Exchange bias phenomena in ferrimagnetic based bilayersF. Canet, S. Mangin, C. Bellouard, M. Piécuch, and A. Schuhl Citation: Journal of Applied Physics 89, 6916 (2001); doi: 10.1063/1.1357148 View online: http://dx.doi.org/10.1063/1.1357148 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A setup combining magneto-optical Kerr effect and conversion electron Mössbauer spectrometry for analysis ofthe near-surface magnetic properties of thin films Rev. Sci. Instrum. 80, 043905 (2009); 10.1063/1.3121215 Influence of an interface domain wall on spin-valve giant magnetoresistance Appl. Phys. Lett. 93, 222503 (2008); 10.1063/1.3041640 Tuning exchange-bias properties by thermal effects in a hard/soft bilayer Appl. Phys. Lett. 91, 022505 (2007); 10.1063/1.2753108 Influence of magnetic domain-wall width and shape on magnetoresistance measurements J. Appl. Phys. 89, 7203 (2001); 10.1063/1.1357114 Positive giant magnetoresistance in ferrimagnetic/Cu/ferrimagnetic films J. Appl. Phys. 89, 7124 (2001); 10.1063/1.1357112

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Exchange bias phenomena in ferrimagnetic based bilayersF. Canet, S. Mangin,a) C. Bellouard, M. Piecuch, and A. SchuhlLaboratoire de Physique de Mate´riaux, UHP-Nancy I, BP 239, F-54506 Vandoeuvre cedex, France

We present positive and negative exchange bias~EB! phenomena observed in twoferrimagnetic-based amorphous bilayers systems: FeSn/FeGd and FeGd/TbFe. In both cases,magnetization measurements exhibit a shift of the loop of one of the ferrimagnetic layer towards apositive or negative field. In FeSn/FeGd, the coupling at the interface between the net magnetizationof the two layers is antiferromagnetic, which leads to a positive shift. However in FeGd/TbFebilayers, the coupling is ferromagnetic and the shift is negative. The EB phenomenon is attributedto the occurrence of a magnetic domain wall present at the interface, which can be ‘‘compressed’’or ‘‘decompressed’’ by an external applied field. A simple model based on the competition betweenthe Zeeman energy and the interface exchange interaction energy permits a good quantitativeevaluation of the observed EB fields. ©2001 American Institute of Physics.@DOI: 10.1063/1.1357148#

I. INTRODUCTION

Over the past decade, a great interest has been shown forunderstanding reversal processes in exchange biasedantiferromagnetic/ferromagnetic~AF/F! bilayers. Indeed, ex-change bias~EB! phenomenon has applications in spin-valvegiant magnetoresistance devices1 and tunneling junctions.2

Its characteristic is that after cooling the sample in high posi-tive field through the Ne´el temperature of the AF layer, ashift of the hysteresis loop from Hv0 to a bias field HE isobserved. The fundamental origin of this field is still underheavy discussion.3–5 In the present article, we studyFerrimagnetic/Ferrimagnetic bilayers systems which exhibitEB. Our goal is to obtain new systems for applications and tocontribute to a better understanding of exchange bias phe-nomena. We present results obtained on amorphousFe55Sn45/Fe60Gd40 and Fe60Gd40/Tb55Fe45 bilayers preparedby coevaporation in a high vacuum on substrates cooled atT577 K in order to grow amorphous alloys. The growthconditions are detailed in Ref. 6. An uniaxial anisotropy axisis induced by specific growth conditions in FeSn and FeGdalloys. In FeSn/FeGd bilayers, Fe55Sn45 is a ferromagneticalloy and Fe60Gd40 is a ferrimagnetic one. In Fe60Gd40, thecoupling between Fe and Gd spins is antiferromagnetic andthe contribution of Gd moments to magnetization is domi-nant. Because the exchange coupling between the layers isdominated by Fe–Fe ferromagnetic interactions, the couplingbetween the net magnetization of the two layers is antiferro-magnetic. In FeGd/TbFe bilayers, FeGd is a soft ferrimag-netic alloy and Tb55Fe45 a hard one. Since the coupling be-tween Fe and the rare earth~RE! is antiferromagnetic, andthe contribution of the RE moment is dominant for bothalloys, the coupling between the two magnetization layers isferromagnetic.

II. RESULTS AND DISCUSSION

A. FeGd „ferrimagnetic …ÕTbFe„ferrimagnetic … bilayers

Figure 1 presents an hysteresis loop obtained on a fieldcooled FeGd~1000 Å!/TbFe~120 Å! bilayer, for an appliedmagnetic field swept fromH52 kOe toH52300 Oe, andthen back toH52 kOe. We clearly observe a shift of theFeGd layer magnetization loop towards a negative exchangebias fieldHE5250 Oe. Figure 2 shows the deduced mag-netic configurations of the bilayer for various applied fields~H!. From high applied field toH5HE , the sample is keptsaturated along the easy axis direction@Fig. 2~a!#. For H5HE2Hc a magnetization drop is observed due to the mag-netization reversal of the FeGd layer. This leads to the for-mation of a 180° domain wall~DW! at the interface which ispinned by the hard TbFe layer@Fig. 2~b!#. Because of theuniaxial anisotropy induced in the FeGd layer, the DW is avery well defined Bloch 180° DW~with a modulation vectorperpendicular to the film plane!, and magnetization lays inplane. Further decrease of the external field leads to the

a!Electronic mail: mangins@lps.u-nancy.fr; author to whom correspondenceshould be addressed at Laboratoire de Physique des Mate´riaux, Universite´Henri Poincare´, B. P. 239 54506, Vandoeuvre-les-Nancy cedex, France.

FIG. 1. Magnetization loop obtained on a FeGd~1500 Å!/TbFe~100 Å! bi-layer atT55 K (H52 kOe→H52300 Oe→H52 kOe), after cooling thesample from 300 to 5 K under a field of 2 kOe. The loop is shifted towarda negative fieldHE , its width is equal to 2Hc .

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001

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‘‘compression’’ of the DW against the TbFe layer whosemagnetization is constrained by anisotropy, and to a decreaseof magnetization@Fig. 2~c!#. When the field is swept back,the magnetization slowly increases due to the DW decom-pression~DW width increases!. For H larger thanH5HE

1Hc , the DW is annihilated and the sample come back tothe saturated state. The width of the minor loop is equal to2Hc whereHc is the coercive field of a single FeGd layer.The presence of the 180° DW located in FeGd has also beenevidenced by ac-susceptibility7 and magnetotransport8 mea-surements, and by polarized neutron reflectometry.9 We canestimate the exchange bias field as:

HE52Ds

2Mst, ~1!

wheret and Ms are, respectively, the layer thickness and thesaturated magnetization of the FeGd alloy andDs is thedomain wall energy. The values obtained from the aboveexpression are in very good agreement with the experimentaldata.

B. FeSn „ferromagnetic …ÕFeGd„ferrimagnetic … bilayers

A quasistatic magnetization measurement performed ona Fe55Sn45(800 Å)/Fe60Gd40(1000 Å) bilayer is shown Fig.3. For an applied magnetic field higher thanHE1Hc1

'50 Oe, the net magnetization of both layers tends to bealigned in the direction of the field. Because of the AF cou-pling at the interface between the net magnetizations, a DWis induced between the layers as sketched on Fig. 4~a!. ThisDW is a 180° DW because of the uniaxial anisotropy. As theapplied field is decreased untilH5HE1Hc1 , one can ob-serve a slow decrease of the magnetization, due to the in-crease of the DW thickness. This DW decompression canalso be observed very well by magnetotransport and ac-susceptibility measurements.10 At H5HE1Hc1 , a first dropof magnetization is observed. It corresponds to the reversalof the net magnetization of the FeSN layer, which is smallerthan the net magnetization of the FeGd layer@Fig. 4~b!#.BetweenHE1Hc1 and HC2 , the net magnetizations of thetwo layers point in opposite directions. Another drop of mag-netization occurs for an applied fieldHC , attributed to theglobal reversal of the magnetization of the bilayer whichkeeps its AF configuration@Fig. 4~c!#. Finally, another drop

of magnetization occurs for an applied fieldHR2 , corre-sponding to the reversal of the magnetization of the FeSnlayer and to the creation of a DW at the interface. The valuesof the relative magnetization drops are consistent with themodel proposed here.

The inset in Fig. 3 shows a minor loop obtained on thesame bilayer by sweeping the field fromH52 kOe to H50 Oe, then back toH52 kOe. This hysteresis loop, attrib-uted to the reversal of the FeSn layer, is shifted away fromthe zero-field axis to a positive fieldH5HE . The width ofthe loop is 2Hc1 whereHc1 is the coercive field of FeSn.This behavior is very similar to the positive exchange biasphenomenon11,12 since it corresponds to the reversal of alayer which is exchange coupled to a second one.

We have systematically studiedHE relative to FeSn andFeGd thicknesses (tFeSnandtFeGd) ranging from 200 to 1500Å. All magnetization loops present a minor loop centered atHE.0 which corresponds to the magnetization reversal ofthe layer with the smallest magnetization per surface unit(Ms* t). The evolution ofHE relative to the lowest magne-tization per surface unit@Min( MFeSntFeSn,MFeGdtFeGd)#, isshown Fig. 5. HE shows a quick decrease as@Min( MFeSntFeSn,MFeGdtFeGd)# increases. If we consider thatEB is due to the competition between the Zeeman energygain when both magnetization layer are parallel to the field

FIG. 2. Magnetic configuration of a FeGd/TbFe bilayer for different appliedfield: (A:12 kOe.H.HE2Hc5250 Oe) the saturated state, (B:H.HE

2Hc) a large DW state, (C:H@HE2Hc Oe) small~compressed! DW.FIG. 3. Hysteresis loop measured on a FeGd~1000 Å!/FeSn~800 Å! at T55 K. The inset represents a minor loop obtained on the same bilayer (H52 kOe→H50 Oe→H52 kOe). The minor loop is shifted toward a posi-tive field HE , its width is equal to 2Hc1 .

FIG. 4. Magnetic configuration of a FeGd~1000 Å!/FeSn~800 Å! bilayer fordifferent applied fields:~a! 12 kOe.H.HE2Hc15250 Oe, ~b! HE

2Hc1.H.Hc2 , and~c! Hc2.H.2HE1Hc2 .

6917J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Canet et al.

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and the exchange interaction at the interface~which is mini-mized by the formation of a domain wall!, we found thatHE

is also given by: HE5Ds/2Mst where Mst5@Min( MFeSntFeSn,MFeGdtFeGd)#. Assuming that the DW islocated in the FeSn layer,Ds is given by: DsFeSn(H)5pA2AFeSn(2MFeSnH1KFeSn), whereK is the constant an-isotropy of FeSn andAFeSn is the exchange constant. Usingthe aforementioned equations, we have been able to fit theevolution of HE . This fit provides a value of the exchangeconstant AFeSn5931028 erg/cm, in agreement with themean field theory.

III. CONCLUSION

In conclusion, ferro/ferrimagnetic FeSn/FeGd bilayerswith an antiferromagnetic interfacial coupling give rise topositive exchange bias, while soft ferri/hard ferri FeGd/TbFebilayers with a ferromagnetic interfacial coupling give rise tonegative exchange bias. These behaviors are very similar to

the one usually observed in AF/F exchange biased bilayers:Antiferromagnetic coupling is a necessary condition to ob-serve positive exchange bias.3–5 For both systems, we high-lighted the influence of the presence of a DW at the interfaceon magnetization reversal processes. The presence of DW inAF/F systems has already been proposed.4,12,13 We haveshown that it is possible to give a good quantitative evalua-tion of the exchange bias field by using a simple model basedon the competition between Zeeman energy and exchangeinteraction which leads to the formation of a DW.

The use of Ferri/F bilayers could be an alternative toAF/F bilayers as the limiting temperature is given by theCurie temperature of the Ferrimagnetic or Ferromagneticmaterial which can be found much higher than usual Ne´eltemperatures.

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FIG. 5. Evolution ofHE relative to the magnetization per surface unit (Mst)of the lowest (Mst) layer @Min( MFeSntFeSn,MFeGdtFeGd)#. Circles are the ex-perimental points and the line corresponds to the fit.

6918 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Canet et al.

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