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High temperature magnetic properties of mechanically alloyed Fe–Zr powder
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Materials Letters 62 (2008) 2640 – 2642 www.elsevier.com/locate/matlet
High temperature magnetic properties of mechanically alloyed Fe–Zr powder Debabrata Mishra, A. Perumal ⁎, A. Srinivasan Department of Physics, Indian Institute of Technology Guwahati, Guwahati — 781 039, India Received 10 November 2007; accepted 3 January 2008 Available online 11 January 2008
Abstract We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe100 − xZrx (x = 20–35) alloys by mechanical alloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effective magnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that the Fe80Zr20 sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solid solution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~ 570 °C, which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than those of melt-spun amorphous ribbons. © 2008 Elsevier B.V. All rights reserved. Keywords: Amorphous phase; Mechanical alloying; Curie temperature; Internal stress; Magnetic moment
1. Introduction
2. Experimental details
Fe rich metallic alloys with disordered structure have stimulated a keen interest over a few decades due to their complex magnetic behaviour as well as their potential application in various magnetic devices [1,2]. These materials can be obtained through two diverse processes viz., melt-spinning [2] and mechanical alloying [3]. Mechanical alloying (MA) has been successful in producing various disordered alloy [4] powders since the pioneering study by Koch et al. [5]. MA has been largely employed to obtain nanometer sized crystallites from microcrystalline phases and to synthesize composite materials with interesting properties [6]. However, there is still a lack of detailed investigation on the phase transformation, thermodynamics, and resulting magnetic properties of these powders obtained by MA. In the present study, we report the preparation and characterization of amorphous and/or non-equilibrium solid solution Fe100 − xZrx (x = 20–35 at.%) alloy powders by MA process. The effect of stress induced during the MA process on structural and high temperature magnetic properties of Fe100 − xZrx powders are also investigated.
Weighed quantities of elemental Fe and Zr powders (99.9% purity) corresponding to nominal compositions of Fe100 − xZrx (x = 20–35 at.%) were sealed in a hardened steel vial filled with high purity Ar gas. Mechanical alloying of the powder mixture was performed in a Insmart planetary ball mill under a milling speed of 500 rpm using 8 mm diameter tempered steel balls and a ball/ powder weight ratio of 20:1. In order to avoid excessive heating, the mill was programmed to halt for 10 min after every 15 min of operation. All the alloy compositions were milled for 60 h. The phases evolved in the as-milled powders were characterized by an X-ray diffractometer (XRD, Seifert 3003T/T), Scanning Electron Microscope (SEM, Leo 1430VP) with EDS attachment and Transmission Electron Microscope (TEM, JEOL2100). The magnetic properties of the powders were characterized using a Vibrating Sample Magnetometer (VSM, LakeShore7410) in applied field range of ±20 kOe.
⁎ Corresponding author. Tel.: +91 361 2582714; fax: +91 361 2690762. E-mail address: [email protected] (A. Perumal), [email protected] (D. Mishra), [email protected] (A. Srinivasan).
Fig. 1 shows the XRD patterns of as-mixed Fe65Zr35 powder and asmilled Fe100 − xZrx alloy powders. The multiple sharp Bragg peaks corresponding to the starting constituents of Fe and Zr are visible in the
0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.004
3. Results and discussion
D. Mishra et al. / Materials Letters 62 (2008) 2640–2642
2641
Fig. 1. XRD patterns of as-mixed Fe65Zr35 and as-milled Fe100 − xZrx alloy powders.
Fig. 3. Room temperature M–H loops and Inset: Variation of effective magnetic anisotropy with Zr substitution in Fe100 − xZrx (x = 20–35 at.%) as-milled alloy powders.
XRD pattern of the as-mixed sample. On milling, the characteristic peaks of Zr disappear. The broadened low intensity peaks corresponding to FeZr amorphous phase centered around 2θ = 33° and nonequilibrium solid solution of Fe(Zr) centered around 2θ = 44°, respectively, appear. This indicates that milling for 60 h results either in the formation of an amorphous phase (for x ≥ 25 at.%) or a structure consisting of an amorphous phase with a finite non-equilibrium solid solution (for x = 20 at.%) with an average crystallite size of about 8 nm. The MA process introduces a lot of strain in the as-milled powders. EDS analyses performed at selected places show that the deviations in the measured compositions of FeZr are within 3% of the nominal starting compositions. This signifies compositional homogeneity in the as-milled powders. Fig. 2 shows a bright-field plane view TEM image and selected area electron diffraction (SAED) pattern of the amorphous Fe65Zr35 alloy powder. The diffused halo ring in the SAED pattern is the typical signature of amorphous structure present in the milled Fe65Zr35 powders. This is in good agreement with the XRD results depicted in
Fig. 1. Fig. 3 shows the room temperature magnetic hysteresis loops of the as-milled Fe100 − xZrx powders. The curves depict the typical soft magnetic behavior, but they do not saturate at 20 kOe applied field. The average magnetic moment of FeZr varies from 0.8 μB per formula unit for x = 20 at.% to 0.25 μB per formula unit for x = 35 at.%. These values are found to be much lower when compared to the value of 2.2 μB for pure iron and somewhat lower than the value of 1.08 μB of the mechanically alloyed powders [7]. Such a large decrease in magnetic moment could be due to more than one of the following reasons: (i) change in the average Fe atomic distance caused by the increased Zr substitution leads to the formation of finite antiferromagnetic (AFM) sites [8], (ii) charge transfer from the alloying element to Fe, which partially fills the d-band of Fe and lowers its magnetic moment [9], (iii) consequence of the change in the hybridization between Fe–Fe d bands and Fe–Zr d bands [10] and (iv) presence of large topological disorder in the amorphous phase. In order to calculate the saturation magnetization and to obtain more specific details about effective magnetic anisotropy in the system, the experimental high field
Fig. 2. TEM bright-field plane view image and the selected area electron diffraction pattern of amorphous Fe65Zr35 sample.
Fig. 4. Thermomagnetization curves measured at 1 kOe applied field. Inset: Thermal derivative of magnetization as a function of temperature for as-milled Fe100 − xZrx (x = 20–35 at.%) alloy powders (Only one-third of the data points are shown in the plots for the sake of clarity).
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D. Mishra et al. / Materials Letters 62 (2008) 2640–2642
magnetization curves were fitted to the law of approach of saturation, defined as [11], a b c M ¼ MS 1 pffiffiffiffi 2 3 : : : þ vH H ð1Þ H H H where, MS is saturation magnetization, H is applied field, χH is high field susceptibility, and a, b, c are constant coefficients. The coefficient b is related to thep effective through the relation ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffimagneticpanisotropy ffiffiffiffi [13], keff ¼ A0 MS 105 b=8. The a= H term arises from point like defects with stress fields varying as σ ∝ 1/r3 [12], while the term b/H2 is attributed to magnetoelastic interaction. Long range stresses, accumulated in the powders, can cause stress anisotropy via magnetoelastic coupling. A detailed least square fitting procedure was carried out to fit the experimental magnetization data, by considering a, b, c, MS and χH as free fitting parameters. Inset in Fig. 3 shows the variation of the effective magnetic anisotropy with Zr substitution in Fe–Zr. It is to be noted that the value of Keff is found to be more than one order higher than that of bulk iron (0.414 × 105 J/m3). Thermomagnetization (M–T) measurement was carried out up to 850 °C under 1 kOe applied field for as-milled Fe100 − xZrx powders and the data are presented in Fig. 4. In order to accommodate all the M–T curves and to compare them directly, normalized magnetization [M(T)/ M30] with respect to the room temperature magnetization of the corresponding samples is plotted along the y-axis. The inset in Fig. 4 shows the derivative of thermomagnetization for all the samples to identify the magnetic phase transition temperature(s). With increasing temperature, all the samples show a similar variation of magnetization with temperature except for Fe80Zr20 sample. The Fe80Zr20 sample clearly shows two magnetic phase transitions: one at 625 °C and another at 760 °C. These two magnetic phase transitions represent the presence of both amorphous and non-equilibrium solid solution phases in this sample, as described in Fig. 1. While the first magnetic phase transition at 625 °C corresponds to the ferromagnetic (FM) to paramagnetic (PM) phase transition in the amorphous phase, the second transition might be due to the FM to PM phase transition (Curie temperature, TC) in Fe rich non-equilibrium solid solution. The value of TC decreases to about 570 °C with increasing Zr from 20 to 25 at.% and then remains almost constant up to 35 at.%. It can also be observed from Fig. 4 that the manner in which thermomagnetization decreases is not smooth. This might be due to the release of the stresses with increasing temperature. It is important to note that these FeZr milled powders exhibit high TC despite having low average magnetic moment at room temperature. Also, the TC of these samples is higher than those of amorphous alloys of similar compositions prepared by the meltspinning technique. Such a difference in TC could be due to the presence of large internal stresses accumulated during MA process and the existence of finite AFM sites in FM matrix. As described earlier, the effective magnetic anisotropy caused by the induced stresses from the MA process is larger and facilitates improvement of the stability of the local magnetic structure with long range magnetic ordering via magnetoelastic coupling [14,15]. In addition, the presence of fluctuations of concentration and exchange integral have also considerable effect on the higher TC, as described in an earlier report of micromagnetic theory of phase transition [15]. The increase of Curie temperature on the applied stress has been reported in a similar (FeZrCuB) system [16]. Internal stress is a consequence of the milling process, while the existence of finite AFM sites in FM matrix can result
from a change in the average Fe atomic distance. Both these factors could have efficiently contributed to the high magnetic stability in these alloy powders and hence the high TC. However, at this moment, it may be difficult to quantify exact contribution of various parameters leading to the observed high TC.
4. Conclusions In summary, we have prepared and characterized the amorphous and/or non-equilibrium solid solution Fe100 − xZrx alloys by MA process. MA of the mixture of elemental powders led to the formation of amorphous or amorphous/nonequilibrium solid solution depending upon the composition. These samples showed high TC of about 570 °C, which is almost constant with increasing Zr substitution. The internal stresses accumulated in the samples during the milling process have strong influences on the magnetic properties. Acknowledgment This work was financially supported by DAE-BRNS, India through a Young Scientist Research Award (2005/20/34/1/ BRNS/376) for AP and DST, India vide project No: SR/S2/ CMP-19/2006. Permission from CIF, IIT Guwahati, India for the use of SEM, TEM (100/IFD/6278/2005–2006) and VSM is gratefully acknowledged. References [1] D.E. Witkin, E.J. Lavernia, Prog. Mater. Sci. 51 (2006) 1–60. [2] M.E. McHenry, M.A. Williard, D.E. Laughlin, Prog. Mater. Sci. 44 (1999) 291–433. [3] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556. [4] J.J. Ipus, J.S. Blaquez, V. Franco, A. Conde, Intermetallics 15 (2007) 1132–1138. [5] C.C. Koch, O.B. Cavin, C.G. Mckamey, J.O. Scarbrough, Appl. Phys. Lett. 43 (1983) 1017–1019. [6] J. Bednarick, E. Burkel, K. Saksl, P. Kollar, S. Roth, J. Appl. Phys. 100 (2006) 1–6. [7] A. Grabias, D. Oleszak, J. Kalinowska, M. Kopcewics, J. Latuch, M. Pekala, J. Alloy. Compd. 434–435 (2007) 493–496. [8] J.S. Zhang, M. Ding, F. Pan, J. Phys., D, Appl. Phys. 33 (2000) 185–194. [9] T.D. Shen, R.B. Schwarz, J.D. Thompson, Phys. Rev., B 72 (2005) 014431. [10] D. Stoeffler, K. Ounadjela, J. Sticht, F. Gautier, Phys. Rev., B 49 (1994) 299–309. [11] R. Herz, H. Kronmuller, J. Magn. Magn. Mater. 15–18 (1980) 1299–1300. [12] H. Kronmuller, M. Fahnle, M. Domann, H. Grimm, R. Grimm, B. Groger, J. Magn. Magn. Mater. 13 (1979) 53–70. [13] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964, p. 277. [14] Y.N. Mitsay, A. Fridman, D.V. Spirin, C.N. Alexeyev, M.S. Kochmanski, Phys., B 292 (2000) 83–88. [15] H. Kronmuler, M. Fahnle, Phys. Status Solidi (b) 97 (1980) 513–520. [16] P. Goria, I. Orue, M.L.F. Gubieda, F. Plazaola, N. Zabala, J.M. Barandiaran, J. Magn. Magn. Mater. 157–158 (1996) 203–204.
Materials Letters 62 (2008) 2640 – 2642 www.elsevier.com/locate/matlet
High temperature magnetic properties of mechanically alloyed Fe–Zr powder Debabrata Mishra, A. Perumal ⁎, A. Srinivasan Department of Physics, Indian Institute of Technology Guwahati, Guwahati — 781 039, India Received 10 November 2007; accepted 3 January 2008 Available online 11 January 2008
Abstract We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe100 − xZrx (x = 20–35) alloys by mechanical alloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effective magnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that the Fe80Zr20 sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solid solution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~ 570 °C, which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than those of melt-spun amorphous ribbons. © 2008 Elsevier B.V. All rights reserved. Keywords: Amorphous phase; Mechanical alloying; Curie temperature; Internal stress; Magnetic moment
1. Introduction
2. Experimental details
Fe rich metallic alloys with disordered structure have stimulated a keen interest over a few decades due to their complex magnetic behaviour as well as their potential application in various magnetic devices [1,2]. These materials can be obtained through two diverse processes viz., melt-spinning [2] and mechanical alloying [3]. Mechanical alloying (MA) has been successful in producing various disordered alloy [4] powders since the pioneering study by Koch et al. [5]. MA has been largely employed to obtain nanometer sized crystallites from microcrystalline phases and to synthesize composite materials with interesting properties [6]. However, there is still a lack of detailed investigation on the phase transformation, thermodynamics, and resulting magnetic properties of these powders obtained by MA. In the present study, we report the preparation and characterization of amorphous and/or non-equilibrium solid solution Fe100 − xZrx (x = 20–35 at.%) alloy powders by MA process. The effect of stress induced during the MA process on structural and high temperature magnetic properties of Fe100 − xZrx powders are also investigated.
Weighed quantities of elemental Fe and Zr powders (99.9% purity) corresponding to nominal compositions of Fe100 − xZrx (x = 20–35 at.%) were sealed in a hardened steel vial filled with high purity Ar gas. Mechanical alloying of the powder mixture was performed in a Insmart planetary ball mill under a milling speed of 500 rpm using 8 mm diameter tempered steel balls and a ball/ powder weight ratio of 20:1. In order to avoid excessive heating, the mill was programmed to halt for 10 min after every 15 min of operation. All the alloy compositions were milled for 60 h. The phases evolved in the as-milled powders were characterized by an X-ray diffractometer (XRD, Seifert 3003T/T), Scanning Electron Microscope (SEM, Leo 1430VP) with EDS attachment and Transmission Electron Microscope (TEM, JEOL2100). The magnetic properties of the powders were characterized using a Vibrating Sample Magnetometer (VSM, LakeShore7410) in applied field range of ±20 kOe.
⁎ Corresponding author. Tel.: +91 361 2582714; fax: +91 361 2690762. E-mail address: [email protected] (A. Perumal), [email protected] (D. Mishra), [email protected] (A. Srinivasan).
Fig. 1 shows the XRD patterns of as-mixed Fe65Zr35 powder and asmilled Fe100 − xZrx alloy powders. The multiple sharp Bragg peaks corresponding to the starting constituents of Fe and Zr are visible in the
0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.004
3. Results and discussion
D. Mishra et al. / Materials Letters 62 (2008) 2640–2642
2641
Fig. 1. XRD patterns of as-mixed Fe65Zr35 and as-milled Fe100 − xZrx alloy powders.
Fig. 3. Room temperature M–H loops and Inset: Variation of effective magnetic anisotropy with Zr substitution in Fe100 − xZrx (x = 20–35 at.%) as-milled alloy powders.
XRD pattern of the as-mixed sample. On milling, the characteristic peaks of Zr disappear. The broadened low intensity peaks corresponding to FeZr amorphous phase centered around 2θ = 33° and nonequilibrium solid solution of Fe(Zr) centered around 2θ = 44°, respectively, appear. This indicates that milling for 60 h results either in the formation of an amorphous phase (for x ≥ 25 at.%) or a structure consisting of an amorphous phase with a finite non-equilibrium solid solution (for x = 20 at.%) with an average crystallite size of about 8 nm. The MA process introduces a lot of strain in the as-milled powders. EDS analyses performed at selected places show that the deviations in the measured compositions of FeZr are within 3% of the nominal starting compositions. This signifies compositional homogeneity in the as-milled powders. Fig. 2 shows a bright-field plane view TEM image and selected area electron diffraction (SAED) pattern of the amorphous Fe65Zr35 alloy powder. The diffused halo ring in the SAED pattern is the typical signature of amorphous structure present in the milled Fe65Zr35 powders. This is in good agreement with the XRD results depicted in
Fig. 1. Fig. 3 shows the room temperature magnetic hysteresis loops of the as-milled Fe100 − xZrx powders. The curves depict the typical soft magnetic behavior, but they do not saturate at 20 kOe applied field. The average magnetic moment of FeZr varies from 0.8 μB per formula unit for x = 20 at.% to 0.25 μB per formula unit for x = 35 at.%. These values are found to be much lower when compared to the value of 2.2 μB for pure iron and somewhat lower than the value of 1.08 μB of the mechanically alloyed powders [7]. Such a large decrease in magnetic moment could be due to more than one of the following reasons: (i) change in the average Fe atomic distance caused by the increased Zr substitution leads to the formation of finite antiferromagnetic (AFM) sites [8], (ii) charge transfer from the alloying element to Fe, which partially fills the d-band of Fe and lowers its magnetic moment [9], (iii) consequence of the change in the hybridization between Fe–Fe d bands and Fe–Zr d bands [10] and (iv) presence of large topological disorder in the amorphous phase. In order to calculate the saturation magnetization and to obtain more specific details about effective magnetic anisotropy in the system, the experimental high field
Fig. 2. TEM bright-field plane view image and the selected area electron diffraction pattern of amorphous Fe65Zr35 sample.
Fig. 4. Thermomagnetization curves measured at 1 kOe applied field. Inset: Thermal derivative of magnetization as a function of temperature for as-milled Fe100 − xZrx (x = 20–35 at.%) alloy powders (Only one-third of the data points are shown in the plots for the sake of clarity).
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D. Mishra et al. / Materials Letters 62 (2008) 2640–2642
magnetization curves were fitted to the law of approach of saturation, defined as [11], a b c M ¼ MS 1 pffiffiffiffi 2 3 : : : þ vH H ð1Þ H H H where, MS is saturation magnetization, H is applied field, χH is high field susceptibility, and a, b, c are constant coefficients. The coefficient b is related to thep effective through the relation ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffimagneticpanisotropy ffiffiffiffi [13], keff ¼ A0 MS 105 b=8. The a= H term arises from point like defects with stress fields varying as σ ∝ 1/r3 [12], while the term b/H2 is attributed to magnetoelastic interaction. Long range stresses, accumulated in the powders, can cause stress anisotropy via magnetoelastic coupling. A detailed least square fitting procedure was carried out to fit the experimental magnetization data, by considering a, b, c, MS and χH as free fitting parameters. Inset in Fig. 3 shows the variation of the effective magnetic anisotropy with Zr substitution in Fe–Zr. It is to be noted that the value of Keff is found to be more than one order higher than that of bulk iron (0.414 × 105 J/m3). Thermomagnetization (M–T) measurement was carried out up to 850 °C under 1 kOe applied field for as-milled Fe100 − xZrx powders and the data are presented in Fig. 4. In order to accommodate all the M–T curves and to compare them directly, normalized magnetization [M(T)/ M30] with respect to the room temperature magnetization of the corresponding samples is plotted along the y-axis. The inset in Fig. 4 shows the derivative of thermomagnetization for all the samples to identify the magnetic phase transition temperature(s). With increasing temperature, all the samples show a similar variation of magnetization with temperature except for Fe80Zr20 sample. The Fe80Zr20 sample clearly shows two magnetic phase transitions: one at 625 °C and another at 760 °C. These two magnetic phase transitions represent the presence of both amorphous and non-equilibrium solid solution phases in this sample, as described in Fig. 1. While the first magnetic phase transition at 625 °C corresponds to the ferromagnetic (FM) to paramagnetic (PM) phase transition in the amorphous phase, the second transition might be due to the FM to PM phase transition (Curie temperature, TC) in Fe rich non-equilibrium solid solution. The value of TC decreases to about 570 °C with increasing Zr from 20 to 25 at.% and then remains almost constant up to 35 at.%. It can also be observed from Fig. 4 that the manner in which thermomagnetization decreases is not smooth. This might be due to the release of the stresses with increasing temperature. It is important to note that these FeZr milled powders exhibit high TC despite having low average magnetic moment at room temperature. Also, the TC of these samples is higher than those of amorphous alloys of similar compositions prepared by the meltspinning technique. Such a difference in TC could be due to the presence of large internal stresses accumulated during MA process and the existence of finite AFM sites in FM matrix. As described earlier, the effective magnetic anisotropy caused by the induced stresses from the MA process is larger and facilitates improvement of the stability of the local magnetic structure with long range magnetic ordering via magnetoelastic coupling [14,15]. In addition, the presence of fluctuations of concentration and exchange integral have also considerable effect on the higher TC, as described in an earlier report of micromagnetic theory of phase transition [15]. The increase of Curie temperature on the applied stress has been reported in a similar (FeZrCuB) system [16]. Internal stress is a consequence of the milling process, while the existence of finite AFM sites in FM matrix can result
from a change in the average Fe atomic distance. Both these factors could have efficiently contributed to the high magnetic stability in these alloy powders and hence the high TC. However, at this moment, it may be difficult to quantify exact contribution of various parameters leading to the observed high TC.
4. Conclusions In summary, we have prepared and characterized the amorphous and/or non-equilibrium solid solution Fe100 − xZrx alloys by MA process. MA of the mixture of elemental powders led to the formation of amorphous or amorphous/nonequilibrium solid solution depending upon the composition. These samples showed high TC of about 570 °C, which is almost constant with increasing Zr substitution. The internal stresses accumulated in the samples during the milling process have strong influences on the magnetic properties. Acknowledgment This work was financially supported by DAE-BRNS, India through a Young Scientist Research Award (2005/20/34/1/ BRNS/376) for AP and DST, India vide project No: SR/S2/ CMP-19/2006. Permission from CIF, IIT Guwahati, India for the use of SEM, TEM (100/IFD/6278/2005–2006) and VSM is gratefully acknowledged. References [1] D.E. Witkin, E.J. Lavernia, Prog. Mater. Sci. 51 (2006) 1–60. [2] M.E. McHenry, M.A. Williard, D.E. Laughlin, Prog. Mater. Sci. 44 (1999) 291–433. [3] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556. [4] J.J. Ipus, J.S. Blaquez, V. Franco, A. Conde, Intermetallics 15 (2007) 1132–1138. [5] C.C. Koch, O.B. Cavin, C.G. Mckamey, J.O. Scarbrough, Appl. Phys. Lett. 43 (1983) 1017–1019. [6] J. Bednarick, E. Burkel, K. Saksl, P. Kollar, S. Roth, J. Appl. Phys. 100 (2006) 1–6. [7] A. Grabias, D. Oleszak, J. Kalinowska, M. Kopcewics, J. Latuch, M. Pekala, J. Alloy. Compd. 434–435 (2007) 493–496. [8] J.S. Zhang, M. Ding, F. Pan, J. Phys., D, Appl. Phys. 33 (2000) 185–194. [9] T.D. Shen, R.B. Schwarz, J.D. Thompson, Phys. Rev., B 72 (2005) 014431. [10] D. Stoeffler, K. Ounadjela, J. Sticht, F. Gautier, Phys. Rev., B 49 (1994) 299–309. [11] R. Herz, H. Kronmuller, J. Magn. Magn. Mater. 15–18 (1980) 1299–1300. [12] H. Kronmuller, M. Fahnle, M. Domann, H. Grimm, R. Grimm, B. Groger, J. Magn. Magn. Mater. 13 (1979) 53–70. [13] S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964, p. 277. [14] Y.N. Mitsay, A. Fridman, D.V. Spirin, C.N. Alexeyev, M.S. Kochmanski, Phys., B 292 (2000) 83–88. [15] H. Kronmuler, M. Fahnle, Phys. Status Solidi (b) 97 (1980) 513–520. [16] P. Goria, I. Orue, M.L.F. Gubieda, F. Plazaola, N. Zabala, J.M. Barandiaran, J. Magn. Magn. Mater. 157–158 (1996) 203–204.