Complex magnetic order in novel heavy fermion compound YbPt5B2: elastic and inelastic neutron scattering studies

Abstract

YbPt5B2 and isostructural LuPt5B2 are new ternary borides [1]. Monoclinic YbPt5B2 exhibits two magnetic phase transitions at Tmag1 ~ 8 K and Tmag2 ~ 4 K, as deduced from specific heat, magnetostriction, electric resistivity and magnetization data. Externally applied magnetic fields are responsible for further field induced phase transitions. The quite complex magnetic features appear to be the result of RKKY interaction and Kondo physics in context of crystalline electric field (CEF) effects. In order to resolve both the magnetic structure of YbPt5B2 and CEF characteristics, elastic and inelastic neutron studies have been carried out on polycrystalline samples at the ISIS Facility, using the high flux, high-resolution time-of-flight (TOF) diffractometer WISH at temperatures down to 1 K and magnetic fields up to 8 T. INS measurements were carried out with the TOF spectrometer MARI, employing several incident neutron energies, and temperatures ranging from 10 K up to room temperature.These temperature and field dependent neutron studies essentially confirmed the previous bulk measurements, revealing antiferromagnetic transitions at TN1 ~ 8 K and TN2 ~ 4 K. While simple AFM was obtained for T < TN2, characterised by a propagation vector kcom = (0,0,0) and Yb-magnetic-moments as large as 3.04 μB (at T = 1.5 K), the propagation vector dramatically modifies for TN2 < T < TN1, together with a change of the ordered magnetic moment, revealing an incommensurate structure kincom = (0.1938725, 0, -0.0446576), with a distinct temperature dependent variation of the respective (hkl) values. Modifications of the magnetic structure and the respective magnetic moments, owing the application of external fields, were derived from elastic neutron studies, too. In order to explicate temperature dependent quantities, such as the specific heat or the magnetic susceptibility of YbPt5B2, the knowledge of crystalline electric field featues are indispensable. The present INS studies allowed to derive the CEF level scheme and associated magnetic moments. A doublet as ground state, with a predominant |7/2> wave function, explains the magnetic moments at low temperature. The respective level scheme, with excited CEF levels at 11, 25 and 34 meV excellently accounts for the almost constant magnetic entropy below 100 K. In this work, neutron data obtained on YbPt5B2 are analyzed, allowing to elucidate the complex magnetic properties indicated above.