Multiple Magnetic Transitions in a Frustrated Heavy-Fermion Antiferromagnet YbAgGe under Magnetic Field and Pressure

Abstract

The magnetization M and magnetoresistance of a heavy-fermion antiferromagnet YbAgGe have been measured under pressures P up to 2.7 GPa and magnetic fields B up to 9.5 T. This compound crystallizing in the hexagonal ZrNiAl-type structure with a quasi-kagome lattice of Yb ions has two magnetically ordered phases, namely, phase I ( T ≤ T M2 = 0.65 K) and phase II ( T M2 ≤ T ≤ T M1 = 0.8 K). With increasing pressure to 2.2 GPa, M ( B || a = 9.5 T) at 0.3 K increases from 1.5 µ B /Yb to 1.8 µ B /Yb, while M ( B || c = 9.5 T) hardly changes. The metamagnetic transition field in M ( B || a ) decreases from 4.6 T for P = 0 to 3.3 T for P = 2.2 GPa. As the pressure exceeds P * = 1.6 GPa, both T M and the initial slope of M ( B || a ) steeply increase and the area of phase I for B || a expands. These observations suggest that the magnetic frustration of the Yb quasi-kagome lattice is partially suppressed by the application of pressure above 1.6 GPa or magnetic field B || a above 4.5 T. Detailed B – T phase diagrams under pressures up to 2.7 GPa are discussed with respect to the magnetic frustration in YbAgGe.