Ns Resolved Techniques

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M. Bonfim, K. Mackay, S. Pizzini, M-L. Arnou,A. Fontaine et al.Citation: J. Appl. Phys. , 5974 (2000); doi: 10.1063/1.372584View online: http://dx.doi.org/10.1063/1.372584View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v87/i9Published by theAmerican Institute of Physics.

The coercivity and domain structure of Sm(CobalFe0.1CuxZr0.033)6.9 (x=0.07, 0.10, 0.13) high temperaturepermanent magnetsJ. Appl. Phys. 112, 033909 (2012)

A spin triplet supercurrent in half metal ferromagnet/superconductor junctions with the interfacial Rashba spin-orbit couplingAppl. Phys. Lett. 101, 062601 (2012)Wireless and passive temperature indicator utilizing the large hysteresis of magnetic shape memory alloys

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Appl. Phys. Lett. 101, 042406 (2012)Influence of rare earth moment ordering on magnetic entropy change in Nd0.5Sr0.5CoO3

Appl. Phys. Lett. 101, 042411 (2012)

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Nanosecond resolved techniques for dynamical magnetization

reversal measurements

M. Bonfim,a) K. Mackay, S. Pizzini, M-L. Arnou, and A. FontaineCNRS, Laboratoire Louis Neel, BP 166, F-38042 Grenoble, France

G. Ghiringhelli, S. Pascarelli, and T. NeisiusEuropean Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France

In this article we present three techniques developed by our group for probing magnetizationdynamics in the nanosecond time scale. All these techniques are based on the magneto-optical

interaction of materials with polarized light. Magnetic excitation is provided by microcoils able to

generate field pulses of some teslas within a few nanoseconds. Standard Kerr/Faraday dynamic

measurements and imaging can be performed as well as time-resolved x-ray magnetic circular

dichroism where chemical selectivity can be achieved. 2000 American Institute of Physics.

S0021-8979 00 91508-1

INTRODUCTION

Magnetization dynamics of magnetic films in the nano-second regime is an essential issue for the future of magnetic

recording and nonvolatile magnetic memories.1,2 Measure-

ments on these time scales involve the ability of generating

fast magnetic or thermal pulses and detecting the magneti-

zation process by means of high frequency techniques. We

have developed microcoils associated with fast current

sources able to generate magnetic pulses as high as 8 T with

rise time of some nanoseconds. Three different systems for

probing the magnetization of thin films on the nanosecond

scale were developed: magneto-optical Kerr effect, magneto-

optical imaging, and x-ray magnetic circular dichroism

XMCD .

MICROCOILS AND PULSED SOURCES

A set of single turn microcoils with inner dimensions

ranging from 50 to 800 m was developed. Figure 1 shows

two types of microcoils. In type a a copper layer of 30 m

is electrochemically deposited over the silicon substrate. The

sample placed on the top of the coil is submitted to a mag-

netic field that is perpendicular to its plane. In type b the

copper is deposited on the inner region of the window made

in the silicon substrate. The applied field is then parallel to

the sample that is placed inside the coil. Characterization ofthe field on these coils was done either by calculation or

measurements with paramagnetic materials.3

The current drivers developed for these microcoils are

based on fast power metaloxide semiconductor field effect

transistor. Peak currents up to 700 A can be obtained with

typical dI/dt on the order of 5 A/ns. This allows fields up to

8 T to be generated with the 50 m microcoils without dam-

age. The repetition rate can go up to 360 kHz for 0.7-T-high

and 25 ns wide pulses. Another current driver based on fast

capacitor discharge has allowed 50 T/30 ns pulses with a

repetition rate of about 1 Hz.

TIME RESOLVED MAGNETO-OPTICAL KERR EFFECT

A simple way to dynamically probe the magnetization in

the nanosecond scale is by means of visible light. Interac-

tions of the light with matter take place in the 1015 s range,

so it can be neglected when probing phenomena slower than

1012 s. In addition, the use of light as the probe makes the

measure relatively immune to the strong electromagnetic

noise EM present in the high current pulse generation. We

have chosen the magneto-optical Kerr and Faraday effects

for probing opaque and transparent materials, respectively.

Figure 2 shows schematically the setup developed for this

purpose. Polarized light from a 5 mW HeNe or diode laser

is shone onto the sample placed in or on the microcoil. The

polarization of the reflected or transmitted light is then ana-

lyzed and its intensity detected with a fast Si photodiode

100 MHz BW . The signal is then measured using a fast

digitizing oscilloscope 500 MHz BW, 1 GS/s and trans-

ferred to a computer. The analyzer is placed at 45 with

respect to the polarizer, where we have the highest sensitivity

a Electronic mail: [email protected]

FIG. 1. Microcoils for applying fields perpendicular a or parallel to the

sample b .

JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000

59740021-8979/2000/87(9)/5974/3/$17.00 2000 American Institute of Physics



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and linearity between the angle and the intensity. Statistical

noise is greatly reduced by averaging some measurements

taken in the same conditions. In order to eliminate the syn-

chronous EM noise, the analyzer is flipped from 45 to

45 between each successive measurement and the differ-ence gives the final value. A resolution of 5 rad on the

polarization plane is achieved in 3 min of acquisition time.

Differently from other dynamic Kerr techniques based on a

pumpprobe scheme,4,5 this real time approach allows single

shot measurements, useful to probe nonreproducible mag-

netic states. Magnetic thin films like garnets, Co, Fe, Ni,

TbFe, PtCo multilayers, amorphous GdCo and TbCo, were

dynamically probed with this setup.

Exploiting the ability to have high pulsed fields, a high

perpendicular anisotropy film was studied. The sam-

ples consist of a 7-m-thick crystalline film of

YGdTmBr 3 FeGa 5O12, grown by liquid phase epitaxy

over a GGG gadolinium gallium garnet substrate. Figure 3shows the magnetization dynamics of this sample for a

pulsed field applied perpendicular to the film 0.24 T/50 ns ,

using the coil shown in Fig. 1 a . The laser circular spot

focalized on the sample has about the same dimensions of

the magnetic domains. The result shown is the average in

time of 1000 single shot measurements it takes about 20 s .

The inset shows two single shot measurements with the same

conditions. Since the applied field is strong enough to satu-

rate the magnetization of the sample, nonreproducible mag-

netic states are found, which can be confirmed by the differ-

ent shape of the reversal in a single shot compared to an

averaged measurement. This is due to the high perfection of

the garnet, where domain nucleation can occur randomly in

the film, once the magnetization was saturated. These results

can be better understood with the dynamic image sequence

shown in the next topic.

TIME RESOLVED MAGNETO-OPTICAL IMAGING

Three different mechanisms are known to take part inthe magnetization reversal dynamics of thin films in the

nanosecond scale:6 domain nucleation, domain wall propaga-

tion, and coherent rotation. The first two mechanisms are

essentially thermally activated7,8 and dominate the reversal

in the low field low frequency regime. Coherent rotation is

not thermally activated and typically occurs for high field

high frequency magnetic fields.6 In order to distinguish and

study these mechanisms, magneto-optical imaging appears to

be a powerful technique for dimensions down to submicron. 4

With the same optical setup used for dynamical Kerr and

Faraday measurements, we have developed a magneto-

optical, imaging system, having as detector a charge-coupled

device camera. It is based on a pumpprobe technique,where the pump is the magnetic pulse and the probe is

a flash of light from a laser diode 850 nm, 5 ns full width at

half-maximum . This is a new and inexpensive way to re-

place the pico- or femtosecond lasers normally used for this

kind of experiment, in detriment of the time resolution sub-

nanosecond laser diodes are already available . This setup

has permitted us to visualize magnetic domain motion with a

spatial resolution of about 0.7 m and a time resolution of 5

ns.

Figure 4 shows a series of images from a

YGdTmBi 3 FeGa 5O12 garnet, taken before and after the

application of a pulsed field 0.2 T/50 ns at room tempera-

FIG. 2. Schematic diagram of the optical setup for dynamical Kerr and

Faraday measurements.

FIG. 3. Time response of a garnet film YGdTmB 3 FeGa 5O12 for a 0.24

T/50 ns pulsed field time averaged of 1000 single shots . Inset shows two

single shot measurements taken with identical conditions, giving evidence

of the nonreproducibility of the magnetization reversal.

FIG. 4. Dynamic image sequence of a garnet before and after a 0.2 T/50 ns

pulse applied at t0 by a 50 m1000 m coil. Each frame measures

50 m100 m.

5975J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Bonfim et al.



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ture. As already discussed previously, once the magnetiza-

tion of this sample is saturated, reversal occurs randomly,

preventing the use of a pumpprobe technique. Therefore,

the amplitude of the applied field was chosen in order to

avoid complete saturation of the probed region.

One can see that, in this system, the magnetization re-

versal process in this time scale is dominated by domain wall

propagation instead of nucleation or coherent rotation. The

domain wall speed after the magnetic pulse duration can beeasily calculated from two subsequent images and has an

average value of 35 m/s. Domains enlarge up to a dimension

a bit bigger than the steady state and then relax to a static

condition. This can be also seen in Fig. 3 negative overshoot

on time response . The coil used here gives rise to a vertical

gradient of field, having its maxima near the borders top and

bottom and a minimum at the center. This explains why the

regions near the borders are completely saturated while in

the center some domains remain. This also explains the

bubblelike domains in the center and ribbonlike domains

near the borders.

TIME RESOLVED XMCD

Magneto-optical analysis with visible light cannot sepa-

rate the contribution of each magnetic layer in heterostruc-

tures such as spin valves.9,10 To overcome this limitation, a

time-resolved technique based on x-ray magnetic circular di-

chroism XMCD was developed by our group at the Euro-

pean Synchrotron Radiation Facility ESRF .11 The tech-

nique takes advantage of the ESRF single bunch structure,

associated with the microcoil setup for the generation of

magnetic field pulses synchronized with the photons bunches

in a pumpprobe scheme. The time dependence is ana-

lyzed by changing the time delay between the pump the

magnetic pulse and the probe the 100-ps-long x-raypulse , which have the same repetition rate 357 kHz . Ele-

ment selectivity is obtained as the x-ray energy can be tuned

to the absorption edge of the desired element. Soft 500

1500 eV or hard 515 keV x rays can be chosen depend-

ing on the element and shell to be analyzed. The absorption

signal is measured in total fluorescence yield for the soft x

rays or in transmission for the hard x rays. In the field of soft

x rays, absorption takes place in a few tens of angstroms,

which makes the measurement feasible for very thin layers

deposited on an unspecified substrate. The acquisition time

for a 400 point dynamic curve is about 5 min, including

change of the photon helicity. The ultimate time resolution is100 ps, limited by the width of the x-ray pulse. Previous

measurements done on GdCo thin films were reported in

Ref. 11. New results obtained for spin valve systems will

soon be reported elsewhere.

CONCLUSION

Three nanosecond resolved magneto-optic based tech-

niques were developed by our group for studying magneti-

zation dynamics of thin films. Magneto-optical Kerr and Far-

aday setups can be used to measure single magnetic layers

and their association with magneto-optical imaging gives a

better understanding of the mechanisms present on the mag-netization reversal domain nucleation and propagation, co-

herent rotation . Time resolved XMCD which provides ele-

ment selectivity, is an extremely powerful tool for studying

separately each magnetic layer in heterostructures like spin

valves and multilayers.

1 W. D. Doyle, S. Stinnett, C. Dawson, and L. He, J. Magn. Soc. Jpn. 22, 91

1998 .2 M. R. Freeman et al., J. Appl. Phys. 79, 5898 1996 .3 M. Bonfim and K. Mackay unpublished .4 M. R. Freeman et al., J. Appl. Phys. 83, 6217 1998 .5 T. J. Silva et al., J. Appl. Phys. 81, 5015 1997 .6 R. H. Koch et al., Phys. Rev. Lett. 81, 4512 1998 .7

L. Neel, Ann. Geofis.5

, 99 1949 .8 W. F. Brown, Phys. Rev. 130, 1677 1963 .9 G. Prinz, Phys. Today 48, 58 1995 .

10 J. S. Moodera et al., Phys. Rev. Lett. 74, 3273 1995 .11 M. Bonfim et al., J. Synchrotron Radiat. 5, 750 1998 .

5976 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Bonfim et al.

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