Nous avons commencer par ´etudier le comportement du champ coercitif HC ainsi que du
champ d’´echange HE de notre structure en fonction de l’´epaisseur des couches P t1 et P t2 (voir
figure A.1). Nous remarquons que µ0HC d´epasse les 5 mT dans tous les cas, ce qui est un
premier facteur limitant dans l’utilisation de telles structures pour des capteurs. Cependant, il est ´egalement possible d’ajuster l’´epaisseur de P t2 afin de d´ecaler les cycles d’hyst´er´esis. La figure A.1.b montre que pour des ´epaisseurs proches de 1 nm, HE est de l’ordre de grandeur de
HC. Dans cette configuration nous observons le retournement de l’aimantation pour des champ
appliqu´es proches de z´ero.
Cette m´ethode nous permettrait alors de choisir le champ de retournement avec une g´eom´e- trie fix´ee. Nous nous sommes malheureusement aper¸cus que l’´elaboration de couches pr´esentant un champ HE d´etermin´e avec pr´ecision pr´esentait un r´eel d´efi technologique avec les moyens
du laboratoire.
L’observation par microscopie Kerr a elle aussi r´ev´el´e des comportements int´eressants. Il a ´
et´e mis en ´evidence que de fortes inhomog´en´eit´es existait sur nos ´echantillons, avec des zones poss´edant des champs coercitifs jusqu’`a deux fois plus importants que la moyenne. De plus, le retournement dans ce type de structures est essentiellement gouvern´e par le d´eplacement des parois. Ce dernier est tr`es lent, de l’ordre de la seconde. En revanche, la relative simplicit´e de la mise en œuvre de la configuration polaire du microscope ainsi que les contrastes observ´es sont de s´erieux avantages des mat´eriaux `a anisotropie perpendiculaire.
L’´etude est suivie du calcul de la constante d’anisotropie par une m´ethode originale passant par l’application d’un champ magn´etique oblique lors des mesures MOKE [104].
A.4. CONCLUSION 129
Fig. A.2 – Images obtenues par microscopie Kerr du retournement d’une couche de Co dans une structure P t1(20˚A) − Co(5˚A) − P t(e) − IrM n(120˚A).
A.4
Conclusion
Les multi-couches `a anisotropie perpendiculaire et couplage d’´echange pr´esentent des pro- pri´et´es int´eressantes pour des applications capteurs :
– les forts contrastes combin´es `a une configuration polaire plus simple `a mettre en place que la configuration longitudinale,
– une sensibilit´e hors du plan.
En revanche, plusieurs difficult´es nous ont pouss´es `a nous tourner vers d’autres couches : – les forts champs coercitifs,
– la difficult´e `a reproduire des ´echantillons avec un mˆeme d´ecalage dˆu au champ d’´echange, – la g´en´eration d’un champ microonde de forte amplitude hors du plan.
Journal of Magnetism and Magnetic Materials 316 (2007) 147–150
Exchange bias and perpendicular anisotropy study of ultrathin
Pt–Co–Pt–IrMn multilayers sputtered on float glass
M. Laval
, U. Lu¨ders, J.F. Bobo
LNMH CNRS ONERA, 2 avenue E. Belin, BP 4025, 31055 Toulouse Cedex 4, France Available online 4 March 2007
Abstract
We have prepared ultrathin Pt–Co–Pt–IrMn polycrystalline multilayers on float-glass substrates by DC magnetron sputtering. We have determined the optimal set of thickness for both Pt layers, the Co layer and the IrMn biasing layer so that these samples exhibit at the same time out-of-plane magnetic anisotropy and exchange bias. Kerr microscopy domain structure imaging evidences an increase of nucleation rate accompanied with inhomogeneous magnetic behavior in the case of exchange-biased films compared to Pt–Co–Pt trilayers. Polar hysteresis loops are measured in obliquely applied magnetic field conditions, allowing us to determine both perpendicular
anisotropy effective constant Keffand exchange-bias coupling JE, which are significantly different from the ones determined by standard
switching field measurements.
r2007 Elsevier B.V. All rights reserved.
PACS: 75.60 Ej; 75.60 Ch; 75.70 Ak
Keywords: Exchange bias; Perpendicular anisotropy; Domain structure; Hysteresis loop modeling
1. Introduction
The feasibility of multilayer systems with perpendicular magnetic anisotropy like Pt–Co–Pt has been evidenced by
many authors [1–6]. More recently, exchange-biased
magnetic films with perpendicular anisotropy were ob-
served, either with Pt2Co2Pt2FeF2 [7] or with
Pt–Co–Pt–CoO multilayers [8]. Such experiments bring a new insight on exchange-biased multilayers, i.e. ferromag- netic–antiferromagnetic (F–AF) systems in which a shift of the hysteresis loop occurs due to coupling between AF and F spins at the interface. Such systems have been more intensively studied with in-plane magnetic easy axis[9]. The observation of exchange bias in out-of-plane magnetized systems is expected to allow a more precise study of the influence of domain structures of AF and F materials on the magnetic behavior, as well as the study of exchange coupling in the case of ultrathin F layers.
For applications, achieving exchange-biased magnetic systems with out-of-plane easy axis is an important
challenge for data storage, for example for MRAM or magnetic nanosensors technology. For sensors applica- tions, one needs either systems with linear magnetic response for analogic reading or systems with controlled switching field for discrete peak magnetic field detection.
The objective of our work concerns this last issue. The results presented in this paper concern the determination of optimal individual layer thicknesses for simultaneous stabilization of exchange bias and out-of-plane anisotropy in the following five layer structures:
float glassFPt12Co2Pt22IrMn2Pt3,
where Pt1 is a buffer layer, Co is the F layer, Pt2 is a spacer layer required for preserving out-of-plane anisotropy[10], IrMn is the AF biasing layer and Pt3 is a 20 ˚A capping layer.
2. Experimental procedures
The samples were prepared by DC magnetron sputtering in 5 mTorr Ar pressure from separate Co, Pt and Ir0:77Mn0:23 (IrMn) targets. The pressure before deposition
was lower than 1 107Torr. Deposition rates were,
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0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.108
Corresponding author.
respectively, 0.16, 0.33 and 0:19 ˚A=s for Co, Pt, and IrMn. In order to induce a preferential bias axis, the substrate- holder is fitted with a permanent magnet which delivers 4 k Oe perpendicular to the plane of the substrate.
A preliminary tIrMn thickness dependence of exchange
bias has evidenced the optimal IrMn thickness to be 120 ˚A to observe loop shift in our samples. This thickness is constant for all samples presented here.
Hysteresis loops were measured by magneto-optic Kerr effect (MOKE) with a 50 Hz AC magnetic field ranging from 7 to +7 k Oe applied either normal to the sample plane (PMOKE) or with an oblique angle a (oblique PMOKE). Images of magnetic domains were acquired by a Kerr effect microscope operating in polar configuration and high power halogen lamp polarized light. The field stability and accuracy of the Kerr microscope was better than 0.1 Oe.
3. Results
Fig. 1 represents the dependence of both coercive (HC)
and exchange-bias (HE) fields as a function of the Pt1 and
Pt2 thicknesses. In Fig. 1a, we note an onset of per- pendicular anisotropy for Pt1 thicker than 10 ˚A, PMOKE loops for thinner buffer layers have low squareness. The coercive field increases monotonicaly up to 160 Oe for tPt1¼35 ˚A. Even larger values of tPt1 provide larger HC,
typically up to 500 Oe for tPt1¼200 ˚A. They are useless for
sensor applications where low coercivity is required and are not presented in the present study. In Fig. 1b, for a constant set of Pt1ð20 ˚AÞ, Coð4 ˚AÞ and IrMnð120 ˚AÞ, we have plotted the tPt2 dependence of both HC and HE.
We observe an onset for perpendicular anisotropy for tPt2729 ˚A. Below this thickness, we assume the 4 A˚-thick
cobalt layer loses out-of-plane anisotropy due to insuffi-
cient interfacial band hybridization with Pt2. However, exchange bias culminates at 7 ˚A Pt2 due to direct coupling with IrMn through pinholes that we suppose to be present in the subnanometer Pt2 layer. For larger tPt2, HCkeeps a
stable value of 60280 Oe while HE decreases to reach a
negligible value for tPt2415 ˚A. This is evidence for indirect
exchange-bias coupling between IrMn and Co through ultrathin platinum layers as reported by others [10]. Note that, due to the dynamic character of PMOKE, HCvalues
are slightly larger than the ones determined in static microscopy measurements.
Kerr microscopy was performed on several samples. For Pt–Co–Pt–IrMn–Pt structures[11], the overall thickness of the top layers of Pt and IrMn is a drawback for direct observation of the domain structure. However, back-side observations through the glass substrate allow the detec- tion of magneto-optic (MO) contrast, as shown inFigs. 2
and 3. The sample presented in Fig. 2 has a completely
shifted hysteresis loop, i.e. with a coercive field smaller than the exchange-bias field. The sample was initially saturated
ARTICLE IN PRESS
Fig. 1. (a) Pt1 thickness dependence of HCand HEfor a series of samples with Pt1–Co(4 A˚)–Pt2(12 A˚)–IrMn(120A˚) deduced from PMOKE; (b) Pt2 thickness dependence of HCand HEfor a series of samples with Pt1ð20 ˚AÞ, Coð4 ˚AÞ and IrMnð120 ˚AÞ.
Fig. 2. Kerr microscopy images recorded between 24 and 16 Oe across the magnetization reversal of the Co layer of a Pt(20 A˚)–Co(4 A˚)–P- t(12 A˚)–IrMn(120 A˚). The size of the pictures is 600 500 mm.
Fig. 3. Kerr microscopy images recorded between 0.2 and 7 Oe across the low field branch of the magnetization reversal of the Co layer of a Pt(20 A˚)–Co(4 A˚)–Pt(8 A˚)–IrMn(120 A˚) sample. The size of the pictures is 600 500 mm.
M. Laval et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 147–150 148
at H ¼ 100 Oe and a reference image was acquired. This reference image is subtracted from all the following images of the up-field ramp. Up to 24 Oe, no significant MO contrast is detected (uniform gray level for the image). At 21:5 Oe, some dark spots appear on the image. They are the signature of nucleation centers. Increasing field evidences a growth of the area of the initially switched parts of the cobalt film by domain wall propagation. This behavior is quite similar for the down field ramp.
In Fig. 3, a sample with the formula Pt(20 A˚)–
Co(4 A˚)–Pt(8 A˚)–IrMn(120 A˚) (with a coercive field nearly
equal to the exchange-bias field) is presented during its almost zero field switch. This result illustrates the high sensitivity of the domain structure of the exchange-biased cobalt film at low field. We believe such structures are good candidates for the detection of sub-Oe magnetic fields with a contact-less MO technique. One can also notice the significant inhomogeneity of switching behavior of differ- ent submillimeter regions of the sample. This suggests that exchange biasing is not constant through the sample surface, either due to AF magnetic domain structure of IrMn or to thickness fluctuations of Pt2.
All these Kerr microscopy data illustrate the important domain nucleation and propagation processes occurring in our samples. We have developed an original method for the determination of anisotropy and exchange bias by polar MOKE hysteresis loops with a quasi-in-plane obliquely applied magnetic field as shown inFig. 4 [12]. For a system with exchange bias and perpendicular anisotropy, if a is small (i.e. H is along a hard axis), the Stoner–Wohlfarth model is valid and the energy of the system is
E ¼ MsH cosðp=2 y aÞ þ Keffsin2y
JE
tCo
cos y. (1) Minimizing (1) with respect to y (angle between the magnetization and the MOKE polar axis) gives the equation that allows fitting of the data:
H ¼ 2Keffcos y þ JE=tCo Mscosðy þ aÞ
sin y. (2)
Such fitted data are presented inFigs. 5 and 6 for two of our samples.Table 1is a summary of our simulations for a set of samples with increasing tPt2. As expected, Keff
increases with increasing tPt2 while JE decreases. One
important feature is that JE determined from fitting to
Eq. (1) yields results of the same order but larger than the ones ðJEÞobtained by the standard loop shift measurement ðJ
E¼MsHEtCoÞ. Our interpretation of this behavior is
that, in the oblique field procedure, we model the magnetic behavior of the cobalt film in a reversible single domain regime, whereas switching field measurements are obtained from irreversible parts of the magnetization curves. Since irreversible effects like domain nucleation and domain wall propagation are present in our samples, this last method is known to provide underestimated values of exchange coupling and uniaxial anisotropy.
In conclusion, we have successfully prepared exchange- biased films with perpendicular anisotropy. Their magnetic
Fig. 4. Principle of MOKE experiments with obliquely applied field.
Fig. 5. Fit (solid line) to Eq. (2) of the hysteresis loop of an unbiased Pt (20 A˚)–Co(4 A˚)–Pt(12 A˚) film ða ¼ 5Þwith the PMOKE loop in inset.
Fig. 6. Fit (solid line) to Eq. (2) of the hysteresis loop of an exchange- biased Pt(20 A˚)–Co(4 A˚)–Pt(11 A˚)–IrMn(120 A˚) film ða ¼ 5Þ with the PMOKE loop in inset.
Table 1
Summary of the anisotropy terms of the series of samples determined either by oblique field fittings or switching fields (*)
tPt2(A˚) Keffðerg=cm3Þ JEðerg=cm2Þ HE(Oe) HC(Oe) JEðerg=cm2Þ
11 2:24 106 2:4 103 54 70 3:0 103
12 2:40 106 2:2 103 36 63 2:0 103
13 2:40 106 2:0 103 18 74 1:0 103
switching reveals complex domain structures. Determina- tion of exchange bias and perpendicular anisotropy by oblique MOKE is, to our mind, the most appropriate way of investigation.
References
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M. Laval et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 147–150 150
Annexe B
Traitement du d´ecalage sub-pixel
B.1
Introduction
Le fonctionnement du microscope Kerr est bas´e sur l’acquisition d’une image de r´ef´erence qui sera ensuite soustraite `a l’ensemble des autre images qui comportent l’information sur la configuration des domaines magn´etiques. L’acquisition de l’ensemble des images d’une mˆeme exp´erience peut s’´etaler sur des temps importants (plusieurs dizaines de minutes). Il est alors ´
evident qu’un d´eplacement micronique de notre ´echantillon peut se produire. Cette d´erive en- traˆıne une perte d’information cons´equente qu’il est n´ecessaire de corriger. Le traitement des images obtenues par la microscopie Kerr permet de faire apparaˆıtre des contrastes sur des clich´es `
a priori inexploitables.
Fig. B.1 – A gauche et au milieu, les images utilis´ees pour les calculs pr´eliminaires. A gauche, la m´ethode utilis´ee pour cr´eer un d´ecalage artificiel sur les pr´ec´edentes images.