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As an in-plane charge current flows in a heavy metal film with spin–orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin–orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe 12 O 19 bilayer where the BaFe 12 O 19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe 12 O 19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe 12 O 19 film by as much as about 500 Oe when the film is switched with an out-of-plane field. By virtue of strong spin-orbit coupling, a current-carrying heavy metal may generate a torque on the magnetization of an interfaced ferromagnetic metal. Here, the authors demonstrate how this effect assists the magnetic reversal of ferromagnetic insulators with perpendicular magnetic anisotropy.

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO 2 , which consists of alternating layers of metallic Pd and Mott-insulating CoO 2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO 2 , we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO 2 layers. Computationally, we find that many-body interactions in Pd and CoO 2 layers are highly different. Holes in the CoO 2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials. PdCoO 2 belongs to a class of materials where both weakly and strongly correlated electrons co-exist side-by-side in different layers of the crystal structure. Here, the authors investigate PdCoO 2 using standing wave photoemission spectroscopy and many-body calculations reporting layer-specific details about the electronic structure.

PdCoO 2 layered delafossite is the most conductive compound among metallic oxides, with a room-temperature resistivity of nearly 2 μ Ω cm , corresponding to a mean free path of about 600 Å. These values represent a record considering that the charge density of PdCoO 2 is three times lower than copper. Although its notable electronic transport properties, PdCoO 2 collective charge density modes (i.e. surface plasmons) have never been investigated, at least to our knowledge. In this paper, we study surface plasmons in high-quality PdCoO 2 thin films, patterned in the form of micro-ribbon arrays. By changing their width W and period 2 W , we select suitable values of the plasmon wavevector q , experimentally sampling the surface plasmon dispersion in the mid-infrared electromagnetic region. Near the ribbon edge, we observe a strong field enhancement due to the plasmon confinement, indicating PdCoO 2 as a promising infrared plasmonic material. Delafossite PdCoO 2 boasts outstanding electronic transport properties, making it an interesting plasmonic material. Here, experimental evidence of surface plasmons in micro-ribbon samples is presented.

We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO2 thin films. For the ab-plane, an optical peak near Ω ≈ 750 cm−1 drives the scattering rate 1/τ(ω) and effective mass m*(ω) of the Drude carrier to increase and decrease respectively for ω ≧ Ω. For the c-axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/εc(ω)]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at Ω results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent 1/τ(ω) and m*(ω). This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared (IR) band and a c-LO. On the contrary, it is the Drude carrier that couples in PdCoO2. The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of PdCoO2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.

We investigate the low temperature electron transport properties of manganese doped lead sulfide films. The system shows variable range hopping at low temperatures that crosses over into an activation regime at even lower temperatures. This crossover is destroyed by an applied magnetic field which suggests a magnetic origin of the hard gap, associated with bound magnetic polarons. Even though the gap forms around the superconducting transition temperature of lead, we do not find evidence of this being due to insulator-superconductor transition. Comparison with undoped PbS films, which do not show the activated transport behavior, suggests that bound magnetic polarons create the hard gap in the system that can be closed by magnetic fields.

The study of diluted magnetic semiconductors (DMS) has been a trend for many decades. Although many DMS materials have been identified, the application to semiconductor technology has remained stagnant. An important aspect of research in this field is the search and application of such materials. For this dissertation, a DMS material - manganese doped lead sulfide (MnPbS) is identified and studied. Specifically, the material has been investigated for the electronic, optical and magnetic properties. This material is found to be useful for studying interesting fundamental physics as well as for possible technological applications. The material has been studied in various ways. First, quantum dots are prepared using pulsed laser deposition and show an increase of the bandgap as well as decrease of absorbance in the visible/NIR range as a result of Mn doping. The quantum dots are also applied as solar energy absorber to construct a quantum dot sensitized solar cell. When used together with a wide bandgap photoanode (Zn2SnO4 nanowire), the system is more efficient in converting photons to electrons and generating higher photocurrent compared to undoped PbS. The results show the promise of MnPbS for the construction of more efficient solar cells. In another study, the magnetotransport properties of MnPbS are compared to PbS. It is found that the films are highly disordered and insulating, and show variable range hopping (VRH) at low temperatures. At very low temperatures, a soft Coulomb gap is found to arise in MnPbS that changes to a hard gap at lower temperatures. The hard gap can be closed by magnetic field and reverts to the soft gap. It is found that the influence of bound magnetic polarons (BMP) results in this behavior. The final study on MnPbS compares the effect of sulfur vacancy and possible effect of the Mn dopants on the magnetic properties. It is found that films with high concentration of sulfur vacancies are ferromagnetic at room temperature while lower concentration of sulfur vacancies result in nonmagnetic films. X-ray absorption spectroscopy reveals the nature of Mn dopants and shows that the Mn does not directly influence the ferromagnetism in the material. Both PbS and MnPbS are thus identified to be d0 ferromagnets.

We report proximity effects of spin-orbit coupling in EuO\\(_{1-x}\\) films capped with a Pt overlayer. Transport measurements suggest that current flows along a conducting channel at the interface between the Pt and EuO. The temperature dependence of the resistivity picks up the critical behaviors of EuO, i.e., the metal-to-insulator transition. We also find an unusual enhancement of the magnetic anisotropy in this structure from its bulk value which results from strong spin-orbit coupling across the Pt/EuO interface.