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Effect of high temperature annealing on ferromagnetism induced by energetic ion irradiation in FeRh alloy
Nuclear Instruments and Methods in Physics Research B 269 (2011) 869–872
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of high temperature annealing on ferromagnetism induced by energetic ion irradiation in FeRh alloy S. Kosugi a, Nao Fujita a, T. Matsui a, F. Hori a, Y. Saitoh b, N. Ishikawa c, Y. Okamoto c, A. Iwase a,⇑ a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Japan Atomic Energy Agency (JAEA-Takasaki), Takasaki, Gunma 370-1292, Japan c Japan Atomic Energy Agency (JAEA-Tokai), Tokai, Ibaraki 319-1195, Japan b
a r t i c l e
i n f o
Article history: Received 16 August 2010 Received in revised form 15 November 2010 Available online 23 December 2010 Keywords: FeRh alloy Magnetic properties Ion irradiation Thermal annealing EXAFS SQUID
a b s t r a c t Effects of thermal annealing on ion-irradiation induced ferromagnetism of Fe–50at.%Rh bulk alloy and the related structural change were investigated by means of superconducting quantum interference device (SQUID) and extended X-ray absorption fine structure (EXAFS), respectively. Depending on the annealing temperature from 100 to 500 °C, the magnetization induced by 10 MeV iodine ion irradiation and the lattice structure of the alloy were remarkably changed. After 500 °C annealing, the magnetization and the lattice ordering of the alloy become similar to the states before the irradiation. The experimental result indicates that the thermal relaxation of irradiation-induced atomic disordering dominates the magnetic state of ion-irradiated Fe–50at.% Rh alloy. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Equiatomic ordered Fe–50at.%Rh alloy with the B2 (CsCl type) structure is of much interest as a magnetic material due to the first order anti-ferromagnetic to ferromagnetic phase transition near the room temperature and the transition from ferromagnetic to paramagnetic phase at 400 °C [1,2]. In addition to the magnetic properties, the crystalline structure of this alloy also depends on the temperature; it is the A1 structure above 1030 °C, and below this temperature, its lattice structure becomes B2 structure [3]. In the previous studies, we have found that energetic ion irradiation induces a ferromagnetic state in Fe–50at.%Rh alloys at low temperatures and this phenomenon is dominated by the energy deposition through elastic collisions [4,5]. In this paper, we show the thermal annealing effects on the magnetization and the lattice structure of ion-irradiated Fe– 50at.%Rh alloy and discuss the relationship between the magnetization and the thermal relaxation of irradiation-induced lattice disordering. 2. Experimental procedure Fe–50at.%Rh bulk samples with a dimension of 5.0 5.0 0.2 mm3 were prepared by cutting from an ingot. These samples ⇑ Corresponding author. Tel./fax: +81 72 254 9810. E-mail address: [email protected] (A. Iwase). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.12.061
were annealed at 1100 °C for 24 h for the processing of homogenization. The samples were irradiated at room temperature with 10 MeV iodine ions by using a tandem accelerator at JAEA-Takasaki. The ion-fluences were 1 1013/cm2 and 5 1013/cm2. Fig. 1 shows the depth dependence of the energy deposited by 10 MeV iodine ion per unit length through the elastic collision in Fe–50at.%Rh, which has been calculated using the SRIM code [6]. The energy deposition is localized only from the surface to the depth of 2.5 lm, which is much smaller than the sample thickness (0.2 mm). Therefore, only the region near the surface was affected by the irradiation. After the irradiations, the samples were isochronally annealed up to 500 °C. The isochronal annealing was performed in vacuum for 100 min at temperature interval of 100 °C. Effects of the thermal annealing on magnetization and lattice structure were measured by using the Quantum Design SQUID magnetometer and the extended X-ray absorption fine structure (EXAFS) measurement, respectively. The magnetic hysteresis (M–H curve) was measured in a field range of 6000 to 6000 Oe at 253 °C. Spectra of EXAFS around the Fe K-absorption edge (7.11 keV) were acquired by the fluorescence method at room temperature at the end of the station of the 27 B beam line of the Photon Factory at High Energy Accelerator Research Organization (KEK-PF). 3. Results and discussion In Fig. 2, values of irradiation-induced saturation magnetization are plotted against the iodine ion-fluence. In the figure, data for the
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Fig. 3. Thermal annealing effect on magnetization-external magnetic field curves at 253 °C for the Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (a), and that for the sample irradiated to the fluence of 1 1013/cm2 (b).
Ion-fluence [x1012/cm2] Fig. 2. Values of saturation magnetization, as a function of 10 MeV iodine ion fluence. Solid squares are the present experimental data and open circles are the previously reported ones [5]. The dashed line is a guide to the eye.
samples irradiated at this experiment are shown as well as the previous ones [5]. As we have reported previously, for low ionfluences, the magnetization increases with increasing ion-fluence, reaches the maximum value, and then it decreases for higher ion-fluences. In the figure, data points indicated by solid squares are those for the samples which were used for the present annealing experiment. Figs. 3 and 4 show the annealing behavior of the magnetizationexternal magnetic field (M H) curves and the k3-weighted Fe K-edge Fourier transform (FT) EXAFS spectra, respectively, for the two ion-irradiated Fe–50at.%Rh samples with different fluences. The values of the magnetization in Fig. 3 are the average values for the irradiated region of the samples, which is shown in Fig. 1
as from the surface to the depth of 2.5 lm. Fig. 5 indicates the dependence of the saturation magnetization, Ms, on the annealing temperature for the two samples. First, we discuss the annealing behavior of the sample irradiated to 5 1013/cm2. Figs. 2 and 3(a) show that the saturation magnetization of this sample before the thermal annealing is 15 emu/g, which is only one-sixth of the maximum value of irradiation-induced magnetization. The FEFF simulation code [7] has revealed that the FT-EXAFS spectrum for the sample before thermal annealing (the top spectrum in Fig. 4(a)) corresponds to A1 structure, i.e., the FCC structure in which Fe and Rh atoms are randomly distributed. As mentioned in the introduction, the paramagnetic A1 structure appears above 1030 °C in the phase diagram. The experimental result implies that the high-temperature paramagnetic phase is produced by the irradiation, causing the strong reduction of magnetization. The reason why the sample shows the finite Ms value (15 emu/g) even after the irradiation is that a small part of the irradiated sample still keeps the
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S. Kosugi et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 869–872
100 1x10 13 /cm2 5x10 13 /cm2
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Temperature [ C] Fig. 5. Saturation magnetization as a function of annealing temperature for Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (solid squares) and that for the fluence of 1 1013/cm2 (solid circles).
sample (the bottom spectrum in Fig. 4(a)) corresponds to B2 (CsCl) structure. As can be seen in Figs. 3(a), 4(a) and 5, the annealing behavior of the magnetization and the lattice structure are quite interesting. The annealing at 100 and 200 °C little changes the magnetization or the lattice structure. The annealing at 300 and 400 °C remarkably increases the magnetization and changes the lattice structure from A1 to B2 structure. After the annealing at 500 °C, the magnetization becomes nearly zero, which is the same as for the unirradiated sample, and the FT-EXAFS spectrum becomes about the same as that for unirradiated sample. This means that by the 500 °C annealing, the magnetization and the lattice structure nearly recover to the state before irradiation, i.e., the anti-ferromagnetic B2 state. Fig. 6 shows the full width half maximum (FWHM) of the first FT-EXAFS peak of
Fig. 4. Thermal annealing effect on Fe K-edge FT-EXAFS spectra for the Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (a), and that for the sample irradiated to the fluence of 1 1013/cm2 (b).
ferromagnetic B2 structure. It is worth noting here that the FEFF simulation also shows that the FT-EXAS spectrum for unirradiated
Fig. 6. Full width half maximum (FWHM) of the first FT-EXAFS peaks of B2 structure as a function of annealing temperature. Dashed line is the FWHM for unirradiated sample.
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B2 structure as a function of annealing temperature. In the figure, the result for the sample irradiated to 1 1013/cm2 is also shown, which will be discussed later. The figure shows that the peak for the annealing at 400 °C is slightly broader than that for the annealing at 500 °C. On the other hand, the value of the saturation magnetization for the annealing at 500 °C is nearly 0 emu/g and that for the annealing at 400 °C is larger than 60 emu/g. These two experimental results imply that the magnetic state of Fe–50at%Rh is quite sensitive to the degree of lattice ordering, and even a slight increase in lattice disordering causes the appearance of the ferromagnetic state. Second, we discuss the annealing behavior of the sample irradiated to 1 1013/cm2. Figs. 2 and 3(b) shows that the saturation magnetization of this sample before the thermal annealing is 62 emu/g which is close to the maximum value of the irradiation-induced magnetization. The FT-EXAFS spectrum for the sample before thermal annealing (the top spectrum in Fig. 4(b)) shows that the lattice structure still remains B2 structure, but as can be seen in Fig. 6, the peak broadening is observed as compared with the spectrum for unirradiated sample (the bottom spectrum in Fig. 4(b)). This result means that the atomic arrangement around Fe atoms is disordered by the irradiation. As can be seen in Figs. 3(b), 4(b) and 5, the annealing behavior of the magnetization and the lattice structure for the sample irradiated to 1 1013/cm2 are quite different from that for the sample irradiated to 5 1013/cm2. The annealing at 100 °C and 200 °C induces a small increase in magnetization, but the FT-EXAF spectra are kept unchanged. Fig. 6 shows that the FWHM values for the first FT-EXAFS peak is about the same for the annealing at 100 and 200 °C. The annealing at 300 and 400 °C remarkably decreases the magnetization and the atomic arrangement around Fe atoms restores the ordering, which can be deduced from a peak narrowing of the FT-EXAFS spectrum (see Fig. 4(b) and Fig. 6). After the annealing at 500 °C, like for the sample irradiated to 5 1013/ cm2, the magnetization, and the lattice structure become similar to the state before irradiation, i.e., the anti-ferromagnetic B2 state. The results of the present annealing experiment clearly indicate that the thermal relaxation of irradiation-induced lattice disordering dominates the state of magnetization of ion-irradiated Fe–50at.%Rh alloy. As long as the lattice structure remains B2 structure, more disordered the lattice structure is, the larger magnetization is observed. The correlation between the magnetic state of FeRh alloy and the lattice disordering is qualitatively consistent with the results which have been observed in other disordered FeRh materials. For example, it has so far been reported that disordered FeRh thin films and ball-milled FeRh show the ferromagnetic state even at low temperatures [8,9]. During the thermal annealing, the irradiation-induced A1 structure is thermally relaxed to the disordered B2 structure, causing the change in magnetic state from paramagnetic to ferromagnetic state. If the ion-fluence is not so large and not A1 structure but disordered B2 structure is induced, this B2 structure is thermally relaxed to ordered B2 structure, causing the change in magnetic state from
ferromagnetic to anti-ferromagnetic state. Until now, however, the quantitative correlation between the lattice disordering and magnetic state still remains uncertain. For example, we have never explained the reason why the values of saturation magnetization after 400 °C annealing is completely different between for the sample irradiated to 1 1013/cm2 and for the sample irradiated to 5 1013/cm2, in spite of nearly the same FT-EXAFS spectra and FWHM values of the first FT-EXAFS peaks. To clarify the details of ion-irradiation and subsequent annealing effects on magnetic states of FeRh alloys, experiments using the soft X-ray magnetic circular dichroism (XMCD) and the magnetic compton profiles (MCP) [10] are now in progress at synchrotron radiation facilities. In our previous paper [11], we have reported that ion irradiation is a useful tool for the modification of magnetic properties of FeRh alloy. The present result suggests that the combination of ion irradiation and thermal annealing can modify the magnetic properties of FeRh alloy much more widely. 4. Summary Thermal annealing experiments were performed for Fe– 50at.%Rh bulk alloy irradiated with 10 MeV iodine ions, and the annealing behavior of magnetization and lattice structure was estimated by using of SQUID magnetometer and EXAFS measurement. The thermal relaxation of irradiation-induced lattice disordering strongly affects the magnetic state of ion-irradiated Fe–50at.%Rh alloy. Acknowledgements This work was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society. The authors thank the staffs of KEK-PF and the accelerator division of JAEA-Takasaki for their help. This work has been performed under the collaboration program between Osaka Prefecture University and Japan Atomic Energy Agency. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
J.S. Kouvel, C.C. Hartelius, J. Appl. Phys. Suppl. 33 (1962) 1343. J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. L.J. Swartzendruber, Binary Alloy Phase Diagram 2 (1992) 1762. Y. Zushi, M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, A. Iwase, Nucl. Instr. Meth. B 256 (2007) 434. S. Kosugi, N. Fujita, Y. Zushi, T. Mastui, N. Ishikawa, Y. Saito, A. Iwase, Nucl. Instr. Meth. B 267 (2009) 1612–1615. J.F. Ziegler. Available from:
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of high temperature annealing on ferromagnetism induced by energetic ion irradiation in FeRh alloy S. Kosugi a, Nao Fujita a, T. Matsui a, F. Hori a, Y. Saitoh b, N. Ishikawa c, Y. Okamoto c, A. Iwase a,⇑ a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Japan Atomic Energy Agency (JAEA-Takasaki), Takasaki, Gunma 370-1292, Japan c Japan Atomic Energy Agency (JAEA-Tokai), Tokai, Ibaraki 319-1195, Japan b
a r t i c l e
i n f o
Article history: Received 16 August 2010 Received in revised form 15 November 2010 Available online 23 December 2010 Keywords: FeRh alloy Magnetic properties Ion irradiation Thermal annealing EXAFS SQUID
a b s t r a c t Effects of thermal annealing on ion-irradiation induced ferromagnetism of Fe–50at.%Rh bulk alloy and the related structural change were investigated by means of superconducting quantum interference device (SQUID) and extended X-ray absorption fine structure (EXAFS), respectively. Depending on the annealing temperature from 100 to 500 °C, the magnetization induced by 10 MeV iodine ion irradiation and the lattice structure of the alloy were remarkably changed. After 500 °C annealing, the magnetization and the lattice ordering of the alloy become similar to the states before the irradiation. The experimental result indicates that the thermal relaxation of irradiation-induced atomic disordering dominates the magnetic state of ion-irradiated Fe–50at.% Rh alloy. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Equiatomic ordered Fe–50at.%Rh alloy with the B2 (CsCl type) structure is of much interest as a magnetic material due to the first order anti-ferromagnetic to ferromagnetic phase transition near the room temperature and the transition from ferromagnetic to paramagnetic phase at 400 °C [1,2]. In addition to the magnetic properties, the crystalline structure of this alloy also depends on the temperature; it is the A1 structure above 1030 °C, and below this temperature, its lattice structure becomes B2 structure [3]. In the previous studies, we have found that energetic ion irradiation induces a ferromagnetic state in Fe–50at.%Rh alloys at low temperatures and this phenomenon is dominated by the energy deposition through elastic collisions [4,5]. In this paper, we show the thermal annealing effects on the magnetization and the lattice structure of ion-irradiated Fe– 50at.%Rh alloy and discuss the relationship between the magnetization and the thermal relaxation of irradiation-induced lattice disordering. 2. Experimental procedure Fe–50at.%Rh bulk samples with a dimension of 5.0 5.0 0.2 mm3 were prepared by cutting from an ingot. These samples ⇑ Corresponding author. Tel./fax: +81 72 254 9810. E-mail address: [email protected] (A. Iwase). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.12.061
were annealed at 1100 °C for 24 h for the processing of homogenization. The samples were irradiated at room temperature with 10 MeV iodine ions by using a tandem accelerator at JAEA-Takasaki. The ion-fluences were 1 1013/cm2 and 5 1013/cm2. Fig. 1 shows the depth dependence of the energy deposited by 10 MeV iodine ion per unit length through the elastic collision in Fe–50at.%Rh, which has been calculated using the SRIM code [6]. The energy deposition is localized only from the surface to the depth of 2.5 lm, which is much smaller than the sample thickness (0.2 mm). Therefore, only the region near the surface was affected by the irradiation. After the irradiations, the samples were isochronally annealed up to 500 °C. The isochronal annealing was performed in vacuum for 100 min at temperature interval of 100 °C. Effects of the thermal annealing on magnetization and lattice structure were measured by using the Quantum Design SQUID magnetometer and the extended X-ray absorption fine structure (EXAFS) measurement, respectively. The magnetic hysteresis (M–H curve) was measured in a field range of 6000 to 6000 Oe at 253 °C. Spectra of EXAFS around the Fe K-absorption edge (7.11 keV) were acquired by the fluorescence method at room temperature at the end of the station of the 27 B beam line of the Photon Factory at High Energy Accelerator Research Organization (KEK-PF). 3. Results and discussion In Fig. 2, values of irradiation-induced saturation magnetization are plotted against the iodine ion-fluence. In the figure, data for the
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S. Kosugi et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 869–872
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700
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Fig. 3. Thermal annealing effect on magnetization-external magnetic field curves at 253 °C for the Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (a), and that for the sample irradiated to the fluence of 1 1013/cm2 (b).
Ion-fluence [x1012/cm2] Fig. 2. Values of saturation magnetization, as a function of 10 MeV iodine ion fluence. Solid squares are the present experimental data and open circles are the previously reported ones [5]. The dashed line is a guide to the eye.
samples irradiated at this experiment are shown as well as the previous ones [5]. As we have reported previously, for low ionfluences, the magnetization increases with increasing ion-fluence, reaches the maximum value, and then it decreases for higher ion-fluences. In the figure, data points indicated by solid squares are those for the samples which were used for the present annealing experiment. Figs. 3 and 4 show the annealing behavior of the magnetizationexternal magnetic field (M H) curves and the k3-weighted Fe K-edge Fourier transform (FT) EXAFS spectra, respectively, for the two ion-irradiated Fe–50at.%Rh samples with different fluences. The values of the magnetization in Fig. 3 are the average values for the irradiated region of the samples, which is shown in Fig. 1
as from the surface to the depth of 2.5 lm. Fig. 5 indicates the dependence of the saturation magnetization, Ms, on the annealing temperature for the two samples. First, we discuss the annealing behavior of the sample irradiated to 5 1013/cm2. Figs. 2 and 3(a) show that the saturation magnetization of this sample before the thermal annealing is 15 emu/g, which is only one-sixth of the maximum value of irradiation-induced magnetization. The FEFF simulation code [7] has revealed that the FT-EXAFS spectrum for the sample before thermal annealing (the top spectrum in Fig. 4(a)) corresponds to A1 structure, i.e., the FCC structure in which Fe and Rh atoms are randomly distributed. As mentioned in the introduction, the paramagnetic A1 structure appears above 1030 °C in the phase diagram. The experimental result implies that the high-temperature paramagnetic phase is produced by the irradiation, causing the strong reduction of magnetization. The reason why the sample shows the finite Ms value (15 emu/g) even after the irradiation is that a small part of the irradiated sample still keeps the
871
S. Kosugi et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 869–872
100 1x10 13 /cm2 5x10 13 /cm2
80
60
40
20
0 0
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200
300
400
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o
Temperature [ C] Fig. 5. Saturation magnetization as a function of annealing temperature for Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (solid squares) and that for the fluence of 1 1013/cm2 (solid circles).
sample (the bottom spectrum in Fig. 4(a)) corresponds to B2 (CsCl) structure. As can be seen in Figs. 3(a), 4(a) and 5, the annealing behavior of the magnetization and the lattice structure are quite interesting. The annealing at 100 and 200 °C little changes the magnetization or the lattice structure. The annealing at 300 and 400 °C remarkably increases the magnetization and changes the lattice structure from A1 to B2 structure. After the annealing at 500 °C, the magnetization becomes nearly zero, which is the same as for the unirradiated sample, and the FT-EXAFS spectrum becomes about the same as that for unirradiated sample. This means that by the 500 °C annealing, the magnetization and the lattice structure nearly recover to the state before irradiation, i.e., the anti-ferromagnetic B2 state. Fig. 6 shows the full width half maximum (FWHM) of the first FT-EXAFS peak of
Fig. 4. Thermal annealing effect on Fe K-edge FT-EXAFS spectra for the Fe–50at.%Rh sample irradiated to the fluence of 5 1013/cm2 (a), and that for the sample irradiated to the fluence of 1 1013/cm2 (b).
ferromagnetic B2 structure. It is worth noting here that the FEFF simulation also shows that the FT-EXAS spectrum for unirradiated
Fig. 6. Full width half maximum (FWHM) of the first FT-EXAFS peaks of B2 structure as a function of annealing temperature. Dashed line is the FWHM for unirradiated sample.
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B2 structure as a function of annealing temperature. In the figure, the result for the sample irradiated to 1 1013/cm2 is also shown, which will be discussed later. The figure shows that the peak for the annealing at 400 °C is slightly broader than that for the annealing at 500 °C. On the other hand, the value of the saturation magnetization for the annealing at 500 °C is nearly 0 emu/g and that for the annealing at 400 °C is larger than 60 emu/g. These two experimental results imply that the magnetic state of Fe–50at%Rh is quite sensitive to the degree of lattice ordering, and even a slight increase in lattice disordering causes the appearance of the ferromagnetic state. Second, we discuss the annealing behavior of the sample irradiated to 1 1013/cm2. Figs. 2 and 3(b) shows that the saturation magnetization of this sample before the thermal annealing is 62 emu/g which is close to the maximum value of the irradiation-induced magnetization. The FT-EXAFS spectrum for the sample before thermal annealing (the top spectrum in Fig. 4(b)) shows that the lattice structure still remains B2 structure, but as can be seen in Fig. 6, the peak broadening is observed as compared with the spectrum for unirradiated sample (the bottom spectrum in Fig. 4(b)). This result means that the atomic arrangement around Fe atoms is disordered by the irradiation. As can be seen in Figs. 3(b), 4(b) and 5, the annealing behavior of the magnetization and the lattice structure for the sample irradiated to 1 1013/cm2 are quite different from that for the sample irradiated to 5 1013/cm2. The annealing at 100 °C and 200 °C induces a small increase in magnetization, but the FT-EXAF spectra are kept unchanged. Fig. 6 shows that the FWHM values for the first FT-EXAFS peak is about the same for the annealing at 100 and 200 °C. The annealing at 300 and 400 °C remarkably decreases the magnetization and the atomic arrangement around Fe atoms restores the ordering, which can be deduced from a peak narrowing of the FT-EXAFS spectrum (see Fig. 4(b) and Fig. 6). After the annealing at 500 °C, like for the sample irradiated to 5 1013/ cm2, the magnetization, and the lattice structure become similar to the state before irradiation, i.e., the anti-ferromagnetic B2 state. The results of the present annealing experiment clearly indicate that the thermal relaxation of irradiation-induced lattice disordering dominates the state of magnetization of ion-irradiated Fe–50at.%Rh alloy. As long as the lattice structure remains B2 structure, more disordered the lattice structure is, the larger magnetization is observed. The correlation between the magnetic state of FeRh alloy and the lattice disordering is qualitatively consistent with the results which have been observed in other disordered FeRh materials. For example, it has so far been reported that disordered FeRh thin films and ball-milled FeRh show the ferromagnetic state even at low temperatures [8,9]. During the thermal annealing, the irradiation-induced A1 structure is thermally relaxed to the disordered B2 structure, causing the change in magnetic state from paramagnetic to ferromagnetic state. If the ion-fluence is not so large and not A1 structure but disordered B2 structure is induced, this B2 structure is thermally relaxed to ordered B2 structure, causing the change in magnetic state from
ferromagnetic to anti-ferromagnetic state. Until now, however, the quantitative correlation between the lattice disordering and magnetic state still remains uncertain. For example, we have never explained the reason why the values of saturation magnetization after 400 °C annealing is completely different between for the sample irradiated to 1 1013/cm2 and for the sample irradiated to 5 1013/cm2, in spite of nearly the same FT-EXAFS spectra and FWHM values of the first FT-EXAFS peaks. To clarify the details of ion-irradiation and subsequent annealing effects on magnetic states of FeRh alloys, experiments using the soft X-ray magnetic circular dichroism (XMCD) and the magnetic compton profiles (MCP) [10] are now in progress at synchrotron radiation facilities. In our previous paper [11], we have reported that ion irradiation is a useful tool for the modification of magnetic properties of FeRh alloy. The present result suggests that the combination of ion irradiation and thermal annealing can modify the magnetic properties of FeRh alloy much more widely. 4. Summary Thermal annealing experiments were performed for Fe– 50at.%Rh bulk alloy irradiated with 10 MeV iodine ions, and the annealing behavior of magnetization and lattice structure was estimated by using of SQUID magnetometer and EXAFS measurement. The thermal relaxation of irradiation-induced lattice disordering strongly affects the magnetic state of ion-irradiated Fe–50at.%Rh alloy. Acknowledgements This work was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society. The authors thank the staffs of KEK-PF and the accelerator division of JAEA-Takasaki for their help. This work has been performed under the collaboration program between Osaka Prefecture University and Japan Atomic Energy Agency. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
J.S. Kouvel, C.C. Hartelius, J. Appl. Phys. Suppl. 33 (1962) 1343. J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. L.J. Swartzendruber, Binary Alloy Phase Diagram 2 (1992) 1762. Y. Zushi, M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, A. Iwase, Nucl. Instr. Meth. B 256 (2007) 434. S. Kosugi, N. Fujita, Y. Zushi, T. Mastui, N. Ishikawa, Y. Saito, A. Iwase, Nucl. Instr. Meth. B 267 (2009) 1612–1615. J.F. Ziegler. Available from: