Hyperfine Interactions 156/157: 151 155, 2004. 2004 Kluwer Academic Publishers. Printed in the Netherlands. 151 Non-Magnetic Stainless Steels Reinvestigated a Small Effective Field Component in External Magnetic Fields T. ERICSSON 1, Y. A. ABDU 1, H. ANNERSTEN 1 and P. NORDBLAD 2 1 Uppsala University, Department of Earth Sciences, Villavägen 16, SE-75236 Uppsala, Sweden; e-mail: Tore.Ericsson@geo.uu.se 2 Uppsala University, Department of Materials Science, Box 534, SE-75121 Uppsala, Sweden Abstract. Three standard non-magnetic stainless steels of composition (wt%) Fe 70 Cr 19 Ni 11, Fe 70 Cr 17 Ni 13 and Fe 69 Cr 18.5 Ni 10.3 Mn 1.8 Ti 0.4 have been investigated by Mössbauer spectroscopy (5 295 K and in external fields 7 T at room temperature) and magnetization measurements (10 300 K) using a SQUID magnetometer. There are indications of a field induced ferromagnetic interaction in the samples at room temperature. Key words: stainless steel, external field, field induced ferromagnetism. 1. Introduction Non-magnetic stainless steels, containing normally Fe, Cr and Ni as major elements and sometimes Ti, Mn as minor elements have the fcc structure, the same as γ -Fe, being thermodynamically stable above 900 C. The γ -phase is stable above 400 CinFe 60 Ni 40 [1]. Introduction of Cr stabilizes the γ -phase to even lower temperatures [2]. The γ -phase may also be retained in the miscibility region at room and lower temperatures in quenched samples, e.g., in small grains [3] and after mechanical alloying producing great amounts of lattice defects, but frequently a transformation to martensite structure occurs. The magnetic phase diagram of fcc FeCrNi stainless steels is very complicated. Increased Ni contents leads to ferromagnetic (FM) phases, decreased to paramagnetic (PM) or antiferromagnetic (AFM) phases. Other phases also occur, e.g., Fe 80 x Ni x Cr 20 is FM for x = 30, FM and spin glass (SG) for x = 26 and 23, field induced ferromagnets (FIFM) and SG for 21 and 19, AFM for x = 17 and 14 [2]. However, the Neél- and Curie temperatures, T N and T C, are low, normally <50 K [2]. The complicated magnetic properties of fcc FeCrNi alloys can, at least partly, be understood introducing mixed exchange interactions. Kondorsky and Sedov [4] introduced the phrase latent antiferromagnetism when studying Fe Ni invar alloys: an AFM coupling between Fe Fe nearest neighbour pairs, but FM exchanges between Fe Ni and Ni Ni pairs. However, there are details still not well understood and we have
152 T. ERICSSON ET AL. here studied three FeCrNi stainless steels, and focused on Mössbauer spectroscopy (MS) in external fields at room temperature. 2. Experimental Three non-magnetic stainless steels of standard commercial type have been investigated using Mössbauer spectroscopy at room and low temperatures and in external fields up to 7 T at room temperature. Magnetization measurements, using a SQUID magnetometer, have been performed as M versus temperature, M(T), from 10 K to 300 K and as M versus magnetizing field H ( 50 koe), M(H), at 10 K and 300 K. Two powder samples ( Ni11, Ni13 ) from Alfa Aesar, Fe 70 Ni 11 Cr 19 (in wt%, type 304-L, -100 mesh) and Fe 70 Ni 13 Cr 17 (type 303-L, -140 mesh) and one foil (25 µm of a type often delivered with 57 Co-sources) of composition Fe 69.0 Ni 10.3 Cr 18.5 Mn 1.8 Ti 0.4 (determined using a Cameca electronmicroprobe) were used in the investigation. A superconducting magnet up to 7 T from Cryogenic Ltd., having a 10 cm open bore hole was used for the Mössbauer meaurements at room temperature. The Mössbauer velocity scale and center shifts CS are given with α-fe at room temperature as reference. 3. Results and discussion Figure 1 shows M(T)-curves at increasing temperatures for the three samples in an external field of 1 koe (=0.1 T). The weak temperature dependence of the curves may look typical to metallic PM samples. Knowing about the existence of a possible low temperature AFM transition, indications of such a transition are seen at low temperatures for the powder samples. The magnetization of the foil only shows a continuous decrease, but this sample had a remanent magnetization of 4.3 emu/g when the external field was switched off. We attribute this remanence to martensite or a ferrite impurity of 2 wt%, not characteristic Figure 1. M(T) curves measured at increasing temperature for zero field cooled (ZFC) samples in an external field of 1 koe.
NON-MAGNETIC STAINLESS STEELS REINVESTIGATED 153 for the fcc stainless steel, and that the temperature dependence of the magnetization is influenced by the decrease of the remanence with increasing temperature. Figure 2 shows magnetization versus applied field curves at 300 K for the three samples. The M(H)-curves measured at 10 K did have a rather similar shape as those in Figure 2: a technical saturation well below 10 koe and then concave curves not at all saturated even at 50 koe. However, the magnetization at higher fields (and slope of the curves) was about 2 3 times stronger than at 300 K, albeit with an initial slope at lower fields (<2 koe) that remained quite weakly temperature dependent (cf. Figure 1). The rapid increase of the magnetization at low fields might be assigned to nano sized regions (few % in volume) of martensitic or ferritic phases. Somewhat larger regions of martensite or ferrite in the foil could then also cause the remanence. Mössbauer spectra (5 300 K) recorded in zero external field showed a singlet that broadened below 50 K. The broadening (in FWHM) was 0.5 mm/s, indicating a PM AFM transition in agreement with the SQUID measurements. However, the saturated moments seem to be very small, 0.1µ B (as 2.2 µ B corresponds to 10.62 mm/s in α-fe), but in line with results for the low moment AFM phase in the Fe Ni Invar region [3]. The CS(T )-curves for the three samples were also quite normal, 0.1 mm/s at 295 K, then a slope in agreement with the second order Doppler-shift (SODS) for a Debye-temperature of 400 K. However, at the PM AFM transition there seems to be a very small extra shift of 0.005 mm/s. The in-field spectra for the Ni11 and Ni13 samples are similar to those obtained for the foil, shown in Figure 3. Due to the high absorption in the middle of the Mössbauer pattern, it seems necessary to use a low field component in fitting the spectra, much too strong to be related to the earlier mentioned martensitic or ferritic phases. Accordingly, the samples cannot be in a pure PM state at room temperature. A possible interpretation could be to introduce a field induced fer- Figure 2. M(H) curves for the three samples measured at 300 K. The remanence detected in the foil is subtracted.
154 T. ERICSSON ET AL. Figure 3. MS-spectra for the foil recorded at room temperature in longitudinal external fields 0 7 T. The spectra are fitted using two quadruplets (lines 2 and 5 missing in an ordinary sextet). The high field (dashed line) and low field (full line) components are also shown. Figure 4. Measured effective fields and intensity of low field component (I2, right scale) versus external field.
NON-MAGNETIC STAINLESS STEELS REINVESTIGATED 155 romagnetic state (FIFM). The measured effective field will then roughly be the magnitude difference between the external field and the induced hyperfine field. Our results above then indicate a heterogeneous situation. A fraction of the sample is PM (high field component) and another part, having an intensity decreasing with increasing field (Figure 4), is FIFM with a magnetic moment component increasing with 0.07 µ B /T external field. Interestingly, we have found the same behavior in Fe Ni (21 27%) fcc alloys and also in the anti-taenite Fe Ni fraction in two chondrites (not published yet). FIFM states have been proposed in fcc dilute FeCu [5] and FeCrNi stainless steels [2] (magnetization study). Metamagnetism has been proposed at low temperature in Fe Ni Invar alloys [6], also from magnetization measurements. References 1. Reuter, K. B., Williams, D. B. and Goldstein, J. I., Metallurgical Transactions A 20 (1989), 719. 2. Majumdar, A. K. and Blanckenhagen, P. V., Phys. Rev. B 29 (1984), 4079. 3. Asano, H. J., Phys. Soc. Japan 27 (1969), 542. 4. Kondorsky, E. I. and Sedov, V. L., J. Appl. Phys. 31 (Suppl.) (1960), 331S. 5. Frankel, R. B., Blum, N. A., Schwartz, B. B. and Kim, D. J., Phys. Rev. Lett. 18 (1967), 1051. 6. Pauthenet, R., Maruryama, H., and Yamada, O., J. Magn. Magn. Mater. 31 34 (1983), 835.