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Magnetic and electrical characteristics of MnAs films grown by metalorganic chemical vapour deposition
ELSEVIER
Journal of Alloys and Compounds 248 (1997) 125-131
Magnetic and electrical characteristics of MnAs films grown by metalorganic chemical vapour deposition M.E.G. Tilsley”, N.A. Smith”, B. Cockayne”, I.R. Han-is”, PA. Laneb, P.E. Ohverb, P.J. Wrightb ‘Schoo/ of Merollurgy and Marriols. Universi~ of Birmingham. PO Box 363. Birmingham. UK ‘DRA. St. Andrew Rd. Malwm. Worcestershire WRIT 3PS, UK Received 5 August 19%
Abstract The magnetic and electrical property changes associated with the phase transition in MnAs have been measured in tmcracked material by using thin films grown on to sapphire substrates by metal organic chemical vapour deposition. The fibns exbibiti a first-order transition Kith Curie points in the range 35-472 I “C. An increase of appmximately 60% in tbe electrical resistivity of the films was measured on heating thmugh the phase transition. The Curie temperature of the films increaud with an applii magttettc field by appmximately 1.2 “C in a field of 600 kA 6’ This shift in Curie temperature with applii magnedc tield also pmduced a magnetoresistance effect with a maximum value of 2.24% at 600 kA II-‘. The values of these parameters are shown to be &pm&m upon the growth conditions of the films. Keywords: MnAs; Fenomsgaetism; First-order phase tiansitton: MOCVD; Magnetoresistawe; Thin film
1. Introduction Bulk MnAs has an abrupt Curie temperature at 40 “C [1,2], which is due to a first-order transition in crystal structure from the hexagonal NiAs ferromagnetic stlucture to the orthorhombicMnP pammagnetic structure [3]. This phase change results in an increase in electrical resistivity from 2X 10M4 to 5X 10e4 n cm [4], thermal hysteresis of 10°C [2], and a 2.1% volume contraction of the lattice [5]. It is known that the temperature of the phase transition can be altered by either the application of a magnetic field (0.004T kA-’ m [6]) or pressure (- I2 “C kbar [7]). Detailed measurements of the electrical resistivity of bulk MnAs [4] have been difficuit fo obtain owing to the presence of micro-cracks and macro-cracks In the material. These cracks are present in the as-grown material and, because MnAs transforms on heating, they enlarge due to tbe intrinsic
volume
MnAs may have scverai advantages over bulk MnAs, such as compatibility with planar device tecbxdogy and a potential re.ductmn in briltkness because of the stability induced by the substrate. In this paper the magnet& magnetoresistive and morphological feanaeS observed in films of MnAs grown on sapphire substmtes by metalorganic chemical vapour deposition (MDCVD) [12] an described and discussed.
of
change
of the material.
On cooling.
these cracks close but still remain visible [a], resulting in a very brittle material. As bulk MnAs is so brittle its application to &vices based on its magnetic properties is limited. Most of the published literature to date [I-S] has considered only the bulk or powdered forms of MnAs. However, epitaxial thin films of MnAs have been grown by molecular beam epitaxy (MBE) [9] [IO.1 11. Thin films 0 1997 Eisevicr S&ace S.A. All rim o!xs-8388/97/517.00 Pff SO925-8388(96)02665-S
twerved
2. Experimental 2.1. Growth of MIAS layers MnAs films were grown on (I i02) sapphire substrates by atmospheric pressure MOCVD as described elsewhere [12]byrhcauthors.The1espcctiveprecursorsforMnand As were nicarbonyhnethykyclopcntadienyl manganese (TCM) and a&e (ASH,). The ASH, and TCM were transported to the reactor in a stream of palladium-purified hydrogen. where MnAs was formed by thermal cracking of the precursors onto the substrate. Two series of films wen grown. In series 1 the role of growth temperature was examined. This was varied from 375 to 600 “C, the growth time was I-80 min. the arsine Slow was 110 seem (5% arsine in Hz) and the hydrogen flow thtvugh the TCM
M.E.G.
Tibley
of Alloys
et al. I Joamal
temperature using a sensitive Curie-Faraday magnetic balance (CFB) [ 131. The samples were heated in the CFB in a vacuum at a rate of 1 “C min-’ and allowed to co.,. naturally at a rate not greater than I “C min-‘. The magnetic susceptibilities of the films in a field of 400 mT were measured, this value being proportional to the amount of magnetic material present in each sample. A*., (see Fig. 1) is the decrease in the value of susceptibility when the MnAs changes from the fully ferromagnetic state to the fully paramagnetic state. It is therefore proportional to the amount of MnAs present in the sample, and since all the films had the same area, A,,, is also proportional to the film thickness providing that the films all consist of single phase magnetic material. The Curie temperatures of the films were determined as the intercept of two lines, one 2 fit to the susceptibility data for the paramagnetic region and one a fit to the ferromagnetic data in the steepest part of the curve. The amount of thermal hysteresis was measured at a point halfway up each susceptibility curve, as indicated in Fig. 1. Where
IO
20
30
40
so
60
Tempm~tre (“0 Fig. 1. Curie-Faraday curve for an MnAs film grown a1475”C.
and
Compounds
248 (1997)
12%131
3. Results and discussion 3. I. Magnetic measurements 3.1.1. Series I firms As discussed in Section 2.1, these films were derived from an initial study aimed at establishing the optimum growth temperature for MnAs. Magnetic measurements on the films obtained by CFB all showed the shape of the typical example given in Fig. I; the asymmetry in the heating and cooling curves could be due to the slightly different heating and cooling rates. Fig. 2(a) plots MnAs growth rate vs. growth temperature [12], the film growth rate being expressed as the weight of the MnAs deposited per minute per unit mass of substrate (to normalise for different substrate areas). The data show that the growth rate of the MnAs films has a slang dependence on the growth temperature, with a maximum growth rate observed at 475 “C. All films in this series were grown for the same duration, and hence the growth rate is proportional to the thickness. Fig. 2(b) plots A,,, vs. growth temperature for the same films and has a similar general form to Fig. 2(a). There is, however, a divergence of the two graphs above 475’C which could suggest that a second, non-ferromagnetic, phase is forming at high growth temperatures. This phase would contribute to the mass of the film but would not contribute to the amount of ferromagnetic material present (i.e. L?~,,).
M.E.G.
Tikky
et al. I Journal
of Alloys
and Compounds
248 (1997)
125-131
127
growth rate (and hence film thickness) and surface lifetimes will be strongly dcpcndent on the grow& temperature. As a result it is -hnpossibk to de&xmine exactly which of these parameters influences the changes in film Curie temperature. However, it sSJ&J be aoted that in the temperature region 375 to 425°C the Curie kmpemt~ increases slightly with incnxed film thiikness. Above 475 “C, where -he films axe beliived to be multi-phase, the Curie temperature falls dramatically with increasing tetnpxature, and hence decreasing tbickness. 350
I 550 I 600 I 4001 So 450
650
Grwtb Tempcrava (T)
350
4lm
45a
WI
ss4
fm
3.1.2. [email protected] Thelilmgfownat475°Cffomseries 1 wasshowatobe single phase magoetic material by X-ray diffmction (XRD) and exqy dispersive X-ray (EDX) analysis [12]. Cottsequently, as discussed in !Sectkm 2.2, A... is proportiona to film thickness for the series 2 films. -Fig. 4 plots A,,, vs. growth time for MnAs films. An approximately linear increase in film tbMness with growth
650
Gmwb Temperatwr (‘C) Fig. 2. (a) MnAs gmwtb rate vs. gmwtb tempcnture: (b) A,., vs. growth temperature.
Fig. 3 illustrates the behaviour of the Curie temperatures on heating for each film. The Curie temperatures increase slightly as the film growth temperature is increased from 375 to 425°C. followed by a subsequent decrease as the temperature is increased in the range 425 to 600°C. As discussed earlier, increasing growth temperature causes the precursors to break down thermally in such a way that the ratio of manganese to arsenic-active spies in the gas varies. In addition, the nucleation density,
-
I
0
i
,kl ii GmwbTime(mim*s)
240
Rg. 4. MnAs film lttickncss (CXptead = 4”‘) vs. B m m
2so
lbw
7
3.2. 3.2.1.
0
so
loo IS0 200 Growth time (minutes)
250
Fig. 5. MnAs Curie
value of 10°C [2]. However, for hysteresis to occur, the MnAs must be free to change its volume on going through the transition [15] The presence of the substrate will hinder this volume change and could therefore be the cause of the reduced hysteresis in the films. The variation of hysteresis with film thickness could be because the filmsubstrate interaction is different for very thin films (less than 30min growth), than it is for thicker films (greater than 60min growth). As shown later, films grown for 30 min or less did not completely cover the substrate, and hence were not electrically conducting, whilst films grown for 60 min or more always covered the substrate. A similar argument explains the variation of Curie temperature with thickness, because the Curie temperature depends upon the Mn-Mn atomic spacing [ 15,161. If this spacing is influenced by the substrate, and this influence is dependent on the film thickness, a variation in the Curie temperature will occur with thickness. Two parts to the Curie temperature curve may be inferred, a low T. part for thin noncontinuous films and a high T, region for thicker films. Indeed, T, does not change much for films grown for more than 120min. which supports this tenet.
Fig. 6. SEM photgnphs
Investigation Series
of jibn
morphology
I films
The morphology of these films. investigated by SEM has already been shown ll2j to be strongly dependent on the growth temperature. In summary, films grown in the low temperature range, 375 to 500°C. have an irregular grain morphology at the lower end of the range, but as the grain size increases systematically with increasing temperature, and hence increasing thickness, the grains become more regular in shape. In contrast, films grown in the highest temperature range studied, 525 to 6OO’C. have a grain size which decreases with increasing growth temperature, and hence decreasing thickness, in which the grains become regular and equiaxed. These observations show that the grain size and shape are strongly dependent on the film growth temperature and thickness. However, since the film thickness is also a function of the growth temperature it is difficult to deconvolute the two effects of growth temperature and thickness for the series 1 films. 3.2.2. Series 2jihns The SEM secondary electron images shown in Fig. 6 are for films grown for 120 and 24Omin. The films have a similar grain morphology, but the average grain size is greater at the larger growth time. This confirms results from series I, which showed that thicker films had larger grains. Both are continuous films, but the film grown for the longer duration is more completely covered by the large flat surface grains. The thinner film contains gaps between the flat surface grains but these are underlayered by smaller grains, which are probably nucleated directly or the substrate. In much thinner films it is expected that the Rat surface grains do not form a continuous layer on the substrate, resulting in a non-conducting film. The SEM and magnetic results complement each other. The 120 and 240 min films are both similar in morphology
of MaAs films grown for diffenat dursticas: (8) 120 nun: (b)240 min.
and magnetic properties. The observed morphology provides a possible explanation for the hysteresis changes shown in Fig. 5. The. low T, region, where the hysteresis is at a relatively high value, corresponds to low thickness films where the films are likely to consist of non-continuous grains nucleated directly onto the substrates. Such grains would have less restriction to changes in volume and thus exhibit greater hysteresis. These films would not be expected to be electrically conducting, which is in agreement with measurements made (see Section 3.3). The high T, region, where the hysteresis is low. corresponds to higher thickness films which, as the SEM data show, consist of continuous grains, dominated by the presence of large flat surface grains. These films are elecirically conducting (see Section 3.3) and would be expected to be resistant to volume changes. thereby accounting for the low values of hysteresis. An important morphological observation is that none of the films exhibited the micro-cracks observed m bulk MnAs.
3.3. Mng:retore.vistmce series 2
and resistance
mvurrrements
for
The thickest films from both series I and 2 were electrically conducting. However, only series 2 films are considered here because of the growth variables discussed earlier for series I. The relevant data are listed in Table I, apart from the film grown for 120 min; this was electrical!y conducting at low temperatures but became non-conducting on heating through the phase transition and so is omitted. For Table I the resistance change due to the first order transition AR is defined as the change in the zero field resistance as the temperature is increased from 2 to 62 “C. Table I also lists the maximum M R ratio (max.MR) recorded for the film, the temperature at which it occurned (T of max.MR), the temperature coincident with the maximum gradient of the zero field resistance vs. temperature curve (T of max.dR/dT) and the temperature coincident with the maximum gradient of the susceptibility vs. temperature cutve (T of max. d.rusldT). Table I shows that the maximum M R determined was 2.24% in the film grown for 150min. This value decrcascs with increasing growth time, and hence increasing film thickness. Additionally. for all the films investigated, the M R always has its largest value at T of max. dRld7’. This temperature also
Ftg.
7. The
gradient
of
suscepibility and mistance cylyes
the
temperature vs. 1empenurc for an MnA.5 film grown for 24Omin.
equates with the maximum d.r&dT. This effect is shown in Fig. 7 for an MnAs film grown for 240 min. film Fig. 8 shows theMRdataobtainedat20°Cforthe grown for 240 min. The resistance decreases approximately linearly with applied field up to the maximum of 640 kA m-‘. The same trend is true for all conducting MnAs films measured in the reuion of the first-order transition (2 to 62 “0. Fig. 9 shows the zero field resistance and the resistance at &tkAm-’ vs. temperature for the same film. This graph was constructed from several isothermal M R runs between 2 and 62°C: however, only the xero field point 5.735
--
~-
--
I
2.730 P
2.725
’
8
2.720
I
,i
2.715
1
f
2.710
-
2.705
!
2.700
r -800
-600
-400
Fig. 8. MR
mice
at 2 O T
I
I
I -200
I 0
AaakedFuldWW for a film
gmm
for 2 4 0 min.
Table I MR
results
Gmwtb
for electrically
time (min)
conductive Resistance
( 20.05 R) 2180 40
150
2.46 3.41 13.10
films
at 2 “C
from
sews
2
MR
T of max.
dRldT
T of max.
Max. MR (~O.I?s)
T of max. (+I”0
(+I “c)
(?I%3
59.4 61.7
I .73 2.01
34.7 35.0
34.4 34.8
60.1
2.24
35.2
34.9
2: 35.6
JR (CZ.O%)
dsnr/dT
M.E.G. T~/.s/eyR 01. I Jnrrmrd rf Alloys and Co~w~,,,nds 248 (1997) IZS-131
130
point IO change, the film volume must change [IS]; as discussed in Section 3.2.2, the substrate is expected to hinder this volume change, and hence to hinder the change in Curie temperature.
4.2
4. Conclusions
lo.0
2.4
70 Fig. 9. Zero and hiSh field rerwmce and MU vs. grown for 240 min.
and one maximum field ooint are olotted at each temoerature. The MR. as defined in R~J. (1) and calculated using these two points, is also shown- as a function of temperature. As the films are heated through the first-order transition (2-62 “C) an increase in the electrical resistivity of approximately 60% occurs. In bulk MnAs [4] there is a large resistance increase (ca. 150%) at this phase change, but this is accompanied by the opening of micro-cracks in the material [8]-which contribute to the increase. The smaller resistance increase in the thin films and the absence of micro-cracks suggests that, in the present case, only the intrinsic change in the resistance of MnAs is
--
ranee over the resistance changes is ‘coincident 4th the temperature range over which the magnetic properties change and “ariaions in both sets of properties exhibit similar trends. The major MR effect is present only at temperatures where. both the ferromagnetic and paramagnetic phases coexist in the Ai discussed in Section I, the application of a magnetic field to bulk MnAs causes an -increase in the Curie temperature 161. As a result, in thin films of MnAs. at
Thin films of MnAs grown by MOCVD have properties substantially similar to those of the bulk material. The absence of micro-cracks in the films does. however, allow intrinsic properties for MnAs to be measured. Nevertheless, some minor differences, which are dependent on the exact film growth conditions, are apparent. Thus, in the thin films, a phase transition occurs at approximately the same temperature as in the bulk material; however, the exact Curie temperature of the films depends on the film morphology and thickness, and very thin films exhibit lower Curie points. Thin films of MnAs exhibit less thermal hysteresis than bulk MnAs; this can also be attributed to the effect of the film substrate, morphology and thickness. The film morphology, as revealed by SEM, is dependent on the film growth temperature and thickness; the film morphology can also be correlated with magnetic properties. The film resistance increases upon heating through the phase transition; this increase is less than that reported for bulk MnAs, and can be attributed, in part at least, to the absence of micro-cracks in the thin film material. A magnetoresistive effect, which is strongly dependent on the fihn temperature, is detected at temperatures where both the ferromagnetic and paramagnetic phases coexist in the film: this MR effect arises from an increase in the film Curie temperature with applied field, and hence with growth of the low resistivity ferromagnetic phase at the expense of the high resistivity paramagnetic phase. An increase in Curie temperature with applied field is observed but is less than that reported for bulk MnAs; this can again be attributed to the effects induced by the presence of a substrate.
Acknowledgments than the pammagnetic s&e this will result in a reduction of film resistance. The change in the Curie point of a film grown for 240 min due to an applied field of 600 kA m-’ has been determined from Fig.* 9. The change was determined as the distance the zero field resi&mce curve needs to be displaced along the x-axis in order to overlap the resistance in the 600 kA mm-’ curve. This change was measured to be I .24 “C; the changes in Curie temperature for the other two films measured from series 2 were, within experimental errors, the same. This change is lower than the 2.4”C per 6OOkAm-’ expected for bulk MnAs, and this lower value could be caused by the effect of the substrate and/or film morphology. In order for the Curie
The authors thank C.L. Reeves (DRA, Malvem) for the SEM data. Financial support provided by EPSRC and DRA (Malvem) in the form of a CASE award is acknowledged by one of us (M.E.G.T.)
References [II L.F. Bales. Phi/m. kg.. 8 (1929) 714. 121 B.T.M. Willis and H.P. Rcmksby. Pmt. Phys. Sot. London Sect. B:. 67 (1954) 290. 131 R.H. Wilson and J.S. Rasper. Acro Cr)rrra//qr.. I7 ( 1964) 95. (41 G. Fischer and W.B. Pearson. Can. 1. Phys.. 36 (IY58) 1010.
M.E.G. Tikley et al. I Journal of Alloys and Compounds 248 (1997) ID-131 ISI T. Suzuki and H. Ido, J. Phys. Sot Jpn.. 51 (1982) 3149. D.S. Rodbell and P.E. Lawrence. 1. APP~ Phys. Suppl., 31 ( 1960) 215. 171 D.S. Rodbell. in F. Bundy. W.R. Hibbard and H.M. Strong (eds.), Progress in Very High Pressure Research. Wiley, New York. 1961. IS1 Z.S. Basinski and W.B. Pearson. Can. 1. Phys.. 36 (1958) 1017. 191 M. Tanaka J.P. Harbison. T. Sands. T.L. Cheeks.V.G. Kemddas and G.M. Rothberg, J. Vur. Sci. Techad. 6. 12 (2) (1994) 1091. [IO] K. Akeun. M. Tanaka. M. Ueki and T. Nishinaga. App!. Phys. Len.. 67 ( 1995) 3349. [III A.M. Grishin. S.I. Khanso and K.V. Rae. Appl. Phys. Lurt.. 68 (1996) 2008.
R
131
[I21 f’.A. Lane. B. Cockaync. PJ. WI@K, P.E. Dliver. M.E.G. likky. N.A. Smith and I.R. Harris, 1. CryrOr Gtvwh. 143 (1994) 237. 1131 D. Jdes, lntrad&u-tion IO Mawtism and Ma.qutic Materials. Chapman and Hall. London, 1991. [I41 R.D. Heap. Heat Pqs. E. .4 F.N. Spa New York. 1979. li.51 C.P. Bean and D.S. Rodbell. Phys. RN.. I.76 (1%2) 104. (I61 J.B. Ckxxknwgh. D.H. Rid&y asd W.A. Newman, Proc. IN. Con/. on Mugnehn, Noftit@atn. 1964. Tke institw of Fbysir and lbc Phy5icnl Society. London, 1965.
Journal of Alloys and Compounds 248 (1997) 125-131
Magnetic and electrical characteristics of MnAs films grown by metalorganic chemical vapour deposition M.E.G. Tilsley”, N.A. Smith”, B. Cockayne”, I.R. Han-is”, PA. Laneb, P.E. Ohverb, P.J. Wrightb ‘Schoo/ of Merollurgy and Marriols. Universi~ of Birmingham. PO Box 363. Birmingham. UK ‘DRA. St. Andrew Rd. Malwm. Worcestershire WRIT 3PS, UK Received 5 August 19%
Abstract The magnetic and electrical property changes associated with the phase transition in MnAs have been measured in tmcracked material by using thin films grown on to sapphire substrates by metal organic chemical vapour deposition. The fibns exbibiti a first-order transition Kith Curie points in the range 35-472 I “C. An increase of appmximately 60% in tbe electrical resistivity of the films was measured on heating thmugh the phase transition. The Curie temperature of the films increaud with an applii magttettc field by appmximately 1.2 “C in a field of 600 kA 6’ This shift in Curie temperature with applii magnedc tield also pmduced a magnetoresistance effect with a maximum value of 2.24% at 600 kA II-‘. The values of these parameters are shown to be &pm&m upon the growth conditions of the films. Keywords: MnAs; Fenomsgaetism; First-order phase tiansitton: MOCVD; Magnetoresistawe; Thin film
1. Introduction Bulk MnAs has an abrupt Curie temperature at 40 “C [1,2], which is due to a first-order transition in crystal structure from the hexagonal NiAs ferromagnetic stlucture to the orthorhombicMnP pammagnetic structure [3]. This phase change results in an increase in electrical resistivity from 2X 10M4 to 5X 10e4 n cm [4], thermal hysteresis of 10°C [2], and a 2.1% volume contraction of the lattice [5]. It is known that the temperature of the phase transition can be altered by either the application of a magnetic field (0.004T kA-’ m [6]) or pressure (- I2 “C kbar [7]). Detailed measurements of the electrical resistivity of bulk MnAs [4] have been difficuit fo obtain owing to the presence of micro-cracks and macro-cracks In the material. These cracks are present in the as-grown material and, because MnAs transforms on heating, they enlarge due to tbe intrinsic
volume
MnAs may have scverai advantages over bulk MnAs, such as compatibility with planar device tecbxdogy and a potential re.ductmn in briltkness because of the stability induced by the substrate. In this paper the magnet& magnetoresistive and morphological feanaeS observed in films of MnAs grown on sapphire substmtes by metalorganic chemical vapour deposition (MDCVD) [12] an described and discussed.
of
change
of the material.
On cooling.
these cracks close but still remain visible [a], resulting in a very brittle material. As bulk MnAs is so brittle its application to &vices based on its magnetic properties is limited. Most of the published literature to date [I-S] has considered only the bulk or powdered forms of MnAs. However, epitaxial thin films of MnAs have been grown by molecular beam epitaxy (MBE) [9] [IO.1 11. Thin films 0 1997 Eisevicr S&ace S.A. All rim o!xs-8388/97/517.00 Pff SO925-8388(96)02665-S
twerved
2. Experimental 2.1. Growth of MIAS layers MnAs films were grown on (I i02) sapphire substrates by atmospheric pressure MOCVD as described elsewhere [12]byrhcauthors.The1espcctiveprecursorsforMnand As were nicarbonyhnethykyclopcntadienyl manganese (TCM) and a&e (ASH,). The ASH, and TCM were transported to the reactor in a stream of palladium-purified hydrogen. where MnAs was formed by thermal cracking of the precursors onto the substrate. Two series of films wen grown. In series 1 the role of growth temperature was examined. This was varied from 375 to 600 “C, the growth time was I-80 min. the arsine Slow was 110 seem (5% arsine in Hz) and the hydrogen flow thtvugh the TCM
M.E.G.
Tibley
of Alloys
et al. I Joamal
temperature using a sensitive Curie-Faraday magnetic balance (CFB) [ 131. The samples were heated in the CFB in a vacuum at a rate of 1 “C min-’ and allowed to co.,. naturally at a rate not greater than I “C min-‘. The magnetic susceptibilities of the films in a field of 400 mT were measured, this value being proportional to the amount of magnetic material present in each sample. A*., (see Fig. 1) is the decrease in the value of susceptibility when the MnAs changes from the fully ferromagnetic state to the fully paramagnetic state. It is therefore proportional to the amount of MnAs present in the sample, and since all the films had the same area, A,,, is also proportional to the film thickness providing that the films all consist of single phase magnetic material. The Curie temperatures of the films were determined as the intercept of two lines, one 2 fit to the susceptibility data for the paramagnetic region and one a fit to the ferromagnetic data in the steepest part of the curve. The amount of thermal hysteresis was measured at a point halfway up each susceptibility curve, as indicated in Fig. 1. Where
IO
20
30
40
so
60
Tempm~tre (“0 Fig. 1. Curie-Faraday curve for an MnAs film grown a1475”C.
and
Compounds
248 (1997)
12%131
3. Results and discussion 3. I. Magnetic measurements 3.1.1. Series I firms As discussed in Section 2.1, these films were derived from an initial study aimed at establishing the optimum growth temperature for MnAs. Magnetic measurements on the films obtained by CFB all showed the shape of the typical example given in Fig. I; the asymmetry in the heating and cooling curves could be due to the slightly different heating and cooling rates. Fig. 2(a) plots MnAs growth rate vs. growth temperature [12], the film growth rate being expressed as the weight of the MnAs deposited per minute per unit mass of substrate (to normalise for different substrate areas). The data show that the growth rate of the MnAs films has a slang dependence on the growth temperature, with a maximum growth rate observed at 475 “C. All films in this series were grown for the same duration, and hence the growth rate is proportional to the thickness. Fig. 2(b) plots A,,, vs. growth temperature for the same films and has a similar general form to Fig. 2(a). There is, however, a divergence of the two graphs above 475’C which could suggest that a second, non-ferromagnetic, phase is forming at high growth temperatures. This phase would contribute to the mass of the film but would not contribute to the amount of ferromagnetic material present (i.e. L?~,,).
M.E.G.
Tikky
et al. I Journal
of Alloys
and Compounds
248 (1997)
125-131
127
growth rate (and hence film thickness) and surface lifetimes will be strongly dcpcndent on the grow& temperature. As a result it is -hnpossibk to de&xmine exactly which of these parameters influences the changes in film Curie temperature. However, it sSJ&J be aoted that in the temperature region 375 to 425°C the Curie kmpemt~ increases slightly with incnxed film thiikness. Above 475 “C, where -he films axe beliived to be multi-phase, the Curie temperature falls dramatically with increasing tetnpxature, and hence decreasing tbickness. 350
I 550 I 600 I 4001 So 450
650
Grwtb Tempcrava (T)
350
4lm
45a
WI
ss4
fm
3.1.2. [email protected] Thelilmgfownat475°Cffomseries 1 wasshowatobe single phase magoetic material by X-ray diffmction (XRD) and exqy dispersive X-ray (EDX) analysis [12]. Cottsequently, as discussed in !Sectkm 2.2, A... is proportiona to film thickness for the series 2 films. -Fig. 4 plots A,,, vs. growth time for MnAs films. An approximately linear increase in film tbMness with growth
650
Gmwb Temperatwr (‘C) Fig. 2. (a) MnAs gmwtb rate vs. gmwtb tempcnture: (b) A,., vs. growth temperature.
Fig. 3 illustrates the behaviour of the Curie temperatures on heating for each film. The Curie temperatures increase slightly as the film growth temperature is increased from 375 to 425°C. followed by a subsequent decrease as the temperature is increased in the range 425 to 600°C. As discussed earlier, increasing growth temperature causes the precursors to break down thermally in such a way that the ratio of manganese to arsenic-active spies in the gas varies. In addition, the nucleation density,
-
I
0
i
,kl ii GmwbTime(mim*s)
240
Rg. 4. MnAs film lttickncss (CXptead = 4”‘) vs. B m m
2so
lbw
7
3.2. 3.2.1.
0
so
loo IS0 200 Growth time (minutes)
250
Fig. 5. MnAs Curie
value of 10°C [2]. However, for hysteresis to occur, the MnAs must be free to change its volume on going through the transition [15] The presence of the substrate will hinder this volume change and could therefore be the cause of the reduced hysteresis in the films. The variation of hysteresis with film thickness could be because the filmsubstrate interaction is different for very thin films (less than 30min growth), than it is for thicker films (greater than 60min growth). As shown later, films grown for 30 min or less did not completely cover the substrate, and hence were not electrically conducting, whilst films grown for 60 min or more always covered the substrate. A similar argument explains the variation of Curie temperature with thickness, because the Curie temperature depends upon the Mn-Mn atomic spacing [ 15,161. If this spacing is influenced by the substrate, and this influence is dependent on the film thickness, a variation in the Curie temperature will occur with thickness. Two parts to the Curie temperature curve may be inferred, a low T. part for thin noncontinuous films and a high T, region for thicker films. Indeed, T, does not change much for films grown for more than 120min. which supports this tenet.
Fig. 6. SEM photgnphs
Investigation Series
of jibn
morphology
I films
The morphology of these films. investigated by SEM has already been shown ll2j to be strongly dependent on the growth temperature. In summary, films grown in the low temperature range, 375 to 500°C. have an irregular grain morphology at the lower end of the range, but as the grain size increases systematically with increasing temperature, and hence increasing thickness, the grains become more regular in shape. In contrast, films grown in the highest temperature range studied, 525 to 6OO’C. have a grain size which decreases with increasing growth temperature, and hence decreasing thickness, in which the grains become regular and equiaxed. These observations show that the grain size and shape are strongly dependent on the film growth temperature and thickness. However, since the film thickness is also a function of the growth temperature it is difficult to deconvolute the two effects of growth temperature and thickness for the series 1 films. 3.2.2. Series 2jihns The SEM secondary electron images shown in Fig. 6 are for films grown for 120 and 24Omin. The films have a similar grain morphology, but the average grain size is greater at the larger growth time. This confirms results from series I, which showed that thicker films had larger grains. Both are continuous films, but the film grown for the longer duration is more completely covered by the large flat surface grains. The thinner film contains gaps between the flat surface grains but these are underlayered by smaller grains, which are probably nucleated directly or the substrate. In much thinner films it is expected that the Rat surface grains do not form a continuous layer on the substrate, resulting in a non-conducting film. The SEM and magnetic results complement each other. The 120 and 240 min films are both similar in morphology
of MaAs films grown for diffenat dursticas: (8) 120 nun: (b)240 min.
and magnetic properties. The observed morphology provides a possible explanation for the hysteresis changes shown in Fig. 5. The. low T, region, where the hysteresis is at a relatively high value, corresponds to low thickness films where the films are likely to consist of non-continuous grains nucleated directly onto the substrates. Such grains would have less restriction to changes in volume and thus exhibit greater hysteresis. These films would not be expected to be electrically conducting, which is in agreement with measurements made (see Section 3.3). The high T, region, where the hysteresis is low. corresponds to higher thickness films which, as the SEM data show, consist of continuous grains, dominated by the presence of large flat surface grains. These films are elecirically conducting (see Section 3.3) and would be expected to be resistant to volume changes. thereby accounting for the low values of hysteresis. An important morphological observation is that none of the films exhibited the micro-cracks observed m bulk MnAs.
3.3. Mng:retore.vistmce series 2
and resistance
mvurrrements
for
The thickest films from both series I and 2 were electrically conducting. However, only series 2 films are considered here because of the growth variables discussed earlier for series I. The relevant data are listed in Table I, apart from the film grown for 120 min; this was electrical!y conducting at low temperatures but became non-conducting on heating through the phase transition and so is omitted. For Table I the resistance change due to the first order transition AR is defined as the change in the zero field resistance as the temperature is increased from 2 to 62 “C. Table I also lists the maximum M R ratio (max.MR) recorded for the film, the temperature at which it occurned (T of max.MR), the temperature coincident with the maximum gradient of the zero field resistance vs. temperature curve (T of max.dR/dT) and the temperature coincident with the maximum gradient of the susceptibility vs. temperature cutve (T of max. d.rusldT). Table I shows that the maximum M R determined was 2.24% in the film grown for 150min. This value decrcascs with increasing growth time, and hence increasing film thickness. Additionally. for all the films investigated, the M R always has its largest value at T of max. dRld7’. This temperature also
Ftg.
7. The
gradient
of
suscepibility and mistance cylyes
the
temperature vs. 1empenurc for an MnA.5 film grown for 24Omin.
equates with the maximum d.r&dT. This effect is shown in Fig. 7 for an MnAs film grown for 240 min. film Fig. 8 shows theMRdataobtainedat20°Cforthe grown for 240 min. The resistance decreases approximately linearly with applied field up to the maximum of 640 kA m-‘. The same trend is true for all conducting MnAs films measured in the reuion of the first-order transition (2 to 62 “0. Fig. 9 shows the zero field resistance and the resistance at &tkAm-’ vs. temperature for the same film. This graph was constructed from several isothermal M R runs between 2 and 62°C: however, only the xero field point 5.735
--
~-
--
I
2.730 P
2.725
’
8
2.720
I
,i
2.715
1
f
2.710
-
2.705
!
2.700
r -800
-600
-400
Fig. 8. MR
mice
at 2 O T
I
I
I -200
I 0
AaakedFuldWW for a film
gmm
for 2 4 0 min.
Table I MR
results
Gmwtb
for electrically
time (min)
conductive Resistance
( 20.05 R) 2180 40
150
2.46 3.41 13.10
films
at 2 “C
from
sews
2
MR
T of max.
dRldT
T of max.
Max. MR (~O.I?s)
T of max. (+I”0
(+I “c)
(?I%3
59.4 61.7
I .73 2.01
34.7 35.0
34.4 34.8
60.1
2.24
35.2
34.9
2: 35.6
JR (CZ.O%)
dsnr/dT
M.E.G. T~/.s/eyR 01. I Jnrrmrd rf Alloys and Co~w~,,,nds 248 (1997) IZS-131
130
point IO change, the film volume must change [IS]; as discussed in Section 3.2.2, the substrate is expected to hinder this volume change, and hence to hinder the change in Curie temperature.
4.2
4. Conclusions
lo.0
2.4
70 Fig. 9. Zero and hiSh field rerwmce and MU vs. grown for 240 min.
and one maximum field ooint are olotted at each temoerature. The MR. as defined in R~J. (1) and calculated using these two points, is also shown- as a function of temperature. As the films are heated through the first-order transition (2-62 “C) an increase in the electrical resistivity of approximately 60% occurs. In bulk MnAs [4] there is a large resistance increase (ca. 150%) at this phase change, but this is accompanied by the opening of micro-cracks in the material [8]-which contribute to the increase. The smaller resistance increase in the thin films and the absence of micro-cracks suggests that, in the present case, only the intrinsic change in the resistance of MnAs is
--
ranee over the resistance changes is ‘coincident 4th the temperature range over which the magnetic properties change and “ariaions in both sets of properties exhibit similar trends. The major MR effect is present only at temperatures where. both the ferromagnetic and paramagnetic phases coexist in the Ai discussed in Section I, the application of a magnetic field to bulk MnAs causes an -increase in the Curie temperature 161. As a result, in thin films of MnAs. at
Thin films of MnAs grown by MOCVD have properties substantially similar to those of the bulk material. The absence of micro-cracks in the films does. however, allow intrinsic properties for MnAs to be measured. Nevertheless, some minor differences, which are dependent on the exact film growth conditions, are apparent. Thus, in the thin films, a phase transition occurs at approximately the same temperature as in the bulk material; however, the exact Curie temperature of the films depends on the film morphology and thickness, and very thin films exhibit lower Curie points. Thin films of MnAs exhibit less thermal hysteresis than bulk MnAs; this can also be attributed to the effect of the film substrate, morphology and thickness. The film morphology, as revealed by SEM, is dependent on the film growth temperature and thickness; the film morphology can also be correlated with magnetic properties. The film resistance increases upon heating through the phase transition; this increase is less than that reported for bulk MnAs, and can be attributed, in part at least, to the absence of micro-cracks in the thin film material. A magnetoresistive effect, which is strongly dependent on the fihn temperature, is detected at temperatures where both the ferromagnetic and paramagnetic phases coexist in the film: this MR effect arises from an increase in the film Curie temperature with applied field, and hence with growth of the low resistivity ferromagnetic phase at the expense of the high resistivity paramagnetic phase. An increase in Curie temperature with applied field is observed but is less than that reported for bulk MnAs; this can again be attributed to the effects induced by the presence of a substrate.
Acknowledgments than the pammagnetic s&e this will result in a reduction of film resistance. The change in the Curie point of a film grown for 240 min due to an applied field of 600 kA m-’ has been determined from Fig.* 9. The change was determined as the distance the zero field resi&mce curve needs to be displaced along the x-axis in order to overlap the resistance in the 600 kA mm-’ curve. This change was measured to be I .24 “C; the changes in Curie temperature for the other two films measured from series 2 were, within experimental errors, the same. This change is lower than the 2.4”C per 6OOkAm-’ expected for bulk MnAs, and this lower value could be caused by the effect of the substrate and/or film morphology. In order for the Curie
The authors thank C.L. Reeves (DRA, Malvem) for the SEM data. Financial support provided by EPSRC and DRA (Malvem) in the form of a CASE award is acknowledged by one of us (M.E.G.T.)
References [II L.F. Bales. Phi/m. kg.. 8 (1929) 714. 121 B.T.M. Willis and H.P. Rcmksby. Pmt. Phys. Sot. London Sect. B:. 67 (1954) 290. 131 R.H. Wilson and J.S. Rasper. Acro Cr)rrra//qr.. I7 ( 1964) 95. (41 G. Fischer and W.B. Pearson. Can. 1. Phys.. 36 (IY58) 1010.
M.E.G. Tikley et al. I Journal of Alloys and Compounds 248 (1997) ID-131 ISI T. Suzuki and H. Ido, J. Phys. Sot Jpn.. 51 (1982) 3149. D.S. Rodbell and P.E. Lawrence. 1. APP~ Phys. Suppl., 31 ( 1960) 215. 171 D.S. Rodbell. in F. Bundy. W.R. Hibbard and H.M. Strong (eds.), Progress in Very High Pressure Research. Wiley, New York. 1961. IS1 Z.S. Basinski and W.B. Pearson. Can. 1. Phys.. 36 (1958) 1017. 191 M. Tanaka J.P. Harbison. T. Sands. T.L. Cheeks.V.G. Kemddas and G.M. Rothberg, J. Vur. Sci. Techad. 6. 12 (2) (1994) 1091. [IO] K. Akeun. M. Tanaka. M. Ueki and T. Nishinaga. App!. Phys. Len.. 67 ( 1995) 3349. [III A.M. Grishin. S.I. Khanso and K.V. Rae. Appl. Phys. Lurt.. 68 (1996) 2008.
R
131
[I21 f’.A. Lane. B. Cockaync. PJ. WI@K, P.E. Dliver. M.E.G. likky. N.A. Smith and I.R. Harris, 1. CryrOr Gtvwh. 143 (1994) 237. 1131 D. Jdes, lntrad&u-tion IO Mawtism and Ma.qutic Materials. Chapman and Hall. London, 1991. [I41 R.D. Heap. Heat Pqs. E. .4 F.N. Spa New York. 1979. li.51 C.P. Bean and D.S. Rodbell. Phys. RN.. I.76 (1%2) 104. (I61 J.B. Ckxxknwgh. D.H. Rid&y asd W.A. Newman, Proc. IN. Con/. on Mugnehn, Noftit@atn. 1964. Tke institw of Fbysir and lbc Phy5icnl Society. London, 1965.