Exchange-coupled Thin Films Research Articles

Non-exchange bias hysteresis loop shifts in dense composites of soft-hard magnetic nanoparticles: New possibilities for simple reference layers in magnetic devices

Exchange bias has been extensively studied in both exchange-coupled thin films and nanoparticle composite systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias is far from being understood. Here, dense soft-hard binary nanoparticle composites are used not only as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift but also as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft maghemite (γ-Fe2O3) nanoparticles (no bias—symmetric reversal) and hard cobalt doped γ-Fe2O3 nanoparticles (large exchange bias—asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe2O3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Stabilization of magnetic helix in exchange-coupled thin films

Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m3. We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator.

Graded/soft/graded exchange-coupled thin films fabricated by [FePt/C]5/Fe/[C/FePt]5 multilayer deposition and post-annealing

We report the fabrication of exchange-coupled graded/soft/graded films with well-isolated magnetic grains. In contrast to the FePt/Fe exchange spring film, a reduction of up to 72% in coercivity value in the graded/soft/graded film can be obtained. Adding C layers with different thicknesses not only tailors the anisotropy gradient, but also refines the grain size and weakens the intergranular exchange interactions. Micromagnetic simulations reveal that the magnetization reversal proceeds by the propagation of a partial domain wall into the hard FePt layers from the center of the soft Fe layer. The formation of partial domain wall can assist the two graded FePt layers to switch simultaneously, resulting in a lower coercivity.

Spin Reorientation Transitions in Perpendicularly Exchange-Coupled Thin Films Studied Using Element Specific Imaging

Spatial variations in the spin-reorientation transition of an exchange-coupled Co-wedge/YIG (Y3Fe5O12) bilayer, from perpendicular to in-plane domain structure, was studied using magnetic force microscopy (MFM) and photo-emission electron microscopy (PEEM). Even though MFM measurements of the YIG film showed perpendicular stripe domains, it was not possible to unambiguously resolve the domain structure of only the top ferromagnetic metal layer because of complications arising from the stray fields of the much thicker YIG underlayer. Hence, using element-specific, X-ray magnetic circular dichroism (XMCD) of the transition metal L3,2 edges (Co and Fe, respectively) for magnetic contrast, PEEM measurements were carried out to resolve the domain structure of the individual Co and YIG layers. The two were identical up to a Co thickness of 4.5 nm, confirming that the Co layer was exchange coupled with the YIG underlayer; however, a transition of the Co domains, from perpendicular to in-plane, was observed at thickness of 4.5-6 nm. For thicker regions of the Co film (>6 nm), a sizeable portion of the Co layer showed in-plane domains; their spins were perpendicular to the domain walls of the stripe domains with Co XMCD values in the range of -5% to +5%-a value much smaller than that of the perpendicular domains

Magnetic characterization of exchange-coupled thin films having competing anisotropies

We have studied the magnetic properties of co-sputtered Ni 0.89 Fe 0.11 single layers (10–100 nm) and Ni 0.89 Fe 0.11 / Ni 0.89 Fe 0.11 bilayers (10 nm/10 nm to 100 nm/100 nm) grown on glass substrates. Hysteresis loops and transverse biased initial susceptibility measurements were performed by transverse magneto-optical Kerr effect from both film/air and film/glass interfaces. The individual layers show in-plane uniaxial easy axes produced by the oblique incidence of incoming atom beams. For thinner bilayers (up to 20 nm/20 nm) the overall magnetic behavior can be justified by competing effective uniaxial and biaxial anisotropies acting over the average magnetization. For thicker bilayers (40 nm/40 nm and above), both transverse magneto-optical Kerr effect measurements and micromagnetic simulations suggest the existence of a Bloch-type magnetic wall extending across the thickness of the bilayer.

A deep look into exchange-coupled films: advances through nuclear resonant scattering of synchrotron radiation

We introduce a new method to image the internal spin structure in exchange-coupled thin films. The technique relies on nuclear resonant scattering of synchrotron radiation from ultrathin layers of Mössbauer isotopes embedded in different depths of the sample. The technique is applied to image the spiral spin structure that forms in exchange-spring bilayers in external magnetic fields.

Imaging the magnetic spin structure of exchange-coupled thin films.

We have investigated the magnetic spin structure of a soft-magnetic film that is exchange-coupled to a hard-magnetic layer to form an exchange-spring layer system. The depth dependence of the magnetization direction was determined by nuclear resonant scattering of synchrotron radiation from ultrathin 57Fe probe layers. In an external field a magnetic spiral structure forms that can be described within a one-dimensional micromagnetical model. The experimental method allows one to image vertical spin structures in stratified media with unprecedented accuracy.

A method for the determination of exchange and magnetocrystalline anisotropies in exchange-coupled thin films

A method for the determination of exchange and magnetocrystalline anisotropies in exchange-coupled bilayer thin films is presented. The interfacial exchange anisotropy is modeled as a magnetic field H E. The ferromagnetic film is taken to be a single crystal with cubic and uniaxial magnetocrystalline anisotropies. The method is based on the torque curve slope. It will be shown how the study of the torque curve slope can lead to the values of the exchange anisotropy field H E, the magnetization M and the cubic or the uniaxial anisotropy fields.

Calculation of Brillouin light scattering intensities from pairs of exchange-coupled thin films

A computer program has been developed that can be used to calculate the intensity of light scattered from thermal magnons in two planar exchange-coupled magnetic films. The static magnetizations have been assumed to be uniform and parallel with an applied magnetic field which lies in the plane of the films. The calculation includes exchange energies, cubic magnetocrystalline anisotropy energies, and surface pinning energies. The results of the calculation compare favorably with the data reported for iron films by Grünberg et al. [Phys. Rev. Lett. 57, 2442 (1986)].

Investigation of anisotropy by ferromagnetic resonance (FMR) in exchange-coupled bilayer films

We have used ferromagnetic resonance (FMR) to investigate magnetic anisotropy in bilayer exchange-coupled thin films. Experiments, using both our 9 and 33 GHz FMR spectrometers, were done on two systems: NiFe/FeMn and NiFe/α-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> . In some of the NiFe/α-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> specimens an additional response was detected at 33 GHz. We attribute this additional resonance to a region formed by interdiffusion of NiFe and α-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> at their common interface. Effective anisotropy field, g-value and the relative volume have been determined for each ferromagnetic region. Auger profile, TEM and CBED (convergent beam electron diffraction) cross-section examinations were performed for an interdiffused specimen, the results are consistent with the presence of a layer containing Fe and Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> located between the NiFe and α-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> regions. A memory effect, due presumably to irreversible spin reorientation at the interface, was observed in some of the NiFe/α-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> specimens.

Increasing the density of planar magnetic domain memories

The propagation of flat domains in soft ferromagnetic thin films with uniaxial anisotropy has been utilized in the past to implement low-cost nonvolatile shift register memories. However the low density of storage, the poor quality of the films, and the complexity of the structure have overwhelmed the inherent low material cost and simple manufacturing tooling. As a result, the devices have not been competitive. A novel, very simplified structure using chemically deposited soft and hard exchange-coupled thin films is described. Localized magnetostatic fields are created through rectangular wide regions of the propagation channels in order to permit the storage of very small domains and the control of the domain propagation when the shift conductors are current pulsed. Experimental work on a 104 kbits/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> density register geometry is reported, and the improvements necessary to reach higher densities are discussed.