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We observe magnetization dynamics induced by spin momentum transfer in the noise spectra of current perpendicular to the plane giant magnetoresistance spin valves. The dynamics are observable only for those combinations of current direction and magnetic configuration in which spin transfer acts to reorient the free layer magnetization away from the direction set by the net magnetic field. Detailed measurements as a function of magnetic configuration reveal an evolution of the noise spectra, going from a spectrum with a well-defined noise peak when the free layer is roughly collinear with the pinned layer to a spectrum dominated by f 1 noise when the free layer is in an orthogonal configuration. Finally, the amplitude of the corresponding resistance noise increases rapidly with increasing current until it saturates at a value that is a substantial fraction of the magnetoresistance between parallel and antiparallel states.

The magnetization state of the ferromagnet is crucial in the cooling process for establishing exchange coupling in ferromagnet/antiferromagnet bilayers. Using special cooling procedures, the value and even the sign of the exchange bias field in several bilayers can be greatly altered. While the coercivity only depends on temperature, the exchange bias field shows an accumulative memory of the thermal and field history of the bilayer. We propose that this is due to the formation of a domain wall with a temperature dependent width in the antiferromagnet. ͓S0163-1829͑99͒02130-X͔ A great deal of experimental and theoretical attention has been focused on the intriguing physics of the exchange coupling between a ferromagnet ͑FM͒ and an antiferromagnet ͑AF͒, and the central role of exchange bias in spin-valve devices. To establish the exchange coupling experimentally, it is a common practice to cool the FM/AF bilayer in a dc magnetic field from TϾT N to lower temperatures, where T N is the Néel temperature of the AF. 1 In the cooling process, the AF order is established while the FM layer is in the single-domain state. The resultant exchange coupling causes the hysteresis loop of the FM layer to shift by the amount of the exchange bias field (H E ), accompanied by a larger coercivity (H c ) than that of the uncoupled FM layer. After this common cooling process, the values of both H E and H c decrease with increasing temperature 1-7 until the so-called blocking temperature (T B ) at which H E vanishes and H c retains its uncoupled FM value. For some AF ͑e.g., CoO͒, the values of T B and T N are essentially the same, whereas in others ͑e.g., NiO͒ T B can be noticeably lower than T N . 2 It has often been taken that once cooling across T N has been accomplished, a unique exchange coupling has been established. We show in this work that both the value and the sign of H E depend on the cooling process, in which the state of the FM layer is of key importance. More importantly, we show that the resultant exchange coupling retains an accumulative memory effect of the entire cooling procedure. Not only the value and sign of H E can be tailored, but the socalled blocking temperature T B can also be manipulated to have virtually any value less than T N . On the other hand, the value of H c is uniquely defined at each temperature, independent of the cooling history and the resultant values of H E and T B . The unidirectional anisotropy, which gives rise to H E , can thus be altered and manipulated, while the uniaxial anisotropy associated with H c remains unchanged.These results are relevant to the microscopic origin of the exchange coupling. It has been generally accepted that the FM/AF coupling is due to the interactions among the FM and the AF moments across the FM/AF interface. Most micromagnetic models, with or without interfacial roughness and defects, assume certain spin structures for the FM and the AF layers or allow the FM and the AF moments to arrive at a spin structure through their interactions. [8][9][10][11][12] The ...