J. C. Eckert

Magnetic structure variations during giant magnetoresistance training in spin valves with picoscale antiferromagnetic layers

Microscopic models of exchange bias focus on the formation of domains in the antiferromagnet or the ferromagnet, or on a small induced moment in the antiferromagnet. Previous giant magnetoresistance (GMR) measurements, however, reveal exchange bias and training effects in CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 0.4 nm. Polarized neutron reflectometry studies of a related spin valve with a 1.6 nm IrMn layer were carried out for several points along the GMR hysteresis curve to probe separately the free and pinned CoFe layers. These measurements confirm that the two ferromagnetic CoFe layers are aligned in parallel in saturating fields. During the first field cyle, regions of high resistance correspond to an antiparallel alignment of the CoFe layers as expected. Significant changes in this antiparallel structure are observed during the second field sweep, and a magnetic spiral forms and persists in the pinned CoFe layer. High-field saturation seems to reduce the effectiveness of ...

Exchange bias and giant magnetoresistance in spin valves with Angstro/spl uml/m-scale antiferromagnetic Layers at 5 K

We have studied the effects on the exchange bias of decreasing the antiferromagnetic layer to the Angstro/spl uml/m-scale regime in order to shed light on the minimum required thickness of the antiferromagnet. We have deposited IrMn layers between 0.2 and 2 nm on spin valves and measured the exchange bias by examining hysteresis loops at 5 K using the giant magnetoresistance of the spin valves. The exchange bias persists for IrMn thicknesses down to 0.4 nm and has a maximum at 1.6 nm. Because the ultra-thin layers create an exchange field, the origin of at least one component of exchange biasing must have a similarly short length scale.

Properties of thin IrMn in exchange biased multilayers

We report striking behavior in thin IrMn exchange-biased heterostructures. We have studied exchange-biased multilayers with IrMn thicknesses from 2 to 80 A with 50-A Ti over- and underlayers. The resistance of spin valves with Si/50-A Ti/40-A NiFe/8-A Co/30-A Cu/30-A Co/tIrMn/50-A Ti as a function of temperature shows anomalous behavior for 12 A<tIrMn<26 A. These features are not seen for spin valves in which the ferromagnetic layers are replaced with CoFe. To isolate the effects of the IrMn, resistance and magnetization versus temperature for structures of 50 A Ti/ tIrMn /50 A Ti were measured. Our measurements are suggestive of a model involving a mixed hcp/fcc phase of IrMn, in which the hcp phase undergoes a magnetic phase transition, and the resistance is influenced by a combination of the magnetic transition and the structural properties of the IrMn layer.

Determining the spin dependent mean free path in Co90Fe10 using giant magnetoresistance

The spin dependent mean free path in Co90Fe10 is determined as a function of temperature down to 5K using two different spin valve structures. At 5K the spin dependent mean free path for one structure was measured to be 9.4±1.4nm, decreasing by a factor of 3 by 350K. For the other structure, it is 7.5±0.5nm at 5K and decreased by a factor of 1.5 by 350K. In both cases, the spin dependent mean free path approaches the typical thickness of ferromagnetic layers in spin valves at room temperature and, thus, has an impact on the choice of design parameters for the development of new spintronic devices.

Low temperature properties of spin valves with extremely thin IrMn

The properties of IrMn spin valves with tIrMn⩽26 A are explored at temperatures down to 5 K. The structure is: Ti(50 A)/NiFe(40 A)/Co(8 A)/Cu(30 A)/Co(30 A)/IrMn(t A)/Ti(50 A) on a Si substrate. The low temperature giant magnetoresistance(GMR) is not greatly affected by the thickness of the IrMn. The thinner the IrMn, the lower the temperature at which the GMR is adversely affected. This is consistent with a reduction in blocking temperature. A number of interesting features in the coercivity and field training are IrMn thickness dependent.

Thickness of the pinned layer as a controlling factor in domain wall formation during training in IrMn-based spin valves

Studies of CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 1.6nm have demonstrated that a domain wall parallel to the surface develops in the pinned layer after training at the magnetoresistance (MR) maximum. To investigate the effects of domain wall formation on the MR, we have studied the depth profile of the vector magnetization in comparable spin valves, with pinned ferromagnetic (FM) layer thicknesses, from 1to15nm, using polarized neutron reflectivity. At the maximum MR achieved after training, the antiparallel magnetization of the pinned layer, in a 2nm sample, is reduced to 5% of its saturation value, suggesting the formation of domain walls perpendicular to the surface. In a 9nm sample, the pinned layer magnetization is instead canted away from the field at the MR maximum. A transition from perpendicular to parallel domain wall formation occurs for pinned layer thicknesses greater than 4nm, and the magnitude of the maximum MR subsequently depends on the type of domain wall th...

Magnetic resistivity measurements in nickel films for CIW and CPW domain geometries

We report magnetoresistance measurements on Ni films with two stripe domain orientations. Measurements were taken for both current parallel to the domain walls (CIW) and current perpendicular to the domain walls (CPW). We observe a negative magnetoresistance for the field applied perpendicular to the plane. For CIW, we observe an added resistance, /spl Delta/R/sub CIW//R(2 T), that vanishes after an applied field removes the ordered domains. The /spl Delta/R/sub CPW//R(2 T) is comparable to the /spl Delta/R/R(2 T) for the disordered domain structure observed after the field is removed. We find /spl Delta/R/sub CPW///spl Delta/R/sub CIW/ is less than one, which is not consistent with previous work.

Determination of Complex Magnetic Structures From Polarized Neutron Reflectivity Data by Flexible Modeling of Depth-Dependent Vector Magnetization

In multilayer systems with exchange-coupled layers such as exchange-spring magnets, interfacial pinning can give rise to spiral domain walls and other complex magnetic structures that are sensitive to temperature, relative layer thicknesses, etc. Though these spin structures develop in subsurface layers, the depth-dependent magnetic profile can be fully characterized using polarized neutron reflectivity (PNR). In order to obtain the profile of the vector magnetization as well as the chemical composition, these data are typically analyzed using software in which the sample is described by a series of flat layers. This approach is cumbersome for continuously varying depth profiles, such as magnetic spirals, since the magnetic layers must be artificially subdivided to mimic the smooth changes in the vector magnetization. Thus, we have developed a flexible PNR fitting program in which users can specify a formula for the model (e.g., flat, power law, or piecewise polynomials). The program can easily be extended to handle simultaneous fitting of multiple data sets from measurements made with different techniques (such as PNR and X-rays) with constraints between the models.

Magnetic Reversal Mechanisms in Spin Valves with Pico-Scale Antiferromagnetic Layers

Giant magnetoresistance (GMR) studies at 5K have verified the presence of exchange bias in spin valves with AFM layers as thin at 0.4 nm. To probe the stability of such systems at these threshold values, a comparable spin valve with AFM thickness 0.4 nm is studied using polarized neutron reflectometry (PNR) and GMR techniques.

Ferromagnetic Relaxation in Spin Valves With Picoscale Antiferromagnetic Layers

We have investigated the mechanism of weak exchange bias for a spin valve with threshold layer antiferromagnetic (AFM) thickness using giant magnetoresistance (GMR) and polarized neutron reflectometry (PNR). Results show that the sample exhibits instantaneous switching of the free layer followed by a gradual reorientation of the magnetization in the pinned ferromagnetic layer via domain wall formation. During subsequent field cycles, we found that relaxation in the pinned ferromagnetic (FM) layer is induced not only by an increasing field, but also in a static field over a relatively long time scale