Rotating black holes with an anisotropic matter field.

We present a family of new rotating black hole solutions to Einstein's equations that generalizes the Kerr-Newman spacetime to include an anisotropic matter. The geometry is obtained by employing the Newman-Janis algorithm. In addition to the mass, the charge and the angular momentum, an additional hair exists thanks to the negative radial pressure of the anisotropic matter. The properties of the black hole are analyzed in detail including thermodynamics. This black hole can be used as a better engine than the Kerr-Newman one in extracting energy.