Insights into Growth of Fe7Pd3 Ferromagnetic Shape Memory Alloy

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Insights into growth of FePd ferromagnetic shape memory alloy thin films A. J. Bischoff, K. Hua, and S. G. Mayr Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01641 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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Crystal Growth & Design

Insights into growth of Fe7Pd3 ferromagnetic shape memory alloy thin films A. J. Bischoff,∗,† K. Hua,† and S. G. Mayr∗,†,‡ †Leibniz Institute for Surface Modification, Permoserstr. 15, 04318 Leipzig, Germany. ‡Division of Surface Physics, University of Leipzig, Leipzig, Germany. E-mail: [email protected]; [email protected]

Abstract Single crystalline-like thin film samples of the ferromagnetic shape memory alloy Fe7 Pd3 enable to study the physics of this promising material system. We demonstrate how to grow such samples with electron beam evaporation and investigate the influence of variable deposition parameters focusing hereby on substrate temperature and deposition interruptions. We are able to produce Fe7 Pd3 thin film samples of 500 nm thickness in austenitic and martensitic phases and the latter with twinning structures visible at the sample surface. The ideal substrate temperature turns out to be around 700 ◦ C whereas deposition at temperatures below 690 ◦ C leads to sample demixing while film quality is decreased at higher temperatures because of lattice misfit effects. In case of direct substrate heating by heat radiation, comparable sample quality is accomplishable if numerous deposition interruptions are included in the growth procedure to homogenize surface temperature so that comparable effective material diffusion, i.e., material relaxation is achieved. Our investigations resulted in identifying a quite easily realizable deposition procedure to grow samples of satisfyingly good quality with closed surfaces and without need for subsequent processing.

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Introduction The ferromagnetic shape memory (FSM) alloy Fe7 Pd3 is a very promising candidate for miniaturized, contact-free actuation devices, e.g., it is a potential material for future integrated on-chip actuators in micro medical applications. 1–3 In view of potential usage in integrated devices, a low dimensional system of this material was further investigated as given by thin film samples. Samples are required to reside in one specific thermodynamic phase, in fct-martensite. In this phase, magnetic anisotropy energy and twin boundary mobility are sufficiently high to observe the FSM effect and thus reversible strains of several percent in an applied magnetic field. Defect-free single crystalline fct-structured thin films promise maximum yield strains at minimum magnetic field, as desirable in potential technical applications. In this work we will demonstrate that this aim is achieved with molecular beam epitaxy, under carefully controlled conditions. During this growth technique, atoms reach the sample surface with thermal energy in contrast to other already applied methods to produce Fe7 Pd3 thin films, such as pulsed laser deposition, 4 DC magnetron sputtering 5,6 or radio frequency magnetron sputtering. 7 The material system Fe7 Pd3 shows four phases of which three are martensitic. The higher temperature phase, fcc-austenite, can undergo a reversible transformation to fct-martensite. Then, the material features two long a-axes and one shorter c-axis, which is the hard axis of magnetization (c/afcc ≈ 0.94). Further phase transformations are non-thermoelastic and √ result in bct- (c/afcc ≈ 0.717) and bcc-martensite (c/afcc = 1/ 2 ≈ 0.707). 8,9 These two martensitic phases cannot exhibit a FSM effect and are therefore not desirable.

Experimental details Sample preparation of Fe7 Pd3 thin films was achieved with molecular beam epitaxy, as basically described previously. 10 The samples were grown from two independent rate-controlled electron beam evaporators (Fig. 1) with a total rate of 0.15 nm/s on MgO (100) substrates. 2


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Deposition took place at ultra-high vacuum conditions with a base pressure lower than 10−9 mbar and at temperatures above 665 ◦ C. Substrates were placed on a sample holder made of molybdenum which had a hole in its center and 3 possible substrate positions as marked with numbers in Fig. 2f. Temperatures measured above substrates were higher at the position above the hole compared to the 2 side positions due to direct thermal radiation and the relatively high transmittance of MgO of more than 80 % in the wavelength range of 2.5-10 µm. 11 Thus, at a set heater temperature of 900 ◦ C only about 845 ◦ C or 695 ◦ C were measured at positions 2 or 1 and 3, respectively. 12 These temperature measurements were performed with a thermocouple which was clamped on the holder at the position above the holder hole instead of a substrate or on a holder without recess on the side turned away from the heater, respectively. Within this work, temperatures given refer to temperatures at substrate positions unless otherwise specified.

sample heater sample holder with substrates

shutter

quartz crystal feedback system

quartz crystal feedback system

cryo-shield shutter

shutter

electron beam evaporator

electron beam evaporator

Figure 1: Sketch of the vacuum chamber in which deposition of Fe7 Pd3 samples took place. Obtaining the desired fct-martensitic phase at room temperature requires accurate adjustment of composition which is done with precise settings of the electron beam evaporators. 3



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Therefore the palladium content has to amount to ≈ 29.3-29.7 at% as determined for bulk samples. 8 To meet this relatively small window, both evaporators are tuned regularly based on sample thickness deviations of calibration thin films of pure Pd and Fe as determined with X-ray reflectivity (XRR) using a Seifert XRD 3003 PTS. Besides, slight phase adjustments are possible in between deposition runs. Internal stresses built up during deposition also influence the phase transformation temperatures as investigated previously. 13 The phase of each sample was determined with X-ray diffraction (XRD) measurements using a Seifert XRD 3003 PTS. In case of surface twinning structures the c/a-axis ratio was calculated additionally based on atomic force microscopy (AFM) data measured with a Veeco Dimension Icon. To categorize sample quality, the fraction of holes and groves per surface area was identified. This was done with image analysis on scanning electron microscopy (SEM) pictures taken with a Carl Zeiss Ultra 55 on which deep going surface structures are distinctly cognizable. As a result, five different quality categories are distinguished denominated as A1, A2, B, C, D. These categories represent the range of surface quality starting from samples with almost no holes (A1) through ones with few holes (A2), several holes and individual grooves (B) and quite many holes and grooves (C) up to samples with connected grooves showing strong remainders from island growth (D) (Fig. 2a-e).

Results and discussion When previously applying our deposition procedure, only a certain portion of films turned out to be of sufficient quality, while another portion would yield rough surfaces with individual island. To track down the physics behind this and reduce unsuccessfully synthesized films to a minimum, i.e., reduce size, depth and amount of grooves and holes on film surface, we investigated the influence of various variable parameters. The two main factors affected by this were substrate temperature and deposition interruptions. Modification of the deposition rate was not investigated because previously performed tests revealed demixing of thin films

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at higher and unstable deposition conditions due to accuracy and feedback loop of our crystal monitors at lower rates. Deposition interruptions are usually introduced for manual melting and sublimation of raw Pd and Fe material. A thorough manual material melting is already performed prior to deposition start but can become necessary again during the process. As deposition is conducted at low rates, only low powers of the electron beams are applied, resulting in changes in evaporation characteristics due to groove formation around the impact point of the electron beam in the course of time. If manual remelting is to be done and when it should happen is decided based on the power range of the quartz crystal feedback system of the electron beam evaporators (Fig. 1). This value slightly but persistently increases and decreases during deposition to guarantee stable deposition rates of both evaporated materials. By reducing the tolerance of this parameter, the average number of deposition interruptions for additional manual material melting increases as well as deposition conditions are stabilized. Until now, this power range tolerance was set to ± 7 % resulting in one or two additional material melting interruptions. We investigated the influence of both additional manual meting interruptions and simple deposition pauses without manual melting. In case of additional material melting, this was done while the sample holder was covered with a shutter and took about 2-5 minutes. Deposition interruptions - no matter if accompanied by additional material melting or not - have a strong influence on surface quality as indicated by the results summarized in Table 1. Comparing the first 3 runs during which the number of melting interruptions was increased from 0 to 4 due to decreasing power range tolerance, a significant increase in sample quality and a strong reduction of the surface height range between material mountains and vales was observed. However, samples grown with the same low power range tolerance but only two additional material melting interruptions showed clearly poorer quality (run 4). This indicates that quality improvement is not directly connected to additional material melting but rather to the number of deposition interruptions. Therefore, during growth run 5 only

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plain deposition interruptions were carried out after every 50 nm deposition lasting about 3 minutes each. This resulted in samples of equivalent quality compared to run 3 with 4 material melting interruptions. Interestingly, no effects on surface quality due to substrate positioning on the sample holder was observed for this deposition run. In general, samples grown at position 2 show noticeably poorer quality as underlined by quality and height range details of corresponding samples of run 2 and 3 (Table 1, marked with h). However, all samples of run 5 are of similar, good surface quality. The poorer quality of samples placed above the sample holder hole can be explained by temperature inhomogeneities on the sample surface during deposition due to the transmittance of MgO to heat radiation. Investigations of the transmittance of thin Fe films on MgO substrates at wavelengths of 2.2-5.5 µm revealed high transmittance of 95-98 % at a Fe film thickness of 1 nm and strongly reduced transmittance of 37-53 % for 4 nm thin films. 14 It can hence be assumed that Fe7 Pd3 material islands show considerably higher temperatures than neighboring yet uncovered substrate regions. At the surface of these material islands heat is lost by radiation emission while layers in substrate proximity are heated up strongly. This results in an inhomogenous temperature distribution throughout the material and relatively strong temperature gradients enhancing diffusive material movement from areas with little to no deposited material towards growing material islands. Furthermore the critical thickness, i.e., the film thickness at which material islands have coalesced increases with substrate temperature and amounts to 50 or 100 nm for 630 or 690 ◦ C, respectively. 10 Assuming this trend in critical thickness increase remains for higher temperatures, a critical thickness of up to 200 nm or more is expected for samples grown above the sample holder hole. Increased misfit effects further support the observed trend towards sample surfaces with deep going holes and groves, i.e., decreased smoothing effects. During the numerous deposition interruptions of run 5, surface temperatures are homogenized on samples placed above the holder hole and growth conditions become comparable to

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those of samples deposited at the side positions. However, constant deposition interruptions as performed for run 5 destabilized considerably deposition and resulted in noticeable fluctuations of deposition rates as deposition was shut down and powered up again frequently. Thus, this approach is not suitable for a common sample growth procedure. To improve sample quality above a hole in the sample holder, a heat absorbing layer should be placed on the backside of substrates. 15 To ensure deposition rates stay stable hereafter power range changes were again used as indication for deposition interruptions. Moreover, the relatively poor quality of the samples grown in run 4 cannot only be related to deposition interruptions as run 2 with also only two interruptions obviously resulted in samples of better surface quality. The relevant difference between these two runs is the timing of the last deposition interruption which was taken 130, 100 and 200 nm before the final thickness of 500 nm was reached for run 2, 3 and 4, respectively. In a next run, the last deposition interruption required because of a power range change of 3 % was introduced 170 nm before deposition end. Then after the growth process was finished, the sample holder was heated for another 45 min. With this post-heating, samples of relatively good quality were grown (run 6). Finally, as more interruptions towards the end of deposition seemingly support film healing, an additional material melting interruption 30 nm before deposition completion was added resulting in satisfactory sample quality (run 7). Therefore, all further samples were prepared with the same deposition conditions: material melting interruptions once the power range of the crystal feedback system changes by ± 3 %, an additional interruption 30 nm before deposition ends and post-deposition heating for 30 minutes. As sample surfaces show alternating hills and vales, we also reviewed an Asaro-TillerGrinfeld instability as a possible contributing effect. 16,17 Calculations of the threshold wavelength λ* for stability of a planar surface shape calculated based on 18 resulted in values smaller than 42 nm (assuming a Poisson ratio of ν ≈ 1/3, a Young’s modulus of E ≈ 120 GPa for Fe7 Pd3 , 19,20 a lattice misfit of 9 and 12 % and surface stiffnesses σ in the range of

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Table 1: Influence of deposition interruptions for material melting (NM ), interruptions with no extra material melting (NB ) and post-deposition heating of the sample holder for a certain time (tph ) on surface quality (Q) and the height range (∆h) of the samples as measured with AFM. For the latter, the height range of an overview AFM-image of 20 x 7 µm2 scanning size was chosen to represent the average height effects of holes and grooves as well as material pile-ups. Deposition interruptions were done based on the power range tolerance (PRT) of the feedback system of the electron beam evaporators or after a certain film thickness. Samples marked with h were grown on substrates above the sample holder hole at 845 ◦ C whereas all other samples were deposited at 695 ◦ C. No. 1 2 3 4 5 2h 3h 5h 6 7

Condition for interruption tph [min] no interruptions ± 5% PRT ± 3% PRT ± 3% PRT + 50 nm (3 min) ± 5% PRT ± 3% PRT + 50 nm (3 min) ± 3% PRT 45 ± 3% PRT, at 470 nm 45

NM 0 2 4 2 0 2 4 0 2 4

NB 0 2 4 2 9 2 4 9 2 4

Q C A2 A1 B A1 D C A2 A2 A1

∆h [nm] 619 190 110 439 133 600 525 171 206 170

up to 13 GPa nm as determined previously by in-situ substrate curvature measurements 10 ). An initially planar surface is unstable for modulations of wavelengths longer than this determined threshold wavelength. This means that in case of an Asaro-Tiller-Grinfeld instability surface modulations in the order of magnitude of several nm are expected and stable. Observed surface modulations of our samples are stable and show wavelengths of about 2 µm, hence, cannot be explained by this instability. In a next series of deposition runs we investigated the influence of temperature changes on surface quality. Substrate temperature is regulated by the settings of the sample holder heater but also by substrate positioning on the sample holder. After deposition, cooling of the samples can be slightly accelerated by extra N2 -cooling of the cryo-shield of the UHV chamber (Fig. 1). Contrarily, post-deposition heating of the sample holder is possible as already analyzed in runs 6 and 7. Besides, the dwell time between the moment when the set deposition temperature is reached and the starting point of actual deposition can be varied. So far, a minimal dwell time of 90 min was proposed to anneal the MgO substrates 9



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which tend to absorb CO2 and water while exposure to ambient air or long storage at UHV conditions. 21 The results of selected runs performed to investigate temperature effects are depicted in Table 2. Table 2: Occurrence and influence of demixing due to additional N2 -cooling (indicated with C) after post-heating (post-heating time tph = 30 min unless otherwise specified) as well as effects of varying dwell time after substrates have reached the set deposition temperature and increased deposition temperatures on surface quality (Q) and the height range (∆h) of the samples. Besides, details on substrate temperature (TS ), deposition interruptions for material melting (NM ) and determined c/a-ratios (in case of surface twins) are given. Samples marked with h were grown on substrates above the sample holder hole. No. 8 9 10 11 12 13 14 14 h

TS [◦ C] 695 695 665 665 695 695 725 875

Comment C 45 min, C C 0h dwell 5h dwell -

NM 3 4 3 3 3 4 4 4

Q A1 A1 A2 A2 A1 A2 A2 D

∆h [nm] 239 222 167 241 172 158 210 470

c/a 0.83 0.95 0.93 0.96 0.96 -

Structure fluffy islands chiseled chiseled islands islands islands islands

Within these temperature studies, the occurrence of α-Fe precipitates was further analyzed. They are usually observed at substrate temperatures below 690 ◦ C and cause different looking surface structures compared to previously grown samples with island-like structure due to Volmer-Weber growth of Fe7 Pd3 thin films (Fig. 2). 10 The precipitates are placed on the substrate surface but are also dispersed throughout the film. In case of relatively small intensities of α-Fe XRD-peaks (Fig. 3), the surface resembles a chiseled-looking surface with sharp, elevated Fe-precipitates whereas samples with higher α-Fe content are covered with fluffy-looking material pile-ups (Fig. 4a-d). In between and on top of these Fe-precipitations twinning structures can be observed if the composition of the sample is in accordance with fct-martensitic phase (Fig. 4b). Demixing of the thin films, thus, does not influence phase formation. Besides, Fe-precipitates were observed when slight temperature changes were enforced upon sample cooling at the end of deposition. At 695 ◦ C substrate temperature and after post-heating for 30 min, the cryo-shield of the UHV chamber was filled with liquid N2 10


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resulting in marginal acceleration of sample cooling and strong demixing, i.e., fluffy-looking samples (run 8). If the additional N2 -cooling was performed after 45 min post-heating only one of two samples showed demixing and a surface with both chiseled- and fluffy-looking areas while the other one exhibited an island-like structure and no precipitations (run 9). Demixing caused by fast cooling seems to be prevented in samples which had more time for diffusive material healing prior to cooling. At 665 ◦ C substrate temperature, additional N2 -cooling (run 11) did not increase Fe-precipitates, i.e., XRD signals of α-Fe, and samples showed equivalent quality and height ranges compared to a previous run at the same temperature (run 10). At substrate temperatures above 805 ◦ C, slight acceleration of sample cooling had no impact on sample structure and quality. This confirms the observation that the cooling effect has marginal effects and is relevant in a small temperature range around 695 ◦ C, i.e., around the critical deposition temperature between demixed and pure Fe7 Pd3 thin films. Besides, the two samples on the side substrate positions differ slightly in Fe content due to different positions in respect to the material pods for Fe and Pd. The sample with a higher atomic percentage of Fe showed for all runs which resulted in samples with demixing at least areas with a fluffy-looking structure as it would be expected. Furthermore, the annealing time of the MgO substrates, i.e., the dwell time between the moment when the set deposition time is reached and actual deposition start has no apparent effect on surface quality and height range of samples. Samples with no dwell time (run 12) were similar to those grown after a dwell time of 2 (as chosen for common growth procedures) or 5 hours (run 13) and surface twinning structures could be observed for all these runs in case of ideally chosen material composition. Finally, deposition at higher temperatures of 725 ◦ C at side positions and 875 ◦ C above the hole was investigate and resulted in closed surfaces but relatively big height ranges (run 14) and poor quality with numerous grooves and holes (run 14 h), respectively. As the critical thickness of samples increases with increasing substrate temperatures which effects

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MgO (200) 42.9°

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𝛼-Fe (220) 98.4° MgO (400) 94.0°

𝛼-Fe (110) 44.5° fct (200) 46.8° fcc (200) 48.3° 𝛼-Fe (200) 64.9° fct (220) 69.1° 665 °C, C

665 °C 695 °C

Figure 3: XRD measurement of Fe7 Pd3 thin films grown at 695 ◦ C within run 6 (bottom curve), at 665 ◦ C within run 10 with some Fe precipitates showing a chiseled-like structure (middle curve) and at 665 ◦ C with extra liquid N2 -cooling within run 11 showing relatively strong demixing resulting in a fluffy-looking surface structure (upper curve). The broadened fcc-fct peaks at 45-50◦ of the two bottom curves are observed due to diffraction on twinning dislocations. Analysis of the sample presented in the upper curve revealed no surface twins.

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b

10 !m

1 !m

c

d

1 !m

1 !m

Figure 4: SEM images of Fe7 Pd3 thin films with some Fe precipitates showing a chiseledlike structure (a, c) and with relatively strong demixing resulting in a fluffy-looking surface structure (b, d). Despite demixing, twinning structures can be observed on the surface of the samples as illustrated by image (b).

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bigger height ranges of the thin films, the ideal substrate temperature for growth of Fe7 Pd3 thin films seems to be around 700 ◦ C .

Conclusions In summary, we were able to grow Fe7 Pd3 thin films in austenitic and martensitic phase. Surface twinning structures were observed at all examined substrate temperatures ranging from 665 ◦ C to around 875 ◦ C regardless of varying deposition parameters and depended solely on material composition. The ideal substrate temperature is around 700 ◦ C while deposition at lower temperatures below 690 ◦ C results in demixing of the samples and at higher temperatures sample quality is reduced due to lattice misfit effects. Samples which were directly heated by heat radiation show decreased sample quality because of inhomogenous surface temperature distributions influencing diffusion. To achieve comparable sample quality provided these conditions, numerous deposition interruptions were needed to reach temperature homogenization, i.e., comparable effective material diffusion. The temperature range for optimal deposition is relatively narrow and already slight variations as introduced by additional N2 -cooling of the cryo-shield of the UHV-chamber after deposition stop can cause demixing and thus lower sample quality. Our investigations resulted in identifying a deposition procedure to grow samples of improved quality with closed surfaces quite easily and without need for additional annealing or other subsequent processing.

Acknowledgement This project was performed within the Research Internships in Science and Engineering (RISE) project of the German Academic Exchange Service (DAAD) and the Leipzig Graduate School for Natural Sciences BuildMoNa, established within the German Excellence Initiative by the German Science Foundation (DFG). Financial support by the DFG-Project "BIOSTRAIN" was gratefully acknowledged. 14


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(17) Grinfeld, M. A. Sov. Phys. Dokl. 1986, 31, 831. (18) Politi, P.; Grenet, G.; Marty, A.; Ponchet, A.; Villain, J. Phys. Rep. 2000, 324, 271. (19) Nakayama, T.; Kikuchi, M.; Fukamichi, K. J. Phys. F 1980, 10, 715. (20) Ma, Y.; Mayr, S. G. Acta Mater. 2013, 61, 6756. (21) Duriez, C.; Chapon, C.; Henry, C. R.; Rickard, J. M. Surf. Sci. 1990, 230, 123.

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For Table of Contents Use Only Insights into growth of Fe7 Pd3 ferromagnetic shape memory alloy thin films A. J. Bischoff1 , K. Hua1 and S. G. Mayr1,2 1

Leibniz Institute for Surface Modification, Permoserstr. 15, 04318 Leipzig, Germany.

2

Division of Surface Physics, University of Leipzig, Leipzig, Germany.

Improved deposition recipe

10 !m

10 !m

A newly optimized and reliably reproducible recipe for synthesis of Fe7 Pd3 ferromagnetic shape memory alloy thin films is developed. Sample quality of the grown single crystalline samples can be improved significantly by carefully choosing sample temperature and deposition interruptions. Thorough analysis of different deposition parameters allows for a better understanding of contributing effects.

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Insights into Growth of Fe7Pd3 Ferromagnetic Shape Memory Alloy

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