Exploring the Effects of Spin Currents in Heavy Metals and Ferromagnetic Metals Using Harmonic Hall and Ferromagnetic Resonance Techniques

Aon, Soumik (2024) Exploring the Effects of Spin Currents in Heavy Metals and Ferromagnetic Metals Using Harmonic Hall and Ferromagnetic Resonance Techniques. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Soumik Aon (16RS050)) 16RS050.pdf - Submitted Version Restricted to Repository staff only Download (58MB)

Abstract

The primary objective of spintronics revolves around enhancing the efficiency of interconversion between spin and charge current, crucial for spintronics device functionality. In the context of heterostructures combining ferromagnetic (FM) and heavy metal (HM) materials, the generation of pure spin current holds importance as it enables faster data processing and advanced storage capabilities. A pure spin current refers to a flow of spin without any accompanying charge movement. Two distinct phenomena, namely the spin Hall effect (SHE) and spin pumping (SP) effect, independently generate and inject pure spin currents into adjacent layers within bilayer devices. Recently, the generation of spin currents within a polycrystalline material has been explained through the symmetry argument. The magnetization within FM breaks certain symmetries, allowing more spin currents to flow within FM materials compared to the non-magnetic (NM) materials. These broken symmetries enable the generation of spin currents with spin polarization transverse to the magnetization. Such spin currents, known as unconventional spin currents, have previously been neglected in the context of spintronics devices. In our investigation of FM/HM bilayers, we explore two complementary phenomena: SHE and spin pumping. Heavy metals (HM) like tungsten (W) and platinum (Pt) are selected for their respective highest values of -θSHE and +θSHE among transition metals, while permalloy (Py) is employed as the FM material. Additionally, the crystalline phase of W films can be adjusted based on growth conditions, such as argon pressure (PAr), during deposition via DC magnetron sputtering. W-films with various crystalline phases of metastable pure β, mixed (α+β), pure α characterized by a wide range of longitudinal resistivity (ρW) have been utilized. The spin Hall effect (SHE) induces a pure spin current perpendicular to an applied current which is not detectable through conventional electrical measurements. An intuitive Hall like measurement is designed to directly measure spin chemical potential by detecting accumulated spins at the edges. This setup involves a HM channel and a unique pair of transverse voltage probes: one FM electrode and one reference metal (RM) electrode. The FM/RM electrode combination produces an additional voltage proportional to spin accumulation potential, which exhibits anti-symmetry with respect to FM orientations. Verification of this measurement scheme involves comparing voltage signs for different HM channels, such as W and Pt. Additionally, the devices are capable of detecting the inverse spin Hall effect (ISHE) by reversing the current and voltage leads, consistent with the reciprocity principle. Furthermore, a series of devices with varying W resistivity (10 - 750) μΩ-cm show that spin Hall resistivity increases with higher HM resistivity. This measurement approach, coupled with extensive HM resistivity variation provides an ideal platform to investigate the microscopic mechanisms underlying SHE/ISHE. In a single-layer ferromagnetic film subjected to an ac current, a characteristic Hall voltage emerges with both first and second harmonic components. This phenomenon is attributed to the existence of spin currents with polarization non-collinear with the film’s magnetization. We utilize 30 nm thick permalloy (Py) films in our study, deposited at an oblique angle relative to the substrate plane. This deposition induces an in-plane easy axis in the magnetization of the initial nucleating layers, distinct from the overall bulk magnetic properties of the film. This unique magnetic texture enables the direct detection of the inverse spin Hall effect, termed Anomalous Inverse Spin Hall Effect (AISHE), in Hall bar-shaped macroscopic devices at room temperature. Control samples, fabricated by standard Py deposition with slow substrate rotation, exhibit a significant reduction in the harmonic Hall signal, supporting our model. Analysis of the second harmonic Hall signal confirms the presence of spin-orbit torque arising from unconventional spin currents in the single-layer ferromagnets. Using the VNA-FMR technique, we have investigated spin induced pumping from Py into β-W across a range of β-W films with ρW varying from (100-1400) μΩ-cm. Our main focus is to analyze how resistivity affects the effective damping (αeff ) in these β-W/Py bilayers. Our findings indicates a strong correlation between αeff and ρW, suggesting that ρW can serve as a controllable parameter for regulating αeff , advantageous for faster magnetization dynamics switching. The spin pumping efficiency can be qualitatively understood by the thickness variation of Py in these bilayers. Moreover, the role of β-W can be explained using thickness dependence of W-films. Low-temperature studies reveal a distinctive temperature dependence in αeff , which can be explained by the torque correlation model.

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