Layered Ising metamagnets are antiferromagnetic (AF) materials consisting of monolayer Ising ferromagnets coupled to each other via interlayer AF interactions. They exhibit rich magnetic phase diagrams, featuring tricritical and critical end points, due to the competing magnetic interactions and the Ising anisotropy. While conventional thermodynamic probes can identify these critical points in bulk Ising metamagnets, achieving this in the two-dimensional (2D) limit, where enhanced fluctuation effects can substantially modify critical phenomena, remains to be realized. Here, we combine a nanomechanical signal proportional to the specific heat capacity ($C_{\mathrm{V}}$) and magnetic circular dichroism measurements to identify these critical points, extract a tricritical exponent, and map out the complete magnetic phase diagram of 2D Ising metamagnetic ${\text{CrI}}_3$. This is achieved in a nanomechanical device of six-layer ${\text{CrI}}_3$, in which a direct measurement of the temperature derivative of its mechanical resonance frequency gives a signal proportional to $C_{\mathrm{V}}$. The tricritical point is identified by the onset of an abrupt spin-flip transition on one side and, on the other side, by a vanishing specific heat $\lambda$ anomaly for a continuous AF phase transition. In contrast, only the spin-flip transition remains near the critical end point. Our results establish nanomechanical calorimetry as a general route to classify metamagnetic phase transitions and to study multicritical phenomena in 2D magnets.

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• Interference & diffraction of light
• Magnetism
• 2-dimensional systems
• Magnetic semiconductors
• Calorimetry
• Critical phenomena
• Optical microscopy