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Linear, time-averaged resistance captures only part of the physics of a magnetic
multilayer. Two further sources of information lie in the response beyond the first order
in the driving current and in the statistics of fluctuations about the mean. This talk
brings these perspectives together through nonlinear Hall measurements and electrical
noise spectroscopy in sputtered, polycrystalline heavy-metal/ferromagnet multilayers.
First, I will present a zero-field second-harmonic transverse response in Ir/Fe/Co/Pt
multilayers that scales quadratically with current and persists from 2 K to room
temperature. Angle-resolved measurements using a 16-terminal disc reveal in-plane
transport anisotropy and a pronounced directional nonlinear response. In Hall bars, the
signal remains essentially unchanged over the measured frequency range and through
perpendicular magnetization reversal. Conductivity scaling and first-principles
calculations provide a framework for discussing the possible coexistence of Berry-
curvature-related and disorder-mediated contributions.
Second, I will examine low-frequency Hall-voltage fluctuations during the evolution
from stripe domains to skyrmion-containing states, as characterized by magnetic force
microscopy. Although the conventional power spectrum remains nearly unchanged,
second-spectrum analysis reveals non-Gaussian fluctuations with pronounced
dependence on field, temperature, current, and frequency. Their association with the
evolving magnetic textures provides insight into slow magnetic dynamics that are not
apparent from the conventional spectrum alone. Together, these studies demonstrate
how nonlinear electrical responses and higher order fluctuation statistics provide
complementary probes of symmetry, disorder and magnetic texture dynamics in
sputtered magnetic multilayers. |