  Emerging altermagnetic materials with vanishing net magnetizations and unique
band structures have been envisioned as an ideal electrode to design
antiferromagnetic tunnel junctions. Their momentum-resolved spin splitting in
band structures defines a spin-polarized Fermi surface, which allows
altermagnetic materials to polarize current as a ferromagnet, when the current
flows along specific directions relevant to their altermagnetism. Here, we
design an Altermagnet/Insulator barrier/Ferromagnet junction, renamed as
altermagnetic tunnel junction (ATMTJ), using RuO$_2$/TiO$_2$/CrO$_2$ as a
prototype. Through first-principles calculations, we investigate the tunneling
properties of the ATMTJ along the [001] and [110] directions, which shows that
the tunneling magnetoresistance (TMR) is almost zero when the current flows
along the [001] direction, while it can reach as high as 6100\% with current
flows along the [110] direction. The spin-resolved conduction channels of the
altermagnetic RuO$_2$ electrode are found responsible for this
momentum-dependent (or transport-direction-dependent) TMR effect. Furthermore,
this ATMTJ can also be used to readout the N\'{e}el vector of the altermagnetic
electrode RuO$_2$. Our work promotes the understanding toward the altermagnetic
materials and provides an alternative way to design magnetic tunnel junctions
with ultrahigh TMR ratios and robustness of the altermagnetic electrode against
external disturbance, which broadens the application avenue for
antiferromagnetic spintronic devices.
