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Giant induced anisotropy ruins the magnetocaloric effect in gadolinium
Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Letter to the Editor
Giant induced anisotropy ruins the magnetocaloric effect in gadolinium S.V. Taskaev a, M.D. Kuz’min b,n, K.P. Skokov c,d, D.Yu. Karpenkov b,d, A.P. Pellenen e, V.D. Buchelnikov a, O. Gutfleisch c a
Physics Faculty, Chelyabinsk State University, 454001 Chelyabinsk, Russia IFW Dresden, Postfach 270116, 01171 Dresden, Germany c Institut f¨ ur Materialwissenschaft, TU Darmstadt, 64287 Darmstadt, Germany d Physics Faculty, Tver State University, 170002 Tver, Russia e National Research South Ural State University, 454080 Chelyabinsk, Russia b
a r t i c l e i n f o
abstract
Article history: Received 1 October 2012 Available online 19 November 2012
Strips of cold-rolled gadolinium have been produced starting from an ingot of 99.9%-pure metal. The rolling has been carried out in a particularly severe regime, the thickness has been reduced 550 times. The rolled strips feature significantly (several times) diminished DT and DS effects, as well as a very large induced magnetic anisotropy, estimated to be between 10 and 15 MJ/m3. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetocaloric effect Gadolinium Magnetic anisotropy
Magnetic refrigerants are always used in comminuted form. This is because the required rapid heat transfer over distances of several centimeters can only be achieved in a two-stage process combining thermal conduction and forced convection. The former is a slow process and a bottle-neck of the whole scheme; it necessitates a close contact between the solid refrigerant and the heat exchange fluid. This was pointed out in the pioneering work of Brown [1], who used 1 mm thick Gd plates. A more recent estimate [2] suggests that the dimensions should be reduced more significantly, down to 0:1 mm. A practical implementation would be a refrigerator bed made of material thinned in either one (foil) or two (wire) dimensions with channels for the heat exchange fluid. Examples are known of cooling devices using foils as thin as 0.076 mm [3]. Alternative solutions include the use of hammered flakes [4] or powders consisting of either spherical [6,5] or irregular [7,8] particles. A fairly complete review of magnetic refrigerators and heat pumps build before 2010 can be found in Ref. [9]. Cold rolling (or drawing) is a simple and cost-efficient way of producing refrigerants reduced in one (or two) dimensions. The downside is that work hardening may affect the magnetocaloric properties of well-characterized bulk materials in an undesirable way. This Letter reports a rather unexpected and dramatic loss of the DT and DS effects in cold-rolled gadolinium. Our main experiment consisted of preparation and characterization of cold-rolled Gd strips. The starting material was 99.9%-
n
Corresponding author. E-mail address: [email protected] (M.D. Kuz’min).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.11.016
pure gadolinium metal purchased from Grirem Advanced Materials, Beijing. It came as a 20 mm-thick ingot. This was heated up to 750 1C in an argon atmosphere and hammered down to a thickness of 5 mm, being reheated twice during the forging. After the forging the slab was etched in 15% HCl solution and cleaned with a wire brush. Further thinning was carried out on two four-high rolling mills. The first one had 100 mm work rolls and was used down to a thickness of 0.4 mm. Thinner strips were produced on a second mill, whose work rolls were 40 mm in diameter. The thickness reduction was between 20% and 50% per run. The rolling was performed at room temperature and interrupted three times for heat treatment (30 min in argon at 750 1C, followed by etching) at thicknesses of 2, 1, and 0.4 mm, in order to reverse hardening and improve ductility. Three representative samples of the rolled material were taken at different stages of the described process. The thicknesses of the samples were as follows: 1.59 mm (Sample 1), 0.057 mm (Sample 2), and 0.036 mm (Sample 3), the numbering being in order of decreasing thickness. Direct measurements of adiabatic temperature change DT ad were carried out in an apparatus described in detail elsewhere [10]. The variable magnetic field was generated by a permanentmagnet assembly and limited to 1.9 T. The field was applied in the plane of the rolled strips, to avoid demagnetization. The acquired DT ad data for Samples 1–3 are plotted against the initial temperature in Fig. 1a. The graph also contains a reference curve measured on polycrystalline Gd (MaTecK GmbH, stated purity 99.9%). One can appreciate that cold rolling has depressed the DT effect in Gd to about one-third of the initial value. Fig. 1b displays the magnetic entropy change for the same samples and the same change of internal magnetic field (0–1.9 T)
S.V. Taskaev et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
Δ
34
Fig. 2. Magnetic field dependence of the maximum DT effect. The curves are labeled as in Fig. 1a.
Fig. 1. (a,b) Magnetocaloric DT and DS effects in gadolinium; the internal magnetic field changes from zero to 1.9 T. (c) Magnetization in a magnetic field of 1 T applied in the direction of the rolling. The numbers on the curves are numbers of the cold-rolled samples. The horizontal scratch and the arrow in panel (b) mark the height and the position of the DS peak in a Gd single crystal according to Ref. [11].
as in Fig. 1a. The entropy change was calculated by means of the Maxwell relation from magnetization curves taken by using a SQUID magnetometer (Quantum Design MPMS-5S) in static magnetic fields up to 4.7 T. Shown in the same Fig. 1b are the height and the position of the maximum of DS for a Gd single crystal, according to Ref. [11]. One observes in Fig. 1b the same trend of depression of the DS effect by cold rolling. The DS peak of the thinnest Sample 3 is 3.5 times lower than that of the single crystal. The trend persists also at other values of the final magnetic field H. This can be observed in Fig. 2, where the height of the DT peaks is plotted versus H2=3 , as well as in Fig. 3a, where a similar dependence is presented for the DS peaks. In both cases the dependence is approximately linear in H2=3 , as expected theoretically [11–13]. It is the slope of the linear dependence that is depressed as a result of the rolling, not the intercept. The latter does not experience any significant systematic change. It has been demonstrated earlier [13] that the intercept is determined by the width of the distribution of Curie points in an inhomogeneous sample—the narrower the distribution, the less the intercept. As opposed to that, the slope is a purely intrinsic property. Thus, a possible broadening of the distribution induced by cold rolling cannot be responsible for the observed depression of the magnetocaloric effect (MCE). Particularly, as one observes in Fig. 1 that
Fig. 3. Magnetic field dependence of the maximum DS effect: (a) main samples labeled as in Fig. 1b, (b) additional samples. The single crystal data are from Ref. [11].
the peaks are not broadened in any significant way, but only reduced in height. Judging by the position of the peaks, the Curie temperature does not change much either. This conclusion is confirmed by the M(T) curves presented in Fig. 1c. An X-ray diffraction analysis revealed no significant change of structure, lattice parameters, or widths of the diffraction lines in the cold-rolled samples. No foreign phases were detected either, apart from traces of gadolinium oxide and hydroxide (most likely, left-overs of an incomplete descaling after the annealing). In order to gain an idea of whether comminution always results in a depression of the MCE, a number of additional samples were studied. Firstly, a piece of very thin (0.025 mm) foil of 99.9%-pure Gd was purchased from Alfa Aesar GmbH. It is known that the foil underwent a heat treatment after it had been rolled, but no details about the heat treatment were available from the supplier. Secondly, some of the polycrystalline gadolinium investigated in
S.V. Taskaev et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
the first part of this work was filed down and the filings were collected for further study. Thirdly, Sample 3 was divided into two parts and one of them was annealed for 1 h in vacuum at 1000 1C. The DSmax -vs-H2=3 plots for the additional samples are presented in Fig. 3b. Also two curves from Fig. 3a are reproduced here for reference. In the graph, the commercial foil features a steep slope — equal roughly to that of Sample 1 — in spite of being thinner than Sample 3. The reason lies, in all likelihood, in the special heat treatment carried out by the manufacturer of the foil. As against that, the filings show a much weaker MCE than the initial polycrystal. The loss has proved reversible: after the filings had been annealed for 1 h in vacuum at 1000 1C, the MCE was restored nearly completely (see Fig. 3b). Likewise, after being annealed for 1 h at 1000 1C, Sample 3 showed a higher MCE, albeit not as high as polycrystalline Gd. A second annealing of Sample 3 for 2 h at 1050 1C, followed by a slow (during 16 h) cooling down to room temperature, produced no visible change in the DS effect. However, a full recovery of both DT and DS effects took place upon remelting. The cause of the drastic reduction of the MCE in cold-rolled Gd becomes clear from Fig. 4. There are magnetization curves of the thinnest Sample 3 in a field pointing in different directions: in the direction of the rolling (curve No. 4), perpendicularly to the sheet (No. 2), and perpendicularly to the previous two (No. 3). No. 1 corresponds to a reference curve measured on the MaTecK polycrystal. The measurements were carried out by means of a vibrating sample magnetometer (Quantum Design PPMS 14) at T¼10 K in static magnetic fields up to 14 T. Curves 1 and 2 were corrected for demagnetization. One sees in Fig. 4 that the cold rolling has induced in polycrystalline Gd a giant magnetic anisotropy. To a good approximation the anisotropy is uniaxial: the direction perpendicular to the sheet is an easy one, whereas the two directions in the plane are practically equivalent and harder. The anisotropy energy was estimated from the area between the curves 2 and 3. To this end the difference of both dependences presented as H vs M was plotted (see the inset of Fig. 4: the solid line indicates the data; the dashed segment was obtained by linear extrapolation; the upper integration limit is the saturation magnetization, Ms ¼ 268 A m2 =kg, Refs. [14,15]). The so-determined induced anisotropy energy is rather large, 9.6 MJ/m3. Yet, it is an underestimation. Apparently the anisotropy is distributed over the sample in such a way that the direction perpendicular to the sheet is preferred on average, but it
Fig. 4. Magnetization curves of cast polycrystalline (No. 1) and cold-rolled (Nos. 2–4) gadolinium. The field is applied either in the direction of the rolling (4), or perpendicularly to the sheet (2), or perpendicularly to the previous two directions (3). The inset illustrates the determination of the area between curves 2 and 3.
35
is not an easy magnetization direction in the true meaning of the word. There is a considerable area between the curves 1 and 2, 2.7 MJ/m3. Speculating, one can imagine that if the local easy axis were oriented the same way in the entire sample, Curve 2 would rise to the position of curve 1, whereas curves 3 and 4 would come down by the same amount. As a result, the area between the magnetization curves in the easy and hard directions would widen up to become as large as 15 MJ/m3 (the area between 2 and 3 plus twice the area between 1 and 2). One can broadly say that the induced anisotropy energy lies between 10 and 15 MJ/m3 (2– 3 meV/atom). This is a very large figure. For comparison, the anisotropy energy of YCo5 equals 6.5 MJ/m3, while the celebrated permanent-magnet material SmCo5 has 24 MJ/m3, both at T¼4.2 K [16]. When compared with single-crystalline Gd, whose magnetocrystalline anisotropy energy is only 0.17 MJ/m3 [17,18], the rolling-induced enhancement of anisotropy is between 1 12 and 2 orders of magnitude. As a consequence, a magnetic field 2 T is grossly insufficient for magnetizing cold-rolled Gd, especially if it is applied perpendicularly to the sheet, as in our earlier experiments. In general, the more severe the rolling, the slower the magnetization growth, the less the MCE. Considering possible reasons of such a strong induced anisotropy in Gd, one can discard right away the mechanism proposed by Chikazumi et al. [19] for Fe–Ni alloys. Firstly, it applies essentially to binary alloys, vanishing in the limit of pure elements. Secondly, the size of the effect in Fe–Ni, 104 J=m3 , is 3 orders of magnitude less than what we have found in Gd. One can equally say that no important role is played by the dipole– dipole interaction. The associated energy density is at the most m0 M2s ¼ 5:6 MJ=m3 , where Ms ¼ 2:11 MA=m is the (volume) saturation magnetization. This is an upper bound, it applies in the extreme case of a needle-shaped sample or to Gd atoms located at a needle-shaped cavity inside the bulk material. It is not inconceivable that fractures of extremely anisotropic shapes might appear during cold rolling and that these fractures would prefer a particular orientation. But even then, only a small proportion of Gd atoms would be located directly at the surface of a cavity. Accordingly, the dipolar anisotropy energy, 5.6 MJ/m3, should be reduced by an appropriate small factor, which makes it irrelevant to the effect under discussion herein ( 10215 MJ=m3 ). In our view, the only anisotropy mechanism able to account for such a high energy density is the single-ion one. Thus, e.g., the anisotropy energy of Tb metal amounts to 88 MJ/m3 at T¼1.8 K [20]. Gadolinium is an exception in this respect: its 4f shell is exactly half-full and has L¼0, it is therefore insensitive to crystal field. We conjecture that this property can be lost by a small proportion of Gd atoms, namely, by those situated near lattice defects. It is unclear at this point what exactly causes such a change. It could be a strong non-centrosymmetric crystal field mixing in excited configurations of different parity, such as 6 1 4f 5d . Or it could be a strongly reduced electron density near 7 1 the defects that makes Gd more akin to Gd2 þ ð4f 5d Þ than to Gd3 þ . Such Gd atoms would possess nonzero orbital moments and make a significant contribution to the net anisotropy energy. Still, the vast majority of the Gd atoms (and thus, the TC) are unaffected by the change and retain L¼0. Taking for the spin– orbit coupling constant of Gd a value of x4f ¼ 0:21 eV [21, Fig. 17], we estimate that in order to account for the observed anisotropy energy (2–3 meV/atom), a mean orbital moment anisotropy of 0:0120:015mB per Gd atom is necessary. This is not implausible. Finally, the annealing reduces the concentration of defects in the lattice, partially restoring the original state of isotropy. In conclusion, severe cold rolling leads to a significant (several times) decrease of both DT and DS magnetocaloric effects in gadolinium. The reduction can be reversed by annealing. The
36
S.V. Taskaev et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
likely cause of the reduction is a giant induced anisotropy, the easy direction being perpendicular to the sheet. The large value of the anisotropy energy, 10215 MJ=m3 , cannot be explained by any of the known mechanisms and is possibly associated with occurrence of significant orbital moments on Gd atoms near lattice defects. References [1] [2] [3] [4] [5] [6] [7] [8]
G.V. Brown, Journal of Applied Physics 47 (1976) 3673. M.D. Kuz’min, Applied Physics Letters 90 (2007) 251916. L.D. Kirol, M.W. Dacus, Advances in Cryogenic Engineering 33 (1988) 757. M.-A. Richard, A.M. Rowe, R. Chanine, Journal of Applied Physics 95 (2004) 2146. C. Zimm, A. Jastrab, A. Sternberg, V.K. Pecharsky, K.A. Gschneidner Jr., M. Osborne, I. Anderson, Advances in Cryogenic Engineering 43 (1998) 1759. T. Okamura, K. Yamada, N. Hirano, S. Nagaya, International Journal of Refrigeration 29 (2006) 1327. C. Zimm, A. Boeder, J. Chell, A. Sternberg, A. Fujita, S. Fujieda, K. Fukamichi, International Journal of Refrigeration 29 (2006) 1302. A. Rowe, A. Tura, International Journal of Refrigeration 29 (2006) 1286.
[9] B. Yu, M. Liu, P.W. Egolf, A. Kitanovski, International Journal of Refrigeration 33 (2010) 1029. ¨ [10] K.P. Skokov, V.V. Khovaylo, K.-H. Muller, J.D. Moore, J. Liu, O. Gutfleisch, Journal of Applied Physics 111 (2012) 07A910. [11] J. Lyubina, M.D. Kuz’min, K. Nenkov, O. Gutfleisch, M. Richter, D.L. Schlagel, T.A. Lograsso, K.A. Gschneidner Jr., Physical Review B 83 (2011) 012403. [12] H. Oesterreicher, F.T. Parker, Journal of Applied Physics 55 (1984) 4334. [13] M.D. Kuz’min, M. Richter, A.M. Tishin, Journal of Magnetism and Magnetic Materials 321 (2009) L1. [14] H.E. Nigh, S. Legvold, F.H. Spedding, Physical Review 132 (1963) 1092. [15] M.D. Kuz’min, A.S. Chernyshov, V.K. Pecharsky, K.A. Gschneidner Jr., A.M. Tishin, Physical Review B 73 (2006) 132403. ¨ [16] H.R. Kirchmayr, E. Burzo, in: H.P.J. Wijn (Ed.), Landolt-Bornstein New Series III, vol. 19/d2, Springer, Berlin, 1990. (Chapter 2.4). ˚ ¨ [17] M. Corarieti-Tosti, S.I. Simak, R. Ahuja, L. Nordstrom, O. Eriksson, D. Aberg, S. Edvardsson, M.S.S. Brooks, Physical Review Letters 91 (2003) 157201. [18] J.J.M. Franse, R. Gersdorf, Physical Review Letters 45 (1980) 50. [19] S. Chikazumi, K. Suzuki, H. Iwata, Journal of the Physical Society of Japan 12 (1957) 1259. ¨ Angewandte Physik 30 (1970) [20] J.L. Fe´ron, G. Hug, R. Pauthenet, Zeitschrift Fur 61. [21] M. Richter, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, vol. 13, North-Holland, Amsterdam, 2001 (Chapter 2).
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Letter to the Editor
Giant induced anisotropy ruins the magnetocaloric effect in gadolinium S.V. Taskaev a, M.D. Kuz’min b,n, K.P. Skokov c,d, D.Yu. Karpenkov b,d, A.P. Pellenen e, V.D. Buchelnikov a, O. Gutfleisch c a
Physics Faculty, Chelyabinsk State University, 454001 Chelyabinsk, Russia IFW Dresden, Postfach 270116, 01171 Dresden, Germany c Institut f¨ ur Materialwissenschaft, TU Darmstadt, 64287 Darmstadt, Germany d Physics Faculty, Tver State University, 170002 Tver, Russia e National Research South Ural State University, 454080 Chelyabinsk, Russia b
a r t i c l e i n f o
abstract
Article history: Received 1 October 2012 Available online 19 November 2012
Strips of cold-rolled gadolinium have been produced starting from an ingot of 99.9%-pure metal. The rolling has been carried out in a particularly severe regime, the thickness has been reduced 550 times. The rolled strips feature significantly (several times) diminished DT and DS effects, as well as a very large induced magnetic anisotropy, estimated to be between 10 and 15 MJ/m3. & 2012 Elsevier B.V. All rights reserved.
Keywords: Magnetocaloric effect Gadolinium Magnetic anisotropy
Magnetic refrigerants are always used in comminuted form. This is because the required rapid heat transfer over distances of several centimeters can only be achieved in a two-stage process combining thermal conduction and forced convection. The former is a slow process and a bottle-neck of the whole scheme; it necessitates a close contact between the solid refrigerant and the heat exchange fluid. This was pointed out in the pioneering work of Brown [1], who used 1 mm thick Gd plates. A more recent estimate [2] suggests that the dimensions should be reduced more significantly, down to 0:1 mm. A practical implementation would be a refrigerator bed made of material thinned in either one (foil) or two (wire) dimensions with channels for the heat exchange fluid. Examples are known of cooling devices using foils as thin as 0.076 mm [3]. Alternative solutions include the use of hammered flakes [4] or powders consisting of either spherical [6,5] or irregular [7,8] particles. A fairly complete review of magnetic refrigerators and heat pumps build before 2010 can be found in Ref. [9]. Cold rolling (or drawing) is a simple and cost-efficient way of producing refrigerants reduced in one (or two) dimensions. The downside is that work hardening may affect the magnetocaloric properties of well-characterized bulk materials in an undesirable way. This Letter reports a rather unexpected and dramatic loss of the DT and DS effects in cold-rolled gadolinium. Our main experiment consisted of preparation and characterization of cold-rolled Gd strips. The starting material was 99.9%-
n
Corresponding author. E-mail address: [email protected] (M.D. Kuz’min).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.11.016
pure gadolinium metal purchased from Grirem Advanced Materials, Beijing. It came as a 20 mm-thick ingot. This was heated up to 750 1C in an argon atmosphere and hammered down to a thickness of 5 mm, being reheated twice during the forging. After the forging the slab was etched in 15% HCl solution and cleaned with a wire brush. Further thinning was carried out on two four-high rolling mills. The first one had 100 mm work rolls and was used down to a thickness of 0.4 mm. Thinner strips were produced on a second mill, whose work rolls were 40 mm in diameter. The thickness reduction was between 20% and 50% per run. The rolling was performed at room temperature and interrupted three times for heat treatment (30 min in argon at 750 1C, followed by etching) at thicknesses of 2, 1, and 0.4 mm, in order to reverse hardening and improve ductility. Three representative samples of the rolled material were taken at different stages of the described process. The thicknesses of the samples were as follows: 1.59 mm (Sample 1), 0.057 mm (Sample 2), and 0.036 mm (Sample 3), the numbering being in order of decreasing thickness. Direct measurements of adiabatic temperature change DT ad were carried out in an apparatus described in detail elsewhere [10]. The variable magnetic field was generated by a permanentmagnet assembly and limited to 1.9 T. The field was applied in the plane of the rolled strips, to avoid demagnetization. The acquired DT ad data for Samples 1–3 are plotted against the initial temperature in Fig. 1a. The graph also contains a reference curve measured on polycrystalline Gd (MaTecK GmbH, stated purity 99.9%). One can appreciate that cold rolling has depressed the DT effect in Gd to about one-third of the initial value. Fig. 1b displays the magnetic entropy change for the same samples and the same change of internal magnetic field (0–1.9 T)
S.V. Taskaev et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
Δ
34
Fig. 2. Magnetic field dependence of the maximum DT effect. The curves are labeled as in Fig. 1a.
Fig. 1. (a,b) Magnetocaloric DT and DS effects in gadolinium; the internal magnetic field changes from zero to 1.9 T. (c) Magnetization in a magnetic field of 1 T applied in the direction of the rolling. The numbers on the curves are numbers of the cold-rolled samples. The horizontal scratch and the arrow in panel (b) mark the height and the position of the DS peak in a Gd single crystal according to Ref. [11].
as in Fig. 1a. The entropy change was calculated by means of the Maxwell relation from magnetization curves taken by using a SQUID magnetometer (Quantum Design MPMS-5S) in static magnetic fields up to 4.7 T. Shown in the same Fig. 1b are the height and the position of the maximum of DS for a Gd single crystal, according to Ref. [11]. One observes in Fig. 1b the same trend of depression of the DS effect by cold rolling. The DS peak of the thinnest Sample 3 is 3.5 times lower than that of the single crystal. The trend persists also at other values of the final magnetic field H. This can be observed in Fig. 2, where the height of the DT peaks is plotted versus H2=3 , as well as in Fig. 3a, where a similar dependence is presented for the DS peaks. In both cases the dependence is approximately linear in H2=3 , as expected theoretically [11–13]. It is the slope of the linear dependence that is depressed as a result of the rolling, not the intercept. The latter does not experience any significant systematic change. It has been demonstrated earlier [13] that the intercept is determined by the width of the distribution of Curie points in an inhomogeneous sample—the narrower the distribution, the less the intercept. As opposed to that, the slope is a purely intrinsic property. Thus, a possible broadening of the distribution induced by cold rolling cannot be responsible for the observed depression of the magnetocaloric effect (MCE). Particularly, as one observes in Fig. 1 that
Fig. 3. Magnetic field dependence of the maximum DS effect: (a) main samples labeled as in Fig. 1b, (b) additional samples. The single crystal data are from Ref. [11].
the peaks are not broadened in any significant way, but only reduced in height. Judging by the position of the peaks, the Curie temperature does not change much either. This conclusion is confirmed by the M(T) curves presented in Fig. 1c. An X-ray diffraction analysis revealed no significant change of structure, lattice parameters, or widths of the diffraction lines in the cold-rolled samples. No foreign phases were detected either, apart from traces of gadolinium oxide and hydroxide (most likely, left-overs of an incomplete descaling after the annealing). In order to gain an idea of whether comminution always results in a depression of the MCE, a number of additional samples were studied. Firstly, a piece of very thin (0.025 mm) foil of 99.9%-pure Gd was purchased from Alfa Aesar GmbH. It is known that the foil underwent a heat treatment after it had been rolled, but no details about the heat treatment were available from the supplier. Secondly, some of the polycrystalline gadolinium investigated in
S.V. Taskaev et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 33–36
the first part of this work was filed down and the filings were collected for further study. Thirdly, Sample 3 was divided into two parts and one of them was annealed for 1 h in vacuum at 1000 1C. The DSmax -vs-H2=3 plots for the additional samples are presented in Fig. 3b. Also two curves from Fig. 3a are reproduced here for reference. In the graph, the commercial foil features a steep slope — equal roughly to that of Sample 1 — in spite of being thinner than Sample 3. The reason lies, in all likelihood, in the special heat treatment carried out by the manufacturer of the foil. As against that, the filings show a much weaker MCE than the initial polycrystal. The loss has proved reversible: after the filings had been annealed for 1 h in vacuum at 1000 1C, the MCE was restored nearly completely (see Fig. 3b). Likewise, after being annealed for 1 h at 1000 1C, Sample 3 showed a higher MCE, albeit not as high as polycrystalline Gd. A second annealing of Sample 3 for 2 h at 1050 1C, followed by a slow (during 16 h) cooling down to room temperature, produced no visible change in the DS effect. However, a full recovery of both DT and DS effects took place upon remelting. The cause of the drastic reduction of the MCE in cold-rolled Gd becomes clear from Fig. 4. There are magnetization curves of the thinnest Sample 3 in a field pointing in different directions: in the direction of the rolling (curve No. 4), perpendicularly to the sheet (No. 2), and perpendicularly to the previous two (No. 3). No. 1 corresponds to a reference curve measured on the MaTecK polycrystal. The measurements were carried out by means of a vibrating sample magnetometer (Quantum Design PPMS 14) at T¼10 K in static magnetic fields up to 14 T. Curves 1 and 2 were corrected for demagnetization. One sees in Fig. 4 that the cold rolling has induced in polycrystalline Gd a giant magnetic anisotropy. To a good approximation the anisotropy is uniaxial: the direction perpendicular to the sheet is an easy one, whereas the two directions in the plane are practically equivalent and harder. The anisotropy energy was estimated from the area between the curves 2 and 3. To this end the difference of both dependences presented as H vs M was plotted (see the inset of Fig. 4: the solid line indicates the data; the dashed segment was obtained by linear extrapolation; the upper integration limit is the saturation magnetization, Ms ¼ 268 A m2 =kg, Refs. [14,15]). The so-determined induced anisotropy energy is rather large, 9.6 MJ/m3. Yet, it is an underestimation. Apparently the anisotropy is distributed over the sample in such a way that the direction perpendicular to the sheet is preferred on average, but it
Fig. 4. Magnetization curves of cast polycrystalline (No. 1) and cold-rolled (Nos. 2–4) gadolinium. The field is applied either in the direction of the rolling (4), or perpendicularly to the sheet (2), or perpendicularly to the previous two directions (3). The inset illustrates the determination of the area between curves 2 and 3.
35
is not an easy magnetization direction in the true meaning of the word. There is a considerable area between the curves 1 and 2, 2.7 MJ/m3. Speculating, one can imagine that if the local easy axis were oriented the same way in the entire sample, Curve 2 would rise to the position of curve 1, whereas curves 3 and 4 would come down by the same amount. As a result, the area between the magnetization curves in the easy and hard directions would widen up to become as large as 15 MJ/m3 (the area between 2 and 3 plus twice the area between 1 and 2). One can broadly say that the induced anisotropy energy lies between 10 and 15 MJ/m3 (2– 3 meV/atom). This is a very large figure. For comparison, the anisotropy energy of YCo5 equals 6.5 MJ/m3, while the celebrated permanent-magnet material SmCo5 has 24 MJ/m3, both at T¼4.2 K [16]. When compared with single-crystalline Gd, whose magnetocrystalline anisotropy energy is only 0.17 MJ/m3 [17,18], the rolling-induced enhancement of anisotropy is between 1 12 and 2 orders of magnitude. As a consequence, a magnetic field 2 T is grossly insufficient for magnetizing cold-rolled Gd, especially if it is applied perpendicularly to the sheet, as in our earlier experiments. In general, the more severe the rolling, the slower the magnetization growth, the less the MCE. Considering possible reasons of such a strong induced anisotropy in Gd, one can discard right away the mechanism proposed by Chikazumi et al. [19] for Fe–Ni alloys. Firstly, it applies essentially to binary alloys, vanishing in the limit of pure elements. Secondly, the size of the effect in Fe–Ni, 104 J=m3 , is 3 orders of magnitude less than what we have found in Gd. One can equally say that no important role is played by the dipole– dipole interaction. The associated energy density is at the most m0 M2s ¼ 5:6 MJ=m3 , where Ms ¼ 2:11 MA=m is the (volume) saturation magnetization. This is an upper bound, it applies in the extreme case of a needle-shaped sample or to Gd atoms located at a needle-shaped cavity inside the bulk material. It is not inconceivable that fractures of extremely anisotropic shapes might appear during cold rolling and that these fractures would prefer a particular orientation. But even then, only a small proportion of Gd atoms would be located directly at the surface of a cavity. Accordingly, the dipolar anisotropy energy, 5.6 MJ/m3, should be reduced by an appropriate small factor, which makes it irrelevant to the effect under discussion herein ( 10215 MJ=m3 ). In our view, the only anisotropy mechanism able to account for such a high energy density is the single-ion one. Thus, e.g., the anisotropy energy of Tb metal amounts to 88 MJ/m3 at T¼1.8 K [20]. Gadolinium is an exception in this respect: its 4f shell is exactly half-full and has L¼0, it is therefore insensitive to crystal field. We conjecture that this property can be lost by a small proportion of Gd atoms, namely, by those situated near lattice defects. It is unclear at this point what exactly causes such a change. It could be a strong non-centrosymmetric crystal field mixing in excited configurations of different parity, such as 6 1 4f 5d . Or it could be a strongly reduced electron density near 7 1 the defects that makes Gd more akin to Gd2 þ ð4f 5d Þ than to Gd3 þ . Such Gd atoms would possess nonzero orbital moments and make a significant contribution to the net anisotropy energy. Still, the vast majority of the Gd atoms (and thus, the TC) are unaffected by the change and retain L¼0. Taking for the spin– orbit coupling constant of Gd a value of x4f ¼ 0:21 eV [21, Fig. 17], we estimate that in order to account for the observed anisotropy energy (2–3 meV/atom), a mean orbital moment anisotropy of 0:0120:015mB per Gd atom is necessary. This is not implausible. Finally, the annealing reduces the concentration of defects in the lattice, partially restoring the original state of isotropy. In conclusion, severe cold rolling leads to a significant (several times) decrease of both DT and DS magnetocaloric effects in gadolinium. The reduction can be reversed by annealing. The
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likely cause of the reduction is a giant induced anisotropy, the easy direction being perpendicular to the sheet. The large value of the anisotropy energy, 10215 MJ=m3 , cannot be explained by any of the known mechanisms and is possibly associated with occurrence of significant orbital moments on Gd atoms near lattice defects. References [1] [2] [3] [4] [5] [6] [7] [8]
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