Piezomagnetism, the linear response between strain and magnetic field, is a relatively unexplored cross correlation but has promising potential as a novel probe of time-reversal symmetry breaking in various classes of materials. Interestingly, there has been no report of piezomagnetism in ferromagnets, most archetypal time-reversal symmetry-broken materials. This half-century absence of piezomagnetic ferromagnets is attributable to complications originating from multiple-domain states, as well as from changes in the magnetic point group by rotation of the magnetic moment. Here, we report characteristic V-shaped magnetostriction in the Ising itinerant ferromagnet URhGe, observed by simultaneous multiaxis strain measurements utilizing optical fiber Bragg grating sensors. This magnetostriction occurs only under fields along the $c$ axis and does not scale with the square of magnetization. Such an unconventional feature indicates piezomagnetism as its origin. Our observation is owing to monodomain switching and Ising magnetization. The obtained piezomagnetic coefficients are fairly large, implying that Ising ferromagnets are promising frontiers when seeking for materials with large piezomagnetic responses.

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• Critical phenomena
• Ferromagnetism
• Magnetic anisotropy
• Magnetic phase transitions
• Magnetism
• Magnetostriction
• Strain
• Thermal expansion
• Magnetic techniques
• Magnetization measurements