Up to now, for the conventional exchange bias (EB) systems there has been one pinning phase and one pinned phase, and the pinning and pinned phases are inherent to the material and do not mutually transform into each other. Interestingly, we show here that EB is observed in a special system (-Fe2O3)0.1-(FeTiO3)0.9 (HI-9) different from the conventional EB system. Neutron powder diffraction and magnetic measurement confirm that for HI-9: i) two types of short-range antiferromagnetic ordering coexist; ii) there are two pinning phases and one pinned phase; iii) the pinned phase is not intrinsic to the structure but can be dynamically produced from the pinning phase with the help of an external magnetic field. Consequently, two anomalous EB behaviors are observed: i) both the coercivity (HC) and the exchange bias field (HE) simultaneously decrease to zero at 30 K; ii) for a high cooling field (Hcool), HE decreases logarithmically with increasing Hcool. Using Arrott plots it is confirmed that the first-order magnetic phase transformation (FOMPT) from the AFM Fe 2+ to ferromagnetic (FM) Fe 2+ and the second-order magnetic phase transformation (SOMPT) for the process whereby the FM Fe 2+ aligns with the external field direction coexist in HI-9. The Morin transition and FOMPT cause the anomalous EB behaviors. This work may provide fresh ideas for research into EB behavior.

Magnetization jumps (MJs) and the exchange bias (EB) effect are simultaneously observed in the mixed-spin oxide (FeTiO3)0.9-(Fe2O3)0.1 at 2.0 K. Dc and ac susceptibility measurements confirm a reentrant spin glass phase with a partially disordered antiferromagnetic (PDA) state below the irreversibility temperature (Tir = 60 K). Antiferromagnetic (AFM) Fe3+ clusters are nested in AFM Fe2+ lattices forming a triangular lattice, in which 2/3 of the magnetic moments order antiferromagnetically with each other leaving the remaining 1/3 “confused.” This geometric frustration in the triangular lattice leads to a PDA state that is the ground state of the AFM triangular configuration. The PDA state, in the presence of a critical trigger field, evolves into a ferromagnetic (FM) state, and induces the AFM spins of the Fe2+ ions to enter a FM state, resulting in the MJs. Meanwhile, the FM spins of Fe2+ can serve as the pinned phase, and the AFM spins of Fe3+ can serve as the pinning phase, resulting in the EB effect. Thus, we point out that the PDA state is very likely to be at the origin of the MJs and the EB effect.

In Mn50Ni41−xSn9Cox ribbons, the exchange bias field is very sensitive to the Co content. Based on both theoretical and experimental studies, it has been found that with increasing Co content, the pinned phase (ferromagnetic phase) remains almost unchanged while the pinning phase is changed from a canonical spin glass to a cluster spin glass and finally to a ferromagnetic phase. Changing the Co content in Mn50Ni41−xSn9Cox alloys has been proven to be an effective way of tuning the magnetic anisotropy and the phase structure of the pinning phase. With different Co contents, a continuous tuning of the exchange bias field from 345 Oe to 3154 Oe is realized.

No abstract

The magnetic configuration of Mn2NiAl ribbon has been investigated. In contrast to Ni2MnAl, the compound Mn2NiAl with considerable disorder does exhibit ferromagnetism and, due to exchange interaction competition, both ferromagnetic and antiferromagnetic moment orientations can coexist between nearest neighbor Mn atoms. This is unexpected in Heusler alloys. Regarding the mechanism of the martensitic transformation in Mn50Ni50−xAlx, it is found that increasing the Al content results in an unusual change in the lattice constant, a decrease of the transformation entropy change, and enhancement of the calculated electron localization. These results indicate that the p-d covalent hybridization between Mn (or Ni) and Al atoms gradually increases at the expense of the d-d hybridization between Ni and Mn atoms. This leads to an increased stability of the austenite phase and a decrease of the martensitic transformation temperature. For 11 ≤ x ≤ 14, Mn50Ni50−xAlx ferromagnetic shape memory alloys are obtained.