L. Malkinski

Iron and Permalloy based magnetic monolithic tunable microwave devices

We fabricated a series of magnetic monolithic tunable microwave notch-filters and phase shifters. In contrast to previous work with molecular beam epitaxy grown metallic ferromagnets, our devices were created by magnetron sputtering. Single crystal GaAs (001) was used as a substrate. Iron and Permalloy were used as magnetic materials in a coplanar waveguide geometry. The transmission characteristics of the filters were observed to depend on substrate quality, film deposition parameters (Argon pressure, growth rate, power, etc.), and grain size. In addition we observed a substantial increase in the resonance frequency for the Fe based notch-filters. This increase in the resonance frequency is due to a growth-induced uniaxial anisotropy field of 40 kA/m in the Fe films. This is an unexpected and important result especially because the observed anisotropy is growth and not field induced. The resonance frequency shifted from 9.3 GHz at zero applied magnetic field to 15 GHz for an applied static magnetic field...

Annealing effects and degradation mechanism of NiFe/Cu GMR multilayers

Abstract Structural, transport and magnetic properties of sputtered Ni80Fe20/Cu multilayers showing giant magnetoresistance (GMR) were studied using X-ray reflectometry and diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect. In particular, mechanisms of the GMR degradation at elevated temperatures were investigated. Multilayers with an individual layer thickness of 2xa0nm show a sharp drop of the GMR after annealing at about 250°C. Whereas below this temperature grain growth and defect reduction contribute to a partial improvement of the GMR, above ∼250°C interdiffusion between Ni and Cu appears to lead to layer intermixing and to the degradation of transport and magnetic properties. Moreover, the initial 〈1xa01xa01〉 texture sharpens, and strong tensile stresses arise in the layer stack. We correlated the structural alterations to changes in the magnetic properties such as the strength of the antiferromagnetic coupling (bilinear and biquadratic) and the magnetic anisotropy. Above 250°C an increasing magnetic inhomogeneity of the Permalloy layers can be inferred from the FMR linewidth broadening.

Proximity effects at epitaxial Co/FeMn thin film systems (invited)

We prepared exchange-biased Co/FeMn structures to examine the interaction between single crystalline ferromagnetic and antiferromagnetic films with well-defined interfaces. Using x-ray magnetic circular dichroism (XMCD) and magneto-optical Kerr effect we studied in situ layered systems of fcc Co and FeMn epitaxially grown onto Cu(001) templates. XMCD studies reveal that in the FeMn layer a certain fraction of Fe is polarized at the Co/FeMn interface. The magnetic moment on the Fe atoms is parallel to the Co magnetization direction. Compared to Fe the measured dichroic signal for Mn was considerably smaller. Experiments with epitaxially grown trilayer systems of Co/FeMn/Co showed that the proximity of the ferromagnetic material influences the properties of the antiferromagnetic material leading, e.g., to a reduction of the temperature at which a marked increase of the coercive field occurs. Strong ferromagnetic and 90° coupling between the Co films observed at different thicknesses of the FeMn spacer do no...

Magnetization reversal of elliptical Co/Cu/Co pseudo-spin valve dots

We present our recent simulated results on Cr (5 nm)/ Cu (5 nm)/ Co (5 nm)/ Cu (3 nm)/ Co (2 nm) pseudo-spin valve dots. The simulated results agree qualitatively with the experimental results. Three different sizes of elliptical dots, 150u2009nm×105u2009nm, 175u2009nm×70u2009nm, and 200u2009nm×60u2009nm, were simulated. Our simulations show that in these types of dots magnetization reversal occurs by the formation of domain walls: 90° for 175u2009nm×70u2009nm and 360° for 200u2009nm×60u2009nm. No domain wall was observed in the reversal of the 150u2009nm×105u2009nm dots. For such dots, the simulated loops show a small two-step reversal pattern with the thin upper layer partially reversing followed by complete reversal of both layers at higher fields. In the larger dots, a two-step reversal is clearly observed both in the simulation and experiment.

Exchange bias in the Fe/KCoF3 system: A comprehensive magnetometry study

Exchange bias was studied in the Fe/KCoF3 ferromagnet/antiferromagnet system. KCoF3 can be deposited onto single crystal of Fe, either in the polycrystalline or single crystal form, depending on growth conditions. The samples were grown by molecular beam epitaxy on Ga-terminated GaAs (100) wafers. We study effects of the crystal state of the fluoride, thickness of the Fe film, crystallographic orientation of the Fe, and temperature on exchange bias. The structures with single crystal KCoF3 show that the exchange bias is well correlated with the coercivity at low temperatures and vanishes at a temperature close to the Neel temperature. Both the magnitude of the exchange bias and the blocking temperature of the samples with the polycrystalline fluoride were significantly reduced compared to the single crystal structures. As the Fe film thickness was increased, the exchange bias decreased for all samples. In contrast, the blocking temperature remained unchanged for the samples with the single-crystal fluorid...

Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Neel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the f...

Thermal stability and degradation mechanism of NiFe/Cu giant magnetoresistance multilayer systems

Ni80Fe20/Cu multilayers show large giant magnetoresistance (GMR) at low magnetic saturation fields. The GMR signal is known to degrade irreversibly at elevated temperatures. Clarification of the relevant deterioration mechanisms refines our basic understanding of the GMR effect and may help to improve the thermal stability of devices. We therefore investigated structural, transport, and magnetic properties of sputtered Ni80Fe20/Cu multilayers in the as-deposited state and after different anneals (up to 600u200a°C) by x-ray techniques, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). Multilayers with the second maximum of the antiferromagnetic (afm) coupling showed a sharp drop of the GMR at about 250u200a°C. The changes of the transport properties were associated with a series of structural alterations. These ranged from grain growth and defect reduction through texture sharpening and stress evolution up to the onset of interdiffusion. Interdiffusion changed the NiFe ...

Ferromagnetic resonance study of interface anisotropies in exchange-biased Fe(001)/KNiF3 bilayers

Abstract Magnetic anisotropies at epitaxial Fe/KNiF3 interfaces were probed by ferromagnetic resonance. Fe(0xa00xa01) films coupled to single crystal KNiF3 exhibit four-fold in-plane anisotropy and a unidirectional bias upon field-cooling. In Fe(0xa00xa01) with polycrystalline KNiF3, the bias direction deviates from the field-cooling direction. Lattice mismatch strain due to polycrystalline KNiF3 also induces uniaxial anisotropy in Fe.

Exchange bias and anisotropy in the Fe/KCoF3 structure

Abstract A new type of ferromagnet/antiferromagnet structure (Fe/KCoF 3 ) was deposited by molecular beam epitaxy. Unidirectional and uniaxial anisotropies of 5.1 and 3.4xa0kA/m were measured at 23xa0K using ferromagnetic resonance. Magnetization measurements at 5xa0K showed a hysteresis loop shift of 6xa0kA/m due to exchange bias. Significant enhancement of four-fold anisotropy was found at low temperatures in the samples with polycrystalline KCoF 3 structure.

Exchange bias system of Fe/KFeF3

Epitaxial Fe/KFeF3 bilayers were grown using molecular beam epitaxy on GaAs (100). FMR measurements revealed two peaks for samples with an Fe seed layer. A peak with fourfold symmetry was identified as the main layer Fe whereas a peak with twofold symmetry appeared to be due to the seed layer Fe. The magnetocrystalline anisotropy for the fourfold peak is ∼400u2009Oe at 300 K whereas a large anisotropy of 1.5 kOe is associated with the seed layer. A large rotatable anisotropy of 280±20u2009Oe was found for a sample with Fe thickness 2.1 nm. All Fe/KFeF3 bilayers checked by superconducting quantum interference device exhibit exchange bias and enhanced coercivity. The blocking temperature of the system is close to the Neel temperature 112 K of KFeF3. One sample has polycrystalline KFeF3 and shows a much larger exchange bias and coercivity than the samples having single crystal KFeF3.