Volume 89, Issue 11, 01 June 2001
Index of content:
- PROCEEDINGS OF THE 8TH JOINT MAGNETISM AND MAGNETIC MATERIALS-INTERMAG CONFERENCE
- Exchange Biasing: Dynamics and Structural Effects I
89(2001); http://dx.doi.org/10.1063/1.1358821View Description Hide Description
The exchange field decays when spin valves are subjected to a field that rotates the pinned layer towards the reverse direction. The decay results from a competition between the torque on the interfacial AF spins from the pinned layer, which lowers the barrier for thermal switching, and the KuV product of the AF grains, which provide stability. Typical values of the decay rates at 125 °C vary between 5–35%/decade, depending upon the AF. A comparison of the thermal decay-rates for many AF’s important for spin valve heads shows that IrMn (blocking temperature, is the most stable, followed by NiMn (350 °C), PtMn (325 °C) and NiO (200 °C). An Arrhenius-type model is presented which fits the data well. This model is used to estimate the anisotropy constants of NiMn and IrMn to be and respectively. Thermally activated reversal of the AF results in often being less than and a peak in the pinned layer coercivity observed near
89(2001); http://dx.doi.org/10.1063/1.1358822View Description Hide Description
We performed a detailed study of the magnetization reversal in polycrystalline exchange-coupled NiO/Co bilayers over 10 decades of field sweep rate for different NiO and Co thicknesses. For all sweep rates and thicknesses, the symmetry of the hysteresis loops shows that an identical pinning strength has to be overcome in both directions of the reversal. At low the reversal is governed by domain wall displacement while domain nucleation is dominant at higher ones. The at which the transition between the two regimes takes place depends on the relative thickness of the NiO and Co layers. It increases (decreases) when the Co (NiO) thickness is increased. Experimentally, it was found that the energy barrier varies linearly with the square root of the area corresponding to the activation (Barkhausen) volume which is consistent with a random walk model of the coupling between antiferromagnetic and ferromagnetic layers. The results can be explained in terms of a thermally activated switching of the NiO magnetization dragged by the Co reversal.
89(2001); http://dx.doi.org/10.1063/1.1360676View Description Hide Description
Magnetic microstructure, exchange induced uniaxial and unidirectional anisotropy and structural transformation have been studied in PtMn/NiFe bilayer films and small elements as a function of annealing time. The relationship between the fcc-fct ordering phase transformation in PtMn and the development of exchange induced magnetic properties in PtMn/NiFe bilayers is complicated by the fact that the transformation occurs throughout the entire volume of the PtMn film, while the exchange between the layers is predominantly an interface effect. Consequently, the development of the exchange anisotropy should depend primarily on the character of the structural transformation at the interface between PtMn and NiFe. The purpose of this article is to correlate the volume phase transformation in PtMn to the development of exchange anisotropy and micromagnetic behavior in PtMn/NiFe bilayers. The interface structure can be inferred from the anisotropy and micromagnetic measurements, leading to a model that explains the relationship between the volume and interface transformation structures in PtMn, and magnetic properties of the bilayers. The structure and magnetic properties were characterized by x-ray diffraction, vibrating sample magnetometry, and magnetic force microscopy.
89(2001); http://dx.doi.org/10.1063/1.1359793View Description Hide Description
A study has been made on the effect of annealing time on magnetization reversal of the pinned layer in NiFe(100 Å)/PtMnCr(500 Å) bilayers. In the as-deposited state, the PtMnCr layer is in a metastable, nonmagnetic, disordered fcc phase. Heating progressively transforms the alloy to the stable fct phase which is antiferromagnetic, providing the pinning layer for the soft ferromagnetic NiFe layer. The samples were annealed in a magnetic field at 250 °C for 1, 1.5, 2, and 8 h. The effect of annealing is to both increase the shift of the loops along the field axis and the coercivity of the pinned ferromagnetic layer. Although the widening of the loop is correlated with the degree of antiferromagnet transformation, the exact mechanism for the increased coercivity is unclear. Measurements of loops made at different field sweep rates and after different waiting times at saturation, on the sample annealed for 1.5 h, indicate two possible mechanisms for the increased coercivity: (i) thermally activated reversal of some of the antiferromagnetic layer (AFM) during the measurement of the hysteresis loop and/or (ii) spin-flop coupling between the AFM and ferromagnet moments at a partially compensated interface.
89(2001); http://dx.doi.org/10.1063/1.1357145View Description Hide Description
Ex situ CrPtMn pinned bilayers and ex situ CrPtMn pinned spin valves have been investigated by exploring the correlation between the pinning and deposition process. It was found that exchange coupling is strongly related to the deposition condition. The ex situ deposited CrPtMn can only reliably exchange couple to the NiFeCo (or CoFe) when CrPtMn is deposited on it with an applied magnetic field. The exchange coupling is not seen as a strong function of the thickness of the removed NiFeCo (or CoFe) surface layer if the backsputter time is longer than 3 s. However, it is found that the giant magnetoresistance(GMR) value decreases with an increase of backsputter time, which is probably due to interface disruption during backsputter. It is found that 3–5 s backsputter can produce GMR values as high as in situ CrPtMn-pinned spin valves. The GMR value is around 8% for ex situ CrPtMn-pinned spin valves with a configuration of Ta(30 Å)/NiFe(45 Å)/CoFe(10 Å)/Cu(30 Å)/CoFe(41 Å)/CrPtMn(300 Å).
89(2001); http://dx.doi.org/10.1063/1.1360677View Description Hide Description
Epitaxial PdMn/Fe bilayer structures, in both a-axis PdMn(100)/Fe(001)/MgO(001) and c-axis PdMn(001)/Fe(001)/MgO(001) orientations, were grown by ion-beamsputtering. The a-axis samples were grown at low temperatures while the c-axis films were stabilized at a higher temperature range Vibrating sample magnetometry measurements show that the as-grown a-axis samples do not have a measurableexchange bias while c-axis samples have an exchange bias field However, annealing at 230 °C for 40 min results in a measurable exchange for a-axis samples due to chemical ordering. The possible cause for the difference of in a-axis and c-axis orientations is also discussed. In addition to the normal structure, inverted structures were obtained epitaxially. The exchange biasing for Fe(001)/PdMn(001)/MgO(001) is as big as 68 Oe.
89(2001); http://dx.doi.org/10.1063/1.1354576View Description Hide Description
The correlation between the exchange field of NiFe/NiMn and the phase transformation of NiMn was investigated. Transmission electron microscopy(TEM) dark-field images, contributed by the order phase of NiMn, were used to identify the location and volume fraction of the order phase. TEMselected area diffraction patterns showed the (110) superlatticediffraction rings of NiMn, verifying the existence of the order phase in the annealed samples. The order volume fraction can be calculated by the dark field image contributed by the (110) diffraction. The exchange field increased almost linearly with increasing order volume fraction. Energy dispersive x-ray spectroscopy attached to TEM indicated that Mn diffused into NiFe for annealing at leading to a larger coercivity and small coercivity squareness. Part of the NiMn still maintains the paramagnetic phase even after annealing at
89(2001); http://dx.doi.org/10.1063/1.1354577View Description Hide Description
In-plane magnetic anisotropy was studied in Co films epitaxially grown on NiMn (001) films. The hysteresis loops measured in-plane perpendicular to the exchange field show double shifted loops, and the magnetization of Co to rotate coherently. Magnetic anisotropy constants can be determined from these double shifted loops. Exchange coupling in NiMn/Co not only induces a unidirectional anisotropy but also a uniaxial anisotropy. This induced uniaxial anisotropy may contribute to the enhancement of Co coercivity.
89(2001); http://dx.doi.org/10.1063/1.1354578View Description Hide Description
Ion irradiation is an excellent tool to modify magnetic properties on the submicrometer scale, without modification of the sample topography. We utilize this effect to magnetically pattern exchange biasdouble layers using resist masks patterned by electron-beam lithography. Ion irradiation through the masks leads to a lateral modification of the magnetization reversal behavior and allows one to study the magnetization reversal as a function of the exchange bias field strength on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and magnetic force microscopy, respectively.
Enhancement of exchange bias in Mn–Ir/Co–Fe based spin valves with an ultrathin Cu underlayer and in situ Mn–Ir surface modification89(2001); http://dx.doi.org/10.1063/1.1357146View Description Hide Description
Enhancement of exchange bias induced at the interface of the antiferromagnetic (AF)/ferromagnetic (F) layers was studied using the bottom “spin-valve films” (SVs) with the Mn–Ir/Co–Fe exchange coupled films.Exchange bias increased using an ultrathin Cu underlayer. Meanwhile, both exchange bias field, and blocking temperature, increased intensively by heating specimens after depositing Mn–Ir film in a high vacuum. These two enhancement effects worked in an additive. As a result, an unidirectional anisotropy constant, of 0.39 erg/cm2 of 1.3 kOe) and of ∼325 °C were obtained for the bottom SVs with a total thickness of 233 Å including an AF layer of 68 Å and a pinned layer of 20 Å where the SVs were field annealed at 320 °C. A microstructural analysis using x-ray diffraction revealed that did not depend on the diffraction intensity from Mn–Ir (111) for the SVs with various underlayers, and no remarkable changes occurred in the microstructure of the SVs with the heating treatment in a vacuum. Therefore, the enhancement effects might result from some changes in the microstructure and/or the morphology of the interface of AF/F layers.