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Cu multilayer
Journal of Magnetism and Magnetic Materials 462 (2018) 58–69
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Research articles
Room temperature superparamagnetism in ternary (Fe50Pt50)0.42Cu0.58 phase at interfaces on annealing of Fe50Pt50/Cu multilayer Surendra Singh a,b,⇑, C.L. Prajapat c, M. Gupta d, S. Basu a,b a
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400095, India c Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India d UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 001, India b
a r t i c l e
i n f o
Article history: Received 13 March 2018 Received in revised form 27 April 2018 Accepted 4 May 2018 Available online 5 May 2018
a b s t r a c t The face- centered- tetragonal (FCT) or L10 phase of Fe50Pt50 (FePt) film is obtained on annealing FePt films at high temperature (600 °C), that usually grows as face-centered- cubic (FCC) phase. Cu as an additive up to 20 atomic%, which form ternary alloy FePtCu on mixing, is used to reduce the ordering temperature. We studied the formation of FePtCu ternary alloy: its structure, magnetic properties and grain growth at the interfaces of a FePt/Cu multilayer, as a function of annealing temperature (100 °C– 600 °C), which is very less as compared to their melting temperatures. The study indicates the growth of a ternary alloy with composition of (FePt)0.42Cu0.58 in FCT phase on annealing the FePt/Cu multilayer at a moderate annealing temperature (400 °C). With a large addition of Cu at 56 atomic% in FePt, the ordering temperature reduces by 200 °C and form a ternary alloy with distinct magnetic properties. We observed rapid growth of ordered FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400 °C–600 °C. Interestingly, from macroscopic magnetization measurements we could identify the existence of a super-paramagnetic (SP) phase at room temperature with a typical particle size of 12 Å of ternary alloy on annealing the multilayer at 400 °C, which is much smaller than the crystallite size (50 Å). Annealing at 600 °C reduces the multilayer to a disordered magnetic phase. The room temperature SP nanoparticles of FePtCu alloy phase formed on annealing the multilayer at 400 °C are potential candidates in biomedical applications. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The ordered face-centered-tetragonal (FCT) or L10 ordered Fe50Pt50 (FePt) films have attracted considerable attention in the field of heat-assisted perpendicular magnetic recording medium because of its high magneto-crystalline anisotropy (Ku), large saturation magnetization (Ms) and the high-temperature gradient of Ku [1–9]. Reducing ordering temperature of the transition from as-deposited face-centered-cubic (FCC) phase to FCT phase, controlling the c-axis alignment perpendicular to the film surface with narrow switching field distribution, and smaller grain size with columnar growth are some of the desirable properties for FePt film in the application of recording media [10–14]. However a key limiting factor has been the high annealing (ordering) temperature (>600 °C) necessary to transform the as-deposited FCC A1 phase into the FCT L10 phase, which result in undesirable large grains that
⇑ Corresponding author at: Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. E-mail address: [email protected] (S. Singh). https://doi.org/10.1016/j.jmmm.2018.05.008 0304-8853/Ó 2018 Elsevier B.V. All rights reserved.
are strongly exchange coupled [1–8]. Efforts were made to reduce the ordering (annealing) temperature of FePt films and to improve the orientation and granular structure by employing different approaches such as monoatomic layer deposition [15], microwave/ion irradiation [16], annealing in hydrogen and magnetic field [17,18] and creating FePt-based ternary alloys by adding different metals [19–23]. In case of possible high-density magnetic storage application of FePt based alloy, superparamagnetic (SP) property of the alloy is a limitation due to strong coupling between the exchange interactions within each grain vs. dipolar interactions across neighboring grains, as it impacts the media performance such as the thermal stability. On the other hand FePt based alloy nanoparticles has shown considerable potential to be next generation magnetic nanoparticles for biomedical application [24–29], because of excellent SP property and their stability combined with the high X-ray absorptions. The possible use of nano particles of FePt based alloy in biomedical application [24–29] include magnetic resonance imaging (MRI) [26,28], targeted drug delivery and in the field of hyperthermia [24,25,27–29].
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Reduction in ordering (FCC to FCT) temperature of FePt film on adding Cu to FePt has been realized by forming (FePt)1xCux ternary alloys (x = 0–30%) [19–23]. These ternary alloys show drastically different magnetic properties [19–23]. Thin films of ternary alloys of Fe, Pt and Cu with different compositions are usually grown by co-deposition of elemental atom targets using rfmagnetron sputtering. Annealing of rf-sputter-deposited multilayers of Fe/Cu/Pt and bilayers of FePt/Cu have also been employed to get (FePt)1xCux ternary alloy [30,31]. The influence of Cu additives on the FCC-FCT transformation is, however, still under debate and there have been mixed reports regarding this transformation [32– 34]. Reduction of the ordering temperature by the addition of Cu was attributed to an increase in the driving force of the FCC-FCT transformation [32] and enhanced ordering kinetics as a result of the higher diffusivity [33] of Cu. Berry et al. [34] has revealed that Cu additions are no more effective in lowering the ordering temperature than equivalent additions of Fe. However a systematic study of evolution of structure and magnetic properties of (FePt)1xCux ternary alloy as a function of annealing temperature has been elusive in these systems. Such a study is important for understanding the phase transformation, evolution of new phases and possible improvement in the properties desirable for technological application of the system. In this work we have studied the structural and magnetic evolution of FePt/Cu multilayer as a function of annealing temperature in the range of 100 °C–600 °C. We investigated, in details, the structural and magnetic properties of the as-deposited and postannealed multilayer by combining various ex situ techniques including, grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), polarized neutron reflectivity (PNR) and superconducting quantum interference device (SQUID) magnetometer. We didn’t observe any drastic change in the layer (depth dependent) structure and magnetism of the multilayer on annealing it up to 200 °C. Small interdiffusion at interfaces started at 200 °C, initiating alloy formation at the interfaces which gradually increased with annealing at temperatures higher than 300 °C. XRR and PNR results also suggested a near-complete mixing of elements across the interfaces and formation of an almost uniform density single alloy layer on annealing at 400 °C. We observed an enhancement of the FCT phase growth from FCC with (0 0 1) texture on annealing FePt/Cu multilayer at 400 °C concomitant with the formation of (FePt)1xCux ternary alloy. Using electron and nuclear densities from XRR and PNR respectively, the composition of ternary alloy was identified as (FePt)0.38Cu0.56, which is very close to theoretically expected phase of ternary (FePt)0.42Cu0.58 alloy, assuming complete mixing of FePt and Cu. The study clearly indicates rapid growth of FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400–600 °C. PNR measurements suggested strong reduction of inplane long range magnetization (ferromagnetism) of (FePt)1xCux ternary alloy phase, formed on annealing at 400 °C. SQUID measurements suggested the existence of a SP phase of particle size distribution with an average of 12.5 Å in ternary alloy at room temperature on annealing the multilayer at 400 °C. This observation is a clear indication of granular magnetism in this system. While ferromagnetic materials embedded in insulating or metallic base material have shown granular magnetism and SP behavior, it is rare for such a crystalline system. Temperature dependent SQUID data from the multilayer annealed at 600 °C indicated the development of a disordered magnetic phase at low temperature and ferromagnetic hysteresis loop at room temperature indicated higher coercivity but reduced average magnetization. Formation of SP nanoparticles of FePtCu alloy phase at room temperature on annealing multilayer at 400 °C didn’t show any perpendicular magnetic anisotropy. However these SP nanoparticles with average size of 12.5 Å may find potential application in biomedical field.
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2. Experimental FePt/Cu multilayer was grown using dc-Magnetron sputtering technique on glass and Si substrates using an AJA Int. Inc. make ATC Orion-8 series sputtering system [35]. We have co-sputtered FePt layer from Fe and Pt targets. Five bilayers of FePt and Cu with thickness 120 Å and 140 Å respectively were deposited. The nominal structure of sample grown on glass substrate can be represented as glass (substrate)/[FePt (120 Å)/Cu(140 Å)]5, where 5 represents the number of repeats of the bilayers. In this paper we have presented results from sample grown on glass substrate only. However we obtained similar structural and magnetic properties of multilayer grown on Si substrate, as obtained for multilayer grown on Glass substrate and discussed here. The exact thicknesses of each layer in the sample were obtained by reflectometry (XRR and PNR) techniques discussed later. The multilayer was ex-situ annealed at different temperatures from 100 °C to 600 °C for 0.5 h, in an inert Ar environment at one atmosphere. After each anneal we carried out SQUID, GIXRD, XRR and PNR measurements to track the structural and magnetic changes. We have used Cu Ka (wavelength = 1.54 Å) radiation for XRR and GIXRD measurements. PNR measurements at room temperature were carried out at Dhruva, BARC, Mumbai [37], using neutrons of wavelength 2.5 Å. An in-plane magnetic field of 1.5 kOe was applied on the samples during PNR measurements. Macroscopic magnetization characterization of the films at room temperature and low temperature was carried out using SQUID magnetometer (Quantum Design, model MPMS5). Specular (angle of incidence = angle of reflection) XRR and PNR are two nondestructive techniques from which depth dependent structure of the sample with nanometer resolution averaged over the lateral dimensions of the sample (typically 100 mm2) can be inferred [36–41]. The specular reflectivity is measured as a function of wave vector transfer Q (i.e., the difference between the outgoing and incoming wave vectors and Q ¼ 4kp sinh, where h is the angle of incidence and k is the wavelength of X-ray/neutron) and it is related to the square of the Fourier transform of the depth dependent (z) scattering length density (SLD) profile qðzÞ (normal to the film surface or along the z-direction) [37–39]. For XRR, qx ðzÞ is proportional to electron density whereas for PNR, qðzÞ consists of nuclear SLD (NSLD) and magnetic SLD (MSLD) such that q ðzÞ ¼ qn ðzÞ CMðzÞ , where C = 2.9109 109 Å2 cm3/emu, and M(z) is the magnetization (emu/cm3) depth profile [37,38]. The sign +() is determined by the condition when the neutron beam polarization is parallel (opposite) to the in-plane magnetization of the sample and corresponds to reflectivities, R±.
3. Results 3.1. GIXRD measurements Fig. 1 shows the GIXRD data measured at a grazing incidence of 1° from the multilayer after each anneal. In Fig. 1(a) the peak positions of reflections from different planes of the FCC phase of FePt and Cu are marked by (D) and FCT phase of FePt and ternary alloy FePtCu are marked by (⁄). Fig. 1(b) shows the GIXRD data from the as-deposited multilayer sample. Both FePt and Cu layers in the asdeposited film were polycrystalline with FCC structure. The peaks at 2h–43.5° and 50.7°corresponds to (1 1 1) and (2 0 0) reflections of FCC Cu (Fig. 1(b)). Similarly peaks at 2h–41.82° and 47.2° corresponds to (1 1 1) and (2 0 0) reflection of FCC (A1) FePt phase (Fig. 1(b)). GIXRD data from as-deposited multilayer also indicate the presence of traces of tetragonal L10 phase of FePt by showing peak corresponding to (0 0 2) reflection of this phase at 2h–49.0°. This may be possible due to some L10 phase formation at the inter-
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Fig. 1. Grazing incidence X-ray diffraction (GIXRD) data at an angle of incidence 1° from the sample. (a) peak position for different planes of FePt and Cu system. (b) GIXRD from as-deposited FePt/Cu multilayer sample. (c–g) GIXRD of sample annealed at different temperatures.
faces during deposition. However the peak around 2h–49.0° in the as-deposited sample is quite broad and it may also include the reflections from FCC phases of Cu and FePt. GIXRD data from multilayer annealed at different temperatures (100 °C–600 °C) for 0.5 h each are shown in Fig. 1(c)–(g). We didn’t find any significant change in GIXRD data from the multilayer up to an annealing temperature of 300 °C. We observed a broad peak at 2h–42.2° in concomitant with the disappearance of Bragg peak of FCC phases of Cu (1 1 1) and FePt (1 1 1) on annealing the multilayer at 400 °C, which corresponds to the FCC phase ((1 1 1) reflection) of the possible ternary alloy (FePt)1xCux (‘FePtCu’). Further annealing at 600 °C, we found increase in FCT L10 phase of the ternary alloy FePtCu and reflections from different planes of FCT ternary alloy (FePtCu), shifted marginally with respect to the reflections from different planes of ordered FCT binary alloy FePt [19]. A comparison of GIXRD data around 2h–49.0° ((0 0 2) reflection of FCT phase of binary and ternary alloy) for as-deposited multi-
layer and multilayer annealed at 400 °C and 600 °C is shown in Fig. 2(a), suggesting enhancement of ordered FCT phase. Evolution of crystalline peaks as a function of annealing temperature were also compared by plotting the ratio of intensity of different reflections from FCT phase of FePt (or FePtCu) to FCC phase of FePt, (i.e. I⁄FePt(0 0 2)/IFePt(1 1 1), where ⁄ indicates both binary and ternary phase) in Fig. 2(b). It is evident from GIXRD data in Figs. 1 and 2 that there are no significant changes in intensity ratio (phase change of FePt) on annealing the sample up to 200 °C. Thus GIXRD data as a function of annealing temperature especially above 300 °C indicates a modification in structural properties as a result of interdiffusion of atoms at interfaces and formation of possible ternary FePtCu alloy. The FCC peaks corresponding to FePt (1 1 1) and Cu (1 1 1) merges to one peak at marginally higher angle than that of FePt (1 1 1). This peak may corresponds to (1 1 1) reflection from FCC ternary alloy FePtCu and marginal shift to higher angle is due to inclusion of Cu, which has a smaller size, at the Fe sites,
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Fig. 2. (a) A comparison of the GIXRD data around 2h–50° for as-deposited sample (black closed star with line) and sample annealed at 400 °C (open triangle with line, red) and 600 °C (open square with line, blue). (b) Ratio of peak intensity of Bragg reflections from FCT phase to FCC phase. Data for annealing at 0 °C in (b) corresponds to asdeposited sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
causing compressive strain in the lattice. Also there is enhancement in the tetragonal L10 (FCT) phase as evidenced by increase in the intensity of FCT to FCC phase of FePt (I⁄FePt(0 0 2)/IFePt(1 1 1) in Fig. 2(b)). This modification is further confirmed by detail depth dependent structure and magnetic properties of the multilayer using XRR and PNR, discussed below. 3.2. Depth dependent structure evolution: XRR measurements Fig. 3(a) shows the specular XRR as a function of Q from asdeposited multilayer and multilayer annealed at different temperatures. XRR data for different annealing temperatures are shifted vertically for better visualization. The electron scattering length density (ESLD) depth profile were inferred from the XRR data by fitting a model q(z) to the XRR data (solid lines in Fig. 3(a)). A model consisted of layers representing regions with different ESLDs [37–41]. The parameters of the model included layer thickness, interface (or surface) roughness and ESLD [37–41]. The reflectivity pattern corresponding to a given set of parameters was calculated using the dynamical formalism of Parratt [42], and parameters of the model were adjusted to minimize the value of weighted measure of the goodness of fit, v2 [37–41]. XRR and PNR data were fitted using a genetic algorithm based program [43] which uses Parratt formalism [42]. Fig. 3(b)–(f) show the ESLD depth profile of as-deposited (Fig. 3(b)) multilayer and multilayer annealed at different temperatures, which best fitted (solid lines
in Fig. 3(a)) the XRR data. We obtained a thickness of 110 ± 5 Å and 135 ± 5 Å for FePt and Cu layers in the as-deposited multilayer from XRR data. However to get best fit for XRR data we had to vary roughness for each interfaces. The average roughness for two interfaces (i.e. FePt/Cu interface and Cu/FePt interface) of as-deposited multilayer sample was 17 ± 4 Å. Inset of Fig. 3(a) shows the schematic of layer model used to analysis of reflectivity data before and after annealing of multilayer. On annealing at 100 °C we observed marginal reduction in interface roughness as well as thickness of the multilayer, indicating compaction of the layers possibly due to vacancy migration, which resulted into a shift of Bragg peak in XRR data to higher Q. On further annealing at 200 °C we observed modification in ESLD near interfaces as a result of interdiffusion of atoms across the interfaces. However the multilayer remains intact on annealing the multilayer up to 200 °C, which is evident from the presence of Bragg peak in XRR data. A significant modification in XRR data, i.e. suppression of Bragg peak and overall reflectivity, was observed on annealing the multilayer at 300 °C, suggesting more interdiffusion and formation of alloy layer at interfaces (ESLD profile in Fig. 3 (e)). We observed almost a single alloy layer of average ESLD of 6.1 ± 0.3 105 Å2 on annealing the multilayer at 400 °C (Fig. 3(f)). However, small variation in ESLD is also observed in depth profile at the position of Cu layer, suggesting non homogeneous single layer formation on annealing at this temperature. We didn’t observe any further significant change in XRR data on
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Fig. 3. (a) X-ray reflectivity (XRR) measurements from FePt/Cu multilayer sample for as-deposited and annealed conditions. The data for different conditions are shifted vertically. Inset shows the schematic of the bilayer model used for analysis of reflectivity data before and after annealing. (b) The electron scattering length density (ESLD) depth profile for as-deposited FePt/Cu multilayer sample. (c–f) ESLD profile for sample annealed at different temperatures. ESLD profiles shown in (b–f) best fitted (solid lines) the XRR data shown in (a).
annealing the multilayer sample at 600 °C (Fig. 3(f)), as both XRR data from annealed sample at 400 °C and 600 °C could be fitted with the same parameters. 3.3. Depth dependent structure - magnetism correlation: PNR measurements In order to investigate the evolution of depth dependent magnetism as a function of annealing temperature, we performed PNR measurements on the as-deposited and multilayer annealed
in the range of 100 °C–600 °C (Fig. 4(a)–(f)). Fig. 4(a)–(f) show the spin dependent reflectivity data (closed and open circles) and corresponding fits (solid lines) to the data. Fig. 4(g)–(l) show NSLD and MSLD depth profiles which best fitted the PNR data from asdeposited multilayer and multilayer annealed at different temperatures. The NSLD depth profiles as a function of annealing temperatures show similar trend of interdiffusion and alloying across interfaces as seen in the case of ESLD profiles obtained from XRR. The MSLD depth profile for as-deposited multilayer and multilayer annealed at 100 °C follow the corresponding NSLDs and indicate
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Fig. 4. Polarized neutron reflectivity data from (a) as-deposited multilayer and (b–f) multilayer annealed at different temperatures. (g–l) Corresponding nuclear and magnetic scattering length density of the sample for different annealing temperature which best fitted the PNR data shown in (a–f).
that the layered structure of the FePt/ Cu multilayer film remained intact. We obtained layers with reduced magnetization at the interfaces of the multilayer as a result of intermixing on annealing the sample at 200 °C (steps in Fig. 4(i)). A drastic modification in NSLD and MSLD was observed on annealing the sample at 300 °C and 400 °C. NSLD profile for sample annealed at 400 °C also corroborates XRR findings and suggests formation of almost single layer with small in-homogeneities along the depth near the positions of Cu layer. These inhomogeneities in NSLD reflects as nearly zero magnetic region (non magnetic) in MSLD profile. Due to alloying there is strong reduction in magnetization of the sample, though it remains ferromagnetic, evident from the split of the R+ and R intensity [inset Fig. 4(e)]. PNR data for sample annealed at 600 °C suggest further decrease in magnetization of the sample.
3.4. Macroscopic magnetization: SQUID measurements Fig. 5 shows the SQUID magnetization, M (H), measurements at room temperature from the multilayer for different conditions, as deposited and annealed at different temperatures. Fig. 5(a) presents the M (H) data in the plane of the film after annealing at different temperatures. Inset of Fig. 5(a) shows the in-plane data expanded for the as-deposited multilayer and the same annealed at 400 °C and 600 °C for comparison. It is evident from the inplane data that the as-deposited sample is a soft ferromagnet (black squares) which shows SP trend (blue stars) after annealing at 400 °C, reducing to a weak ferromagnet with highly reduced saturated magnetic moment (brown open stars) after annealing at 600 °C. The out-of-plane hysteresis loop data shown in Fig. 5(b)
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Fig. 5. M(H) curves for FePt/Cu multilayer at different annealing temperature along the (a) in-plane direction and (b) out of plane direction.
for different annealing temperatures, confirm that the sample has a hard axis along this direction, since there is no saturation in magnetization up to the highest applied field. The multilayer in all the cases, as-deposited and annealed at different temperatures, suggest easy (hard) axis of magnetization along the plane (perpendicular) of the film. This is in contrast to the properties of ternary FePtCu alloy with a Cu doping of <20%, which show perpendicular magnetic anisotropy (PMA) [23]. Fig. 6(a) and (b) show the variation of Hc and the ratio of remanence magnetization (Mr) and saturation magnetization (Ms) for the multilayer sample in the plane of the film, respectively, for different annealing temperatures (zero annealing temperature corresponds to as-deposited multilayer). Fig. 6 highlights that there is a dip in coercive field and in remnant magnetization to saturation magnetization ratio for the in-plane magnetization after anneal at 400 °C. In the annealing temperature range of 300 °C–400 °C, we obtained large magnetization but drastic reduction in the ratio of Mr/Ms ratio (0.05 at 400 °C). The reduction in both Hc and Mr/ Ms ratio (both tending to 0) for sample annealed at 400 °C (Fig. 6) clearly indicate the SP phase in the sample [24,44]. On annealing the multilayer at 500 °C, we obtained similar values of Hc and Mr/Ms ratio as obtained for annealed multilayer at 400 °C (Fig. 6), suggesting no significant change in macroscopic magnetization properties of the multilayer on annealing at 500 °C. However a
large increase in Hc and Mr/Ms ratio for sample annealed at 600 °C was observed, suggesting growth of a phase with higher inplane anisotropy. The SP behaviors and related particle size distributions in SP state have been evaluated from the analysis of SQUID magnetization data of annealed multilayer at 400 °C (Fig. 7). In the classical limit, the magnetization can be described by the Langevin function [44]. The Langevin function taking the log-normal particle size distribution into account is described by 3 3 n P [44]: M ¼ M S 43p d2i f ðdi ÞLðai Þ , with ai ¼ MkBSTH 43p d2i , where i¼1
Ms is saturation magnetization of sample, H is the applied field, T is the temperature, the particle diameter, di, has a log-normal distribution function, f(di) [43] and L(ai) = coth (ai)1/ai, is the Langevin function. Using this expression we fitted magnetization curve for sample annealed at 400 °C. The solid curve in Fig. 7, which gives best fit to magnetization data, has the particle size distribution shown in the inset of Fig. 7. The curve shows large value of saturation magnetization and zero coercivity, confirming the SP nature of the alloy grains. We obtained an average particles size of 12.3 Å, which is much smaller than the crystallite size 50 Å obtained from GIXRD measurements at 400 °C. This has implications regarding the alloying process as discussed later.
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Fig. 6. Variation of (a) in-plane coercive field, HC, and (b) ratio of in-pane remnant magnetization and saturated magnetization (MR /MS) as a function of annealing temperature.
Fig. 7. Normalized magnetization (M/Ms) versus applied field divided by temperature (H/T) curve from sample on annealing at 400 °C. The solid curve represents the calculation which gives the best fit to experiments. Inset show the particle size distribution with an average size, davg = 12.3 Å, used in the calculation.
Further, to confirm the nature of macroscopic in-plane magnetic properties of the multilayer, we recorded temperature dependent dc magnetization of the multilayer, annealed at 400 °C and 600 °C. Fig. 8(a) shows the temperature dependence of dc magne-
tization (M (T)), curves recorded from sample annealed at 400 °C and 600 °C under field-cooled (FC) and zero-field-cooled (ZFC) conditions. All the measurements were carried out in heating cycle under a specific magnetic field for FC and ZFC condition as men-
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Fig. 8. In-plane dc magnetization (M(T)) FC and ZFC data from multilayer annealed at (a) 400 °C and at (b) 600 °C. Insets show the M(T)/M(300 K), ZFC data from annealed multilayer at different magnetic fields.
tioned in the plots. The field mentioned for ZFC run is the magnetic field under which measurements were done in the heating cycle. For FC condition samples were cooled from 300 K to 5 K at same field as mentioned in plots. The low magnetization for ZFC at lower temperature is due to slow reorientation dynamics of the SP particle and the separation of FC and ZFC is a signature of SP phase of sample. In case of FC runs the magnetization showed saturation at lower temperature, indicating alignment of the SP particles and strong magnetic interaction between them. The temperature at the peak in ZFC is known as blocking temperature TB, where time scale of dynamics of the magnetism is same as the time scale of the measurement. Inset of Fig. 8(a) shows a comparison of normalized ZFC magnetization (M (T)/M (300 K)) measurements done under two applied fields (100 Oe and 1000 Oe). At higher field thermodynamic equilibrium occurs at lower temperature because the energy barrier between easy directions is reduced and the TB shifts to 125 K from room temperature. These results are another confirmation of the SP behavior [24,45,46] of FePtCu ternary alloy phase, formed on annealing the FePt/Cu multilayer at 400 °C. Fig. 8(b) depicts the temperature dependence of in-plane FC and ZFC curves of the sample after annealing at 600 °C in an applied inplane magnetic field of 100 Oe. M(H) curve (Fig. 5(a)) suggested
that the sample becomes a weak ferromagnetic after annealing at 600 °C. It is also evident from the peak magnetization value of the FC curve in Fig. 8(b), which is about 25% of the value that we obtained in case of sample annealed at 400 °C. Inset of Fig. 8(b) shows a comparison of normalized ZFC magnetization (M (T)/M (300 K)) for measurements done under two applied fields (100 Oe and 1000 Oe). Though the magnetization curve shows a peaked nature, it is quite different from what we observed in case of a SP system. The peak in the ZFC magnetization does not show any shift to lower temperature at higher magnetic field. Rather the magnetism shows a sharp step at 125 K for the measurement under a field of 100 Oe, which gets less enhanced for the measurement at 1000 Oe. In contrast with the typical behavior of SP ensembles, in which the ZFC and FC curves converge just above TB, the separation between the ZFC and FC curves persists even at 300 K, far above the perceived blocking temperature; Instead of the expected monotonic increase with decreasing temperature, the FC magnetization reaches a maximum at TB 125 K and then decreases below this temperature with a very broad minima followed by increase at lower temperature. A drop in the FC magnetization below a critical temperature is usually associated with the collective freezing of the system. This feature persists even on applying a higher field
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( 1000 Oe). For ZFC measurements we obtained an increase in ZFC magnetization from 5 K to 125 K. A small cusp in ZFC magnetization for multilayer annealed at 600 °C was observed at T = 125 K (TB), which didn’t change significantly on applying higher magnetic field. However, there is a bifurcation of the ZFC and FC curves near 300 K, and this magnetic irreversibility becomes more obvious below 300 K. These results imply a magnetic frustration around T = 125 K. Therefore the ZFC and FC measurements from multilayer annealed at 600 °C clearly indicate growth of a FePtCu alloy phase, which shows magnetic disorder and frustration at low temperature. 4. Discussion GIXRD, XRR and PNR data clearly indicated formation of a ternary alloy (FePtCu) on annealing the multilayer at temperatures greater than 300 °C. Using Debye-Scherrer formula: 0:9k [particle size ¼ bcosh ; where k, b and hb are wavelength of X-ray, b
line broadening and Bragg angle from GIXRD data], we estimated the crystallite sizes of FePt, Cu and ternary alloy of FePtCu as a function of annealing temperature as shown in Fig. 9(a). We have used well defined Bragg reflections (1 1 1) of the FCC phases for FePt, Cu and the ternary alloy. Similarly the crystallite size of FCT phase of ternary alloy has also been estimated from (0 0 2) reflection. It is evident from the data that both FCC and FCT phase of the ternary alloy grew on annealing beyond 200 °C. In general, we observed the growth of the FCT phase of the ternary alloy hap-
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pened at the cost of its FCC phase. For the as-deposited multilayer we obtained a crystallite size of 60 Å for FCC phase of both FePt and Cu, which remains same up to an annealing temperature of 200 °C. GIXRD data from multilayer annealed at 400 °C (Fig. 1(e)) showed emergence of new peaks suggesting growth of FCT phase of FePtCu alloy. We estimated the crystalline size of 50 Å for both the FCC and FCT phases (Fig. 9(a)) for sample annealed at 400 °C. We observed a large increase (3 times) in crystallite size of FCT [(0 0 2) reflection] phase of the FePtCu alloy on increasing the annealing temperature from 400 °C to 600 °C, accompanied by small decrease in the crystallite size of the FCC [(1 1 1) reflection] phase of FePtCu alloy. The modification in structure was caused due to diffusion of the atoms at the interfaces, generating the alloy phase. Reduction of the Bragg peak reflectivity in PNR bears the signature of diffusion of the elements across interfaces. Fig. 9(b) shows the drop in intensity of the PNR Bragg peak as a function of annealing temperature. We have plotted the ratio of reflectivity of the Bragg peaks for spin up (R+) neutrons in annealed and as-deposited case. Using intensity ratio of the Bragg peaks before and after annealing in PNR measurement [47] we have also estimated the diffusion constant at different annealing temperature and shown in Fig. 9(c). We obtained a diffusion constant, D = 7.6 1019 m2/sec at 400 °C, which is few order higher than the self diffusivity [5 1022 m2/sec] of Cu and Fe at this temperature [48,49]. The composition of the alloy layer formed on complete mixing of elements on annealing of a multilayer as a result of diffusion
Fig. 9. (a) The crystallite size of FePt, Cu and FePtCu alloy grains, respectively, deduced from the FePt(1 1 1), Cu(1 1 1) and FePtCu (0 0 2) Bragg reflections. (b) Variation of reflectivity of Bragg peak for spin up neutron. (c) Diffusivity at different temperature of annealing.
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at interfaces can be estimated using their density and thickness [39,50]. With a particle density of n(FePt) = 7.25 1022 cm3 and n(Cu) = 8.43 1022 cm3, for FePt and Cu, respectively, and using the average thickness of 110 Å and 135 Å for each layer of FePt and Cu in our multilayer (measured from reflectivity), the composition of homogeneous alloy would be (Fe50Pt50)42Cu58 on complete mixing [39,50]. For binary alloy it is possible to get exact composition experimentally using XRR and PNR measurements [38–41,51]. By fitting spin dependent neutron reflectivity data (R+ and R), the q(z)+ and q(z) SLD’s are estimated for each layer. Using these spin dependent SLD’s, the nuclear SLD for each layer can be extracted from qn = (q+–q)/2. Thus on comparing qn obtained from PNR and qx obtained from XRR data for the alloy layer with theoretical value of corresponding SLDs, the stoichiometry of the binary alloys formed at the interfaces can be obtained [39,40,51]. However for ternary alloy it may not possible to use such approach to get exact estimation of the composition. Though assuming FePt as a single entity, we tried a similar approach, as mentioned above as well as adopted for binary alloy [39,40,51], for estimating the composition of ternary alloy, using XRR and PNR data from multilayer annealed at 400 °C. We obtained a composition of (FePt)0.38Cu0.56, which is close to the expected composition of [(FePt)0.42Cu0.58], calculated assuming complete mixing of the atoms, as discussed above. Previous studies clearly indicate the existence of an ordered FCT and disordered FCC phase of Cu rich ternary alloy ((FePt)1xCux even with x = 0.58) on annealing at 600 °C [52]. It is evident from the combined study of GIXRD, XRR and PNR that Cu alloying with FePt indeed reduces the ordering temperature for formation of FCT phase from as-deposited FCC phase. Even by addition of 58 atomic percent of Cu in FePt, as a result of interdiffusion at interfaces, we observed approximately a 200 °C reduction in the ordering temperature. It is the faster volume diffusion of Cu atoms in FePt system that helps to reduce the ordering temperature [32]. But we did not observe any desirable magnetic anisotropy normal to the film plane as seen in previous studies. From the magnetic hysteresis loop we found that the sample had an in-plane magnetic easy axis. However we observed a hysteresis in magnetization curve [M(H) curve from SQUID] in multilayer annealed at 400 °C (in-plane), consistent with the results showing reduced in-plane Hc for FePt sample on doping Cu more than 50% [27]. Further annealing of the multilayer at 600 °C a small increase in diffusivity was observed. Though annealing at this temperature considerably increases the FCT phase with higher crystallite size. The sample also degrades magnetically. Interestingly the magnetic hysteresis loop after annealing at 400 °C clearly showed a SP behavior not seen earlier. The temperature dependent dc-magnetization (ZFC and FC) measurements also confirmed the SP nature of the sample annealed at 400 °C. On annealing multilayer at 600 °C we obtained ordered FCT phase of FePtCu alloy with larger crystallites as compared to that of multilayer annealed at 400 °C. In contrast out-ofplane direction was still the hard axis for the multilayer for annealing temperature to 600 °C. The absence of PMA for FePtCu ternary alloy formed on annealing multilayer sample at higher temperature (400 °C) and presence of unusual SP behavior might have resulted due to higher% of Cu in this case. An interesting and unique observation in the present case is the large difference in the size of the SP particles (12 Å) vis-à-vis the physical grain size obtained from GIXRD studies (50 Å–100 Å), after on annealing the multilayer at 400 °C. This indicates that Cu diffusion is along the grain boundaries of the FePt particles, which makes the surface (not to be confuse with the interface between layers) of the ternary alloy grains magnetically dead with the magnetic moment confined to a small core. The high magnetization at room temperature of SP particles of size 12.3 Å can make them a potential candidate
for biomedical application. On the other hand on annealing multilayer at 600 °C, we obtained large crystallite size (150 Å) of ternary alloy but frustrated magnetic behavior at low temperature. The strong structure-magnetic correlation as a function of annealing temperature in the FePt/Cu system is clear from the present studies. The possible reason for getting different size of magnetic phase (12.3 Å) and crystalline phase (50–100 Å) may be due to presence of mixed phase and small depth dependent inhomogeneities in NSLD/ESLD, in the annealed sample. This may suggest a kind of embedding of smaller ferromagnetic material in an insulating or metallic base material. While ferromagnetic materials embedded in insulating or metallic base material have shown granular magnetism and SP behavior, it is rare for such a crystalline system. Thus this observation is a clear indication of granular magnetism in this system on annealing at 400 °C. However, other possible reason cannot be neglected. 5. Conclusion In summary, depth dependent structural and magnetic properties of FePt/Cu multilayers as a function of annealing temperature in the range of 100–600 °C suggest large structural and magnetic modifications in the sample. Especially on annealing the sample above 300 °C resulted to high interdiffusion of components of the multilayer and formation of alloy layers at interfaces. The additional reflection peaks observed in GIXRD data from sample annealed at 400 °C clearly suggest formation of ordered (FePt)1xCux alloy phase. Formation of ternary (FePt)1xCux alloy on addition of even 58 atomic% of Cu in FePt film and transformation of FCC phase to ordered FCT phase on annealing of a FePt/Cu multilayer at 400 °C has been confirmed. This is 200 °C less than the observed transition temperature of FePt system. XRR and PNR further confirmed the formation of an alloy layer on annealing the multilayer at 400 °C. Maximum diffusion of elements at interface was observed on annealing the multilayer at 400 °C. The alloy layer formed on annealing the multilayer at 400 °C showed SP behavior of magnetization. SQUID data indicates a magnetically soft multilayer in the as-deposited state, which changed to SP behavior in concomitant of alloying on annealing the sample. The multilayer annealed at 600 °C showed higher coercivity (10 times that of as-deposited multilayer) at room temperature and disordered frustrated magnetic phase at low temperature. Increase in ordered phase with crystallite size of 3 times was also observed on annealing multilayer at 600 °C. The magnetization in out-of-plane direction remains hard axis at all the temperature of annealing. Large average magnetization of SP phase at room temperature on annealing multilayer at 400 °C may find possible application of FePtCu ternary alloy phase in biomedical field. Acknowledgements We acknowledge the help of V B Jayakrishnan for XRR and GIXRD experiments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jmmm.2018.05.008. References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] C.L. Zha, R.K. Dumas, Y.Y. Fang, V. Bonanni, J. Nogués, J. Åkerman, Appl. Phys. Lett. 97 (2010) 182504.
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Research articles
Room temperature superparamagnetism in ternary (Fe50Pt50)0.42Cu0.58 phase at interfaces on annealing of Fe50Pt50/Cu multilayer Surendra Singh a,b,⇑, C.L. Prajapat c, M. Gupta d, S. Basu a,b a
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400095, India c Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India d UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 001, India b
a r t i c l e
i n f o
Article history: Received 13 March 2018 Received in revised form 27 April 2018 Accepted 4 May 2018 Available online 5 May 2018
a b s t r a c t The face- centered- tetragonal (FCT) or L10 phase of Fe50Pt50 (FePt) film is obtained on annealing FePt films at high temperature (600 °C), that usually grows as face-centered- cubic (FCC) phase. Cu as an additive up to 20 atomic%, which form ternary alloy FePtCu on mixing, is used to reduce the ordering temperature. We studied the formation of FePtCu ternary alloy: its structure, magnetic properties and grain growth at the interfaces of a FePt/Cu multilayer, as a function of annealing temperature (100 °C– 600 °C), which is very less as compared to their melting temperatures. The study indicates the growth of a ternary alloy with composition of (FePt)0.42Cu0.58 in FCT phase on annealing the FePt/Cu multilayer at a moderate annealing temperature (400 °C). With a large addition of Cu at 56 atomic% in FePt, the ordering temperature reduces by 200 °C and form a ternary alloy with distinct magnetic properties. We observed rapid growth of ordered FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400 °C–600 °C. Interestingly, from macroscopic magnetization measurements we could identify the existence of a super-paramagnetic (SP) phase at room temperature with a typical particle size of 12 Å of ternary alloy on annealing the multilayer at 400 °C, which is much smaller than the crystallite size (50 Å). Annealing at 600 °C reduces the multilayer to a disordered magnetic phase. The room temperature SP nanoparticles of FePtCu alloy phase formed on annealing the multilayer at 400 °C are potential candidates in biomedical applications. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The ordered face-centered-tetragonal (FCT) or L10 ordered Fe50Pt50 (FePt) films have attracted considerable attention in the field of heat-assisted perpendicular magnetic recording medium because of its high magneto-crystalline anisotropy (Ku), large saturation magnetization (Ms) and the high-temperature gradient of Ku [1–9]. Reducing ordering temperature of the transition from as-deposited face-centered-cubic (FCC) phase to FCT phase, controlling the c-axis alignment perpendicular to the film surface with narrow switching field distribution, and smaller grain size with columnar growth are some of the desirable properties for FePt film in the application of recording media [10–14]. However a key limiting factor has been the high annealing (ordering) temperature (>600 °C) necessary to transform the as-deposited FCC A1 phase into the FCT L10 phase, which result in undesirable large grains that
⇑ Corresponding author at: Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. E-mail address: [email protected] (S. Singh). https://doi.org/10.1016/j.jmmm.2018.05.008 0304-8853/Ó 2018 Elsevier B.V. All rights reserved.
are strongly exchange coupled [1–8]. Efforts were made to reduce the ordering (annealing) temperature of FePt films and to improve the orientation and granular structure by employing different approaches such as monoatomic layer deposition [15], microwave/ion irradiation [16], annealing in hydrogen and magnetic field [17,18] and creating FePt-based ternary alloys by adding different metals [19–23]. In case of possible high-density magnetic storage application of FePt based alloy, superparamagnetic (SP) property of the alloy is a limitation due to strong coupling between the exchange interactions within each grain vs. dipolar interactions across neighboring grains, as it impacts the media performance such as the thermal stability. On the other hand FePt based alloy nanoparticles has shown considerable potential to be next generation magnetic nanoparticles for biomedical application [24–29], because of excellent SP property and their stability combined with the high X-ray absorptions. The possible use of nano particles of FePt based alloy in biomedical application [24–29] include magnetic resonance imaging (MRI) [26,28], targeted drug delivery and in the field of hyperthermia [24,25,27–29].
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Reduction in ordering (FCC to FCT) temperature of FePt film on adding Cu to FePt has been realized by forming (FePt)1xCux ternary alloys (x = 0–30%) [19–23]. These ternary alloys show drastically different magnetic properties [19–23]. Thin films of ternary alloys of Fe, Pt and Cu with different compositions are usually grown by co-deposition of elemental atom targets using rfmagnetron sputtering. Annealing of rf-sputter-deposited multilayers of Fe/Cu/Pt and bilayers of FePt/Cu have also been employed to get (FePt)1xCux ternary alloy [30,31]. The influence of Cu additives on the FCC-FCT transformation is, however, still under debate and there have been mixed reports regarding this transformation [32– 34]. Reduction of the ordering temperature by the addition of Cu was attributed to an increase in the driving force of the FCC-FCT transformation [32] and enhanced ordering kinetics as a result of the higher diffusivity [33] of Cu. Berry et al. [34] has revealed that Cu additions are no more effective in lowering the ordering temperature than equivalent additions of Fe. However a systematic study of evolution of structure and magnetic properties of (FePt)1xCux ternary alloy as a function of annealing temperature has been elusive in these systems. Such a study is important for understanding the phase transformation, evolution of new phases and possible improvement in the properties desirable for technological application of the system. In this work we have studied the structural and magnetic evolution of FePt/Cu multilayer as a function of annealing temperature in the range of 100 °C–600 °C. We investigated, in details, the structural and magnetic properties of the as-deposited and postannealed multilayer by combining various ex situ techniques including, grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), polarized neutron reflectivity (PNR) and superconducting quantum interference device (SQUID) magnetometer. We didn’t observe any drastic change in the layer (depth dependent) structure and magnetism of the multilayer on annealing it up to 200 °C. Small interdiffusion at interfaces started at 200 °C, initiating alloy formation at the interfaces which gradually increased with annealing at temperatures higher than 300 °C. XRR and PNR results also suggested a near-complete mixing of elements across the interfaces and formation of an almost uniform density single alloy layer on annealing at 400 °C. We observed an enhancement of the FCT phase growth from FCC with (0 0 1) texture on annealing FePt/Cu multilayer at 400 °C concomitant with the formation of (FePt)1xCux ternary alloy. Using electron and nuclear densities from XRR and PNR respectively, the composition of ternary alloy was identified as (FePt)0.38Cu0.56, which is very close to theoretically expected phase of ternary (FePt)0.42Cu0.58 alloy, assuming complete mixing of FePt and Cu. The study clearly indicates rapid growth of FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400–600 °C. PNR measurements suggested strong reduction of inplane long range magnetization (ferromagnetism) of (FePt)1xCux ternary alloy phase, formed on annealing at 400 °C. SQUID measurements suggested the existence of a SP phase of particle size distribution with an average of 12.5 Å in ternary alloy at room temperature on annealing the multilayer at 400 °C. This observation is a clear indication of granular magnetism in this system. While ferromagnetic materials embedded in insulating or metallic base material have shown granular magnetism and SP behavior, it is rare for such a crystalline system. Temperature dependent SQUID data from the multilayer annealed at 600 °C indicated the development of a disordered magnetic phase at low temperature and ferromagnetic hysteresis loop at room temperature indicated higher coercivity but reduced average magnetization. Formation of SP nanoparticles of FePtCu alloy phase at room temperature on annealing multilayer at 400 °C didn’t show any perpendicular magnetic anisotropy. However these SP nanoparticles with average size of 12.5 Å may find potential application in biomedical field.
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2. Experimental FePt/Cu multilayer was grown using dc-Magnetron sputtering technique on glass and Si substrates using an AJA Int. Inc. make ATC Orion-8 series sputtering system [35]. We have co-sputtered FePt layer from Fe and Pt targets. Five bilayers of FePt and Cu with thickness 120 Å and 140 Å respectively were deposited. The nominal structure of sample grown on glass substrate can be represented as glass (substrate)/[FePt (120 Å)/Cu(140 Å)]5, where 5 represents the number of repeats of the bilayers. In this paper we have presented results from sample grown on glass substrate only. However we obtained similar structural and magnetic properties of multilayer grown on Si substrate, as obtained for multilayer grown on Glass substrate and discussed here. The exact thicknesses of each layer in the sample were obtained by reflectometry (XRR and PNR) techniques discussed later. The multilayer was ex-situ annealed at different temperatures from 100 °C to 600 °C for 0.5 h, in an inert Ar environment at one atmosphere. After each anneal we carried out SQUID, GIXRD, XRR and PNR measurements to track the structural and magnetic changes. We have used Cu Ka (wavelength = 1.54 Å) radiation for XRR and GIXRD measurements. PNR measurements at room temperature were carried out at Dhruva, BARC, Mumbai [37], using neutrons of wavelength 2.5 Å. An in-plane magnetic field of 1.5 kOe was applied on the samples during PNR measurements. Macroscopic magnetization characterization of the films at room temperature and low temperature was carried out using SQUID magnetometer (Quantum Design, model MPMS5). Specular (angle of incidence = angle of reflection) XRR and PNR are two nondestructive techniques from which depth dependent structure of the sample with nanometer resolution averaged over the lateral dimensions of the sample (typically 100 mm2) can be inferred [36–41]. The specular reflectivity is measured as a function of wave vector transfer Q (i.e., the difference between the outgoing and incoming wave vectors and Q ¼ 4kp sinh, where h is the angle of incidence and k is the wavelength of X-ray/neutron) and it is related to the square of the Fourier transform of the depth dependent (z) scattering length density (SLD) profile qðzÞ (normal to the film surface or along the z-direction) [37–39]. For XRR, qx ðzÞ is proportional to electron density whereas for PNR, qðzÞ consists of nuclear SLD (NSLD) and magnetic SLD (MSLD) such that q ðzÞ ¼ qn ðzÞ CMðzÞ , where C = 2.9109 109 Å2 cm3/emu, and M(z) is the magnetization (emu/cm3) depth profile [37,38]. The sign +() is determined by the condition when the neutron beam polarization is parallel (opposite) to the in-plane magnetization of the sample and corresponds to reflectivities, R±.
3. Results 3.1. GIXRD measurements Fig. 1 shows the GIXRD data measured at a grazing incidence of 1° from the multilayer after each anneal. In Fig. 1(a) the peak positions of reflections from different planes of the FCC phase of FePt and Cu are marked by (D) and FCT phase of FePt and ternary alloy FePtCu are marked by (⁄). Fig. 1(b) shows the GIXRD data from the as-deposited multilayer sample. Both FePt and Cu layers in the asdeposited film were polycrystalline with FCC structure. The peaks at 2h–43.5° and 50.7°corresponds to (1 1 1) and (2 0 0) reflections of FCC Cu (Fig. 1(b)). Similarly peaks at 2h–41.82° and 47.2° corresponds to (1 1 1) and (2 0 0) reflection of FCC (A1) FePt phase (Fig. 1(b)). GIXRD data from as-deposited multilayer also indicate the presence of traces of tetragonal L10 phase of FePt by showing peak corresponding to (0 0 2) reflection of this phase at 2h–49.0°. This may be possible due to some L10 phase formation at the inter-
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S. Singh et al. / Journal of Magnetism and Magnetic Materials 462 (2018) 58–69
Fig. 1. Grazing incidence X-ray diffraction (GIXRD) data at an angle of incidence 1° from the sample. (a) peak position for different planes of FePt and Cu system. (b) GIXRD from as-deposited FePt/Cu multilayer sample. (c–g) GIXRD of sample annealed at different temperatures.
faces during deposition. However the peak around 2h–49.0° in the as-deposited sample is quite broad and it may also include the reflections from FCC phases of Cu and FePt. GIXRD data from multilayer annealed at different temperatures (100 °C–600 °C) for 0.5 h each are shown in Fig. 1(c)–(g). We didn’t find any significant change in GIXRD data from the multilayer up to an annealing temperature of 300 °C. We observed a broad peak at 2h–42.2° in concomitant with the disappearance of Bragg peak of FCC phases of Cu (1 1 1) and FePt (1 1 1) on annealing the multilayer at 400 °C, which corresponds to the FCC phase ((1 1 1) reflection) of the possible ternary alloy (FePt)1xCux (‘FePtCu’). Further annealing at 600 °C, we found increase in FCT L10 phase of the ternary alloy FePtCu and reflections from different planes of FCT ternary alloy (FePtCu), shifted marginally with respect to the reflections from different planes of ordered FCT binary alloy FePt [19]. A comparison of GIXRD data around 2h–49.0° ((0 0 2) reflection of FCT phase of binary and ternary alloy) for as-deposited multi-
layer and multilayer annealed at 400 °C and 600 °C is shown in Fig. 2(a), suggesting enhancement of ordered FCT phase. Evolution of crystalline peaks as a function of annealing temperature were also compared by plotting the ratio of intensity of different reflections from FCT phase of FePt (or FePtCu) to FCC phase of FePt, (i.e. I⁄FePt(0 0 2)/IFePt(1 1 1), where ⁄ indicates both binary and ternary phase) in Fig. 2(b). It is evident from GIXRD data in Figs. 1 and 2 that there are no significant changes in intensity ratio (phase change of FePt) on annealing the sample up to 200 °C. Thus GIXRD data as a function of annealing temperature especially above 300 °C indicates a modification in structural properties as a result of interdiffusion of atoms at interfaces and formation of possible ternary FePtCu alloy. The FCC peaks corresponding to FePt (1 1 1) and Cu (1 1 1) merges to one peak at marginally higher angle than that of FePt (1 1 1). This peak may corresponds to (1 1 1) reflection from FCC ternary alloy FePtCu and marginal shift to higher angle is due to inclusion of Cu, which has a smaller size, at the Fe sites,
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Fig. 2. (a) A comparison of the GIXRD data around 2h–50° for as-deposited sample (black closed star with line) and sample annealed at 400 °C (open triangle with line, red) and 600 °C (open square with line, blue). (b) Ratio of peak intensity of Bragg reflections from FCT phase to FCC phase. Data for annealing at 0 °C in (b) corresponds to asdeposited sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
causing compressive strain in the lattice. Also there is enhancement in the tetragonal L10 (FCT) phase as evidenced by increase in the intensity of FCT to FCC phase of FePt (I⁄FePt(0 0 2)/IFePt(1 1 1) in Fig. 2(b)). This modification is further confirmed by detail depth dependent structure and magnetic properties of the multilayer using XRR and PNR, discussed below. 3.2. Depth dependent structure evolution: XRR measurements Fig. 3(a) shows the specular XRR as a function of Q from asdeposited multilayer and multilayer annealed at different temperatures. XRR data for different annealing temperatures are shifted vertically for better visualization. The electron scattering length density (ESLD) depth profile were inferred from the XRR data by fitting a model q(z) to the XRR data (solid lines in Fig. 3(a)). A model consisted of layers representing regions with different ESLDs [37–41]. The parameters of the model included layer thickness, interface (or surface) roughness and ESLD [37–41]. The reflectivity pattern corresponding to a given set of parameters was calculated using the dynamical formalism of Parratt [42], and parameters of the model were adjusted to minimize the value of weighted measure of the goodness of fit, v2 [37–41]. XRR and PNR data were fitted using a genetic algorithm based program [43] which uses Parratt formalism [42]. Fig. 3(b)–(f) show the ESLD depth profile of as-deposited (Fig. 3(b)) multilayer and multilayer annealed at different temperatures, which best fitted (solid lines
in Fig. 3(a)) the XRR data. We obtained a thickness of 110 ± 5 Å and 135 ± 5 Å for FePt and Cu layers in the as-deposited multilayer from XRR data. However to get best fit for XRR data we had to vary roughness for each interfaces. The average roughness for two interfaces (i.e. FePt/Cu interface and Cu/FePt interface) of as-deposited multilayer sample was 17 ± 4 Å. Inset of Fig. 3(a) shows the schematic of layer model used to analysis of reflectivity data before and after annealing of multilayer. On annealing at 100 °C we observed marginal reduction in interface roughness as well as thickness of the multilayer, indicating compaction of the layers possibly due to vacancy migration, which resulted into a shift of Bragg peak in XRR data to higher Q. On further annealing at 200 °C we observed modification in ESLD near interfaces as a result of interdiffusion of atoms across the interfaces. However the multilayer remains intact on annealing the multilayer up to 200 °C, which is evident from the presence of Bragg peak in XRR data. A significant modification in XRR data, i.e. suppression of Bragg peak and overall reflectivity, was observed on annealing the multilayer at 300 °C, suggesting more interdiffusion and formation of alloy layer at interfaces (ESLD profile in Fig. 3 (e)). We observed almost a single alloy layer of average ESLD of 6.1 ± 0.3 105 Å2 on annealing the multilayer at 400 °C (Fig. 3(f)). However, small variation in ESLD is also observed in depth profile at the position of Cu layer, suggesting non homogeneous single layer formation on annealing at this temperature. We didn’t observe any further significant change in XRR data on
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Fig. 3. (a) X-ray reflectivity (XRR) measurements from FePt/Cu multilayer sample for as-deposited and annealed conditions. The data for different conditions are shifted vertically. Inset shows the schematic of the bilayer model used for analysis of reflectivity data before and after annealing. (b) The electron scattering length density (ESLD) depth profile for as-deposited FePt/Cu multilayer sample. (c–f) ESLD profile for sample annealed at different temperatures. ESLD profiles shown in (b–f) best fitted (solid lines) the XRR data shown in (a).
annealing the multilayer sample at 600 °C (Fig. 3(f)), as both XRR data from annealed sample at 400 °C and 600 °C could be fitted with the same parameters. 3.3. Depth dependent structure - magnetism correlation: PNR measurements In order to investigate the evolution of depth dependent magnetism as a function of annealing temperature, we performed PNR measurements on the as-deposited and multilayer annealed
in the range of 100 °C–600 °C (Fig. 4(a)–(f)). Fig. 4(a)–(f) show the spin dependent reflectivity data (closed and open circles) and corresponding fits (solid lines) to the data. Fig. 4(g)–(l) show NSLD and MSLD depth profiles which best fitted the PNR data from asdeposited multilayer and multilayer annealed at different temperatures. The NSLD depth profiles as a function of annealing temperatures show similar trend of interdiffusion and alloying across interfaces as seen in the case of ESLD profiles obtained from XRR. The MSLD depth profile for as-deposited multilayer and multilayer annealed at 100 °C follow the corresponding NSLDs and indicate
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Fig. 4. Polarized neutron reflectivity data from (a) as-deposited multilayer and (b–f) multilayer annealed at different temperatures. (g–l) Corresponding nuclear and magnetic scattering length density of the sample for different annealing temperature which best fitted the PNR data shown in (a–f).
that the layered structure of the FePt/ Cu multilayer film remained intact. We obtained layers with reduced magnetization at the interfaces of the multilayer as a result of intermixing on annealing the sample at 200 °C (steps in Fig. 4(i)). A drastic modification in NSLD and MSLD was observed on annealing the sample at 300 °C and 400 °C. NSLD profile for sample annealed at 400 °C also corroborates XRR findings and suggests formation of almost single layer with small in-homogeneities along the depth near the positions of Cu layer. These inhomogeneities in NSLD reflects as nearly zero magnetic region (non magnetic) in MSLD profile. Due to alloying there is strong reduction in magnetization of the sample, though it remains ferromagnetic, evident from the split of the R+ and R intensity [inset Fig. 4(e)]. PNR data for sample annealed at 600 °C suggest further decrease in magnetization of the sample.
3.4. Macroscopic magnetization: SQUID measurements Fig. 5 shows the SQUID magnetization, M (H), measurements at room temperature from the multilayer for different conditions, as deposited and annealed at different temperatures. Fig. 5(a) presents the M (H) data in the plane of the film after annealing at different temperatures. Inset of Fig. 5(a) shows the in-plane data expanded for the as-deposited multilayer and the same annealed at 400 °C and 600 °C for comparison. It is evident from the inplane data that the as-deposited sample is a soft ferromagnet (black squares) which shows SP trend (blue stars) after annealing at 400 °C, reducing to a weak ferromagnet with highly reduced saturated magnetic moment (brown open stars) after annealing at 600 °C. The out-of-plane hysteresis loop data shown in Fig. 5(b)
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Fig. 5. M(H) curves for FePt/Cu multilayer at different annealing temperature along the (a) in-plane direction and (b) out of plane direction.
for different annealing temperatures, confirm that the sample has a hard axis along this direction, since there is no saturation in magnetization up to the highest applied field. The multilayer in all the cases, as-deposited and annealed at different temperatures, suggest easy (hard) axis of magnetization along the plane (perpendicular) of the film. This is in contrast to the properties of ternary FePtCu alloy with a Cu doping of <20%, which show perpendicular magnetic anisotropy (PMA) [23]. Fig. 6(a) and (b) show the variation of Hc and the ratio of remanence magnetization (Mr) and saturation magnetization (Ms) for the multilayer sample in the plane of the film, respectively, for different annealing temperatures (zero annealing temperature corresponds to as-deposited multilayer). Fig. 6 highlights that there is a dip in coercive field and in remnant magnetization to saturation magnetization ratio for the in-plane magnetization after anneal at 400 °C. In the annealing temperature range of 300 °C–400 °C, we obtained large magnetization but drastic reduction in the ratio of Mr/Ms ratio (0.05 at 400 °C). The reduction in both Hc and Mr/ Ms ratio (both tending to 0) for sample annealed at 400 °C (Fig. 6) clearly indicate the SP phase in the sample [24,44]. On annealing the multilayer at 500 °C, we obtained similar values of Hc and Mr/Ms ratio as obtained for annealed multilayer at 400 °C (Fig. 6), suggesting no significant change in macroscopic magnetization properties of the multilayer on annealing at 500 °C. However a
large increase in Hc and Mr/Ms ratio for sample annealed at 600 °C was observed, suggesting growth of a phase with higher inplane anisotropy. The SP behaviors and related particle size distributions in SP state have been evaluated from the analysis of SQUID magnetization data of annealed multilayer at 400 °C (Fig. 7). In the classical limit, the magnetization can be described by the Langevin function [44]. The Langevin function taking the log-normal particle size distribution into account is described by 3 3 n P [44]: M ¼ M S 43p d2i f ðdi ÞLðai Þ , with ai ¼ MkBSTH 43p d2i , where i¼1
Ms is saturation magnetization of sample, H is the applied field, T is the temperature, the particle diameter, di, has a log-normal distribution function, f(di) [43] and L(ai) = coth (ai)1/ai, is the Langevin function. Using this expression we fitted magnetization curve for sample annealed at 400 °C. The solid curve in Fig. 7, which gives best fit to magnetization data, has the particle size distribution shown in the inset of Fig. 7. The curve shows large value of saturation magnetization and zero coercivity, confirming the SP nature of the alloy grains. We obtained an average particles size of 12.3 Å, which is much smaller than the crystallite size 50 Å obtained from GIXRD measurements at 400 °C. This has implications regarding the alloying process as discussed later.
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Fig. 6. Variation of (a) in-plane coercive field, HC, and (b) ratio of in-pane remnant magnetization and saturated magnetization (MR /MS) as a function of annealing temperature.
Fig. 7. Normalized magnetization (M/Ms) versus applied field divided by temperature (H/T) curve from sample on annealing at 400 °C. The solid curve represents the calculation which gives the best fit to experiments. Inset show the particle size distribution with an average size, davg = 12.3 Å, used in the calculation.
Further, to confirm the nature of macroscopic in-plane magnetic properties of the multilayer, we recorded temperature dependent dc magnetization of the multilayer, annealed at 400 °C and 600 °C. Fig. 8(a) shows the temperature dependence of dc magne-
tization (M (T)), curves recorded from sample annealed at 400 °C and 600 °C under field-cooled (FC) and zero-field-cooled (ZFC) conditions. All the measurements were carried out in heating cycle under a specific magnetic field for FC and ZFC condition as men-
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Fig. 8. In-plane dc magnetization (M(T)) FC and ZFC data from multilayer annealed at (a) 400 °C and at (b) 600 °C. Insets show the M(T)/M(300 K), ZFC data from annealed multilayer at different magnetic fields.
tioned in the plots. The field mentioned for ZFC run is the magnetic field under which measurements were done in the heating cycle. For FC condition samples were cooled from 300 K to 5 K at same field as mentioned in plots. The low magnetization for ZFC at lower temperature is due to slow reorientation dynamics of the SP particle and the separation of FC and ZFC is a signature of SP phase of sample. In case of FC runs the magnetization showed saturation at lower temperature, indicating alignment of the SP particles and strong magnetic interaction between them. The temperature at the peak in ZFC is known as blocking temperature TB, where time scale of dynamics of the magnetism is same as the time scale of the measurement. Inset of Fig. 8(a) shows a comparison of normalized ZFC magnetization (M (T)/M (300 K)) measurements done under two applied fields (100 Oe and 1000 Oe). At higher field thermodynamic equilibrium occurs at lower temperature because the energy barrier between easy directions is reduced and the TB shifts to 125 K from room temperature. These results are another confirmation of the SP behavior [24,45,46] of FePtCu ternary alloy phase, formed on annealing the FePt/Cu multilayer at 400 °C. Fig. 8(b) depicts the temperature dependence of in-plane FC and ZFC curves of the sample after annealing at 600 °C in an applied inplane magnetic field of 100 Oe. M(H) curve (Fig. 5(a)) suggested
that the sample becomes a weak ferromagnetic after annealing at 600 °C. It is also evident from the peak magnetization value of the FC curve in Fig. 8(b), which is about 25% of the value that we obtained in case of sample annealed at 400 °C. Inset of Fig. 8(b) shows a comparison of normalized ZFC magnetization (M (T)/M (300 K)) for measurements done under two applied fields (100 Oe and 1000 Oe). Though the magnetization curve shows a peaked nature, it is quite different from what we observed in case of a SP system. The peak in the ZFC magnetization does not show any shift to lower temperature at higher magnetic field. Rather the magnetism shows a sharp step at 125 K for the measurement under a field of 100 Oe, which gets less enhanced for the measurement at 1000 Oe. In contrast with the typical behavior of SP ensembles, in which the ZFC and FC curves converge just above TB, the separation between the ZFC and FC curves persists even at 300 K, far above the perceived blocking temperature; Instead of the expected monotonic increase with decreasing temperature, the FC magnetization reaches a maximum at TB 125 K and then decreases below this temperature with a very broad minima followed by increase at lower temperature. A drop in the FC magnetization below a critical temperature is usually associated with the collective freezing of the system. This feature persists even on applying a higher field
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( 1000 Oe). For ZFC measurements we obtained an increase in ZFC magnetization from 5 K to 125 K. A small cusp in ZFC magnetization for multilayer annealed at 600 °C was observed at T = 125 K (TB), which didn’t change significantly on applying higher magnetic field. However, there is a bifurcation of the ZFC and FC curves near 300 K, and this magnetic irreversibility becomes more obvious below 300 K. These results imply a magnetic frustration around T = 125 K. Therefore the ZFC and FC measurements from multilayer annealed at 600 °C clearly indicate growth of a FePtCu alloy phase, which shows magnetic disorder and frustration at low temperature. 4. Discussion GIXRD, XRR and PNR data clearly indicated formation of a ternary alloy (FePtCu) on annealing the multilayer at temperatures greater than 300 °C. Using Debye-Scherrer formula: 0:9k [particle size ¼ bcosh ; where k, b and hb are wavelength of X-ray, b
line broadening and Bragg angle from GIXRD data], we estimated the crystallite sizes of FePt, Cu and ternary alloy of FePtCu as a function of annealing temperature as shown in Fig. 9(a). We have used well defined Bragg reflections (1 1 1) of the FCC phases for FePt, Cu and the ternary alloy. Similarly the crystallite size of FCT phase of ternary alloy has also been estimated from (0 0 2) reflection. It is evident from the data that both FCC and FCT phase of the ternary alloy grew on annealing beyond 200 °C. In general, we observed the growth of the FCT phase of the ternary alloy hap-
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pened at the cost of its FCC phase. For the as-deposited multilayer we obtained a crystallite size of 60 Å for FCC phase of both FePt and Cu, which remains same up to an annealing temperature of 200 °C. GIXRD data from multilayer annealed at 400 °C (Fig. 1(e)) showed emergence of new peaks suggesting growth of FCT phase of FePtCu alloy. We estimated the crystalline size of 50 Å for both the FCC and FCT phases (Fig. 9(a)) for sample annealed at 400 °C. We observed a large increase (3 times) in crystallite size of FCT [(0 0 2) reflection] phase of the FePtCu alloy on increasing the annealing temperature from 400 °C to 600 °C, accompanied by small decrease in the crystallite size of the FCC [(1 1 1) reflection] phase of FePtCu alloy. The modification in structure was caused due to diffusion of the atoms at the interfaces, generating the alloy phase. Reduction of the Bragg peak reflectivity in PNR bears the signature of diffusion of the elements across interfaces. Fig. 9(b) shows the drop in intensity of the PNR Bragg peak as a function of annealing temperature. We have plotted the ratio of reflectivity of the Bragg peaks for spin up (R+) neutrons in annealed and as-deposited case. Using intensity ratio of the Bragg peaks before and after annealing in PNR measurement [47] we have also estimated the diffusion constant at different annealing temperature and shown in Fig. 9(c). We obtained a diffusion constant, D = 7.6 1019 m2/sec at 400 °C, which is few order higher than the self diffusivity [5 1022 m2/sec] of Cu and Fe at this temperature [48,49]. The composition of the alloy layer formed on complete mixing of elements on annealing of a multilayer as a result of diffusion
Fig. 9. (a) The crystallite size of FePt, Cu and FePtCu alloy grains, respectively, deduced from the FePt(1 1 1), Cu(1 1 1) and FePtCu (0 0 2) Bragg reflections. (b) Variation of reflectivity of Bragg peak for spin up neutron. (c) Diffusivity at different temperature of annealing.
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at interfaces can be estimated using their density and thickness [39,50]. With a particle density of n(FePt) = 7.25 1022 cm3 and n(Cu) = 8.43 1022 cm3, for FePt and Cu, respectively, and using the average thickness of 110 Å and 135 Å for each layer of FePt and Cu in our multilayer (measured from reflectivity), the composition of homogeneous alloy would be (Fe50Pt50)42Cu58 on complete mixing [39,50]. For binary alloy it is possible to get exact composition experimentally using XRR and PNR measurements [38–41,51]. By fitting spin dependent neutron reflectivity data (R+ and R), the q(z)+ and q(z) SLD’s are estimated for each layer. Using these spin dependent SLD’s, the nuclear SLD for each layer can be extracted from qn = (q+–q)/2. Thus on comparing qn obtained from PNR and qx obtained from XRR data for the alloy layer with theoretical value of corresponding SLDs, the stoichiometry of the binary alloys formed at the interfaces can be obtained [39,40,51]. However for ternary alloy it may not possible to use such approach to get exact estimation of the composition. Though assuming FePt as a single entity, we tried a similar approach, as mentioned above as well as adopted for binary alloy [39,40,51], for estimating the composition of ternary alloy, using XRR and PNR data from multilayer annealed at 400 °C. We obtained a composition of (FePt)0.38Cu0.56, which is close to the expected composition of [(FePt)0.42Cu0.58], calculated assuming complete mixing of the atoms, as discussed above. Previous studies clearly indicate the existence of an ordered FCT and disordered FCC phase of Cu rich ternary alloy ((FePt)1xCux even with x = 0.58) on annealing at 600 °C [52]. It is evident from the combined study of GIXRD, XRR and PNR that Cu alloying with FePt indeed reduces the ordering temperature for formation of FCT phase from as-deposited FCC phase. Even by addition of 58 atomic percent of Cu in FePt, as a result of interdiffusion at interfaces, we observed approximately a 200 °C reduction in the ordering temperature. It is the faster volume diffusion of Cu atoms in FePt system that helps to reduce the ordering temperature [32]. But we did not observe any desirable magnetic anisotropy normal to the film plane as seen in previous studies. From the magnetic hysteresis loop we found that the sample had an in-plane magnetic easy axis. However we observed a hysteresis in magnetization curve [M(H) curve from SQUID] in multilayer annealed at 400 °C (in-plane), consistent with the results showing reduced in-plane Hc for FePt sample on doping Cu more than 50% [27]. Further annealing of the multilayer at 600 °C a small increase in diffusivity was observed. Though annealing at this temperature considerably increases the FCT phase with higher crystallite size. The sample also degrades magnetically. Interestingly the magnetic hysteresis loop after annealing at 400 °C clearly showed a SP behavior not seen earlier. The temperature dependent dc-magnetization (ZFC and FC) measurements also confirmed the SP nature of the sample annealed at 400 °C. On annealing multilayer at 600 °C we obtained ordered FCT phase of FePtCu alloy with larger crystallites as compared to that of multilayer annealed at 400 °C. In contrast out-ofplane direction was still the hard axis for the multilayer for annealing temperature to 600 °C. The absence of PMA for FePtCu ternary alloy formed on annealing multilayer sample at higher temperature (400 °C) and presence of unusual SP behavior might have resulted due to higher% of Cu in this case. An interesting and unique observation in the present case is the large difference in the size of the SP particles (12 Å) vis-à-vis the physical grain size obtained from GIXRD studies (50 Å–100 Å), after on annealing the multilayer at 400 °C. This indicates that Cu diffusion is along the grain boundaries of the FePt particles, which makes the surface (not to be confuse with the interface between layers) of the ternary alloy grains magnetically dead with the magnetic moment confined to a small core. The high magnetization at room temperature of SP particles of size 12.3 Å can make them a potential candidate
for biomedical application. On the other hand on annealing multilayer at 600 °C, we obtained large crystallite size (150 Å) of ternary alloy but frustrated magnetic behavior at low temperature. The strong structure-magnetic correlation as a function of annealing temperature in the FePt/Cu system is clear from the present studies. The possible reason for getting different size of magnetic phase (12.3 Å) and crystalline phase (50–100 Å) may be due to presence of mixed phase and small depth dependent inhomogeneities in NSLD/ESLD, in the annealed sample. This may suggest a kind of embedding of smaller ferromagnetic material in an insulating or metallic base material. While ferromagnetic materials embedded in insulating or metallic base material have shown granular magnetism and SP behavior, it is rare for such a crystalline system. Thus this observation is a clear indication of granular magnetism in this system on annealing at 400 °C. However, other possible reason cannot be neglected. 5. Conclusion In summary, depth dependent structural and magnetic properties of FePt/Cu multilayers as a function of annealing temperature in the range of 100–600 °C suggest large structural and magnetic modifications in the sample. Especially on annealing the sample above 300 °C resulted to high interdiffusion of components of the multilayer and formation of alloy layers at interfaces. The additional reflection peaks observed in GIXRD data from sample annealed at 400 °C clearly suggest formation of ordered (FePt)1xCux alloy phase. Formation of ternary (FePt)1xCux alloy on addition of even 58 atomic% of Cu in FePt film and transformation of FCC phase to ordered FCT phase on annealing of a FePt/Cu multilayer at 400 °C has been confirmed. This is 200 °C less than the observed transition temperature of FePt system. XRR and PNR further confirmed the formation of an alloy layer on annealing the multilayer at 400 °C. Maximum diffusion of elements at interface was observed on annealing the multilayer at 400 °C. The alloy layer formed on annealing the multilayer at 400 °C showed SP behavior of magnetization. SQUID data indicates a magnetically soft multilayer in the as-deposited state, which changed to SP behavior in concomitant of alloying on annealing the sample. The multilayer annealed at 600 °C showed higher coercivity (10 times that of as-deposited multilayer) at room temperature and disordered frustrated magnetic phase at low temperature. Increase in ordered phase with crystallite size of 3 times was also observed on annealing multilayer at 600 °C. The magnetization in out-of-plane direction remains hard axis at all the temperature of annealing. Large average magnetization of SP phase at room temperature on annealing multilayer at 400 °C may find possible application of FePtCu ternary alloy phase in biomedical field. Acknowledgements We acknowledge the help of V B Jayakrishnan for XRR and GIXRD experiments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jmmm.2018.05.008. References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] C.L. Zha, R.K. Dumas, Y.Y. Fang, V. Bonanni, J. Nogués, J. Åkerman, Appl. Phys. Lett. 97 (2010) 182504.
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