Propagation of exchange bias in Co Fe ∕ Fe Mn ∕ Co Fe trilayersD. N. H. Nam, W. Chen, K. G. West, D. M. Kirkwood, J. Lu, and S. A. Wolf Citation: Applied Physics Letters 93, 152504 (2008); doi: 10.1063/1.2999626 View online: http://dx.doi.org/10.1063/1.2999626 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/93/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Amorphous FeCoSiB for exchange bias coupled and decoupled magnetoelectric multilayer systems: Real-structure and magnetic properties J. Appl. Phys. 116, 134302 (2014); 10.1063/1.4896662 Setting temperature effect in polycrystalline exchange-biased IrMn/CoFe bilayers J. Appl. Phys. 113, 17D704 (2013); 10.1063/1.4795211 Asymmetric stochasticity of magnetization reversal dynamics in exchange-biased IrMn/CoFe Film J. Appl. Phys. 111, 07D731 (2012); 10.1063/1.3694022 Magnetization profile of Ir in a MnIr/CoFe exchange bias system evaluated by hard x-ray resonant magneticreflectivity J. Appl. Phys. 106, 123919 (2009); 10.1063/1.3273313 Enhanced exchange bias in sub-50-nm IrMn/CoFe nanostructure Appl. Phys. Lett. 94, 082503 (2009); 10.1063/1.3085965

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Propagation of exchange bias in CoFe/FeMn/CoFe trilayersD. N. H. Nam,a� W. Chen, K. G. West, D. M. Kirkwood, J. Lu, and S. A. WolfDepartment of Materials Science and Engineering, University of Virginia,Charlottesville, Virginia 22904, USA

�Received 10 September 2008; accepted 22 September 2008; published online 14 October 2008�

CoFe /FeMn, FeMn /CoFe bilayers and CoFe /FeMn /CoFe trilayers were grown in magnetic fieldand at room temperature. The exchange bias field HEB depends strongly on the order of depositionsand is much higher at CoFe /FeMn than at FeMn /CoFe interfaces. By combining the two bilayerstructures into symmetric CoFe /FeMn�tFeMn� /CoFe trilayers, HEB

t and HEBb of the top and bottom

CoFe layers, respectively, are both enhanced. Reducing tFeMn of the trilayers also results inenhancements of both HEB

b and HEBt . These results evidence the propagation of exchange bias

between the two CoFe /FeMn and FeMn /CoFe interfaces mediated by the FeMn antiferromagneticorder. © 2008 American Institute of Physics. �DOI: 10.1063/1.2999626�

The magnetization loop M�H� of the ferromagnetic �FM�layer in a FM/antiferromagnetic �FM/AF� thin film structurecan be shifted from zero field if it is grown or cooled in thepresence of an external magnetic field. This phenomenon,called exchange bias �EB� or unidirectional exchange aniso-tropy, was discovered by Meiklejohn and Bean more than50 years ago.1 Today, exchange biased ferromagnets arewidely used as a key component in spintronic devices2 suchas spin-valves and magnetic tunneling junctions. Despitehaving been long discovered, widely used in practical appli-cations, and under intense investigations, the underlyingphysics of this intriguing effect still remains open to debate.

The EB phenomenon has been generally considered asan interfacial effect, implying that only interfacial spins areresponsible for the unidirectional pinning of the magnetiza-tion of the FM layer. However, there have been a number ofexperimental evidences for an important role the AF bulkeffect may play in EB. For instance, the EB field HEB wasfound to be strongly dependent on the thickness of the AFlayer even for large thicknesses,3 or EB can persist evenwhen a layer of nonmagnetic spacer such as Ag, Au, or Cuwas inserted in between the AF and FM layers.4 In FM/AF/FM trilayers, a bulk characteristic of EB would implythat EB could propagate from one AF/FM �or FM/AF� inter-face to the other. However, recent studies on trilayers gavedifferent results depending on material specifics and mag-netic heat treatments. Using a field cooling �FC� procedure atthe plateau field that separates the minor switching loops ofthe two FM layers in Py /FeMn /Co trilayers, Yang andChien5 observed that the top and bottom EB systems arecoupled via a spiraling spin structure across the interveningFeMn layer. Leung and Blamire6 later suggested that such amacroscopic AF spin spiral was due to the specific FC treat-ment employed in the work, but not a universal feature of EBtrilayers. While no sign of bias propagation was observed inNiFe /FeMn /Co trilayers grown in a low field ��5 Oe�,7 itwas seen in those that were field cooled in 1 kOe from abovethe blocking temperature.6 Blamire et al.8 reported that nopropagation of spin order was observed between the inter-faces in Co /FeMn /CuNi structures grown in H=200 Oe.

Our work on bilayers shows that there is a large differ-ence in EB between FeMn /CoFe and CoFe /FeMn bilayerstructures due to the influence of the magnetized CoFe un-derlayer on the establishment of the AF order of FeMn. Re-markably, we observe that adding a seed CoFe to theFeMn /CoFe bilayer significantly improves the bias of thetop CoFe layer. On the other hand, deposition of a top CoFeon the CoFe /FeMn bilayer enhances the bias field of thebottom CoFe layer. The top and bottom CoFe layers in trilay-ers both show an enhanced bias with decreasing thickness ofthe shared FeMn layer. These results support the presenceof significant propagation of EB in the FeMn layer inCoFe /FeMn /CoFe trilayers.

The CoFe /FeMn, FeMn /CoFe bilayers and symmetricCoFe /FeMn /CoFe trilayers were grown at room temperature�RT�, in an in situ field of 50 Oe applied along the substratesurface, using a biased-target ion beam deposition system9

with a base pressure of �2�10−7 Torr. The CoFe and FeMntarget compositions are Co95Fe5 and Fe50Mn50, respectively.All the samples were grown on Si wafers, whose surface wascovered by a seed layer of Ta�5� �for CoFe /FeMn bilayersand trilayers� or Cu�5� �for FeMn /CoFe bilayers�, andcapped by a Ta�5� layer except for CoFe /FeMn bilayers thatuse a cap of 5 nm of Cu �all the thickness units are in na-nometers�. The use of Cu layers was aimed at promoting the�-fcc FeMn phase required for EB. The samples subject toFC were heated from RT to 230 °C in 4.5 min and thenfurnace cooled to RT in a field of 3 kOe. The whole heating/cooling process was performed in a flowing mixture of N2+5% H2. The EB fields HEB= �Hc1+Hc2� /2 were measured at305 K for all the samples in both as-deposited and field-cooled states. Here, Hc1 and Hc2 are the coercivities deter-mined on the opposite field sweeping directions that are mea-sured at M /Ms=0 for the bilayers and at M /Ms= �0.5separately for the two CoFe layers in the trilayers.

Typical M�H� data of our bilayers and trilayers are pre-sented in Fig. 1 for FeMn�10�/CoFe�4�, CoFe�4�/FeMn�10�,and CoFe�4�/FeMn�10�/CoFe�4� samples. As for as-deposited bilayers, we have commonly observed that EB ismuch stronger in CoFe /FeMn than in FeMn /CoFe struc-tures. That trend is clearly demonstrated in Figs. 1�a� and1�b� where a large difference in HEB between FeMn�10�/CoFe�4� �HEB=44 Oe� and CoFe�4�/FeMn�10� �HEB

a�Electronic mail: daonhnam@yahoo.com.

APPLIED PHYSICS LETTERS 93, 152504 �2008�

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=190 Oe� bilayers is observed. This effect seems to becaused by changes in micromagnetic structure due to mag-netic interactions rather than in the crystalline structure andtexture of the FeMn layer associated with the underlayer ef-fect, since a seed Cu layer was used for the FeMn /CoFebilayer. In the case of CoFe /FeMn, because of a strong po-larizing field produced by the CoFe surface magnetizationthat competes against the formation of the AF order, uncom-pensated spins are easily created when FeMn is grown on thesaturated CoFe in magnetic field. On the other hand, depos-iting CoFe on a stable FeMn layer may have a minimal affecton its well established AF order, resulting in a significantlyweaker EB. A much higher in situ field may be required tocreate as strong an EB in FeMn /CoFe as that in CoFe /FeMnbilayers. Within this context, the concentration of uncompen-sated spins may be one factor that determines the exchangecoupling between AF and FM layers across their interface.

For the trilayers, by varying the thickness of the CoFelayers, we can easily identify the minor loops for the top andbottom CoFe layers. The hysteresis loop of the as-depositedCoFe�4�/FeMn�10�/CoFe�4� trilayer in Fig. 1�c� shows onlya slight increase in the EB field HEB

b of the bottom CoFe�4�layer �lower minor loop; HEB

b =196 Oe� in comparison to thatof the corresponding CoFe /FeMn bilayer �HEB=190 Oe,Fig. 1�b��. In contrast, a huge change is induced in the topCoFe layer. The broad magnetization reversals of the topCoFe layer �upper minor loop� indicates that its exchangecoupling with the FeMn layer is not uniform and partiallyrandomized. A major part of the layer seems to switch itsmagnetic moment synchronously with the bottom one. The

nominal EB field HEBt measured at M /Ms=0.5 is 109 Oe,

which is much higher than the value of 44 Oe obtained forthe corresponding FeMn /CoFe bilayer �Fig. 1�a��. These re-sults unambiguously indicate that EB is strongly improved inthe top CoFe layer in the presence of the bottom CoFe layer.It is unlikely that the bottom CoFe layer would cause a largerimprovement of the �-fcc AF phase in the FeMn layer thanby a Cu underlayer. On the other hand, the presence of thetop CoFe is not expected to make any change in the crystal-line structure of the FeMn layer underneath. Therefore, theimprovements of EB of both CoFe layers must be indicativeof a magnetic coupling between the two CoFe /FeMn andFeMn /CoFe systems sharing the same intervening FeMnlayer.

Nevertheless, the increase of 6 Oe �or 3%� in EB field ofthe bottom CoFe layer from bilayer �Fig. 1�b�� to trilayer�Fig. 1�c�� is somewhat too small to be conclusive. If the topand bottom EB systems are magnetically coupled, HEB

t andHEB

b would increase with decreasing the FeMn layer thick-ness tFeMn. Figure 2 plots the hysteresis loops of similar bi-layer and trilayer structures but with a thinner �6 nm� FeMnlayer. As expected, with decreasing tFeMn from 10 to 6 nm,HEB

t increases from 109 to 128 Oe and HEBb from

196 to 221 Oe �compare Figs. 1�c� and 2�c��. Moreover, HEBb

of the tFeMn=6 nm trilayer is now 32 Oe �or 17%� higherthan that of the corresponding CoFe layer in bilayer. It isworth noting here that while dipolar �or “orange-peel” type�coupling between the two CoFe layers may not be avoidablein our trilayers, that cannot be responsible for the increase ofHEB

b with decreasing tFeMn. Our data �not shown here� indi-

-1

0

1

Heb=44 Oe

FeMn/CoFe

(a)

-1

0

1

M/Ms

Heb=190 Oe

CoFe/FeMn

(b)

-1

0

1

-200 0 200

Heb=196 Oe

Heb=109 Oe

CoFe/FeMn/CoFe

(c)

b

t

Heb=65 Oe

(d)

FeMn/CoFe

Heb=207 Oe

CoFe/FeMn

(e)

-200 0 200

Heb=221 Oeb

Heb=121 Oet

CoFe/FeMn/CoFe

(f)

as-deposited field-cooled

H (Oe)

FIG. 1. �Color online� M�H� loops of the ��a� and �d�� Si/Cu�5�/FeMn�10�/CoFe�4�/Ta�5�, ��b� and �e�� Si/Ta�5�/CoFe�4�/FeMn�10�/Cu�5� bilayers, and��c� and �f�� Si/Ta�5�/CoFe�4�/FeMn�10�/CoFe�4�/Ta�5� trilayers. �a�–�c� arefor as-deposited samples. �d�–�f� are for field-cooled samples. The crossesmark the center of the main or minor loops where HEB is determined.

-1

0

1

Heb=10 Oe

FeMn/CoFe

(a)

-1

0

1

M/Ms

CoFe/FeMn

Heb=189 Oe

(b)

-1

0

1

-200 0 200CoFe/FeMn/CoFe

Heb=128 Oe

Heb=221 Oe

(c)

b

t

FeMn/CoFe

Heb=24 Oe

(d)

CoFe/FeMn

Heb=199 Oe

(e)

-200 0 200CoFe/FeMn/CoFeHeb=237 Oe

Heb=166 Oe(f)

b

t

H (Oe)

as-deposited field-cooled

FIG. 2. �Color online� M�H� loops of the ��a� and �d�� Si/Cu�5�/FeMn�6�/CoFe�4�/Ta�5�, ��b� and �e�� Si/Ta�5�/CoFe�4�/FeMn�6�/Cu�5� bilayers, and��c� and �f�� Si/Ta�5�/CoFe�4�/FeMn�6�/CoFe�4�/Ta�5� trilayers. �a�–�c� arefor as-deposited samples. �d�–�f� are for field-cooled samples. The crossesmark the center of the main or minor loops where HEB is determined.

152504-2 Nam et al. Appl. Phys. Lett. 93, 152504 �2008�

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cate that due to the dipolar coupling, HEBb starts to decrease

when tFeMn is decreased to below 6 nm �e.g., HEBb =204 and

173 Oe for tFeMn=5 and 4 nm, respectively�. A huge increasein EB is also obtained for the top CoFe layer in trilayer �Fig.2�c�� with reference to the bilayer �Fig. 2�a��. It is very in-teresting that as tFeMn decreases, while EB fields are reducedin both top and bottom CoFe bilayers, they are strongly in-creased in the corresponding trilayers. All of these facts con-vincingly suggest that there exists a mutual propagation ofEB between the top and bottom interfaces through the inter-mediate FeMn. Even for tFeMn of up to 25 nm, the top CoFelayer EB is still induced by the bottom one. Although theshape of the M�H� loops of the trilayers in Figs. 1�c� and 2�c�look rather similar to those observed by Yang and Chien,5

our angular measurements indicate that both the CoFe layershave the same easy axis as that initially created by the depo-sition field, thus avoiding any possibility of a spiraling mag-netic structure in these samples.

As shown in Figs. 1�d�–1�f� and 2�d�–2�f� it is surprisingthat FC the samples from 230 °C and in H=3 kOe onlyslightly improves their EB fields, indicating that the EBstates established in the as-deposited samples were alreadyclose to equilibrium. The biggest change is observed for thetop CoFe layer in both the FeMn�10� �Fig. 1�f�� and FeMn�6��Fig. 2�f�� trilayers, where EB becomes uniform and im-proved, due to a realignment of randomized uncompensatedspins by the FC process. Qualitatively, the behaviors of thefield-cooled samples are in general the same as that of theas-deposited ones. We varied the FC temperature from180 to 300 °C and observed no significant change in our re-sults. The large difference in EB between field-cooledCoFe /FeMn and FeMn /CoFe bilayers �and between the topand bottom interfaces in trilayers as well� is a striking featurethat would imply that the FC may have reset the coupling ofthe CoFe layers and uncompensated spins without signifi-cantly increasing their concentration. It is therefore possiblethat a 3 kOe cooling field is still far from enough to bringabout equal EB fields for the two EB systems whether inbilayers or trilayers.

Regarding the shape of the M�H� curves of the as-deposited trilayers �Figs. 1�c� and 2�c��, one would have aconcern that the apparent increase in HEB

b is probably anartifact of the large coercivity and broad loop of the topCoFe layer. However, a numerical deconvolution of thetrilayer M�H� curves, using the loop of the CoFe /FeMn bi-

layer �Figs. 1�b� and 2�b�� as that of the bottom CoFe layer,gives us an unreasonably distorted loop for the top CoFelayer though less distortion is obtained when HEB of theCoFe /FeMn bilayer is shifted toward HEB

b . This would implythat the increase in HEB

b is an intrinsic change of the bottomCoFe layer in the trilayer structure, as is also supported bythe results of FC �Figs. 1�f� and 2�f�� where a substantialincrease in HEB

b is observed while the top CoFe layer loop issharp, narrow, and has HEB

t significantly below HEBb .

In summary, our results have shown that uncompensatedspins are created favorably when an antiferromagnet is de-posited on a magnetized FM layer. Although the uncompen-sated spins are created near the FM/AF interface, they alsospread over the AF layer to the top AF/FM interface, wherecreation of uncompensated spins is less favored, leading to astrong improvement of EB of the top FM layer. On the otherhand, the top FM layer may also contribute a certain amountof uncompensated spins, resulting in an increase of the bot-tom FM layer EB. Our results here underline the importanceof the concentration, as well as the distribution, of uncom-pensated spins in EB systems and demonstrate that thereexists a propagation of EB within the AF layer in FM/AF/FM trilayer structures.

This work is supported by DMEA under Contract No.H94003-08-2-0803 and ONR under Contract No. N00014-06-1-0428.

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152504-3 Nam et al. Appl. Phys. Lett. 93, 152504 �2008�

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