DSpace at VNU: Coexistence of positive and negative exchange bias in CrMn Co bilayers

Journal of Magnetism and Magnetic Materials 298 2006 43–47Coexistence of positive and negative exchange bias in CrMn/Co bilayers Nguyen Nguyen Phuoca,b, , Nguyen Phu Thuya,c, Nguyen Anh

Journal of Magnetism and Magnetic Materials 298 (2006) 43–47

Coexistence of positive and negative exchange bias in

CrMn/Co bilayers Nguyen Nguyen Phuoca,b, , Nguyen Phu Thuya,c, Nguyen Anh Tuana,

Le Thanh Hunga, Nguyen Trung Thanha, Nguyen Thanh Nama

a

International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam

b Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan

c College of Technology, Vietnam National University, Hanoi, Vietnam Received 7 January 2005; received in revised form 19 February 2005

Available online 24 March 2005

Abstract

Exchange-biased CrMn/Co bilayers with various thicknesses of Co sputtered onto Si(1 0 0) substrates by the RF sputtering system have been studied Double-shifted loops have been observed with the thickness of Co layer in a narrow range and become single-shifted loops after some cycles of measurement Those results are interpreted as the association of positive and negative exchange bias

r2005 Elsevier B.V All rights reserved

PACS: 75.70.Cn; 75.70.
i; 75.25.+z; 75.30.Gw

Keywords: Exchange bias; Magnetic thin film; Double-shifted loop; Training effect

1 Introduction

Discovered in 1956 [1], the phenomenon of

exchange bias between an antiferromagnet (AF)

and ferromagnet (FM) is of great interest due to its

widespread application in spin valves and

mag-netic tunnel junctions Nevertheless, its physical origin remains unanswered[2]

Usually, exchange bias is described as an additional unidirectional anisotropy induced by the AF into the FM via exchange coupling at the interface, producing a single magnetic hysteresis loop shifted along the magnetic field axis after field cooling procedure through the Ne´el point of the

AF The magnitude of this shift is named exchange bias field (HE) and in almost all cases, the magnetic hysteresis loop is shifted in the negative field if one defines the direction of the cooling field (HFC) as

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doi:10.1016/j.jmmm.2005.03.006

Corresponding author Information Storage Materials

Laboratory, Toyota Technological Institute, 2-12-1 Hisakata,

Tempaku, Nagoya 468-8511, Japan Tel.: +81 52 809 1872; fax:

+81 52 809 1874.

E-mail address: nnguyenphuoc@yahoo.com (N.N Phuoc).

the positive direction This case is referred to as

negative exchange bias The phenomenon of

positive exchange bias was first observed in 1996

by Nogue´s et al.[3]when studying the systems of

Fe/FeF2 and Fe/MnF2 They found that the sign

of exchange bias field changes from negative to

positive as the cooling field increases Very

recently, Roshchin et al [4] have found that the

state of coexistence of positive and negative

exchange bias can be achieved by cooling the

sample of FeF2/Co in a properly chosen constant

applied magnetic field This state manifests itself as

a double hysteresis loop

In this paper, we report the observation of the double-shifted loop in CrMn/Co bilayers with a proper thickness of Co layer A training-like effect has been observed in the sample exhibiting the double-shifted hysteresis loops, which becomes a single-shifted hysteresis loop after some cycles of measurement The results are interpreted as the association of positive and negative exchange bias,

in which the portion of positive exchange bias

-0.0010

-0.0005

0.0000

0.0005

0.0010

tCo = 36 nm

-0.00030

-0.00015

0.00000

0.00015

0.00030

tCo = 18 nm

-0.00150

-0.00075

0.00000

0.00075

0.00150

tCo = 54 nm

-0.002 -0.001 0.000 0.001 0.002

tCo = 72 nm

-200 0 200 400 -0.0030

-0.0015 0.0000 0.0015 0.0030

tCo = 108 nm

H (Oe)

-0.002 -0.001 0.000 0.001 0.002

tCo = 90 nm

-400

H (Oe) -400

-1000

H (Oe)

H (Oe) -1000

-2000

H (Oe)

-400

H (Oe) -1000 -2000

Fig 1 Hysteresis loops of the CrMn(12 nm)/Co (x nm) samples measured at 123 K The thickness of Co layer is indicated in the figure.

gradually reduces with the cycle of measurement

causing the observed training-like effect

2 Experiment

The samples used in this work with the structure

of Si(1 0 0)/CrMn (12 nm)/Co (x nm) were

depos-ited at room temperature by the RF sputtering

The CrMn layer was sputtered from a composite target constituting of Cr target with Mn chips placed on it The base pressure was about

10
6mbar whereas the Ar pressure during deposi-tion was 10
3mbar The deposition was carried out without applying a magnetic field The composition of the CrMn films, identified by energy dispersive X-ray spectroscopy (EDS), is

Cr45Mn55 The samples were then annealed in high

T = 173 K

T = 173 K

-0.00030

-0.00015

0.00000

0.00015

0.00030

T = 223 K

-0.00030

-0.00015

0.00000

0.00015

0.00030

-0.00030

-0.00015

0.00000

0.00015

0.00030

H (Oe)

-0.002 -0.001 0.000 0.001 0.002

T = 223 K

-0.002 -0.001 0.000 0.001 0.002

-0.002 -0.001 0.000 0.001 0.002

200

H (Oe)

-400 -200 0 200 400

H (Oe)

-400 -200 0 200 400

H (Oe)

-2000

-1000 0 1000 2000

H (Oe) -2000

H (Oe) -2000

Fig 2 Hysteresis loops of the CrMn(12 nm)/ Co(18 nm) (left panel) and CrMn(12 nm)/ Co(90 nm) (right panel) bilayers at various temperatures.

vacuum oven (10
5mbar) at the temperature of

300 1C for 1 h They were subsequently cooled in

the magnetic field of 5 kOe to room temperature

Magnetic properties of the annealed bilayers were

characterized by vibrating sample magnetometer

(VSM) in the temperature range from 123 K to

room temperature

3 Results and discussion

The hysteresis loops of the annealed CrMn

(12 nm)/Co (x nm) (x ¼ 18; 36, 54, 72, 90, 108 nm)

bilayers, measured at 123 K were shown inFig 1

We have observed a very large exchange bias

(HE600 Oe) It is noted that all the

magnetiza-tion curves in Fig 1 show double-loops and this

effect is especially clear in the sample with tCo¼72

and 90 nm Moreover, there is a strong correlation

between the appearance of exchange bias and

double-shifted loops as shown inFig 2 On the left

panel of Fig 2 are the hysteresis loops of the

annealed CrMn (12 nm)/Co (18 nm) bilayer

mea-sured at different temperatures while on the right

are those of the CrMn (12 nm)/Co (90 nm) bilayer

measured at the same corresponding temperatures

It is worth noting that the width of the kink in the

curve with double-shifted loops decreases as the

temperature increases in the same manner with the

diminution of exchange bias and at the

tempera-ture of 223 K, both double-shifted loops and

exchange bias disappear This suggests that the

double-shifted loop may result from exchange

bias

Fig 3 shows a representative of magnetic

hysteresis loops of CrMn (12 nm)/Co (90 nm) film

after some cycles of measurement It is remarkable

that the right-hand loop becomes smaller and the

left-hand loop becomes bigger with increasing n

After nine cycles of hysteresis measurements, the

right-hand loop disappears, making a

single-shifted loop in the magnetization curve This effect

seems to be similar to the training effect often

observed in exchange bias system, in which

exchange bias field decreases with the number of

measurement [5] However, in our case, only the

shape of the magnetization curve is changed with

the cycle of measurement while the value of

-0.002 -0.001 0.000 0.001 0.002

H (Oe)

n = 1

n = 5

n = 9

Fig 3 Representative of hysteresis loops of CrMn(12 nm)/Co (90 nm) film M–H loops for the cycles 1, 5, and 9 are shown.

AF FM Negative exchange bias Positive exchange bias

AF domain wall (c)

AF FM

Negative exchange bias

(a)

HFC

(b) AF FM

Positive exchange bias

Fig 4 Schematic diagram of the spin configurations of FM/

AF bilayers in three cases of H FC : (a) Small H FC making negative exchange bias, (b) Large H FC making positive exchange bias, (c) Intermediate H FC making a double-shifted loop.

exchange bias field is nearly constant Moreover,

after 9 cycles of measurement, further

measure-ment was carried out but the shift of the single

loop was not changed any more This feature

indicates that the physical origin of this

training-like effect is possibly different from that of the

normal training effect

The manifestation of double-shifted loop is

presumably attributed to the overlap of positive

and negative exchange bias Positive exchange bias

is believed to be due to the fact that the interfacial

interaction is antiferromagnetic and the origin of

such antiferromagnetic coupling as proposed has

been system specific [6] For small HFC, the

magnetic hysteresis loop exhibits negative

ex-change bias with the AF spins at the interface

aligned in the negative direction as inFig 4(a) If

HFC is large enough to align the AF surface

magnetization along HFC as shown in Fig 4(b),

thus overcoming the interface AF–FM interface

magnetic interaction, the magnetization curve will

exhibit a positive shift For intermediate HFC, the

AF moments at the interface are partially aligned

with the cooling field as sketched in Fig 4(c)

causing the formation of two AF domains, one

makes negative exchange bias and the other makes

positive exchange bias The magnetization curve

thus consists of two hysteresis loops, one shifted to

negative and the other to positive, making a

double-shifted loop The height of the positive

loop depends on the portion of the AF moments

aligned with the cooling field The fact that the

right-hand loop, corresponding to positive

ex-change bias becomes smaller with the cycle of

measurement suggests that this state is metastable

The height of the positive loop decreases with the

number of measurements as shown in Fig 3,

implies that the area of the AF domain causing

positive exchange bias diminishes correspondingly

The training-like effect can therefore be described

as the movement of the AF domain wall toward

the positive-exchange-biased domain making this

domain shrink and finally be suppressed It is well-known that the positive exchange bias is in a high-energy state so it is less stable than the negative exchange bias, which may explain why the training effect seems to affect only the right-hand side subloop

4 Conclusion

In summary, we have fabricated CrMn/Co bilayers with varied thickness of the Co layer and have observed the double-shifted loop in CrMn/Co bilayers with the thickness of Co layer

in a narrow range This double-shift loop is assumed to be due to the coexistence of positive and negative exchange bias This assumption is consistent with a training-like effect observed for the first time in the sample exhibiting a double-shifted loop

Acknowledgements

This work is supported by the State Programs

on Fundamental Research of Vietnam under the Grant No 811604

References

[1] W.H Meiklejohn, C.P Bean, Phys Rev 102 (1956) 141 [2] J Nogue´s, I.K Schuller, J Magn Magn Mater 192 (1999) 203.

[3] J Nogue´s, D Lederman, T.J Moran, I.K Schuller, Phys Rev Lett 76 (1996) 4624.

[4] I.V Roshchin, O Petracic, R Morales, Z.P Li, X Batlle, I.K Schuller, Los Alamos National Laboratory, Preprint Archive, Condensed Matter (2004), 1–14, arXiv:cond-mat/

0411014 ( http://xxx.lanl.gov/pdf/cond-mat/0411014 ) [5] K Zhang, T Zhao, H Fujuwara, J Appl Phys 89 (2001) 6910.

[6] X Ke, M.S Rzchowski, L.J Belenky, C.B Eom, Appl Phys Lett 84 (2004) 5458.