The samples (La1-xEux)4/3Sr5/3Mn2O7 (x=0, 0.15) were prepared by the traditional solid-state reaction, and their magnetic and electrical properties were investigated. The magnetzation measurement reveals that as temperature lowers, all the samples undergo a complex magnetic transition process: they transform from the two-dimensional short-range ferromagnetic order at T* into the three-dimensional long-range ferromagnetic state at TC. With the increase of Eu doping, T* and TC are both reduced, and the sample (La0.85Eu0.15)4/3Sr5/3Mn2O7 exhibits spin-glass-like behaviour in a low temperature region. Electrical property measurements show that with the increase of Eu concentration, resistivity sharply increases, the metal-insulator transition temperature decreases and the magnetoresistance peak increases. These effects are attributed to the decrease of the average ionic radius diminution and the lattice distortion due to the substitution of the smaller Eu3+ ions for La3+ ions. In addition, the small-sized Eu3+ ion preferentially occupies the R site in the rock-salt layer, then the distributions of La3+, Sr3+, Eu3+ ions in the sample (La0.85Eu0.15)4/3Sr5/3Mn2O7 should be more orderly, so there is only one peak in the ρ-T curve of the sample with x=0.15.

La 0.8 Sr 0.2 Mn 1-x Co x O 3 (x = 0, 0.1, 0.3) samples were prepared via the conventional high-temperature solid-state reaction method. The effects of Co doping on Griffiths phase, magnetic entropy change, critical behavior, and electrical transport properties of La 0.8 Sr 0.2 MnO 3 (LSMO) polycrystalline samples were systematically investigated. Results show that the prepared polycrystalline samples all have rhombohedral symmetry structures with Griffiths phase above the low temperature magnetic transition temperature (T C2). When magnetic field is applied to the La 0.8 Sr 0.2 Mn 1-x Co x O 3 (x = 0, 0.1, 0.3) samples, the maximum magnetic entropy change ΔS max for 7 T is −2.88, −2.05, and −2.75 J/(kg•K), respectively. Doping of Co element makes ΔS max decrease first and then increase. The critical behavior of the parent phase fits best with the mean field model, and that of the sample after doping fits best with the 3D Heisenberg model. The mother phase is a semiconductor material, and the metal insulator transition appears near the low temperature magnetic transition temperature (T C2) when the Co element doping amount reaches 0.1. The conductivity of the three samples in high temperature region satisfies the small polaron model.

La 0.9 Eu 0.1 ) 4/3 Sr 5/3 Mn 2 O 7 polycrystalline sample was prepared by solid state reaction method. The magnetic and magnetocaloric properties have been studied. The competition between FM and AFM interactions was observed at low temperature range, and the coexistence of FM and PM was found in the temperature range of T c 3D -T c 2D for this compound. In addition, the magnetic entropy change under 1 T magnetic field is up to 1.69 J/(kg K), which suggests that such materials may be applied to magnetic refrigeration in the temperature range of liquid hydrogen.