$\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ represents a borderline case in the range of solid solutions formed in the pseudobinary system $\mathrm{LaC}{\mathrm{o}}_2{\mathrm{P}}_2\text{−}\mathrm{CeC}{\mathrm{o}}_2{\mathrm{P}}_2$. The material undergoes ferromagnetic ordering at ∼225 K followed by a structural collapse at ∼190 K, which leads to a strong suppression of magnetization. The structural phase transition manifests itself in a gradual decrease in the parameter $c$ and a relatively smaller increase of the parameter $a$ of the tetragonal lattice. Interestingly, a combination of magnetic measurements and nonpolarized and polarized neutron scattering experiments suggests that the structural collapse does not lead to an antiferromagnetically ordered state, observed in samples with the higher Ce content. On the contrary, $\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ appears to enter a disordered, spin glass state, with gradual dissipation of the ferromagnetic ordering taking place simultaneously with the structural collapse, as evidenced by temperature-dependent measurements of the depolarization factor for a polarized neutron beam passing through the sample. The observed behavior is analogous to that reported for so-called reentrant spin glasses. In the present case, however, the appearance of the reentrant spin glass regime is caused not by tuning the chemical composition but by the structural phase transition. Electronic structure calculations confirm that the loss of magnetic ordering is caused by the subtle change to the density of states at the Fermi level due to the variation of the crystal structure of the material.

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• Density of states
• Electronic structure
• Magnetic order
• Magnetic phase transitions
• Magnetism
• Phase transitions
• Structural properties
• Ferromagnets
• Spin glasses
• AC susceptibility measurements
• Band structure methods
• Crystallography
• DC susceptibility measurements
• Density functional theory
• Magnetization measurements
• Neutron diffraction
• Neutron scattering
• Neutron techniques
• Specific heat measurements
• Susceptibility measurements
• X-ray diffraction