DSpace at VNU: Magnetization and magnetostriction studies of TbFeCo YFeCo multilayers

2006 Abstract Exchange-spring TbFeCo/YFeCo multilayers exhibit interesting magnetic and magnetostrictive properties that are rather promising for application in microsystems.. In this pa

Magnetization and magnetostriction studies

of TbFeCo/YFeCo multilayers

D T Huong Giang&N H Duc&J Juraszek&J Teillet

Published online: 20 December 2006

# Springer Science + Business Media B.V 2006

Abstract Exchange-spring TbFeCo/YFeCo multilayers exhibit interesting magnetic and magnetostrictive properties that are rather promising for application in microsystems In this paper, we present a study of the effect of the exchange coupling on the magnetic properties

of these magnetostrictive multilayers An exchange bias phenomenon, revealed by a shift of the minor hysteresis loop along the applied field axis, is found as the result of the creation

of interfacial domain walls This effect strongly depends on the magnetic properties of the soft YFeCo layer, and becomes more pronounced at low temperatures due to the enhancement of the magnitude of the exchange coupling between the layers

Key words RE–MT multilayers magnetostriction magnetization process

microstructure Mössbauer

1 Introduction

When two grains are directly in contact with each other, the magnetic moments at the grain interface cause exchange-coupling interaction between the two grains The fundamental understanding of the exchange-coupling mechanism has been successfully applied to the so-called low-field giant magnetostrictive exchange-spring TbFeCo/YFeCo multilayers, in which giant magnetostrictive (e.g., TbFeCo) and soft magnetic (e.g., FeCo) layers alternate [1, 2] The structural, magnetic and magnetostrictive properties of TbFeCo/YFeCo

Hyperfine Interact (2006) 169:1337 –1342

DOI 10.1007/s10751-006-9448-5

D T Huong Giang ( *):J Juraszek:J Teillet

Groupe de Physique des Matériaux, Université de Rouen, UMR CNRS 6634,

76801 St Etienne du Rouvray, France

e-mail: dth.giang@univ-rouen.fr

D T Huong Giang

Faculty of Physics, Vietnam National University,

Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

N H Duc

College of Technology, Vietnam National University,

Hanoi, 144 Xuan Thuy Road, Hanoi, Vietnam

multilayers have been previously studied in relation with concentration as well as the annealing treatments [2] Most of recent results for such magnetostrictive materials have attracted much attention mainly in magnetic and magnetostrictive softness improvement However, the exchange bias phenomenon caused by the formation of the interfacial domain wall has not been taken into account

In this article, we have focused on the interfacial exchange-coupling interaction and exchange bias phenomenon and their correlation with magnetization and magnetostriction process in the sputtered Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8and multilayers

2 Experimental procedures

{Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8}50and {Tb(Fe0.55Co0.45)1.5/Y0.2(Fe0.7Co0.3)0.8}50multilayers with the individual layer thicknesses tTbFeCo=12 nm and tYFeCo=10 nm were alternately deposited by RF-magnetron sputtering method on glass microscope cover slips with a nominal thickness of 200 μm The typical power during sputtering was 200 W, the high-purity argon gas was used and deposition pressure was 10−2mbar

Structure analyses were done using transmission electron microscopy (TEM) and magnetization curves were measured with a superconductor quantum interference device (SQUID) in applied fields up to 5 T Magnetic behavior of layers was also observed by the conversion electron Mössbauer spectroscopy (CEMS) and magnetic force microscopy (MFM) Magnetostriction was measured by using an optical deflectometer (resolution of 5×10−6rad)

3 Results and discussion

The bright-field cross-sectional TEM image of the as-sputtered TbFeCo/Y0.2Fe0.8sample in Fig.1a shows clearly the layered structure with well-defined boundaries between TbFeCo (bright) and YFe (dark) layers A high degree of smooth stripe periodicity, smooth interfaces and no evidence of the existence of any crystalline phase are visible in the Fig 1 Bright field TEM (a) and electron diffraction pattern (b) of TbFeCo/Y 0.2 Fe 0.8 multilayers

micrograph This indicates that the amorphous state exists in the whole sample, i.e., in both the magnetostrictive TbFeCo and the soft magnetic YFe layers The amorphous structure of sample is further revealed in Fig.1b by bright spread rings from the inside diffraction spot, the first one characteristics of the amorphous TbFeCo layers and the others of the amorphous YFe layers The same behavior was also observed in the TbFeCo/Y0.2(FeCo)0.8

film

Although the microstructure is the same, these two samples exhibit a significantly different magnetostrictive behavior (Fig 2) For the TbFeCo/YFe multilayer, the magnetostriction curve has a parabolic shape and does not reach saturation in the field range of magnetostriction measurement This behavior is characteristic of films with perpendicular magnetic anisotropy [3] In the case of the TbFeCo/YFeCo multilayer, a first enhancement of low-field magnetostriction, which is characteristic of in-plane anisotropic materials, is followed by a highly negative slope at high magnetic fields Such behavior is ascribed to the formation of interfacial extended domain walls (EDW) The difference between the two systems is due to the different magnetic behavior between YFe and YFeCo layers Amorphous Y0.2Fe0.8 alloy is paramagnetic at room temperature [4] Thus, the coupling between successive TbFeCo/YFe layers is weak and the magnetostrictive layers preserve their intrinsic perpendicular magnetic anisotropy, as already found in TbFeCo single layer films [5] On the other hand, partial substitution of Fe by Co makes the

Y0.2(FeCo)0.8 layers becoming magnetic at room temperature and the exchange coupling between the layers is strengthened In this case, the effective spring-magnet configuration results in an enhancement of the low-field magnetostriction

The difference in the magnetic anisotropy between TbFeCo/Y0.2Fe0.8 and TbFeCo/

Y0.2(FeCo)0.8samples is also evidenced in MFM images (Fig.3) For the TbFeCo/Y0.2Fe0.8 sample, a stripe domain structure indicates the presence of magnetic domains with dif-ferent directions of magnetization oriented perpendicular to the film plane (Fig 3a) In contrast, for the TbFeCo/Y0.2(FeCo)0.8 specimen, the existence of perpendicular stripe domains is no longer clearly revealed (Fig.3b) reflecting the in-plane magnetic anisotropy

in this sample

Further evidence for above statement is given by the Mössbauer analysis of the TbFeCo/

Y0.2Fe0.8sample (Fig.4) The spectrum was fitted with three components: (1) a doublet (δ= 0.2 mm s−1, < B >=1.5 T and 40% of the total spectrum area) attributed to a non-magnetic

Fig 2 Parallel magnetostriction

of TbFeCo/Y 0.2 Fe 0.8 and

TbFeCo/Y 0.2 (Fe 0.7 Co 0.3 ) 0.8

multilayers

part of YFe phase (Fe-poor), (2) a broad sextet (δ=0.094 mm s−1

, < Bhf>=32 T and 13% of the total spectrum area) attributed to a magnetic part of YFe phase (Fe-rich) and (3) a broad distribution of sextet with same isomer shift (<δ >=0.094 mm s−1, < Bhf>=16 T, and 47%

of the total spectrum area) attributed to the different Fe environments in amorphous TbFeCo layers The value of the average“cone-angle” between the direction of γ-ray and

Fe spins is 35°, evidencing for out of plane magnetic anisotropy

For a better understanding of the exchange-coupling in the TbFeCo/YFeCo multilayer, the magnetization data measured at 5 K are presented in Fig.5 When sweeping the field from high positive to negative field, the hysteresis loop exhibits the field-induced magnetization transitions The corresponding magnetization configurations in the magne-tization process are illustrated in the inserted figures The magnemagne-tization process in this system is governed by the competition between three energies: exchange coupling, magnetic anisotropy and Zeeman energy [6] For a very high applied field, the Zeeman energy dominates and the sample is saturated In this state, the FeCo moments of TbFeCo

Fig 4 Mössbauer spectrum (a) and hyperfine-field distribution (b) of the TbFeCo/Y 0.2 Fe 0.8 multilayer at

300 K

Fig 3 MFM images of TbFeCo/Y 0.2 Fe 0.8 (a) and TbFeCo/Y 0.2 (FeCo) 0.8 (b) multilayers in zero field

(dominated by Tb) and YFeCo layers are directed in opposite directions and EDW is formed at the interfaces (state a) When the field is decreased, the EDW vanishes by the reversal of the YFeCo layer (state b) In this case, because KTbFeCois very large compared

to KYFeCo, meanwhile MTbFeCois comparable to MYFeCo, the TbFeCo-layers would be still more strongly pinked along the field direction than FeCo-layers when the applied field decreases Only the YFeCo magnetization rotates easily by antiferromagnetic interaction between these two layers As the field is further decreased, a EDW in which TbFeCo moments rotate out of the external field direction is formed (state c) This results in a negative contribution to the parallel magnetostriction and a decrease of magnetostriction at high fields [7]

The minor hysteresis loops have been recorded at 5, 25 and 50 K for TbFeCo/YFeCo multilayer (Fig.6) The measurement was obtained by sweeping the field from 5 T to a small negative value (μ0H=−0.3 and −0.4 T for 5 and 25 K, respectively) in the hysteresis loops that corresponds only to the reversal of the soft layers (state b; see in Fig 5) The width of the loops can be roughly considered as the coercive field of the soft magnetic YFeCo-layer The shift of the minor hysteresis loop is defined as the exchange bias field HE

that is usually found in antiferromagnetic (AF)/ferromagnetic (F) bilayer systems [8] HEis found to decrease from 165 to 115 and 36 mT when the temperature increases from 5 to 25

Fig 5 Hysteresis loops and

magnetization process at 5 K for

TbFeCo/YFeCo multilayers

Fig 6 Magnetization ( dotted line) and minor loop (closed circles) at 5 K (a) and 25 K (b) for TbFeCo/ YFeCo film

and 50 K, respectively This observation is in good accordance with the decreasing tendency of magnetization and magnetic anisotropy of constituted layers [6]

4 Conclusion

In this work, we have studied the magnetization and magnetostriction in the spring-magnet TbFeCo/YFe and TbFeCo/YFeCo multilayers Among them, the later system with the soft YFeCo layer exhibits an exchange-bias phenomenon and interfacial domain wall as a result

of antiferromagnetic exchange-coupling interaction between layers

Acknowledgement This work was partially supported by CNRS (France) through a PICS program The authors would like to thank Dr S Schulze from Institute of Physics, Chemnitz University of Technology (Germany) for his help in TEM investigations.

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