J.C.S. Kools

Exchange-biased spin-valves for magnetic storage

An overview is given of the material properties of exchange-biased spin-valves. More specifically, we discuss the microstructure and magnetic properties of these materials as relevant for (future) industrial application in magnetoresistive read heads for rigid disk and tape recording and Magnetic Random Access Memory (MRAM) devices.

Gas flow dynamics in laser ablation deposition

The gas flow dynamics of laser ablation plumes is investigated experimentally and theoretically. Experimentally, angular‐resolved time‐of‐flight (ARTOF) measurements are performed on a model system (laser etching of copper in a chlorine environment). The TOF spectra obtained can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Theoretically, an analytical model is constructed, based on the hydrodynamical problem of an expanding elliptical gas cloud. The model allows semiquantitative prediction of ARTOF distributions and angular intensity distributions. Observed trends in laser ablation deposition such as independence of the angular intensity distribution on mass of the atom and laser fluence, and dependence of the angular distribution on spot dimensions are explained.

Monte Carlo simulations of the transport of laser-ablated atoms in a diluted gas

The interaction between gas clouds, created by laser ablation, and a diluted gas background is studied by means of Monte Carlo simulations. More specifically, the effect of elastic collisions between ablated atoms and background gas atoms on the kinetic energy and spatial distributions of atoms arriving at the substrate is investigated. The pressure regime under study is typically the range 0.1–200 mTorr. Thermalization is observed in a pressure range that varies over two orders of magnitude, dependent on the initial energy of the atoms and the ratio of the background gas and ablated atom molecular masses. During the first stages of thermalization, forward focusing is found to occur.

On the ferromagnetic interlayer coupling in exchange-biased spin-valve multilayers

The ferromagnetic interlayer coupling in sputter-deposited permalloy/copper/permalloy exchange-biased spin valve multilayers has been measured as a function of the copper thickness. The variation with thickness may, for t/sub cu/>1.7 nm, be analyzed in terms of the Neel model for magnetostatic coupling due to correlated interface roughness, using parameters which are consistent with the observed microstructure.

Thermally assisted reversal of exchange biasing in NiO and FeMn based systems

The stability of the exchange bias field Heb has been studied for magnetron sputtered NiO/Ni66Co18Fe16 and Ni66Co18Fe16/FeMn bilayers. A forced antiparallel alignment of the ferromagnetic magnetization to Heb results in a gradual decrease of Heb as a function of time for NiO as well as FeMn based samples. The observed decrease of Heb increases with temperature and is interpreted as a thermally assisted reversal of magnetic domains in the antiferromagnetic layer.

Effect of energetic particle bombardment during sputter deposition on the properties of exchange‐biased spin‐valve multilayers

The effect of energetic particle bombardment during sputter deposition on the physical (magnetoresistance and interlayer coupling) and microstructural (roughness and interface thickness) properties of exchange‐biased spin‐valve multilayers is investigated. An increasing pressure leads, through enhanced stopping of energetic particles by collisions with the background gas, to a decrease of the interfacial intermixing by collisions during growth, and to a more rough, void‐rich structure. These microstructural changes lead to an increase of the transmissivity of the interfaces for conduction electrons, as well as to an increase of the ferromagnetic interlayer coupling with increasing pressure.

Incongruent transfer in laser deposition of FeSiGaRu thin films

The laser ablation and deposition of FeSiGaRu is studied. The deposited thin films are analyzed with Auger electron spectroscopy and Rutherford backscattering spectrometry. It is found that the gallium and ruthenium content of the thin films is strongly dependent on the laser fluence. At high laser fluences (6 J/cm2) the thin films are depleted of gallium due to preferential sputtering of the gallium atoms from the thin film. Near the threshold fluence (1.9 J/cm2) the films contain an excess of gallium due to preferential evaporation of gallium from the target. The latter conclusions are based on time‐of‐flight studies of ablated atoms and ions and on measurements of the atoms that are sputtered from the substrate by the incoming flux.

1/f noise in anisotropic and giant magnetoresistive elements

Microfabricated magnetoresistive elements based on either the anisotropic or the giant magnetoresistance effect were tested for their frequency dependent resistance noise behavior at room temperature in a dc magnetic field, using a dc sense current. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in saturation is comparable to the 1/f noise level found in nonmagnetic metals; the excess noise has a magnetic origin. The variation of the excess noise level with the applied dc magnetic field can be explained qualitatively using a simple model based on thermal excitations of the magnetization direction.

Magnetization reversal mechanisms in NiFe/Cu/NiFe/FeMn spin‐valve structures

Transmission electron microscopy is used to provide a detailed description of how magnetization reversals take place in NiFe/Cu/NiFe/FeMn spin‐valves. Direct observation is made of how both NiFe layers respond to an applied field. Marked differences from the behavior observed in single Permalloy layers of the same thickness are identified. Complex 360° wall structures frequently form and are studied in some detail. A description of their structure is given and a theory involving the compensation of charges from the biased layer is suggested to explain stability. The work has a direct bearing on the performance of spin‐valves as sensors as the way the magnetization changes under the influence of an applied field affects the noise characteristics.

MAGNETORESISTANCE IN NI80FE20/CU/NI80FE20/FE50MN50 SPIN VALVES WITH LOW COERCIVITY AND ULTRAHIGH SENSITIVITY

We present magnetoresistance measurements on Ni80Fe20/Cu/Ni80Fe20/Fe50Mn50 spin valves with crossed anisotropies: the easy magnetization direction of the unbiased Ni80Fe20 layer is perpendicular to the exchange biasing field which effectively works on the second Ni80Fe20 layer due to the interaction with the Fe50Mn50 layer. The hysteresis in the low‐field magnetoresistance is less than 0.03 kA/m, which is ten times smaller than the hysteresis for similar materials with parallel anisotropies. The sensitivity (∂R/R∞)/∂H ranges from 8% to 18%/(kA/m), depending on the preparational method. To our knowledge this combination of very high sensitivity and very low coercivity has not been reported before.