List of abbreviations/Symbols
1.7 Exchange Bias Effect
In 1956, Meiklejohn and Bean discovered the exchange bias (EB) effect when studying a system where Co particles were embedded in CoO [79]. In this experiment where Co is FM and CoO is AFM, the system is cooled from above the Néel temperature, TN of AFM CoO under static applied magnetic field. They had observed a shift in M-H loops along the field axis during this experiment and discovered a new exchange anisotropy which is responsible for such shifting in M-H loops [79,80]. The EB field can be qualitatively defined in terms of the M-H loop shifting which is given as:
Where HC+ and HC- are the positive and negative coercive fields of the M-H loop, respectively.
Since the discovery of EB in 1956, it was it was observed in various systems containing FM- AFM interfaces but still continued to be of fundamental interest with some other interface systems like AFM-FIM and FM-FIM [28]. EB effect has been observed in different bulk oxide materials [81]. Systems with EB effects have various useful technological applications for example magnetic recording devices [27,37–39,41], magnetic field sensors [28,44],
magnetoresistive random access memories (MRRAM) [82–87], and in permanent magnets [88–
90].
Figure 1.10 Schematic representation of EB effect in bilayer structure with appearance of a pinned ferromagnetic layer at the interface with antiferromagnetic layer having uncompensated spins (b). The hysteresis loop without shifting (a) and shifted hysteresis loop (c) presenting the exchange bias effect in the system.
Typical EB M-H loop along with normal M-H loop is demonstrated in Fig. 1.10 where AFM interface layer has uncompensated spin and coupled to the spins at the FM interface layer via the exchange interaction. Such interactions which developed from the exchange correlations at the FM-AFM interface in the cooling process of the materials were assumed to be the origin of the EB phenomenon [91]. We can discuss the origin of EB effect with a model as shown in Fig. 1.10 where FM and AFM substances are in contact with each other having their magnetic ordering temperature TC and TN, respectively where TC of FM component is greater than that of the TN of AFM component. When a static magnetic field is applied on such system at a temperature much above of the TN but below TC and then cooled through TN down to much low temperature i.e. T << TN, the FM spins at the interface of the AFM spins are getting coupled to the uncompensated AFM spins of the system. In other words, the AFM spins at the interface exert a pinning force (microscopic torque) on the ordered FM spins to keep them well aligned at the interface. During this process few layers of FM spins at the interface are pinned in to the AFM spin side, hence due to this pinning there will be some difference in the energy required to
rotate the FM spins in positive and negative field directions which is responsible for shifting in the M-H loop. Depending on the depth of this FM pinning at interface which depends on the strength of anisotropy in AFM substance, one can observe an increase or decrease in the loop shifting in M-H measurements. Such existing coupling between FM and AFM substances at interface gives rise to a displacement of the M-H loop, and a sin θ component in the torque curve which is the typical manifestations of the EB effect [91].
Figure 1.11 Temperature dependence of (a) the effective coercive field ( ), and (b) the EB field (HEB) of single crystalline Nd0.75Ho0.25Al2 [26].
Although EB effect is basically explained on the basis of interface between the FM and AFM layers and assumed to be an interfacial phenomenon, but such effect has also been observed in other materials namely, spin glasses, magnetically disordered systems, core-shell nanostructures and irregular nanostructures of metal-metal oxides etc. [92–95]. Bulk materials such as binary alloys, Heusler alloys, intermetallic compounds along with oxide materials are also known to reveal EB effect [81]. Structurally single-phase materials having the analogous structure of AFM-FM interfaces in multilayers and composites also exhibit the phenomenon of EB. Such discovery of EB effect in the single-phase alloys and compounds has renewed interest
in research community. Still there are not many reports on the investigations on the EB effects in structurally single phase materials. There are very few reports on EB effects on structurally single phase materials in charge ordered manganite having the spontaneous interfaces between short range FM clusters embedded in the AFM matrix [92].
Further, the observation of the sign change of EB with temperature across Tcomp in an ideally homogeneous single phase system [26] is the most stunning feature of this effect. This is called the tunable EB with respect to the change in temperature. Fig. 1.11 (b) depicts the change in sign of EB effect across Tcomp. Recently similar observation of sign reversal of both EB field and magnetization across Tcomp is also observed in few other bulk single phase oxides [96–98].
There are a few reports where magnetic field (or cooling field, HFC) induced tunable EB is also observed for example Sr2YbRuO6, NdMnO3 and in bilayer system of FeF2/Fe etc. [97–99]. The finding of EB effect in single phase material is due to the coexistence of FM and AFM orderings which arises because of competing interactions between the transition metal ions. Origin of EB effect in these single phase materials is quite different from other interfacial systems for example multilayers etc. Further, a sign change in EB is observed across the Tcomp which is due to both the presence of different magnetic ordering and change of exchange anisotropy between the FM and AFM spin configuration across the magnetically compensated state. Although lot of research has been done on EB effect, but the exact origin of tunable EB is under debate. Consequently, a detailed investigation is required to explore new materials, with magnetically compensated state, to observe the tunable EB effect.