Antiferromagnetic Coupling Maximum Research Articles

Interlayer exchange coupling in perpendicularly magnetized Pt/Co/Ir/Co/Pt structures

The occurrence of both interlayer exchange coupling and perpendicular magnetic anisotropy is evidenced in Pt/Co/Ir/Co/Pt magnetron sputtered structures. We point out the effect of lattice strains on stabilizing the perpendicular magnetic anisotropy. The surface magnetic anisotropy constant is around 1.76 erg cm−2 and around 1.5 erg cm−2 for the Co layers, depending on their positioning within the stack. We demonstrate a relatively high interlayer exchange constant of −2.5 erg cm−2 at the first antiferromagnetic coupling maximum corresponding to an exchange field larger than 12 kOe. The interlayer exchange coupling shows remarkable annealing endurance being preserved for annealing temperatures up to 400 °C.

Temperature dependence of interlayer coupling and magnetization in amorphous-FeNiB/Ru multilayers

For amorphous-FeNiB (2.0 nm)/Ru multilayers, an oscillatory interlayer coupling is observed with the variation of the Ru spacer thickness. At 0.8 nm thick Ru layer, near the first antiferromagnetic coupling maximum, the energy of the effective interlayer coupling at low temperature T is found to change as a linear function of T 3 / 2 . At the same time, the spontaneous magnetization of the samples at low temperature scales as a linear function of T. Therefore, the temperature dependence of the energy of the effective coupling might be controlled by thermal spin waves in the ferromagnetic layers.

Comparison of a prototype magnetoresistive biosensor to standard fluorescent DNA detection

We present a comparative analysis of a magnetoresistive biosensor to standard fluorescent DNA detection. The biosensor consists of giant magnetoresistive (GMR) type Cu/Ni 80Fe 20 multilayers in the second antiferromagnetic coupling maximum. Each of the 206 elements of the magnetoresistive biosensor is patterned into a spiral-shaped line that can cover the area of a typical DNA spot (70 μm diameter). The probe DNA is assembled on top of the sensor elements in different concentrations ranging from 16 pg/μl to 10 ng/μl. Complementary biotin-labeled analyte DNA is hybridized to the probe DNA at a concentration of 10 ng/μl. A number of different commercially available magnetic microspheres are investigated to determine the most appropriate markers. The experimental comparison shows that the relative sensitivity of the magnetoresistive biosensor is superior to the fluorescent detection at low probe DNA concentrations.

Magnetoresistance and interlayer coupling in spin valves employing very thin Cu spacer

Magnetoresistance and interlayer coupling behavior near the first antiferromagnetic coupling maximum in underlayer 55 Å/NiFe 10 Å/CoFe/Cu/CoFe/Ru 4 Å/CoFe/IrMn 70 Å/overlayer 50 Å top spin valve structures are reported. As the Cu spacer thickness decreased, interlayer coupling turned to antiferromagnetic at 10 Å of Cu thickness and the coupling field reached maximum near 8 Å of Cu thickness, when the thickness of the CoFe layers was ∼20 Å. Magnetoresistance ratio reached a local minimum at 12 Å of Cu thickness where the ferromagnetic coupling is maximum, and at 10 Å where the coupling field is nearly zero. The sheet resistance change increased slightly at very thin Cu thickness for films having CoFe thickness of ∼20 Å, but increased significantly (by 0.5 Ω/□ from 2.2 to 2.7 Ω/□) for films having very thin (∼10 Å) CoFe layers due to considerable decrease of current shunting. These thin CoFe and Cu layers may be utilized for very small devices where the RKKY coupling of the layers is dominated by high demagnetizing fields.

Magnetic Properties of Permalloy/Cu-Au Multilayers

The interlayer exchange coupling and GMR effect of (permalloy/Cu x Au1-x )30 (Py = Ni83Fe17; 0.29≤x≤0.75) sputtered multilayers (MLs) were investigated. The strength of the antiferromagnetic (AF) interlayer coupling J AF was determined from M(H) and/or R(H) curves. GMR effect and AF coupling was found in entire investigated concentration range of Cu x Au1-x . For x<0.65 the J AF values at the first maximum of AF coupling (1.3≤t Cu-Au≤1.6 nm) were smaller than 3×10-6 J/m2 and for x>0.65 J AF increased to a value characteristic of Py/Cu MLs (J AF≈10-5 J/m2). The second maximum of AF coupling (J AF≈10-7 J/m2) was only found for x≈0.75 at t Cu-Au≈2.6 nm.

Annealing of CoxCu1-x / Cu Multilayers

ABSTRACTIn the multilayer system cobalt / copper at the second antiferromagnetic coupling maximum (2. AFM) with a copper thickness of dCu = 2,2 nm it is possible to reduce magnetoresistive hysteresis by the use of either very thin Co-layers or by alloyed magnetic layers Co1-xCux. It was possible to achieve values for the giant magnetoresistance effect of GMR ≈ 20 % for as prepared samples. A heat treatment was applied to study the degeneration of the system. For annealing at moderate temperatures (Tanneal ≤ 250°C) an increase up to GMR ≈ 25 % was observed. Annealing at slightly higher temperatures lead to an rapid decrease in the GMR effect. To study the structural changes the method of x-ray reflectivity was utilized showing changes in interface roughness as well as in bilayer thickness.

Enhanced GMR amplitude and temperature stability of copper/permalloy combination multilayers

The high effect amplitude of {Cu/Py} multilayers at first antiferromagnetic coupling maximum (AFCM) can be combined with the temperature stability of multilayers at second AFCM by using both corresponding copper layer thicknesses in one multilayer stack. Multilayers that consist of alternating blocks of 1st and 2nd AFCM with at least two double layers each reveal the same temperature stability as multilayers at 2nd AFCM but retain higher GMR amplitude. Knowing the GMR characteristics of the multilayers at 1st and 2nd AFCM as well as of the multilayer with directly alternating copper layer thicknesses (1st, 2nd) the GMR characteristics of combination multilayers with enhanced temperature stability can be predicted.

Interplay of antiferromagnetic coupling in copper/permalloy combination multilayers

The evolution of the giant magnetoresistance (GMR) effect in sputtered combination multilayers (CMLs) of type Py1.8nm//{(Cu1.8nm/Py1.6nm)N/(Cu0.9nm/Py1.6nm)N}y with Py (permalloy=Ni81Fe19) has been investigated at room temperature. It is shown that the GMR characteristic of these CMLs can be phenomenologically predicted, if the physical properties are known, i.e., the GMR effect amplitude, double layer conductance, and bilinear and biquadratic antiferromagnetic exchange coupling constants of the two underlying {Cu/Py}N base systems at the first (Cu0.9nm) and second (Cu1.8nm) antiferromagnetic coupling maximum (AFCM). The GMR characteristic of the simplest CML with N=1 is, e.g., determined by averaging the bilinear and biquadratic exchange coupling constants of the two base systems. The GMR characteristics of CML with N⩾2 are a superposition of that of the underlying {Cu/Py}N base systems weighted by the fraction of the corresponding double layer conductance. Furthermore, it is demonstrated that the CMLs are interesting from an application point of view since they combine the temperature stability of the underlying {Cu/Py}N base system at the second AFCM with a larger GMR effect amplitude.

Structural Studies of Co/Cu and Co/Ru Interfaces Using59Co NMR Method

The structural and magnetic properties of three magnetron sputtered multilayers at the 2nd maximum of antiferromagnetic coupling were studied using 59 Co NMR spectroscopy: [Cο(20 A)/Cu(20 A)]20, [Cο(20 A)/ Ru(18 A)]20 and a heterostructure, in which non-magnetic spacers alternated between Cu and Ru. Co/Cu interfaces were found to be relatively sharp, extended over 3 monolayers, while the Co/Ru interfaces extend over 6 monolayers due to the good miscibility of Co and Ru. The NMR restoring field in the heterostructure is found to be the average of the restoring fields in both reference samples. PACS numbers: 76.60.Lz, 75.70.Ρa, 75.70.Cn, 75.20.En

Evolution of the GMR-effect amplitude in copper/permalloy-multilayered thin films

The evolution of the giant magnetoresistance (GMR) effect in thin film multilayers based on {Cu/Ni 81Fe 19} N has been investigated. The maximum effect amplitude at room temperature reaches about 20% and about 10% in first and second antiferromagnetic maximum, respectively. The magnetoresistance vs field dependence does not show a hysteresis. A simple magnetization reversal model has been applied to separate contributions of bilinear and biquadratic exchange energies to the magnetic coupling. It is shown, that this model can also be used to predict saturation fields for different systems. Multilayers with Cu layer thickness corresponding to the second maximum of the antiferromagnetic coupling survive heat spikes of up to 200°C retaining their sensitivity of >0.16%/Oe. Investigations of multilayers comprised of blocks of first and second antiferromagnetic coupling maximum revealed the field dependence of the coupling between these blocks and it is shown that the GMR effect is simply additive.

Characterisation of interfacial properties in sputtered Co/Cu multilayers: X-ray reflectometry compared with TEM and AFM

Combining localised and global structural information we suggest that differences in the magnetoresistive effect in the first maximum of antiferromagnetic coupling in Co/Cu multilayers can be attributed to the number of magnetic shortcuts localised at grain boundaries. In particular evidence is given that the increased rms-roughness in samples without buffer as compared to samples grown on a Fe buffer can be attributed to a break off of the multilayer structure in adjacent grains.

Structure of Co layers in Co/Cu multilayers at the first antiferromagnetic maximum studied by nuclear magnetic resonance

Using Co59 nuclear magnetic resonance, a structural study has been performed on a series of Co/Cu multilayers with Cu thickness of 9 Å (corresponding to the first maximum of antiferromagnetic coupling) and Co thickness varying between 10 and 50 Å. The structure of the Co layer is fcc for small Co thicknesses. With increasing Co thickness, a rapid increase of hcp Co content is observed, particularly for samples between 10 and 30 Å Co. The equivalent thickness of the deposited Co involved in the mixed interface planes is estimated to be a total of about 5 Å for both interfaces of the Co layer. The interface maintains a relatively sharp concentration profile similar to that observed in samples at the second maximum of antiferromagnetic coupling. However, a certain degree of topological disorder in the interface is introduced by the onset of hcp stacking.