L. E. Fernandez-Outon

Measurement of the anisotropy constant of antiferromagnets in metallic polycrystalline exchange biased systems

A method for the measurement of the anisotropy constant of the antiferromagnet (AF) KAF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature ⟨TB⟩ of a CoFe∕IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5±0.5)×106erg∕cc has been obtained at room temperature for a 4nm thick IrMn layer.

Antiferromagnetic grain volume effects in metallic polycrystalline exchange bias systems

In this work we present a new interpretation of the role of the antiferromagnetic (AF) grain size in polycrystalline exchange bias thin films. It is found that at a finite temperature AF grains with sizes below a given critical volume VC do not contribute to the loop displacement because they are not thermally stable. There is a second critical volume VSET above which the AF grains cannot be set due to their anisotropy energy being too large. Therefore, only grains in the window between VC and VSET contribute to the loop displacement. Using this interpretation we can explain both the increase and the decrease in the exchange field with the AF grain size and the layer thickness.

Control of the setting process in CoFe/IrMn exchange bias systems

The time and temperature dependences of the exchange field achieved by control of the setting conditions in CoFe/IrMn exchange couples have been investigated. This has been achieved by varying the temperature at which the antiferromagnet was set (TSET) and by measuring the time dependence of the exchange field (Hex) at each temperature. The exchange field was observed to vary logarithmically with setting time, indicating that a wide distribution of energy barriers is present in the antiferromagnetic (AF) layer. The results are consistent with an independent AF grain model of exchange bias where the energy barrier distribution to reversal is governed by the grain volume distribution and the temperature dependence of the magnetocrystalline anisotropy. Calculations of the time dependence of the setting process are in good agreement with the grain volume model.

Large Exchange Bias IrMn/CoFe for Magnetic Tunnel Junctions

We report on the enhancement of the exchange field, Hex , in IrMn/CoFe systems as a result of the application of field annealing procedures. The exchange field is enhanced up to a 37% on application of a 2 Tesla field for 90 minutes at 225degC. This setting process improves the magnitude of HEX without significant changes in the coercivity, HC, or the median blocking temperature. The sample with largest exchange bias field measured, 3.7 kOe and HC = 0.2 kOe at 20degC, seems to have the largest ratio of HEX to HC ever reported for an IrMn/CoFe exchange bias system. This makes these structures good candidates as pinned layers in magnetic tunnel junctions. The samples show good thermal stability with median blocking temperatures up to 200degC. The results also indicate that the improvement in the quality of the interfaces via the degree of alignment of the interfacial spins improves the coupling across the interface and hence the magnitude of the exchange bias field.

Interfacial spin effects on Hex in metallic polycrystalline exchange biased systems

In this work, we show how the magnitude of the annealing field affects the antiferromagnetic (AF) spin structure in an exchange biased trilayer with two ferromagnetic layers with different thicknesses pinned by a single AF layer. The value of Hex is moderated by the interfacial spin structure, while the mean blocking temperature remains constant. We find a 24% increase in Hex when the setting field (Hset) used in the field cooling procedure is increased from 0.25to20kOe, while all other setting conditions are kept constant. We show that the reversal of the order in the AF gives the same mean blocking temperature, 442±2K, while the maximum value of Hex increases with Hset from 220to286Oe. The superposition of the variation of Hex with the degree of order of the AF reveals the dependence of the blocking temperature TB on the bulk of the AF, while the magnitude of the exchange field is shown to be dependent not only on the order in the AF but also on the degree of alignment of the interfacial spins determine...

Bulk and interfacial effects in exchange bias systems

In this work we report on an extensive and detailed study of exchange bias systems consisting of two ferromagnetic layers deposited with an antiferromagnetic layer of IrMn between them. Systems with two different ferromagnetic layers have been studied in which sample 1 had CoFe layers of different thicknesses and sample 2 had not only ferromagnetic layers of different thicknesses but also the composition of the upper ferromagnetic layers was changed from CoFe to NiFe. In both the samples the antiferromagnetic layer thickness was maintained constant at 5 nm, NiCr seed and capping layers (5 nm) were used on all samples. Such a system is of considerable interest as the properties of each ferromagnetic layer are affected by the same antiferromagnet. However differences in behaviour will occur due to the nature of the interfaces between the different layers as well as other parameters such as the ferromagnetic layer thickness. We have conducted a study of thermal activation effects in these systems where both or a single ferromagnetic layer can be reversed whilst the system is heated. We find that we can differentiate between bulk and interface effects indicating that the role of spin disorder at the interface is crucial in determining the final value of the exchange bias.

Effect of the Ferromagnetic Layer Thickness on the Blocking Temperature in IrMn/CoFe Exchange Couples

In this work we present a study of the thermal stability of IrMn/CoFe exchange couples as a function of the thickness of the ferromagnetic (F) layer tF. We find a monotonic decrease in the average blocking temperature (TB) with tF of up to 29% for tF = 12 nm. We have quantified the effect of the exchange field (H*) from the F layer on the antiferromagnetic (AF) layer and its effect on the thermal stability of the AF. H* can increase the degree of thermal activation by up to 44%.

Magnetic Properties of Nanocrystalline Co Thin Films Grown on Glass

Co thin films have been deposited by sputtering on glass substrates. The film thickness ranges from 10 to 170 nm. Structural results show that the films are polycrystalline with a dominant hcp structure. An ultrafine distribution of crystallites with diameters about 5 nm is found for the thinnest samples, which is also detected by a smooth surface. The grain size grows as the thickness increases in consequence an evolution occurs from a well-defined uniaxial anisotropy to a more isotropic behavior. This gives rise to an increase in the coercivity, domain wall pinning strength, and its dispersion. The magnetization reversal mechanism is also influenced by the film thickness. Using transverse susceptibility measurements we have found that the maximum domain wall pinning strength increases significantly as the film thickness increases.

Influence of seed layer on magnetic properties of laminated Co65Fe35 films

CoFe alloys have important applications in recording heads since they have a high MS which enables writing to high coercivity media. They also can have a high coercivity which can hinder applications. Previous studies have attempted to reduce the coercivity by the use of seed layers, process conditions, or lamination. We have used a high target utilization sputtering system that allows control of grain size to study laminated Co35Fe65 films with 15A Al2O3 spacer layers. Samples were fabricated with Ru, Ni81Fe19, and Ta seed layers, as a single layer, a bilayer, or a trilayer. For samples with Ru and Ni81Fe19 seed layers, the grain size was reduced with increasing lamination resulting in a significantly reduced coercivity. Samples with a Ta seed layer showed the opposite trend and an increase of coercivity was found as the number of laminations increased.

Thermal activation of bulk and interfacial order in exchange biased systems

In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately ∼50% of the exchange bias, while the remaining 50% is due to the or...