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Study on ferromagnetic ordering of FeRh thin films induced by energetic heavy ion irradiation by means of X-ray Magnetic Circular Dichroism
Nuclear Instruments and Methods in Physics Research B 314 (2013) 99–102
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study on ferromagnetic ordering of FeRh thin films induced by energetic heavy ion irradiation by means of X-ray Magnetic Circular Dichroism Kazuma Aikoh a, Atsushi Tohki a, Shuichi Okuda b, Yuichi Saitoh c, Tomihiro Kamiya c, Tetsuya Nakamura d, Toyohiko Kinoshita d,e, Akihiro Iwase a, Toshiyuki Matsui f,⇑ a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Radiation Research Center, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Gumma 370-1292, Japan d Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198, Japan e CREST-JST, Kawaguchi, Saitama 332-0012, Japan f Research Organization of the 21st Century, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan b c
a r t i c l e
i n f o
Article history: Received 29 November 2012 Received in revised form 28 April 2013 Accepted 3 May 2013 Available online 24 June 2013 Keywords: Ion beam irradiation MCD FeRh thin film Modification of magnetic properties
a b s t r a c t We investigated the ion irradiation induced ferromagnetic state of FeRh thin films with 10 MeV I ion beam by the measurements of a superconducting quantum interference device (SQUID) magnetometer as well as of soft X-ray Magnetic Circular Dichroism (XMCD). It was clearly shown in the magnetization loops by SQUID measurements that the ion irradiation induced the ferromagnetic state in the FeRh thin films even below room temperature. This was also confirmed by the Fe L2,3-edge XMCD measurements for the irradiated FeRh film samples. However, we found that the irradiation ion fluence dependence on the magnetization was totally different between two measurement techniques. We also revealed by XMCD sum rule analysis that the ferromagnetism in the ion irradiated FeRh thin films was mainly dominated by the spin moment. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction B2(CsCl)-type FeRh has attracted much attention to several potential technological advantages for magnetic devices, such as thermally assisted magnetic recording media, due to its unique first order low temperature antiferromagnetic (AF) – high temperature ferromagnetic (FM) phase transition [1,2]. In our previous researches, we successfully induced the FM state by the energetic ion-beam irradiation in the Fe-50 at.% Rh bulk and film samples even at low temperatures where they were originally in AF state [3–6]. In addition, we also reported that the deposited energy through elastic collisions between ion species and the samples significantly influenced the change in magnetizations [7,8]. Actually, we have been able to successfully modify the magnetic state of FeRh bulk and film samples by the ion-beam irradiation with various accelerated voltage [9]. In the present studies, we have tried to evaluate the nature of the magnetic state of the FeRh thin films irradiated with 10 MeV Iodine ion beam by means of soft X-ray Magnetic Circular Dichroism (XMCD) with synchrotron radiation. XMCD is well known to be an effective technique for measuring magnetic state at the sample ⇑ Corresponding author. Tel.: +81 72 254 9311; fax: +81 72 254 9912. E-mail address: [email protected] (T. Matsui). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.031
surface. Hence, the surface magnetic state modified by ion beam irradiation can be effectively evaluated by that technique, even if it cannot be measured by ordinary measurement techniques, such as SQUID magnetometer, due to its small values of the net magnetization. In addition to this, XMCD provides information on the magnetization of one element even though the sample is composed of more than two elements. Moreover, the analysis of the MCD integrated intensity is capable of quantitative evaluation of orbital and spin magnetic moment using the well-established XMCD sum rule. Hence, it is possible to know more detailed magnetic natures of the samples rather than those obtained from SQUID measurement. In this paper, we discuss the results of the XMCD measurement for the 10 MeV I ion irradiated FeRh thin films, in conjunction with those of the SQUID measurements. In addition, we also quantitatively discuss ferromagnetism induced by such irradiation by the well-established XMCD sum rule for the L edge. 2. Experimental procedure Fe47Rh53 thin films about 80 nm thick were deposited on MgO (1 0 0) substrates at 973 K by means of ion beam sputtering from an alloy target of Fe50Rh50. The base pressure of the sputtering chamber was about 2.0 10 6 Pa. The total deposition rate was
100
K. Aikoh et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 99–102
The results of the structural characterization for the unirradiated and irradiated samples with various ion fluences are shown in the XRD scans in Fig. 1. The unirradiated sample was composed of the a’-FeRh phase with a B2-type structure and the c-FeRh phase with an A1 (disordered face centered cubic)-type structure, both of which exhibit the [0 0 1] preferential orientation. As for the irradiation effect on the structural properties, the B2-type FeRh phase still existed up to the ion fluence until 2 1013 cm 2 of the 10 MeV I ion-beam irradiation. The relative intensity of the B2 phase gradually decreased with increasing in the ion irradiation fluence. This is considered the fairly amount of the lattice defects are introduced in the B2 type phase by the ion irradiation. On
the other hand, the sample irradiated with 4 1014 cm 2 did not exhibit the XRD peaks for the B2 phase any more. We can apparently see only the XRD peaks for the A1 type phase except those from the MgO substrates. This result indicates that the excess ion irradiation caused the decomposition of the B2 phase, resulting in the formation of high temperature A1 phase. Fig. 2 shows the magnetic-hysteresis loops measured at 20 K for the unirradiated and irradiated samples with various ion fluences, 2 1012–4 1014 cm 2. Even in the unirradiated sample, a small magnetization of about 8 emu/g existed prior to the ion exposure. This small magnetization can be ascribed to the lattice defects and/ or lattice distortion in the B2-type FeRh phase, which was already discussed in the previous papers [7,8]. As clearly seen in the figure, the ion irradiation increases the magnetization of the films. The maximum value of saturation magnetization, 57 emu/g, was obtained in the sample irradiated with an ion fluence of 1– 2 1013 cm 2, which is 43% of the saturation magnetization of the ferromagnetic FeRh bulk samples (50 at.%) bulk samples at room temperature. According to our previous paper, the lattice defects introduced by ion irradiation can make the ferromagnetic state stabilize in the B2 type FeRh compounds. Hence, the ferromagnetic orders are considered to be enhanced by the 10 MeV ion beam irradiations. Further increase in the ion fluence decreased the magnetization of the films. This can be ascribed to the decomposition of the B2 phase, that is, the formation of the paramagnetic A1 phase. However, the small magnetic moments was still observed in this sample. This is considered due to the B2 phase remained in this irradiation condition that cannot be detected by XRD. The detailed relations between the structural and magnetic properties have been discussed elsewhere [7]. Fig. 3 shows XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the unirradiated samples. As can be seen in the figure, the small XMCD signal can be observed, which is consistent with the results of the SQUID measurements. The effect of the ion irradiation on the XMCD spectra is indicated in Fig. 4. As is clearly seen in the figure, the increasing in the ion fluence up to 1 1013 cm-2, causes little changes in the Fe L2,3-edge XMCD spectra. These facts suggest that the surface magnetization of the samples irradiated with the ion fluence smaller than about 1 1013 cm 2 almost unchanged, whereas the net magnetization of the entire films has started to increase from the 2 1012 cm 2 irradiation as indicated in Fig. 2. Hereafter, the XMCD signal for the sample irradiated with 2 1013 cm-2 became notable, which means the irradiation-induced ferromagnetism can be observed at the sample surface. It should be noted here that the shapes of the XAS spectra were totally unchanged for the samples with the irradiation fluence up to 2 1013 cm 2 as is indicated in Fig. 5.
Fig. 1. XRD scans for the samples unirradiated and irradiated with various ion fluences.
Fig. 2. M–H curves at 20 K for the samples unirradiated and irradiated with a 10 MeV I ion-beam.
kept at 0.006 nm/s. The films were subsequently annealed at 1073 K for 4 h under a pressure of 4 10 4 Pa after the deposition. The composition of the films determined by an electron probe micro analyzer is Fe47Rh53. According to the magnetic phase diagram of Fe-Rh system, AF state is stable at and below room temperature. Irradiation with 10 MeV iodine ions was performed at room temperature with an ion fluence ranging from 2 1012 to 4 1014 cm 2 using a tandem accelerator at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of Japan Atomic Energy Agency (JAEA). Considering the ion range of the 10 MeV I ion beam, the ions completely pass through the films and are accumulated within the substrates. In addition, the deposited energy through the elastic collisions is speculated to be uniformly given to the 80 nm thick FeRh films along the thickness direction. The crystal structure of the samples was characterized by X-ray diffraction (XRD). The net magnetization of the samples was measured by using a superconducting quantum interference device (SQUID) magnetometer. The in-plane magnetic hysteresis (M–H) curves were measured in the range of 0.6 to 0.6 T at 20 K. XMCD measurements were carried out at 20 K at the soft-X-ray undulator beam line BL25SU at SPring-8. The incident angle of synchrotron radiation to sample surface is 30°. The applied magnetic field parallel to the film plane was 1.9 T. The XMCD spectra in Total Electron Yield (TEY) mode were obtained by calculating the difference between two X-ray absorption spectra (XAS) with right- and left-circularly polarized light. Since the penetration depth is typically about 2 nm in the case of TEY, XMCD gives only the information on magnetic moments which originate from the upper sample surface. Furthermore, the XMCD spectra were normalized to the absorption edge jump for each sample, which made the data perfectly comparable.
3. Results and discussion
K. Aikoh et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 99–102
Fig. 3. XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the unirradiated samples measured at SPring-8 BL25SU.
101
Fig. 5. XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the samples irradiated with 10 MeV I ion measured at SPring-8 BL25SU.
Fig. 4. XMCD spectra of the samples unirradiated and irradiated with 10 MeV I ion measured at SPring-8 BL25SU.
Fig. 6. Magnetic moment per Fe atom for the samples plotted as a function of the ion fluence obtained from XMCD sum-rule analysis. The magnetization data by SQUID measurement are also included for reference.
Accordingly, the change in the surface state of the film samples, such as surface oxidation, is considered to be negligible. Further increase in the ion fluence up to 4 1014 cm 2 causes the drastic decreasing in the XMCD signal. Since the XAS spectra for this samples become totally different from those for the samples (see Fig. 5), the film composition at the film surface has been notably changed due to the ion irradiation. Above all, these results indicate that the surface magnetization induced 10 MeV I ion irradiation exhibits different ion fluence dependence from the net-magnetization of the films that can be measured by a SQUID magnetometer. To discuss in detail about magnetic nature, we tried to estimate magnetic moment of Fe atom using the well-established XMCD sum rule for the L edge quantitatively [10,11]. In this evaluation, magnetic moment of the sample in the depth direction irradiated with 10 MeV I ion was assumed homogeneous. Since the magnetic dipole term in the spin sum rule was not taken into account, the obtained spin magnetic moment is effective (mspineff = mspin + 7 < Tz>). The sum-rules were applied the results of XMCD measurement of the samples unirradiated and irradiated with an ion fluence ranging from 2 1012 to 2 1013 cm 2. The number of valence holes of Fe was used the values of 3.1 [12]. The sample irradiated with 4 1014 cm 2 was excluded from the sum-rule analysis. Fig. 6 shows the effective spin magnetic moment and the orbital magnetic moment of the unirradiated and irradiated areas with 10 MeV I ion by varying the ion fluence. It can be seen that after ion beam irradiation as large as 1 1013 cm 2, the total magnetic
moments almost unchanged. On the other hand, the irradiation with the ion fluence of 2 1013 cm 2 significantly increased the magnetic moment. The maximum value of the total magnetic moment obtained in this sample was 0.92 lB/atom, which is 30% of the magnetic moment of Fe atom of the ferromagnetic FeRh bulk samples (50 at.%) at room temperature. In addition, the contribution of the orbital magnetic moment to the total magnetic moment of the irradiated area was found to be less than 20%. Therefore it is also noticed that the irradiation induced ferromagnetism in the FeRh thin films mainly ascribed to the spin magnetic moment. This fact is consistent with the results of the magnetic Compton scattering measurement for FeRh bulk samples [13]. In order to know the difference between the surface magnetization and the net magnetization date induced by ion beam irradiation for FeRh thin films, the observed magnetization measured by a SQUID magnetometer are also included in Fig. 6. As is clearly in the figure, the both data exhibit totally different ion fluence dependence. In contrast that the maximum net magnetization can be obtained by the ion irradiation with 1 1013 cm 2, the surface magnetization required the ion fluence of 2 1013 cm 2. This simply indicates that the magnetic state could not be homogeneously modified along the film thickness direction. Of course, we know that the magnetic polarization for both of the Fe and Rh atoms can be measured by a SQUID magnetometer, whereas XMCD is element sensitive (Fe in the present case). Hence, for the samples irradiated with 2 1013 cm 2 the different magnetization values between two different measurement techniques can be
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understandable. Because the contribution of Rh to the magnetic moments was reported to be roughly 30% of the contribution of Fe [14]. However, we still see huge differences between the magnetization data for the samples irradiated with 2 1012 cm 2 and 1 1013 cm 2. That is, the irradiation-induced magnetization at the film surface seems to be much smaller than the net magnetization of the irradiated films. We have not yet clarified the definite reasons of this. But one possible reason may be the difference in the amount of lattice defects between at the surface side and at the substrate side of the films. We have ever revealed that the films initially containing a fairly large amount of lattice defects can be easily modified magnetically by ion beam irradiation rather than the films with small amount of lattice defects [15]. Since the present FeRh films grow in epitaxial-like form on the MgO substrates, the substrate side of the films possibly contains a fairly large amount of lattice defects due to the lattice mismatch between the film and the substrate. So the net magnetization may be notable rather than the surface magnetization of the films for the sample with small amount of ion fluence below 1 1013 cm 2. Another possible reason may be due to the presence of the surface itself, such as the effect of surface anisotropy. Further experiments, for examples, XMCD measurements varying the incidence angle, are required to clarify this view. 4. Summary FeRh thin films were irradiated with 10 MeV I ions at room temperature. The ion irradiation induced the ferromagnetic state in the FeRh thin films even below room temperature, which was confirmed by the Fe L2,3-edge XMCD measurements as well as by the SQUID measurements. We found that the irradiation ion fluence dependence on the magnetization was totally different between two measurement techniques. The maximum net magnetization measured by SQUID can be obtained by the ion irradiation with
1 1013 cm 2, the surface magnetization measured by XMCD required the ion fluence of 2 1013 cm 2. We also revealed by the XMCD sum-rule analysis that the ferromagnetism in the ion irradiated FeRh thin films was mainly dominated by the spin moment. Acknowledgements The XMCD experiment was performed under the approval of SPring-8 proposal 2012A1174 as well as 2010B1708. A part of this work has been also financially supported by Wakasa-Wan Energy Research Center. Financial support from Grant-in-Aid for Challenging Exploratory Research Grant No. 23656592 from Japan Society for the Promotion of Science is also gratefully acknowledged. References [1] M. Fallot, R. Hocart, Rev. Sci. 77 (1939) 498. [2] J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. [3] M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, S. Komatsu, A. Iwase, Nucl. Instr. Meth. B 230 (2005) 269. [4] M. Fukuzumi, Y. Chimi, N. Ishikawa, M. Suzuki, M. Takagaki, J. Mizuki, F. Ono, R. Neumann, A. Iwase, Nucl. Instr. Meth. B 245 (2006) 161. [5] A. Iwase, M. Fukuzumi, Y. Zushi, M. Suzuki, M. Takagaki, N. Kawamura, Y. Chimi, N. Ishikawa, J. Mizuki, F. Ono, Nucl. Instr. Meth. B 256 (2007) 429. [6] Y. Zushi, M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, A. Iwase, Nucl. Instr. Meth. B 256 (2007) 434. [7] Nao Fujita, S. Kosugi, Y. Saitoh, Y. Kaneta, K. Kume, T. Batchuluun, N. Ishikawa, T. Matsui, A. Iwase, J. Appl. Phys. 107 (2010) 09E302. [8] K. Aikoh, S. Kosugi, T. Matsui, A. Iwase, J. Appl. Phys. 109 (2011) 07E311. [9] N. Fujita, T. Matsui, S. Kosugi, T. Satoh, Y. Saitoh, K. Takano, M. Koka, T. Kamiya, S. Seki, A. Iwase, Jpn. J. Appl. Phys. 49 (2010) 060211-1–060211-3. [10] B.T. Thole, P. Carra, F. Sette, G. Van Der Laan, Phys. Rev. Lett. 68 (1992) 1943. [11] P. Carra, M. Altarelli, X. Wang, Phys. Rev. Lett. 70 (1993) 694. [12] W.L. O’Brien, B.P. Tonner, Phys. Rev. B 50 (1994) 17. [13] S. Kosugi, T. Matsui, N. Ishikawa, M. Itou, Y. Sakurai, K. Aikoh, K. Shimizu, Y. Tahara, F. Hori, A. Iwase, J. Appl. Phys. 109 (2011) 07B737. [14] J. Chaboy, Fernando Bartolomé, M.R. Ibarra, C.I. Marquina, P.A. Algarabel, Andrei Rogalev, Claus Neumman, Phys. Rev. B 59 (1999) 3306. [15] K. Aiko, K. Yoneda, A. Tohki, R. Ishigami, A. Iwase, T. Matsui, in preparation.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study on ferromagnetic ordering of FeRh thin films induced by energetic heavy ion irradiation by means of X-ray Magnetic Circular Dichroism Kazuma Aikoh a, Atsushi Tohki a, Shuichi Okuda b, Yuichi Saitoh c, Tomihiro Kamiya c, Tetsuya Nakamura d, Toyohiko Kinoshita d,e, Akihiro Iwase a, Toshiyuki Matsui f,⇑ a
Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Radiation Research Center, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Gumma 370-1292, Japan d Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198, Japan e CREST-JST, Kawaguchi, Saitama 332-0012, Japan f Research Organization of the 21st Century, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan b c
a r t i c l e
i n f o
Article history: Received 29 November 2012 Received in revised form 28 April 2013 Accepted 3 May 2013 Available online 24 June 2013 Keywords: Ion beam irradiation MCD FeRh thin film Modification of magnetic properties
a b s t r a c t We investigated the ion irradiation induced ferromagnetic state of FeRh thin films with 10 MeV I ion beam by the measurements of a superconducting quantum interference device (SQUID) magnetometer as well as of soft X-ray Magnetic Circular Dichroism (XMCD). It was clearly shown in the magnetization loops by SQUID measurements that the ion irradiation induced the ferromagnetic state in the FeRh thin films even below room temperature. This was also confirmed by the Fe L2,3-edge XMCD measurements for the irradiated FeRh film samples. However, we found that the irradiation ion fluence dependence on the magnetization was totally different between two measurement techniques. We also revealed by XMCD sum rule analysis that the ferromagnetism in the ion irradiated FeRh thin films was mainly dominated by the spin moment. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction B2(CsCl)-type FeRh has attracted much attention to several potential technological advantages for magnetic devices, such as thermally assisted magnetic recording media, due to its unique first order low temperature antiferromagnetic (AF) – high temperature ferromagnetic (FM) phase transition [1,2]. In our previous researches, we successfully induced the FM state by the energetic ion-beam irradiation in the Fe-50 at.% Rh bulk and film samples even at low temperatures where they were originally in AF state [3–6]. In addition, we also reported that the deposited energy through elastic collisions between ion species and the samples significantly influenced the change in magnetizations [7,8]. Actually, we have been able to successfully modify the magnetic state of FeRh bulk and film samples by the ion-beam irradiation with various accelerated voltage [9]. In the present studies, we have tried to evaluate the nature of the magnetic state of the FeRh thin films irradiated with 10 MeV Iodine ion beam by means of soft X-ray Magnetic Circular Dichroism (XMCD) with synchrotron radiation. XMCD is well known to be an effective technique for measuring magnetic state at the sample ⇑ Corresponding author. Tel.: +81 72 254 9311; fax: +81 72 254 9912. E-mail address: [email protected] (T. Matsui). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.031
surface. Hence, the surface magnetic state modified by ion beam irradiation can be effectively evaluated by that technique, even if it cannot be measured by ordinary measurement techniques, such as SQUID magnetometer, due to its small values of the net magnetization. In addition to this, XMCD provides information on the magnetization of one element even though the sample is composed of more than two elements. Moreover, the analysis of the MCD integrated intensity is capable of quantitative evaluation of orbital and spin magnetic moment using the well-established XMCD sum rule. Hence, it is possible to know more detailed magnetic natures of the samples rather than those obtained from SQUID measurement. In this paper, we discuss the results of the XMCD measurement for the 10 MeV I ion irradiated FeRh thin films, in conjunction with those of the SQUID measurements. In addition, we also quantitatively discuss ferromagnetism induced by such irradiation by the well-established XMCD sum rule for the L edge. 2. Experimental procedure Fe47Rh53 thin films about 80 nm thick were deposited on MgO (1 0 0) substrates at 973 K by means of ion beam sputtering from an alloy target of Fe50Rh50. The base pressure of the sputtering chamber was about 2.0 10 6 Pa. The total deposition rate was
100
K. Aikoh et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 99–102
The results of the structural characterization for the unirradiated and irradiated samples with various ion fluences are shown in the XRD scans in Fig. 1. The unirradiated sample was composed of the a’-FeRh phase with a B2-type structure and the c-FeRh phase with an A1 (disordered face centered cubic)-type structure, both of which exhibit the [0 0 1] preferential orientation. As for the irradiation effect on the structural properties, the B2-type FeRh phase still existed up to the ion fluence until 2 1013 cm 2 of the 10 MeV I ion-beam irradiation. The relative intensity of the B2 phase gradually decreased with increasing in the ion irradiation fluence. This is considered the fairly amount of the lattice defects are introduced in the B2 type phase by the ion irradiation. On
the other hand, the sample irradiated with 4 1014 cm 2 did not exhibit the XRD peaks for the B2 phase any more. We can apparently see only the XRD peaks for the A1 type phase except those from the MgO substrates. This result indicates that the excess ion irradiation caused the decomposition of the B2 phase, resulting in the formation of high temperature A1 phase. Fig. 2 shows the magnetic-hysteresis loops measured at 20 K for the unirradiated and irradiated samples with various ion fluences, 2 1012–4 1014 cm 2. Even in the unirradiated sample, a small magnetization of about 8 emu/g existed prior to the ion exposure. This small magnetization can be ascribed to the lattice defects and/ or lattice distortion in the B2-type FeRh phase, which was already discussed in the previous papers [7,8]. As clearly seen in the figure, the ion irradiation increases the magnetization of the films. The maximum value of saturation magnetization, 57 emu/g, was obtained in the sample irradiated with an ion fluence of 1– 2 1013 cm 2, which is 43% of the saturation magnetization of the ferromagnetic FeRh bulk samples (50 at.%) bulk samples at room temperature. According to our previous paper, the lattice defects introduced by ion irradiation can make the ferromagnetic state stabilize in the B2 type FeRh compounds. Hence, the ferromagnetic orders are considered to be enhanced by the 10 MeV ion beam irradiations. Further increase in the ion fluence decreased the magnetization of the films. This can be ascribed to the decomposition of the B2 phase, that is, the formation of the paramagnetic A1 phase. However, the small magnetic moments was still observed in this sample. This is considered due to the B2 phase remained in this irradiation condition that cannot be detected by XRD. The detailed relations between the structural and magnetic properties have been discussed elsewhere [7]. Fig. 3 shows XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the unirradiated samples. As can be seen in the figure, the small XMCD signal can be observed, which is consistent with the results of the SQUID measurements. The effect of the ion irradiation on the XMCD spectra is indicated in Fig. 4. As is clearly seen in the figure, the increasing in the ion fluence up to 1 1013 cm-2, causes little changes in the Fe L2,3-edge XMCD spectra. These facts suggest that the surface magnetization of the samples irradiated with the ion fluence smaller than about 1 1013 cm 2 almost unchanged, whereas the net magnetization of the entire films has started to increase from the 2 1012 cm 2 irradiation as indicated in Fig. 2. Hereafter, the XMCD signal for the sample irradiated with 2 1013 cm-2 became notable, which means the irradiation-induced ferromagnetism can be observed at the sample surface. It should be noted here that the shapes of the XAS spectra were totally unchanged for the samples with the irradiation fluence up to 2 1013 cm 2 as is indicated in Fig. 5.
Fig. 1. XRD scans for the samples unirradiated and irradiated with various ion fluences.
Fig. 2. M–H curves at 20 K for the samples unirradiated and irradiated with a 10 MeV I ion-beam.
kept at 0.006 nm/s. The films were subsequently annealed at 1073 K for 4 h under a pressure of 4 10 4 Pa after the deposition. The composition of the films determined by an electron probe micro analyzer is Fe47Rh53. According to the magnetic phase diagram of Fe-Rh system, AF state is stable at and below room temperature. Irradiation with 10 MeV iodine ions was performed at room temperature with an ion fluence ranging from 2 1012 to 4 1014 cm 2 using a tandem accelerator at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of Japan Atomic Energy Agency (JAEA). Considering the ion range of the 10 MeV I ion beam, the ions completely pass through the films and are accumulated within the substrates. In addition, the deposited energy through the elastic collisions is speculated to be uniformly given to the 80 nm thick FeRh films along the thickness direction. The crystal structure of the samples was characterized by X-ray diffraction (XRD). The net magnetization of the samples was measured by using a superconducting quantum interference device (SQUID) magnetometer. The in-plane magnetic hysteresis (M–H) curves were measured in the range of 0.6 to 0.6 T at 20 K. XMCD measurements were carried out at 20 K at the soft-X-ray undulator beam line BL25SU at SPring-8. The incident angle of synchrotron radiation to sample surface is 30°. The applied magnetic field parallel to the film plane was 1.9 T. The XMCD spectra in Total Electron Yield (TEY) mode were obtained by calculating the difference between two X-ray absorption spectra (XAS) with right- and left-circularly polarized light. Since the penetration depth is typically about 2 nm in the case of TEY, XMCD gives only the information on magnetic moments which originate from the upper sample surface. Furthermore, the XMCD spectra were normalized to the absorption edge jump for each sample, which made the data perfectly comparable.
3. Results and discussion
K. Aikoh et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 99–102
Fig. 3. XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the unirradiated samples measured at SPring-8 BL25SU.
101
Fig. 5. XAS with right- and left-circularly polarized light XMCD spectra at the Fe L2,3-edge of the samples irradiated with 10 MeV I ion measured at SPring-8 BL25SU.
Fig. 4. XMCD spectra of the samples unirradiated and irradiated with 10 MeV I ion measured at SPring-8 BL25SU.
Fig. 6. Magnetic moment per Fe atom for the samples plotted as a function of the ion fluence obtained from XMCD sum-rule analysis. The magnetization data by SQUID measurement are also included for reference.
Accordingly, the change in the surface state of the film samples, such as surface oxidation, is considered to be negligible. Further increase in the ion fluence up to 4 1014 cm 2 causes the drastic decreasing in the XMCD signal. Since the XAS spectra for this samples become totally different from those for the samples (see Fig. 5), the film composition at the film surface has been notably changed due to the ion irradiation. Above all, these results indicate that the surface magnetization induced 10 MeV I ion irradiation exhibits different ion fluence dependence from the net-magnetization of the films that can be measured by a SQUID magnetometer. To discuss in detail about magnetic nature, we tried to estimate magnetic moment of Fe atom using the well-established XMCD sum rule for the L edge quantitatively [10,11]. In this evaluation, magnetic moment of the sample in the depth direction irradiated with 10 MeV I ion was assumed homogeneous. Since the magnetic dipole term
moments almost unchanged. On the other hand, the irradiation with the ion fluence of 2 1013 cm 2 significantly increased the magnetic moment. The maximum value of the total magnetic moment obtained in this sample was 0.92 lB/atom, which is 30% of the magnetic moment of Fe atom of the ferromagnetic FeRh bulk samples (50 at.%) at room temperature. In addition, the contribution of the orbital magnetic moment to the total magnetic moment of the irradiated area was found to be less than 20%. Therefore it is also noticed that the irradiation induced ferromagnetism in the FeRh thin films mainly ascribed to the spin magnetic moment. This fact is consistent with the results of the magnetic Compton scattering measurement for FeRh bulk samples [13]. In order to know the difference between the surface magnetization and the net magnetization date induced by ion beam irradiation for FeRh thin films, the observed magnetization measured by a SQUID magnetometer are also included in Fig. 6. As is clearly in the figure, the both data exhibit totally different ion fluence dependence. In contrast that the maximum net magnetization can be obtained by the ion irradiation with 1 1013 cm 2, the surface magnetization required the ion fluence of 2 1013 cm 2. This simply indicates that the magnetic state could not be homogeneously modified along the film thickness direction. Of course, we know that the magnetic polarization for both of the Fe and Rh atoms can be measured by a SQUID magnetometer, whereas XMCD is element sensitive (Fe in the present case). Hence, for the samples irradiated with 2 1013 cm 2 the different magnetization values between two different measurement techniques can be
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understandable. Because the contribution of Rh to the magnetic moments was reported to be roughly 30% of the contribution of Fe [14]. However, we still see huge differences between the magnetization data for the samples irradiated with 2 1012 cm 2 and 1 1013 cm 2. That is, the irradiation-induced magnetization at the film surface seems to be much smaller than the net magnetization of the irradiated films. We have not yet clarified the definite reasons of this. But one possible reason may be the difference in the amount of lattice defects between at the surface side and at the substrate side of the films. We have ever revealed that the films initially containing a fairly large amount of lattice defects can be easily modified magnetically by ion beam irradiation rather than the films with small amount of lattice defects [15]. Since the present FeRh films grow in epitaxial-like form on the MgO substrates, the substrate side of the films possibly contains a fairly large amount of lattice defects due to the lattice mismatch between the film and the substrate. So the net magnetization may be notable rather than the surface magnetization of the films for the sample with small amount of ion fluence below 1 1013 cm 2. Another possible reason may be due to the presence of the surface itself, such as the effect of surface anisotropy. Further experiments, for examples, XMCD measurements varying the incidence angle, are required to clarify this view. 4. Summary FeRh thin films were irradiated with 10 MeV I ions at room temperature. The ion irradiation induced the ferromagnetic state in the FeRh thin films even below room temperature, which was confirmed by the Fe L2,3-edge XMCD measurements as well as by the SQUID measurements. We found that the irradiation ion fluence dependence on the magnetization was totally different between two measurement techniques. The maximum net magnetization measured by SQUID can be obtained by the ion irradiation with
1 1013 cm 2, the surface magnetization measured by XMCD required the ion fluence of 2 1013 cm 2. We also revealed by the XMCD sum-rule analysis that the ferromagnetism in the ion irradiated FeRh thin films was mainly dominated by the spin moment. Acknowledgements The XMCD experiment was performed under the approval of SPring-8 proposal 2012A1174 as well as 2010B1708. A part of this work has been also financially supported by Wakasa-Wan Energy Research Center. Financial support from Grant-in-Aid for Challenging Exploratory Research Grant No. 23656592 from Japan Society for the Promotion of Science is also gratefully acknowledged. References [1] M. Fallot, R. Hocart, Rev. Sci. 77 (1939) 498. [2] J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. [3] M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, S. Komatsu, A. Iwase, Nucl. Instr. Meth. B 230 (2005) 269. [4] M. Fukuzumi, Y. Chimi, N. Ishikawa, M. Suzuki, M. Takagaki, J. Mizuki, F. Ono, R. Neumann, A. Iwase, Nucl. Instr. Meth. B 245 (2006) 161. [5] A. Iwase, M. Fukuzumi, Y. Zushi, M. Suzuki, M. Takagaki, N. Kawamura, Y. Chimi, N. Ishikawa, J. Mizuki, F. Ono, Nucl. Instr. Meth. B 256 (2007) 429. [6] Y. Zushi, M. Fukuzumi, Y. Chimi, N. Ishikawa, F. Ono, A. Iwase, Nucl. Instr. Meth. B 256 (2007) 434. [7] Nao Fujita, S. Kosugi, Y. Saitoh, Y. Kaneta, K. Kume, T. Batchuluun, N. Ishikawa, T. Matsui, A. Iwase, J. Appl. Phys. 107 (2010) 09E302. [8] K. Aikoh, S. Kosugi, T. Matsui, A. Iwase, J. Appl. Phys. 109 (2011) 07E311. [9] N. Fujita, T. Matsui, S. Kosugi, T. Satoh, Y. Saitoh, K. Takano, M. Koka, T. Kamiya, S. Seki, A. Iwase, Jpn. J. Appl. Phys. 49 (2010) 060211-1–060211-3. [10] B.T. Thole, P. Carra, F. Sette, G. Van Der Laan, Phys. Rev. Lett. 68 (1992) 1943. [11] P. Carra, M. Altarelli, X. Wang, Phys. Rev. Lett. 70 (1993) 694. [12] W.L. O’Brien, B.P. Tonner, Phys. Rev. B 50 (1994) 17. [13] S. Kosugi, T. Matsui, N. Ishikawa, M. Itou, Y. Sakurai, K. Aikoh, K. Shimizu, Y. Tahara, F. Hori, A. Iwase, J. Appl. Phys. 109 (2011) 07B737. [14] J. Chaboy, Fernando Bartolomé, M.R. Ibarra, C.I. Marquina, P.A. Algarabel, Andrei Rogalev, Claus Neumman, Phys. Rev. B 59 (1999) 3306. [15] K. Aiko, K. Yoneda, A. Tohki, R. Ishigami, A. Iwase, T. Matsui, in preparation.