Journal of Magnetism and Magnetic Materials 251 (2002) 323–326

Exchange bias in patterned FeMn/NiFe bilayers

Z.B. Guoa,*, K.B. Lia, G.C. Hana, Z.Y. Liua, P. Luoa, Y.H. Wua,b

aData Storage Institute, 5 Engineering Drive 1, National University of Singapore, Singapore 117608, SingaporebDepartment of Electrical and Computer Engineering, National University of Singapore, Singapore 119260, Singapore

Received 13 June 2002; received in revised form 9 August 2002

Abstract

Electron beam lithography and ion beam etching have been used to pattern a wire-like array in FeMn/NiFe bilayers.

The variation of hysteresis loops with the etching depth in FeMn layer has been presented, and it has been found that

with increasing etching depth the coercivity increases and M–H loops show an asymmetric kink. Detailed studies of the

magnetic behaviors of the asymmetric kink in the patterned sample with 3.5 nm thick FeMn layer have been performed,

and a magnetization component perpendicular to the wire direction has been observed.

r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Exchange bias; Lithography; Hysteresis loops; Training effects; Wire-like array

In recent years, there has been extensive interestin the exchange coupling between a ferromagnet(FM) and an antiferromagnet (AFM) due totechnological applications of exchange bias inmagnetoresistive heads [1] and spin valve-baseddevices [2]. A lot of interesting phenomena, such asspin-flop coupling [3–5] and positive exchange bias[6], have been discovered. Due to the interfacenature of exchange bias, recent interest hasbeen focused on the studies of the correlationbetween interfacial spin structures and exchangebias [7–12].In the studies of AFM thickness dependence of

exchange bias, it has been found that as the AFMthickness decreased to a certain value, the ex-change bias sharply decreased and coercivitysharply increased due to the presence of an

irreversible transition of AFM moments [13,14].Inspired by these phenomena, for FeMn/NiFebilayers, we patterned a wire-like array in theFeMn layer by partially etching the FeMn layer.Under a certain applied magnetic field, a nonuni-form magnetic structure can be obtained, when anirreversible transition is achieved in the etchedareas, and is avoided in the nonetched areas. Themagnetic behaviors related to the nonuniformmagnetic structure in the patterned sample werestudied.The samples with the structure of Ta 5 nm/

Fe50Mn50 22 nm/Ni81Fe19 20 nm/Ta 5 nm/Si(1 0 0)substrate were prepared at room temperature byhelicon sputtering deposition under the processpressure of 9� 10�5 Torr. The base pressure ofthe system is 5� 10�10 Torr. The NiFe layer wasdeposited in a magnetic field of 100Oe in order toset a bias field direction. The Ta and NiFe layerswere present to obtain the g phase of FeMn. A

*Corresponding author.

E-mail address: zbguo@dsi.a-star.edu.sg (Z.B. Guo).

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 8 8 5 - 5

wire-like array along the exchange bias direction,i.e., the applied magnetic field direction during filmdeposition, was fabricated by using electron beamlithography and ion beam etching. The dimensionof the patterned sample is about 1 cm� 1 cm:Fig. 1 presents a scanning electron microscopyimage of a representative wire-like array patternedsample. The profile of the sample measured byatomic force microscope is shown in the inset ofFig. 1. The width of the etched areas andnonetched areas is B1:32 and 0:68 mm, respec-tively. The etching depth is B23:5 nm; that meansa thickness of FeMn B3:5 nm remains in theetched areas.The magnetization (M) was measured by a

Lakeshore vibrating sample magnetometer atroom temperature. M–H loops in Fig. 2 weremeasured with fields along the direction of themagnetic field applied during film deposition.Shown in Fig. 2a are M–H loops of uniformFeMn 22 nm/NiFe 20 nm and FeMn 3.5 nm/NiFe20 nm bilayers, the former exhibits exchange biasB50 Oe with coercivity B1:2 Oe: In contrast, thelatter exhibits no exchange bias with coercivityB3 Oe: This is because all atomic moments of theNiFe and FeMn layers are rotated to theiropposite easy directions by the reversal magnetic

fields (740 Oe). Therefore, it can be understoodthat the patterned sample of Fig. 1 is composed oftwo parts: one is termed as free areas for the FeMn3.5 nm/NiFe 20 nm, the other is termed as biasedareas for the FeMn 22 nm/NiFe 20 nm.Shown in Fig. 2b–f are the hysteresis loops of

the wire-like array patterned sample with 10.9, 7.2,5.3, 3.5, and 0 nm thick FeMn layers left in theetched areas, respectively. With increasing etchingdepth, the coercivity increases and M–H loopsbecome more and more asymmetric as theremained thickness of FeMn layer decreasingto 3.5 nm in the etched areas. When the FeMnlayer was etched completely in the etched areas,the M–H loop presents a combination of NiFemagnetization from biased and nonbiased areas(Fig. 2f).First focus on the magnetic behavior of the

sample in Fig. 2e, whose surface profile has beenshown in Fig. 1. The sample exhibits an asym-metric kinked hysteresis loop, and in the kink themagnitude of magnetization along the wire direc-tion is close to zero. All of these are much differentwith those of FeMn 22 nm/NiFe 20 nm bilayers, asshown in Fig. 2a.In order to reveal the magnetic state at zero field

in the kink (point A of Fig. 2e), magneticmeasurements were performed by using theprocedure as follows:At first, the film was magnetized with a field of

�120 Oe; and then the field was increased to zerowith the measurement configuration of Fig.2e,which resulted in the state of point A of Fig. 2e.Afterwards, the film was rotated 901 about an

axis perpendicular to the sample plane.Finally, the initial magnetization curve and

M–H loops were measured.The initial magnetization curve and virgin M–H

loop are shown in Fig. 3. The initial magnetizationat zero field is nearly equal to the remanentmagnetization of M–H loops, which is about 75%of saturation magnetization. This phenomenonindicates magnetization in the patterned FeMn/NiFe at point A of Fig. 2e has a componentperpendicular to the wires, which is different fromthe case at point B of Fig. 2e, where themagnetization is along the wire direction. Trainingeffects, which have been extensively observed in

Fig. 1. Scanning electron microscopy image of a wire-like array

patterned Ta 5 nm/Fe50Mn50 22 nm/Ni81Fe19 20 nm/Ta 5 nm/

Si(1 0 0) substrate sample. Inset: Etching profile of the patterned

sample measured by atomic force microscope.

Z.B. Guo et al. / Journal of Magnetism and Magnetic Materials 251 (2002) 323–326324

exchange bias system [15,16], also have been foundin the present patterned sample. For example,shown in the inset of Fig. 3b are M–H loop cycles1, 2 and 3, with decreasing coercivity for increasingnumber of loop cycles.The above asymmetric kinked hysteresis loop is

similar to that observed in MnF2/Fe bilayers,which has been attributed to the twinned natureof MnF2 and coherent rotation process [17]. Forour patterned FeMn/NiFe of Fig. 2e, a negative

reversal magnetic field causes an irreversibletransition of AFM moments in the etched areas.However, the irreversible transition cannot beobtained in the nonetched areas, and thereforethe AFM magnetic configuration should bestable at zero field as the field increases fromnegative saturation, and results in the kink.In summary, magnetization behaviors in the

patterned FeMn/NiFe bilayers were studied, andwith increasing etching depth the coercivity

Fig. 2. (a) Hysteresis loops of uniform FeMn 22 nm/NiFe 20 nm, and FeMn 3.5 nm/NiFe 20 nm. (b)–(f) Hysteresis loops of the wire-

like array patterned sample with 10.9, 7.2, 5.3, 3.5, and 0 nm thick FeMn layers left in the etched areas, respectively.

Z.B. Guo et al. / Journal of Magnetism and Magnetic Materials 251 (2002) 323–326 325

increased and M–H loops became more and moreasymmetrical. Detailed studies of the asymmetrickink in the patterned sample with 3.5 nm thickFeMn left, revealed that there is a magnetizationcomponent perpendicular to the wire direction inthe kink. These phenomena are attributed to thenonuniform magnetic structure induced by irre-versible transition in the etched areas of FeMnlayer.

We are grateful to Y.K. Zheng, J.J. Qiu, D.You, and Y.T. Shen for helpful discussions.

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Fig. 3. (a) The initial magnetization curve (solid symbol) and virgin (cycle 1) hysteresis loops (open symbol) of the wire-like array

patterned sample. (b) The virgin (cycle 1) and trained (cycles 2 and 3) hysteresis loops.

Z.B. Guo et al. / Journal of Magnetism and Magnetic Materials 251 (2002) 323–326326