Mössbauer Study on Fe Ag and Fe Ni Ag Super-Laminates Prepared by Repeated Rolling and Treated by Gas Nitriding

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1 Hyperfine Interactions 148/149: , Kluwer Academic Publishers. Printed in the Netherlands. 307 Mössbauer Study on Fe Ag and Fe Ni Ag Super-Laminates Prepared by Repeated Rolling and Treated by Gas Nitriding K. NOMURA 1, S. KIKUCHI 2, M. YASUDA 2, K. TOKUMITSU 1 and Y. UJIHIRA 1 1 School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo , Japan 2 Department of Materials Science, University of Shiga Prefecture, 2500 Hassaka, Hikone , Japan Abstract. Both Ag and Fe are mutually immiscible elements and this feature made it possible to produce nano-scaled multilayer laminates. Super-laminates of [Ag/Fe] 10,000 layers and [Ag/FeNi 3 ] 17,000 layers of several thicknesses were successfully prepared by repeated rolling. We studied the structure and interface of Ag/Fe and Ag/FeNi 3 multilayers by X-ray diffraction, electron microscopy and transmission Mössbauer spectrometry (TMS). [Ag/Fe] laminates were further treated by nitriding. These surface layers were characterized by conversion electron Mössbauer spectrometry (CEMS). Key words: Ag/FeNi 3, Ag/Fe, multilayer, repeated rolling, nitriding, crystal orientation, Mössbauer spectroscopy, CEMS. 1. Introduction Ag and Fe are known to be mutually immiscible elements [1]. The maximum solubility is at.% Ag in α-fe, and at.% Fe in Ag [1]. This feature makes it possible to produce nano-scaled multilayer laminates. The methods to produce multilayers have been usually MBE, and the sputtering method among others [2], but the repeated rolling method has not been well established for production of multilayers. Shingu et al. succeeded in the preparation of Fe Ag multilayers by this last method [3]. The hetero-interface between fcc and bcc phases may be affected by rolling and annealing, however, it has not been analyzed by Mössbauer spectroscopy as in the case of mechanically alloyed Fe 81.3 Ag 18.7 [4] and Fe Ni Ag powders [5]. In these works mechanical alloying was used to produce the Fe Ni Ag alloys consisting of finely divided magnetic Fe Ni particles separated by Ag, but superparamagnetic phases were not obtained [5]. Nano-scaled multilayers consisting of non-magnetic and magnetic phases are known to show the highest strength, and to exhibit giant magnetic resistance (GMR) [6].

2 308 K. NOMURA ET AL. The multilayer structures obtained by repeated rolling were not as simple as those obtained by evaporation methods. However, the rolling method has many merits of fabrication such as the possibility of large production without special equipments. The GMR properties of Ag/Fe [3, 4, 7], the strength of Ag/Fe [3] and Ag/Ni [8] and the structural changes of Ar ion irradiated Fe/Ag multilayers [9 11] have been already investigated. From the study on the magnetic and chemical orderdisorder phenomena in Fe 3 Ni, FeNi and FeNi 3, it is known that FeNi 3 alloy has the highest Curie temperature among these compounds [12]. In this paper, superlaminates of [Ag/Fe] 10,000 and [Ag/FeNi 3 ] 17,000 layers of several thicknesses were prepared by repeated rolling, and the interface and coagulation between Ag and both α-fe(bcc) and FeNi 3 (fcc) layers are studied by Mössbauer spectrometry, X-ray diffraction and magnetic resistance measurements. Nitride compounds were so hard that we could not apply the rolling method directly to the fabrication of the laminate. Consequently, the super-laminated films were treated by plasma and gas nitriding. 2. Experimental 2.1. PREPARATION OF SUPER LAMINATES Ag/Fe and Ag/FeNi 3 super-laminates with nano-scale dimensions were prepared by repeated rolling and annealing. Starting laminates of [Ag (10 µm)/fe (30 µm)] and [Ag (10 µm)/fe (10 µm)/ni (20 µm)] were used. Procedure of preparation described in Figure 1 is as followed. (1) Micro foils were cut into pieces of the same size (30 25 mm 2 ). (2) Fifty pieces of each foil were stacked alternately. (3) The stacked foils were pressed with about 50 MPa at 773 K in vacuum. (4) The pressed stack was annealed at 1073 K for 3.6 ks in vacuum. Figure 1. Schematic fabrication of [FeNi 3 /Ag] super laminates by repeated rolling.

3 Fe Ag AND Fe Ni Ag SUPER-LAMINATES 309 (5) The annealed stack was rolled up to 50 µm in thickness at room temperature in air. (6) The rolled sample was cut into pieces of the same size. (7) The pieces were piled up again for the next cycle of rolling. The cycle operations from (3) to (7) were repeated up to several times. The employed number of stacked laminates was fifty before rolling, twenty after first rolling and ten after second rolling. Finally 10,000 layers of [Ag/Fe] were obtained, which were expressed as [Ag/Fe] 10,000. Analogously, 17,000 layers of [Ag/FeNi 3 ] with 5, 10 and 20 µm in total thickness were prepared by the above method although the stack number was different MEASUREMENTS X-ray diffraction and pole figure X-ray analysis were used in order to obtain the lattice parameters and the crystallographic orientation of Ag and Fe or FeNi 3. Transmission electron microscopy was used to observe the cross section structure of the laminates. The magneto-resistant (MR) ratio was measured with four points probes at room temperature. The current was parallel to the layer plane, and the magnetic field was applied parallel to the layer plane along the current direction (I H, Plane H ) or parallel to the plane and perpendicular to the current (I H, Plane H ). MR ratio is defined by (ρ ρ sat )/ρ sat,whereρ sat is the saturation resistivity. 57 Fe Mössbauer spectra were measured in the transmission and the conversion electron Mössbauer modes (TMS and CEMS) [13]. Differential CEMS spectra were obtained by selecting three different energy ranges of conversion electrons with a He + 5%CH 4 flowing counter. Detecting the electrons with the lower energy revealed a thickness of the layers of about 300 nm. 3. Results 3.1. CHARACTERIZATION OF MULTILAYERS BY XRD From X-ray pole figures of Ag/Fe super laminate, aggregated textures were developed by cold rolling, whereas the Ag crystal was arrayed to {001}[100] to reduce the mismatch of each crystal at the interface. After repeated rolling more than three times, FeNi 3 compounds were grown as shown in X-ray patterns of Figure 2. In the case of Ag/FeNi 3 annealing did not affect the change of crystal direction, but the gradient interface was grown to reduce mismatching. The crystal growth is different between Ag/Fe and Ag/FeNi 3 because of the different textures of Fe and FeNi 3 layers respect to Ag layer. The XRD peak of Ag [220] was shifted to higher diffraction angles to fit lattice parameter of Ag to each phase by annealing at 673 and 773 K. By heating at higher temperatures, XRD peak returned back near to the original angle as shown in Figure 2 (bottom). It is suggested that the annealing at high temperatures corrupted the interface of laminates and broke the layer structures.

4 310 K. NOMURA ET AL. Figure 2. X-ray diffraction patterns of [Ag/FeNi 3 ] 17,000 layers with 5 mm thickness as rolled and XRD peak shits of Ag [200] in [Ag(10)/FeNi 3 (30)] by the annealing. The magnetoresistance ratio increased with the reduction of the thickness between layers, especially for laminates within 20 µm thickness. The ratio of magnetoresistance showed 0.35%, 0.45% and 1% for 10 µm, 5 µm and 1 µm in thickness at room temperature, respectively, as shown in Figure 3. It is confirmed that Ag/FeNi 3 multilayers prepared by repeated rolling show the GMR effect FeNi 3 /Ag LAYERS CHARACTERIZED BY MÖSSBAUER SPECTRA Mössbauer spectra of FeNi 3 /Ag multilayers showed two ferromagnetic components Fe1 and Fe2, as shown in Figure 4. The Mössbauer parameters are listed in Table I. Mössbauer parameters were not so much different before and after the annealing as well as among the FeNi 3 /Ag laminates with 5 µm, 10 µm and 20 µm thickness. The area intensity of Fe1 and Fe2 changed after annealing, which may be due to the gradient interface grown to reduce the mismatching.

5 Fe Ag AND Fe Ni Ag SUPER-LAMINATES 311 Figure 3. MR ratio of [Ag/FeNi 3 ] 17,000 multilayers after annealing at 773 K for 7.2 ks. Magnetic field//electric current. Figure 4. TMS spectra of FeNi 3 /Ag super laminate after annealing at 500 C for 2 hours.

6 312 K. NOMURA ET AL. Table I. Mössbauer parameters of FeNi 3 /Ag multilayers Subspectra IS QS Magnetic fields Line width Area int. Before annealing Fe mm/s 0.05 mm/s 28.7 T 0.45 mm/s 58% Fe mm/s 0.05 mm/s 30.9 T 0.45 mm/s 42% After annealing at 500 C for 2 hours Fe mm/s 0.06 mm/s 28.9 T 0.45 mm/s 52% Fe mm/s 0.03 mm/s 31.3 T 0.45 mm/s 48% Figure 5. CEMS spectra of plasma nitrided surface of Fe/Ag laminate film. (a) gray part and (b) black part NITRIDING AND OXIDATION OF Fe/Ag MULTILAYERS Plasma nitriding and oxidation [Fe/Ag] 10,000 multilayer with the thickness of 50 µm was treated by plasma nitriding. The surface was not uniform in color because shiny black and gray parts were observed. CEMS spectra of the two different parts were shown in Figure 5. The black parts showed 2 broad sextets (B h = 41.7 T,IS = 0.40 mm/s, line width = 0.92 mm/s, B h = 46.4 T,IS = 0.38 mm/s, line width = 0.92 mm/s). Both sextets were assigned to magnetite-like oxide with many defects or small particle size. The sharp peaks are due to α-fe (B h = 31.4 T)/Ag multilayer. This laminate was heated in air at 400 C for 1 hour, and the differential CEMS spectra are shown in Figure 6. The magnetic component with B h = 51.2 T observed is due to large grains of hematite oriented to the surface because the peak intensity ratio was

7 Fe Ag AND Fe Ni Ag SUPER-LAMINATES 313 Figure 6. DCEMS spectra of plasma nitrided and oxidized surface of Fe/Ag multilayers. 3 : 3 : 1 : 1 : 3 : 3. The paramagnetic peaks (IS = 0.39 mm/s, QS = 1.2 mm/s, line width = 1.3 mm/s) are considered to be due to the super-paramagnetism of small grains. The intensity ratio of paramagnetic components was 21% for top spectrum (obtained by detecting the high-energy electrons emitted), 24% for middle spectrum (by detecting the middle energy electrons), and 30% for bottom spectrum (by detecting the low energy electrons), respectively. The ratio increased with layer deep. It suggests that the Fe oxides produced between Ag layers are small oxide grains Gas nitriding of Fe/Ag multilayers Figure 7 shows the TMS and CEMS spectra of [Fe/Ag] 10,000 treated in NH 3 gas at 400 C for 2, 4 and 6 hours. The substrate of Fe was observed in TMS spectra of Fe/Ag multilayers, nitrided for less than 4 hours, whereas only nitride products were observed in CEMS even for less than 2 hours. The difference between TMS and CEMS was observed especially for the sample treated for 4 hours. In the CEMS spectrum, almost all products were paramagnetic components although the relaxation components of magnetic nitride were included on the surface layers. These results show that the nitrogen is concentrated on the top of layers. The

8 314 K. NOMURA ET AL. Figure 7. TMS and CEMS spectra of Fe/Ag multilayers treated by gas nitriding.

9 Fe Ag AND Fe Ni Ag SUPER-LAMINATES 315 magnetic components of Fe nitride compounds are known to be α -martensite (N) (33 T), α -Fe 16 N 2 (37.7 T, 35.3 T and 29.5 T), γ -Fe 4 N(B h = 26 T) and ε-fe 3+x N (B h = 20 T, 11 T) [14, 15]. The hyperfine fields of nitride products observed in CEMS and TMS are around 21 T, 19 T and 11 T. The main product was ε-fe 3+x N after 2 hours nitriding and the hyperfine fields become smaller, depending on the amount of nitrogen doped. Paramagnetic components of nitrides are Fe 2+x N(IS = 0.35 mm/s and QS = 0.30 mm/s), ε(ζ)fe 2 N(IS = 0.4 mm/s), and austenitic γ -FeN x (IS = 0.09 mm/s, IS = 0.18 mm/s and QS = 0.19 mm/s) [16]. By nitriding for more than 6 hours, the whole laminates of Fe/Ag multilayers were uniform although austenitic γ -FeN x was not detected. Each Ag layer is considered to play a role of a barrier against nitrogen diffusion in the laminate, so nitrogen atoms are considered to be concentrated on the surface layers and to be diffused repeatedly into the deeper layers. The laminate volume was enlarged by absorption of nitrogen. 4. Conclusions The super-laminates of FeNi 3 /Ag multilayers were more easily fabricated than the Fe/Ag multilayers by repeated rolling. The laminates of FeNi 3 /Ag also exhibited the magneto-resistance phenomenology as well as the Ag/Fe super-laminates. The super-laminate of FeNi 3 /Ag multilayers has fundamentally the same texture as normally pressed texture. By annealing, Ag texture is easily oriented to each Fe or FeNi 3 phase, respectively. The measurement of Mössbauer spectra clarified that the FeNi 3 phases in multilayers are little affected by annealing. It is found that the oxidation of Fe/Ag multilayers provides us with nano-scaled grains of Fe oxides between Ag layers, and the nitriding of multilayers proceeds by diffusing nitrogen step by step into the bulk layers. Acknowledgement Authors express thanks to Monbushou (Ministry of Education, Culture, Sports, Science and Technology) for supporting our study. References 1. Swartzendruber, L. J., Bull. Alloy Phase Diagrams 5 (1984), Parkin,S.S.P.,Appl. Phys. Lett. 61 (1992), Shingu, P. H., Ishihara, K. N., Otsuki, A., Hashimoto, M., Hasegawa, N., Daigo, I. and Huang, B., J. Met. Nanocryst. Mater. 2 6 (1999), Yoshioka, T., Yasuda, M., Miyamura, H., Kikuchi, S. and Tokumitsu, K., Mater. Sci. Forum (2002), Bennett, L. H., Takacs, L., Swartzendruber, L. J., Weissmuller, J., Bendersky, L. A. and Shapiro, A. J., Senpta Metallurgica et Materialia 33 (1995), Baibich, M. N., Broto, J. M., Fert, A., Nguyen, F., Van Dau, Petroff, F., Eitenne, P., Greuzet, G., Friederich, A. and Chazelas, J., Phys. Rev. Lett. 61 (1988), 2472.

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