- Email: [email protected]
low fluence ion irradiation
Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tuning the antiferromagnetic to ferromagnetic phase transition in FeRh thin films by means of low-energy/low fluence ion irradiation A. Heidarian, R. Bali, J. Grenzer, R.A. Wilhelm, R. Heller, O. Yildirim, J. Lindner, K. Potzger ⇑ Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 13 March 2015 Received in revised form 22 June 2015 Accepted 24 June 2015 Available online 11 July 2015 Keywords: Ion irradiation FeRh Magnetic phase transition
a b s t r a c t Ion irradiation induced modifications of the thermomagnetic properties of equiatomic FeRh thin films have been investigated. The application of 20 keV Ne+ ions at different fluencies leads to broadening of the antiferromagnetic to ferromagnetic phase transition as well as a shift of the transition temperature towards lower temperatures with increasing ion fluence. Moreover, the ferromagnetic background at low temperatures generated by the ion irradiation leads to pronounced saturation magnetisation at 5 K. Complete erasure of the transition, i.e. ferromagnetic ordering through the whole temperature regime was achieved at a Ne+ fluence of 3 1014 ions/cm2. It does not coincide with the complete randomization of the chemical ordering of the crystal lattice. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Binary metallic alloys consisting of a 3d ferromagnetic metal and a non-ferromagnetic alloying element, such as FeAl, FeV and FeRh may show magnetic properties that depend strongly on the degree of their chemical order. In such alloys, varying the degree of chemical order can be a lever to modify the magnetic behaviour [1–4]. In particular, the chemically disordered state, where the atomic site occupancies have been randomized, is metastable. The chemically ordered state usually occurs in thermal equilibrium, the metastable disordered state can be achieved by ion irradiation [1] or mechanical milling [2]. In particular, ion irradiation technique offers a quantitative control of the degree of chemical disorder simply by adjusting the ion fluence applied, while the penetration depth of the disordered phase can be adjusted by the ion-energy. Moreover, for chemical disordering only low ion fluencies are needed to achieve full magnetic modification of the material [1]. Therefore, magnetic modification due to chemical disordering can be considered as a fast and low-cost method. Moreover, ion-irradiation is suited for magnetic film patterning on micro- and nanoscale, i.e. using shadow masks or focused ion beam techniques [1,3,5]. A model system to study disorder induced magnetic phenomena is Fe1 xAlx with composition around x = 0.4 [1]. For such a composition, FeAl thin films are paramagnetic in the chemically ordered and ferromagnetic in the disordered state. Ion beam patterning by means of Ne+ ion irradiation revealed highly resolved para- and ferromagnetic areas [1,3]. By adjusting ⇑ Corresponding author. http://dx.doi.org/10.1016/j.nimb.2015.06.027 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
the dimensions of the ferromagnetic regions, reprogrammable parallel and antiparallel magnetisation configurations in zero field have been achieved [3]. On the other hand, chemically ordered (B2 or CsCl-type) FeRh alloys at equi-atomic composition exhibit a antiferromagnetic (AFM) to ferromagnetic (FM) phase transition at around 370 K [6–9]. An associated hysteresis between the cooling and heating cycles is directly connected to the first order nature of the phase transition. This hysteresis is further evidenced in thin-film systems, where defects and associated deviations from the bulk-like structure produce a continuous like transition with a hysteresis of approximately 10 K [10]. The material therefore is considered as a potential candidate for heat assisted magnetic recording (HAMR), where the transition temperature can be tuned by means of Cu, Pd, Pt or Ir co-alloying [11–14]. Recently it was found that chemical disorder introduced into FeRh thin films by means of ion irradiation with either H (1 MeV) or I (10 MeV) ions leads to an increasing ferromagnetic signal at low temperatures. The saturation magnetisation measured at 20 K increases monotonically with fluence before decreasing due to the formation of the A1 phase [15]. It was found that already a small decrease of the ordering parameter of about 9% achieved, e.g. by a 10 MeV I+ ion fluence of 1 1013 ions/cm2, leads to a partial chemical randomization. This is evidenced by a ferromagnetic saturation magnetisation, which is about 20 times larger as compared to the unirradiated sample. At larger ion fluencies, the saturation magnetisation decreases again. This behaviour was interpreted as the formation of the A1 structure, which is known to be paramagnetic. It has also been shown that low energy Ga+ ion irradiation at an energy of
252
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
30 keV leads to similar results [16]. Consequently, commercial low-energy ion implanters or focused ion beam facilities which are commonly used in semiconductor or microelectronics industries, can be used for magnetic modification of FeRh thin films. These results have been exploited for magnetic patterning of FeRh thin films by high energetic microbeam H+ and He+ ion irradiation [17]. It was found that room-temperature ferromagnetic patterns with a defined shape and sizes of 1–2 lm can be embedded in an antiferromagnetic FeRh thin film. In this paper we present investigations on tuning the shape and transition temperature of the antiferromagnetic to ferromagnetic phase transition in FeRh thin films by means of low-energy/low fluence Ne+ ion irradiation. As a motivation, laterally patterned magnetic regions with different phase transition temperatures would offer the opportunity for temperatureand field-programmable magnetic states.
2. Experimental An FeRh thin films with a thickness of 35 nm were deposited on MgO(001) single crystals from CRYSTEC/Berlin by means of molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) at a base pressure below 1 10 10 mbar. Prior to deposition, the MgO single crystals were annealed at 873 K for 8 h in order to remove surface contamination. Deposition of FeRh was performed at room temperature followed by post annealing. For the adjustment of the composition, Fe and Rh were evaporated from two individual sources. Flux calibration was applied using a quartz microbalance. Post annealing was performed up to 1123 K for 4 h. Since the film composition is critical for the presence of the antiferromagnetic to ferromagnetic phase transition, it was controlled by ex-situ Rutherford backscattering spectroscopy (RBS) (Fig. 1). By analysing the spectra using the SIMNRA program [18], the composition was found to be Fe50Rh50 with an uncertainty of around 1%. After RBS and magnetic characterisation, the sample was stepwise implanted with Ne+ ions at an energy of 20 keV applying different total ion fluencies of 5 1012, 7.5 1012, 1 1013, 2.5 1013, and 7.5 1013 ions/cm2. A maximum fluence of 3 1014 ions/cm2 was applied to a control sample of the same composition with the aim to completely erase the AFM to FM phase transition. The ion energy has been chosen in such way that the projected range of the implanted ions (not shown) as well as the maximum displacements per atom (dpa) are located 20 nm below the surface of the FeRh films. The displacement profile was calculated by SRIM/TRIM software [19] (inset in Fig. 1).
Magnetometry was performed for the as-prepared sample as well as after each implantation step using a Quantum Design SQUID/VSM. The maximum achievable temperature was 400 K. For the measurement of the antiferromagnetic to ferromagnetic phase transition, as well as revealing a possible ferromagnetic background, we applied the zero-field-cooled/field-cooled (ZFC/FC) protocol at a constant magnetic field of 150 Oe. For that purpose, samples firstly were cooled down to 5 K in zero field starting at room temperature. Subsequently, they were warmed in the field (ZFC curve) up to 400 K and immediately cooled down again in the field (FC curve). Splitting of the curves points towards thermomagnetic irreversibility induced by a deblocking of ferromagnetic domains. Fig. 2a shows the ZFC/FC curves while Fig. 2b shows the ion fluence dependence of the saturation magnetisation recorded at a temperature of 5 K. The control sample before and after implantation also was analysed by X-ray diffraction (XRD, Fig. 3). For that purpose, using Cuja–radiation a h 2h diffractometer equipped with a Goebel mirror on the source side and a parallel-plate collimator on the detector side, XRD patterns were recorded in glancing angle geometry. The energy window of the detector was carefully calibrated in order to suppress the Fe fluorescence signal. The incidence angle was kept above the critical angle (ac = 0.42 degree) at 1 degree. Additionally we used X-ray reflectivity to verify the film thickness before and after irradiation. 3. Results and discussion The onset of the antiferromagnetic to ferromagnetic phase transition for equi-atomic FeRh is usually expected at about 350 K. For the unirradiated film, in our case the onset is located at about 375 K which is likely related to a slight deviation from the exact equiatomic stoichiometry lying within the error of RBS measurements (Fig. 2a). Due to the temperature limitations of the magnetometer minor thermal loops were recorded. From Fig. 2a it is evident that Ne+ irradiation at a fluence of 5 1012 ions/cm2 is sufficient to induce a pronounced shift of the onset of the phase transition by 45 K, as compared to the unirradiated film. Increasing the ion fluence above this value, however, leads a larger splitting between the ZFC and FC curves as well as pronounced positive magnetisation values at temperatures below the transition temperature. This indicates an increasing ferromagnetic background induced by the ongoing chemical disordering. This explains the drastic increase of the saturation magnetisation at 20 K as observed by other groups [15,16]. For our films, the increase of the ferromagnetic signal, i.e. the saturation magnetisation measured at 5 K is displayed in Fig. 2b. For fluencies of 5 1012,
Fig. 1. Left: RBS of the as-prepared FeRh film. The composition was found to be Fe50Rh50. Right: Depth profile of the dpa per implanted ion for 20 keV Ne+ ions implanted into a 35 nm FeRh film deposited on MgO calculated using SRIM/TRIM [19].
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
253
Fig. 2. (a) ZFC/FC thermomagnetic measurements recorded at a field of 150 Oe. The Ne+ ion fluence applied at an ion energy of 20 keV is indicated in ions/cm2. (b) Displays the increase of the saturation magnetisation recorded at 5 K.
Fig. 3. XRD diffraction pattern measured at an incidence angle of 1degree of the control sample in the non-irradiated state (black), after Ne+ irradiation at a fluence of 3 1014 ions/cm2 (red). The bold dots show the calculated theoretical integral intensities taking into account the S parameter and the additional Debye–Waller factor. The discrepancy at the (2 0 0) peak, non-irradiated sample, is probably due to an influence of the substrate MgO (2 2 0) reflection. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7.5 1012, and 1 1013 ions/cm2 two overlapping thermal hysteresis loops corresponding to phases with two different transition temperatures can be observed (Fig. 2a). We believe that this effect is related to regions of slightly different chemical ordering which are created due to the ion irradiation. These regions might arise due to the distribution of the dpa within the film thickness as shown in Fig. 1. Since the transition temperature is very sensitive to the chemical ordering, a lower dpa, e.g. in the vicinity of the tail of the distribution function close to the interface, can lead to a higher transition temperature as compared to the surface region. The AFM to FM phase transition is completely erased at a fluence of 3 1014 ions/cm2, i.e. the film becomes ferromagnetic in the whole temperature range. This striking effect supports the observation presented in Refs. [15,16]. XRD did not give hints towards pronounced epitaxial ordering of the as-grown film. Consequently, assuming an almost polycrystalline sample glancing angle XRD measurements at an incidence angle of 1degree have been performed before and after Ne+ irradiation at a fluence of 3 1014 ions/cm2. Fig. 3 shows the corresponding diffraction patterns. The irradiation leads to a drop of the (integral) diffraction peak intensities demonstrating the introduction of an additional disorder due to implantation that affects the whole lattice but most strongly destroys the chemical order inside the unit cell. Common explanations for the increase of
ferromagnetic order with chemical disordering, e.g. for FeAl alloys, are either the increase of the number of magnetic Fe atoms within their nearest neighbourhood due to the disordering or the increase of the lattice parameter [20]. In Refs. [15,16], no change of lattice constant due to ion irradiation was reported. The same fact is valid for our samples. Besides, the film thickness of 35 nm that did not change with irradiation the lattice constant (2.985 Å) value and the crystallite size (20 nm) were almost not influenced by the irradiation. At higher fluencies (>1 1015 ions/cm2) and/or using other ions a growth of the crystallite size and an increase of the lattice parameter due to implantation should be carefully taken into account [21]. In accordance with [15,16], the intensity of the (0 0 1) B2 superstructure peak is influenced more strongly than the peaks that are not sensitive to the chemical order. This change of the chemical order can be qualitatively estimated from the XRD data using the ratio between the integrated intensities of the superstructure peaks and the fundamental peaks. This is normally described as the long-range order parameter S [22,23]. However, all other diffraction peaks are as well influenced by the irradiation induced disorder. Therefore we can introduce additionally a static Debye– Waller factor that describes the disorder introduced by this irradiation in the complete unit cell [22]. Comparing the integral peak intensities of both measurements, a reduction of the S parameter
254
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
3 1014 ions/cm2 the AFM phase completely disappears and FM dominates the investigated temperature regime. We also found that there is no straightforward relation between the structure and the described changes of the magnetic properties. The role of the (not fully) chemical disordering of the lattice, the lowering of the crystalline quality observed by XRD as well as changes of the nearest neighbourhood of the lattice atoms due to the ion irradiation has to be clarified in future experiments . On the other hand, ion beam patterned areas could be used for thermally programmed AFM/AFM, AFM/FM or FM/FM micro- or nanoscale regions. References
Fig. 4. Evolution of the saturation magnetisation with increasing peak-dpa compared with data reproduced from Fig. 2 of Ref. [16].
by more than 50% was observed. Taking into account the relative large error of about 10% (only one super structure (0 0 1) peak is visible) the order parameter S drops from about 90% to below 50%. The ion beam induced Debye–Waller factor was estimated to about 0.2 A. For comparison of our results with those of Ref. [16], the displacements created by the Ne+ irradiation have been recalculated in terms of the peak-dpa using SRIM/TRIM. The dependence of the saturation magnetisation on the dpa is presented in Fig. 4. The evolution of the saturation magnetisation with increasing dpa shows a similar trend as compared to Ref. [16]. The somewhat higher values for the 20 keV Ne+ implanted films might originate from the lower measurement temperature or slightly different stoichiometry. 4. Conclusions Applying 20 keV Ne+ ion irradiation with stepwise increase of the ion fluence to nearly equiatomic FeRh thin films leads to a shift of the AFM to FM phase transition temperature towards lower temperatures as well as the formation of an increasing ferromagnetic background phase. After irradiation with a fluence of
[1] E. Menendez, M.O. Liedke, J. Fassbender, T. Gemming, A. Weber, L.J. Heyderman, K.V. Rao, S.C. Deevi, S. Surinach, M.D. Baro, J. Sort, J. Nogues, Small 5 (2009) 229–234. [2] Q. Zeng, I. Baker, Intermetallics 14 (2006) 396–405. [3] R. Bali, S. Wintz, F. Meutzner, R. Huebner, R. Boucher, A.A. Uenal, S. Valencia, A. Neudert, K. Potzger, J. Bauch, F. Kronast, S. Facsko, J. Lindner, J. Fassbender, Nano Lett. 14 (2014) 435–441. [4] J.C. Krause, J. Schaf, M.I. da Costa Jr., C. Paduani, Phys. Rev. B 61 (2000) 6196. [5] O.D. Roshchupkina, J. Grenzer, T. Strache, J. McCord, M. Fritzsche, A. Mücklich, C. Baehtz, J. Fassbender, J. Appl. Phys. 112 (2012) 033901. [6] L.J. Swartzendruber, Bull. Alloy Phase Diag. 5 (5) (1984). [7] M. Fallot, R. Horcart, Rev. Sci. 77 (1939) 498. [8] J.S. Kouvel, C.C. Hartelius, J. Appl. Phys. 33 (1962) 1343. [9] L. Zsoldos, Phys. Status Sol. 20 (1967) K25. [10] S. Maat, J.-U. Thiele, Eric E. Fullerton, Phys. Rev. B 72 (2005) 21443. [11] J.U. Thiele, S. Maat, E.E. Fullerton, Appl. Phys. Lett. 82 (2003) 2859–2861. [12] J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. [13] Radhika Barua, Felix Jimenez-Villacorta, L.H. Lewis, J. Appl. Phys. 115 (2014) 17A903. [14] W. Lu, N.T. Nam, T. Suzuki, IEEE T. Magn. 45 (2009) 2716–2719. [15] Nao Fujita, S. Kosugi, Y. Sa Saitoh, Y. Kaneta, K. Kume, T. Batchuluun, N. Ishikawa, T. Matsui, A. Iwase, J. Appl. Phys. 107 (2010). 09E302. [16] K. Aikoh, S. Kosugi, T. Matsui, A. Iwase, J. Appl. Phys. 109 (2011) 07E311. [17] T. Koide, T. Satoh, M. Kohka, Y. Saitoh, T. Kamiya, T. Ohkouchi, M. Kotsugi, T. Kinoshita, T. Nakamura, A. Iwase, T. Matsui, Jpn. J. Appl. Phys. 53 (2014). 05FC06. [18] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, Germany, 1997. [19] F. Ziegler,
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Tuning the antiferromagnetic to ferromagnetic phase transition in FeRh thin films by means of low-energy/low fluence ion irradiation A. Heidarian, R. Bali, J. Grenzer, R.A. Wilhelm, R. Heller, O. Yildirim, J. Lindner, K. Potzger ⇑ Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 13 March 2015 Received in revised form 22 June 2015 Accepted 24 June 2015 Available online 11 July 2015 Keywords: Ion irradiation FeRh Magnetic phase transition
a b s t r a c t Ion irradiation induced modifications of the thermomagnetic properties of equiatomic FeRh thin films have been investigated. The application of 20 keV Ne+ ions at different fluencies leads to broadening of the antiferromagnetic to ferromagnetic phase transition as well as a shift of the transition temperature towards lower temperatures with increasing ion fluence. Moreover, the ferromagnetic background at low temperatures generated by the ion irradiation leads to pronounced saturation magnetisation at 5 K. Complete erasure of the transition, i.e. ferromagnetic ordering through the whole temperature regime was achieved at a Ne+ fluence of 3 1014 ions/cm2. It does not coincide with the complete randomization of the chemical ordering of the crystal lattice. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Binary metallic alloys consisting of a 3d ferromagnetic metal and a non-ferromagnetic alloying element, such as FeAl, FeV and FeRh may show magnetic properties that depend strongly on the degree of their chemical order. In such alloys, varying the degree of chemical order can be a lever to modify the magnetic behaviour [1–4]. In particular, the chemically disordered state, where the atomic site occupancies have been randomized, is metastable. The chemically ordered state usually occurs in thermal equilibrium, the metastable disordered state can be achieved by ion irradiation [1] or mechanical milling [2]. In particular, ion irradiation technique offers a quantitative control of the degree of chemical disorder simply by adjusting the ion fluence applied, while the penetration depth of the disordered phase can be adjusted by the ion-energy. Moreover, for chemical disordering only low ion fluencies are needed to achieve full magnetic modification of the material [1]. Therefore, magnetic modification due to chemical disordering can be considered as a fast and low-cost method. Moreover, ion-irradiation is suited for magnetic film patterning on micro- and nanoscale, i.e. using shadow masks or focused ion beam techniques [1,3,5]. A model system to study disorder induced magnetic phenomena is Fe1 xAlx with composition around x = 0.4 [1]. For such a composition, FeAl thin films are paramagnetic in the chemically ordered and ferromagnetic in the disordered state. Ion beam patterning by means of Ne+ ion irradiation revealed highly resolved para- and ferromagnetic areas [1,3]. By adjusting ⇑ Corresponding author. http://dx.doi.org/10.1016/j.nimb.2015.06.027 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
the dimensions of the ferromagnetic regions, reprogrammable parallel and antiparallel magnetisation configurations in zero field have been achieved [3]. On the other hand, chemically ordered (B2 or CsCl-type) FeRh alloys at equi-atomic composition exhibit a antiferromagnetic (AFM) to ferromagnetic (FM) phase transition at around 370 K [6–9]. An associated hysteresis between the cooling and heating cycles is directly connected to the first order nature of the phase transition. This hysteresis is further evidenced in thin-film systems, where defects and associated deviations from the bulk-like structure produce a continuous like transition with a hysteresis of approximately 10 K [10]. The material therefore is considered as a potential candidate for heat assisted magnetic recording (HAMR), where the transition temperature can be tuned by means of Cu, Pd, Pt or Ir co-alloying [11–14]. Recently it was found that chemical disorder introduced into FeRh thin films by means of ion irradiation with either H (1 MeV) or I (10 MeV) ions leads to an increasing ferromagnetic signal at low temperatures. The saturation magnetisation measured at 20 K increases monotonically with fluence before decreasing due to the formation of the A1 phase [15]. It was found that already a small decrease of the ordering parameter of about 9% achieved, e.g. by a 10 MeV I+ ion fluence of 1 1013 ions/cm2, leads to a partial chemical randomization. This is evidenced by a ferromagnetic saturation magnetisation, which is about 20 times larger as compared to the unirradiated sample. At larger ion fluencies, the saturation magnetisation decreases again. This behaviour was interpreted as the formation of the A1 structure, which is known to be paramagnetic. It has also been shown that low energy Ga+ ion irradiation at an energy of
252
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
30 keV leads to similar results [16]. Consequently, commercial low-energy ion implanters or focused ion beam facilities which are commonly used in semiconductor or microelectronics industries, can be used for magnetic modification of FeRh thin films. These results have been exploited for magnetic patterning of FeRh thin films by high energetic microbeam H+ and He+ ion irradiation [17]. It was found that room-temperature ferromagnetic patterns with a defined shape and sizes of 1–2 lm can be embedded in an antiferromagnetic FeRh thin film. In this paper we present investigations on tuning the shape and transition temperature of the antiferromagnetic to ferromagnetic phase transition in FeRh thin films by means of low-energy/low fluence Ne+ ion irradiation. As a motivation, laterally patterned magnetic regions with different phase transition temperatures would offer the opportunity for temperatureand field-programmable magnetic states.
2. Experimental An FeRh thin films with a thickness of 35 nm were deposited on MgO(001) single crystals from CRYSTEC/Berlin by means of molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) at a base pressure below 1 10 10 mbar. Prior to deposition, the MgO single crystals were annealed at 873 K for 8 h in order to remove surface contamination. Deposition of FeRh was performed at room temperature followed by post annealing. For the adjustment of the composition, Fe and Rh were evaporated from two individual sources. Flux calibration was applied using a quartz microbalance. Post annealing was performed up to 1123 K for 4 h. Since the film composition is critical for the presence of the antiferromagnetic to ferromagnetic phase transition, it was controlled by ex-situ Rutherford backscattering spectroscopy (RBS) (Fig. 1). By analysing the spectra using the SIMNRA program [18], the composition was found to be Fe50Rh50 with an uncertainty of around 1%. After RBS and magnetic characterisation, the sample was stepwise implanted with Ne+ ions at an energy of 20 keV applying different total ion fluencies of 5 1012, 7.5 1012, 1 1013, 2.5 1013, and 7.5 1013 ions/cm2. A maximum fluence of 3 1014 ions/cm2 was applied to a control sample of the same composition with the aim to completely erase the AFM to FM phase transition. The ion energy has been chosen in such way that the projected range of the implanted ions (not shown) as well as the maximum displacements per atom (dpa) are located 20 nm below the surface of the FeRh films. The displacement profile was calculated by SRIM/TRIM software [19] (inset in Fig. 1).
Magnetometry was performed for the as-prepared sample as well as after each implantation step using a Quantum Design SQUID/VSM. The maximum achievable temperature was 400 K. For the measurement of the antiferromagnetic to ferromagnetic phase transition, as well as revealing a possible ferromagnetic background, we applied the zero-field-cooled/field-cooled (ZFC/FC) protocol at a constant magnetic field of 150 Oe. For that purpose, samples firstly were cooled down to 5 K in zero field starting at room temperature. Subsequently, they were warmed in the field (ZFC curve) up to 400 K and immediately cooled down again in the field (FC curve). Splitting of the curves points towards thermomagnetic irreversibility induced by a deblocking of ferromagnetic domains. Fig. 2a shows the ZFC/FC curves while Fig. 2b shows the ion fluence dependence of the saturation magnetisation recorded at a temperature of 5 K. The control sample before and after implantation also was analysed by X-ray diffraction (XRD, Fig. 3). For that purpose, using Cuja–radiation a h 2h diffractometer equipped with a Goebel mirror on the source side and a parallel-plate collimator on the detector side, XRD patterns were recorded in glancing angle geometry. The energy window of the detector was carefully calibrated in order to suppress the Fe fluorescence signal. The incidence angle was kept above the critical angle (ac = 0.42 degree) at 1 degree. Additionally we used X-ray reflectivity to verify the film thickness before and after irradiation. 3. Results and discussion The onset of the antiferromagnetic to ferromagnetic phase transition for equi-atomic FeRh is usually expected at about 350 K. For the unirradiated film, in our case the onset is located at about 375 K which is likely related to a slight deviation from the exact equiatomic stoichiometry lying within the error of RBS measurements (Fig. 2a). Due to the temperature limitations of the magnetometer minor thermal loops were recorded. From Fig. 2a it is evident that Ne+ irradiation at a fluence of 5 1012 ions/cm2 is sufficient to induce a pronounced shift of the onset of the phase transition by 45 K, as compared to the unirradiated film. Increasing the ion fluence above this value, however, leads a larger splitting between the ZFC and FC curves as well as pronounced positive magnetisation values at temperatures below the transition temperature. This indicates an increasing ferromagnetic background induced by the ongoing chemical disordering. This explains the drastic increase of the saturation magnetisation at 20 K as observed by other groups [15,16]. For our films, the increase of the ferromagnetic signal, i.e. the saturation magnetisation measured at 5 K is displayed in Fig. 2b. For fluencies of 5 1012,
Fig. 1. Left: RBS of the as-prepared FeRh film. The composition was found to be Fe50Rh50. Right: Depth profile of the dpa per implanted ion for 20 keV Ne+ ions implanted into a 35 nm FeRh film deposited on MgO calculated using SRIM/TRIM [19].
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
253
Fig. 2. (a) ZFC/FC thermomagnetic measurements recorded at a field of 150 Oe. The Ne+ ion fluence applied at an ion energy of 20 keV is indicated in ions/cm2. (b) Displays the increase of the saturation magnetisation recorded at 5 K.
Fig. 3. XRD diffraction pattern measured at an incidence angle of 1degree of the control sample in the non-irradiated state (black), after Ne+ irradiation at a fluence of 3 1014 ions/cm2 (red). The bold dots show the calculated theoretical integral intensities taking into account the S parameter and the additional Debye–Waller factor. The discrepancy at the (2 0 0) peak, non-irradiated sample, is probably due to an influence of the substrate MgO (2 2 0) reflection. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
7.5 1012, and 1 1013 ions/cm2 two overlapping thermal hysteresis loops corresponding to phases with two different transition temperatures can be observed (Fig. 2a). We believe that this effect is related to regions of slightly different chemical ordering which are created due to the ion irradiation. These regions might arise due to the distribution of the dpa within the film thickness as shown in Fig. 1. Since the transition temperature is very sensitive to the chemical ordering, a lower dpa, e.g. in the vicinity of the tail of the distribution function close to the interface, can lead to a higher transition temperature as compared to the surface region. The AFM to FM phase transition is completely erased at a fluence of 3 1014 ions/cm2, i.e. the film becomes ferromagnetic in the whole temperature range. This striking effect supports the observation presented in Refs. [15,16]. XRD did not give hints towards pronounced epitaxial ordering of the as-grown film. Consequently, assuming an almost polycrystalline sample glancing angle XRD measurements at an incidence angle of 1degree have been performed before and after Ne+ irradiation at a fluence of 3 1014 ions/cm2. Fig. 3 shows the corresponding diffraction patterns. The irradiation leads to a drop of the (integral) diffraction peak intensities demonstrating the introduction of an additional disorder due to implantation that affects the whole lattice but most strongly destroys the chemical order inside the unit cell. Common explanations for the increase of
ferromagnetic order with chemical disordering, e.g. for FeAl alloys, are either the increase of the number of magnetic Fe atoms within their nearest neighbourhood due to the disordering or the increase of the lattice parameter [20]. In Refs. [15,16], no change of lattice constant due to ion irradiation was reported. The same fact is valid for our samples. Besides, the film thickness of 35 nm that did not change with irradiation the lattice constant (2.985 Å) value and the crystallite size (20 nm) were almost not influenced by the irradiation. At higher fluencies (>1 1015 ions/cm2) and/or using other ions a growth of the crystallite size and an increase of the lattice parameter due to implantation should be carefully taken into account [21]. In accordance with [15,16], the intensity of the (0 0 1) B2 superstructure peak is influenced more strongly than the peaks that are not sensitive to the chemical order. This change of the chemical order can be qualitatively estimated from the XRD data using the ratio between the integrated intensities of the superstructure peaks and the fundamental peaks. This is normally described as the long-range order parameter S [22,23]. However, all other diffraction peaks are as well influenced by the irradiation induced disorder. Therefore we can introduce additionally a static Debye– Waller factor that describes the disorder introduced by this irradiation in the complete unit cell [22]. Comparing the integral peak intensities of both measurements, a reduction of the S parameter
254
A. Heidarian et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 251–254
3 1014 ions/cm2 the AFM phase completely disappears and FM dominates the investigated temperature regime. We also found that there is no straightforward relation between the structure and the described changes of the magnetic properties. The role of the (not fully) chemical disordering of the lattice, the lowering of the crystalline quality observed by XRD as well as changes of the nearest neighbourhood of the lattice atoms due to the ion irradiation has to be clarified in future experiments . On the other hand, ion beam patterned areas could be used for thermally programmed AFM/AFM, AFM/FM or FM/FM micro- or nanoscale regions. References
Fig. 4. Evolution of the saturation magnetisation with increasing peak-dpa compared with data reproduced from Fig. 2 of Ref. [16].
by more than 50% was observed. Taking into account the relative large error of about 10% (only one super structure (0 0 1) peak is visible) the order parameter S drops from about 90% to below 50%. The ion beam induced Debye–Waller factor was estimated to about 0.2 A. For comparison of our results with those of Ref. [16], the displacements created by the Ne+ irradiation have been recalculated in terms of the peak-dpa using SRIM/TRIM. The dependence of the saturation magnetisation on the dpa is presented in Fig. 4. The evolution of the saturation magnetisation with increasing dpa shows a similar trend as compared to Ref. [16]. The somewhat higher values for the 20 keV Ne+ implanted films might originate from the lower measurement temperature or slightly different stoichiometry. 4. Conclusions Applying 20 keV Ne+ ion irradiation with stepwise increase of the ion fluence to nearly equiatomic FeRh thin films leads to a shift of the AFM to FM phase transition temperature towards lower temperatures as well as the formation of an increasing ferromagnetic background phase. After irradiation with a fluence of
[1] E. Menendez, M.O. Liedke, J. Fassbender, T. Gemming, A. Weber, L.J. Heyderman, K.V. Rao, S.C. Deevi, S. Surinach, M.D. Baro, J. Sort, J. Nogues, Small 5 (2009) 229–234. [2] Q. Zeng, I. Baker, Intermetallics 14 (2006) 396–405. [3] R. Bali, S. Wintz, F. Meutzner, R. Huebner, R. Boucher, A.A. Uenal, S. Valencia, A. Neudert, K. Potzger, J. Bauch, F. Kronast, S. Facsko, J. Lindner, J. Fassbender, Nano Lett. 14 (2014) 435–441. [4] J.C. Krause, J. Schaf, M.I. da Costa Jr., C. Paduani, Phys. Rev. B 61 (2000) 6196. [5] O.D. Roshchupkina, J. Grenzer, T. Strache, J. McCord, M. Fritzsche, A. Mücklich, C. Baehtz, J. Fassbender, J. Appl. Phys. 112 (2012) 033901. [6] L.J. Swartzendruber, Bull. Alloy Phase Diag. 5 (5) (1984). [7] M. Fallot, R. Horcart, Rev. Sci. 77 (1939) 498. [8] J.S. Kouvel, C.C. Hartelius, J. Appl. Phys. 33 (1962) 1343. [9] L. Zsoldos, Phys. Status Sol. 20 (1967) K25. [10] S. Maat, J.-U. Thiele, Eric E. Fullerton, Phys. Rev. B 72 (2005) 21443. [11] J.U. Thiele, S. Maat, E.E. Fullerton, Appl. Phys. Lett. 82 (2003) 2859–2861. [12] J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. [13] Radhika Barua, Felix Jimenez-Villacorta, L.H. Lewis, J. Appl. Phys. 115 (2014) 17A903. [14] W. Lu, N.T. Nam, T. Suzuki, IEEE T. Magn. 45 (2009) 2716–2719. [15] Nao Fujita, S. Kosugi, Y. Sa Saitoh, Y. Kaneta, K. Kume, T. Batchuluun, N. Ishikawa, T. Matsui, A. Iwase, J. Appl. Phys. 107 (2010). 09E302. [16] K. Aikoh, S. Kosugi, T. Matsui, A. Iwase, J. Appl. Phys. 109 (2011) 07E311. [17] T. Koide, T. Satoh, M. Kohka, Y. Saitoh, T. Kamiya, T. Ohkouchi, M. Kotsugi, T. Kinoshita, T. Nakamura, A. Iwase, T. Matsui, Jpn. J. Appl. Phys. 53 (2014). 05FC06. [18] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, Germany, 1997. [19] F. Ziegler,