  Magnetic tunnel junctions (MTJs) are key components of spintronic devices,
such as magnetic random-access memories. Normally, MTJs consist of two
ferromagnetic (FM) electrodes separated by an insulating barrier layer. Their
key functional property is tunneling magnetoresistance (TMR) that is a change
in MTJ's resistance when magnetization of the two electrodes alters from
parallel to antiparallel. Here, we demonstrate that TMR can occur in MTJs with
a single FM electrode, provided that the counter electrode is an
antiferromagnetic (AFM) metal that supports a spin-split band structure and/or
a N\'eel spin current. Using RuO$_{2}$ as a representative example of such
antiferromagnet and CrO$_{2}$ as a FM metal, we design all-rutile
RuO$_{2}$/TiO$_{2}$/CrO$_{2}$ MTJs to reveal a non-vanishing TMR. Our
first-principles calculations predict that magnetization reversal in CrO$_{2}$
significantly changes conductance of the MTJs stacked in the (110) or (001)
planes. The predicted giant TMR effect of about 1000% in the (110) oriented
MTJs stems from spin-dependent conduction channels in CrO$_{2}$ (110) and
RuO$_{2}$ (110), whose matching alters with CrO$_{2}$ magnetization
orientation, while TMR in the (001) oriented MTJs originates from the N\'eel
spin currents and different effective TiO$_{2}$ barrier thickness for the two
magnetic sublattices that can be engineered by the alternating deposition of
TiO$_{2}$ and CrO$_{2}$ monolayers. Our results demonstrate a possibility of a
sizable TMR in MTJs with a single FM electrode and offer a practical test for
using the altermagnet RuO$_{2}$ in functional spintronic devices.
