Field-Induced Local Excitations Causing Zero-Magnetization Plateaus in Antiferromagnets of Antiferromagnetic Spin Dimers under Magnetic Field

The zero-magnetization plateau refers to the phenomenon that the magnetization of an antiferromagnet under a magnetic field remains at zero when the field increases from 0 to a certain critical value μ0Hc. Certain antiferromagnets composed of antiferromagnetic (AFM) spin dimers exhibit a zero-magnetization plateau despite the fact that the single-ion anisotropy of their magnetic ions is negligible. To investigate the cause for this finding, we analyzed how a magnetic field affects the energy spectrum of an AFM chain composed of AFM spin dimers made up of two S = 1/2 ions under the supposition that each spin dimer counteracts external field, according to Le Chatelier’s principle, to introduce a small amount of spin into the ground state (S = 0) by mixing the excited state (S = 1). First, we show that this concept of field-induced local excitations explains several puzzling observations concerning the zero-magnetization plateaus of TlCuCl3 and KCuCl3, which are antiferromagnets made up of molecular anions Cu2Cl62– (i.e., spin dimers of S = 1/2 ions Cu2+). Then, we analyze the magnetic entropy associated with the zero-magnetization plateaus occurring in the field region between 0 and μ0Hc to find that this field region is divided into two subregions of different magnetic entropy; the 0 – μ0Hm region where two different magnetic species coexist, the distribution of which in the spin–lattice generates the magnetic entropy described largely by the binomial coefficients, and the μ0Hm – μ0Hc region where there exists only one magnetic species, leading to zero magnetic entropy. We carried out specific heat measurements for KCuCl3 as a function of magnetic field between 0–9 T at 2 K and verified the predictions of our analysis concerning the field-dependence of specific heat for the two-phase region of a zero-magnetization plateau.