K. Zhang

Training effect of exchange biased iron–oxide/ferromagnet systems

Substantial training effect in hysteresis loops was observed in ferromagnet (F)–antiferromagnet (AF) exchange coupled systems of the structure, Si(100)/iron–oxide(tAF)/F layer (5 nm)/Ta(10 nm), prepared by magnetron sputtering. The exchange bias field and the coercivity decrease with increasing number of loop cycles. The initially asymmetric hysteresis loop becomes more symmetric as the number of loop cycles increases. The effect is more prominent in the descending curve starting from the saturation in the positive (parallel to the pinned direction) direction than in the ascending curve of the hysteresis loop. The value of the ascending curve switching field |Hsw+| is rather unchanged while the switching field |Hsw−| in the descending curve decreases drastically with repeated hysteresis loop cycling as was observed in some CoO/Co and NiO/NiFe systems. Those phenomena can be interpreted by a modification of the extended model of Fulcomer and Charap’s: the essential modification being the introduction of po...

Training effect in ferro (F)/antiferromagnetic (AF) exchange coupled systems: Dependence on AF thickness

Systematic investigation on the training effect in polycrystalline ferro (F)/antiferromagnetic (AF) exchange coupled systems using sputtered NiFe/IrMn bilayers has revealed that the type of the training effect changes from one type (Type I) to another (Type II) with increasing the AF layer thickness. In Type I the hysteresis loop in the pinned direction shrinks from both sides with the cycle of the applied field, while in Type II it shifts toward the positive pinned direction accompanied by loop shrinkage. This phenomenon can be understood as the stabilization of both AF grains contributing to the enhancement of coercivity and to the exchange biasing with cycles of applied field. This conclusion is based on our recently proposed extended Fulcomer and Charap’s model in which direct positive and negative exchange coupling between the AF grains is incorporated.

Effect of direct exchange coupling between antiferromagnetic grains on magnetic behavior of ferro/antiferromagnetic exchange coupled polycrystalline layer systems

Two fundamental properties, unidirectional rotational hysteresis and training effect, of ferro/antiferromagnetic exchange coupled polycrystalline layer systems are discussed in which a direct exchange coupling between antiferromagnetic grains is found to play a decisive role. A new model is proposed to describe the systems. The analysis of some experimental data suggests the existence of preferential angular distribution of anisotropy axes of antiferromagnetic grains.

Rotational hysteresis of torque curves in polycrystalline ferro/antiferromagnetic systems

Rotational hysteresis of ferromagnetic (F)/antiferromagnetic (AF) exchange coupled systems was studied by using NiFe/IrMn, NiFe/FeMn, and NiFe/NiMn samples sputter deposited under almost the same conditions, although the sample of NiFe/NiMn was annealed later to obtain the antiferromagnetic phase for the NiMn layer. The rotational hysteresis of each sample exhibited quite a different feature from each other, especially in the rotational hysteresis–magnetization angle curves. The NiFe/IrMn bilayer showed a dip at around the antiparallel direction to the applied field direction during sample preparation (pinning direction), while the NiFe/NiMn bilayer exhibited a large peak in that direction. The NiFe/FeMn bilayer did not show any noticeable structure in the rotational hysteresis–magnetization curve. Based on the analysis of those data done by using the model recently proposed by the authors, the following was inferred: (1) The distribution of the anisotropy easy axes of IrMn of the NiFe/IrMn bilayer grains...

Biquadratic coupling effect on magnetoresistance response curves of spin valves with a synthetic antiferromagnet

Both normal and inverse magnetoresistance (MR) response curves were observed for synthetic spin valves with the structure of Si(100)/Ru/Co(t1)/Ru(0.7u200anm)/Co(t2)/Cu(3u200anm)/Co(t3)/Ru. Under the assumption of a coherent rotation of the magnetization in the three Co layers, the hysteresis loops, magnetization response, and MR response curves were calculated as a function of the parameters of the system. The parameters include antiferromagnetic coupling of Co(t1) and Co(t2) through Ru layer, a weak ferromagnetic coupling of Co(t2) and Co(t3) through Cu spacer, giant magnetoresistance of the Co/Cu/Co and Co/Ru/Co systems, and AMR contribution of each Co layer. The uniaxial anisotropy of each Co layer and a distribution of the coupling strength of Co(t1)/Ru/Co(t2) were also included. To fit the experimental data well, it was necessary to include a biquadratic coupling in the Co(t1)/Ru/Co(t2) trilayer.

Annealing effect on the recovery of training effect in FeO/sub x//CoFe polycrystalline systems

Summary form only given. In our previous work (Zhang et al. JMMM vol.235, p.319-328, (2001)), we reported that the training effect observed in iron-oxide/ferromagnet (F) exchange coupled bilayer systems is caused by the combination of locking, releasing, and stabilizing of the net surface moments of the AF grains during the training cycles, and pointed out that the thermal effect also plays some role in the training process. In the same systems we observed some recovery of training effect at room temperature. To investigate this phenomenon we performed an annealing experiment. A sample with the structure of Si(100)/FeO/sub x/(70 nm)/CoFe(5 nm)/Ta(10 nm) after 25 cycles of training was annealed at 70/spl deg/C with a magnetic field applied along the pinned direction up to 32 hours with successive interruptions for the measurement of the exchange bias field H/sub eb/, coercivity H/sub c/ and switching fields H/sub sw//sup +/ (pinned direction) and H/sub sw//sup -/ (opposite direction). The training effect after annealing was confirmed to be similar to the as-deposited state with some difference in detail. The activation energy for the recovering process was estimated to be about 1.13 eV by curve fitting of H/sub sw//sup -/, indicating that the distribution of activation energy responsible for the annealing effect is rather narrow. This is consistent with the report by Hou (1999), in which vacancy relocation model was proposed as a mechanism for changing the effective net surface spin moment of each AF-grain by low temperature annealing. This implies that the thermal switching effect (switching of whole spin configuration in a gain) in training and the thermal effect in recovery is different from each other.

Thermal relaxation in exchange coupled ferromagnet/antiferromagnet bilayers

Summary form only given. Stability of exchange-coupled F/AF bilayers is a very important issue for magnetoresistive (MR) read head for high density magnetic recording. Many different effects, such as training effect, sweeping-rate dependent coercivity, thermally-assisted switching of AF spins and annealing effect through atomic diffusion at F/AF interface, are involved in this issue. Strong thermal effect was observed in Py(10nm)/IrMn(xnm) bilayer system, for example, in sample with x=5, the easy-axis hysteresis loop changed drastically after putting the sample at a hard-direction remanent state for several months at room temperature. It implies that the pinned directions of some parts of the sample reversed during the waiting period. This can be ascribed to thermally-assisted switching of the AF surface spins driven by the interface F/AF exchange coupling in the regions (magnetic domains) with the F moment antiparallel to the initial pinned direction. Dependence of switching field on the waiting time at the saturation state was measured to probe the switching mechanism of the AF surface spins. The switching field first shows a linear dependence on the logarithm of the waiting time and then tends to saturate. The linear part, extending three decades, indicates a wide distribution of activation energy. Thermal dynamics of the AF spins was studied by measuring the samples with different AF layer thickness at different temperatures. The domain structures at different magnetizing states were observed using magnetic force microscope.

Angular dependence of remanence of exchange-coupled CoFe/NiO bilayers

The magnetic properties of Co90Fe10(5u200anm)/NiO(1–5u200anm) bilayers fabricated by rf/dc magnetron sputtering were studied. The bilayers show enhanced uniaxial anisotropy, but no unidirectional anisotropy. The angular dependence of the remanence of these samples shows hysteresis around the hard direction, which increases with increasing NiO layer thickness and was ascribed to switching of the surface spins of the antiferromagnetic layer driven by the exchange coupling between CoFe and NiO layers. This phenomenon can be simulated using a model considering the preferential distribution of the easy axes of the antiferromagnetic grains. A method for extracting information about interlayer coupling and anisotropy of antiferromagnetic layers from the angular dependence of remanence is proposed.