Giant magnetoresistance and magnetism of heterogeneous CoCu produced by ion-beam techniques

Giant magnetoresistance and magnetism of heterogeneous CoCu produced by ion-beam techniques

Sensors and Actuators A 91 (2001) 169±172 Giant magnetoresistance and magnetism of heterogeneous CoCu produced by ion-beam techniques U.K. RoÈûlera,*...

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Sensors and Actuators A 91 (2001) 169±172

Giant magnetoresistance and magnetism of heterogeneous CoCu produced by ion-beam techniques U.K. RoÈûlera,*, J. Noetzelb, A. Tselevc, K. Nenkova,1, A. Handsteina, D. Eckerta, K.-H. MuÈllera a

b

IFW Dresden, Institut fuÈr Metallische Werkstoffe, P.O.B. 270016, D-01171 Dresden, Germany Forschungszentrum Rossendorf, Institut fuÈr Ionenstrahlphysik und Materialforschung, P.O.B. 510119, D-01314 Dresden, Germany c Institut fuÈr Werkstoffwissenschaften, Technische UniversitaÈt Dresden, D-01062 Dresden, Germany

Abstract Magnetic properties and giant magnetoresistance (GMR) of granular CoCu layers produced by ion-beam mixing were investigated. Laser-deposited CoCu multilayers with an integral composition of Co20Cu80 were ion-beam mixed with 150 keV Cu‡. At a maximum ¯uence of 5  1015 Cu/cm2 the multilayer structure is completely replaced by a granular microstructure. Thermal magnetization curves indicate a broad distribution of Co-cluster sizes with superparamagnetic contributions. The magnetoresistance increases strongly by ionbeam mixing. There is evidence for strong magnetic interaction effects related with irradiation damages. The interaction effects can be reduced by annealing, which further increases the magnetoresistance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Giant magnetoresistance (GMR); CoCu; Ion-beam mixing; Granular metals

1. Introduction The giant magnetoresistance (GMR) of heterogeneous metallic structures built from immiscible alloy systems such as CuCo or AgCo depends crucially on various geometrical and morphological parameters which are dif®cult to control, e.g. interface roughness in multilayer systems. Granular systems may show magnetoresistance effects as high as those of multilayers [1]. The mechanism for GMR in these granular alloys is thought to be similar to that of multilayers with current perpendicular to the planes (CPP) [2]. Usually, these granular systems own low sensitivity of the resistance dR(H)/dH(H) in small ®elds. However, very high ®eldsensitivities for metallic systems were demonstrated by discontinuous multilayers, where the ferromagnetic layers have been transformed to a granular structure through annealing [3]. Therefore, speci®c preparation methods for granular metals are of interest for producing highly sensitive GMR materials. Ion-beam techniques can be used to mix and demix heterostructures by athermal processes. Such techniques can be used to modify or tailor heterostructures. Recently, there were successes with lateral structuring of miscible multilayers by ion-irradiation through lithographic *

Corresponding author. On leave from International Laboratory of High Magnetic Fields, Wroclaw and ISSP, BAS Sofia, Bulgaria. 1

masks [4]. These techniques allow the patterning of magnetic ®lms with laterally varying magnetic properties on length scales of a few tens of nanometers. Here, we use homogeneous irradiation to modify and destroy a multilayer structure of an immiscible alloy system CoCu. These experiments may provide the basis for more advanced routes to patterned heterostructures from such alloys. The experiments also provide insight into the relation between structural and magnetotransport properties of granular alloys. Especially, the role of rough interfaces for the spin-dependent scattering is dif®cult to assess. Cai et al. found that the GMR of irradiated CoCu multilayers decreases due to increasingly ferromagnetic coupling [5]. Similar results were found for irradiated CoAg multilayers, which were transformed into granular structures under irradiation [6,7]. In these experiments, the volume fraction of Co was high because the as-prepared multilayers had thin non-ferromagnetic interlayers. The high Co-concentration favours ferromagnetic behaviour and suppresses the GMR after ion-mixing. Therefore, we used multilayers with thick Cu interlayers which effectively decouple the ferromagnetic Co layers in as-prepared state. The weak magnetoresistance in these multilayers must be ascribed to the granularity of the layers and a CPP-like scattering mechanism by spin-accumulation. Irradiation by Cu-ions destroys the layered structure. But no complete mixing of Co in Cu was achieved [8]. Here, we investigate the magnetic and magnetoresistance

0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 4 8 3 - 6

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properties of these samples. Additionally, annealing effects on these properties are investigated. For this investigation, magnetoresistance (MR) measurements are a highly sensitive probe to uncover small changes in the granular structures. 2. Experimental Several multilayers were prepared by crossed-beam laserdeposition [9]. Details of preparation and a characterization by combined X-ray techniques are reported elsewhere [8]. From X-ray re¯ectivity curves, the structure of as-prepared multilayers was determined as 8  ‰Co …2:1 nm)/ Cu (6.3 nm)] on Si-substrates. Proton induced X-ray emission yields the integral composition Co20Cu80 for the multilayers. There is a high jaggedness of the as-prepared multilayers which seems to stem from columnar growth of the multilayers. Samples with this structures were ionbeam mixed using 150 keV Cu‡ with ¯uences (1± 5†  1015 Cu/cm2. Irradiation induces a change of the microstructure in particular grain-growth sets. From X-ray re¯ectivity measurements it was concluded that at the maximum ¯uence the multilayer structure is completely destroyed. MR was measured by standard four-probe geometry with the ®eld parallel to the current. Magnetization of as-prepared and irradiated samples was measured by a SQUID-magnetometer. 3. Results Field-cooling (FC) and zero-®eld-cooling (ZFC) magnetization measurements for the as-deposited sample and for the sample irradiated with maximum ¯uence are shown in Fig. 1. The as-deposited sample shows ferro behaviour. For the irradiated sample, we still ®nd superparamagnetic block-

Fig. 1. Field-cooled (FC) and zero-field-cooled (ZFC) magnetization in 100 mT for a laser-deposited 8  ‰Co (2.1 nm)/Cu (6.3 nm)] multilayer. Upper two curves for the layer in as-prepared state and lower two curves for layer after irradiation with maximum fluence of Cu-ions.

Fig. 2. Demagnetization curves at two temperatures for the as-prepared multilayer and the layer irradiated with maximum fluence.

ing at high temperatures. The bifurcation between FC- and ZFC-magnetization sets in above 250 K. We recently demonstrated that a homogenous layer of Co25Cu75 with 50 nm thickness produced by direct implantation of Co into Cu displays a spin-glass-like freezing with the bifurcation between FC- and ZFC-magnetization below 25 K in considerably weaker ®elds [10]. The totally different behaviour of the ion-mixed multilayer in Fig. 1 demonstrates the granularity of their microstructure. The broad maximum of the ZFC magnetization indicates a broad distribution of Co-cluster sizes. Fig. 2 shows demagnetization curves at room temperature and at 5 K for the same two samples. The as-prepared layer has a higher saturation magnetization. Temperature has little in¯uence on the demagnetization curve. The reduced magnetization and the stronger temperature effect for the sample irradiated with maximum ¯uence also indicate the granular microstructure of this layer. At high temperatures, superparamagnetic behaviour of a part of the Co-clusters reduces the achievable magnetization. However, the increased coercive ®elds seen for this samples indicates an increase of magnetic anisotropy, respectively, interaction effects between Co-granules compared to the Co layers in as-prepared state. The magnetoresistance ratios MR…H† ˆ …R…H† R…0††=R…0† at low temperatures are shown in Fig. 3. The overall effect in high ®elds is strongly enhanced by ion-mixing. The effect becomes comparable to that of concentrated granular CoCu ®lms [1] or of bulk CoCu produced by melt-spinning [11]. The strong increase of the magnetoresistance in high ®elds for the irradiated samples is due to the decay of the multilayers into a granular structure, which means that there are granular layers for irradiation with lower ¯uence. This should increase the CPP-like contribution to magnetoresistance of the single Co layer. However, in lower ®elds, we see that the magnetoresistance sets in only in ®elds above a threshold of about 0:05 T 0:1 T (inset of Fig. 3). This indicates the presence of strong couplings between Co-granules with antiparallel magnetization which can be overcome only by stronger ®elds to reverse them into

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Fig. 3. Magnetoresistance ratio at T ˆ 5 K for 8  ‰Co (2.1 nm)/ Cu (6.3 nm)] multilayers irradiated at various fluences with 150 keV Cu‡. The inset shows the same data in magnification for lower magnetic fields.

Fig. 5. Magnetoresistance of the irradiated (curves with circles) and annealed (curves with triangles) layer at low and high temperature, respectively.

ferromagnetic con®gurations with lower resistance. By annealing the irradiated samples, this threshold for the onset of magnetoresistance can be suppressed, see Fig. 4. For these results an irradiated same sample was annealed in two steps with increasing temperature and the magnetoresistance measured after each step. The effect on the low-®eld magnetoresistance is visible already for the ®rst annealing step at T a ˆ 200 C. At the annealing temperatures chosen, bulk interdiffusion of Co in Cu is slow. Thus, precipitation and growth of Co-clusters from Co-solute in the Cu-matrix should be relatively unimportant. Thus, the annealing treatments should not change the granular microstructure on larger length scale. The main effect concerns only local rearrangement without long-range diffusion, such as mobile Co-interstitials in Cu and vice versa and smoothing of the surfaces of Co-granules. Changing the roughness of the CoCu interfaces also changes the spin-dependent scattering mechanism that causes the GMR-effect. However, this effect is largely independent of temperature as it affects the

microscopic scattering mechanism. In Fig. 5, we have plotted the MR for high and low temperature for an irradiated sample before and after annealing. The corresponding resistivities in zero ®eld for the as-irradiated sample are about 65 (92) mO cm and for the annealed sample 63 (88) mO cm at 5 (300) K, respectively. The small effect of annealing on MR at 300 K compared to the stronger effect at 5 K corroborates that annealing does mainly change magnetic coupling between the Co-granules and, therefore, the magnetic correlations between them. The magnetic correlations between Co-clusters clearly depend strongly on temperature and in¯uence the MR correspondingly. Recently, Slonczewski developed a theory for a coupling mechanism between ferromagnetic constituents in metallic heterostructures which is based on loose spins of paramagnetic ions in the non-ferromagnetic matrix or at rough interfaces [12]. This mechanism can explain the strong magnetic couplings in the irradiated granular layers and the annealing effects which we have observed. 4. Conclusions

Fig. 4. Magnetoresistance at T ˆ 5 K in lower fields. Effect of annealing in two steps with increasing temperature on the layer irradiated with maximum fluence.

In conclusion, we have investigated magnetic properties and magnetoresistance for irradiated CoCu multilayers. The irradiation leads to a granular alloy structure and properties similar to those of granular metals produced by thermal decomposition of supersaturated solid solutions with composition Co20Cu80. In particular, the irradiated samples show increased magnetoresistance. Annealing these samples further increases the overall magnetoresistance at low temperatures. There is evidence for rather strong magnetic coupling between the ferromagnetic granules, which can be weakened by thermal annealing. These couplings seem to be related with magnetic defect structures which are incurred by irradiation damage and can be annealed at relatively low temperatures. If our assumption is right that

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annealing also smoothes the CoCu-interfaces, then the interface roughness does not strongly in¯uence the magnetoresistance at higher temperature in the granular structure. Acknowledgements Supported by DFG through SFB 422 and project MU 1015/7-1. References [1] J.Q. Xiao, J.S. Jiang, C.L. Chien, Giant magnetoresistance in nonmultilayer magnetic systems, Phys. Rev. Lett. 68 (1992) 3749±3752. [2] S.F. Zhang, P.M. Levy, Conductivity and magnetoresistance in magnetic granular films, J. Appl. Phys. 73 (1993) 5315±5319. [3] T.L. Hylton, K.R. Coffey, M.A. Parker, J.K. Howard, Giant magnetoresistance at low fields in discontinuous NiFe±Ag multilayer thin films, Science 261 (1993) 1021±1024. [4] C. Chappert, H. Bernas, J. FerreÂ, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, H. Launois, Planar patterned magnetic media obtained by ion irradiation, Science 280 (1998) 1919±1922. [5] M. Cai, T. Veres, S. Roorda, R.W. Cochrane, R. Abdouche, M. Sutton, MeV ion irradiation of CoCu multilayers, J. Appl. Phys. 81 (1997) 5200±5202. [6] T. Veres, M. Cai, R.W. Cochrane, S. Roorda, Ion-beam modification of CoAg multilayers. I. Structural evolution and magnetic response, J. Appl. Phys. 87 (2000) 8504±8512.

[7] T. Veres, M. Cai, S. Germain, M. Rouabhi, F. Schiettekatte, S. Roorda, R.W. Cochrane, Ion-beam modification of CoAg multilayers. II. Variation of structural and magnetic properties with Co layer thickness, J. Appl. Phys. 87 (2000) 8513±8521. [8] J. Noetzel, U.K. RoÈûler, A. Tselev, F. Prokert, D. Eckert, K.-H. MuÈller, E. Wieser, W. MoÈller, Preparation of granular CoCu by ionbeam mixing of laser-deposited multilayers, Appl. Phys. A 71 (2000) 105±107. [9] J. Noetzel, A. Handstein, A. MuÈcklich, F. Prokert, H. Reuther, J. Thomas, E. Wieser, W. MoÈller, CoCu solid solution prepared by ion implantation, J. Magn. Magn. Mater. 205 (1999) 177±183. [10] A.A. Gorbunov, W. Pompe, A. Sewing, S.V. Gaponov, A.D. Akhsakhalyan, I.G. Zabrodin, I.A. Kaskov, E.B. Klyenkov, A.P. Morozov, N.N. Salaschenko, R. Dietsch, H. Mai, S. Vollmar, Ultrathin-film deposition by pulsed-laser ablation using crossed beams, Appl. Surf. Sci. 96 (1996) 649±655. [11] J. Wecker, R. von Helmholt, L. Schultz, K. Samwer, Magnetoresistance in bulk CuCo-based alloys, IEEE Trans. Mag. 93 (1993) 3087± 3089. [12] J.C. Slonczewski, Overview of interlayer exchange theory, J. Magn. Magn. Mater. 150 (1995) 13±24.

Biography U.K. RoÈûler earned his doctoral degree in physics in 1995 at the University of GoÈttingen, Germany. He is currently researcher at the IFW Dresden. His main interest is in computer simulations of materials properties and numerical statistical physics for models of magnetic systems. He is also working on experimental magnetic fine particle systems and giant magnetoresistive materials.