Random spin-exchange collisions in dense thermal alkali vapors can induce coherent mode-mixing dynamics, enabling a minority species to inherit the majority species’ gyromagnetic ratio and long coherence time. However, conventional polarization Bloch equations rely on a phenomenological slowing-down factor $q(P)$ that does not fully capture transient dynamical coupling effects in the spin-exchange relaxation-free regime for multispecies systems. Here we introduce a hyperfine-resolved Bloch equation framework that explicitly retains hyperfine-resolved alkali-alkali spin-exchange channels and alkali-nuclear Fermi-contact couplings. In a thermal ${}^{39}\mathrm{K}\text{−}{}^{85}\mathrm{Rb}\text{−}{}^3\mathrm{He}$ vapor, we demonstrate that ${}^{39}\mathrm{K}$ coherently inherits the ${}^{85}\mathrm{Rb}$ precession frequency over a wide range of longitudinal field $B_z$, with the nuclear spins inducing rapid relaxation and nonlinear gyromagnetic-ratio shifts near a magnetically tunable critical point. Time- and frequency-domain measurements with species-selective readout show that the hyperfine-resolved model reproduces the measured field-dependent mode frequencies and critical hybridization of the trispin system. By resolving the mode frequencies, damping rates, and optical-readout response near the dynamical critical point, the present framework provides a practical response-function calibration tool for strongly coupled alkali-noble-gas sensors.

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• Coherent control
• Collective effects in atomic physics
• Nuclear & electron resonance
• Optical pumping
• Quantum information with atoms & light
• Quantum sensing
• Alkali metals
• Atomic gases