Spintronic terahertz (THz) emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) thin films, have demonstrated remarkable potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. Since the discovery that novel FM/NM heterostructures can be utilized as sources of THz radiation, researchers have endeavored to find the optimum combination of materials to produce idealized spintronic emitters capable of generating pulses of THz radiation over a large spectral bandwidth. In the last decade, researchers have investigated the influence of a wide range of material properties, including the choice of materials and thicknesses of the layers, the quality of the FM/NM interface, and the stack geometry upon the emission of THz radiation. It has been found that particular combinations of these properties have greatly improved the amplitude and bandwidth of the emitted THz pulse. Significantly, studying the material properties of spintronic THz emitters has increased the understanding of the spin-to-charge current conversion processes involved in the generation of THz radiation. Ultimately, this has facilitated the development of spintronic heterostructures that can emit THz radiation without the application of an external magnetic field. In this review, we present a comprehensive overview of the experimental and theoretical findings that have led to the development of spintronic THz emitters, which hold promise for use in a wide range of THz applications. We summarize the current understanding of the mechanisms that contribute to the emission of THz radiation from the spintronic heterostructures and explore how the material properties contribute to the emission process.

The first order antiferromagnetic to ferromagnetic metamagnetic phase transition of equiatomic FeRh offers new opportunities for novel antiferromagnetic memories and spintronic devices with the caveat that it can be utilized in thin film structures (<50 nm). Here, we report a polarized neutron reflectivity (PNR) study for three representative film thicknesses (5, 20, and 50 nm) aimed at determining the physical and magnetic structure of FeRh at room temperature and partway through the transition. The PNR results are analyzed with reference to X-ray diffraction, X-ray reflectivity, and atomic force microscopy data which together provide a consistent description of the magnetic and physical state of the FeRh thin films. The data demonstrate that the nucleation of the ferromagnetic phase initiates at the MgO substrate, and results from structural and magnetic measurements demonstrate that the magnetic behavior and strain properties of FeRh correlate with the evolving topography of the three films investigated.

The equiatomic alloy FeRh is of great scientific and technological interest due its highly unusual first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition. Here we report an exploration of the interplay between topography and phase evolution with a comprehensive magnetic force microscopy study of nominal 50 nm thick FeRh thin films and subtractively patterned wires of width 0.2 µm–2 µm. In continuous films where the surface morphology had not been optimised for smoothness, the topographical variation was observed to dominate the distribution of the magnetic transition temperatures and dictates the nucleation and growth of the magnetic phases. This observation was repeated for patterned elements, where the effects of surface morphology were more significant than those of spatial confinement. These results have clear implications for future studies of low-dimensional FeRh films, as surface topography must be considered when analysing and comparing the transition behaviour of FeRh thin films.

The ability to tune the competition between the in-plane and out-of-plane orientation of magnetization provides a means to construct thermal sensors with a sharp spin reorientation transition at specific temperatures. We have observed such a tuneable, temperature driven spin reorientation in structurally amorphous, ferrimagnetic rare earth-transition metal (RE-TM) alloy thin films using scanning transmission X-ray microscopy (STXM) and magnetic measurements. The nature of the spin reorientation transition in FeGd can be fully explained by a non-equilibrium, nanoscale modulation of the chemical composition of the films. This modulation leads to a magnetic domain pattern of nanoscale speckles superimposed on a background of in-plane domains that form Laudau configurations in micron-scale patterned elements. It is this speckle magnetic structure that gives rise to a sharp two step-reversal mechanism that is temperature dependent. The possibility to balance competing anisotropies through the temperature opens opportunities to create and manipulate topological spin textures.

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