Large Exchange Bias Effect Research Articles

Exchange bias effect in the martensitic state of Ni-Co-Mn-Sn film

We report a relatively large exchange bias (EB) effect in epitaxially grown Ni-Co-Mn-Sn film. EB effect has been observed at low temperature below 60 K with a maximum EB field of 139 Oe at 5 K which is high compared to Ni-Mn-Sn thin films. Systematic studies on the magnetic properties show that both EB field and coercivity decreased with increase in temperature. Martensitic transformation at room temperature was confirmed from the atomic force microscopy micrograph exhibiting twin pattern. The observed EB behavior in Ni45.0Co4.4Mn41.5Sn9.1 film is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system.

Magnetic properties and exchange bias effect of the layered manganese oxychalcogenides La2O3Mn2Se2

The magnetic properties and exchange bias effect is investigated in the polycrystalline sample La2O3Mn2Se2 synthesized by conventional solid-state reaction. The susceptibility data show a broad maximum near 260 K, indicating the existence of two-dimensional short-range ordering in this compound. The sharp cusp in the zero field-cooled susceptibility at about 162 K signifies the antiferromagnetic (AFM) phase transition, and the upturn means the existence of the weak ferromagnetic (FM) magnetization. Moreover, the temperature dependence of the Laudé g-factor obtained by the electron spin resonance spectrum shows a sharp upturn at about 165 K close to the Néel temperature TN determined by the susceptibility measurement. These results further demonstrate the coexistence of the AFM and FM magnetization in the compound. Through measuring the hysteresis loops, a large exchange bias effect is observed in the compound implying the potentially technological applications of La2O3Mn2Se2. The exchange coupling at the FM/AFM interface is responsible for the exchange bias phenomena.

Magnetic field-induced martensitic transformation, superspin glass and exchange bias in Heusler alloys NiCoMnSn

The crystal structures and magnetic properties of Ni50-xCoxMn39Sn11 (8≤x≤10) Heusler alloys are investigated. As a result, we achieve the magnetic field induced martensitic transformation in Ni42Co8Mn39Sn11. It is found that the saturation magnetic moments of alloys increase with Co content increasing. Moreover, a superspin glass behavior and a large exchange bias effect are also found in samples with Co content being higher than 8. We confirm the coexistence of superspin glass of the martensite and ferromagnetic parent phase, which is the physical origin of the exchange bias effect. On the other hand, we propose that the origin of superspin glass in our NiMnCoSn system is due to the occurence of Mn-Mn cluster as reported by Ma et al. in Heusler Mn2Ni1.6Sn0.4 alloys [Ma L, Wang W H, Liu J B, Li J Q, Zhen M, Hou D L and Wu G H 2011 Appl. Phys. Lett. 99 182507].

Exchange bias effects in epitaxial Fe3O4/BiFeO3 heterostructures

High quality Fe3O4 (5–20 nm)/BiFeO3 (BFO) heterostructures have been grown by pulsed laser deposition on (001) SrTiO3 (STO) substrates. Large exchange bias (EB) effects have been observed below 200 K in Fe3O4/BFO/STO structures compared with Fe3O4/STO structures. The exchange bias field decreases when increasing temperature or the thickness of Fe3O4 layer, and does not change after annealing in 0.1 T field from 700 K to room temperature across Neel temperature of BFO. A possible mechanism was proposed based on the interface spin coupling between Fe3O4 and BFO. Our work shed new light on the study of exchange bias effect in BFO system as well as the application for magnetoelectric devices.

Large exchange bias obtainable through zero-field cooling from an unmagnetized state in Ni-Mn-Sn alloys

We report a large unusual exchange bias effect occurred after zero-field cooling from an unmagnetized state (ZEB) in Ni-Mn-Sn alloys, in which the maximum ZEB field is about twice as large as that observed in Ni-Mn-In alloys at moderate field range (< 40 kOe). The ZEB field can be tuned by tuning the magnitude of the initial magnetization fields and its sign is strongly dependent on the direction of the initial magnetization fields. Furthermore, the ZEB effect only exists in alloys with volume fraction of superparamagnetic domains less than the percolation limitation in three-dimensional system (∼16%). The present results confirm the universality of this unusual ZEB effect and suggest that NiMn-based alloys provide a platform to obtain this unusual ZEB effect. Such ZEB effect may also exist in other systems with similar magnetic properties.

Stabilization of surface spin glass behavior in core-shell Fe67Co33–CoFe2O4 nanoparticles

Magnetic properties of Co33Fe67–CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at TF∼175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature (TF(H)) following the well known Almeida–Thouless line, δTF∼H2/3. Additionally, the particles exhibit a large exchange bias (HEB∼1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell.

Manipulating the Magnetic Structure of Co Core/CoO Shell Nanoparticles: Implications for Controlling the Exchange Bias

We present an experimental study of the effects of oxidation on the magnetic and crystal structures of exchange biased epsilon-Co/CoO core-shell nanoparticles. Transmission electron microscopy measurements reveal that oxidation creates a Co-CoO interface which is highly directional and epitaxial in quality. Neutron diffraction measurements find that below a Néel temperature TN of approximately 235 K the magnetization of the CoO shell is modulated by two wave vectors, q1=(1/2 1/2 1/2)2pi/a and q2=(100)2pi/a. Oxidation affects the q1 component of the magnetization very little, but hugely enhances the q2 component, resulting in the magnetic decompensation of the core-shell interface. We propose that the large exchange bias effect results from the highly ordered interface between the Co core and CoO shell, and from enhanced core-shell coupling by the uncompensated interface moment.

Optimized exchange biasing by controlled in situ oxidation

A new, simple method is presented for the preparation of thin Co/CoO bilayers exhibiting considerably improved characteristics of the magnetic behavior and large exchange bias effects. The ultrathin antiferromagnetic layer is formed at room temperature by controlled in situ exposure of a clean metal layer to pure oxygen gas. The exposure amounts only 100–10 000 L at an oxygen pressure of 10−5 mbar. High purity gases can be used. This method allows to maintain optimum ultrahigh vacuum conditions in the preparation chamber which is an indispensable condition for the preparation of complex layer systems.

Large exchange bias and its connection to interface structure in FeF2–Fe bilayers

Large exchange bias effects (ΔE∼1.1 erg/cm2) were observed in antiferromagnetic (FeF2)–ferromagnetic (Fe) bilayers grown on MgO. The FeF2 grows along the spin-compensated (110) direction. The FeF2–Fe interface roughness was characterized using specular and diffuse x-ray diffraction and atomic force microscopy. The magnitude of the exchange bias field HE increases as the interface roughness decreases. These results imply that magnetic domain creation in the antiferromagnet plays an important role.

The role of interface crystalline and magnetic structure in exchange anisotropy (abstract)

Large exchange bias effects were measured in antiferromagnetic FeF2-ferromagnetic Fe bilayers. The interface spin structure and roughness was controlled by using different substrates and growth temperatures. The samples were characterized ex situ using low and high angle x-ray diffraction, in-plane x-ray diffraction, and atomic force microscopy. We discovered that the exchange bias is largest in the (110) FeF2 spin-compensated surface films, in contradiction with simple models that predict a low exchange anisotropy for spin-compensated surfaces. For these samples, the exchange anisotropy decreases as the interface roughness increases.