Spin Transport and Proximity-Induced Magnetism in Graphene-Based van der Waals Structures

Spintronics is a promising field to meet the future requirements to information technology. The term describes the use of the spin degree of freedom, a quantum mechanical property, as information carrier. While spin related effects in metallic systems are already used in hard disks for several years, spin-based logic devices are still in an early research stage. The realization of such devices requires the tackling of several experimental challenges such as an efficient manipulation of spins while simultaneously maintaining a long spin lifetime. In metallic and semiconductor films, the spin lifetime is often limited to a few picoseconds at room temperature. On the contrary, graphene is a promising platform for spin-based logic devices due to a relatively weak spin scattering rate and a predicted spin lifetime up to microseconds. Graphene consists of carbon atoms, arranged in a hexagonal lattice and only one atom thick. Given the low intrinsic spin-orbit coupling in carbon, graphene is predicted to provide spin lifetimes several orders of magnitude above typical values for conventional metallic and semiconductor systems. However, the small spin-orbit coupling strength and the lack of a band gap makes the electrical control of spins in graphene rather inefficient. This thesis addresses two current topics of graphene spintronics: the efficient control of spin by inducing magnetism into graphene and the spin transport in fully hBN encapsulated high quality. The high sample quality of encapsulated graphene allows the first measurement of the coupling between the spin and valley degree of freedom in pristine bilayer graphene

Efficient spin injection into graphene through trilayer hBN tunnel barriers

We characterize the spin injection into bilayer graphene fully encapsulated in hBN using trilayer (3L) hexagonal boron nitride (hBN) tunnel barriers. As a function of the DC bias, the differential spin injection polarization is found to rise up to -60% at -250 mV DC bias voltage. We measure a DC spin polarization of $\sim$ 50%, a 30% increase compared to 2L-hBN. The large polarization is confirmed by local, two terminal spin transport measurements up to room temperature. We observe comparable differential spin injection efficiencies from Co/2L-hBN and Co/3L-hBN into graphene and conclude that possible exchange interaction between cobalt and graphene is likely not the origin of the bias dependence. Furthermore, our results show that local gating, arising from the applied DC bias is not responsible for the DC bias dependence. Carrier density dependent measurements of the spin injection efficiency are discussed, where we find no significant modulation of the differential spin injection polarization. We also address the bias dependence of the injection of in-plane and out-of-plane spins and conclude that the spin injection polarization is isotropic and does not depend on the applied bias.Comment: 8 pages, 6 figures, 5 supplementary note

Comparison of the magneto-Peltier and magneto-Seebeck effects in magnetic tunnel junctions

Understanding heat generation and transport processes in a magnetic tunnel junction (MTJ) is a significant step towards improving its application in current memory devices. Recent work has experimentally demonstrated the magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction varies as the magnetic configuration changes from a parallel (P) to an anti-parallel (AP) configuration. Here we report the study on its as-yet-unexplored reciprocal effect, the magneto-Peltier effect, where the heat flow carried by the tunneling electrons is altered by changing the magnetic configuration of the MTJ. The magneto-Peltier signal that reflects the change in the temperature difference across the junction between the P and AP configurations scales linearly with the applied current in the small bias but is greatly enhanced in the large bias regime, due to higher-order Joule heating mechanisms. By carefully extracting the linear response which reflects the magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements performed on the same device, we observe results consistent with Onsager reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear regime using a three-dimensional thermoelectric model. Our result opens up the possibility of programmable thermoelectric devices based on the Peltier effect in MTJs

Spin Transport and Proximity-Induced Magnetism in Graphene-Based van der Waals Structures

Spintronics is a promising field to meet the future requirements to information technology. The term describes the use of the spin degree of freedom, a quantum mechanical property, as information carrier. While spin related effects in metallic systems are already used in hard disks for several years, spin-based logic devices are still in an early research stage. The realization of such devices requires the tackling of several experimental challenges such as an efficient manipulation of spins while simultaneously maintaining a long spin lifetime. In metallic and semiconductor films, the spin lifetime is often limited to a few picoseconds at room temperature. On the contrary, graphene is a promising platform for spin-based logic devices due to a relatively weak spin scattering rate and a predicted spin lifetime up to microseconds. Graphene consists of carbon atoms, arranged in a hexagonal lattice and only one atom thick. Given the low intrinsic spin-orbit coupling in carbon, graphene is predicted to provide spin lifetimes several orders of magnitude above typical values for conventional metallic and semiconductor systems. However, the small spin-orbit coupling strength and the lack of a band gap makes the electrical control of spins in graphene rather inefficient. This thesis addresses two current topics of graphene spintronics: the efficient control of spin by inducing magnetism into graphene and the spin transport in fully hBN encapsulated high quality. The high sample quality of encapsulated graphene allows the first measurement of the coupling between the spin and valley degree of freedom in pristine bilayer graphene

Microwave control of thermal-magnon spin transport

We observe that an rf microwave field strongly influences the transport of incoherent thermal magnons in yttrium iron garnet. Ferromagnetic resonance in the nonlinear regime suppresses thermal magnon transport by 95%. The transport is also modulated at nonresonant conditions in two cases, both related to the magnon band minimum. Firstly, a strong enhancement of the nonlocal signal appears at a static magnetic field below the resonance condition. This increase only occurs at one field polarity and can be as large as 800%. We attribute this effect to magnon kinetic processes, which give rise to band-minimum magnons and high-energy chiral surface modes. Secondly, the signal increases at a static field above the resonance condition, where the rf frequency coincides with the magnon band minimum. Our study gives insight into the interplay between coherent and incoherent spin dynamics: the rf field modifies the occupation of relevant magnon states and, via kinetic processes, the magnon spin transport.</p

Bias dependent spin injection into graphene on YIG through bilayer hBN tunnel barriers

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator Yttrium-Iron-Garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 k$\Omega\mu$m$^2$. Depending on an applied DC bias current, the magnitude of the non-local spin signal can be increased or suppressed below the noise level. The spin injection efficiency reaches values from -60% to +25%. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in sample A to be (85 $\pm$ 30) mT and in sample B close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.Comment: 9 pages, 6 figures, 5 supplementary figure