Micro structuring of the stack layer was done by irradiating the sample

with focused femtosecond laser radiation in a single line-scan. The pulse

overlap was varied in a wide range, down to separated single pulses.

The threshold fluence for material ablation was estimated of 0.11 J/cm².

High pulse overlap (99%), shown in Fig. 3a, resulted in complete

ablation of the stack layer, but irregular edge structures occurred, which

were formed out of molten and solidified material. Also the substrate was

damaged and micro-scaled substructures (ripples) emerged. In contrast,

a low pulse overlap of 80 % yielded incomplete material removal due to

beginning pulse separation (Fig. 3c). Within a narrow process window

(87% overlap, Fig. 3b) the visible amount of molten material was

reduced to a minimum, while it formed well defined saw tooth like

structures at the track edges. The periodicity of these structures

corresponded exactly to the spatial pulse distance of 1.8 µm. The track

width was 5.9 µm, while the applied peak fluence was in the range of the

threshold fluence. Isolated film patches were processed by area ablation

with overlapping lines (Fig. 4).

Investigation of micro structuring and magnetizing of spintronic layer stacks using laser irradiation Isabel Berthold, Mathias Mueller, Robby Ebert, Horst Exner | Laserinstitut at the University of Applied Sciences Mittweida

Senoy Thomas, Patrick Matthes, M. Albrecht | Institute of Physics, Chemnitz University of Technology

Contact

Prof. Dr.-Ing. Dr. h.c. H. ExnerLaserinstitut der Hochschule MittweidaTechnikumplatz 1709648 MittweidaGermanyTel. +49 3727 58-1413exner@hs-mittweida.de

Motivation

Summary and outlookBy applying femtosecond laser radiation in line scan ablation, a track width of 5.9 µm together with uniform edge structures was obtained. Optimized parameter were

applied to fabricate isolated film patches for magnetizing experiments on the spintronic layer stack. The successful setting of the magnetization orientation of the

reference layer induced by laser radiation in combination with an external magnetic field was demonstrated. The MOKE-hysteresis loops confirmed the results

obtained with the magnetic micro sensor. In the future, the feasibility of this technology in GMR-sensor fabrication will be investigated.

The fabrication of highly resolved GMR sensors requires along with the structuring of a spintronic layer stack a selective alignment of the exchange bias fieldoccurring in an antiferromagnetic/ferromagnetic bilayer system, which is utilized to pin the magnetic direction in the reference ferromagnetic layer. Laser-basedprocesses offer promising technologies for both requirements.

www.laserinstitut.org

Experimental setup

A lamp pumped Nd:YAG laser with a wavelength of 1064 nm and a continuous laser power up to 5 W was used for the local

heating of the laser fabricated film patches (Fig.4) above the Neel-temperature (approximately 500 K) of the

antiferromagnetic IrMn layer. The laser beam was focused to a focal radius of 13.4 µm (86% power inclusion) by using a

f-theta optic with a focal distance of 80 mm. The patches were irradiated by the fast scanned continuous laser beam while

the pattern was exposed to an external magnetic field of at least 100 mT. The hatch distance between the scanned lines

was 13.4 µm, corresponding to an overlap of 50 %. The magnetic field was supplied by a permanent magnet, directed in

opposite direction to the initially set exchange bias field of the spin valve system.

The magnetization orientation of the pinned ferromagnetic layer was determined by using a magnetic microsensor (STJ-001

from Micro Magnetics) implemented as a scanning probe technique (Fig. 6). As the magnetic flux lines pass out and reenter

the layer stack vertically at the pattern edges, they are aligned in the direction of sensitivity of the sensor.

Results and discussion

The results achieved with the magnetic microsensor demonstrated that laser intensities between 380 kW/cm2 (scan velocity

1000 mm/s) and 420 kW/cm2 (scan velocity 4000 mm/s) caused a complete realignment of the magnetic pinned layer. Fig. 8

summarizes the magnetic behavior.

Magneto-optical Kerr effect (MOKE) measurements confirmed the results obtained with the magnetic microsensor. Fig. 9

shows typical magnetic hysteresis loops of the spin valve layer stack before and after laser irradiation. The hysteresis loop

of the non-irradiated sample reveals two-step reversal with the free layer exhibiting a reversal at low field and the reference

pinned layer switching at higher magnetic fields of about 70 kA/m. Applying a laser radiation at small laser intensity in an

external magnetic reverse field leads to a partial setting of the exchange bias in the opposite direction as indicated by the

appearance of the magnetization reversal of the pinned layer in the negative field direction. At higher laser intensities, a

complete shift of the reference layer to the opposite direction can be achieved.

Fig. 6: Left: magnetic micro sensor.Right: lateral view of the samplegeometry with magnetic flux lines andrelative motion of the sensor.

Fig. 5: Experimental setup.

Fig. 8: Setting of the exchange bias field as afunction of scan velocity and laser beam intensity.Results achieved with the magnetic microsensor.

Fig. 9: MOKE hysteresis loops at room temperature before and aftercontinuous laser irradiation in an external magnetic field applyingdifferent laser beam intensities using a scan velocity of 1000 mm/s.

Fig. 1: Schematic of the

spintronic stack layer sputtered

on a silicon substrate, total film

thickness is 23 nm, exchange

bias field initially set during the

growth process.

Spin valve Selective magnetizing by laser heating

Micro structuring by laser ablationrelative motiondirection

of sensitivity

sensor with direction of

sensitivity

‐40

‐20

0

20

40

0,385 0,485 0,585 0,685 0,785 0,885 0,985 1,085 1,185 1,285 1,385 1,485 1,585

relative m

agn. flux density [µT]

position [mm]0.2 0.4 0.6 0.80 1.0 1.2

Fig. 7: Measurement curve recorded withthe magnetic microsensor, scheme of thefilm patches in the background. Peaks atthe patch edges: maxima - magnetic fluxlines pass out, minima - magnetic fluxlines reenter.

scanner

optic

(f = 80 mm;w0,86 = 13.4 µm)

sample

permanent magnet (in the processing plane:

B = 100 – 200 mT)

sample table

Nd:YAG laser

λ = 1064 nmcw or q-switched

beam attenuation

Fig. 2: Machining workstation.

Fig. 3: Processing examples, obtained with a peak fluence of 0.12 J/cm².

- repetition rate 128 kHz

- pulse duration 180 fs

- wavelength 1030 nm

- focusing objective f = 30 mm

- focal diameter w0,86 = 18.2 µm

- fast galvanometer beam

deflection system v > 3.0 m/s

Fig. 4: Test pattern for magnetizing experiments.

5 µmc)

b)

a)

200 µm