Magnetic states of the Kondo lattice Ce$_2$PdSi$_3$ and their pressure evolution

Frustrated Kondo lattices are ideal platforms for exploring unconventional forms of quantum criticality, as well as magnetism and other emergent phases. Here we report the magnetic properties of the candidate frustrated heavy fermion compound Ce$2$PdSi$3$, and map their evolution upon applying magnetic fields and hydrostatic pressure. We find that at ambient pressure Ce$2$PdSi$3$ exhibits two distinct magnetic phase transitions, a ferromagnetic-like transition at $T{\mathrm{M1}}=3.8$ K and an incommensurate antiferromagnetic transition at $T{\mathrm{M2}}=2.9$ K. Upon applying pressure, $T{\mathrm{M1}}$ is continuously suppressed and becomes undetectable above 4.2 GPa, whereas $T{\mathrm{M2}}$ increases and remains robust up to at least 7.5 GPa. The observed pressure evolution of magnetic order in Ce$_2$PdSi$_3$ suggests the presence of competing magnetic orders, and cannot be simply encapsulated by the Doniach phase diagram, motivating further investigations for its origin, including discerning the role of geometric frustration.

💡 Research Summary

The authors present a comprehensive study of the magnetic behavior of the heavy‑fermion compound Ce₂PdSi₃, focusing on how its magnetic phases evolve under applied magnetic fields and hydrostatic pressure. Single crystals grown by the optical floating‑zone technique were characterized using electrical resistivity, AC magnetic susceptibility, specific heat, and single‑crystal neutron diffraction. At ambient pressure the material exhibits two distinct low‑temperature magnetic transitions: a ferromagnetic‑like transition at T_M1 ≈ 3.8 K and an incommensurate antiferromagnetic transition at T_M2 ≈ 2.9 K. The ferromagnetic transition is identified by a sharp drop in resistivity, a ZFC/FC bifurcation in χ(T), hysteresis in M(H), and a heat‑capacity peak that shifts to higher temperature with increasing magnetic field. Neutron diffraction reveals magnetic Bragg peaks with propagation vector k₁ = 0, confirming a ferromagnetic component that is commensurate with a structural 2 × 2 × 4 superlattice observed at 8 K.

The lower‑temperature transition displays characteristics of antiferromagnetism: the heat‑capacity peak moves to lower temperature under field, and neutron diffraction uncovers additional magnetic reflections with an in‑plane incommensurate propagation vector k₂ = (0.15, 0, 0). These k₂ peaks appear below T_M2 and are decoupled from the structural modulation along the c‑axis, indicating a distinct magnetic order coexisting with the ferromagnetic component.

Applying hydrostatic pressure up to 7.5 GPa dramatically alters the balance between these two orders. AC specific‑heat measurements show that T_M1 is continuously suppressed with pressure, vanishing above ≈ 4.2 GPa, whereas T_M2 is enhanced, reaching ≈ 3.8 K at 7.5 GPa. Electrical‑resistivity data corroborate this trend: the ferromagnetic‑like anomaly disappears beyond 4.2 GPa, while a new anomaly associated with the antiferromagnetic order becomes prominent. Near 4.2 GPa an additional pressure‑independent feature (T′ ≈ 2.5 K) emerges, whose origin is unclear but may signal a change in the low‑temperature magnetic structure or the onset of a topological spin texture.

The divergent pressure responses of the two transitions cannot be captured by the conventional Doniach picture, which treats the competition between Kondo screening and RKKY exchange as a single‑parameter (J_cf) problem. Instead, the data suggest that geometric frustration inherent to the triangular Ce sublattice, pressure‑induced modifications of the crystal‑field scheme, and changes in the electronic band structure all play crucial roles. The ferromagnetic order, likely favored by certain RKKY pathways, is weakened under compression, while the incommensurate antiferromagnetic order, possibly stabilized by frustration and anisotropic exchange, is reinforced.

The authors draw parallels with the isostructural Gd₂PdSi₃, where skyrmion lattices have been observed. In Ce₂PdSi₃, the high‑pressure regime directly connects the paramagnetic state to the incommensurate antiferromagnetic phase, raising the intriguing possibility of pressure‑induced skyrmions or other topological textures, especially given the emergence of the field‑independent T′ anomaly.

In conclusion, Ce₂PdSi₃ hosts competing ferromagnetic and incommensurate antiferromagnetic orders whose relative stability can be tuned by pressure, revealing a rich phase diagram that lies beyond the simple Doniach scenario. The material therefore provides an excellent platform for exploring the interplay of Kondo physics, geometric frustration, and emergent magnetic textures. Future work involving high‑resolution neutron scattering, μSR, NMR, and possibly resonant X‑ray techniques under pressure will be essential to resolve the microscopic spin configurations and to search for topological spin states in this system.