Spintronics with Ferrimagnetic and Antiferromagnetic Insulators

The discovery that electric currents can excite spin waves and reverse the magnetization of a magnetic material by transporting spin angular momentum has had a significant impact on magnetism research and technology [1,2]. While the initial focus of the field was on ferromagnetic materials and magnetic tunnel junctions, new research opportunities are with materials and nanostructures with different types of magnetic order, electronic structure and geometries. In this talk, I will provide an overview of my group’s experiments investigating current-induced excitation and reorientation of the Néel vector in antiferromagnetic materials, including experiments in my group on hematite (a-Fe₂O₃) thin films [3-5]. I will also discuss a means of quantifying spin torques in antiferromagnetic thin films using the harmonic Hall effect, which makes it possible to determine the form and magnitude of the spin torques that act on the Néel vector [4,5]. Further, I will discuss a new type of spin Hall nanooscillator based on hybrid magnetic nanostructures, specifically ferromagnetic metal/ferrimagnetic insulator heterostructures and permalloy/epitaxial lithium aluminum ferrite [6]. These heterostructures are particularly interesting due to the very low damping of the ferrimagnetic insulators, which enables the excitation of spin waves at lower currents and oscillators with higher output power and quality factors. This new type of spin Hall nanooscillator has potential applications in neuromorphic computing by enabling electrically isolated oscillators to communicate through spin waves in an extended insulating ferrimagnetic film.<br/><br/><b>References</b><br/>[1] A. Brataas, A. D. Kent and H. Ohno, “Current-Induced Torques in Magnetic Materials,” Nature Materials <b>11</b>, 372 (2012)<br/>[2] A. D. Kent and D. C. Worledge, “A new spin on magnetic memories,” Nature Nanotechnology <b>10</b>, 187 (2015)<br/>[3] E. Cogulu, N. N. Statuto, Y. Cheng, F. Yang, R. V. Chopdekar, H. Ohldag, and A. D. Kent, “Direct Imaging of Electrical Switching of Antiferromagnetic Néel Order in α-Fe₂O₃ Epitaxial Films,” Physical Review B <b>103</b>, L100405 (2021)<br/>[4] E. Cogulu, H. Zhang, N. N. Statuto, Y. Cheng, F. Yang, R. Cheng, and A. D. Kent, “Quantifying Spin-Orbit Torques in Antiferromagnet/Heavy Metal Heterostructures” Physical Review Letters <b>128</b>, 247204 (2022)<br/>[5] Y. Cheng, E. Cogulu, R. D. Resnick, J. J. Michel, N. N. Statuto, A. D. Kent & F. Yang, “Third harmonic characterization of antiferromagnetic heterostructures,” Nature Communications <b>13</b>, 3659 (2022)<br/>[6] H. Ren, X. Y. Zheng, S, Channa, G. Wu, D. A. O’Mahoney, Y. Suzuki, and A. D. Kent, “Hybrid spin Hall nano-oscillators based on ferromagnetic metal/ferrimagnetic insulator heterostructures” Nature Communications <b>14</b>, 1406 (2023)