  We discuss neutron $^{3}P_{2}$ phases in the core of neutron stars in strong
magnetic field (magnetars). The neutron $^{3}P_{2}$ pairing provides a wide
variety of condensates, such as the uniaxial nematic and (D$_{2}$ and D$_{4}$)
biaxial nematic, with different symmetries stemming from the combinations of
spin and momentum. Based on the spin-orbital angular momentum coupling and the
spin-magnetic field coupling of the neutrons, we derive the Ginzburg-Landau
equation containing higher order terms of the magnetic field. We investigate
the phase diagram of the neutron $^{3}P_{2}$ superfluidity, and find that the
D$_{2}$ biaxial nematic phase is extended by the higher order terms of the
magnetic field. We also discuss the thermodynamic properties, the heat capacity
and the spin susceptibility.
