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Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide
Liedke, M. O.; Anwand, W.; Bali, R.; Cornelius, S.; Butterling, M.; Trinh, T. T.; Wagner, A.; Salamon, S.; Walecki, D.; Smekhova, A.; Wende, H.; Potzger, K.;
Magnetic phase transition in the Fe60Al40 transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000°C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation and Mössbauer spectroscopy. The positron annihilation spectroscopy (PAS) has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis (AIDA) that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate importance of the open volume defects onto the magnetic phase transition.
Magneto-optical measurements of the ordered Fe60Al40 as well as disordered sample after each annealing step have been done ex-situ at room temperature. A set of magnetization reversal loops is presented in Figure 1(a). The ordered sample shows no magnetic signal at room temperature at all, whereas the disordered one is represented by a magnetic reversal curve with coercivity of about 65 Oe. Due to annealing both the remanence and coercivity drop significantly and already at 500°C the magnetic signal vanishes. The SEM images [see inset of Fig. 1(a)] reveal continuous film surface at 500°C, whereas at 1000°C darker and bright island-like regions were found that possibly correspond to the former FeAl film and Al-AlO segregations, respectively. The light gray color surrounding the island-like regions in the 1000°C case represents most probably the SiO2 substrate.

Two different positron annihilation spectroscopy (PAS) measurement types have been utilized for defect analysis after each temperature step: (i) room temperature standard Doppler broadening as a function of positron energy E for depth profiling, (ii) RT coincident Doppler broadening measurements at fixed energy after each temperature step that can in detail reveal information of the chemical environment of defects with higher energy resolution [Fig. 1(b)]. In both cases two specific annihilation line parameters have been extracted: (i) the shape parameter S that corresponds to the fraction of positrons annihilating with the low-momentum electrons localized close to the middle of the annihilation line, and (ii) the wing parameter W that takes into appoint positron annihilation with high-momentum electrons at the outer region of the annihilation line. In general, the S parameter is sensitive to the open volume defects amount and their size, whereas the W parameter is a fingerprint of the annihilation site chemical environment, thus defects decoration by neighboring matrix elements [1].

cDB results are summarized in Figure 1(b), where the annihilation line parameters are plotted as a function of the annealing temperature. We can clearly see that up to 600°C both S and W parameters are more or less constant, whereas a jump followed by another plateau in the S parameter value for higher temperatures is visible. The jump of the S parameter can correspond to a slight increase of the open volume number and/or its size, whereas the decrease of W shifts the defect decoration in direction of Al. The S parameter difference at the jump is of about 1.5% that can be considered as low, nevertheless a step-like dependence is visible. A drop of about 5.5% in W cannot be neglected. Moreover W recovers to its original value at 1000°C that suggests larger occupation of the neighboring defect sites with Fe as well as a slight change of film stoichiometry due to annealing.

Summarizing, no evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600°C) has been found, whereas higher temperatures showed an increase in the vacancy concentration. Still, in the surface region of the sample reasonably large annihilation line parameters variations have been found showing sufficient sensitivity of our experimental tools, and ruling out possible signal saturation due to too high vacancies concentration. At higher temperatures a slightly larger defect concentration as well as different defect decorations were found that is likely a result of temperature driven material decomposition leading to a complete film melt at 1000°C. Magnetic and structural changes have been tracked using PAS, XRD, and CEMS measurements. The magnetic phase transition from para- to ferro-magnetic behavior appears to be driven by chemical disordering alone, and is independent of the vacancy concentration. These results help understand the role of defects in materials that show disorder-induced ferromagnetism.

This work has been partially financed by the Initiative and Networking Fund of the German Helmholtz Association, Helmholtz Virtual Institute MEMRIOX (VH-VI-442).

[1] - R. Krause-Rehberg and H. Leipner, Positron Annihilation in Semiconductors, Solid-State Sciences. Berlin: Springer, vol. 127 (1999)
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