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Effects of Ar pressure on magnetic and magnetocaloric properties of sputtered Er–Co thin films
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Effects of Ar pressure on magnetic and magnetocaloric properties of sputtered Er–Co thin films Miri Kim a, Myung-Hwa Jung b, Chung Man Kim b, Sang Ho Lim a,c,n a
Department of Nano Semiconductor Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Physics, Sogang University, Seoul 121-742, Republic of Korea c Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2014 Received in revised form 24 March 2015 Accepted 28 March 2015
The magnetic and magnetocaloric properties of Er–Co thin films fabricated by sputtering under an Ar pressure (PAr) varying from 2 to 10 mTorr in steps of 2 mTorr are investigated. The content of Co in the sputtered thin films decreases slightly as the PAr increases from 2 to 10 mTorr, resulting in a film composition varying from ErCo1.07 to ErCo0.93. Extensive changes in the magnetic and magnetocaloric properties are observed. As the PAr increases, the compensation temperature increases substantially from 83 to 141 K, as well as the temperature at which the maximum change in the magnetic entropy occurs, particularly in the low PAr range. Considering the small change observed in the composition of the thin films as a function of PAr, the changes in the magnetic and magnetocaloric properties are extensive, indicating that the PAr plays an important role in affecting the aforementioned properties in amorphous magnetic thin films. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sputtered thin films Er–Co alloys Ar pressure Magnetic properties Magnetocaloric effects
1. Introduction Amorphous materials are characterized by the lack of longrange ordering, although they may exhibit short-range ordering at the atomic length scale [1]. Thus, the physical properties of amorphous materials may depend on their short-range ordering. This is particularly true for magnetic materials with magnetic properties extremely sensitive to their structural correlation length (L) [2]. Chudnovsky demonstrated that the magnetic permeability varies with L 6, whereas the coercivity varies with L6; Herzer reported a similar trend for nanocrystalline magnetic materials [3]. These results indicate that the magnetic properties of amorphous materials can be tuned over a large scale by controlling the degree of amorphization (or short-range ordering) during their fabrication. In case of sputtered thin films, this can be achieved relatively easily by varying the Ar pressure (PAr) during the sputtering process, as its variation directly affects the thermalization process, which causes sputtered particles to lose momentum (mechanical potential energy) during their movement from the target to the substrate. A larger scattering between sputtered particles and Ar atoms is expected to occur at a higher PAr, resulting in a small number of energetic particles reaching the n Corresponding author at: Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea. Fax: þ82 29283584. E-mail address: [email protected] (S.H. Lim).
substrate. As a result of this, a lower degree of short-range ordering (or structural correlation length) is expected at a higher PAr, thus resulting in a higher magnetic softness [2]. Although no study has been carried out so far to examine the dependence between PAr and the short-range ordering of magnetic thin films, with the resultant magnetic properties, there are several scattered reports in the literature regarding sputtered thin films, including CoCrPt [4], TbFe [5], and FePt [6]; in the case of TbFe thin films, for example, large changes in the coercivity, in the sensitivity to magnetization under an applied magnetic field and in magnetostriction were found to depend on the PAr. The aim of the present study was to investigate the effects of PAr on the magnetic and magnetocaloric properties of sputtered Er–Co thin films.
2. Experimental details Amorphous thin films were deposited on a Si/SiO2 substrate using a direct-current magnetron sputtering system, in order to achieve a multilayer film with the structure Ta/Er–Co/Ta. In the direct-current magnetron sputtering technique [7,8], the cathode electrode was target material which was bombarded with ionized Ar atoms and the substrate was placed on anode. Magnetrons were used to utilize strong magnetic field to confine charged plasma particles close to the surface of the target. In a magnetic field, electrons follow helical paths around magnetic field lines undergoing more ionizing collisions with Ar gas near the target
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surface. The base pressure of the sputtering chamber was 3 10 7 Torr. The sputtering power was 100 W and the target-tosubstrate distance was 60 mm. These sputtering conditions were kept constant, whereas PAr was varied from 2 to 10 mTorr in steps of 2 mTorr. A target consisting in an Er/Co alloy with a 1:2 M ratio was used to fabricate the Er–Co thin films. The film thickness was measured using a surface profiler. The M–T curves (where M and T are the magnetization and the temperature, respectively) were obtained from a superconducting quantum interference device (SQUID), by varying the applied magnetic field (H), which was set at 1, 10, 30, 50, and 70 kOe. The curves were obtained under the field-cooled (FC) condition. At the lowest value of H, namely 1 kOe, the curves were obtained also under the zero-field-cooled (ZFC) condition. The microstructure of the films was examined using the X–ray diffraction (XRD) technique, and their composition was analyzed using the inductively coupled plasma method.
3. Results and discussion The content of Co in the sputtered films decreased with increasing PAr, resulting in films with the composition ErCo1.07 at 2 mTorr, ErCo1.04 at 4 mTorr, ErCo1.00 at 6 mTorr, ErCo0.96 at 8 mTorr, and ErCo0.93 at 10 mTorr. These compositions differ greatly from the composition of the target (ErCo2), with the content of Co being significantly reduced with respect to that of Er. This can be explained with the fact that Co atoms have a reduced mass (58.98 g/mol) compared to Er atoms (167.25 g/mol), therefore they are subject to a stronger scattering with the Ar atoms during the thermalization process. As a result, the number of sputtered Co atoms eventually reaching the substrate will be lower compared to the Er atoms (in other words, Co atoms will have a lower sputter yield), leading to the observed composition of the films. The relative content of Co in the thin films is approximately half of its content in the alloy target, indicating a similar ratio in the sputter yield of Co and Er. The relatively light dependence of the Co content on PAr can also be explained with the stronger thermalization process taking place at higher PAr and resulting in the observed changes in the composition of the thin films. Not only the relative content of Co, but also the thickness of the sputtered films can be affected by the thermalization process and, hence, by PAr; indeed, the thickness of the films decreases from 380 to 230 nm for a given sputtering time, as PAr increases from 2 to 10 mTorr. No crystalline peaks were observed in the XRD analysis of all the Er–Co thin films examined in the present study and one set of results are shown in the inset of Fig. 1(a). This indicates that their microstructure consists entirely in an amorphous phase. Due to this particular microstructure, the magnetic thin films prepared in the present work are expected to be magnetically soft, as demonstrated by Fig. 1(a)–(c), showing the M–T curves obtained at different values of H, from 1 to 70 kOe, for three selected samples fabricated at a PAr of 2, 6, and 10 mTorr. The variation of the M–T curves with PAr was monotonic, and hence, the curves obtained for the samples fabricated at 4 and 8 mTorr are not shown in the figures just for clarity in presentation. At the lowest value of H, 1 kOe, measurements were collected over a wide range of temperatures, from 5 to 300 K, whereas at higher values of H a narrower range of temperatures was employed to save time and resources, reaching a temperature just slightly higher than the relative compensation temperature (Tcp). In the case of samples fabricated at 6 and 10 mTorr, measurements were not reached Tcp at 30 and 50 kOe. It is noted that in the measured temperature range, the variation of magnetization with temperature is monotonous and furthermore is similar to that measured at 10 and 70 kOe. At a given T, M was found to be high
already at the lowest value of H, 1 kOe; with a further increase of H, a small increase in M was observed. This indicates that the present amorphous thin films were magnetically soft; viz., they were easily magnetized showing a large value of M even at the lowest value of H. Generally, no difference was observed for the M–T curves measured under FC and under ZFC, except for the curves obtained at the lowest value of H, 1 kOe. The difference was particularly evident for the sample fabricated at 10 mTorr, for which the curve obtained under ZFC started to deviate from the curve obtained under FC at approximately 50 K. A magnetic phase transition may occur at this temperature, although the details of this phenomenon are not known at the moment. One important feature to be noted in the M–T curves is the compensation behavior, indicated by the occurrence of a minimum in M at an intermediate temperature. For a given PAr, the Tcp is independent on H, although M at that temperature progressively increases with increasing H. It is rather surprising to observe a significant variation of Tcp with PAr, as clearly seen in Fig. 2, where Tcp is plotted as a function of PAr for all the samples considered in the present study. Tcp increases rather substantially from 83 to 141 K as PAr increases from 2 to 4 mTorr, whereas a much slower increase is observed for a higher PAr. The described compensation behavior often occurs in ferrimagnetic or sperimagnetic materials consisting of two magnetic elements, such as the Er–Co thin films considered in the present study, when the dependence of the magnetization on the temperature is different for the two magnetic elements. Both ferrimagnetic and sperimagnetic materials have a similar magnetic configuration, with the magnetic moment of one element opposite to that of the other. The main difference between a ferrimagnetic material and a sperimagnetic material consists in the fact that in the former the magnetic moments within the same atoms are completely aligned, whereas in the latter they are tilted at specific angles. This means that the intra-molecular fields within the same elements are weaker in a sperimagnetic material than in a ferrimagnetic one. Considering the large variation of M with H at the relative values of Tcp, particularly for the sample fabricated at 2 mTorr, the intra-molecular fields of the thin films considered in the present work are weak. This is in contrast with the behavior commonly observed in magnetic materials, showing the compensation behavior when M is nearly zero (the magnetization is “compensated”) at Tcp [9,10]. Therefore, from the M–T curves in Fig. 1(a)–(c) it is reasonable to conclude that the thin films prepared in the present work show a sperimagnetic behavior, in agreement with what found in the literature for amorphous Er–Co thin films with different compositions [11,12]. Initially, the large change of Tcp with PAr was considered to be due to the change in the chemical composition of the thin films. However, this had to be ruled out because the variation of the composition with PAr is very small, as clearly seen in the inset of Fig. 2. It is reasonable that the change in the magnetic properties, including Tcp, results from microstructural characteristics of the thin films, such as the short-range ordering, which possibly affects the canting angle of the magnetic moment or even the Curie temperature. The observed results (large change in Tcp with PAr) are consistent with the prediction of Chudnovsky, stating that the magnetic properties are extremely sensitive to changes in the structural correlation length (or short-range ordering) [2]. The magnetocaloric properties can be characterized by the change in the magnetic entropy (ΔSM), which is a measure of the heat transferred between the system and its surroundings during the magnetic field sweep at a given temperature. The ΔSM value during the field sweep was calculated from M–T curves [13]. The equation is given by
Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i
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Fig. 1. M–T curves obtained for samples fabricated at (a) 2 mTorr, (b) 6 mTorr, and (c) 10 mTorr. The numbers on the curves indicate H (in kOe) during the measurement. Measurements were carried out over a wide range of temperatures, between 5 and 300 K, at the lowest H, 1 kOe, whereas at higher H they were carried out over a narrower range (mainly up to a slightly higher T than the respective Tcp). The curves were measured under the FC condition (solid lines), although at the lowest H, 1 kOe, they were also measured under the ZFC condition (dotted lines). In the inset of (a) is shown the XRD pattern.
3
Fig. 3. ΔSM as a function of T for samples fabricated at (a) 2 mTorr, (b) 6 mTorr, and (c) 10 mTorr. The curves were obtained under four different magnetic field sweeps: 0–10 kOe (circles), 0–30 kOe (triangles), 0–50 kOe (squares), and 0–70 kOe (diamonds).
⎛ ∂S ⎞ ⎛ ∂M ⎞ ⎟ ⎜ ⎟ = ⎜ ⎝ ∂T ⎠H ⎝ ∂H ⎠T
(2)
Eq. (1) can be re–written as
ΔSM(T , ΔH) =
∫0
H
⎛ ∂M ⎞ ⎜ ⎟ dH ⎝ ∂T ⎠H
The numerical calculation of following equation:
ΔSM =
∑ i
Fig. 2. Variation of Tcp with PAr. The inset shows the variation of the Co/Er molar ratio with PAr.
ΔSM(T , ΔH) = SM(T , H) − SM(T , 0) =
By using the Maxwell relation
∫0
H
⎛ ∂S ⎞ ⎜ ⎟ dH ⎝ ∂H ⎠T
(1)
(3)
ΔSM was carried out by using the
Mi − Mi + 1 ΔHi Ti + 1 − Ti
(4)
where Mi and Mi + 1 are the magnetization values measured at Ti and Ti + 1 in a magnetic field sweep ΔHi , respectively. The dependence of ΔSM on the temperature is shown in Fig. 3(a)–(c) for the same set of samples shown in Fig. 1(a)–(c). A broad maximum, characteristic of an amorphous microstructure [14–16], is observed for all the samples, although it is particularly evident for the samples fabricated at higher PAr. The temperature at which the maximum in ΔSM occurred increased with increasing PAr. This behavior is, in a sense, similar to the behavior observed for Tcp. However, the temperatures of the peak were generally lower than the temperatures observed for the respective values of Tcp; at 10 mTorr, for example, the Tcp value was 185 K, whereas the peak temperature was 100 K. This was actually expected, because the maximum in ΔSM occurs at the inflection point of the M–T curve, where the slope of the curve reaches its
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magnetocaloric properties were investigated. The Co content decreased slightly with increasing PAr. All the samples exhibited a good magnetic softness, which is expected in an amorphous microstructure, and a compensation behavior, with Tcp continuously increasing from 83 to 141 K in the PAr range of 2–10 mTorr. The dependence of ΔSM on the temperature showed a broad maximum in all the samples, another characteristic of the amorphous microstructure. The temperature at which a maximum in ΔSM occurred increased appreciably with increasing PAr, particularly in the low PAr range. A similar behavior was also observed for ΔSM. Considering the small change occurring in the chemical composition of the thin films as a function of PAr, the changes in the magnetic and magnetocaloric properties are significant, demonstrating the determining role played by the short-range ordering in affecting the magnetic properties. Fig. 4. ΔSM as a function of T measured under the magnetic field sweep of 0– 70 kOe for samples fabricated at 2 mTorr (circles), 4 mTorr (triangles), 6 mTorr (squares), 8 mTorr (diamonds), and 10 mTorr (inverse triangles). The temperatures at which a maximum in ΔSM occurs are indicated by arrows.
maximum. A similar tendency is also observed in the variation of ΔSM with PAr, with ΔSM increasing with increasing PAr. Specifically, ΔSM increases continuously from 1.56 to 3.32 J/kg K as the PAr value increases from 2 to 10 mTorr during the field sweep from 0 to 70 kOe. The variation of the peak temperature and ΔSM with PAr can be observed more clearly in Fig. 4, where the dependence of ΔSM on T is shown for all the samples at the same field sweep, 0–70 kOe. The peak temperature monotonically increases with increasing PAr, although the increment is very small at higher PAr. Note that from Eq. (3), ΔSM should be zero at Tcp, where the slope of the M–T curve is zero. The values of ΔSM obtained for the Er–Co thin films obtained in the present work are similar to those reported in the literature. For example, ΔSM is 5.97 J/kg K in case of an amorphous Co35Er65 ribbon for a field sweep of 0– 50 kOe [17]. No ΔSM values were reported for thin films of the same materials used in the present work, but the following results should be noted: ΔSM was 1.6 J/kg K in case of a Gd thin film for a field sweep of 0–10 kOe [18] and 3.4 J/kg K in case of a Gd/W heterogeneous thin film for a field sweep of 0–30 kOe [19]. At temperatures above Tcp, where the slope of the M–T curve is positive, so that the magnetization increases with increasing T, an inverse magnetocaloric effect (the entropy increases with the application of H) was observed. A similar behavior was observed in other ferrimagnetic or sperimagnetic materials showing the compensation behavior [20], or in antiferromagnetic materials below the Néel temperature, showing a positive slope in the M–T curve [21].
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014M3C1A8048841). MK and SHL wish to thank Mr. Seok Jin Yun for his help in the design of the alloys during the initial stage of the work.
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[14] [15] [16] [17] [18] [19]
4. Conclusions [20]
Binary Er–Co thin films were fabricated by magnetron sputtering from a target of ErCo2 alloy under different values of PAr, varying from 2 to 10 mTorr, and their magnetic and
[21]
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Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i
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Effects of Ar pressure on magnetic and magnetocaloric properties of sputtered Er–Co thin films Miri Kim a, Myung-Hwa Jung b, Chung Man Kim b, Sang Ho Lim a,c,n a
Department of Nano Semiconductor Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Physics, Sogang University, Seoul 121-742, Republic of Korea c Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2014 Received in revised form 24 March 2015 Accepted 28 March 2015
The magnetic and magnetocaloric properties of Er–Co thin films fabricated by sputtering under an Ar pressure (PAr) varying from 2 to 10 mTorr in steps of 2 mTorr are investigated. The content of Co in the sputtered thin films decreases slightly as the PAr increases from 2 to 10 mTorr, resulting in a film composition varying from ErCo1.07 to ErCo0.93. Extensive changes in the magnetic and magnetocaloric properties are observed. As the PAr increases, the compensation temperature increases substantially from 83 to 141 K, as well as the temperature at which the maximum change in the magnetic entropy occurs, particularly in the low PAr range. Considering the small change observed in the composition of the thin films as a function of PAr, the changes in the magnetic and magnetocaloric properties are extensive, indicating that the PAr plays an important role in affecting the aforementioned properties in amorphous magnetic thin films. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sputtered thin films Er–Co alloys Ar pressure Magnetic properties Magnetocaloric effects
1. Introduction Amorphous materials are characterized by the lack of longrange ordering, although they may exhibit short-range ordering at the atomic length scale [1]. Thus, the physical properties of amorphous materials may depend on their short-range ordering. This is particularly true for magnetic materials with magnetic properties extremely sensitive to their structural correlation length (L) [2]. Chudnovsky demonstrated that the magnetic permeability varies with L 6, whereas the coercivity varies with L6; Herzer reported a similar trend for nanocrystalline magnetic materials [3]. These results indicate that the magnetic properties of amorphous materials can be tuned over a large scale by controlling the degree of amorphization (or short-range ordering) during their fabrication. In case of sputtered thin films, this can be achieved relatively easily by varying the Ar pressure (PAr) during the sputtering process, as its variation directly affects the thermalization process, which causes sputtered particles to lose momentum (mechanical potential energy) during their movement from the target to the substrate. A larger scattering between sputtered particles and Ar atoms is expected to occur at a higher PAr, resulting in a small number of energetic particles reaching the n Corresponding author at: Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea. Fax: þ82 29283584. E-mail address: [email protected] (S.H. Lim).
substrate. As a result of this, a lower degree of short-range ordering (or structural correlation length) is expected at a higher PAr, thus resulting in a higher magnetic softness [2]. Although no study has been carried out so far to examine the dependence between PAr and the short-range ordering of magnetic thin films, with the resultant magnetic properties, there are several scattered reports in the literature regarding sputtered thin films, including CoCrPt [4], TbFe [5], and FePt [6]; in the case of TbFe thin films, for example, large changes in the coercivity, in the sensitivity to magnetization under an applied magnetic field and in magnetostriction were found to depend on the PAr. The aim of the present study was to investigate the effects of PAr on the magnetic and magnetocaloric properties of sputtered Er–Co thin films.
2. Experimental details Amorphous thin films were deposited on a Si/SiO2 substrate using a direct-current magnetron sputtering system, in order to achieve a multilayer film with the structure Ta/Er–Co/Ta. In the direct-current magnetron sputtering technique [7,8], the cathode electrode was target material which was bombarded with ionized Ar atoms and the substrate was placed on anode. Magnetrons were used to utilize strong magnetic field to confine charged plasma particles close to the surface of the target. In a magnetic field, electrons follow helical paths around magnetic field lines undergoing more ionizing collisions with Ar gas near the target
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Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i
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surface. The base pressure of the sputtering chamber was 3 10 7 Torr. The sputtering power was 100 W and the target-tosubstrate distance was 60 mm. These sputtering conditions were kept constant, whereas PAr was varied from 2 to 10 mTorr in steps of 2 mTorr. A target consisting in an Er/Co alloy with a 1:2 M ratio was used to fabricate the Er–Co thin films. The film thickness was measured using a surface profiler. The M–T curves (where M and T are the magnetization and the temperature, respectively) were obtained from a superconducting quantum interference device (SQUID), by varying the applied magnetic field (H), which was set at 1, 10, 30, 50, and 70 kOe. The curves were obtained under the field-cooled (FC) condition. At the lowest value of H, namely 1 kOe, the curves were obtained also under the zero-field-cooled (ZFC) condition. The microstructure of the films was examined using the X–ray diffraction (XRD) technique, and their composition was analyzed using the inductively coupled plasma method.
3. Results and discussion The content of Co in the sputtered films decreased with increasing PAr, resulting in films with the composition ErCo1.07 at 2 mTorr, ErCo1.04 at 4 mTorr, ErCo1.00 at 6 mTorr, ErCo0.96 at 8 mTorr, and ErCo0.93 at 10 mTorr. These compositions differ greatly from the composition of the target (ErCo2), with the content of Co being significantly reduced with respect to that of Er. This can be explained with the fact that Co atoms have a reduced mass (58.98 g/mol) compared to Er atoms (167.25 g/mol), therefore they are subject to a stronger scattering with the Ar atoms during the thermalization process. As a result, the number of sputtered Co atoms eventually reaching the substrate will be lower compared to the Er atoms (in other words, Co atoms will have a lower sputter yield), leading to the observed composition of the films. The relative content of Co in the thin films is approximately half of its content in the alloy target, indicating a similar ratio in the sputter yield of Co and Er. The relatively light dependence of the Co content on PAr can also be explained with the stronger thermalization process taking place at higher PAr and resulting in the observed changes in the composition of the thin films. Not only the relative content of Co, but also the thickness of the sputtered films can be affected by the thermalization process and, hence, by PAr; indeed, the thickness of the films decreases from 380 to 230 nm for a given sputtering time, as PAr increases from 2 to 10 mTorr. No crystalline peaks were observed in the XRD analysis of all the Er–Co thin films examined in the present study and one set of results are shown in the inset of Fig. 1(a). This indicates that their microstructure consists entirely in an amorphous phase. Due to this particular microstructure, the magnetic thin films prepared in the present work are expected to be magnetically soft, as demonstrated by Fig. 1(a)–(c), showing the M–T curves obtained at different values of H, from 1 to 70 kOe, for three selected samples fabricated at a PAr of 2, 6, and 10 mTorr. The variation of the M–T curves with PAr was monotonic, and hence, the curves obtained for the samples fabricated at 4 and 8 mTorr are not shown in the figures just for clarity in presentation. At the lowest value of H, 1 kOe, measurements were collected over a wide range of temperatures, from 5 to 300 K, whereas at higher values of H a narrower range of temperatures was employed to save time and resources, reaching a temperature just slightly higher than the relative compensation temperature (Tcp). In the case of samples fabricated at 6 and 10 mTorr, measurements were not reached Tcp at 30 and 50 kOe. It is noted that in the measured temperature range, the variation of magnetization with temperature is monotonous and furthermore is similar to that measured at 10 and 70 kOe. At a given T, M was found to be high
already at the lowest value of H, 1 kOe; with a further increase of H, a small increase in M was observed. This indicates that the present amorphous thin films were magnetically soft; viz., they were easily magnetized showing a large value of M even at the lowest value of H. Generally, no difference was observed for the M–T curves measured under FC and under ZFC, except for the curves obtained at the lowest value of H, 1 kOe. The difference was particularly evident for the sample fabricated at 10 mTorr, for which the curve obtained under ZFC started to deviate from the curve obtained under FC at approximately 50 K. A magnetic phase transition may occur at this temperature, although the details of this phenomenon are not known at the moment. One important feature to be noted in the M–T curves is the compensation behavior, indicated by the occurrence of a minimum in M at an intermediate temperature. For a given PAr, the Tcp is independent on H, although M at that temperature progressively increases with increasing H. It is rather surprising to observe a significant variation of Tcp with PAr, as clearly seen in Fig. 2, where Tcp is plotted as a function of PAr for all the samples considered in the present study. Tcp increases rather substantially from 83 to 141 K as PAr increases from 2 to 4 mTorr, whereas a much slower increase is observed for a higher PAr. The described compensation behavior often occurs in ferrimagnetic or sperimagnetic materials consisting of two magnetic elements, such as the Er–Co thin films considered in the present study, when the dependence of the magnetization on the temperature is different for the two magnetic elements. Both ferrimagnetic and sperimagnetic materials have a similar magnetic configuration, with the magnetic moment of one element opposite to that of the other. The main difference between a ferrimagnetic material and a sperimagnetic material consists in the fact that in the former the magnetic moments within the same atoms are completely aligned, whereas in the latter they are tilted at specific angles. This means that the intra-molecular fields within the same elements are weaker in a sperimagnetic material than in a ferrimagnetic one. Considering the large variation of M with H at the relative values of Tcp, particularly for the sample fabricated at 2 mTorr, the intra-molecular fields of the thin films considered in the present work are weak. This is in contrast with the behavior commonly observed in magnetic materials, showing the compensation behavior when M is nearly zero (the magnetization is “compensated”) at Tcp [9,10]. Therefore, from the M–T curves in Fig. 1(a)–(c) it is reasonable to conclude that the thin films prepared in the present work show a sperimagnetic behavior, in agreement with what found in the literature for amorphous Er–Co thin films with different compositions [11,12]. Initially, the large change of Tcp with PAr was considered to be due to the change in the chemical composition of the thin films. However, this had to be ruled out because the variation of the composition with PAr is very small, as clearly seen in the inset of Fig. 2. It is reasonable that the change in the magnetic properties, including Tcp, results from microstructural characteristics of the thin films, such as the short-range ordering, which possibly affects the canting angle of the magnetic moment or even the Curie temperature. The observed results (large change in Tcp with PAr) are consistent with the prediction of Chudnovsky, stating that the magnetic properties are extremely sensitive to changes in the structural correlation length (or short-range ordering) [2]. The magnetocaloric properties can be characterized by the change in the magnetic entropy (ΔSM), which is a measure of the heat transferred between the system and its surroundings during the magnetic field sweep at a given temperature. The ΔSM value during the field sweep was calculated from M–T curves [13]. The equation is given by
Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i
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Fig. 1. M–T curves obtained for samples fabricated at (a) 2 mTorr, (b) 6 mTorr, and (c) 10 mTorr. The numbers on the curves indicate H (in kOe) during the measurement. Measurements were carried out over a wide range of temperatures, between 5 and 300 K, at the lowest H, 1 kOe, whereas at higher H they were carried out over a narrower range (mainly up to a slightly higher T than the respective Tcp). The curves were measured under the FC condition (solid lines), although at the lowest H, 1 kOe, they were also measured under the ZFC condition (dotted lines). In the inset of (a) is shown the XRD pattern.
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Fig. 3. ΔSM as a function of T for samples fabricated at (a) 2 mTorr, (b) 6 mTorr, and (c) 10 mTorr. The curves were obtained under four different magnetic field sweeps: 0–10 kOe (circles), 0–30 kOe (triangles), 0–50 kOe (squares), and 0–70 kOe (diamonds).
⎛ ∂S ⎞ ⎛ ∂M ⎞ ⎟ ⎜ ⎟ = ⎜ ⎝ ∂T ⎠H ⎝ ∂H ⎠T
(2)
Eq. (1) can be re–written as
ΔSM(T , ΔH) =
∫0
H
⎛ ∂M ⎞ ⎜ ⎟ dH ⎝ ∂T ⎠H
The numerical calculation of following equation:
ΔSM =
∑ i
Fig. 2. Variation of Tcp with PAr. The inset shows the variation of the Co/Er molar ratio with PAr.
ΔSM(T , ΔH) = SM(T , H) − SM(T , 0) =
By using the Maxwell relation
∫0
H
⎛ ∂S ⎞ ⎜ ⎟ dH ⎝ ∂H ⎠T
(1)
(3)
ΔSM was carried out by using the
Mi − Mi + 1 ΔHi Ti + 1 − Ti
(4)
where Mi and Mi + 1 are the magnetization values measured at Ti and Ti + 1 in a magnetic field sweep ΔHi , respectively. The dependence of ΔSM on the temperature is shown in Fig. 3(a)–(c) for the same set of samples shown in Fig. 1(a)–(c). A broad maximum, characteristic of an amorphous microstructure [14–16], is observed for all the samples, although it is particularly evident for the samples fabricated at higher PAr. The temperature at which the maximum in ΔSM occurred increased with increasing PAr. This behavior is, in a sense, similar to the behavior observed for Tcp. However, the temperatures of the peak were generally lower than the temperatures observed for the respective values of Tcp; at 10 mTorr, for example, the Tcp value was 185 K, whereas the peak temperature was 100 K. This was actually expected, because the maximum in ΔSM occurs at the inflection point of the M–T curve, where the slope of the curve reaches its
Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i
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magnetocaloric properties were investigated. The Co content decreased slightly with increasing PAr. All the samples exhibited a good magnetic softness, which is expected in an amorphous microstructure, and a compensation behavior, with Tcp continuously increasing from 83 to 141 K in the PAr range of 2–10 mTorr. The dependence of ΔSM on the temperature showed a broad maximum in all the samples, another characteristic of the amorphous microstructure. The temperature at which a maximum in ΔSM occurred increased appreciably with increasing PAr, particularly in the low PAr range. A similar behavior was also observed for ΔSM. Considering the small change occurring in the chemical composition of the thin films as a function of PAr, the changes in the magnetic and magnetocaloric properties are significant, demonstrating the determining role played by the short-range ordering in affecting the magnetic properties. Fig. 4. ΔSM as a function of T measured under the magnetic field sweep of 0– 70 kOe for samples fabricated at 2 mTorr (circles), 4 mTorr (triangles), 6 mTorr (squares), 8 mTorr (diamonds), and 10 mTorr (inverse triangles). The temperatures at which a maximum in ΔSM occurs are indicated by arrows.
maximum. A similar tendency is also observed in the variation of ΔSM with PAr, with ΔSM increasing with increasing PAr. Specifically, ΔSM increases continuously from 1.56 to 3.32 J/kg K as the PAr value increases from 2 to 10 mTorr during the field sweep from 0 to 70 kOe. The variation of the peak temperature and ΔSM with PAr can be observed more clearly in Fig. 4, where the dependence of ΔSM on T is shown for all the samples at the same field sweep, 0–70 kOe. The peak temperature monotonically increases with increasing PAr, although the increment is very small at higher PAr. Note that from Eq. (3), ΔSM should be zero at Tcp, where the slope of the M–T curve is zero. The values of ΔSM obtained for the Er–Co thin films obtained in the present work are similar to those reported in the literature. For example, ΔSM is 5.97 J/kg K in case of an amorphous Co35Er65 ribbon for a field sweep of 0– 50 kOe [17]. No ΔSM values were reported for thin films of the same materials used in the present work, but the following results should be noted: ΔSM was 1.6 J/kg K in case of a Gd thin film for a field sweep of 0–10 kOe [18] and 3.4 J/kg K in case of a Gd/W heterogeneous thin film for a field sweep of 0–30 kOe [19]. At temperatures above Tcp, where the slope of the M–T curve is positive, so that the magnetization increases with increasing T, an inverse magnetocaloric effect (the entropy increases with the application of H) was observed. A similar behavior was observed in other ferrimagnetic or sperimagnetic materials showing the compensation behavior [20], or in antiferromagnetic materials below the Néel temperature, showing a positive slope in the M–T curve [21].
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014M3C1A8048841). MK and SHL wish to thank Mr. Seok Jin Yun for his help in the design of the alloys during the initial stage of the work.
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[14] [15] [16] [17] [18] [19]
4. Conclusions [20]
Binary Er–Co thin films were fabricated by magnetron sputtering from a target of ErCo2 alloy under different values of PAr, varying from 2 to 10 mTorr, and their magnetic and
[21]
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Please cite this article as: M. Kim, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.03.028i