H. A. M. van den Berg

Ultrafast precessional magnetization reversal by picosecond magnetic field pulse shaping

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.

GMR sensor scheme with artificial antiferromagnetic subsystem

A magnetoresistive GMR-sensor scheme is demonstrated and analyzed in which the hard magnetic layers are replaced by Artificial Antiferromagnetic Subsystems (AAFs). These consist of ferromagnetic layers antiferromagnetically coupled via interlayers. The magnetic rigidity of this AAF is improved by an order of magnitude compared to the individual magnetic layers. Operational field windows for 360/spl deg/-angle detectors 20 kA/m have been realized. The sensor signal /spl Delta//spl rho///spl rho/ is 6%. The temperature-operation range extends itself up to 150/spl deg/C. The angle resolution is 1/spl deg/.

GMR angle detector with an artificial antiferromagnetic subsystem (AAF)

Abstract A magnetoresistive GMR sensor scheme for detection of angular positions is demonstrated and analyzed in which the hard magnetic layers are replaced by artificial antiferromagnetic subsystems (AAFs). These consist of ferromagnetic layers (Co) that are antiferromagnetically coupled via interlayers (Cu or Ru). The rigidity of this AAF is improved by an order of magnitude as compared to the individual Co layers. Operational field windows for 360° angle detectors of 15–20 kA/m have been realized. The maximal sensor signal Δϱ/ϱ is 6%. The temperature operation range extends up to 150°C. The angle resolution is about 1°.

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

Abstract The d.c. plasma jet CVD process is one of the most promising coating processes used for the production of polycrystalline diamond films. In comparison with other CVD processes, its obtainable linear growth rates, in the range of 100 μm/h, are much higher than growth rates of microwave or hot filament CVD (1–10 μm/h). One interesting application is the diamond coating of cutting tools. The main problem here is the poor adhesion of the films. Therefore, a mechanical or chemical pretreatment or intermediate layers are used to improve the adhesion. In these investigations the influence of mechanical pretreatment by grinding and polishing with diamond powder of different grain sizes as well as chemical etching are examined on WC-Co hardmetals. Sputtered metallic interlayers of different thicknesses and arc-ion plated amorphous carbon films are deposited on these substrates, and diamond films were synthesized on these pretreated cutting tools by d.c. plasma jet CVD. Adhesion and wear resistance of the diamond films have been examined by dry turning tests on very abrasive metal-matrix composites. Distinct improvement in adhesion of diamond coatings on hard metal substrates was achieved by two methods of substrate surface pretreatment: etching with Murakamis solution and following ultrasonically seeding with diamond particles or using an amorphous carbon film as intermediate layer.

Tunnel magnetoresistance in magnetic tunnel junctions with a ZnS barrier

We report on the junction magnetoresistance in magnetic tunnel junctions of the hard–soft type with magnetic layers separated by a ZnS barrier. The hard magnetic bottom electrode consists of an artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. The samples were grown by sputtering on Si (111) wafers at room temperature and have the following structure: Fe6 nmCu30 nm(CoFe)1.8 nmRu0.8 nm(CoFe)3 nmZnSx(CoFe)1 nmFe4 nmCu10 nmRu3 nm. The square tunnel elements, with lateral sizes of 10, 20, 50, and 100μm, exhibit typical tunnel resistance of 2–3 kΩ μm2 and nonlinear zero field current–voltage (J–V) variation. The most interesting result is the observation of junction magnetoresistance of about 5% at room temperature with a 2 nm thick ZnS barrier.

Artificial antiferromagnetic tunnel junction sensors based on Co/Ru/Co sandwiches

A novel method is used for pinning the magnetization of the magnetically hard subsystem in micron-size magnetic tunnel junctions: the so-called artificial antiferromagnetic structure. The latter uses the strong antiparallel exchange coupling between two Co layers through a Ru spacer layer to ensure a high rigidity of the hard subsystem magnetization. The tunnel barriers were formed by sputter etching previously deposited Al layers in a rf Ar/O2 plasma. Wafers, 3 in. in diameter, were patterned into arrays of square junctions with lateral sizes of 20 and 50 μm. All junctions of a given size show resistances reproducible within several percents. The tunnel magnetoresistance (TMR) is found to be independent of the junction size and TMR ratios of 14%–16% are achieved at room temperature.

Contactless potentiometer based on giant magnetoresistance sensors

We present a magnetic sensor based on the giant magnetoresistance (GMR) effect, which can be used as a contactless potentiometer. The sensor consists of a novel GMR sensor scheme with an artificial antiferromagnetic subsystem. The sensor gives a sinusoidal signal in dependence on the direction of a rotating external magnetic field Hrot. With two sensors in a planar setup, the whole 360° angle range can easily be covered. The amplitude ΔR/R of the signal is about 5%. The signal amplitude runs through a very flat maximum and changes only by 5% within a field range of about 4.4–27.2 kA/m. This large field range, the magnetic window (Hw), is the main advantage of this sensor compared to other magnetic sensor physical principles for contactless potentiometers. Therefore, large mounting tolerances and variations in the field strength of the rotating permanent magnet can be accepted. The temperature dependence of the sensor is linear both for the ground resistance (R0) and the signal amplitude (ΔR), allowing sim...

Giant magnetoresistance, antiferromagnetic volume fraction and saturation field in Co/Cu multilayers

A series of Co/Cu multilayers is prepared exhibiting giant magnetoresistance in the first maximum of the oscillatory exchange coupling regime. The size of the magnetoresistance strongly depends on the thickness of the layers involved, the substrate material, the buffer layer and the sputtering conditions. The analysis of resistive and magnetic data reveals that to a first approximation the magnetoresistance linearly depends on the volume fraction of Co coupled antiferromagnetically. This volume fraction can be derived from the extrapolated remanent magnetization. The authors investigate under which conditions and to what extent the linearity applies. A maximum of the magnetoresistivity of 72% at room temperature is reached under optimized conditions. >

Magnetization dynamics in NiFe thin films induced by short in-plane magnetic field pulses

The magnetization dynamics in a thin NiFe film was investigated by applying short in-plane magnetic field pulses while probing the response using a time-resolved magneto-optical Kerr effect setup. In-plane magnetic field pulses, with duration shorter than the relaxation of the system, were generated using a photoconductive switch and by subsequent propagation of current pulses along a waveguide. The field pulses with typical rise and decay times of 10–60 and 500–700 ps, respectively, have a maximum field strength of 9 Oe, by which Permalloy elements of 16 nm thickness and lateral dimensions of 10×20 μm were excited. The observed coherent precession of a ferromagnetic NiFe system had precession frequencies of several GHz and relaxation times on a nanosecond time scale. The dynamic properties observed agree well the Gilberts’s precession equation and the static magnetic properties of the elements

Local investigation of thin insulating barriers incorporated in magnetic tunnel junctions

In this work we study properties of very thin insulating Al oxide films, used as barriers in magnetic tunnel junctions. For the small barrier thicknesses required for technological applications (∼10 A), the presence of pinholes (direct contact between the ferromagnetic metals through the barrier), or oxidation inhomogeneities, are the major factors for vanishing of the tunnel magnetoresistance effect. We have produced and characterized very thin, pinhole-free Al oxide layers, incorporated in magnetic tunnel junctions. The transport properties of the different barriers were analyzed by barrier impedance scanning microscopy and were correlated with the magnetotransport properties of the patterned microsized junctions.