Effect of the sensing layer resistivity on sensitivity in DSAWR sensors

SAW sensors have become important for gas detection applications in recent years. One of the parameters affecting the sensitivity of SAW sensors is the resistivity of the sensing layer. In this study, it is tried to determine the sensitivities of the sensors for different resistance values for the designed DSAWR. In accordance with this purpose, instead of using actual sensing layers whose resistance can be changed by parameters such as temperature etc., 8 different fixed resistors in the range of 3 to 4M ohms were used to fully demonstrate the effect of the resistance. It has been found that the DSAWR has better sensitivity for active layer resistances between kΩ to a few hundred kΩ

Independence of spin-orbit torques from the exchange bias direction in Ni81_{81}Fe19_{19}/IrMn bilayers

We investigated a possible correlation between spin Hall angles and exchange bias in Ni$_{81}$Fe$_{19}$/IrMn samples by performing spin torque ferromagnetic resonance measurements. This correlation is probed by patterning of Ni$_{81}$Fe$_{19}$/IrMn bilayers in different relative orientations with respect to the exchange bias direction. The measured voltage spectra allow a quantitative determination of spin Hall angles, which are independent of the orientation around 2.8\pm0.3%.Comment: 10 page

String Phase in an Artificial Spin Ice

One-dimensional strings of local excitations are a fascinating feature of the physical behavior of strongly correlated topological quantum matter. Here we study strings of local excitations in a classical system of interacting nanomagnets, the Santa Fe Ice geometry of artificial spin ice. We measured the moment configuration of the nanomagnets, both after annealing near the ferromagnetic Curie point and in a thermally dynamic state. While the Santa Fe Ice lattice structure is complex, we demonstrate that its disordered magnetic state is naturally described within a framework of emergent strings. We show experimentally that the string length follows a simple Boltzmann distribution with an energy scale that is associated with the system's magnetic interactions and is consistent with theoretical predictions. The results demonstrate that string descriptions and associated topological characteristics are not unique to quantum models but can also provide a simplifying description of complex classical systems with non-trivial frustration

Entropy-driven order in an array of nanomagnets

Long-range ordering is typically associated with a decrease in entropy. Yet, it can also be driven by increasing entropy in certain special cases. Here we demonstrate that artificial spin-ice arrays of single-domain nanomagnets can be designed to produce such entropy-driven order. We focus on the tetris artificial spin-ice structure, a highly frustrated array geometry with a zero-point Pauling entropy, which is formed by selectively creating regular vacancies on the canonical square ice lattice. We probe thermally active tetris artificial spin ice both experimentally and through simulations, measuring the magnetic moments of the individual nanomagnets. We find two-dimensional magnetic ordering in one subset of these moments, which we demonstrate to be induced by disorder (that is, increased entropy) in another subset of the moments. In contrast with other entropy-driven systems, the discrete degrees of freedom in tetris artificial spin ice are binary and are both designable and directly observable at the microscale, and the entropy of the system is precisely calculable in simulations. This example, in which the system’s interactions and ground-state entropy are well defined, expands the experimental landscape for the study of entropy-driven ordering

SPIN TRANSPORT AND SPIN-ORBIT TORQUES IN ANTIFERROMAGNETS

The electron has two fundamental degrees of freedom, i.e., charge and spin. Existing semiconductor electronics utilizes the charge degree of freedom in its functionalities. Spintronics seeks, in addition, to exploit the spin degree of freedom, which can suggest promising pathways for low-power and faster operations. In conventional spintronics devices, ferromagnetic materials (FMs) have been employed as active components. However, it has recently been recognized that antiferromagnetic materials (AFMs) can also play an active role in spintronic devices. Antiferromagnets have several advantages over ferromagnets; for instance, they have net zero magnetization so that they are invisible to external magnetic fields. Also, they show resonances in the terahertz frequency range. Towards this end, this thesis focuses on spin transport and spin-orbit torques in various antiferromagnetic materials. With respect to the former, I demonstrated that spin currents can be transmitted efficiently through a metallic antiferromagnet FeMn. I detect two distinctly different spin transport regimes, which can be associated with electronic and magnonic spin currents. With respect to the latter, I investigated a possible correlation between two important spintronics concepts, i.e., spin-orbit torques and exchange bias since the ferromagnetic/antiferromagnetic interface is crucial for both phenomena. The measured spin Hall angles suggest that these two effects are independent of each other, although it is worthy to mention that there are still strong spin-orbit torques even when the antiferromagnet is directly exchange coupled to the ferromagnet. Furthermore, I discuss anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) in another metallic antiferromagnet, FeRh, which undergoes a temperature driven antiferromagnetic-to-ferromagnetic phase transition. The temperature dependent results show a drastic suppression of both AHE and ANE signals in the antiferromagnetic phase. Interestingly, these non-vanishing signals are opposite in sign compared to their ferromagnetic counterparts, which can suggest changes of inherent symmetries in the electronic structure of FeRh across its magnetic phase transition