Conventionally, the parallelism between the physical properties of Ce and Yb<br />
based magnets and heavy fermions has been considered to arise as a consequence<br />
of the electron-hole symmetry known for the band theory in condensed matter<br />
physics. Namely, the trivalent state of Ce has $4f^{1}$ configuration, while<br />
$4f^{13}$ configuration for Yb$^{3+}$ can be viewed as a state carrying one<br />
4$f$ hole. However, in the light of this parallelism, the magnetism of Yb based<br />
intermetallic compounds has been a mystery over decades; the transition<br />
temperature of the Yb based compounds is normally very small, as low as $\sim$<br />
1 K or even lower, while Ce counterparts may often have the transition<br />
temperature well exceeding 10 K. Here, we report our experimental discovery of<br />
the transition temperature reaching 20 K for the first time in an Yb based<br />
compound. Our single crystal study using transport, thermomagnetic and<br />
photoemission spectroscopy measurements reveals that the Mn substitution at the<br />
Al site in an intermediate valence state of $\alpha$-YbAlB$_{4}$ not only<br />
induces antiferromagnetic transition at a record high temperature of 20 K but<br />
also transform the heavy fermion liquid state in $\alpha$-YbAlB$_{4}$ into a<br />
highly resistive metallic state proximate to a Kondo insulator. The high<br />
temperature antiferromagnetism in a system close to a Kondo insulator is<br />
unusual and suggest a novel type of itinerant magnetism in the Yb based heavy<br />
fermion systems.