Critical Phenomena at the Antiferromagnetic Transition in MnO
- PDF / 1,811,165 Bytes
- 6 Pages / 415.8 x 637.2 pts Page_size
- 81 Downloads / 166 Views
ABSTRACT The specific heat of a polycrystalline sample of MnO was measured from Tz 1 K to T • 400 K using two different experimental apparatuses at zero applied pressure. Features revealed by the data include a hyperfine contribution due to the Mn nuclei, a T 2 temperature dependence at low temperatures due to the type-II antiferromagnetic magnon contribution, and a sharp but well defined antiferromagnetic transition (TN = 117.7095 K) that is clearly second order in nature. The critical exponent, a, deduced from the transition is consistent with a two dimensional Ising model. The specific heat of MnO is also compared with recent results on the type-A antiferromagnet LaMnO 3 . INTRODUCTION Recent specific-heat measurements1 on the colossal magnetoresistance materials Lal.xSrxMnO 3, where x = 0.0, 0.1, 0.2, and 0.3, revealed a low-temperature T 2 term in the undoped parent compound, LaMnO 3, due to spin waves associated with a type-A antiferromagnetic transition at 110 K. Contrary to most magnetic transitions, however, the transition in LaMnO 3 showed only a small anomaly with no excess specific heat. In order to better understand the magnetic properties of LaMnO 3 we decided to measure the specific heat of MnO, a material with a similar magnetic structure and ordering temperature. MnO has long been considered one of the classic antiferromagnets and, consequently,
many of its properties have been studied extensively. At room temperature, MnO is paramagnetic with a NaCl crystal structure and Fm3m space group but, below 118 K, it transforms to an antiferromagnetic phase. The full magnetic structure of the antiferromagnetic state in MnO was determined by Shull, Strausser, and Wollan 2 and by Li. 3 These studies showed that the magnetic Mn spins align ferromagnetically within a given (111) plane and these planes are stacked antiferromagnetically in the direction normal to the given (111) plane; thus, MnO is classified as a type-II antiferromagnet. Some twenty years later, interest in MnO was renewed by numerous theoretical and experimental investigations of tricritical phase transitions. In 1973 Bloch and Maury4 used thermal expansion measurements on a single crystal of MnO to show that while the paramagnetic to antiferromagnetic transition is second order at ambient pressure it becomes first order when a uniaxial stress greater than -200 bar is applied along the (111) direction. The application of the uniaxial stress removed the three T-domains not aligned with (111) thereby revealing the first order character intrinsic to the transition. EXPERIMENTAL The MnO sample used for all measurements was purchased from Alfa Aesar, lot number K13G04, with a stated purity of better than 99.5%. The lot assay reported MnO2 as the only significant impurity at a concentration of 0.1% by weight. All sample masses have been 245 Mat. Res. Soc. Symp. Proc. Vol. 602 0 2000 Materials Research Society
corrected for the MnO 2 impurity, and the specific heat has been corrected for the MnO2 5 contribution (at most 0.05% of the total specific
Data Loading...
