Multiferroic thin film circulator with tunable bandpass behavior

Multiferroic composite materials consisting both magnetic and ferroelectric phase are of great interests, as its magnetoelectric (ME) coupling property may lead to novel RF devices. Multiferroic thin film devices have the advantage of the low tuning voltage requirement which is essential for compact and power efficient devices. In addition, thin film material is easier to fabricate with nano fabrication techniques such as sputtering or MBE growth. It can also be precisely patterned with lithography and made compatible with IC technology. By applying magnetic biases magnetic material, magnetostatic wave (MSW) modes can be well controlled in the media. This has led to non-reciprocal devices (J. Wu, X. Yang, et al., IEEE. T-MTT, 60, 3959-3968, 2012) that are highly desirable in the microwave/RF circuits. In this letter, a 3-port multiferroic thin film circulator is proposed. Compared to traditional bulk ferrite circulators it is possible to reduce the size of the device as well as provide a voltage tuning approach, which is more compact and power efficient. The circulator consists of a four-layer structure: piezoelectric/conductor/dielectric&magnetic thin film/conductor. The area is about 2.5 mm² with a total thickness less than 15 μm. All parameters are listed in figure 1. With a magnetic bias excitation perpendicular to the device plane, a magnetostatic back volume wave (MSBVW) is first excited in the magnetic media. The MSBVW modes set the central frequency (f_c), and the central circular magnetic patch produces the circulation. The simulated (HFSS v15) insertion loss, isolation at fc, and the return loss are 3.5 dB, 25 dB, and 10 dB, respectively. In the meantime, the overall isolation (S12) is over 12 dB, and the off-band rejection is blow 20 dB. By optimizing the device parameters, it is possible to realize a good transmission as well as a good isolation. The device can work at different frequencies/bands depending on the magnet- c bias applied. For further effort, the magnetic property of the thin film can be tuned via ME coupling by adding a voltage of several volts on the piezoelectric.