studied. Susceptibility, neutron scattering and exact diagonalization calculations suggest that effective S = 1=2 degrees of freedom emerge on the triangles. Weaker inter-triangle interactions, in turn, lead to a condensation of these effective spins into a singlet state at lower temperature. The behavior of the 24 spin system resembles a spin liquid behavior, and due to the small size the boleite system is defined as a quantum spin droplet. The investigation of the type-I multiferroic material h-YMnO3 is described in the main text of the thesis, and a manuscript on the work is appended. With inelastic neutron scattering an avoided crossing has been found between magnon and phonon modes close to the boundary of the Brillouin zone. This is further confirmed by use of neutron polarization analysis. A theoretical description of this magnon-phonon hybridization using a Heisenberg model of localized spins, acoustic phonon modes and a coupling via the single-ion magnetostriction allows to calculate the spectra and the measured cross-section. An external magnetic field along the c-axis reveals a linear splitting of one spin wave branch which allows an exclusion of several proposed magnetic ground states based on the theoretical model and a comparison to the measurements.