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Magnetic and electrical properties of Co2ScSn
Journal of Magnetism North-Holland
and Magnetic
Materials
116 (1992) 355-360
Magnetic and electrical properties
of Co,ScSn
C.L. Lin, T. Mihalisin Department of Physics, Temple Unir’ersiTy, Philadelphia, PA 19122, USA
and N. Bykovetz ’ Department of Materials Science and Engineering, University of Pennsylcania, Philadelphia, PA 19104, USA Received
14 February
1992; in revised
form 8 April
1992
Magnetization measurements as a function of temperature and applied magnetic field indicate that the Heusler compound Co,ScSn undergoes a magnetic transition at T, = 238 K. Magnetization versus field curves at constant temperatures suggest a helical state for zero and small applied fields in the 100 K I T I 238 K temperature range. Magnetization versus temperature curves for constant applied fields indicate that for low fields the sample undergoes a second transition to a spin glass state at temperatures T,,(H) < T,. For H = 0, Tsg is about 100 K. Ts8 is depressed by an to the electrical resistivity appears to applied field and goes to zero for H = 1.6 kOe. The estimated magnetic contribution be proportional to T2 up to 200 K and shows a Fisher-Langer type anomaly near 238 K.
1. Introduction
The magnetic properties of cubic L,, Heusler alloys of form X,YZ, where X and Y are transition metals and Z is an sp element such as Al, Ga, Sn, In, etc., have been thoroughly investigated [ 11. More recently X,YZ compounds, where Y is a rare-earth element R rather than a transition metal, have been studied including Pd,RSn [2,3], Ag,RIn 141, Au,RIn [5], and Cu,RIn [6]. Most X,YZ compounds order magnetically. A few are superconducting and at least one namely Pd,YbSn [2,7] appears to have coexisting superconductivity and magnetism. Correspondence to: Prof. C.L. Lin, Department of Physics, Temple University, Philadelphia, PA 19122, USA. Tel.: + l215-787-8636. ’ On a one-year leave from the Spring Garden College, Philadelphia, PA. 0304-8853/92/$05.00
0 1992 - Elsevier
Science
Publishers
Malik et al. [S] have reported that Co,ScSn forms as a Heusler compound and reported a Curie temperature of 270 K and a moment of 0.55 pB per Co atom. Their results were based on ac susceptibility vs. temperature measurements, as well as, dc magnetization vs. temperature at an applied field of 5 kOe, and a magnetization vs. magnetic field measurement at a temperature of 5 K. In addition, they measured a Sn hyperfine field of 40 kOe at 8 K. We have extended the magnetic measurements of Malik et al. in several respects utilizing a Co,ScSn sample that underwent a more prolonged anneal at a higher temperature. X-ray diffraction measurements indicate that this sample is of higher quality than the sample fabricated by Malik et al. In particular we see no evidence of the presence of a second crystallographic phase. We have measured the magnetization vs. field at 5 K, and at several higher temperatures in the
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356
C.L. Lin et al. / Magnetic and
ferromagnetic state, e.g. 100, 200, 225 and 236 K. Moreover, we have measured magnetization versus temperature for H = 5 kOe and several lower fields, e.g. 1 and 0.5 kOe. In addition, we have measured the electrical resistivity of Co,ScSn and Ni,ScSn at H = 0 and 5 kOe. We find a saturation moment of 0.51 pn per Co atom which is reasonably close to the value of 0.55 pa per Co atom reported by Malik et al. Both values are well below the value of 1.7 pn per Co observed for pure Co metal. However, Arrot plots for T = 233, 235, 238, 241 and 243 K indicate that T, = 238 K, a value significantly lower than the 270 K value reported by Malik et al. Moreover, the initial slopes for the M versus H curves are identical for T = 100, 200, 225 and 236 K suggesting that at low fields and at temperatures in the range 100 K < T < 238 K the sample may be in an itinerant helical state [9-111. Our magnetization versus temperature results suggest that for fields less than 1.6 kOe the sample undergoes a transition to a spin glass state for temperatures T I Tsg. At H = 0, Tsg is about 100 K. As H is increased Tsg is lowered to 90 K for H = 0.5 kOe and to 60 K for H = 1 kOe. Assuming a depression of Tsg that is proportional to Hz gives a critical field for driving Tsg to zero of about 1.6 kOe. We have used the resistivity of Ni,ScSn to estimate the non-magnetic resistivity of Co,ScSn. Ni,ScSn shows no magnetism but is also a L,, Heusler compound with a lattice constant that @ reasonably close to thai of Co,ScSn (i.e. 6.215 A for Ni,ScSn and 6.190 A for Co,ScSn) [8,12]. The resulting magnetic component of resistivity for Co,ScSn shows a T2 behavior over a very large temperature interval up to 200 K.
electricalproperties of CozScSn
Powdered X-ray diffraction measurements at room temperature indicated that the samples which were annealed for 8 weeks were singly phase, with lattice constants of 6.190 and 6.215 A for Co,ScSn and Ni,ScSn, respectively. These values are in good agreement with those previously published [8,12]. However, the unannealed samples and samples annealed for less than 1 week, showed some extra weak-intensity lines which could be indexed with a CsCl type structure probably due to a disorder of the three different atoms on the sublattices. The resistivity measurements were performed using the standard dc 4-probe technique for 1.2 K < T < 300 K. The magnetization was measured from 2 to 400 K in magnetic fields up to 5.5 T using a SQUID magnetometer. Only measurements on the well annealed samples are presented here.
3. Results and discussion Arrott plots CM2 vs. H/M) for Co,ScSn at various temperatures are shown in fig. 1. The Curie temperature is defined as the temperature for which a linearly extrapolated curve on the Arrott plot goes through the origin. We find T, = (238 k A) K, where A is less than 3 K. This
co,scsn
T-233K
t
2. Experimental Polycrystalline Co,ScSn and Ni,ScSn samples were prepared in an inert atmosphere arc furnace with appropriate care taken to compensate for the weight loss of the more volatile Sn. The samples wer.e wrapped in Ta foils and were annealed under high vacuum at 800°C for different time intervals ranging from 1 day to 8 weeks.
H/M
(mole-Oe/emu)
Fig. 1. Arrott plots of M* vs. H/M
at T =
C. L. Lin et al. / Magnetic and electrical properties of Co,ScSn
value is significantly lower than the T, = 270 K report’ed by Malik et al. [8]. The field dependence of the magnetization M(H) at various temperatures for Co,ScSn annealed at 800°C for 8 weeks is shown in fig. 2. At T = 100 K, M(H) increases almost linearly with increasing applied field up to a field H, of about 400 G, then it starts to saturate when an applied field is further increased. As the temperature increases, e.g. 200-236 K, M(H) still shows the same behavior except the field H, decreases, as can be seen in fig. 2. At T = 240 K, the linear behavior disappears and M(H) shows a negative curvature over the entire range applied fields shown. For even higher temperatures M(H) exhibits linear behavior for all fields as one would expect for the paramagnetic regime. The behavior described above, i.e. that a constant magnetic susceptibility (linear M vs. H) occurs at temperatures below the critical temperature T, up to a field H,, has been observed previously for other itinerant magnets, e.g. MnSi and Fe,Co,_,Si [lO,ll]. This behavior was explained in terms of a helical magnetic structure below T, for zero and low applied fields. This interpretation was confirmed by neutron diffraction [ill and electron spin resonance [13] measurements and is consistent with theoretical calculations [14,15] based on
Co,ScSn
1OOK
: .E I
2500
0 0
2500
H (Oel Fig. 2. Magnetization M vs. field H at T = 100, 200, 22.5, i28, 232,236,240,245 and 260 K.
0
~h~@.~.~.~.@.~.~
~.~...~.~~..~.~‘.~~~.” 0
357
100
200
300
‘loo
T (K1
Fig. 3. Magnetization M vs. temperature T for applied fields of (a) 500 Oe, (b) 1 kOe and (c) 5 kOe.
a competition between a Heisenberg type exchange interaction and a weak DzyaloshinskyMoriya coupling which can occur in the noncentrosymmetric cubic structure. Microscopic studies, e.g. neutron diffraction and electron spin resonance, must be performed on Co,ScSn in order to fully understand the magnetic structure at zero field. The data of fig. 1 (Arrott plot) are only for T close to T, = 238 K. The M vs. H data of fig. 2 are only shown for temperatures just above T, and down to T = 100 K. The reason for not displaying M vs. H for T < 100 K in fig. 2 will become apparent from the data shown in fig. 3. Figure 3 shows M vs. T from 10 to 400 K, for H = 500 Oe, 1 and 5 kOe. The H = 500 Oe curve shows a maximum near 90 K while the H = 1 kOe curve has a less pronounced maximum near 60 K. The H = 5 kOe curve does not show a maximum but rather increases monotonically as one lowers the temperature. Data were taken first by cooling the sample in zero field and then applying the field and measuring the magnetization vs. temperature from T = 10 to 380 K. Then a second measurement is taken by cooling the sample in field from 380 down to 10 K. Careful examination of the H = 500 Oe and the H = 1 kOe data shows that below the M vs. T maximum the field cooled data are slightly higher
C.L. Lin et al. / Magnetic and electrical properties of Co,ScSn
358
than the zero-field cooled data. The maximum in M vs. T at least for the H = 500 Oe and 1 kOe data and the hysteresis with temperature are indicative of spin glass behavior. If one assumes that an applied field suppresses the spin glass transition in a quadratic manner, i.e. TSp= TSspo th en one finds that TSg goes to (1 -Hz/H;‘) zero at H,E = 1.6 kOe, and TSpo= 100 K. We note that such a rapid suppression of the transition with magnetic field rules out the possibility that the transition is from a ferromagnetic to an antiferromagnetic state. Similar behavior has been reported in the case of some other transitionmetal based alloys, e.g. (Fe,Ni, _x179P,3B8, FeXAIIcx,_, and FexSn,,,,_, [16]. The spin glass behavior at temperatures below T, has been explained as due to the existence of a ferromagnetic exchange interaction between nearest neighbors and an antiferromagnetic superexchange interaction between next-nearest neighbors. Figure 4 shows M vs. T for an applied field of 10 Oe. Note the more pronounced temperature hysteresis and that the temperature at which the zero-field curve data show a maximum is indeed close to 100 K. In addition to the H and T dependence of the magnetization we have also measured the electri-
L 0 0
““““.~“~~~~‘~~~~““‘~.‘(‘...‘~~~~ 100
200
Boa
..%0.OAOIO 300
400
T (K)
Fig. 4. Magnetization M vs. temperature T measured from 10 to 380 K in an applied field of 10 Oe after the sample was cooled in zero field, and subsequently field cooled from 380 down to 10 K.
0
200
100
300
T (K)
Fig. 5. (a) Total resistivity p vs. temperature T in zero applied field for Co,ScSn and Ni,ScSn. (b) Magnetic resistivity Ap as estimated by subtracting p(T) for Ni,ScSn from p(T) for CozScSn vs. temperature T. The insets show dp/dr vs. T, and Ap vs. T2 up to 200 K.
cal resistivity of Co,ScSn. The temperature dependence of the resistivity p(T) for the Co,ScSn and Ni,ScSn samples, both annealed at 800°C for 8 weeks, is shown in fig. 5(a). It is assumed that the phonon contribution to the resistivity of Co,ScSn is similar to p(T) of Ni,ScSn because Co,ScSn and Ni,ScSn have the same crystal structure and their lattice constants differ by less than 0.5%. Moreover, Ni,ScSn remains a weak paramagnet down to T = 1.2 K [17]. It can be seen from fig. 5 that Ni,ScSn has the typical resistivity behavior that one expects for non-magnetic compound. In particular, unlike p(T) for Co,ScSn, when p(T) for Ni,ScSn at high temperatures, e.g. 200 to 300 K, is linearly extrapolated to T = 0 it intercepts the p axis at the measured residual resistivity. Ap(T) of Co,ScSn, which is obtained by subtracting p(T) of Ni,ScSn from p(T) of Co,ScSn, is shown in fig. 5(b). T, is about (235 + 5) K as determined by the temperature where Ap shows a downward deviation from its constant high-temperature behavior. The anomaly is qualitatively consistent with a Fisher-Langer type behavior. Hence T, from p measurements is
C.L. Lin et al. / Magnetic and electrical properties of Co,ScSn
consistent with the 238 K value we obtained from Arrott plots but far below the 270 K value reported by Malik et al. Ap(7’) below T, decreases monotonically with decreasing temperature due to a reduction of magnetic scattering. Although Ap(T) drops about 65 ufi cm from T = T, to T -+ 0, the residual resistivity, i.e. p(T -+ 01, is still very high. A high residual resistivity occurs in many other Heusler alloys [18]. In the inset of fig. 5(b) we show Ap(T) vs. T2 for Co,ScSn. It should be noted that a T2 dependence of Ap, like that predicted by the SCR theory [19] for itinerant ferromagnets below T,, holds over a wide range of temperatures. In particular, Ap N T2 from T = 100 to 200 K. Below 100 K clear deviations from Ap N T2 are seen. Since the M(T) data of fig. 4 suggest that a transition to a spin glass state occurs at this temperature (for H = 0) it is natural to speculate that the spin glass transition is responsible for this deviation. However, it should be noted that Ap(T) measurements in a field of 5 kOe are essentially identical to those shown in fig. 5(b), even though the data of fig. 3 suggest that 5 kOe is sufficient to suppress the spin glass transition. The linear coefficient of the T2 term, R,, is 1.13 X lo-’ @ cm/K2 for Co,ScSn which is much smaller than that for other itinerant ferromagnets 1201,e.g. R, = 47 X 10e3 l~fi cm/K2 for ZrZn, (T, = 23 K) or R, = 9.7 X 1O-3 ~JL! cm/K2 for Ni,Al CT, = 45-60 K). All of these R, values seem to follow a trend but do not agree quantitatively with the theoretical prediction, R, = TcP213. The deviation from the TcP213 relation might be due to the choice of the temperature range which was chosen for a determination of the T* coefficient. It has been shown that for the Heusler alloys the crystal structure, transport, thermodynamic and magnetic properties are influenced significantly by heat treatment [21]. We have also studied the X-ray diffraction, resistivity and magnetization of Co,ScSn samples annealed for shorter times and/or lower temperatures and found that they have disordered crystal structures and ilI-defined transitions and include free Co metal. We have studied the temperature dependence of the resistivity and the temperature and mag-
359
netic field dependence of the magnetization of Co,ScSn. Our results show that at zero field and in low applied fields Co,ScSn undergoes a transition from a paramagnetic state to a possible helimagnetic state at T, = 238 K and exhibits a spin glass behavior below 100 K. At higher applied fields, the system becomes ferromagnetic below T,. The freezing temperature decreases with increasing applied field and is completely suppressed by H = 5 kG. To better understand this interesting material, we are performing Mossbauer-effect measurements in order to obtain a microscopic view of its magnetic structures. Presumably, studies of neutron diffraction will also provide useful information as to the nature of the magnetic properties of this system.
Acknowledgements
We acknowledge the support of the Materials Research Center and Grant-in-Aid and Faculty Summer Research Fellowship programs at Temple University.
References [l] P.J. Webster, Contemp. Phys. 10 (1969) 559. [2] M. lshikawa, J.L. Jorda and A. Junod, Superconductivity in d-and f-band Metals, Proc. of the IV Conf., Karlsruhe, 1982, eds. W. Buckel and W. Weber, Kernforschungszentrum, Karlsruhe (1982) p. 141. [3] SK. Malik, A.M. Umarji and G.K. Shenoy, Phys. Rev. B 31 (1985) 6971. [4] R.M. Galera, J. Pierrie, E. Siaud and A.P. Murani, J. Less-Common Met. 97 (1984) 151. [5] M.J. Besnus, J.P. Kappler, M.F. Ravet, A. Meyer, R. Lahiouel, J. Pierre, E. Siaud, G. Nieva and J. Sereni, J. Less-Common Met. 120 (1986) 101. [6] 1. Felner, Solid State Commun. 56 (1985) 315. [7] H.A. Kierstead, B.D. Dunlap, S.K. Malik, A.M. Umarji and G.K. Shenoy, Phys. Rev. B 32 (1985) 135. [8] S.K. Malik, P.L. Paulose, R. Nagarajan, A.E. Dwight, P.P. Vaishnava and C.W. Kimball, Hyperfine Interactions 34 (1987) 427. [9] K. Makoshi and T. Moriya, J. Phys. Sot. Jpn. 44 (1978) 80. [lo] K. Shim& H. Maruyama, H. Yamazaki and H. Watanabe, J. Phys. Sot. Jpn. 59 (1990) 305. [ll] J. Beille, J. Voiron, F. Towfiq, M. Roth and Z.Y. Zhang, J. Phys. F 11 (1981) 2153.
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[12] [13] [14] [15] [16] [17]
C.L. Lin et al. / Magnetic and electrical properties of Co,ScSn J. Beille, J. Voiron and M. Roth, Solid State Commun. 47 (1983) 399. A.E. Dwight and C.W. Kimball, J. Less-Common Met. 127 (1987) 179. H. Watanabe, Y. Tazuke and H. Nakajima, J. Phys. Sot. Jpn. 54 (1985) 3978. 0. Nakanishi, A. Yanase and A. Hasegawa, J. Magn. Magn. Mater. 15-18 (1980) 879. P. Bak and M.H. Jensen, J. Phys. C 13 (1981) L881. K. Moorjani and J.M.D. Coey, in: Magnetic Glasses (Elsevier, New York, 1984) chap. VII, p. 299. C.L. Lin and T. Mihalisin, to be published.
[18] T.T. Palstra, G.J. Nieuwenhuys, R.F.M. Vlastuin, J. van den Berg, J.A. Mydosh and K.H.J. Buschow, J. Magn. Magn. Mater. 67 (1987) 331. [19] T. Moriya, in: Spin Fluctuations in Itinerant Electron Magnetism, Springer Series in Solid State Sciences 56 (Springer-Verlag, Berlin, 1985) p. 102. [20] H. Sasakura, K. Suzuki and Y. Masuda, J. Phys. Sot. Jpn. 53 (1984) 352. [21] N. Bykovetz, W.H. Herman, T. Yuen, C. Jee, C.L. Lin and J.E. Crow, J. Appl. Phys. 63 (1988) 4127, and refs. therein.
and Magnetic
Materials
116 (1992) 355-360
Magnetic and electrical properties
of Co,ScSn
C.L. Lin, T. Mihalisin Department of Physics, Temple Unir’ersiTy, Philadelphia, PA 19122, USA
and N. Bykovetz ’ Department of Materials Science and Engineering, University of Pennsylcania, Philadelphia, PA 19104, USA Received
14 February
1992; in revised
form 8 April
1992
Magnetization measurements as a function of temperature and applied magnetic field indicate that the Heusler compound Co,ScSn undergoes a magnetic transition at T, = 238 K. Magnetization versus field curves at constant temperatures suggest a helical state for zero and small applied fields in the 100 K I T I 238 K temperature range. Magnetization versus temperature curves for constant applied fields indicate that for low fields the sample undergoes a second transition to a spin glass state at temperatures T,,(H) < T,. For H = 0, Tsg is about 100 K. Ts8 is depressed by an to the electrical resistivity appears to applied field and goes to zero for H = 1.6 kOe. The estimated magnetic contribution be proportional to T2 up to 200 K and shows a Fisher-Langer type anomaly near 238 K.
1. Introduction
The magnetic properties of cubic L,, Heusler alloys of form X,YZ, where X and Y are transition metals and Z is an sp element such as Al, Ga, Sn, In, etc., have been thoroughly investigated [ 11. More recently X,YZ compounds, where Y is a rare-earth element R rather than a transition metal, have been studied including Pd,RSn [2,3], Ag,RIn 141, Au,RIn [5], and Cu,RIn [6]. Most X,YZ compounds order magnetically. A few are superconducting and at least one namely Pd,YbSn [2,7] appears to have coexisting superconductivity and magnetism. Correspondence to: Prof. C.L. Lin, Department of Physics, Temple University, Philadelphia, PA 19122, USA. Tel.: + l215-787-8636. ’ On a one-year leave from the Spring Garden College, Philadelphia, PA. 0304-8853/92/$05.00
0 1992 - Elsevier
Science
Publishers
Malik et al. [S] have reported that Co,ScSn forms as a Heusler compound and reported a Curie temperature of 270 K and a moment of 0.55 pB per Co atom. Their results were based on ac susceptibility vs. temperature measurements, as well as, dc magnetization vs. temperature at an applied field of 5 kOe, and a magnetization vs. magnetic field measurement at a temperature of 5 K. In addition, they measured a Sn hyperfine field of 40 kOe at 8 K. We have extended the magnetic measurements of Malik et al. in several respects utilizing a Co,ScSn sample that underwent a more prolonged anneal at a higher temperature. X-ray diffraction measurements indicate that this sample is of higher quality than the sample fabricated by Malik et al. In particular we see no evidence of the presence of a second crystallographic phase. We have measured the magnetization vs. field at 5 K, and at several higher temperatures in the
B.V. All rights
reserved
356
C.L. Lin et al. / Magnetic and
ferromagnetic state, e.g. 100, 200, 225 and 236 K. Moreover, we have measured magnetization versus temperature for H = 5 kOe and several lower fields, e.g. 1 and 0.5 kOe. In addition, we have measured the electrical resistivity of Co,ScSn and Ni,ScSn at H = 0 and 5 kOe. We find a saturation moment of 0.51 pn per Co atom which is reasonably close to the value of 0.55 pa per Co atom reported by Malik et al. Both values are well below the value of 1.7 pn per Co observed for pure Co metal. However, Arrot plots for T = 233, 235, 238, 241 and 243 K indicate that T, = 238 K, a value significantly lower than the 270 K value reported by Malik et al. Moreover, the initial slopes for the M versus H curves are identical for T = 100, 200, 225 and 236 K suggesting that at low fields and at temperatures in the range 100 K < T < 238 K the sample may be in an itinerant helical state [9-111. Our magnetization versus temperature results suggest that for fields less than 1.6 kOe the sample undergoes a transition to a spin glass state for temperatures T I Tsg. At H = 0, Tsg is about 100 K. As H is increased Tsg is lowered to 90 K for H = 0.5 kOe and to 60 K for H = 1 kOe. Assuming a depression of Tsg that is proportional to Hz gives a critical field for driving Tsg to zero of about 1.6 kOe. We have used the resistivity of Ni,ScSn to estimate the non-magnetic resistivity of Co,ScSn. Ni,ScSn shows no magnetism but is also a L,, Heusler compound with a lattice constant that @ reasonably close to thai of Co,ScSn (i.e. 6.215 A for Ni,ScSn and 6.190 A for Co,ScSn) [8,12]. The resulting magnetic component of resistivity for Co,ScSn shows a T2 behavior over a very large temperature interval up to 200 K.
electricalproperties of CozScSn
Powdered X-ray diffraction measurements at room temperature indicated that the samples which were annealed for 8 weeks were singly phase, with lattice constants of 6.190 and 6.215 A for Co,ScSn and Ni,ScSn, respectively. These values are in good agreement with those previously published [8,12]. However, the unannealed samples and samples annealed for less than 1 week, showed some extra weak-intensity lines which could be indexed with a CsCl type structure probably due to a disorder of the three different atoms on the sublattices. The resistivity measurements were performed using the standard dc 4-probe technique for 1.2 K < T < 300 K. The magnetization was measured from 2 to 400 K in magnetic fields up to 5.5 T using a SQUID magnetometer. Only measurements on the well annealed samples are presented here.
3. Results and discussion Arrott plots CM2 vs. H/M) for Co,ScSn at various temperatures are shown in fig. 1. The Curie temperature is defined as the temperature for which a linearly extrapolated curve on the Arrott plot goes through the origin. We find T, = (238 k A) K, where A is less than 3 K. This
co,scsn
T-233K
t
2. Experimental Polycrystalline Co,ScSn and Ni,ScSn samples were prepared in an inert atmosphere arc furnace with appropriate care taken to compensate for the weight loss of the more volatile Sn. The samples wer.e wrapped in Ta foils and were annealed under high vacuum at 800°C for different time intervals ranging from 1 day to 8 weeks.
H/M
(mole-Oe/emu)
Fig. 1. Arrott plots of M* vs. H/M
at T =
C. L. Lin et al. / Magnetic and electrical properties of Co,ScSn
value is significantly lower than the T, = 270 K report’ed by Malik et al. [8]. The field dependence of the magnetization M(H) at various temperatures for Co,ScSn annealed at 800°C for 8 weeks is shown in fig. 2. At T = 100 K, M(H) increases almost linearly with increasing applied field up to a field H, of about 400 G, then it starts to saturate when an applied field is further increased. As the temperature increases, e.g. 200-236 K, M(H) still shows the same behavior except the field H, decreases, as can be seen in fig. 2. At T = 240 K, the linear behavior disappears and M(H) shows a negative curvature over the entire range applied fields shown. For even higher temperatures M(H) exhibits linear behavior for all fields as one would expect for the paramagnetic regime. The behavior described above, i.e. that a constant magnetic susceptibility (linear M vs. H) occurs at temperatures below the critical temperature T, up to a field H,, has been observed previously for other itinerant magnets, e.g. MnSi and Fe,Co,_,Si [lO,ll]. This behavior was explained in terms of a helical magnetic structure below T, for zero and low applied fields. This interpretation was confirmed by neutron diffraction [ill and electron spin resonance [13] measurements and is consistent with theoretical calculations [14,15] based on
Co,ScSn
1OOK
: .E I
2500
0 0
2500
H (Oel Fig. 2. Magnetization M vs. field H at T = 100, 200, 22.5, i28, 232,236,240,245 and 260 K.
0
~h~@.~.~.~.@.~.~
~.~...~.~~..~.~‘.~~~.” 0
357
100
200
300
‘loo
T (K1
Fig. 3. Magnetization M vs. temperature T for applied fields of (a) 500 Oe, (b) 1 kOe and (c) 5 kOe.
a competition between a Heisenberg type exchange interaction and a weak DzyaloshinskyMoriya coupling which can occur in the noncentrosymmetric cubic structure. Microscopic studies, e.g. neutron diffraction and electron spin resonance, must be performed on Co,ScSn in order to fully understand the magnetic structure at zero field. The data of fig. 1 (Arrott plot) are only for T close to T, = 238 K. The M vs. H data of fig. 2 are only shown for temperatures just above T, and down to T = 100 K. The reason for not displaying M vs. H for T < 100 K in fig. 2 will become apparent from the data shown in fig. 3. Figure 3 shows M vs. T from 10 to 400 K, for H = 500 Oe, 1 and 5 kOe. The H = 500 Oe curve shows a maximum near 90 K while the H = 1 kOe curve has a less pronounced maximum near 60 K. The H = 5 kOe curve does not show a maximum but rather increases monotonically as one lowers the temperature. Data were taken first by cooling the sample in zero field and then applying the field and measuring the magnetization vs. temperature from T = 10 to 380 K. Then a second measurement is taken by cooling the sample in field from 380 down to 10 K. Careful examination of the H = 500 Oe and the H = 1 kOe data shows that below the M vs. T maximum the field cooled data are slightly higher
C.L. Lin et al. / Magnetic and electrical properties of Co,ScSn
358
than the zero-field cooled data. The maximum in M vs. T at least for the H = 500 Oe and 1 kOe data and the hysteresis with temperature are indicative of spin glass behavior. If one assumes that an applied field suppresses the spin glass transition in a quadratic manner, i.e. TSp= TSspo th en one finds that TSg goes to (1 -Hz/H;‘) zero at H,E = 1.6 kOe, and TSpo= 100 K. We note that such a rapid suppression of the transition with magnetic field rules out the possibility that the transition is from a ferromagnetic to an antiferromagnetic state. Similar behavior has been reported in the case of some other transitionmetal based alloys, e.g. (Fe,Ni, _x179P,3B8, FeXAIIcx,_, and FexSn,,,,_, [16]. The spin glass behavior at temperatures below T, has been explained as due to the existence of a ferromagnetic exchange interaction between nearest neighbors and an antiferromagnetic superexchange interaction between next-nearest neighbors. Figure 4 shows M vs. T for an applied field of 10 Oe. Note the more pronounced temperature hysteresis and that the temperature at which the zero-field curve data show a maximum is indeed close to 100 K. In addition to the H and T dependence of the magnetization we have also measured the electri-
L 0 0
““““.~“~~~~‘~~~~““‘~.‘(‘...‘~~~~ 100
200
Boa
..%0.OAOIO 300
400
T (K)
Fig. 4. Magnetization M vs. temperature T measured from 10 to 380 K in an applied field of 10 Oe after the sample was cooled in zero field, and subsequently field cooled from 380 down to 10 K.
0
200
100
300
T (K)
Fig. 5. (a) Total resistivity p vs. temperature T in zero applied field for Co,ScSn and Ni,ScSn. (b) Magnetic resistivity Ap as estimated by subtracting p(T) for Ni,ScSn from p(T) for CozScSn vs. temperature T. The insets show dp/dr vs. T, and Ap vs. T2 up to 200 K.
cal resistivity of Co,ScSn. The temperature dependence of the resistivity p(T) for the Co,ScSn and Ni,ScSn samples, both annealed at 800°C for 8 weeks, is shown in fig. 5(a). It is assumed that the phonon contribution to the resistivity of Co,ScSn is similar to p(T) of Ni,ScSn because Co,ScSn and Ni,ScSn have the same crystal structure and their lattice constants differ by less than 0.5%. Moreover, Ni,ScSn remains a weak paramagnet down to T = 1.2 K [17]. It can be seen from fig. 5 that Ni,ScSn has the typical resistivity behavior that one expects for non-magnetic compound. In particular, unlike p(T) for Co,ScSn, when p(T) for Ni,ScSn at high temperatures, e.g. 200 to 300 K, is linearly extrapolated to T = 0 it intercepts the p axis at the measured residual resistivity. Ap(T) of Co,ScSn, which is obtained by subtracting p(T) of Ni,ScSn from p(T) of Co,ScSn, is shown in fig. 5(b). T, is about (235 + 5) K as determined by the temperature where Ap shows a downward deviation from its constant high-temperature behavior. The anomaly is qualitatively consistent with a Fisher-Langer type behavior. Hence T, from p measurements is
C.L. Lin et al. / Magnetic and electrical properties of Co,ScSn
consistent with the 238 K value we obtained from Arrott plots but far below the 270 K value reported by Malik et al. Ap(7’) below T, decreases monotonically with decreasing temperature due to a reduction of magnetic scattering. Although Ap(T) drops about 65 ufi cm from T = T, to T -+ 0, the residual resistivity, i.e. p(T -+ 01, is still very high. A high residual resistivity occurs in many other Heusler alloys [18]. In the inset of fig. 5(b) we show Ap(T) vs. T2 for Co,ScSn. It should be noted that a T2 dependence of Ap, like that predicted by the SCR theory [19] for itinerant ferromagnets below T,, holds over a wide range of temperatures. In particular, Ap N T2 from T = 100 to 200 K. Below 100 K clear deviations from Ap N T2 are seen. Since the M(T) data of fig. 4 suggest that a transition to a spin glass state occurs at this temperature (for H = 0) it is natural to speculate that the spin glass transition is responsible for this deviation. However, it should be noted that Ap(T) measurements in a field of 5 kOe are essentially identical to those shown in fig. 5(b), even though the data of fig. 3 suggest that 5 kOe is sufficient to suppress the spin glass transition. The linear coefficient of the T2 term, R,, is 1.13 X lo-’ @ cm/K2 for Co,ScSn which is much smaller than that for other itinerant ferromagnets 1201,e.g. R, = 47 X 10e3 l~fi cm/K2 for ZrZn, (T, = 23 K) or R, = 9.7 X 1O-3 ~JL! cm/K2 for Ni,Al CT, = 45-60 K). All of these R, values seem to follow a trend but do not agree quantitatively with the theoretical prediction, R, = TcP213. The deviation from the TcP213 relation might be due to the choice of the temperature range which was chosen for a determination of the T* coefficient. It has been shown that for the Heusler alloys the crystal structure, transport, thermodynamic and magnetic properties are influenced significantly by heat treatment [21]. We have also studied the X-ray diffraction, resistivity and magnetization of Co,ScSn samples annealed for shorter times and/or lower temperatures and found that they have disordered crystal structures and ilI-defined transitions and include free Co metal. We have studied the temperature dependence of the resistivity and the temperature and mag-
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netic field dependence of the magnetization of Co,ScSn. Our results show that at zero field and in low applied fields Co,ScSn undergoes a transition from a paramagnetic state to a possible helimagnetic state at T, = 238 K and exhibits a spin glass behavior below 100 K. At higher applied fields, the system becomes ferromagnetic below T,. The freezing temperature decreases with increasing applied field and is completely suppressed by H = 5 kG. To better understand this interesting material, we are performing Mossbauer-effect measurements in order to obtain a microscopic view of its magnetic structures. Presumably, studies of neutron diffraction will also provide useful information as to the nature of the magnetic properties of this system.
Acknowledgements
We acknowledge the support of the Materials Research Center and Grant-in-Aid and Faculty Summer Research Fellowship programs at Temple University.
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