The magnetic behavior of the compound, Gd3Ru4Al12, which has been reported to crystallize in a hexagonal structure (space group P63/mmc) about two decades ago, had not been investigated in the past literature despite interesting structural features (that is, magnetic layers and triangular as well as Kagome-lattice features favoring frustrated magnetism) characterizing this compound. We report here the results of magnetization, heat-capacity and magnetoresistance studies in the temperature (T) range 1.8 -300 K. The results establish that there is a longrange magnetic order of an antiferromagnetic type below (TN= ) 18.5 K, despite a much larger value (~ 80 K) of paramagnetic Curie temperature with a positive sign characteristic of ferromagnetic interaction. We attribute this to geometric frustration. The most interesting finding is that there is an additional magnetic anomaly below ~55 K before the onset of long range order in the magnetic susceptibility data. Concurrent with this observation, the sign of isothermal entropy change, ΔS= S(0)-S(H), where H is the externally applied magnetic field, remains positive above TN with a broad peak above TN. This observation indicates the presence of ferromagnetic clusters before the onset of long range magnetic order. Thus, this compound may serve as an example for a situation in which magnetic frustration due to geometrical reasons is faced by competition with such magnetic precursor effects. There is also a reversal of the sign of -ΔS in the curves for lower final fields (H<30 kOe) on entering into magnetically ordered state consistent with the entrance to antiferromagetic state. The magnetoresistance behavior is consistent with above conclusions.

The layered compound with distorted Kagome nets, Dy3Ru4Al12, was previously reported to undergo antiferromagnetic ordering below (TN=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of ac and dc magnetic susceptibility (χ), isothermal remnant magnetization (MIRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of TN, AC χ shows a prominent frequency dependence and, below TN, MIRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-flyshaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.

Spin reorientation is a magnetic phase transition in which rotation of the magnetization vector with respect to the crystallographic axes occurs upon a change in the temperature or magnetic field. For example, SmFeO 3 shows a magnetization rotation from the c axis above 480 K to the a axis below 450 K, known as the À 4 ! À 2 transition. This work reports the successful synthesis of the new single-crystal perovskite SmFe 0.75 Mn 0.25 O 3 and finds interesting spin reorientations above and below room temperature. In addition to the spin reorientation of the À 4 ! À 2 magnetic phase transition observed at around T SR2 = 382 K, a new spin reorientation, À 2 ! À 1 , was seen at around T SR1 = 212 K due to Mn doping, which could not be observed in the parent rare earth perovskite compound. This unexpected spin configuration has complete antiferromagnetic order without any canting-induced weak ferromagnetic moment, resulting in zero magnetization in the low-temperature regime. M-T and M-H measurements have been made to study the temperature and magnetic-field dependence of the observed spin reorientation transitions.

We report a spin reorientation from Γ4(Gx, Ay, Fz) to Γ1(Ax, Gy, Cz) magnetic configuration near room temperature and a re-entrant transition from Γ1(Ax, Gy, Cz) to Γ4(Gx, Ay, Fz) at low temperature in TbFe1−xMnxO3 single crystals by performing both magnetization and neutron diffraction measurements. The Γ4 − Γ1 spin reorientation temperature can be enhanced to room temperature when x is around 0.5 ~ 0.6. These new transitions are distinct from the well-known Γ4 − Γ2 transition observed in TbFeO3, and the sinusoidal antiferromagnetism to complex spiral magnetism transition observed in multiferroic TbMnO3. We further study the evolution of magnetic entropy change (−ΔSM) versus Mn concentration to reveal the mechanism of the re-entrant spin reorientation behavior and the complex magnetic phase at low temperature. The variation of −ΔSM between a and c axes indicates the significant change of magnetocrystalline anisotropy energy in the TbFe1−xMnxO3 system. Furthermore, as Jahn-Teller inactive Fe3+ ions coexist with Jahn-Teller active Mn3+ ions, various anisotropy interactions, compete with each other, giving rise to a rich magnetic phase diagram. The large magnetocaloric effect reveals that the studied material could be a potential magnetic refrigerant. These findings expand our knowledge of spin reorientation phenomena and offer the alternative realization of spin-switching devices at room temperature in the rare-earth orthoferrites.