G. Gieres

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/.

Remanence enhancement due to exchange coupling in multilayers of hard- and softmagnetic phases

Nd-Fe-B/Fe/Nd-Fe-B trilayers with individual layer thicknesses between 10 and 100 nm have been grown by sputter deposition. The aim of our study is to investigate remanence enhancement due to exchange coupling between the soft- and hard-magnetic layers as a function of the Fe-interlayer thickness d. It is shown that for decreasing d the remanence of the trilayer system is significantly increased due to exchange coupling. The coercivity of the trilayer is also increased for d<40 nm which can be explained with a higher nucleation field for magnetization reversal.

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°.

Growth characteristics and magnetic properties of sputtered NdFeB thin films

Abstract Thin NdFeB films (200 to 2000 nm) with good hard magnetic properties have been produced by magnetron sputtering in UHV. We have investigated the influence of substrate temperature and sputtering gas pressure on the structural and magnetic properties of the films. It was found that the composition of the films strongly depends on the argon pressure. This gives us the possibility to vary the Nd content in the range of 13 to 32 at%. Highly textured films can be produced at low gas pressure (0.03 mbar) and low substrate temperatures (≈ 675 K). These films exhibit a low coercivity (100 to 300 kA/m) and low remanence, but have a high saturation polarization of about 1.5 T, measured vertical to the film plane. High coercive films ( H cj ≈ 800–1200 kA/m) with a remanence of about 1.05 T can be achieved through sputtering at high argon pressures (0.1 to 0.2 mbar) and a substrate temperature in the range of 800 to 900 K.

Magnetic properties and growth texture of high-coercive Nd–Fe–B thin films

We have investigated the growth texture and magnetic properties of sputtered Nd–Fe–B thin films with thicknesses from 5 to 350 nm. Films deposited directly onto a quartz substrate grow with a pronounced c-axis texture perpendicular to the film plane and have a coercivity of about 160 kA/m. If the films thickness is reduced below 150 nm, a significant part of the Nd is oxidized by substrate oxygen and the coercivity decreases to values below 20 kA/m. The deposition of a 80 nm Cr buffer layer between substrate and Nd–Fe–B film prevents oxidation of the film and decreases the growth texture of the films. This enables us to fabricate Nd–Fe–B films as thin as 20 nm with good hard magnetic properties [Hcj=800 kA/m, Jr=1.1 T, and a maximum energy product (BH)max=190 kJ/m3 perpendicular to film plane]. An inverse relationship between growth texture and coercivity is found which can be understood in terms of domain wall motion. The deposition of a Cr buffer leads to an island type of growth and a high surface roug...

Evolution of the dielectric breakdown in Co/Al2O3/Co junctions by annealing

The temperature and dielectric stability of magnetic tunnel junctions are important requirements for magnetic memory devices and their integration in the semiconductor process technology. We have investigated the changes of the tunneling magnetoresistance (TMR), the barrier properties (height, thickness, and asymmetry) and the dielectric stability upon isochronal annealing up to 410 °C in Co/Al2O3/Co junctions with an artificial antiferromagnet as a pinning layer. Besides a small decrease of the TMR signal after annealing up to 230 °C, a strong decrease between 300 and 350 °C is found. According to Auger and transmission electron microscopy investigations, this decrease is mainly due to interdiffusion of the metallic layers. The dielectric breakdown is characterized by voltage ramp experiments. The size-averaged breakdown voltage improves from 1.35 V for the as prepared junctions to 1.55 V by annealing at 300 °C. At higher temperatures the breakdown voltage decreases strongly to 0.8 V (at 380 °C). Simulta...

Temperature stability of Co/Al2O3/Co junctions

The temperature stability of magnetic tunnel junctions is an important requirement for the fabrication of magnetic memory devices and the integration in the semiconductor process technology. We have investigated the temperature evolution of the tunneling magnetoresistance (TMR) and the structural properties by isochronal annealing experiments up to 750 K. The magnetically hard electrode of the junction consists of an artificial antiferromagnet Co/Cu/Co, the soft electrode of a Co/Fe bilayer. The tunnel barriers are formed by plasma oxidized Al. The tunnel junctions have TMR signals up to 22% at room temperature. Besides a small increase of the TMR signal after annealing up to 480 K, a first decrease at 530 K and a breakdown beyond 600 K are found. This behavior can be attributed to structural changes of the junctions. Auger depth profiles show an unaltered Al2O3 barrier up to 600 K, but the beginning of interdiffusion processes within the magnetic electrodes already at 540 K and above.

Low tunnel magnetoresistance dependence versus bias voltage in double barrier magnetic tunnel junction

We report on the magnetic and transport properties of [IrMn8/CoFe1.5]/AlOx1.2/[CoFe1/NiFe5/CoFe1]/AlOx1.2/[CoFe1.5/IrMn8] (nanometer) double magnetic tunnel junctions (DMTJs) deposited by magnetron sputtering and patterned using optical lithography. The tunnel magnetoresistance (TMR) versus the bias voltage presents a symmetric characteristic, which indicates a good and similar quality of both AlOx barriers. The junctions show a resistance-area product about 35 kΩ μm2, a high TMR at room temperature of 49.5%, and a high bias voltage at which the TMR signal is decreased to half of its maximum value, V1/2DMTJ=1.33 V. Both hard magnetic layers are rigid in negative field up to 51.5 kA/m, while the coercive field of the soft layer is around 1.1 kA/m. The large difference of coercive fields, combined with the large TMR and V1/2, makes these systems very promising for spin electronic devices.

Size dependence of switching field of magnetic tunnel junctions down to 50 nm scale

Tunnel magnetoresistance curves were measured in very small tunnel junctions from scales of 1 μm to 50 nm using conductive atomic force microscopy. The junction arrays were prepared by a simple fabrication process using electron beam lithography. In large size junctions, the minor loops shifted in the negative field direction corresponding to ferromagnetic coupling between free and pinned layers. With decreasing size, the shift changed to the positive field direction corresponding to antiparallel coupling. The dependence of the shift was quantitatively explained by a model taking account of both Neel-type and dipole coupling. The minor loops showed asymmetric shape depending on field sweep directions.

Epitaxial YBa2Cu3Ox thin films on sapphire using a Y‐stabilized ZrO2 buffer layer

Epitaxial, c‐oriented YBa2Cu3Ox thin films were deposited by dc sputtering on (1102)‐sapphire substrates with an intermediate buffer layer of Y‐stabilized ZrO2 (YSZ), which was grown by rf magnetron sputtering. The epitaxy of the YSZ and the YBa2Cu3Ox films was proved by Rutherford backscattering spectrometry combined with ion channeling. The YBa2Cu3Ox films exhibited transition temperatures of 90 K and had critical current densities exceeding 1.2×106 A/cm2 at 77 K in zero magnetic field.