Complex magnetic phase diagram of a geometrically frustrated Sm lattice: Magnetometry and neutron diffraction study of SmPd₂Al₃

Magnetism in SmPd₂Al₃ was investigated on a single crystal by magnetometry and neutron diffraction. SmPd₂Al₃ represents a distinctive example of a Sm magnetism exhibiting complex magnetic behavior at low temperatures with four consecutive magnetic phase transitions at 3.4, 3.9, 4.4, and 12.5 K. The rich magnetic phase diagram of this compound reflects the specific features of the Sm³⁺ ion, namely, the energy nearness of the ground-state multiplet J = 5/2 and the first excited multiplet J = 7/2 in conjunction with strong crystal field influence. Consequently, a significantly reduced Sm magnetic moment in comparison with the theoretical Sm³⁺ free-ion value is observed. Despite the strong neutron absorption by natural samarium and the small Sm magnetic moment (∼0.2 μ _B), we have successfully determined the magnetic k vector (1/3, 1/3, 0) of the phase existing in the temperature interval 12.5–4.4 K. This observation classifies the SmPd₂Al₃ compound as a magnetically frustrated system. The complex magnetic behavior of this material is further illustrated by kinetic effects of the magnetization, inducing a rather complicated magnetic structure with various metastable states.