The anisotropic Hamiltonian, $H=\ensuremath{-}\frac{1}{2}\ensuremath{\Sigma}({J}_{x}{{\ensuremath{\sigma}}_{l}}^{x}{{\ensuremath{\sigma}}_{l+1}}^{x}+{J}_{y}{{\ensuremath{\sigma}}_{l}}^{y}{{\ensuremath{\sigma}}_{l+1}}^{y}+{J}_{z}{{\ensuremath{\sigma}}_{l}}^{z}{{\ensuremath{\sigma}}_{l+1}}^{z})\ensuremath{-}m\mathcal{H}\ensuremath{\Sigma}{{\ensuremath{\sigma}}_{l}}^{z},$ of the linear spin array in the Heisenberg model of magnetism is examined. The eigenstate and the partition function for the case
Jz=0 are obtained exactly for a finite system and for an infinite system with the aid of annihilation and creation operators, and the free energy
F of the latter is given by $\ensuremath{-}\frac{F}{\mathrm{NkT}}=(\frac{1}{\ensuremath{\pi}})\ensuremath{\int}{0}^{\ensuremath{\pi}}\mathrm{ln}{2cosh{[{{K}_{x}}^{2}+{{K}_{y}}^{2}+2{K}_{x}{K}_{y}cos2\ensuremath{\omega}\ensuremath{-}2C({K}_{x}+{K}_{y})cos\ensuremath{\omega}+{C}^{2}]}^{\frac{1}{2}}}d\ensuremath{\omega},$ where
Kx=2kTJx,
Ky=2kTJy,
C=kTmH. The case
Jx=Jy=Jz=J is discussed with the aid of a high-temperature expansion and of analysis of small systems. Specific heats and susceptibilities in special cases: (i)
Jx=Jy=J,
Jz=0, (ii)
Jx=J,
Jy=Jz=0, (
iiif)
Jx=Jy=0,
Jz=J>0, (
iiia)
Jx=Jy=0,
Jz=J<0, (
ivf)
Jx=Jy=Jz=J>0, (
iva)
Jx=Jy=Jz=J<0 are compared and it is shown that (i), (
iiia), and (
iva) have the characteristic features of the observed parallel susceptibility of an antiferromagnetic substance, (ii) those of perpendicular susceptibility, and (
iiif) and (
ivf) those of paramagnetic susceptibility, even though they have no singularities. The distribution of the zeros of the partition function is also discussed.