Direct phase probing and mapping via spintronic Michelson interferometry

Incommensurate Spin Density Wave in Antiferromagnetic RuO_{2} Evinced by Abnormal Spin Splitting Torque

Since the discovery of antiferromagnetism, metallic oxide RuO_{2} has exhibited numerous intriguing spintronics properties such as the anomalous Hall effect and anisotropic spin splitting effect. However, the microscopic origin of its antiferromagnetism remains unclear. By investigating the spin splitting torque in RuO_{2}/Py, we found that metallic RuO_{2} exhibits a spatially periodic spin structure which interacts with the spin waves in Py through interfacial exchange coupling. The wavelength of such structure is evaluated within 14-20 nm depending on the temperature, which is evidence of an incommensurate spin density wave state in RuO_{2}. Our work not only provides a dynamics approach to characterize the antiferromagnetic ordering in RuO_{2}, but also offers fundamental insights into the spin current generation due to anisotropic spin splitting effect associated with spin density wave.

Direct-Current Electrical Detection of Surface-Acoustic-Wave-Driven Ferromagnetic Resonance

Surface acoustic waves (SAW) provide a promising platform to study spin-phonon coupling, which can be achieved by SAW-driven ferromagnetic resonance (FMR) for efficient acoustic manipulation of spin. Although the magneto-elastic effective field model has achieved great success in describing SAW-driven FMR, the magnitude of the effective field acting on the magnetization induced by SAW still remains hard to access. Here, by integrating ferromagnetic stripes with SAW devices, direct-current detection for SAW-driven FMR based on electrical rectification is reported. By analyzing FMR rectified voltage, the effective fields are straightforwardly characterized and extracted, which exhibits the advantages of better integration compatibility and lower cost than traditional methods such as vector-network analyzer-based techniques. A large nonreciprocal rectified voltage is obtained, which is attributed to the coexistence of in-plane and out-of-plane effective fields. The effective fields can be modulated by controlling the longitudinal and shear strains within the films to achieve almost 100% nonreciprocity ratio, demonstrating the potential for electrical switches. Besides its fundamental significance, this finding provides a unique opportunity for a designable spin acousto-electronic device and its convenient signal readout.

Electric and Magnetic Tuning of Gilbert Damping Constant in LSMO/PMN-PT(011) Heterostructure

Electric field control of magnetodynamics in magnetoelectric (ME) heterostructures has
been the subject of recent interest due to its fundamental complexity and promising applications in
room temperature devices. The present work focuses on the tuning of magnetodynamic parameters
of epitaxially grown ferromagnetic (FM) La_0.7Sr_0.3MnO₃(LSMO) on a ferro(piezo)electric (FE)
Pb(Mg_0.33Nb_0.67)O₃-PbTiO₃(PMN-PT) single crystal substrate. The uniaxial magnetic anisotropy
of LSMO on PMN-PT confirms the ME coupling at the FM/FE heterointerface. The magnitude of
the Gilbert damping constant (α) of this uniaxial LSMO film measured along the hard magnetic axis
is significantly small compared to the easy axis. Furthermore, a marked decrease in the α values of
LSMO at positive and negative electrical remanence of PMN-PT is observed, which is interpreted
in the framework of strain induced spin dependent electronic structure. The present results clearly
encourage the prospects of electric field controlled magnetodynamics, thereby realising the room
temperature spin-wave based device applications with ultra-low power consumption.

Microwave resonances of magnetic skyrmions in thin film multilayers

Non-collinear magnets exhibit a rich array of dynamic properties at microwave frequencies. They can host nanometre-scale topological textures known as skyrmions, whose spin resonances are expected to be highly sensitive to their local magnetic environment. Here, we report a magnetic resonance study of an [Ir/Fe/Co/Pt] multilayer hosting Néel skyrmions at room temperature. Experiments reveal two distinct resonances of the skyrmion phase during in-plane ac excitation, with frequencies between 6-12 GHz. Complementary micromagnetic simulations indicate that the net magnetic dipole moment rotates counterclockwise (CCW) during both resonances. The magnon probability distribution for the lower-frequency resonance is localised within isolated skyrmions, unlike the higher-frequency mode which principally originates from areas between skyrmions. However, the properties of both modes depend sensitively on the out-of-plane dipolar coupling, which is controlled via the ferromagnetic layer spacing in our heterostructures. The gyrations of stable isolated skyrmions reported in this room temperature study encourage the development of new material platforms and applications based on skyrmion resonances. Moreover, our material architecture enables the resonance spectra to be tuned, thus extending the functionality of such applications over a broadband frequency range.

Level Attraction Due to Dissipative Magnon-Photon Coupling

We report dissipative magnon-photon coupling caused by the cavity Lenz effect, where the magnons in a magnet induce a rf current in the cavity, leading to a cavity backaction that impedes the magnetization dynamics. This effect is revealed in our experiment as level attraction with a coalescence of hybridized magnon-photon modes, which is distinctly different from level repulsion with mode anticrossing caused by coherent magnon-photon coupling. We develop a method to control the interpolation of coherent and dissipative magnon-photon coupling, and observe a matching condition where the two effects cancel. Our work sheds light on the so-far hidden side of magnon-photon coupling, opening a new avenue for controlling and utilizing light-matter interactions.

Simultaneous Life Detection and Localization Using a Wideband Chaotic Signal with an Embedded Tone

A hybrid life detection radar system which transmits a wideband chaotic signal containing an embedded single-tone is proposed. The chaotic signal is used for target localization by the time-domain correlation method and synthetic aperture technique, and the single-tone signal is used to measure the frequencies of breathing and heartbeat based on an on-chip split-ring integrated sensor and Michelson interference principle. Experimental results in free space and in through-wall scenarios demonstrate that the system can realize human detection and localization simultaneously with high range resolution, high sensitivity, and large dynamic range without complex signal processing. The range resolution is about 10 cm, and the dynamic range is 35 dB for the respiration signal detection and 25 dB for the heartbeat signal detection. Due to its good immunity to interference/jamming and high spectrum efficiency, the proposed system is suitable for post-disaster rescue, elder/infant/patient vitality monitoring, and anti-terrorism enforcement applications.

Magnetic field-dependent shape anisotropy in small patterned films studied using rotating magnetoresistance

Based on the electric rotating magnetoresistance method, the shape anisotropy of a Co microstrip has been systematically investigated. We find that the shape anisotropy is dependent not only on the shape itself, but also on the magnetization distribution controlled by an applied magnetic field. Together with micro-magnetic simulations, we present a visualized picture of how non-uniform magnetization affects the values and polarities of the anisotropy constants K1 and K2. From the perspective of potential appliantions, our results are useful in designing and understanding the performance of micro- and nano-scale patterned ferromagnetic units and the related device properties.

Spin Hall voltages from a.c. and d.c. spin currents

In spin electronics, the spin degree of freedom is used to transmit and store information. To this end the ability to create pure spin currents--that is, without net charge transfer--is essential. When the magnetization vector in a ferromagnet-normal metal junction is excited, the spin pumping effect leads to the injection of pure spin currents into the normal metal. The polarization of this spin current is time-dependent and contains a very small d.c. component. Here we show that the large a.c. component of the spin currents can be detected efficiently using the inverse spin Hall effect. The observed a.c.-inverse spin Hall voltages are one order of magnitude larger than the conventional d.c.-inverse spin Hall voltages measured on the same device. Our results demonstrate that ferromagnet-normal metal junctions are efficient sources of pure spin currents in the gigahertz frequency range.

Note: electrical detection and quantification of Spin Rectification Effect enabled by shorted microstrip transmission line technique

We describe a shorted microstrip method for the sensitive quantification of Spin Rectification Effect (SRE). SRE for a Permalloy (Ni80Fe20) thin film strip sputtered onto SiO2 substrate is demonstrated. Our method obviates the need for simultaneous lithographic patterning of the sample and transmission line, therefore greatly simplifying the SRE measurement process. Such a shorted microstrip method can allow different contributions to SRE (anisotropic magnetoresistance, Hall effect, and anomalous Hall effect) to be simultaneously determined. Furthermore, SRE signals from unpatterned 50 nm thick Permalloy films of area dimensions 5 mm × 10 mm can even be detected.

Ground penetrating detection using miniaturized radar system based on solid state microwave sensor

We propose a solid-state-sensor-based miniaturized microwave radar technique, which allows a rapid microwave phase detection for continuous wave operation using a lock-in amplifier rather than using expensive and complicated instruments such as vector network analyzers. To demonstrate the capability of this sensor-based imaging technique, the miniaturized system has been used to detect embedded targets in sand by measuring the reflection for broadband microwaves. Using the reconstruction algorithm, the imaging of the embedded target with a diameter less than 5 cm buried in the sands with a depth of 5 cm or greater is clearly detected. Therefore, the sensor-based approach emerges as an innovative and cost-effective way for ground penetrating detection.

Seebeck rectification enabled by intrinsic thermoelectrical coupling in magnetic tunneling junctions

An intrinsic thermoelectric coupling effect in the linear response regime of magnetic tunneling junctions (MTJ) is reported. In the dc response, it leads to a nonlinear correction to Ohm's law. Dynamically, it enables a novel Seebeck rectification and second harmonic generation, which apply for a broad frequency range and can be magnetically controlled. A phenomenological model on the footing of the Onsager reciprocal relation and the principle of energy conservation explains very well the experimental results obtained from both dc and frequency-dependent transport measurements performed up to GHz frequencies. Our work refines previous understanding of magnetotransport and microwave rectification in MTJs. It forms a new foundation for utilizing spin caloritronics in high-frequency applications.