Program & Abstracts

Hydrogen is a ubiquitous impurity in most semiconductors, as a result of processing, and is usually electrically active. The low concentration makes direct measurements difficult. Many technologically important oxide materials and insulators also contain hydrogen as an impurity. The positive muon can be considered as a light isotope of hydrogen, and is used as a model for hydrogen to determine its site and electrical behaviour. The measurement uses the hyperfine coupling between the muon spin and an unpaired electron or hole bound to it, so revealing the details and symmetry of the site.

Muons and hydrogen can behave as deep compensating centres with a compact electron wavefunction, or as a shallow donor or acceptor. Varying the doping or temperature, or illuminating the sample, will change the carrier concentrations and the resulting muon site charge changes can be detected.

If the muon response is well characterised, it can then be used to probe the excess carrier density. This allows measurements of the carrier lifetime following injection of excess carriers with a short laser pulse. Bulk and surface recombination mechanisms can be separated.

Light can also interact directly with the muon centre, ionising the bound electron into the conduction band. This spectroscopic technique gives another method to measure the energy levels of acceptors or donors.

In non-magnetic fluorides, an implanted muon will stop very close to the highly electronegative fluoride ion, or very often stop between two of them. The dipolar interaction between the fluorine nuclei spins and the muon spin gives rise to a characteristic signature which characterises the state [1]. The remaining fluorine nuclei are more distant and usually ignored, since their coupling to the muon is weaker. We show that taking them properly into account allows one to model the data in much greater quantitative detail, understand the stopping site more accurately, and to explore how the quantum information held by the state evolves with time [2].  Modelling these rather subtle decoherent effects is only possible due to advances in high-rate beamlines which allow for counting statistics ~10⁹ muons per run in reasonable time.  The possibilities opened up by high-rate muon experiments will be discussed and provide a strong motivation for high-rate beamlines in future muon sources.

[work carried out with John M. Wilkinson]

[1] J. H. Brewer, S. R. Kreitzman, D. R. Noakes, E. J. Ansaldo, D.R. Harshman, and R. Keitel, Phys. Rev. B 33, 7813 (1986).
[2] J. M. Wilkinson and S. J. Blundell, Phys. Rev. Lett. 125, 087201 (2020).

Variants of conventional nuclear magnetic resonance (NMR), μSR and β detected NMR techniques offer a sensitive probe of the local magnetic environment in condensed matter.  Using examples of compounds which are of fundamental interest and may also have practical applications for data storage and transfer, I describe how radioactive species like muons or 8Li nuclei may be used to probe the nature of the magnetic excitations:

Europium chalcogenides have long been recognised as experimental realisations of classic 3D Heisenberg spin systems, with simple rock salt crystalline structures. The compounds EuO and EuS are also rare examples of ferromagnetic semiconductors, where strong indirect exchange interactions between the localised Eu²⁺ ions are mediated by charge carriers rather than superexchange. More recently, artificial magnetic semiconducting heterostructures have generated tremendous interest, due to their potential for spintronics applications.  One candidate, EuO_1−x / n-Si:As combines a model ferromagnet with the dominant semiconductor used for practical devices.

The controlled manipulation of a nonequilbrium spin population is central to the field of spintronics. Typical devices making use of giant magnetoresistance (like hard drive read heads) or spin transfer torque effects are based on spin polarised electron currents. As such, the spin and electron transport are not separated and devices are subject to the constraints of circuit capacitance, heat generation and electron migration. Recently, attention has turned to developing ideas and systems based on pure spin or angular momentum currents, which may be generated through spin pumping, which relies on the precession of the magnetisation induced in a ferromagnet excited by a radio-frequency magnetic field on resonance (FMR). The magnetisation precession is damped via the emission of a polarised spin current into the neighbouring normal metal or “spin sink”.  Widely used in microwave applications, ferrimagnetic Y₃Fe₅O₁₂ (T_C=550 K) has very low magnetic losses and is the protypical material for the generation of pure spin currents. 

Crystalline materials without inversion symmetry may exhibit a variety of exotic magnetically ordered states with canted spin structures like helices, conical states or a periodic array of “Skyrmions”, a hedgehog-like spin structure. The presence of interfaces and confined geometries like thin films leads to the greatly enhanced stability of Skyrmion phases, although the mechanism is not fully understood. While thermal fluctuations are thought to stabilise the Skyrmion phase in bulk samples,  the evolution of the magnetic excitation spectrum with film thickness is largely unexplored. Due to the ability to move Skyrmions easily, their topological stability, their small size (possibly down to 1 nm) and the ability to write and erase individual Skyrmions, these materials are promising for information processing.

The low frequency spin dynamics may be investigated locally both within the film and also in a proximal manner using depth resolved low energy μSR.  Complementary β-NMR measurements, sensitive to spin fluctuations on much longer millisecond timescales, enable the investigation of the dynamical response of the underlying substrate.  Within a wider context, the spin resonance phenomena are compared with extensive neutron scattering investigations. As such, a comprehensive account is presented, using techniques sensitive to both frequency and reciprocal space.