Magnetostriction and Compositional Gradients in BoatEvaporated PermalloyFilmsT. C. Penn and F. G. West Citation: Journal of Applied Physics 38, 2060 (1967); doi: 10.1063/1.1709829 View online: http://dx.doi.org/10.1063/1.1709829 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/38/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetostriction characteristics of ultrathin permalloy films Appl. Phys. Lett. 68, 2885 (1996); 10.1063/1.116320 The saturation magnetostriction of permalloy films J. Appl. Phys. 52, 2474 (1981); 10.1063/1.328971 Composition Gradients in Ni–Fe Alloy Films Produced by Vapor Deposition from a Tungsten Boat J. Vac. Sci. Technol. 7, 573 (1970); 10.1116/1.1315879 Departure from Free Molecular Flow During the Boat Evaporation of Permalloy J. Vac. Sci. Technol. 7, 347 (1970); 10.1116/1.1315855 Stress Effects in Evaporated Permalloy Films J. Appl. Phys. 35, 828 (1964); 10.1063/1.1713494

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2060 H. J. BORCHARDT AND P. E. BIERSTEDT

in Fig. 1 gave particular difficulties as did DY2(Mo04) 3.

In all cases, however, where a boule was grown, it was found to have ferroelectric domain patterns as in Fig. 2, and it exhibited a ferroelectric hysteresis loop.

The ferroelectric properties listed in Table I show relatively little variation from material to material. It is perhaps surprising that even the 15% substitution of W for Mo has as little effect as it does.

Nassau, Levinstein, and Loiacona4 have reported on rare-earth molybdates but did not note ferroelectricity. Also there are apparent discrepancies between our findings and theirs. They find the room-temperature form of Sm, Eu, and Gd molybdates isotypic and of unknown symmetry but the Tb and Dy molybdates of another, tetragonal, form. We find these materials all to be isotypic, probably orthorhombic but definitely not tetragonal. Bernstein6 has reported Gd2(Mo04)s to be tetragonal and our findings conflict with this also.

In attempting to reconcile these results, we note that Nassau et al.4 show the molybdates of Pr, Nd, Sm, Eu, Gd, Tb, and Dy to be isotypic at lO00°C with a tetragonal unit cell and that the molybdates of Pr through Gd undergo a phase transformation on cooling.

, J. L. Bernstein, Z. Krist. 122, 315 (1965).

JOURNAL OF APPLIED PHYSICS

It is possible that quenching from 1000°C leaves only the molybdates of Sm through Dy isotypic. To com­plete the reconciliation requires that the tetragonal assignment by Nassau et al. as well as Bernstein is in error. It should be noted here that the x-ray diffraction patterns look very much like ones produced by a tetra­gonal unit cell and that the nontetragonal nature is indicated by rather subtle differences in the intensity distribution, hence an error can be easily made.

If the above is correct we have a rather interesting situation with Gd2(Mo04)s in that a readily reversible phase transformation, namely, the ferroelectric tran­sition at the Curie temperature, occurs in a material that is unstable with respect to another structure. This seems unusual, but it is not a self-contradictory finding since the lattice mobility required for the ferroelectric transition can be far less than that required for the transition at '""-'750°C reported by Nassau et at.

ACKNOWLEDGMENTS

Dr. C. T. Prewitt and F. J. Baum kindly aided with x-ray and optical crystallography. We thank Dr. F. J. Darnell for helpful discussions and comments on the manuscript.

VOLUME 38, NUMBER 5 APRIL 1967

Magnetostriction and Compositional Gradients in Boat-Evaporated Permalloy Films

T. C. PENN AND F. G. WEST

Texas Instruments Incorporated, Dallas, Texas

(Received 14 October 1966)

Magnetostriction measurements on Permalloy films prepared by vacuum evaporation from a tungsten boat indicate an average Fe enrichment of about 2% above the original melt composition. In addition it is shown that the film composition varies continuously through the thickness from 4% to 7% per 1000 A for typical evaporation conditions. Compositional gradients of even smaller magnitude are capable of pinning spin waves. Since filament and boat evaporation are used widely in preparing laboratory films, the anomalous spin-wave results frequently reported in the literature may be due to thickness-compositional gradients.

INTRODUCTION

FOR exploratory purposes Permalloy films having low dispersion (iX90= 0.3 to 0.6 deg) have been made

in this laboratory by evaporation from tungsten boats. The technique is a useful expedient because high deposition rates (100 to 200 A/sec) are easily obtained. The requirements on attainable vacuum are, therefore, somewhat eased. The work reported here was initially performed in order to determine the relative film com­position compared to the original NiFe melt in support of other experiments in progress. Relative composition was to be determined by magnetostriction measure­ments. While performing the measurements, variations in the film thickness were noted to affect the strain sensitivity. This weakened our confidence in using the strain coefficient of anisotropy 'YJ as a measure of film composition. A shutter was installed in the vacuum

system to obtain samples at various times in the course of a typical evaporation. A significant change in vapor composition with time was observed. This was followed by film-etching experiments to insure that the film­thickness compositional gradient was similar to that observed during the shutter-sampling experiments. U sing the conventional evaporation technique described, a variation in composition through the film thickness of from 4% to 7% was observed in 1000 A films near zero-average magnetostriction.

Fe ENRICHMENT IN EVAPORATED NiFe FILMS

Figure 1 illustrates the variation of the strain co­efficient of anisotropy'YJ vs melt composition from 81 % to 87% Ni in 0.5% increments for NiFe films prepared and measured in our laboratory. These data represent measurements on approximately one hundred and sixty

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1\1 A G NET 0 S 'r R t C T tON AND COM P 0 SIT ION t N PER MAL LOY F t L M S 2061

films, each datum point being the average for all the films (usually four or six) of a particular deposition. When 'T/ is converted to saturation magnetostriction A., the data agree reasonably wen with bulk data if an Fe enrichment of from 1.5% to 2% is assumed from melt to film.l,2 Specifically, the data of Fig. 1 suggest about 1.5% to 2% (based on the 'T/=O point)3 iron en­richment in the average composition of films in which there is a compositional gradient through the thickness. The points shown as triangles in Fig. 1 were obtained by means of radio-frequency ferromagnetic resonance.! Conventionally,

(1)

!lHk being the incremental change in anisotropy field for an increment of strain !It applied perpendicular or parallel to the easy axis (in the plane of the film). The points shown as circles were obtained by means of magnetoresistance2 measurements with strain being applied in the plane of the film at 1f/4 rad with re­spect to the easy axis. For this method it is shown in Ref. 2 that TJ is given by

TJ = 2!::J.H de/ !le, (2)

where !lH de is the incremental field applied perpendic­ular to the easy axis (in the plane of the film) required to realign the easy axis after the film has been strained by an amount !le (in the plane of the film) at an angle of 1f /4 with respect to the easy axis.

Others have shown both theoretically4 and experi­mentally5 that the magnitude of TJ obtained by this strain geometry should be of the order of 20% lower than that yielded by strain perpendicular or parallel to the easy axis. As it was impossible to perform measure­ments on the same film using both the resonance and magnetoresistance methods because of the different

28-

." 0

-I

-3

81 82 83 84 85 86 87

WT % fii IN MELT

FIG. 1. Strain coefficient of anisotropy for different melt compositions as measured by resonance A, and magnetoresistance 0; films approximately 1000 A thick.

1 T. C. Penn, Rev. Sci. Instr. 37, 1134 (1966). 2 T. C. Penn and F. G. West, Rev. Sci. Instr. 37, 1137 (1966). 3 In Refs. 1 and 2 the authors quoted an Fe enrichment of 4%

based on the curves in R. M. Bozorth's Ferromagnetism (D. Van Nostrand Company, Princeton, New Jersey, 1951), pp. 667-669. A check of the original data in Ref. 11 shows that )..=0 occurs at about 82.5% Ni for alloys formed from electrolytic Fe and Ni.

4 N. Goldberg, J. Appl. Phys. 36, 966 (1965). liT. R. Long, J. App!. Phys. 37,1470 (1966).

requirements on film-substrate geometry, six films were deposited simultaneously with three films oriented for each method. The three-film average of 'T/ for the magnetoresistive method was only 7.5% lower than that for the resonance method.

Although the method of deposition was conventional, the details are presented here because of their influence on both the Fe enrichment and the compositional gradient through the thickness of the film. The 2S0-mg melt was cut from a 99.99% NiFe-alloy strip and placed at the center of a flat tungsten boat measuring 7. 7X 1.9XO.OSS cm which was heated in vacuum just enough to wet the boat. Subsequently, the melt (which covered approximately one quarter of the boat surface) was placed downward with the boat located 30 cm above the substrate holder. A piezoelectric crystal­thickness monitor was located near the substrates. The substrates were soft glass. No shutter was used between the boat and substrates. The conditions just prior to applying boat power were a pressure of 1{)-6 to 1{)-7 Torr, substrate temperature of about 275°C and a uniform static magnetic field in the plane of the sub­strates of 100 De. The evaporation was performed to "completion"; that is, the boat power source was turned off when the strip chart recorded output of the thickness monitor became constant. The melt adhering or alloying with the boat amounted to 25% to 30% of the original melt for films of 800 to 1100 A. An evaporation time of approximately 10 sec resulted. Substrates were masked for a film size of approximately 2.5 X 13.5 mm.

Blois" deposited films from large melts in alumina crucibles using rf induction heating at 10 000 A/min and observed a film enrichment of about 2% Fe. Smith7 used essentially the same technique and also observed Fe enrichment of 2%. Moore and Young,S using evaporation methods similar to Blois, state they observed 2% Fe enrichment depending on the rate of evaporation (not specified). Prosen et al.,s using an unspecified melt size in a Morganite crucible with resistance heating at an evaporation rate of 1000 A/min, observed an Fe enrichment of 4%. Tolman et a[.1o pre­pared films by wrapping Ni-Fe wire about a tungsten filament and observed a 3% Fe enrichment of the films . It should be mentioned that where Fe enrichment is measured by magnetostriction methods, published bulk datal! for TJ =0 varies in composition from 81 % to 82.5% Ni.

From these experimental observations it appears that larger melts which change composition very little during deposition tend to give films which are Fe enriched approximately 2%. The amount of Fe enrichment to be expected from melts which change composition (as

6 M. S. Blois, Jr., J. App!. Phys. 26, 975 (1955). 7 D. O. Smith, J. Appl. Phys. 30, 264S (1959). 8 A. C. Moore and A. S. Young, J. App!. Phys. 31, 279S (1960). 9 R. J. Prosen, J. O. Holmen, B. E. Gran, and T. J. Cebula,

J. App!. Phys. 33, 1150 (1962). 10 C. H. Tolman, P. E. Oberg, and S. M. Rubens, Rev. Sci.

lnstr. 35, 738 (1964). llA. Schulze, Z. Physik. 50,448 (1928).

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2062 T. C. PEN~ A~D F. G. WEST

ours did) during deposition is affected by the melt size, evaporation rate and to some extent the boat geometry. All of these factors affect the amount of Ni-rich melt which remains on the boat after an evaporation.

We also measured films prepared by the Blois technique with the addition of a shutter to allow vapor composition to stabilize before exposing the substrates and observed a 0.5% to 1 % Fe enrichment. On the other hand, sputtered films were found to be of the same composition as the anode. No films were prepared by the "flash evaporation" method described by Harris and Siegel,t2

COMPOSITIONAL GRADIENT THROUGH THE FILM THICKNESS

In alloy films there have been several observations concerning thickness-compositional gradients. Tolman et al.lO reported observing the magnetostriction of NiFe films varying from positive to negative during deposi­tion yielding films having zero-average magneto­striction. No magnitudes were mentioned. This observa­tion is closely analogous to that which we observe and appears to be a continuous composition change through the film thickness rather than a lamella, strata, or layer structure. Prosen et al.9 report a substrate stratum of aFe203 and NiFe204 on films deposited relatively slowly (1000 A/min) in moderate vacuum (10-5 to 10-6 Torr) . Alloying of two metal films successively deposited was observed by Belser!3 when films of approximately 1000 A were heated to near their recrystallization temperature. Alloys of Ni-Fe were not included. More recently Morrison et al,t4 have deposited successive Ni and Fe layers and observed "spontaneous" alloying provided that the layers were less than about 30 A thick.

To sample the vapor at various times in the course of a normal. deposition, a shutter was installed in the vacuum system to selectively expose two substrates ,on our holder while two other substrates were exposed continuously as control films. The thickness recording was used to choose the actuation time of the solenoid­operated shutter. The data of Fig. 2 are the result of seven different evaporations made from a melt composi­tion of 84% Ni-16% Fe. All of these runs had control films with TJ approximately zero. Comparing TJ with the composition values of Fig. 1 it appears that the vapor composition varies approximately 6% from substrate to film surface. It will be noted that the composition variation is approximately linear and that the 200-A sample from the middle of the run had zero-average magnetostriction.

Since there is the possibility that the film composi­tional gradient will be different from that of the vapor due to the annealing and alloying effects previously referenced, films prepared in the normal manner were

12 L. Harris and B. M. Siegel, J. App!. Phys. 19, 739 (1948). 13 R. B. Belser, J. App!. Phys. 31, 562 (1960). 14 R. D. Morrison, M. M. Hanson, and P. E. Oberg, J. App!.

Phys. 37, 1476 (1966).

etched and measured for changes in magnetostriction with thickness. Initial attempts to etch uniformly through the entire film thickness failed for several etches, various concentrations, temperatures, agitation and handling techniques. It was finally possible to etch most films uniformly through about half the film thickness by an electropolishing technique. The patent etch MirroFe was used (without an oxidizing agent) with deionized water at 1 part to 75, respectively. Etching was performed at room temperature with continuous agitation provided by a magnetic stirrer. Leads were soldered to each end of the film strip with In solder, and the solder beads were covered with wax. The film leads were connected in parallel to the positive terminal of a constant current source of 500 p.A re­sulting in a current density at the film surface of ap­proximately 1.48 A/cm2. The cathode was gold foil immersed in the etch bath approximately 5 cm from the film. After each etch the film resistance and mag­netostriction were measured, and a visual inspection for pinholes, cloudiness or discoloration was made.

After observing the variation of TJ during etch for normal films, several experiments were performed to insure this gradient was not due to unknown factors occuring during the etch. The shape and character of the low-frequency resonance-absorption peaks! give some indication of the magnetic quality of the film. For some films the resonant absorption was observed im­mediately after etching without noticing changes other than amplitude (because of decreased film thickness) and peak separation (which may be due to either or both the change in anisotropy field Hk or magnetization ripple15). In addition, films which should show different compositional gradients than films evaporated from a boat were etched. Films prepared by the Blois technique, except for shutter control, showed less than 0.3 X 104 change in TJ per 1000 A during etch. Sputtered films showed no measurable change in TJ during etch. Doubling the melt weight while using a shutter to control the thickness in our normal evaporation procedure produced 1000-A-thick films which showed half the normal variation of TJ during etch.

... 2 ~" ,

I ... ... .... ... ,

x 104 "- , 7] 0

.... .... ... , -I ....

-2 ... --=......, ...

-3 o 0,2 0.4 0,6 0,8 1.0

FRACTIONAL FILM THICKNESS

FIG. 2. Strain coefficient of anisotropy of film segments obtained by shuttered samples during the course of a typical evaporation of a 1000-A film; measurement by magnetoresistance.

16 E. Feldtkeller, Z. Physik 176,510 (1963).

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MAG NET 0 S T RIC T ION AND COM P 0 SIT ION I N PER MAL LOY F I L M S 2063

Figure 3 compares the experimental observations of etched films with the average from the shutter experi­ments. The abscissa is the ratio of the initial resistance to the etched resistance. Optical-thickness measure­ments by multiple-beam interference were compared to the final etched film resistance. Not all of the films were measured optically, but those which agreed to within S% of the thickness predicted by resistance measure­ments are separated as solid circles. It can be seen that the data scatter is no less for these supposedly more uniform etches than the other samples. In general all films within a particular evaporation tend to follow smooth, similar curves. This negates arguments of non­uniform etch to explain the lower curves. For example, the etch concentration and rate would be expected to cause different pinhole and edge effects. However, as Fig. 4 illustrates, films evaporated simultaneously but etched in solutions of half, normal, and double con­centration showed the same behavior. Substrate tem­perature seemed of no importance as evidenced by the data shown in Fig. S which shows the results of etching

1.4 x 104

1.2

1.0

0.8

'fJ 0.6

0.4

0.2

0.9 0.8 0.7 0.6 0.5

R (INITIAL) / R (ETCH)

FIG. 3. Change of strain coefficient of anisotropy vs relative change of film resistance during etch with dotted line showing shuttered results. Initial film thickness was approximately 1000 A.

three films prepared simultaneously on substrates at approximately 30°C. Films having an average 71 which is initially nonzero (prepared from 82% and 86% melts) show the same A71 vs thickness during etch as do the films for which 71=0. We therefore conclude from the scatter of the data in Fig. 3 that the composition gradient in the thickness direction probably varies from run to run. This result seems reasonable from an intuitive standpoint in view of the unpredictable way the melt will boil and migrate on the boat during the relatively short evaporation time.

One result which was observed but not explained was the etching behavior of films stored in a dry box from six months to a year. These films etched uniformly to a much thinner thickness than films fresh from the evaporator. These films also showed a A71 vs thickness which agrees quite closely with the shutter experimental data.

1.0

0.8 'T)

0.6

0.4

0.2

o ~~--~--~ __ -L __ -L __ ~ __ ~ __ L-__ L-~

0.9 0.8 0.7 0.6 0.5

R (INITIAL) / R (ETCH)

FIG. 4. Change of strain coefficient of anisotropy vs relative change of film resistance during etch for half e, normal. and double ... concentration.

CONCLUSIONS

When Permalloys having compositions in the region of zero magnetostriction are evaporated from tungsten boats, the resulting films are magnetically similar to those deposited by other techniques. The strain co­efficient 71 per percent change in melt composition agrees closely with that expected from the bulk alloy. However, films WOO-A thick, evaporated at a rate of 100 A/sec from a 2S0-mg melt, have a compositional charge through the thickness of from four to seven per­cent. If the thickness and/or the initial weight of the change on the boat varies, so will the compositional change. Similarly, deposition rate and amount of charge remaining on the boat have an effect. Reproducibility requires taking all of these factors into consideration.

Spin-pinning mechanism have been widely discussed to explain the spin-wave spectra observed in ferro­magnetic films. Some have presumed that the pinning is due primarily to surface effects and have prepared films with surface treatments (such as etching, scratch-. ing or oxidizing) to show spin-pinning.16 It has been shown theoretically that a gradient in magnetization M through the film thickness is sufficient to cause spin­pinning.17 •18 For films 1000 A thick, Portis17 estimates that a 10% change in M distributed parabolically

1.2 ,104

1.0

0.8

0.6 'fJ

0.4

0.2

R (INITIAL) / R (ETCH)

FIG. 5. Change of strain coefficient of anisotropy vs relative change of film resistance during etch for films prepared on sub­strates at 30°C.

16 C. W. Searle, A. H. Morrish, and R. J. Prosen, Physica 29, 1219 (1963).

17 A. M. Portis, Appl. Phys. Letters 2, 69 (1963). 18 E. Schlomann, J. Appl. Phys. 36, 1193 (1965).

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2064 T. C. PENN AND F. G. WEST

through the thickness could explain some of the ob­served spin spectra. Using the linear thickness gradient in M proposed by SchlOmann18 we calculate that a 5% change in M should be adequate to cause pinning in 1Ooo-A-thick films. A change in M of 5% to 10% cor­responds to a film composition change of only 1 % to 2% based on the bulk values of M for Permalloy. It is evident that films prepared by evaporating a NiFe alloy to "completion" from a boat or wire will possess more than adquate gradient in M to cause spin-pinning. The results of Nissenoff and Terhune19 are fairly typical for films which were not deliberately surface contaminated. We speculate, as did SchlOmann, that the observed pinning may be due to a linear gradient in M; but, further that the gradient in M could be caused by the deposition process in the case of alloys. Spin-wave resonances have been reported recently20 in films which remained in the vacuum system after deposition to prevent spurious surface effects. These films were pre­pared by filament evaporation (although it was referred to as "flash" evaporation) and could be expected to possess a gradient in composition through their thickness.

19 M. Nisenoff and R. W. Terhune, J. App!. Phys. 35, 806 (1964).

20 G. 1. Lykken, W. L. Harman, and E. N. Mitchell, J. App!. Phys. 37,3353 (1966).

JOURNAL OF APPLIED PHYSICS

Rapid evaporation from a filament is frequently labeled "flash" evaporation because of the agreement of the average film composition with the original melt composition even though a composition gradient prob­ably exists. In true flash evaporation12 where small granules of an alloy are dropped on a hot surface well above the melting point of both Ni and Fe such that each flake evaporates to completion, not only does the average film composition agree with the melt but the film is free from gradients in composition as well.

ACKNOWLEDGMENTS

Within the Physics Research Laboratory we are grateful to D. J. Squiers for valuable assistance with the melt vs film-composition measurements, C. L. Simmons for helpful discussions and preparation of the sputtered films, T. E. Hasty for assistance with the radio-frequency resonance measurements, L. L. Rey­nolds and J. W. Taylor for film preparation and optical­thickness measurements, and H. L. Trammell for his assistance with the etching experiments. In addition we are grateful to K. W. Kreiselmaier of the Memory Systems Branch for crucible film depositions and Isaac Trachtenberg of the Basic Electrochemistry Group for his helpful suggestions concerning etches.

VOLUME 38, NUMBER 5 APRIL 1967

Boron Detection in Metals by Alpha-Particle Tracking

J. S. ARMIJO AND H. S. ROSENBAUM

General Electric Company, Vallecitos Nuclear Center, Nltcleonics Laboratory, Pleasanton, California

(Received 14 July 1966; in final form 22 November 1966)

Track-etching techniques were used to detect boron in austenitic steels. The technique involved placing polymers which were known to be sensitive to alpha particles in intimate contact with the polished metal surface, exposing the specimen to neutrons, and then etching the polymer detector. Alpha tracks that result from the lOB (n, a)1Li reaction were dev.e1oped in the polymer by etching in an aqueous NaOH solution. The boron concentration in the steels was quantitatively determined and the results compared with more conventional analytical techniques. It is seen that in addition to measuring the total boron content, the track-etch method allows one to map the distribution of boron within the microstructure of the metaL; alloy. The detector materials used were cellulose nitrate and cellulose acetate butyrate. The technique is use­ful for measuring boron concentrations in the parts-per-million range.

INTRODUCTION

WHEN a recoiling fission fragment passes through an insulating material (either crystalline or non­

crystalline), it can leave an atomically and electroni­cally disturbed region in its path. Whether or not the fragment leaves this disturbed region depends on the ionization density along the particle path in the absorber. If this value is greater than the critical ionization density for the absorber, the particle will leave a disturbed path which can be made optically visible by etching in a suitable solution. l

1 R. L. Fleischer, P. B. Price, and R. M. Walker, J. App!. fhys.36,!3645 (1965). .

In a series of papers, Fleischer, Price, and Walker2- 6

have described tracking experiments in many materials. They have used these tracks for a variety of studies including the dating of geological samples and measure­ments of extremely low concentrations of uranium in micas. Plastic films (such as cellulose nitrate) have

2 R. L. Fleischer, P. B. Price, and R. M. Walker, Science 149, 383 (1965).

3 R. L. Fleischer, C. W. Naeser, P. B. Price, R. M. Walker, and U. B. Marvin, Science 148, 629 (1965).

4 P. B. Price and R. M. Walker, App!. Phys. Letters 2, 23 (1963) .

• P. B. Price and R. M. Walker, J. Geophys. Res. 68, 4847 (1963) .

• R. L. Fleischer and P. B, Price, J, Geophys. Res. 69, 331 (1964),

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