Magnetic Structures of the Rare-Earth Platinum AluminidesRPtAl (R=Ce, Pr, Nd)

Abstract

The rare-earth (R) platinum aluminidesRPtAl crystallize in the orthorhombic TiNiSi-type structure (space group Pnma,Z=4), where magnetic rare-earth atoms form a network of chains parallel to thea-axis and parallel to theb-axis. Magnetic structures and phase transitions ofRPtAl (R=Ce, Pr, Nd) compounds were investigated by systematic measurements of magnetic susceptibility, specific heat, and neutron diffraction on polycrystalline samples. The results reveal a large magnetocrystalline anisotropy and magnetic structures that are dominated by a ferromagnetic component parallel to one of the two chain directions: thea-axis for CePtAl and PrPtAl and theb-axis for NdPtAl. The complex magnetism of CePtAl with three successive magnetic phase transitions (TC=5.9 K,T2=4.3 K,T3=2.5 K) and two coexisting propagation vectors (k1=0 forT≤TC, k2i=[0, 0.46, 0] forT2≤T≤TC, k2=[0, 1/2, 0] forT≤T2) is confirmed to be exceptional amongRPtAl compounds. PrPtAl has a nonmagnetic crystalline-electric field (CEF) ground-state singlet separated by 21 K from the first-excited state CEF singlet and magnetic exchange interactions are strong enough to induce long-range magnetic order (Curie temperatureTC=5.8 K, propagation vector k1=0, magnetic group Pnm′a′, ordered saturation momentm1=1.00(7)μB). NdPtAl is a simple ferromagnet (TC=19.2 K, k1=0, Pn′ma′,m1=2.08(4)μB).