Enhancement of magnetic anisotropy for L10-(0 0 1) FePt films grown on SrTiO3 substrate

Chemical Physics Letters 654 (2016) 135–138

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Research paper

Enhancement of magnetic anisotropy for L10-(0 0 1) FePt films grown on SrTiO3 substrate A.M. Zhang a,b, X.S. Wu a,⇑, S.L. Tang a, S.M. Zhou c a

National Laboratory of Solid State Microstructures & Department of Physics, Nanjing University, Nanjing 210093, China College of Science, Hohai University, Nanjing 210098, China c Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology and School of Physics Science and Engineering, Tongji University, Shanghai 200092, China b

a r t i c l e

i n f o

Article history: Received 3 February 2016 Revised 29 April 2016 In final form 3 May 2016 Available online 4 May 2016 Keywords: Magnetic anisotropy Metals and alloys Magnetic force microscopy (MFM) Lattice strain

a b s t r a c t Ordered L10-FePt (0 0 1)-oriented films are deposited on (0 0 1) SrTiO3 by magnetic sputtering. The films are composed of a strained and a relaxed L10-FePt phase. The content of the relaxed phase, the magnetic anisotropy, as well as the perpendicular coercivity of the ordered L10-FePt films increase with increasing the growth temperature. The improvement of the magnetic properties is attributed to the relaxed phase content and the detailed structure variations. Our results show that high quality (0 0 1)-textured granular L10-FePt film may be well combined with the perovskite-like SrTiO3, which might open a roadmap in fabricating oxide-based devices. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction L10-FePt (0 0 1)-oriented films with high perpendicular magnetocrystalline anisotropy (PMA) have been studied extensively for their scientific interests and potential applications in the highdensity magnetic recording media [1–3]. The magnetic anisotropy energy Ku is predicted to reach a value of >108 erg/cc for fully ordering FePt film with (0 0 1) epitaxial orientation [4,5]. Much effort has been made to obtain ordered (0 0 1)-oriented FePt films at a relatively low temperature, which may be due to the interfacial reaction or diffusions [6–8]. Results have proved that tensile strain introduced by the mismatch between the film and substrate is beneficial to enhance the ordering of FePt film and (0 0 1)orientation growth [9]. Many substrates are used to accelerate the ordering of FePt film such as MgO [10,11], KTaO3 [12], SrTiO3 [12], LaAlO3 [12,13], and Si/SiO2 [14,15]. Lattice mismatch between substrate and FePt film may be expected to enhance the PMA Ku. The competition among the composition of FePt, the orientation degree and the strain state of the FePt film, etc., might decide the final magnetic properties of the FePt films. In many experiments, strain effect on the PMA is still an open question and no consistent conclusion has been obtained because of the complex structures of FePt films, such as dislocations, and defects. How to obtain a high value of Ku (>108 erg/cc) is still a challenge. The results reported by ⇑ Corresponding author. E-mail address: [email protected] (X.S. Wu). http://dx.doi.org/10.1016/j.cplett.2016.05.011 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

Hotta et al. have shown that there is no marked difference between PMA and strain [16]. Dong et al. showed that large tensile strain resulted in large Ku for FePt on MgO compared with that on SrTiO3 [12]. Other experiments also have proved that PMA of L10-FePt film decreases with increasing the substrate lattice parameter, such as silver [17] or platinum [18,19]. Therefore, the contradictory results make the effect of strain on PMA more confused by now. In theory, strain has been found to have great effect on PMA of ordered FePt. Burkert et al. [20] found that a strong influence on the PMA is expected from the modification of c/a, with 9% enhancement of PMA originated from 3% increase of c/a. Lukashev et al. [21] and Chepulskii et al. [22] reported that a small shrinkage (1.5% and 4%) of the in-plane lattice parameter would lead to a considerable increase of PMA (21% and 40–76%). The hybridization between the Pt 5d orbitals with large spin–orbit interaction and the highly spinpolarized Fe ones is the consensus for the strong PMA in FePt [20]. By increasing the c/a of ordered FePt, the PMA is enhanced due to the modification of the orbital moment anisotropy (OMA) of Fe through increasing the hybridization between Fe 3d and Pt 5d [23]. MgO (0 0 1) single crystal is the most popular substrate used to prepare (0 0 1)-oriented L10-FePt film [16]. However, the large mismatch (9.39%) between MgO and FePt will induce different microstructures such as defects and dislocation in FePt film, which makes the micro-mechanism of the strain effect on PMA still elusive. Therefore, it is urgent to clarify the effect of tetragonal lattice distortion on PMA in experiments and to find an effective way to enhance it. According the theoretical results, a substrate with

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o

400 C

76

o

500 C 76

Fig. 1 shows the XRD patterns of the films grown at 400 °C (FP400) and 500 °C (FP500). Peaks of (0 0 1), (0 0 2) and (0 0 3) indicate that both samples crystallize in L10 phase with (0 0 1)-oriented texture. It is interesting to find the splitting of the diffraction peaks of (0 0 2), and (0 0 3). In order to separate the two peaks, the reflections are fitted using Lorenz function (seen the inset of Fig. 1). The out-of-plane lattice parameters (along c-axis) corresponding to the two splitting reflections were calculated to be 3.75 Å and 3.69 Å for FP400, and 3.72 Å and 3.69 Å for FP500, respectively, which are all close to the value of the lattice parameter c (3.724 Å) of the bulk L10-FePt phase [25,26]. Because the lattice mismatch of the FePt films on STO substrate is smaller than that on MgO substrate, the fully-relaxed thickness of FePt on STO is 60 nm which is much larger than that on MgO of 20 nm [12]. The thickness of the FePt film

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2 θ (degree) Fig. 1. X-ray diffraction patterns for FePt films grown on SrTiO3 at 400 °C (a) and 500 °C (b). Insets of (a) and (b) expand the (0 0 3) reflection from the L10-FePt (0 0 1) films, which is analyzed by Lorenz function fitting.

in present study is checked to be 27 nm by X-ray reflectivity (XRR), in which strain may be partly relaxed. Therefore, relaxed (continuous film or granular film) and strained (middle states connected the continuous film and the granular film) phase may coexist in the films. The slightly larger lattice parameter along c-axis may correspond to the relaxed phase, or grain-shaped L10-FePt (0 0 1) phase, while the smaller one is of the phase in strain status. The percentage of the relaxed phase is 30% for FP400, and 84% for FP500, respectively, which is evaluated from the integral intensity of the (0 0 3) reflections (inset of Fig. 1). Grazing incident X-ray diffraction (GIXRD) tells the depth dependence of the in-plane lattice parameters, which is shown in Fig. 2. The average in-plane lattice parameters of a of the film are 3.866 Å for FP400, and 3.857 Å for FP500, respectively, which is consistent with the out-of-plan lattice parameter c. The ordering parameter S is defined as following [30]:

S2 ¼

1  ðc=aÞ ; 1  ðc=aÞSf

where ðc=aÞSf is the axial ratio for the fully ordered phase which is determined to be 0.961. Considering the coexistence of the strained

interface

3.866

In-plane lattice parameter a( Å)

3. Results and discussions

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(a)

2. Experimental details Sixteen periodic Fe/Pt bilayers, i.e., [Fe (1.2 nm)/Pt (0.8 nm)]16 multilayers, are grown on (0 0 1) STO by magnetron sputtering and in-situ annealed at the substrate temperature up to higher than 300 °C, after calibrating the growth temperature. The base pressure of deposition chamber is better than 5
105 Pa. Before sample preparation, growth chamber is washed several times by argon, which is used as the sputtering atmosphere (with the Ar pressure of 0.55 Pa) during the films growth. The sputtering powers are set to be 30 W for Fe, and 15 W for Pt, individually, after careful calibrating the rate of growth. The stoichiometric Fe/Pt ratio of the films is checked using X-ray Photoelectron Spectroscopy (XPS) to be nearly 1/1. Structure of the films is performed on Beamline BL14B1 at Shanghai Synchrotron Radiation Facility (SSRF). The surface morphology and the magnetic domain structures are observed on Atomic Force Microscope (AFM), and Magnetic Force Microscope (MFM) using a commercial scanning probe microscope (Digital Instruments, NanoScope V, Veeco). The MFM data is detected using silicon cantilevers with tips magnetized along the tip axis, which was perpendicular to the specimen surface. MFM senses the vertical component of the derivative of force between film and tip. The image signal is detected as the phase shift of an oscillating cantilever. The tips are coated with Co/Cr films with the normal resonance frequency of 78 Hz and spring constant of 2.8 N/m (MESP). The coating produces a coercivity of about 400 Oe. Magnetic properties of the films are measured at room temperature by vibrating sample magnetometry (VSM) and superconductor quantum interference device (SQUID).

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comparatively smaller lattice parameter should be favorable for improving the film quality and enhancing the PMA. Meanwhile, L10-FePt film with high quality is necessary to single out the strain effect on PMA. Cubic SrTiO3 (STO) (a = 3.905 Å) is a well-used substrate in fabricating perovskite-like materials, such as high-temperature superconducting cuprates [24], colossal magnetoresistence manganites [25,26], and ferroelectric materials [27]. Materials well grown on STO may link their applications in the special oxides-based devices. Small lattice mismatch between SrTiO3 (STO) and the L10-FePt (only 1.38%) may results in the (0 0 1)-oriented L10-FePt films well grow on the (0 0 1) STO substrates at relatively low temperature of 350–380 °C [11,12,28]. The well (0 0 1)-oriented film has the high Ku of about 1.5
107 erg/cm3. Even so, the value of Ku is lower than the desired value for completely ordering FePt (>108 erg/cm3) [4,5]. Here we grew (0 0 1)-oriented L10-FePt films on (0 0 1) STO using multilayers growing technique at substrate temperature higher than 300 °C, expecting to enhance the PMA.

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A.M. Zhang et al. / Chemical Physics Letters 654 (2016) 135–138

Intensity (CPS)

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3.864 3.862 3.860 3.858 3.856 o

400 C o 500 C

3.854 3.852 3.850

0

50

100 150 200 250 300 350

penetration depth (Å) Fig. 2. Depth dependence of the in-plane parameter of a for L10-FePt films grown at 400 °C and 500 °C is shown.

A.M. Zhang et al. / Chemical Physics Letters 654 (2016) 135–138

and relaxed phase in L10-FePt film, the average values of lattice parameters c are 3.708 Å and 3.715 Å for FP400 and FP500, respectively. The order parameters S are calculated to be 1.02 and 0.97, respectively, which indicates that FP400 and FP500 are fully chemical ordering. The tetragonality ratios (TR) of c/a are 0.959 and 0.963 for FP400 and FP500, respectively. Theoretical studies have shown that the increase of TR will modify the orbital moment anisotropy (OMA) of Fe through increasing the hybridization between Fe 3d and Pt 5d and enhance the PMA [20]. Magnetic hysteresis loops of FP400 and FP500 are shown in Fig. 3. The magnetic fields are applied in perpendicular and parallel to the film plane. Coercivity (Hc), saturated magnetic Moment (Ms) and magnetic squareness (Mr/Ms) (MS) are obtained. The out-ofplane coercivity of FP500 is about 20 kOe, which is about 8 times larger than that of FP400 (about 2.5 kOe). The magnetization curves show the Mr/Ms values are 0.95, 0.96 for FP400 and FP500, respectively, which indicate that the films exhibit strong PMA with the magnetic easy axis perpendicular to the film plane, consisting with the (0 0 1) texture L10-FePt films [11]. Because of the extremely high anisotropy field Hk of over 100 kOe, the accurate value is difficult to obtain by the conventional methods [28]. The magnetization does not saturate due to the limitation of the magnetic field of 6T in our case. The anisotropy field Hk may be obtained by fitting the in-plane and out-of-plane magnetization curves at high field and extrapolating the straight lines. The error bar in Hk is estimated to be about 10% based on the fitting process. The anisotropy (Ku) value is estimated from the hysteresis loop using formula: K u ¼ HK M S =2 þ 2pM 2S [29], where Hk is the anisotropy field and Ms is the saturated moment. The demagnetization energy could be negligible because it is two orders of magnitude less 2p than that of the first term. The Ku is about 1.1
108 ergs/cc for FP400, while it increases drastically to be 6.1
108 ergs/cc for FP500. Our previous results have reported that the PMA values of L10-FePt films deposited on MgO (0 0 1) substrate are 4.5
107 ergs/cc and 7.9
107 ergs/cc for FePt films grown at substrate temperature of 400 °C and 500 °C, respectively [31]. The PMA value for FePt grown on STO substrate is therefore almost one order of magnitude higher than that on MgO substrate. Our results are well consistent with the prediction from the theoretical calculations [20–22]. The increased PMA of L10-FePt film on STO proves the theoretical prediction that larger TR value is favorable for enhancing PMA of L10-FePt film by changing the orbital moment anisotropy

M (emu/cm3)

M 2S

1200 800 400 0 -400 -800 -1200

Out-of-plane In-plane o

400 C

-20k

-10k

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10k

20k

Magnetic field (Oe) 1200 800 400 0 -400 -800 -1200

o

500 C -60.0k -30.0k

0.0

30.0k

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Magnetic field (Oe) Fig. 3. Hysteresis loops for FePt films grown at 400 °C, and 500 °C, respectively, with magnetic field applied in perpendicular and parallel direction to the film plane.

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(OMA) of Fe through increasing the hybridization between Fe 3d and Pt 5d [21]. The magnetic anisotropy is well known mainly controlled by the magnetocrystalline anisotropy. The magneto-elastic anisotropy due to magnetostriction effect may also sometimes affect the magnetic anisotropy as well. Based on the calculation method of magneto-elastic anisotropy [32,33], the magnetic anisotropy from magnetostriction effect Kr can be given by K r ¼ 3kr r=2 [32], where r is the stress due to difference of thermal expansion and that from the misfit of FePt film and substrate and kr is the magnetostriction coefficient of FePt. The stress from the difference of thermal expansion coefficients between the film and substrate may be calculated using the formula: rT ¼ DaDTE=ð1  lÞ, where Da is the thermal expansion coefficient difference between FePt and STO substrate (10.5
106 K1 and 9
106 K1, respectively), DT is the temperature difference between grown temperature and room temperature, and l is the Poission’s ration (0.33). The other component from the elastic strain due to the mismatch between the FePt and the STO substrate may be estimated using the formula ri ¼ Y e, where Y is Young’s modulus of FePt thin film (180 GPa) and e is the elastic strain of FePt with e = (a  a0)/a0. In our present work, the calculated rT values are 160 MPa and 201 MPa, and ri values are 654 MPa and 233 MPa, for films grown at 400 °C and 500 °C, individually. The stress r values are 494 MPa and 32 MPa for L10-FePt films grown at 400 °C and 500 °C, respectively. The magnetostriction coefficient kr of FePt has not been measured by now. If the estimated value (50
106) based on that of FePd alloy is used [32], the calculated Kr values are about 7:4
105 and 0:24
105 ergs/cc for L10-FePt films grown at 400 °C and 500 °C, which are three orders of magnitude less than those of the experimental ones (108 ergs/ cc). Therefore, the magnetostriction effect is not sufficiently important for explaining this perpendicular anisotropy. The large perpendicular anisotropy of L10-FePt is mainly ascribed to the large spin–orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states. In addition, although the strained and relaxed states in films may induce different Ku values, there is no kink in the loop of FP400. For FP500, however, a slight kink behavior around H = 0 Oe is found as reported in FePt films with grain structure in previous studies [33,34]. These results may be related to the different morphology of the films grown at different temperatures. Surface morphology and the distribution of magnetic domain on the surface are observed by AFM and MFM, respectively, which are shown in Fig. 4. The surface structure for FP400 is continuous with the average roughness of 3.05 nm, while that for FP500 forms particles with clear boundary with average roughness of 12.8 nm, and grains are corresponding to the magnetic domain structures. Obvious perpendicular magnetic anisotropy is observed from the perpendicular magnetic structure by MFM. The perpendicular magnetic domain of FP500 is obviously improved in comparison with that of FP400. The magnetic domain of FP500 shows granular structure with size of about 300 nm, corresponding to the critical single domain size of L10-FePt alloy (350 nm). Compared with that of the FP400 film, the coercivity of FP500 is drastically increased by the combination of the granular distribution and the large Ku. There may be magnetostatic coupling and/or exchange coupling between domains during rotation for FP400 with continuous structure, which could remove the kink effect in the loop. For FP500, the broken symmetry on the surface of grain induces the slight kink in the loop around H = 0 Oe [34]. The lattice mismatch between L10-FePt and STO (0 0 1) substrate (1.38%) is much smaller than that between L10-FePt and MgO of 9.39%, which is suggested to explain the enhancement in the PMA because of the increase of TR value. The shape anisotropy is negligible because of the high

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BK20140839), and the Fundamental Research Funds for the Central Universities (2015B19714). The authors thank beam line BL14B1 (Shanghai Synchrotron Radiation Facility). References

Fig. 4. AFM ((a) and (c)) and MFM ((b) and (d)) images for L10-FePt films on STO grown at 400 °C and 500 °C, respectively. The (a) and (b) are AFM and MFM images from FP400, and the (c) and (d) are AFM and MFM images from FP500, respectively.

magnetocrystalline anisotropy for L10-FePt with fct structure [22]. Therefore, L10-FePt particles have improved (0 0 1) texture and low dislocation density due to the strain relaxation on STO substrate with little mismatch ratio. 4. Conclusions (0 0 1)-oriented L10-FePt films grown on (0 0 1) SrTiO3 substrates at high temperature may well crystalize in (0 0 1) texture and high PMA. Relaxed and strained L10-FePt phases coexist in the films. The concentration of the relaxed phase in L10-FePt film is 30% and 84% for FP400 and FP500 respectively. The PMA Ku increases from 1.1
108 ergs/cc to 6.1
108 ergs/cc, and the perpendicular coercivity increase from 2.5 kOe to 20 kOe, with increasing the grown temperature form 400 to 500 °C. The improvement of the magnetic properties may be attributed to the increase of tetragonal ratio c/a for film of FP500. Our results prove that STO substrate with suitable in-plane lattice parameter is advantageous in increasing the PMA. These results may be helpful to investigate multifunctional heterostructures containing L10-FePt and perovskite materials. Acknowledgements The financial support for this project was from the National Natural Science Foundation of China (Grant Nos. 11404091 and U1332205), Natural Science Foundation of Jiangsu (Grant No.

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