Angle-dependent electron spin resonance of YbRh${}_2$Si${}_2$ measured with planar microwave resonators and in-situ rotation

We present a new experimental approach to investigate the magnetic properties of the anisotropic heavy-fermion system YbRh${}_2$Si${}_2$ as a function of crystallographic orientation. Angle-dependent electron spin resonance (ESR) measurements are performed at a low temperature of 1.6 K and at an ESR frequency of 4.4 GHz utilizing a superconducting planar microwave resonator in a ${}^4$He-cryostat in combination with in-situ sample rotation. The obtained ESR g-factor of YbRh${}_2$Si${}_2$ as a function of the crystallographic angle is consistent with results of previous measurements using conventional ESR spectrometers at higher frequencies and fields. Perspectives to implement this experimental approach into a dilution refrigerator and to reach the magnetically ordered phase of YbRh${}_2$Si${}_2$ are discussed.