Magnetic Crosstie Formation Driven by In-situ Radio Frequency Excitation

Radio frequency (RF) ranging from 20kHz to 300 GHz has numerous applications including radio transmission, long distance communication and wireless network technologies. RF signals are also playing a crucial role in spintronics and quantum information science used in spin nano-oscillators, superconducting and NV center qubits, and other emerging technologies. However, until now RF signals are inaccessible in TEM with commercial biasing sample holders. High frequency signals can be reflected or absorbed by the electrical reactance and resistance of the holder that the RF must travel through. At high frequencies, the losses are even more pronounced, resulting in low transmission efficiency. With the recently developed RF compatible TEM holder at Brookhaven National Laboratory [1], efficient GHz excitation of TEM has become a reality. The signals are carried by a coaxial waveguide with an impedance match and sent to a sample carrier, where a coplanar waveguide further reduces the transmission losses (Figure 1 (a)). Compared to previous studies [2, 3], the transmission efficiency is greatly improved, particularly at higher frequencies (Figure 1 (b)). In this study, ribbon-patterned permalloy (Py) thin films (20nm thick) were deposited on top of a SiN TEM window with an Au waveguide line across, using e-beam lithography and e-beam evaporation. The Au waveguide was wire bonded to the sample carrier to make a RF circuit (Figure 1 (c)). During the experiment, RF signal was supplied from a generator and sent to the sample via the custom holder at a frequency of 4.85 GHz. This excitation frequency is highest transmission efficiency for the sample used in this experiment as determined by a vector network analyzer. The magnetic structure is tuned to have a Néel type domain wall on the center of the Py film before excitation (time = t0⁠), then 17 dBm RF signal was turned on to continuously excite the sample. Shortly after the excitation began, crosstie domain walls [4] (Figure 2 (a)) nucleated near the signal line (time = t1⁠), as time goes, more crosstie domains were generated (time = t2⁠), as shown in overfocused Lorentz TEM images (Figure 2 (b)). Once the sample has been excited for 30 mins, the number of crosstie walls in the Py sample does not increase, indictive of a stable magnetic structure. Additionally, the crosstie structure remains after the removal of the RF excitation. This study shows that the crosstie domain wall can be created near the signal line under RF excitation, with good stability. The crosstie formation driven by RF excitation enables a possible new approach for domain-wall racetrack memory [6], which will be studied further. Experimental setups of in-situ RF excitation in TEM. (a) Schematic of the RF compatible TEM holder. A coaxial signal line run through the center of the holder (left) that carries the excitation current to a sample carrier and sample (right). (b) RF transmission efficiency of the RF holder, compared to previous work. (c) Diagram of the RF circuit used for the experiment. An RF generator supplies the excitation through the holder and across an Au signal line that runs over two Py strips that is impedance matches by adding a 50Ω resister. Magnetic crosstie structure driven by in-situ RF excitation. (a) Induction schematic of the crosstie structure in permalloy thin films, where white and black circles represent magnetic vortices and anti-vortices. (b) Overfocused Lorentz TEM images of the permalloy sample before excited with RF (⁠t0⁠), after 3 mins of 17 dBm (4.85 GHz) excitation (⁠t1⁠) and 30 mins of continuous excitation (⁠t2⁠). Note that the number of crosstie domain walls increase slowly even though the excitation power and frequency is constant.