Abstract
Highly Co-doped ZnO shows uncompensated antiferromagnetic order and a vertical exchange bias shift even in the absence of a ferromagnet. Therefore, it is an ideal model system to study the behavior of uncompensated antiferromagnetic moments, which play a crucial role in the description of conventional exchange bias. Temperature- and cooling-field-dependent magnetometry measurements provide further information on the compensated and uncompensated antiferromagnetic configurations in Co-doped ZnO, revealing that observed effects of both vertical exchange shift and open hysteresis stem from similar magnetic configurations. This transition is evidenced by the increase of the vertical exchange shift on the expense of the hysteresis opening by lowering the temperature.
Highlights
In a system consisting of a ferromagnetic (FM) and an antiferromagnetic (AFM) layer, the exchange bias effect may be observed [1,2,3,4]
The vertical shift, extracted according to the definition given in the inset of Fig. 1(a), is given in percent of the magnetization at +5 T to account for different sample sizes and different sample volumes
While the vertical shift is constantly increasing by lowering the temperature, the hysteresis opening reaches a maximum at around 3 K for the lower Co concentrations
Summary
In a system consisting of a ferromagnetic (FM) and an antiferromagnetic (AFM) layer, the exchange bias effect may be observed [1,2,3,4]. The coercivity Hc increases and the hysteresis loop shifts horizontally along the magnetic field axis, usually in opposite direction of the cooling field [5,6]. Higher magnetic fields are needed to reverse the magnetization of the exchange-biased FM layer compared to the nonbiased one. This is used in technical applications to pin FM layers in a certain direction, for example, in magnetic recording heads [7]. Exchange bias has been reported in nanoparticle-composite systems [8,9] and in rare cases in the absence of a FM [10,11]
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