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Intrinsic pinning of domain walls in Dy(Fe1−xMx)2 (M = Ga, Al; x⩽0.2)
Journal of Magnetism and Magnetic Materials 74 (1988) 39-42 North-Holland, Amsterdam
INTRINSIC Wending Department
PINNING ZHONG,
OF DOMAIN
WALLS
39
IN Dy(Fei_,M,),
(M = Ga, Al; x < 0.2) *
Jian LAN, Zun-xiao LIU and Guo-zhong LI
of Physics, Peking
University, Beijing, P.R. China
Received 14 October 1987; in revised form 7 April 1988
The magnetization and magnetization reversal process in pseudobinary compounds Dy(Fe,_,M,), (M = Ga, Al; x < 0.2) have been investigated. A stepwise increase of magnetization with field can best be viewed on the virgin curves at low temperature while the staircase behavior of the demagnetization curve is observed only at 1.5 K. The temperature dependence of coercivity H,(T) decays exponentially with increasing temperature from 4.2 K up to room temperature. Pronounced thermomagnetic history effects are also observed in the temperature dependence of the magnetization. It has been possible to account for the observed behavior in terms of the intrinsically pinned domain walls.
1. Introduction Recently, there has been considerable interest in the study of the magnetic properties of pseudobinary Laves phase intermetallics Dy(Fe,_,M,), (M = Al, Ga). Although much work has been done on the crystallographic and magnetic properties of these pseudobinary compounds [l-7], a thorough study of the magnetization and magnetization reversal process at very low temperatures is lacking. In this paper we present the results which are observed during the magnetization and magnetization reversal process on cubic Laves compounds Dy(Fe,_,M,), (M = Al, Ga; x < 0.2).
2. Experimental Polycrystalline samples were prepared by arc melting in a water-cooled copper crucible under a purified argon protective atmosphere (see ref. [7] for details). The purity of the constituents was 99.0% for Dy, and better than 99.99% for Fe, Al and Ga. All the samples were checked by the X-ray powder technique and found to be single
* Project supported by the National science Foundation of China.
phase with the cubic MgCu, (C15) type crystal structure. These compounds do not show appreciable secondary phases from SEM combined with EDAX analysis. Magnetic measurements were carried out by an induction method between 1.5 and 300 K in fields up to 80 kOe produced by a superconducting magnet.
3. Results and discussion Fig. 1 shows the virgin M(H) curves and the downhill branch of the hysteresis loops of the compound Dy(Fe,,Ga,,), at four temperatures. The virgin curves of Dy(Fe,,,Ga,,), show two steps on increasing the field: a first step of about 10 emu/g below the critical field, and a second step of a dramatic increase up to 100-100 emu/g at the critical field for all four temperatures. Critical fields H,,, were determined as the point of discontinuity in the dependence of magnetization on the field. Values of H,,, are 16, 15.7, 14 and 9.8 kOe for temperatures of 1.5, 2.9, 4.2 and 10 K, respectively. At still higher fields the magnetization still rises considerably. The curves for decreasing field are then almost identical for all four temperatures. The demagnetization curves of Dy(Fea,Ga a.* ) z are clearly distinguishable at various temperatures.
0304-8853/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W.-d Zhong et al. / Intrinsic pinning of domain walls in Dy(Fe, _ xMx)l
40
-i2
-10
-8
-6
-4
-2
H (hoe)
Fig.
2.
of Dy(Fe,,.,Ga,,), (A), (0) at 1.5 K. Ga,,,), These curves except for Dy(Fer,,,Ga,,,), also show a staircase behavior. WF%8AM2
-20
-15
-i0
-5
0 10 HCxOe)
20
30
40
50
60
Demagnetization (X)
and
curves
WFeo.95
78
Fig. 1. Virgin curves and downhill branch of hysteresis loops of Dy(Fe,.,Ga,,), at various temperatures: T= 1.5 K (X), 2.9 K (Cl), 4.2 K (0) and 10 K (A). The curve for the decreasing field is identical for all four temperatures. A staircase behavior is observed only in the 1.5 K demagnetization curve.
The magnetization reverses continuously with increasing applied field at above 2.8 K, while a staircase behavior is observed only at 1.5 K. However it should be noted that the critical fields and the corresponding coercivities are practically the same at temperatures of 2.9, 4.2 and 10 K and the critical field is larger than the coercivity at 1.5 K. The demagnetization curves of Dy(F%,gGa ,,J 2 and Dy(Fe,.,Al,,,), compounds, except for the
compound also exhibit the WF%.95Gao.05)2 staircase behavior at 1.5 K, as shown in fig. 2. Fig. 3 presents the dependence of magnetization on the field for the Dy(FeO.,Ga,,), compound at 1.5 K. All the data were obtained after cooling the sample from room temperature in the external field H,,,. The arrows show the variation in the applied field. This ‘hysteresis loop’ has three interesting features, namely: (i) After prior exposure to fields H,,, at temperatures from 300 to 1.5 K, a relatively high remanence is left but the downhill branch of the ‘hysteresis loop’ does not reach saturation until an applied field of 75 kOe (fig. 3a).
Fig. 3. Magnetization vs. field for Dy(Fe,,sGa,,2)2 at 1.5 K. (a) Curves were obtained by field-cooling in 30 kOe( 75 k&(a) respectively; (b) curves by 9.8(a), lO( X) and 15 kOe(0) cooling.
x),
50 kOe(*) and
W.-d. Zhong et al. / Intrinsic pinning of domain walls in Dy(Fe, _ xMx)2
7.5
c
52 -
5.5
P 3.5
1.5 0
10
20
30
40
50
60
T (KI Fig. 4. Top: Thermal variation of the coercive force for Dy(Fe,,,Al,,,), (0) and Dy(Fe,,Ga,,,), (X). Bottom: magnificated representation for T d 50 K.
(ii) If, when the magnetization reaches any point (e.g. a, b, c, etc., of fig. 3b) on the demagnetization curve, the applied field is reduced and subsequently reversed, the magnetization remains almost constant until a point at which there occurs a large jump. (iii) The staircase shapes of the demagnetization curves depend upon the order of measurements, while the coercivity is essentially identical (fig. 3b). It is well known that the shapes of hysteresis loops which depend on the value of the external field H,,, convey much information about the mag-
41
netization reversal process, making a sharp distinction between nucleation-dominated and wall pinning-dominated reversal [8]. For domain wall pinning the coercive force is almost the same, and only the remanence changes with H,,, (e.g. lower part of the hysteresis loop in fig. 3a). Therefore the magnetization as a function of applied field behaves as in fig. 3. The temperature dependence of the coercivity of Dy(Fe,,M,,), (M = Al, Ga) is plotted in fig. 4. A linear dependence of the logarithm of H, with increasing T is observed in the temperature range 4.2-150 K. Such a linear dependence is interpreted in terms of a thermal activation process of domain walls. However, anomalous behavior of the coercivity is observed at very low temperatures with H, decreasing with decreasing temperature (see also fig. 1). A small peak of H,(T) exists at about 3 K in Dy(Fe,,9M,,), (M = Al, Ga). An unusual picture is provided by the M(T) curves of Dy(Fe,,,Ga,,), for various fields as shown in fig. 5. All M versus T curves were measured while cooling the sample without an external field. In this case the magnetization first increases with increasing temperature and then decreases after passing through a broad maximum. The maximum appears at a lower temperature, the higher the applied field. Finally, the variation of magnetization with temperature is common to cooling or heating the sample in the corresponding field (fig. 6). The anomalies at low temperatures can be understood by considering the temperature dependence of the coercivity. At low temperatures, where the coercive force is close to or even higher than the applied field, the domain wall displacement is hampered which leads to low or zero it4 values. Owing to the exponential decrease of H, with T, a slight increase in temperature results in a large increase in magnetization. It appears from the results described in the above sections that the unusual stepwise increases of magnetization with field and the staircase shapes of the demagnetization curves can be explained as an intrinsic pinning effect of the domain walls by the substitutional inhomogeneities. It is assumed that the differences in the ionic radii of the substituting elements and pronounced fluctuations of
W.-d. Zhong et al. / Intrinsic pinning of domain walls in Dy(Fet _ xMx)2
42 140.
-“‘..“/“““-I,“““”
rua
Fig. 5. Temperature dependence of magnetization for Dy(Fe,,sGa,,,), cooled to 1.5 K in the absence (0) and presence (A) of an external magnetic field.
Fig. 6. M(T) of Dy(Fe,,sGa,,), measured in increasing (0) decreasing (A) and repeat increasing (X) temperatures after cooling in zero field.
the exchange, as well as of the local crystal fields are responsible for these pinning effects, which lead to the creation of energetic barriers influencing the process of magnetization and magnetization reversal at low temperature. The critical field H,, is then understood as serving as a pinning field of domain walls at structural defects, clusters, Peierls energy barrier, etc. For H .=xH,,,, the inertia of the domain wall will be very large and no motion will occur, thus the variation of magnetization with field will remain almost constant. However, as H --, H,,, the wall inertia will decrease rapidly and for certain conditions some large jumps are found on the virgin and demagnetization curves. The staircase behavior and the anomalous H,(T) observed in the compounds studied during the magnetization reversal process at very low temperatures are very similar to those of single crystal SmCo3,5Cu,,5 [9]. This unusual behavior at very low temperatures in these compounds may be
due to a quantum mechanical motion of domain walls [lo]. References [l] H. Oesterreicher [2] [3] [4] [5]
[6] [7] [8] (91 [lo]
and R. Pitts, J. Appl. Phys. 43 (1972) 5174. R. Grossinger, W. Steiner and K. Krec, J. Magn. Magn. Mat. 2 (1976) 196. J. Bara, A. Pedziwiatr, W. Zarek, D. Konopka and U. Gaoek, J. Magn. Magn. Mat. 27 (1982) 159. A. Kasprzyk, W. Zarek and A. Slebaski, J. Less-Common Metals 105 (1985) 231. V. Sims, Z. Smetana, V. Sechovsky, R. Grlissinger and J.J.M. Franse, J. Magn. Magn. Mat. 31-34 (1983) 201. V. Sima, R. Griissinger, V. Sechovsky, Z. Smetana and H. Sassik, J. Phys. F 14 (1984) 981. V.N. Moskalef, A V. Deryagin, N.V. Kydrevatykh and S.V. Terenteef, Phys. Met. MetaIl. 54 (1982) 54. W.D. Zhong, J. Lan and Z.X. Liu, J. Magn. Magn. Mat. 68 (1987) 197. J.J. Becker, IEEE Trans. Magn. MAG-12 (1976) 965. M. Uehara, J. Magn. Magn. Mat. 31-34 (1983) 1017. M. Uehara and B. Barbara, J. de Phys. 47 (1986) 235.
INTRINSIC Wending Department
PINNING ZHONG,
OF DOMAIN
WALLS
39
IN Dy(Fei_,M,),
(M = Ga, Al; x < 0.2) *
Jian LAN, Zun-xiao LIU and Guo-zhong LI
of Physics, Peking
University, Beijing, P.R. China
Received 14 October 1987; in revised form 7 April 1988
The magnetization and magnetization reversal process in pseudobinary compounds Dy(Fe,_,M,), (M = Ga, Al; x < 0.2) have been investigated. A stepwise increase of magnetization with field can best be viewed on the virgin curves at low temperature while the staircase behavior of the demagnetization curve is observed only at 1.5 K. The temperature dependence of coercivity H,(T) decays exponentially with increasing temperature from 4.2 K up to room temperature. Pronounced thermomagnetic history effects are also observed in the temperature dependence of the magnetization. It has been possible to account for the observed behavior in terms of the intrinsically pinned domain walls.
1. Introduction Recently, there has been considerable interest in the study of the magnetic properties of pseudobinary Laves phase intermetallics Dy(Fe,_,M,), (M = Al, Ga). Although much work has been done on the crystallographic and magnetic properties of these pseudobinary compounds [l-7], a thorough study of the magnetization and magnetization reversal process at very low temperatures is lacking. In this paper we present the results which are observed during the magnetization and magnetization reversal process on cubic Laves compounds Dy(Fe,_,M,), (M = Al, Ga; x < 0.2).
2. Experimental Polycrystalline samples were prepared by arc melting in a water-cooled copper crucible under a purified argon protective atmosphere (see ref. [7] for details). The purity of the constituents was 99.0% for Dy, and better than 99.99% for Fe, Al and Ga. All the samples were checked by the X-ray powder technique and found to be single
* Project supported by the National science Foundation of China.
phase with the cubic MgCu, (C15) type crystal structure. These compounds do not show appreciable secondary phases from SEM combined with EDAX analysis. Magnetic measurements were carried out by an induction method between 1.5 and 300 K in fields up to 80 kOe produced by a superconducting magnet.
3. Results and discussion Fig. 1 shows the virgin M(H) curves and the downhill branch of the hysteresis loops of the compound Dy(Fe,,Ga,,), at four temperatures. The virgin curves of Dy(Fe,,,Ga,,), show two steps on increasing the field: a first step of about 10 emu/g below the critical field, and a second step of a dramatic increase up to 100-100 emu/g at the critical field for all four temperatures. Critical fields H,,, were determined as the point of discontinuity in the dependence of magnetization on the field. Values of H,,, are 16, 15.7, 14 and 9.8 kOe for temperatures of 1.5, 2.9, 4.2 and 10 K, respectively. At still higher fields the magnetization still rises considerably. The curves for decreasing field are then almost identical for all four temperatures. The demagnetization curves of Dy(Fea,Ga a.* ) z are clearly distinguishable at various temperatures.
0304-8853/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W.-d Zhong et al. / Intrinsic pinning of domain walls in Dy(Fe, _ xMx)l
40
-i2
-10
-8
-6
-4
-2
H (hoe)
Fig.
2.
of Dy(Fe,,.,Ga,,), (A), (0) at 1.5 K. Ga,,,), These curves except for Dy(Fer,,,Ga,,,), also show a staircase behavior. WF%8AM2
-20
-15
-i0
-5
0 10 HCxOe)
20
30
40
50
60
Demagnetization (X)
and
curves
WFeo.95
78
Fig. 1. Virgin curves and downhill branch of hysteresis loops of Dy(Fe,.,Ga,,), at various temperatures: T= 1.5 K (X), 2.9 K (Cl), 4.2 K (0) and 10 K (A). The curve for the decreasing field is identical for all four temperatures. A staircase behavior is observed only in the 1.5 K demagnetization curve.
The magnetization reverses continuously with increasing applied field at above 2.8 K, while a staircase behavior is observed only at 1.5 K. However it should be noted that the critical fields and the corresponding coercivities are practically the same at temperatures of 2.9, 4.2 and 10 K and the critical field is larger than the coercivity at 1.5 K. The demagnetization curves of Dy(F%,gGa ,,J 2 and Dy(Fe,.,Al,,,), compounds, except for the
compound also exhibit the WF%.95Gao.05)2 staircase behavior at 1.5 K, as shown in fig. 2. Fig. 3 presents the dependence of magnetization on the field for the Dy(FeO.,Ga,,), compound at 1.5 K. All the data were obtained after cooling the sample from room temperature in the external field H,,,. The arrows show the variation in the applied field. This ‘hysteresis loop’ has three interesting features, namely: (i) After prior exposure to fields H,,, at temperatures from 300 to 1.5 K, a relatively high remanence is left but the downhill branch of the ‘hysteresis loop’ does not reach saturation until an applied field of 75 kOe (fig. 3a).
Fig. 3. Magnetization vs. field for Dy(Fe,,sGa,,2)2 at 1.5 K. (a) Curves were obtained by field-cooling in 30 kOe( 75 k&(a) respectively; (b) curves by 9.8(a), lO( X) and 15 kOe(0) cooling.
x),
50 kOe(*) and
W.-d. Zhong et al. / Intrinsic pinning of domain walls in Dy(Fe, _ xMx)2
7.5
c
52 -
5.5
P 3.5
1.5 0
10
20
30
40
50
60
T (KI Fig. 4. Top: Thermal variation of the coercive force for Dy(Fe,,,Al,,,), (0) and Dy(Fe,,Ga,,,), (X). Bottom: magnificated representation for T d 50 K.
(ii) If, when the magnetization reaches any point (e.g. a, b, c, etc., of fig. 3b) on the demagnetization curve, the applied field is reduced and subsequently reversed, the magnetization remains almost constant until a point at which there occurs a large jump. (iii) The staircase shapes of the demagnetization curves depend upon the order of measurements, while the coercivity is essentially identical (fig. 3b). It is well known that the shapes of hysteresis loops which depend on the value of the external field H,,, convey much information about the mag-
41
netization reversal process, making a sharp distinction between nucleation-dominated and wall pinning-dominated reversal [8]. For domain wall pinning the coercive force is almost the same, and only the remanence changes with H,,, (e.g. lower part of the hysteresis loop in fig. 3a). Therefore the magnetization as a function of applied field behaves as in fig. 3. The temperature dependence of the coercivity of Dy(Fe,,M,,), (M = Al, Ga) is plotted in fig. 4. A linear dependence of the logarithm of H, with increasing T is observed in the temperature range 4.2-150 K. Such a linear dependence is interpreted in terms of a thermal activation process of domain walls. However, anomalous behavior of the coercivity is observed at very low temperatures with H, decreasing with decreasing temperature (see also fig. 1). A small peak of H,(T) exists at about 3 K in Dy(Fe,,9M,,), (M = Al, Ga). An unusual picture is provided by the M(T) curves of Dy(Fe,,,Ga,,), for various fields as shown in fig. 5. All M versus T curves were measured while cooling the sample without an external field. In this case the magnetization first increases with increasing temperature and then decreases after passing through a broad maximum. The maximum appears at a lower temperature, the higher the applied field. Finally, the variation of magnetization with temperature is common to cooling or heating the sample in the corresponding field (fig. 6). The anomalies at low temperatures can be understood by considering the temperature dependence of the coercivity. At low temperatures, where the coercive force is close to or even higher than the applied field, the domain wall displacement is hampered which leads to low or zero it4 values. Owing to the exponential decrease of H, with T, a slight increase in temperature results in a large increase in magnetization. It appears from the results described in the above sections that the unusual stepwise increases of magnetization with field and the staircase shapes of the demagnetization curves can be explained as an intrinsic pinning effect of the domain walls by the substitutional inhomogeneities. It is assumed that the differences in the ionic radii of the substituting elements and pronounced fluctuations of
W.-d. Zhong et al. / Intrinsic pinning of domain walls in Dy(Fet _ xMx)2
42 140.
-“‘..“/“““-I,“““”
rua
Fig. 5. Temperature dependence of magnetization for Dy(Fe,,sGa,,,), cooled to 1.5 K in the absence (0) and presence (A) of an external magnetic field.
Fig. 6. M(T) of Dy(Fe,,sGa,,), measured in increasing (0) decreasing (A) and repeat increasing (X) temperatures after cooling in zero field.
the exchange, as well as of the local crystal fields are responsible for these pinning effects, which lead to the creation of energetic barriers influencing the process of magnetization and magnetization reversal at low temperature. The critical field H,, is then understood as serving as a pinning field of domain walls at structural defects, clusters, Peierls energy barrier, etc. For H .=xH,,,, the inertia of the domain wall will be very large and no motion will occur, thus the variation of magnetization with field will remain almost constant. However, as H --, H,,, the wall inertia will decrease rapidly and for certain conditions some large jumps are found on the virgin and demagnetization curves. The staircase behavior and the anomalous H,(T) observed in the compounds studied during the magnetization reversal process at very low temperatures are very similar to those of single crystal SmCo3,5Cu,,5 [9]. This unusual behavior at very low temperatures in these compounds may be
due to a quantum mechanical motion of domain walls [lo]. References [l] H. Oesterreicher [2] [3] [4] [5]
[6] [7] [8] (91 [lo]
and R. Pitts, J. Appl. Phys. 43 (1972) 5174. R. Grossinger, W. Steiner and K. Krec, J. Magn. Magn. Mat. 2 (1976) 196. J. Bara, A. Pedziwiatr, W. Zarek, D. Konopka and U. Gaoek, J. Magn. Magn. Mat. 27 (1982) 159. A. Kasprzyk, W. Zarek and A. Slebaski, J. Less-Common Metals 105 (1985) 231. V. Sims, Z. Smetana, V. Sechovsky, R. Grlissinger and J.J.M. Franse, J. Magn. Magn. Mat. 31-34 (1983) 201. V. Sima, R. Griissinger, V. Sechovsky, Z. Smetana and H. Sassik, J. Phys. F 14 (1984) 981. V.N. Moskalef, A V. Deryagin, N.V. Kydrevatykh and S.V. Terenteef, Phys. Met. MetaIl. 54 (1982) 54. W.D. Zhong, J. Lan and Z.X. Liu, J. Magn. Magn. Mat. 68 (1987) 197. J.J. Becker, IEEE Trans. Magn. MAG-12 (1976) 965. M. Uehara, J. Magn. Magn. Mat. 31-34 (1983) 1017. M. Uehara and B. Barbara, J. de Phys. 47 (1986) 235.