Positive exchange biasing inGdFe∕NiCoObilayers with antiferromagnetic coupling

Abstract

Positive exchange biasing ͑EB͒ has first been observed in a variety of antiferromagnet ͑AF͒/ferromagnet ͑FM͒ bilayers with AF materials fluorides, in which the FM and AF layers were argued to have antiferromagnetic coupling at the interface.1 It has been detected recently in ferrimagnet/ ferrimagnet, FM/ferrimagnet bilayers with antiferromagnetic coupling and other systems. [2][3][4][5][6][7] These bilayers have two additional distinguished features. First, the exchange field H E has a crossover from negative values to positive values at a critical value of the cooling field H CF 0 . Second, the coercivity H C has a maximum at H CF 0 , in addition to the normal enhancement due to the EB. 8 Apparently, the EB strongly depends on the magnitude of the cooling field H CF . It is quite different from conventional FM/AF bilayers with ferromagnetic coupling, in which H E and H C are almost independent of H CF if it is larger than the saturation field of the FM layer. 9,10 In the strict sense, the positive EB has been found only in a few FM/AF systems.2-7 Therefore, extensive studies on the mechanism of the positive EB have been hindered and more experiments are required.In this paper, we will study the EB phenomena in FM/AF bilayers by using GdFe͑=Gd 45 Fe 55 ͒ / NiCoO͑=Ni 46 Co 54 O͒ bilayers, where NiCoO and GdFe are typical AF material and ferrimagnetic alloys, respectively.2,11 The atomic magnetic moment of Gd comes from spin and orbital angular momentums and both of them are parallel to the atomic magnetic moment. The atomic magnetic moment of Fe is contributed only from the spin angular momentum because the orbital angular momentum is almost quenched. Due to antiferromagnetic coupling between the spins of Gd and Fe, the atomic magnetic moment of Gd is aligned antiparallel to that of Fe and the macroscopic magnetization of Gd 45 Fe 55 alloys is parallel to that of Gd atoms since the latter is dominant. More remarkably, the magnetizations of NiCoO and GdFe layers can have antiferromagnetic coupling at the interface when the contribution of Gd atoms is larger than that of Fe atoms. This is because the atomic magnetic moment of Gd should also be coupled to Co and Ni atoms in AF bilayers antiferromagnetically at the interface. The compositions of the NiCoO and GdFe layers are selected so that the Néel temperature of the AF layer is lower than the Curie temperature of the GdFe layer and FM and AF layers are coupled antiferromagnetically, where the Néel temperature of the NiCoO layer is 400 K and the Curie temperature of the GdFe layer is 430 K. [11][12][13] A large specimen of GdFe/ NiCoO ͑20 nm͒ bilayer was deposited on Si͑100͒ at ambient temperature by magnetron sputtering system. The base pressure was 2 ϫ 10 −5 Pa and the Ar pressure 0.33 Pa during deposition. GdFe and NiCoO layers were made from GdFe and NiCoO composite targets by dc and rf sputtering, respectively. In experiments, small Gd pieces were put on Fe target and small pieces of CoO on NiO target to form GdFe and NiCoO composite targets. The growth rates ...

The origin of noncollinear anisotropies and asymmetric magnetization reversal in FM/AFM bilayer system

“…When h cool 4h 2 , the cooling field dominates the broken symmetry of AFM so that a net magnetization of AFM along the cooling field is expected, which leads to positive exchange bias field. This result agrees well with the experimental observation [25][26][27].…”

The effect of a cooling field on exchange bias in a nanoparticle system

“…3. After cooling, the spins in the AFM matrix at low temperature may be in a metastable state [37], and as a result different net magnetizations may appear on the AFM surface [38,39]. First, an unexpected phenomenon, namely The effect of a cooling field on exchange bias non-zero H E , can be observed in this system after cooling with an absent external field; similar results were obtained in reference [40].…”

Controllable positive exchange bias via redox-driven oxygen migration

“…Henceforth, for simplicity, Ni 0.47 Co 0.53 O and Gd x Fe 1− x will be identified as NiCoO and GdFe, respectively. The GdFe Curie temperature T C was measured to be around 480 K for samples A and B, and around 350 K for samples C and D. To establish exchange bias, the samples were heated to 420 K (above the AF NiCoO Néel temperature of T N =401 K) 26 27 in a helium flow furnace and then cooled to room temperature in the presence of an in-plane cooling field μ o H FC . Room-temperature hysteresis loops for sample A ( x =0.42) are shown in Fig.…”