Modeling of Acoustic Wave Propagation in Layered Solids and its Application in Heat Assisted Magnetic Recording

In multi-layered solids, an acoustic wave is partially reflected and partially transmitted at boundaries where the acoustic impedance changes. With a large number of layers, an acoustic wave propagates by splitting and re-merging at each layer interface, which renders a too complex wave pattern to be predicted with analytical models. A Finite Element Method (FEM) based numerical model is developed to predict the acoustic wave propagation in multi-layered solids, where an ANSYS acoustic fluid element is adopted to solve this problem. This element was originally designed for homogeneous solids, and includes an assumption of negligible density gradients across the boundaries. To overcome this limitation, equivalent acoustic speed ci′ and density ρ i′ are derived to capture the density variation, i.e., ci′ = ci ρ i, ρ i′ = 1; and in order to keep the travel time constant inside each layer, the layer thickness hi′ is changed accordingly to compensate the acoustic speed ci′, i.e., hi′= hi ρ i. The numerical results are benchmarked with analytical computation for both a homogenous case and a two-layer solid case. The model is applied to study the pump-probe transient reflectivity measurements on Heat Assisted Magnetic Recording (HAMR) media, where the thermo-elastic waves are isolated and then subtracted from the composite reflectivity change measurement. As a result, the reflectivity change caused by the thermal decay is separated from the thermo-elastic waves, allowing a more accurate prediction and measurement of the thermal properties of HAMR media.

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