2 00 1 Field - induced magnetic transitions in the quasi - two - dimensional

Field-induced magnetic transitions in the quasi-two-dimensional heavy-fermion antiferromagnets Ce n RhIn 3n+2 (n = 1 or 2) Abstract We have measured the field-dependent heat capacity in the tetragonal antifer-romagnets CeRhIn 5 and Ce 2 RhIn 8 , both of which have an enhanced value of the electronic specific heat coefficient γ ∼ 400 mJ/mol-Ce K 2 above T N. For T < T N , the specific heat data at zero applied magnetic field are consistent with the existence of an anisotropic spin-density wave opening a gap in the Fermi surface for CeRhIn 5 , while Ce 2 RhIn 8 shows behavior consistent with a simple antiferromagnetic magnon. From these results, the magnetic structure, in a manner similar to the crystal structure, appears more two-dimensional in CeRhIn 5 than in Ce 2 RhIn 8 where only about 12% of the Fermi surface remains ungapped relative to 92% for Ce 2 RhIn 8. When B||c, both compounds behave in a manner expected for heavy fermion systems as both T N and the electronic heat capacity decrease as field is applied. When the field is applied in the tetragonal basal plane (B||a), CeRhIn 5 and Ce 2 RhIn 8 have very sim-1 ilar phase diagrams which contain both first-and second-order field-induced magnetic transitions .