Yuyi Wei

Combining topological insulators (TIs) and magnetic materials in heterostructures is crucial for advancing spin‐based electronics. Magnetic insulators (MIs) can be deposited on TIs using the spin‐spray process, which is a unique nonvacuum, low‐temperature growth process. TIs have highly reactive surfaces that oxidize upon exposure to atmosphere, making it challenging to grow spin‐spray ferrites on TIs. In this work, it is demonstrated that a thin titanium capping layer on TI, followed by oxidation in atmosphere to produce a thin TiO_x interfacial layer, protects the TI surface, without significantly compromising spin transport from the magnetic material across the TiO_x to the TI surface states. First, it is demonstrated that in Bi₂Te₃/TiO_x/Ni₈₀Fe₂₀ heterostructures, TiO_x provides an excellent barrier against diffusion of magnetic species, yet maintains a large spin‐pumping effect. Second, the TiO_x is also used as a protective capping layer on Bi₂Te₃, followed by the spin‐spray growth of the MI, Ni_xZn_yFe₂O₄ (NZFO). For the thinnest TiO_x barriers, Bi₂Te₃/TiO_x/NZFO samples have antiferromagnetic (AFM) disordered interfacial layer because of diffusion. With increasing TiO_x barrier thickness, the diffusion is reduced, but still maintains strong interfacial magnetic exchange‐interaction. These experimental results demonstrate a novel method of low‐temperature growth of magnetic insulators on TIs enabled by interface engineering.

Since the discovery of strong magnetoelectric (ME) coupling in two‐phase ME laminate composites, strain mediated ME heterostructures have found practical applications in magnetic sensors, tunable inductors, tunable filters, miniaturized antennas, magnetic memories, and nanoscale motors. Thin film ME sensors, in particular, have become promising candidates in biomagnetic sensing, due to their high sensitivity, CMOS compatibility, room temperature operation, and high spatial resolution. In this article, an overview is presented on different types of thin film ME sensors and their applications in biomagnetic measurement. First, the coupling structures, materials selection, and fabrication processes are introduced. Three different mechanisms of recently emerged ME sensors, including the magnetic modulation, electrical modulation, and delta‐E effect are presented. Their performance and noise analysis are also compared and discussed. Finally, real‐time applications of ME sensors in the detection of magnetic fields from different part of human body are presented and discussed. The review concludes with an outlook on future perspectives and challenges of thin film ME sensors for biomagnetic applications.

Mutual inductance highly determines the performance of dual-coil magnetic coupled resonance wireless power transfer (MCR-WPT). Due to the frequency splitting, the system gives inferior energy transfer efficiency in overcoupled state, where the two coils are close with each other and a large mutual inductance is presented. This raises the need to finely tune the mutual inductance for frequency splitting elimination and consequently performance optimization. Meanwhile, with the built MCR-WPT system, the mutual inductance remains constant given a fixed distance between the coils, which limits its potential application. In this article, the resonant coils with magnetoelectric composites core consisting of Metglas/lead zirconate titanate/Metglas were adopted to tune the mutual inductance. A large tunability of 56.5% in the low-frequency range and a tunability of 16.6% in the high-frequency range were achieved. The tunability of the mutual inductance indicates its great potential and provides a new route to eliminate the frequency splitting for high efficient MCR-WPT.

In order to advance the potential of thick on-chip hard magnets for the micro-electro-mechanical system (MEMS), we investigate a new silicon molding technique to fabricate dry-packed NdFeB magnets, including a silicon compression tool, which enables the pressing step during silicon-compatible processing. This process delivers samples with a remanence of 0.42 T and an energy product of 38 kJ/m ³ . Further studies of metal molding show that, for wax-bonding powder-based NdFeB magnets, the optimum fabrication condition is 300 °C and 425 MPa, giving a remanence of 0.54 T and an energy product of 61.7 kJ/m ³ .

The nonreciprocal bandpass filter has been achieved by utilizing the nonreciprocity characteristics of the magnetostatic surface wave (MSSW) in planar ferrite slabs. In this article, the dispersion relation, impedance, and attenuation of MSSW between the pure and metallized ferrite slabs have been analyzed and compared. Theoretical analysis indicates that the ferrite slab metallized by a thin conductive layer would introduce significant attenuation on the metallized interface, which is MSSW propagating on the isolated direction, while MSSW on the other surface is barely affected. Therefore, nonreciprocal bandpass filter with enhanced isolation can be achieved with metallized ferrite slab. Simulated S-parameters show that a notch in the isolating direction can be obtained at the passing band of the transmitting direction. Experimental results show that the nonreciprocal filter with metallized yttrium iron garnet (YIG) slab (by depositing 100-nm copper film) exhibits more than 10-dB higher isolation than that with pure YIG slab, while the center frequency, insertion loss, and 3-dB bandwidth maintain almost the same. The center frequency of the filter with metallized YIG slab can be tuned from 7.26 to 8.38 GHz with insertion loss of less than 3 dB, and in-band isolation of more than 30 dB as the dc bias magnetic field is increased from 1.8 to 2.3 kOe. Besides, the nonreciprocal filter has ultra-wide out-band suppression since it utilizes the narrow frequency band of MSSW. The -10-dB out-band rejection exceeds 36 GHz in the upper band and 1 MHz in the lower band.

Non-invasive deep brain stimulation has been a major challenge in the field of neuroscience and brain stimulation in the past three decades. Current brain stimulation technologies suffer from such hurdles as the inability to do deep brain stimulation, poor spatial resolution, and invasiveness. Transcranial magnetic stimulation (TMS) technique, for instance, cannot target brain regions deeper than ∼2cm and has a poor spatial resolution, impacting a large area of the peripheral region and leading to various side effects. Implantable electrodes, even though effective for deep brain stimulation, are invasive and carry various drawbacks related to the surgery and site infection. In this paper, we propose a new concept that relies on temporal interference of two high- frequency magnetic fields generated by two electromagnetic coils. The neural system does not respond to each of these high-frequency magnetic fields alone because of the intrinsic low-pass filtering properties of the neural membrane. The peripheral areas of the brain are impacted only by the high-frequency magnetic fields that cannot stimulate the nerves, while the deep brain area where the two fields interfere experiences a magnetic field that contains a low-frequency envelope and therefore the nerves can be stimulated. This technique can noninvasively focus a magnetic or electric beam at any depth inside the brain with a high resolution, without impacting the peripheral regions.

Since the revival of multiferroic laminates with giant magnetoelectric (ME) coefficients, a variety of multifunctional ME devices, such as sensor, inductor, filter, antenna etc. have been developed. Magnetoelastic materials, which couple the magnetization and strain together, have recently attracted ever-increasing attention due to their key roles in ME applications. This review starts with a brief introduction to the early research efforts in the field of multiferroic materials and moves to the recent work on magnetoelectric coupling and their applications based on both bulk and thin-film materials. This is followed by sections summarizing historical works and solving the challenges specific to the fabrication and characterization of magnetoelastic materials with large magnetostriction constants. After presenting the magnetostrictive thin films and their static and dynamic properties, we review micro-electromechanical systems (MEMS) and bulk devices utilizing ME effect. Finally, some open questions and future application directions where the community could head for magnetoelastic materials will be discussed.

The strong strain-mediated magnetoelectric (ME) coupling found in thin-film ME heterostructures has attracted an ever-increasing interest and enables realization of a great number of integrated multiferroic devices, such as magnetometers, mechanical antennas, RF tunable inductors and filters. This paper first reviews the thin-film characterization techniques for both piezoelectric and magnetostrictive thin films, which are crucial in determining the strength of the ME coupling. After that, the most recent progress on various integrated multiferroic devices based on thin-film ME heterostructures are presented. In particular, rapid development of thin-film ME magnetometers has been seen over the past few years. These ultra-sensitive magnetometers exhibit extremely low limit of detection (sub-pT/Hz^1/2) for low-frequency AC magnetic fields, making them potential candidates for applications of medical diagnostics. Other devices reviewed in this paper include acoustically actuated nanomechanical ME antennas with miniaturized size by 1–2 orders compared to the conventional antenna; integrated RF tunable inductors with a wide operation frequency range; integrated RF tunable bandpass filter with dual H- and E-field tunability. All these integrated multiferroic devices are compact, lightweight, power-efficient, and potentially integrable with current complementary metal oxide semiconductor (CMOS) technology, showing great promise for applications in future biomedical, wireless communication, and reconfigurable electronic systems.