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Probe beam deflection technique with liquid immersion for fast mapping of thermal conductance

Frequency-domain probe beam deflection (FD-PBD) is an experimental technique for measuring thermal properties that combines heating by a modulated pump laser and measurement of the temperature field via thermoelastic displacement of the sample surface. In the conventional implementation of FD-PBD, the data are mostly sensitive to the in-plane thermal diffusivity. We describe an extension of FD-PBD that introduces sensitivity to through-plane thermal conductance by immersing the sample in a dielectric liquid and measuring the beam deflection created by the temperature field of the liquid. We demonstrate the accuracy of the method by measuring (1) the thermal conductivity of a 310 nm thick thermally grown oxide on Si, (2) the thermal boundary conductance of bonded interface between a 3C-SiC film and a single crystal diamond substrate, and (3) the thermal conductivities of several bulk materials. We map the thermal boundary conductance of a 3C-SiC/diamond interface with a precision of 1% using a lock-in time constant of 3 ms and dwell time of 15 ms. The spatial resolution and maximum probing depth are proportional to the radius of the focused laser beams and can be varied over the range of 1–20 μm and 4–80 μm, respectively, by varying the 1/e2 intensity radius of the focused laser beams from 2 to 40 μm. FD-PBD with liquid immersion thus enables fast mapping of spatial variations in thermal boundary conductance of deeply buried interfaces.
Publications

Electric field control of magnon spin currents in an antiferromagnetic insulator

Applying heat to a magnetoelectric antiferromagnet enables an electric field control of magnon spin currents.
Publications

Tunable magnetic confinement effect in a magnetic superlattice of graphene

AbstractTwo-dimensional van der Waals materials such as graphene present an opportunity for band structure engineering using custom superlattice potentials. In this study, we demonstrate how self-assemblies of magnetic iron-oxide (Fe3O4) nanospheres stacked on monolayer graphene generate a proximity-induced magnetic superlattice in graphene and modify its band structure. Interactions between the nanospheres and the graphene layer generate superlattice Dirac points in addition to a gapped energy spectrum near the K and K′ valleys, resulting in magnetic confinement of quasiparticles around the nanospheres. This is evidenced by gate-dependent resistance oscillations, observed in our low temperature transport measurements, and confirmed by self-consistent tight binding calculations. Furthermore, we show that an external magnetic field can tune the magnetic superlattice potential created by the nanospheres, and thus the transport characteristics of the system. This technique for magnetic-field-tuned band structure engineering using magnetic nanostructures can be extended to a broader class of 2D van der Waals and topological materials.
Publications

Multiple magnetic droplet solitions from exotic spin–orbit torques

Materials with large spin–orbit interactions generate pure spin currents with spin polarizations parallel to the interfacial surfaces that give rise to conventional spin–orbit torques. These spin–orbit torques can only efficiently and deterministically switch magnets with in-plane magnetization. Additional symmetry breaking, such as in non-collinear antiferromagnets, can generate exotic, unconventional spin–orbit torques that are associated with spin polarizations perpendicular to the interfacial planes. Here, we use micromagnetic simulations to investigate whether such exotic spin–orbit torques can generate magnetic droplet solitions in out-of-plane magnetized geometries. We show that a short, high current pulse followed by a lower constant current can nucleate and stabilize magnetic droplets. Through specific current pulse lengths, it is possible to control the number of droplets in such a system, since torques are generated over a large area. Additionally, the nucleation current scales with the out-of-plane component of the spin polarization and is linear as a function of magnetic field strength.

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