Intermetallics 19 (2011) 1605e1611
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Martensitic transformation and magnetic properties in ferromagnetic shape memory alloy Ni43Mn46Sn11xSix Zhuhong Liu a, *, Zhigang Wu b, Hong Yang b, Yinong Liu b, Wenhong Wang c, Xingqiao Ma a, Guangheng Wu c a b c
Department of Physics, University of Science and Technology Beijing, Beijing 100083, China School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6009, Australia Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2010 Received in revised form 13 June 2011 Accepted 15 June 2011 Available online 20 July 2011
This study investigated the effects of Si addition and heat treatment on the martensitic transformation and magnetic properties of Ni43Mn46Sn11xSix (x ¼ 1, 2, 3) alloys. The martensitic transformation temperatures were found to increase with increasing Si content in the alloy. A magnetic field-induced martensite-to-austenite transformation was found in Ni43Mn46Sn10Si1. Ageing of the Ni43Mn46Sn10Si1 alloy at 673 K resulted in the formation of a (Mn,Ni)eSiSn precipitate. The precipitate contains very low Sn content, causing the increase of the Sn content in the matrix phase and a decrease of martensitic transformation temperature. Ageing at 573 K is found to increase the Curie temperature and the saturation magnetization. This is attributed to the increase of atomic ordering of the matrix. Solution treatment of the aged samples at 1073 K was effective to restore the original transformation behaviour and magnetic properties. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic intermetallics B. Martensitic transformations C. Heat treatment B. Magnetic properties B. Thermal stability
1. Introduction Off-stoichiometric Heusler Ni50Mn50xZx (Z ¼ In, Sn, Sb) alloys have attracted much attention due to their promise for magnetic field-induced martensitic transformations [1e4]. In these alloys, martensitic transformation occurs between a ferromagnetic austenite and a low-magnetization martensite, which may be paramagnetic, antiferromagnetic or ferrimagnetic [1e4]. The large difference in magnetization between the two phases (ΔM) provides strong thermodynamic driving force for the martensitic transformation to be induced by a magnetic field. In contrast, martensitic transformations in all other designated ferromagnetic shape memory alloys (FSMAs) have only been found to be inducible by temperature or stress, but not magnetic field. This is because that in those alloys the magnetic energy ΔM$H makes minor contributions to the Gibbs free energy change of the transformations due to the small values of ΔM between the two phases. For example, ΔM is only 11 emu/g in Ni2MnGa [5], compared to 106 emu/g in a Ni50Mn34In16 alloy [1]. In addition to magnetoactuation, these
* Corresponding author. Tel.: þ86 10 62332139; fax: þ86 10 62327283. E-mail address:
[email protected] (Z. Liu). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.06.007
alloys also exhibit a few other useful properties, i.e., giant magnetoresistance effect [1,6] and giant magnetocaloric effect [7,8] accompanying the magnetic field-induced transformation. Early studies have found that the martensitic transformation behaviour of Ni50Mn50xZx (Z ¼ In, Sn, Sb) alloys is sensitive to alloy composition. Sutou et al. reported that the martensitic transformation temperatures of Ni50Mn50x(In, Sn, Sb)x are strongly dependent on the alloy composition [9], generally decreasing with increasing In, Sn and Sb contents. Wachtel et al. reported that NieMneSn Heusler alloys of low Sn contents (below 16.5%) usually exhibit martensitic transformation [10] whereas those with higher Sn contents do not. To date, explanations of the effects of alloying on the transformation behaviour have been given on the basis of electron concentration. However, it is known that in addition to electron concentration, magnetic properties and martensitic transformation behaviours of Heusler alloys are highly sensitive to composition, atomic ordering and internal stresses in the matrix [11,12]. Ageing is influential to all these factors. Martensitic transformation and Curie transition of NieFeeGaeCo are found to respond to ageing treatment via the formation of g-precipitates and changes in chemical ordering [13]. Ageing is also found to cause multi-step martensitic transformation behaviour in Ni-rich NieMneGa alloys due to the
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formation of different types of martensite and the creation of internal stress [14]. In order to exclude the factor of electron concentration on the martensitic transformation, it is necessary to study the iso-electron substitution alloy system. So we choose Si doped partial Sn in NiMnSn alloy as the research target. This study was conducted to investigate the effects of substitution of Si for Sn and ageing on the martensitic transformation and magnetic properties of NiMnSnSi. Such knowledge is essential for the development of these alloys into viable functional engineering materials. 2. Material and methods Three alloy ingots with Ni43Mn46Sn11xSix (Si ¼ 1, 2, 3) nominal compositions, hereafter referred to as Si1, Si2 and Si3 respectively, were prepared by means of arc-melting from high purity metals under argon atmosphere in water-cooled Cu crucibles. Homogenization was achieved by sealing the ingots under argon atmosphere in quartz ampoules and annealing at 1073 K for 48 h, followed by quenching in ice water. Si1 alloy was selected for ageing at 573 K and 673 K. Martensitic transformation behaviour of the specimens was characterized using a TA Instrument Q10 differential scanning calorimeter (DSC) in a nitrogen atmosphere with a heating/cooling rate of 10 K/min. Ac magnetic susceptibility was measured in a 77 Hz, 5 Oe magnetic field over a temperature range of 77e400 K. Powder X-ray diffraction was performed using a Regaku D/Max 2200 PC X-ray diffractometer with Cu Ka radiation. The magnetization curves were measured using a superconducting quantum interface device (SQUID) magnetometer. Microstructures of the samples were studied with scanning electron microsopy (SEM) using both secondary and back-scattered electrons signals and the compositions were determined by means of X-ray energy dispersive spectrometry (EDS). 3. Results and discussion 3.1. Martensitic transformation Fig. 1 shows DSC measurement of the transformation behaviour of the three Ni43Mn46Sn11xSix (x ¼ 1, 2, 3) alloys. It is seen that all three alloys showed single-step Austenite 4 Martensite transformation with well-defined thermal flux peaks on cooling and
heating. From these measurements, the critical temperatures of the martensitic transformation, including the start (Ms) and finish (Mf) temperatures for the forward transformation on cooling and the start (As) and finish (Af) temperatures for the reverse transformation on heating, and the enthalpy change of the transformation on cooling (DHc) and on heating (DHh) are determined. These values are summarized in Table 1. Also listed in the table are the transformation equilibrium temperature To, defined as To ¼ 1=2ðMs þ Af Þ, and the transformation entropy change, estimated based on DS ¼ DH=T0 . It has been reported the martensitic transformation temperature in Si-free sample, Ni43Mn46Sn11, is about 200 K, nearly the same as our Si1 sample [8]. Averagely, it is evident that the transformation temperatures increased progressively with increasing Si content. This implies that Si weakens the stability of the cubic parent phase. It is generally considered that, for the Ni50Mn50xSnx system, martensitic transformation temperatures increase with increasing electron concentration e/a [15]. However, in this work, substitution of Sn by Si, which has the same number of valence electrons, leads to increase of transformation temperatures at constant e/a ratio. This implies that e/a ratio is not the sole factor to influence martensitic transformation temperatures. This is possibly related to the effect on transformation volume change, as has been observed in NiMnInSb [16]. It is evident that DH increased with increasing Si content, obviously related to the increase of transformation temperature, obeying DH ¼ To DS. This is consistent with the observation of Ni50Mn50xSnx alloys [15]. 3.2. Magnetic properties Fig. 2(a) shows the magnetization behaviour of the three samples during thermal cycling, under the influence of a magnetic field of 50 Oe. Alloy Si1 showed a sharp increase in magnetization upon cooling to 268 K, indicating the Curie transition of the austenite (TC(A)), which is about 22 K lower than that of Ni43Mn46Sn11 sample [8]. The transformation from the ferromagnetic austenite to paramagnetic martensite occurred at 198 K on cooling, as indicated by the Ms. The paramagnetic martensite underwent the Curie transition at 173 K (TC(M)). Further cooling led to progressive increase of magnetization of the martensite. Upon heating, the sample showed a completely reversible behaviour, with the reverse martensitic transformation occurring at 189 K on heating, leaving a thermal hysteresis of 9 K above the forward transformation. It is noticed that the Curie transition in our Si1 sample is shaper than that of the Si-free sample [8]. Alloy Si2 showed a similar transformation behaviour, exhibiting all three transformations. The critical temperatures of the three transformations are determined to be TC(A) ¼ 264 K, TC(M) ¼ 183 K, and Ms ¼ 217 K. Further increasing Si content to 3, as demonstrated by Si3, continued the trend to increase the martensitic transformation temperatures. It is obvious that in this sample the martensitic transformation temperature was very close to the Curie temperature of the austenite and the martensitic transformation started before the completion of the Curie transition of the austenite. Table 1 Critical temperatures, enthalpy changes, entropy changes and magnetization changes of martensitic transformation in Ni43Mn46Sn11xSix alloys.
Fig. 1. Transformation behaviour of Ni43Mn46Sn11xSix (x ¼ 1, 2, 3).
Alloy Ms
Mf
As
Af
To
DHc DHh DSc
(K)
(K)
(K)
(K)
(K)
(J/g) (J/g) (Jg1K1) (Jg1K1) (emu/g)
x ¼ 1 198 179 189 211 205 3.5 x ¼ 2 217 191 200 234 225 5.2 x ¼ 3 273 225 241 283 278 7.6
3.9 5.5 7.9
0.017 0.023 0.027
DSh 0.019 0.024 0.028
DM (50 kOe) 39 28 12
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3.0 Si1 Si2 Si3
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Ms
M (emu/g)
2.0 1.5 TC(M)
1.0 TC(A) 0.5 0.0
As 0
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T (K)
b
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Si1
50
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Fig. 3(a) shows isothermal magnetization behaviour of Si1 at different temperatures. At 5 K, the martensite showed a typical soft magnetization behaviour, reaching a saturation magnetization of 28 emu/g at 70 kOe. At 210 K, which is close to the Af temperature, the austenite magnetized rapidly to above 40 emu/g at below 1 kOe and then gradually increased to a maximum magnetization of 59 emu/g at 70 kOe. In between these two temperatures, at 185 K, 4 K below the As temperature, the martensite magnetized gradually to a maximum of 21 emu/g at 70 kOe. The reduced maximum magnetization compared to that at 5 K is obviously related to the decrease of spontaneous magnetization with increasing temperature. At 195 K, which is 6 K above the As temperature but 16 K below the Af temperature, the sample magnetized almost identically to the behaviour at 185 K at below 20 kOe and then increased more rapidly afterwards, indicating the occurrence of the transformation from the martensite to austenite, as induced by the magnetic field. It is apparent that the transformation was incomplete at 70 kOe. As the temperature was increased to 200 K, which is in the middle between As and Af, the magnetization occurred in two stages corresponding to the initial magnetization of the martensite/austenite matrix, as evidenced by the higher magnetization reached at below 10 kOe compared to that at 195 K, and then the transformation of the remaining martensite to the austenite. It is evident that the transformation occurred at a much lower magnetic field strength
M (emu/g)
40
a
Si2
30
60
Si1 210 K
50
20 10 0
Si3 0
50
100
150
200
250
300
350
M (emu/g)
195 K
30
5K 190 K
20
T (K)
185 K
10
Fig. 2. Thermomagnetization curves measured under the magnetic field of (a) 50 Oe, and (b) 50 kOe.
0
0
10
20
30
40
50
60
70
H (kOe)
b
60 50
M (emu/g)
Fig. 2(b) shows magnetization behaviour of the alloys under the influence of a magnetic field of 50 kOe. The maximum magnetiA Þ reached in alloy Si1 is 59 emu/g and zation of the austenite ðMmax M Þ reached is the minimum magnetization of the martensite ðMmin 20 emu/g, giving a magnetization difference of ΔM ¼ 39 emu/g between the two phases. Si2 showed a magnetization difference of A M ¼ 45 emu=g and Mmin ¼ 17 emu=g. ΔM ¼ 28 emu/g, with Mmax Similarly, ΔM for Si3 sample is 12 emu/g. ΔM values of the three alloys are also summarized in Table 1. It is known that the critical temperature and the critical magnetic field for inducing the martensitic transformation obey a Clausius-Clapeyron type relation, expressed as dT=dH ¼ DM=DS. According to this relation, combination of a large DM and a small DS implies a higher sensitivity of transformation temperature to magnetic field, thus is desirable for magnetic field-induced martensitic transformation. Therefore, the optimization of the alloy composition to obtain large DM and small DS is important. It is seen in Table 1 that Si1 has the highest ΔM/DS ratio with the largest DM and lowest DS, thus the highest potential to achieve magnetic field-induced martensitic transformation. Therefore, Si1 was chosen to investigate the magnetic field-induced martensitic transformation behaviour and the effect of ageing on the martensitic transformation.
200 K
40
40 30 20
1st cycling 2nd cycling
10 0
0
10
20
30
40
50
60
70
H (kOe) Fig. 3. Magnetization curves of as-annealed Si1 sample: (a) measured at different temperatures, (b) in two consecutive cycles at 200 K.
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compared to the measurement at 195 K and that the transformation reached completion at 40 kOe. It is also obvious that the initial magnetization susceptibility at 200 K is higher than that at 195 K. This is obviously related to the pre-existing austenite at 200 K. Fig. 3(b) shows the magnetization behaviour of Si1 in two consecutive cycles at 200 K. It is seen that the martensitic transformation was induced upon the first magnetization to 20 kOe. Upon removal of the external magnetic field, the sample demagnetized in a typical soft magnetic behaviour, showing no sign of a reverse transformation. On the second magnetization cycle, the sample exhibited a typical soft magnetization behaviour identical to that of the austenite shown in Fig. 3(a). This demonstrates the absence of a phase transformation, confirming that there was no reverse transformation upon demagnetization in the first cycle. 3.3. Effect of ageing on transformation behaviour Fig. 4 shows DSC measurements of the transformation behaviour of Si1 aged at two different temperatures for various times, with (a) showing the cooling transformation and (b) showing the heating transformation after ageing at 573 K, and (c) showing the cooling transformation and (d) showing the heating transformation after ageing at 673 K. It is seen that ageing caused obvious changes to the transformation behaviour, with the transformation heat flux peak diminishing after prolonged ageing. Fig. 5 shows the effect of ageing time on the peak temperatures of the forward transformation (Mp) and the reverse transformation (Ap) as determined from DSC measurement. It is seen that ageing at 573 K caused a small initial decrease of both Mp and Ap and then a continuous increase with prolonged ageing. The overall increase of Ap is greater than that of Mp, resulting in an increased thermal hysteresis for the transformation. It is seen from Fig. 4, the heat flux peak decreased with increasing ageing time, indicating the enthalpy change decreased with prolonged ageing time. As the
transformation equilibrium temperature To in sample aged at 573 K increased with ageing time, and hence DS decreased with ageing time according to DH ¼ To DS. This resulted in the increase of the transformation hysteresis because the hysteresis is inversely proportional to the DS. Ageing at 673 K, on the other hand, caused progressive decrease of both Mp and Ap. The transformation hysteresis also increased with ageing time. Fig. 6(a) shows XRD spectra of three Si1 samples at room temperature. It is clear that the annealed sample contained a B2 phase with a lattice constant of a ¼ 0.5947 nm. The (111) peak, which is characteristic of the Heusler structure, emerged in the two aged samples, implying that the ageing has improved Heusler atomic ordering of the matrix phase. Graph (b) shows XRD spectra recorded in step scanning mode within 41e45 , to reveal details of the (220) peak. It is seen that after ageing at 673 K for 210 min, an extra rather weak diffused peak developed at 42.03 , suggesting the precipitates may developed in the matrix. In addition, the (220) Heusler phase peak has obviously shifted to a lower diffraction angle after ageing at 673 K for 210 min. This indicates that the unit cell of the matrix Heusler phase has been enlarged with the formation of precipitate. For the sample aged at 573 K, neither peak shift nor extra peak was observed. On the other hand, the intensity of the Heusler (220) peak increased and the (111) peak emerged. These observations suggest that no precipitates were formed, little change of the lattice parameter occurred but atomic shuffling for Heusler ordering has occurred. This is believed to be responsible for the increase of Ap in the sample aged at 573 K. This is consistent with previous studies on NiMnGa, which confirmed that improving of atomic ordering can lead to increase of martensitic transformation temperatures [14]. In order to confirm the precipitate in the Si1 sample aged at 673 K for 210 min, back-scattered SEM micrographs of the microstructures was taken, as shown in Fig. 7. The sample showed a continuous matrix in light contrast and dispersed second phase
Fig. 4. Effect of ageing on the transformation behaviour of alloy Si1: (a) cooling transformation after ageing at 573 K; (b) heating transformation after ageing at 573 K; (c) cooling transformation after ageing at 673 K; (d) heating transformation after ageing at 673 K.
Z. Liu et al. / Intermetallics 19 (2011) 1605e1611
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Fig. 7. Back-scattered electron image of Si1 sample aged at 673 K for 210 min. Fig. 5. Evolution of transformation temperatures during ageing at 573 K and 673 K for Si1 sample.
a
(422)
(400)
(111)
X-ray Intensity
(220)
(a) as-annealed (b) aged at 573 K for 690 minutes (c) aged at 673 K for 210 minutes
(c) (b)
(a)
20
30
40
50
60
70
80
90
2 (degree)
41
as-annealed aged at 573 K for 690 min aged at 673 K for 210 min
second phase
X-ray Intensity
b
42
43
44
45
2 (degree) Fig. 6. XRD spectra of annealed and aged Si1 samples: (a) normal scans measured at room temperature; (b) XRD spectra carried out by step scanning mode.
particles in dark contrast. Compositions of the phases in the samples as determined by quantitative EDS analysis. The composition for the matrix phase and the second phase are Ni42.3Mn46.1Sn10.6Si1.0, and Ni33.6Mn50.3Sn0.8Si15.3, respectively. From the composition, it is likely that the second phase belongs to Mn3Ni2Si-type phase [17]. It is obvious that precipitation of the Mn3Ni2Si-type phase from the Heusler matrix enriches the Sn concentration in the Heusler phase, which will increase the lattice parameters of Heusler alloy [15], consistent with the XRD results in Fig. 6. It is known that increase in Sn content in the Heusler phase leads to decrease of the martensitic transformation temperatures and the transformation enthalpy change [15]. This is also consistent with the observation of the sample aged at 673 K. 3.4. Reversibility of the effect of ageing Fig. 8(a) shows ac susceptibility (c) of several Si1 samples during thermal cycling. Sample 1 was in the original condition after the homogenization treatment at 1073 K for 48 h. The sample underwent Curie transition at 268 K, followed by the martensitic transformation occurring at Ms ¼ 198 K during cooling and the reverse transformation at As ¼ 189 K during heating, leaving a temperature hysteresis of 9 K. The transformation to paramagnetic martensite was accompanied by a rapid decrease of ac susceptibility. Soon after the transformation, c increased rapidly, signifying the Curie transition of the martensite. The ac susceptibility of the martensite reached a maximum at 161 K and then decreased to settle at a stable level. The final level of ac susceptibility corresponds to the ferromagneticeantiferromagnetic mixed state of the matrix, and the apparent peak at 161 K is related to frustration of magnetization during the Curie transition. Similar ac susceptibility abnormality peak has also been observed in a Ni50Mn35In15 alloy [18], due to magnetic heterogeneity of the alloy matrix. It has been reported that the matrix of Ni-based high-Mn Heusler alloys, like Ni50Mn34Sn16 [19], is magnetically inhomogeneous, containing both ferromagnetic and antiferromagnetic interactions. In these alloys, excessive Mn atoms occupy Sn-sites. The MneMn atoms at Mn-sites form ferromagnetic coupling, whereas the Mn atoms at Sn-sites (Mn(Sn)) and the Mn atoms at Mn-sites (Mn(Mn)) form antiferromagnetic coupling [19], due to the short MneSn site distance in thepHeusler structure. In Heusler ffiffiffi structure, MneMn site distance is 2 times that of MneSn site distance. Interaction between ferromagnetic and antiferromagnetic coupling results in random spin orientations was frozen at the temperature below 161 K, which causes the decrease of c.
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a
curve shape practically overlapped with that of the original sample. This demonstrates that the effect of ageing is completely reversible. Fig. 8(b) shows the magnetization behaviour at 5 K of Si1 samples subjected to different heat treatments. The annealed sample showed a saturation magnetization of 23 emu/g. This value increased to 34 emu/g after ageing at 573 K for 690 min. The increase of magnetization after ageing at 573 K is attributed to the increase of Heusler atomic ordering of the matrix. The saturation magnetization was restored to 23 emu/g after the second solution treatment at 1073 K for 3 min.
200
1 2
150
(a. u.)
3 Tp
100
50
4. Conclusions
4 0
Based on the experimental evidences and the discussions presented above, the following may be concluded:
100
150
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250
300
T (K)
b
40
M (emu/g)
30
20 as-annealed sample ageing at 573 K for 690 minutes solution treated at 1073 K for 3 minutes after ageing
10
0
0
10
20
30
40
50
H (KOe) Fig. 8. (a) Temperature dependence of ac susceptibility of Si1 samples subjected to different heat treatment conditions: (1) homogenized at 1073 K for 48 h; (2) aged at 573 K for 690 min; (3) aged at 673 K for 210 min; (4) solution treated at 1073 K for 3 min after ageing. (b) Magnetization curves measured at 5 K of Si1 samples heat treated under different conditions.
The sample aged at 573 K showed an increased Curie transition temperature to 293 K (Sample 2). The martensitic transformation occurred at Ms ¼ 200 K and As ¼ 229 K, giving an enlarged hysteresis of 29 K. It is evident that the Curie transition of martensite has disappeared, and the martensite formed into the mixed ferromagneticeantiferromagnetic state directly. Consequently, the abnormal ac susceptibility peak at 161 K has also disappeared. This also implies that the Curie transition temperature of the martensite has increased to above that of the martensitic transformation, consistent with the increase of the Curie transition temperature of the Austenite. The increased Curie temperature suggests increased ferromagnetic interaction between MneMn atoms. As discussed above, ageing at 573 K improves the Heusler atomic ordering from the B2 structure. This is believed to be responsible for the increase of MneMn interactions. The sample aged at 673 K (Sample 3) showed a two-step Curie transition, implying the existence of a second phase in the matrix. This is consistent with the XRD observation presented above. The martensitic transformation becomes weak and broad, apparently also related to the formation of the precipitate. Sample 4 was aged at 673 K and then solution treated at 1073 K for 3 min. It is seen that the second solution treatment restored the original behaviour as the homogenized sample. The peak at 161 K reappeared and the
(1) Ni43Mn46Sn11xSix (x ¼ 1, 2, 3) alloys exhibit thermoelastic martensitic transformation. The martensitic transformation temperatures increase with increasing Si content. Magnetic field-induced martensite to austenite transition was observed in Si1 sample. (2) Ageing the sample at 573 K raises the reverse transformation temperature, Curie temperature and magnetization. XRD revealed that the intensity for (111) peak of Heusler phase increased. Together with magnetic measurement results, it is believed ageing the sample at this temperature enhanced the sample’s chemical ordering. (3) The sample decomposed into Heusler phase and the second Mn3Ni2Si-typed phase with very low amount of Sn content after ageing at 673 K, which increases the Sn ratio in the Heusler phase matrix and hence decreases the transformation temperature. (4) Solution treated the ageing sample at 1073 K followed by quenching was effective in restoring the martensitic transformation and Curie temperature. Acknowledgement The authors wish to acknowledge the financial supports by National Basic Research Program of China (973 Program, 2010CB833102), National Natural Science Foundation of China in Grant No. 51001010, and by Department of Innovation Industry, Science and Research in ISL Grant CH070136. References [1] Yu SY, Liu ZH, Liu GD, Chen JL, Cao ZX, Wu GH, et al. Appl Phys Lett 2006;89: 162503. [2] Kainuma R, Imano Y, Ito W, Sutou Y, Morito H, Okamoto S, et al. Nature 2006; 439:957. [3] Koyama K, Watanabe K, Kanomata T, Kainuma R, Oikawa K, Ishida K. Appl Phys Lett 2006;88:132505. [4] Yu SY, Ma L, Liu GD, Liu ZH, Chen JL, Cao ZX, et al. Appl Phys Lett 2007;90: 242501. [5] Ullakko K, Huang JK, Kantner C, O’Handley RC, Kokorin VV. Appl Phys Lett 1996;69:1966. [6] Sharma VK, Chattopadhyay MK, Shaeb KHB, Chouhan A, Roy SB. Appl Phys Lett 2006;89:222509. [7] Han ZD, Wang DH, Zhang CL, Tang SL, Gu BX, Du YW. Appl Phys Lett 2006;89: 182507. [8] Han ZD, Wang DH, Zhang CL, Xuan HC, Gu BX, Du YW. Appl Phys Lett 2007;90: 042507. [9] Sutou Y, Imano Y, Koeda N, Omori T, Kainuma R, Ishida K, Oikawa K. Appl Phys Lett 2004;85:4358. [10] Wachtel E, Henninger F, Predel B. J Magn Magn Mater 1983;38:305. [11] Sánchez-Alarcos V, Recarte V, Pérez-Landazábal JI, Cuello GJ. Acta Mater 2007; 55:3883. [12] Pons J, Seguí C, Chernenko VA, Cesari E, Ochin P, Portier R. Mater Sci Eng A 1999;273e275:315.
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