Low-temperature properties of quantum antiferromagnetic chains with alternating spins and

Abstract

We study the low-temperature properties of S = 1 and 1/2 alternating spin chains with antiferromagnetic nearest-neighbor exchange couplings using analytical techniques as well as a quantum Monte Carlo method. The spin-wave approach predicts two different low-lying excitations, which are gapped and gapless, respectively. The structure of low-lying levels is also discussed by perturbation theory in the strength of the Ising anisotropy. These analytical findings are compared with the results of quantum Monte Carlo calculations and it turns out that spin-wave theory well describes the present system. We conclude that the quantum ferrimagnetic chain exhibits both ferromagnetic and antiferromagnetic aspects.

Magnetic properties of quantum ferrimagnetic spin chains

“…Therefore quantum ferrimagnetism was not necessarily the main subject there. Though there also appeared theoretical investigations [1,8] in close contact with experiments, the argument was restricted to the spin combinations (S, 1/2) and the larger spin S was sometimes treated as classical.In such circumstances, several renewed interests [9][10][11][12][13][14][15][16][17][18][19], have recently been introduced into the field. Alcaraz and Malvezzi [9] predicted the universal low-energy physics of anisotropic ferrimagnetic chains and the distinct behavior of isotropic ones.…”

“…In such circumstances, several renewed interests [9][10][11][12][13][14][15][16][17][18][19], have recently been introduced into the field. Alcaraz and Malvezzi [9] predicted the universal low-energy physics of anisotropic ferrimagnetic chains and the distinct behavior of isotropic ones.…”

“…Another noteworthy subject, which is the main interest here, is the coexistence of the ferromagnetic and antiferromagnetic features in quantum ferrimagnets. The author et al pointed out in their recent study [12,14,17] on the model (1) with (S, s) = (1, 1/2) that at low temperatures the model acts as a ferromagnet, while at intermediate temperatures it behaves like a gapped antiferromagnet. In this article, we quantitatively verify to what extent this scenario is valid discussing various combinations of (S, s).…”

“…We take µ B as the Bohr magneton and set the g factors of both spins S and s equal to g. We assume that S > s. The numerical procedure of the quantum Monte Carlo method employed has been detailed elsewhere [21]. Since the correlation length of the system is considerably small [10,12], the thermal quantities in general show no significant size dependence. We have calculated the chains of N = 24 and N = 32, but we find no difference beyond the numerical uncertainty between them in the temperature region treated.…”

“…Quantum ferrimagnets exhibit two distinct branches of elementary excitations, one of which reduces the magnetization M ≡ j (S z j + s z j ) and is thus called ferromagnetic, and the other of which enhances that and is thus called antiferromagnetic. The lowest-order spin-wave approach [10,12] to the model (1) with…”