Title: Angle-dependent electron spin resonance of YbRh$_2$Si$_2$ measured with planar microwave resonators and in-situ rotation,
Abstract: 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.