Influence of Mn moments on the properties of RMn2 compounds (R=Y and light rare earths)

Abstract

The influence of Mn moments on the magnetism of RMn2 compounds, where R=Y, Pr, Nd, and Sm, has been investigated using SQUID magnetometry, thermal expansion, electrical resistivity, and magnetoresistance measurements. RMn2 compounds, with R=Y, Pr, Nd, and Sm, order antiferromagnetically at 116, 115, 105, and 94 K, respectively. According to thermal expansion data all these materials undergo a first-order phase transition from a paramagnetic to a magnetically ordered state, and this effect is believed to be caused by the collapse of the Mn moment at the transition. For the YMn2 compound, the resistivity has a T3 temperature dependence below 38 K and a positive magnetoresistance in the ordered phase, indicating that the Mn moments are localized below TN in this compound. Above TN, however, the Mn moments become itinerant which is reflected as a gradual increase in susceptibility χ with increasing temperature. In PrMn2, NdMn2, and SmMn2 the resistivity does not follow a simple temperature power law and the magnetoresistance varies from negative to positive values depending on the temperature and field. The transition temperatures in these materials differ significantly from the de Gennes factor and it is believed that the ordering is brought about by antiferromagnetic coupling between Mn moments.