Low-temperature properties and ESR in the quasi-one-dimensional random compoundMnMgB2O5

Abstract

Low-temperature magnetization, ac susceptibility, and specific heat have been measured on powdered ${\mathrm{MnMgB}}_{2}{\mathrm{O}}_{5}$ between 100 mK and 6.8 K. A power-law temperature dependence of the susceptibility has been found down to 2 K in agreement with previous measurements. The exponent describing this behavior is close to that found for the warwickite ${\mathrm{ScMnOBO}}_{3},$ another ${\mathrm{Mn}}^{2+}$ compound with a similar low dimensional random magnetic arrangement of Mn ions. The magnetic and specific-heat measurements indicate that, below 600 mK, the ${\mathrm{MnMgB}}_{2}{\mathrm{O}}_{5}$ pyroborate freezes in a glassy state. Electron-spin resonance measurements (ESR) on a single crystal of this compound are also presented. The linewidth and the g factor show different temperature dependencies for different directions of the applied field. Finally a theory is presented for ESR in random exchange Heisenberg antiferromagnetic chain (REHAC) compounds.