The antiferromagnetic to ferromagnetic phase transition in B2-ordered FeRh is imaged in laterally confined nanopatterned islands using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. The resulting magnetic images directly detail the progression in the shape and size of the FM phase domains during heating and cooling through the transition. In 5 µm square islands this domain development during heating is shown to proceed in three distinct modes: nucleation, growth, and merging, each with subsequently greater energy costs. In 0.5 µm islands, which are smaller than the typical final domain size, the growth mode is stunted and the transition temperature was found to be reduced by 20 K. The modification to the transition temperature is found by high resolution scanning transmission electron microscopy to be due to a 100 nm chemically disordered edge grain present as a result of ion implantation damage during the patterning. FeRh has unique possibilities for magnetic memory applications; the inevitable changes to its magnetic properties due to subtractive nanofabrication will need to be addressed in future work in order to progress from sheet films to suitable patterned devices.

Magnetic materials without structural inversion symmetry can display the Dzyaloshinskii-Moriya interaction, which manifests itself as chiral magnetic ground states. These chiral states can interact in complex ways with applied fields and boundary conditions provided by finite sample sizes that are of the order of the lengthscale of the chiral states. Here we study epitaxial thin films of FeGe with a thickness close to the helix pitch of the helimagnetic ground state, which is about 70 nm, by conventional magnetometry and polarized neutron reflectometry. We show that the helix in an FeGe film reverses under the application of a field by deforming into a helicoidal form, with twists in the helicoid being forced out of the film surfaces on the way to saturation. An additional boundary condition was imposed by exchange coupling a ferromagnetic Fe layer to one of the interfaces of an FeGe layer. This forces the FeGe spins at the interface to point in the same direction as the Fe, preventing node expulsion and giving a handle by which the reversal of the helical magnet may be controlled.

Equi-atomic FeRh is a very interesting material as it undergoes a magnetostructural transition from anEquiatomic iron-rhodium (Fe 48 Rh 52 to Fe 56 Rh 44 ) has attracted considerable attention due to its magnetostructural transition from its antiferromagnetic (AF) to ferromagnetic (FM) phase 1 . This ordered α' alloy adopts a CsCl structure and undergoes a first-order phase transition from its room-temperature AF state to FM between ~75 to 105 °C, and can hence present phase AF/FM co-existence and hysteresis 2 . The co-existing phases are separated by a phase-boundary domain wall (DW) and effective control over the creation and motion of these phase-boundary DWs are considered desirable for potential application in a new generation of novel nanomagnetic or spintronic devices 3 . Previous studies have shown that the DWs can be created and driven in FeRh films by combining heating with differential gradients of chemical doping 4,5 . However, our knowledge of the dynamic behaviour of DWs in FeRh is often limited to bulk magnetic measurements or low magnification imaging (resolution in the order of 10 s of nm), i.e. magnetic force microscopy 6,7 , x-ray magnetic circular dichroism (XMCD) 7 . For example, XMCD photoelectron emission microscopy (PEEM) has been used to observe the phase coexistence in FeRh thin films and to show the first order transition from the nucleation of domains regime to be distinct from the domain growth regime 4,8 . The magnetostructural transition has also been followed in situ through scanning electron microscopy with polarisation analysis (SEMPA) and suggested that the interfacial ferromagnetism coexisting with the AF phase inside the film is an intrinsic property of the FeRh (001) surface 9 . Nevertheless, these techniques are often limited to a spatial resolution of ~20-30 nm in typical cases 10,11 and ~5 nm for SEMPA in specialised SEM instruments 12 , as well as being restricted in penetration depth to a few nm 10,13 . These experimental limitations prevent imaging of the localised dynamic evolution of the aformentioned nucleation and growth stages of the magnetostructural transition, as well as precise quantitative analysis from individual domains. Hence, in order to fully understand the magnetostructural transition and dynamic motion of DWs in FeRh thin films, it is necessary to investigate their associated mechanisms at an even more localised scale through their entire thickness, whilst applying external stimuli, i.e. in situ heating.

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Ferromagnetic nanoparticles in the 10-14 nm size range are examined for their size and interaction dependent magnetic properties. From X-ray magnetic circular dichroism the orbital-to-spin magnetic moment ratio is determined and found to decrease significantly with particle size. This is in accordance with previous complementary studies on smaller particles and highlights the difficulty of fitting to a simple core-shell model. Vibrating sample magnetometry experiments on samples with more than 1000 particles per square micron show a wide distribution of blocking temperatures from 50 to greater than 650 K. This is attributed to the dipole-dipole magnetic coupling forces between particles. The blocking temperatures show an unexpected negative correlation with increasing particle density.