Physica B 327 (2003) 334–336 Low-field magnetoresistance of Fe/Cr multilayers N.H Duca,*, N.A Tuana, N.T Nama, N.H Sinha, J Teilletb, A Fnidikib a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam b GPM-UMR 6634, Universit!e de Rouen, 76821 Mont-Saint-Aignan, France Abstract Sputtered {Fe/Cr} multilayers with a fixed Cr individual layer thickness tCr ¼ nm and variable Fe individual layer thickness (1 nmptFe p6 nm) are investigated by means of X-ray diffraction, magnetoresistance and magnetisation measurements At room temperature, the initial magnetoresistive susceptibility of the as-deposited samples is almost constant However, the saturation field increases with decreasing Fe-layer thickness, therefore, a maximal magnetoresistance ratio DR=R of 0.7% is reached in the sample with tFe=1 nm After annealing at 3501C, a DR=R value as large as 2.3% was obtained Further annealing causes a reduction of magnetoresistance As the temperature is decreased, the DR=R ratio measured in m0 H ¼ 0:3 T increases linearly At 77 K, the magnetoresistance ratio is about four times larger than that at 300 K Results are discussed in terms of the scattering located at interfaces and the formation of a ferromagnetic state at high-temperature heat treatments r 2002 Elsevier Science B.V All rights reserved PACS: 75.70.Nt; 75.70.Cn; 75.70.Pa Keywords: Multilayers; Giant magnetoresistance; Magnetic coupling Intensive study of spin-dependent transport in magnetic multilayers has been stimulated by the discovery of a giant magnetoresistance (GMR) effect [1] Nowadays, on the basis of this effect, various types of device such as sensors, read heads, high-density magnetic random access memories, etc have been realised, see for example Ref [2] It is well known that the origin of GMR is the spindependent scattering of conduction electrons However, there is controversy on the exact location of the scattering centers They can occur at the interfaces and/or in the bulk of the ferromagnetic layers In addition, with respect to applications the question arises to what extent the *Corresponding author E-mail address: duc@netnam.org.vn (N.H Duc) GMR survives at elevated temperatures In order to tackle this point, in this paper, we consider the GMR effect of sputtered Fe/Cr multilayers with a fixed Cr individual layer thickness and variable Fe individual layer thickness The {Fe/Cr}n mutilayers with a number of periods n ¼ 60 and with a fixed Cr individual layer thickness, tCr ¼ nm, and a variable Fe individual layer thickness, tFe ¼ 1; 2, and nm, were prepared by RF-magnetron sputtering The typical power during sputtering was 100 W and the Ar pressure was 10À2 mbar The substrates were glass with a nominal thickness of 0.5 mm Both target and sample holder were water-cooled The samples were annealed at temperatures from TA ¼ 200–5001C for h in a vacuum of  10À5 mbar The crystalline structure of the 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N.H Duc et al / Physica B 327 (2003) 334–336 films was investigated by X-ray diffraction using Cu Ka radiation (Siemens D5000 diffractometer) The magnetoresistance was measured by the fourpoint technique in current-in-plane configuration and longitudinal geometry The magnetisation is measured in fields up to 1.3 T using a VSM The room temperature GMR ratio DR=Rð0Þ ẳ RHị R0ịị=R0ịị; where R0ị and RHị are the resistance in zero field and in applied field m0 H; respectively) of the as-deposited Fe/Cr multilayers is presented in Fig The results show that the initial magnetoresistive susceptibility of the as-deposited samples is almost constant wR (=(DR/R)/m0H)E13% TÀ1 The saturation field, however, increases with decreasing Fe-layer thickness In this way, a maximal magnetoresistance ratio DR/R of 0.7 % is reached in the sample with tFe=1 nm This finding shows that the volume Fe-fraction increases, i.e the volume/interface fraction ratio increases, while the GMR effect decreases The result seems to support the assumption that the scattering centers are located at interfaces Annealing effects on the GMR are presented in Fig for the Fe/Cr multilayers with tFe ¼ nm The GMR ratio initially increases with increasing the annealing temperature and reaches a maximum value of 2.3% at TA ¼ 3501C With further increasing TA ; the GMR ratio decreases, e.g after annealing at 5001C, the GMR ratio equals 0.3% only A similar result was observed for samples with tFe ¼ nm Such a tendency of GMR was recently reported by Hecker et al [3] These results can be explained as follows The annealing at TA p3501C is usually thought to modify the multilayer structure due to the interdiffusion and the broadening of the interfaces This leads to an increasing interface/volume fraction and then to the enhancement of the GMR The annealing at 5001C, however, is assumed to cause a further breakup of the layers, leading to the formation of heterogeneous structures of small particles This argument was proposed earlier by Flores et al [4] XRD results of the Fe (1 nm)/Cr (2 nm) multilayers (Fig 3) strongly support the above argument At TA p3501C, the stability of individual Fe- and Cr-layers is well evidenced by the (1 0) BCC-Fe and (110) BCC-Cr reflections At TA ¼ 5001C, however, a broadened Bragg peak is observed indicating the formation of fine Fig GMR data for annealed Fe (1 nm)/Cr films GMR (%) -0.2 nm -0.4 nm -0.6 nm -0.8 -0.3 tFe = nm -0.2 -0.1 0.1 µoH (T) 0.2 0.3 Fig GMR data for Fe/Cr multilayers at 300 K 335 Fig XRD patterns of Fe (1 nm)/Cr multilayers N.H Duc et al / Physica B 327 (2003) 334–336 336 1.2 500°C M (arb unit) 0.8 350 °C 0.4 -0.4 -0.8 TA = 30 °C -1.2 -0.2 -0.1 µ0H (T) 0.1 0.2 Fig Hysteresis loops of Fe (1 nm)/Cr samples T = 300 K GMR (%) -2 The GMR curves measured at different temperatures are presented in Fig for a sample with tFe ¼ nm annealed at TA ¼ 3501C Note that the GMR ratio measured in m0 H ¼ 0:3 T increases linearly with decreasing temperature and reaches a value as large as 7.7% at 77 K This GMR ratio is about four times larger than that at room temperature It may be related to the enhancement of the antiferromagnetic coupling at lower temperatures In conclusion, our investigation suggests an important role of the scattering at the interfaces It reveals also that the layer structure of sputtered Fe/Cr multilayers remains stable during annealing up to 3501C At higher temperatures, the multilayer structure is modified and the onset of ferromagnetic coupling is found, leading to the reduction of the GMR signal 233 K 184 K -4 145 K -6 77 K -8 -0.3 -0.2 -0.1 0.1 µoH (T) 0.2 Acknowledgements This work was granted by the State Program for Fundamental Researches of Vietnam, within the project 420.301 0.3 Fig Low-temperature GMR data of Fe (1 nm)/Cr samples annealed at 3501C particles of BCC-CrFe phases In this state, the antiferromagnetic coupling breaks down and the ferromagnetic one is established (see Fig 4) The system, thus, can no longer switch between an antiparallel (ground state) and a parallel aligned state (applied field) References [1] A Barth!el!emy, A Fert, F Petroff, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 12, Elsevier Science, Amsterdam, 1999, pp 1–96 [2] K.Y Kim, J.E Evetts, J Magn Magn Mater 198–199 (1999) 92 [3] M Hecker, D Tietjen, D Elefant, C.M Schneider, J Appl Phys 89 (2001) 7113 [4] W.H Flores, S.R Teixeira, J.B.M da Cunha, M.C.M Alves, H Tolentino, A Traverse, J Magn Magn Mater 233 (2001) 100 ... 420.301 0.3 Fig Low-temperature GMR data of Fe (1 nm) /Cr samples annealed at 3501C particles of BCC-CrFe phases In this state, the antiferromagnetic coupling breaks down and the ferromagnetic one is... (%) -0.2 nm -0.4 nm -0.6 nm -0.8 -0.3 tFe = nm -0.2 -0.1 0.1 µoH (T) 0.2 0.3 Fig GMR data for Fe/ Cr multilayers at 300 K 335 Fig XRD patterns of Fe (1 nm) /Cr multilayers N.H Duc et al / Physica... effects on the GMR are presented in Fig for the Fe/ Cr multilayers with tFe ¼ nm The GMR ratio initially increases with increasing the annealing temperature and reaches a maximum value of 2.3% at