Muons and magnets

Abstract:

Recently, a new class of quantum magnets, the so-called breathing pyrochlore spin systems, have attracted much attention due to their potential to host exotic emergent phenomena. Here, we present magnetometry, heat capacity, thermal conductivity, muon-spin relaxation, and polarized inelastic neutron scattering measurements performed on high-quality single crystal samples of the breathing pyrochlore compound Ba3Yb2Zn5O11. We interpret these results using a simplified toy model and provide insight into the low-energy physics of this system beyond the single tetrahedron physics proposed previously.

Abstract:

KAgF₃ is a quasi-one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, μSR spectroscopy, and density functional theory calculations to elucidate the low-temperature magnetic phases. Below T_N1 = 29 K we find that the material orders as an A-type antiferromagnet with an ordered moment of 0.52 μ B . Both neutrons and muons provide evidence for an intermediate phase at temperatures T_N1 < T < T_N2 with T_N2 ≈ 66 K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem.

Abstract:

A software extension to the Windows Muon Data Analysis software package WiMDA has been developed for the analysis of µ⁻SR data, which has been dubbed Negative-WiMDA. In designing Negative-WiMDA, some key features were considered: it should be easily accessed from the main analysis window of WiMDA; it should be able to account for multiple elements in a sample; it should be able to subtract signals treated as unwanted background, allowing the user to focus on a particular element of interest; it should be able to handle transverse-field (TF) data, and perhaps most importantly, it should be intuitive to use. The main features of Negative-WiMDA are presented here with a few examples of their use.

Abstract:

Molecular magnets are one of the key research themes of µSR, but locating the muon stopping site in these compounds using density functional theory is often very challenging as their unit cells tend to contain a very large number of atoms. Nevertheless, many molecular magnets contain the [PF₆]⁻ and [BF₄]⁻ molecular ions, which, due to their fluorine nuclei, produce a distinctive µSR spectrum, which can give information about the muon stopping site. This paper details the calculation of the muon sites in the much simpler materials KPF₆ and KBF₄, providing insights which can be applied to situations where these molecular ions are found in complicated molecular magnets.

Abstract:

The fluorinated ionomer Nafion, first discovered by the Du Pont company, is a material that provides efficient proton conducting membranes for application in important technological areas such as hydrogen fuel cells. Although many aspects of the polymer have been studied in relation to these applications, the microscopic mechanisms for proton transport in this polymer are still only poorly understood. We have therefore applied implanted muon techniques to the study of Nafion, aiming to gain information about these mechanisms via the muon acting as a local spin probe. Our results indicate that the muon is highly sensitive to the hydration state of the polymer and to the dynamical processes of the various sub-phases within the material. A three component model is found to describe the data well. This model has one F-µ-F component, reflecting muons in the PTFE-like polymer matrix, a second component representing quasi-static environments dominated by a single close proton and a third component encompassing highly dynamic proton-coupled environments. The properties have been studied within this modelling framework as a function of temperature for both dehydrated and fully hydrated samples.