Ludwig Bär

Nonvolatile field programmable spin-logic for reconfigurable computing

We have fabricated field programmable spin-logic gates based on spin-dependent tunneling (SDT) elements. Here we show their feasibility down to a width of 0.6 μm of the SDT elements that form spin-logic gates. We further demonstrate the clocked operation of a hybrid spin-logic gate consisting of SDT elements and a semiconductor-based sense amplifier. Apart from the nonvolatility of the inputs, the output and the programming information, the experimentally demonstrated concept seems to be suitable for reconfigurable computing operations.

Multichannel DC SQUID sensor array for biomagnetic applications

A biomagnetic multichannel system for medical diagnosis of the brain and heart has been developed. 37 axial first order gradiometers (manufactured as flexible superconducting printed circuits) are arranged in a circular flat array of 19 cm in diameter. Additionally, three orthogonal magnetometers are provided. The DC SQUIDs are fabricated in all-Nb technology, ten on a chip. The sensor system is operated in a shielded room with two layers of soft magnetic material and one layer of Al. The everyday noise level is 10 fT/Hz/sup 1/2/ at frequencies above 10 Hz. Within two years of operation in a normal urban surrounding, useful clinical applications have been demonstrated (e.g., for epilepsy and heart arrhythmias). For the first time current sources of sporadic events causing epilepsy or ventricular extrasystoles have been localized from coherent recordings of complete biomagnetic field distributions with spatial resolution of millimeters and temporal resolution of 1 ms.

Field programmable spin-logic based on magnetic tunnelling elements

Spin-dependent tunnelling elements are widely studied due to their possible application in electronic devices. Here we focus on field programmable logic devices. We introduce the concepts and demonstrate experimentally the functionality of reprogrammable logic gates based on spin-dependent tunnelling elements. We further extend this demonstration to logic gates consisting of micron-sized tunnelling elements.

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.

Magnetic tunnel sensors with Co-Cu artificial antiferromagnetic (AAF) hard subsystem

Micron-size magnetic tunnel junctions are studied, in which the pinning of the hard magnetic reference layer is accomplished by the so-called Artificial Antiferromagnetic Subsystem (AAF). In the AAF, two Co layers are antiferromagnetically coupled by a 1 nm thick Cu layer, thus giving rise to stability windows larger than 2 kA/m for the AAF. Al/sub 2/O/sub 3/-tunnel barriers of 1 and 1.5 nm Al thickness were put on top of the AAF and at its opposite surface Fe detection layers are located. The signal could be enhanced by a factor of two to 22% by inserting a 1 nm Co-coating between the barrier and the Fe. Thus sensors were realized with signals four times higher than for the GMR counterparts.

Single layer YBaCuO-gradiometer

A single layer YBaCuO-gradiometer galvanically coupled to DC-SQUIDs were prepared on bicrystal substrates. The devices were operated at 77 K without any shielding. The best performance obtained was a field gradient resolution of 0.5 pT/cm/spl radic/(Hz) for a device with a baseline of 7 mm and a pickup-area of 2 cm/sup 2/.<<ETX>>

Ultra low noise all niobium DC-SQUIDs

The noise and signal properties of SQUIDs with amorphous silicon barriers and Al/sub 2/O/sub 3/ barriers are studied. The barrier material is found to be of great importance for the value of the 1/f noise component. The best results were obtained for SQUIDs with Al/sub 2/O/sub 3/ barriers and a 1/f noise level at 1 Hz of about 1*10/sup -6/ Phi / square root Hz was found. After integration of coupling coils onto the SQUIDs, a signal limitation and a dramatic increase of the noise were found. Implementation of a damping circuitry over the coupling coil results in optimized signals ( Delta V( Phi /sub 0//2) approximately=I/sub c/R) and a white noise level comparable to the white noise level without a coupling coil. The 1/f noise component for SQUIDs with a damped coupling coil is higher than for 1/f noise component of SQUIDs without a coupling coil. For SQUIDs with Al/sub 2/O/sub 3/ barriers, the 1/f noise level keeps below 3*10/sup -6/ Phi /sub 0// square root Hz at 1 Hz. For SQUIDs with an amorphous silicon barrier the 1/f noise component changes per cooling cycle in an irregular way. The stability for thermal cycling and room-temperature storage is very good for all the devices.

Creation and annihilation of 360° domain walls in magnetic tunnel junctions with exchange-biased artificial ferrimagnet

In state-of-the-art read heads, exchange-biased artificial ferrimagnet hard layers are extensively used. In this paper, we analyze different magnetic tunnel junction stacks with respect to the formation and annihilation of 360° walls in their hard subsystem, visualized using magnetic force microscopy. The existence of 360° walls in a magnetic tunnel junction has a microscopic origin, i.e., the local competition between different coupling mechanisms in the layered system, and can qualitatively be interpreted from a strong but reversible reduction in minor loop tunnel magnetoresistance. Therefore, the magnetic history of a tunnel junction is of great importance in the interpretation of minor loop measurements.

Single-layer and integrated YBCO gradiometer coupled SQUIDs

For many SQUID applications such as biomagnetism or non-destructive evaluation it is convenient or even necessary to work without the restrictions of a magnetically shielded room. This contribution deals with two sensors appropriate for this purpose. In the first concept we present a flip chip arrangement of a single-layer flux transformer and a single-layer SQUID, taking advantage of a simple technology. The SQUID was prepared on a bicrystal and located in the common line of two-parallel-loop arrangements. The flipped antenna was designed as a two-parallel-loop gradiometer with 26 mm baseline on a single-crystal substrate. A field gradient sensitivity of was obtained. We could demonstrate a field gradient resolution of at 1 kHz in an unshielded environment. In the second concept we integrated both the flux antenna and the SQUID on a bicrystal. The tighter coupling scheme results in smaller devices for similar field gradient sensitivities. The integrated SQUID is designed as a device on a bicrystal substrate. The remaining space is used for test structures and SQUIDs without antennae, in order to control the technology as well as the SQUID design. Parallel processed dummy substrates were used to monitor the quality of film growth by x-ray analysis. The quality of our SQUID design will be discussed on the basis of the measured field gradient sensitivity and noise. The reliability of the devices is demonstrated by an NDE type measurement.

Precessional direct-write switching in micrometer-sized magnetic tunnel junctions

We have implemented direct-write and toggle switching in the precessional limit on micron-sized magnetic tunnel junctions. We have measured the amplitudes and duration of orthogonal applied magnetic fields leading to reliable switching for pulse durations as short as 178 ps. We have shown that the final magnetization state can be well understood by solving the Landau–Lifshitz–Gilbert equation in the macrospin approximation. We finally have compared the size of the writing window in two scenarios of orthogonal field timing: synchronous pulses or imbricated pulses (easy-axis field lasting longer than hard-axis field). Imbricated pulses lead to sizable increase of the writing window.