  In this study, we report a conceptually novel broadband high-frequency
electron spin resonance (HFESR) spectroscopic technique. In contrast to the
ordinary force-detected ESR technique, which detects the magnetization change
due to the saturation effect, this method measures the magnetization change due
to the change of the sample temperature at resonance. To demonstrate its
principle, we developed a silicon nitride nanomembrane-based force-detected ESR
spectrometer, which can be stably operated even at high magnetic fields. Test
measurements were performed for samples with different spin relaxation times.
We succeeded in obtaining a seamless ESR spectrum in magnetic fields of 15~T
and frequencies of 636~GHz without significant spectral distortion. A high spin
sensitivity of $10^{12}$~spins/G$\cdot$s was obtained, which was independent of
the spin relaxation time. These results show that this technique can be used as
a practical method in research fields where the HFESR technique is applicable.
