Dirk Meyners

Exchange biasing of magnetoelectric composites

Magnetoelectric composite materials are promising candidates for highly sensitive magnetic-field sensors. However, the composites showing the highest reported magnetoelectric coefficients require the presence of external d.c. magnetic bias fields, which is detrimental to their use as sensitive high-resolution magnetic-field sensors. Here, we report magnetoelectric composite materials that instead rely on intrinsic magnetic fields arising from exchange bias in the device. Thin-film magnetoelectric two-two composites were fabricated by magnetron sputtering on silicon-cantilever substrates. The composites consist of piezoelectric AlN and multilayers with the sequence Ta/Cu/Mn(70)Ir(30)/Fe(50)Co(50) or Ta/Cu/Mn(70)Ir(30)/Fe(70.2)Co(7.8)Si(12)B(10) serving as the magnetostrictive component. The thickness of the ferromagnetic layers and angle dependency of the exchange bias field are used to adjust the shift of the magnetostriction curve in such a way that the maximum piezomagnetic coefficient occurs at zero magnetic bias field. These self-biased composites show high sensitivity to a.c. magnetic fields with a maximum magnetoelectric coefficient of 96 V cm(-1) Oe(-1) at mechanical resonance.

Giant magnetoelectric effect in vacuum

Magnetoelectric (ME) thin film cantilever type sensors made of AlN and FeCoSiB are operated in vacuum, reducing air damping and thus increasing the ME coefficient and improving the limit of detection (LOD) for ac-magnetic fields. Depending on the sensor geometry, the response is increased by a factor of 5 resulting in a ME coefficient of 20 kV/cmOe at 152 Hz and by a factor of 11 with 12 kV/cmOe at 4.7 kHz and an improvement in LOD by an order of magnitude. Modelling these cantilevers reveals dominant contributions of viscoelastic and molecular damping above and intrinsic damping below 10−2 mbar, respectively.

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature resul...

Pressure sensor based on magnetic tunnel junctions

Magnetic tunnel junctions consisting of a magnetically stable reference layer plus a magnetostrictive sense layer separated by an insulating MgO tunnel barrier have been fabricated and characterized particularly with regard to their application as strain and pressure sensors. Using a four-point-bending apparatus it has been proven that the application of mechanical stress causes the magnetization of the sense layer to rotate, while simultaneously the resistance of the strained magnetic tunnel junction changes. So far gauge factors up to 840 have been reached in resistance versus strain measurements. It has been demonstrated that these magnetic tunnel junctions also work as highly sensitive pressure gauges when placed on bulk or surface micromachined membranes.

Influence of boundary roughness on the magnetization reversal in submicron sized magnetic tunnel junctions

The reproducible magnetic switching of submicron magnetic tunnel junctions (MTJ’s) is an important requirement for their application in highly integrated magnetic memory devices. We have investigated the switching of small MTJ’s by atomic and magnetic force microscopy (AFM/MFM) combined with micromagnetic numerical simulations. The latter are carried out with the real (AFM) shape as input mask, including the boundary roughness of the MTJ’s. MFM reveals S-, C-, and K- shaped magnetization patterns for rectangular submicron sized junctions in saturation. In general, the magnetization loops and switching fields are different for individual junctions. The simulations show that the detailed boundary shape, which is specific for each junction, has a significant influence on the nucleation and location of domain walls and vortices, and hence, on the magnetic switching.

Reliability of field programmable magnetic logic gate arrays

In concepts for logic circuits using hysteretic magnetoresistive effects, one gains advantages of magnetic thin-film elements, e.g., nonvolatility, radiation hardness and nondestructive readout. The requirements concerning reproducibility of resistance and magnetoresistance, however, are much more stringent than for memory applications. Using experimental data of magnetic logic circuits, we show that the variances of resistance and magnetoresistance, and the amplitude of the magnetoresistance, determine the yield of working logic gates. The current magnetoresistance of 75% for CoFeB∕Al2O3 junctions is close to the minimum required value. More than 250% obtained with MgO barriers, however, will allow a fault tolerant production.

Exchange biased magnetoelectric composites for magnetic field sensor application by frequency conversion

A comparison is presented between magnetoelectric composite sensors based on AlN and FeCoSiB with and without exchange bias coupling. All layer stacks were fabricated by thin film deposition on Si substrates. Whereas sensors without exchange bias exhibit a low limit of detection in the 1 pT/√Hz regime for resonant AC fields, such sensors fail at the detection of low frequency signals. Accordingly, their detection limit increases to about 10 nT/√Hz for alternating magnetic fields with 10 Hz frequency and an integration time equal to 3 s. A frequency conversion technique based on magnetic modulation of the sensors improves their detection limit by about one order of magnitude. However, frequency conversion can be applied more effectively to magnetoelectric sensors with exchange biased multilayers as a magnetostrictive phase. As a result, their limit of detection is about 180 pT/√Hz for 10 Hz signals and an integration time of 1 s. This is in contrast to the magnetoelectric coefficient αME of the two types o...

Phase modulated magnetoelectric delta-E effect sensor for sub-nano tesla magnetic fields

We present a resonant micromechanical magnetic field sensor, which utilizes the magnetically induced change in elastic modulus, i.e., the delta-E effect. The sensor is based on magnetoelectric thin film composites, resulting in high sensitivity at room temperature and at low frequencies. The cantilever is electrically excited and read out by a 2 μm AlN piezoelectric layer. Depending on its magnetization, the 2 μm thin film of amorphous (Fe90Co10)78Si12B10 changes its elasticity, which results in a shift of the cantilevers resonance frequency. The sensor is operated in the first or second transversal bending mode at 7.6 kHz or 47.4 kHz. With a limit of detection of 140 pTHz−0.5 at 20 Hz under a magnetic bias field and 1 nTHz−0.5 without external bias field, this sensor exceeds all comparable designs by one order of magnitude.

Exchange biased magnetoelectric composites for vector field magnetometers

Magnetoelectric (ME) vector field sensors can be realized by combining individual sensors with anisotropic sensitivity. Following this approach, exchange biased ME thin film composites were incorporated into two dimensional vector magnetometers. These self biased composites do not need magnetic dc bias fields for their operation, which allows a high degree of miniaturization since interference of individual bias fields is avoided. The successful detection of a two dimensional magnetic vector field opens the path to full three dimensional vector sensing.

Inverted spin polarization of heusler alloys for spintronic devices

A magnetic logic concept overcomes the major limitations of field programmable gate arrays while having a 50% smaller unit cell than conventional designs utilizing magnetic tunnel junctions with one Heusler alloy electrode. These show positive and negative tunneling magnetoresistance values at different bias voltages at room temperature which generally add an additional degree of freedom to all spintronic devices.