Nonlinear magnetization of the one-dimensionalS=1 antiferromagnet

Abstract

It is known that the one-dimensional S=1 antiferromagnet has an energy gap and therefore has no magnetization up to a finite field ${\mathit{H}}_{\mathit{c}}$. We find that the magnetization appears nonlinearly above ${\mathit{H}}_{\mathit{c}}$. So the field derivative dm/dH diverges at ${\mathit{H}}_{\mathit{c}}$. This explains the cusp in dm/dH observed experimentally in the quasi-one-dimensional S=1 material Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$ ${\mathrm{ClO}}_4$.