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Magnetism and associated exchange bias in Ni2−xCoxMn1.4Ga0.6
Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetism and associated exchange bias in Ni2 xCoxMn1.4Ga0.6 Ramakanta Chapai, Mahmud Khan n Department of Physics, Miami University, Oxford, Ohio 45056, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 18 October 2015 Received in revised form 26 November 2015 Accepted 27 November 2015 Available online 28 November 2015
A series of Ni2 xCoxMn1.4Ga0.6 Heusler alloys have been systematically investigated by x-ray diffraction, dc magnetization, and ac susceptibility measurements. For all Co concentration, the alloys exhibit the L10 martensitic structure at room temperature. Interestingly, Co doping simultaneously causes a reduction in the ferromagnetic exchange interaction and enhancement of magnetic anisotropy in Ni2 xCoxMn1.4Ga0.6. Exchange bias effects under both zero field cooled and field cooled condition have been observed in all alloys for x o 0.3. The ac susceptibility data show frequency dependence that changes with increasing Co concentration, indicating a change of ground state from spin glass to super spin glass. The experimental results are explained considering the atomic radii of Ni and Co and the fundamental magnetic interactions in Heusler alloys. & 2015 Elsevier B.V. All rights reserved.
1. Introduction The exchange bias (EB) effect, discovered in 1956 by Meiklejohn and Bean [1], is of great interest both due to its fundamental aspects [2] and its application in technologies including spin valves, magnetic recording read heads, and giant magnetoresistive sensors [3,4]. The phenomenon is traditionally observed in layered or composite materials with ferromagnetic (FM) and antiferromagnetic (AFM) interfaces, and is described as a shift of the magnetic hysteresis loop along the magnetic field axis [5]. The reason of such shift is generally attributed to a unidirectional exchange anisotropy that is induced in the FM layer of the FM/AFM interface. Although the EB effect was initially observed in nanoparticles and thin films, EB has also been reported in several bulk polycrystalline materials including several Mn rich Ni2Mn1 þ xZ1 x (Z ¼Ga, In, Sn, Sb) Heusler alloys. [6,7,8–11] The EB in the Mn-rich Huesler alloys is observed at low temperatures where the Mn–Mn exchange interaction within the regular Mn sublattices is ferromagnetic (FM), while the excess Mn atoms occupying the Z sites couple antiferromagnetically with the Mn atoms on the regular sites [12,13]. Such co-existence of FM and AFM interactions in the alloys often creates magnetic disorder in the system, giving rise to spin-glass (SG) phase at low temperatures. Conventionally, the EB effect is observed after the material is cooled in the presence of a magnetic field to temperatures below the Neel temperature of the AFM layer. Interestingly, the EB effect after zero magnetic field cooling (ZFC) has been reported in several n
Corresponding author. E-mail address: [email protected] (M. Khan).
http://dx.doi.org/10.1016/j.jmmm.2015.11.086 0304-8853/& 2015 Elsevier B.V. All rights reserved.
Heusler alloys [7,11]. EB under both ZFC and FC conditions has been reported recently in Ni2Mn1.4Ga0.6 and a series of Ni2 xMn1.4 þ xGa0.6 alloys [8,14]. The observation of EB in these alloys were explained in terms of a model suggested by Lázpita et al [15]. According to the model, the Mn atoms may occupy the regular Mn sites, Ni sites, and Ga sites. The respective Mn atoms are referred to as Mn/Mn (for regular site), Mn/Ni (for Ni site) and Mn/Ga (for Ga site). In the absence of the Mn/Ni atoms, the coupling between the Mn/Mn atoms and the Mn/Ga atoms is AFM, which was apparently the case for Ni2Mn1.4Ga0.6. When Ni is partially replaced with Mn in Ni2 xMn1.4 þ xGa0.6, the excess Mn atoms occupy the Ni sites. In this case, the Mn/Ni atoms become the closest neighbor to the Mn/ Mn atoms, and thus the stronger AFM coupling between Mn/Ni and Mn/Mn atoms overcome the AFM coupling between the Mn/ Mn and Mn/Ga atoms, resulting in FM coupling between the Mn/ Mn and the Mn/Ga atoms. This results in an enhancement of overall ferromagnetic exchange interactions in Ni2 xMn1.4 þ xGa0.6 [14]. Considering the observation of EB in Ni2Mn1.4Ga0.6 and Ni2 xMn1.4 þ xGa0.6 alloys, it is interesting to explore the magnetic properties of Ni2 xCoxMn1.4Ga0.6, where Ni is partially replaced with Co. The substitution of Ni by Co will apparently keep the amount of Mn/Mn and Mn/Ga atoms same while changing the interatomic distances between the atoms. Since the magnetic interactions between the Mn atoms at different crystalline sites are strongly dependent on the interatomic distances, it is interesting to explore, how the magnetic properties and EB will be effected due to Co substitution in Ni2 xCoxMn1.4Ga0.6. Therefore, in this paper we report an experimental study on a series of Ni2 xCox Mn1.4Ga0.6 Heusler alloys. The idea is to explore the change in the
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R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
magnetic interactions and the associated EB phenomena in the alloys caused by the systematic substitution of Ni by Co.
Table 1 Lattice parameters and c/a ratios of Ni2 xCoxMn1.4Ga0.6 as evaluated from the XRD data. Co concentration (x)
a (Å)
c (Å)
c/a
0.0 0.05 0.10 0.2 0.3
7.6328 7.6393 7.6319 7.6459 7.6468
6.7836 6.7750 6.7734 6.7902 6.7872
0.88874 0.88686 0.88751 0.88808 0.88759
2. Experimental techniques The Ni2 xCoxMn1.4Ga0.6 (0 rxr 0.3) alloys were prepared by arc melting under argon atmosphere using high-purity Ni, Mn, Co, and Ga metals. For homogenization, the as-prepared sample were sealed in partially evacuated (partially filled with inert gas) vycor tubes and annealed at 1123 K for 72 h. After annealing, the samples were rapidly cooled down to room temperature by quenching in cold water. To determine the phase purity and crystal structure of the alloys, x-ray diffraction (XRD) measurements were performed on a Scintag Pad X-ray powder diffractometer with Cu-Kα radiation. The diffraction patterns were indexed using a computer program known as PowderCell [16]. Magnetization (M) measurements as a function of temperature and applied magnetic field were conducted on a Physical Property Measurement System (PPMS) made by Quantum Design Inc. Before obtaining the zero field cooled (ZFC) temperature dependence of magnetization, M(T), data, the samples were cooled from 300 K to 5 K in a zero magnetic field. When the temperature stabilized at 5 K, a magnetic field of 1000 Oe was applied, and the ZFC M (T) data was recorded as a function of increasing temperature. The field cooled cooling (FCC) M(T) data were obtained while sweeping down the temperature from 300 K to 5 K in the presence of 1000 Oe field. Prior to each ZFC magnetization versus field, M(H), measurement, the samples were cooled from 300 K to 5 K in zero magnetic field. Before the field cooled FC M(H) measurements, each sample was cooled down to 5 K in the presence of 50 kOe magnetic field. The ac magnetic susceptibility measurements at an ac field of 10 Oe and frequencies of 10 Hz, 100 Hz, 1000 Hz, and 10,000 Hz were also conducted on the PPMS.
3. Results and discussion The XRD patterns of Ni2 xCoxMn1.4Ga0.6 (0.05 rx r 0.3) alloys obtained at room temperature are shown in Fig. 1. The alloys are single phase and exhibit the L10 non-modulated tetragonal martensitic structure at room temperature. The major peaks observed in the XRD data are labeled in Fig. 1. In the vicinity of 49° (next to the 400 peak) and 74° (next to the 044 peak), minor humps are
Fig. 2. The temperature dependence of magnetization of Ni2 xCoxMn1.4Ga0.6 (a) x¼ 0 and (b) x ¼0.3, measured in a magnetic field of 1000 Oe. (c) TC and Tp of Ni2 xCoxMn1.4Ga0.6 as a function of Co concentration x.
Fig.1. X-ray diffraction patterns temperature.
of
Ni2 xCoxMn1.4Ga0.6
obtained
at
room
observed in the XRD data of some samples. These humps are most likely related to the hkl miller indices 213 (49°) and 600 (74°), respectively. The lattice parameters and c/a ratio of all samples are listed in Table 1. The roughly estimated error in the lattice parameters vary from 70.003 to 70.011. The alloy with x ¼0 has the
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
99
Fig. 3. Magnetization as a function of field, M(H), of Ni2 xCoxMn1.4Ga0.6 obtained at 5 K under ZFC condition.
following lattice parameters at room temperature: a ¼b¼ 7.6328 Å and c ¼6.7836 Å. With increasing Co concentration, the lattice parameters changes marginally. The lattice parameters for the alloy with x ¼0.3 are, a ¼b¼ 7.6468 Å and c¼6.7872 Å. The small change in the lattice parameters of the Ni2 xCoxMn1.4Ga0.6 alloys may be attributed to the small difference in the atomic radii of Ni (1.246 Å) and Co (1.252 Å) considering the coordination number, CN¼ 12 [17]. The ZFC and FCC M(T) data of Ni2 xCoxMn1.4Ga0.6 (x ¼0 and 0.3) samples measured in a magnetic field of 1 kOe are shown in Fig. 2a and 2b. As shown in Fig. 2a, the ZFC magnetization of Ni2 xCoxMn1.4Ga0.6 (x¼ 0) initially increases with increasing temperature until it peaks at TP ¼97 K. Beyond this temperature the magnetization decreases with increasing temperature due to the ferromagnetic–paramagnetic transition. From the derivative (dM/ dT) plot of the M(T) data, the ferromagnetic transition temperature, TC, of Ni2 xCoxMn1.4Ga0.6 (x¼ 0) has been determined to be ∼165 K. The FCC M(T) data exhibit similar behavior with the exception of the thermomagnetic irreversibility below ∼130 K. Such an irreversibility is usually observed in these alloys due to the pinning of the ferromagnetic domains below the freezing temperature of the SG phase. The M(T) data of the alloy with x ¼0.3 exhibit similar behavior with the exception that TC and TP are observed at lower temperatures. This decrease of TC and TP with increasing Co concentration is systematically observed in Ni2 xCox
Mn1.4Ga0.6, as shown in Fig. 2c. The decrease of TC demonstrates a reduction in the ferromagnetic exchange interactions in the system with increasing Co concentration. Fig. 3 shows the M(H) data of Ni2 xCoxMn1.4Ga0.6 (x ¼0, 0.05, 0.1, and 0.2) measured at 5 K under ZFC conditions. The hysteresis loops were measured in magnetic fields from 50 kOe to 50 kOe. For clearer visualization of the lower field region, only the magnetization curves from 2 kOe to 2 kOe are shown. As shown in Fig. 3a, the ZFC M(H) data of the Ni2 xCoxMn1.4Ga0.6 (x ¼0) exhibit a non-symmetric frustrated feature in the vicinity of the center of the hysteresis loop, which is shifted towards the negative field direction demonstrating an EB effect with an exchange bias field, HEB ¼ 422 Oe. The observed behavior is consistent with existing literature [8]. As shown in Fig. 3b, 3c and 3d, the alloys with Co exhibit more symmetric M(H) loops and higher coercive field. For all alloys with Co concentration xr0.2, the ZFC M(H) data demonstrates EB. As shown in Fig. 4, unlike the ZFC data, for all Co concentration the M(H) loops of the Ni2 xCoxMn1.4Ga0.6 alloys, obtained under FC condition, exhibit normal symmetric features. It is interesting to note that when compared to the alloy with x¼ 0, the squareness of the FC M(H) loops is enhanced in the Co containing alloys. This improvement of the squareness indicates that the substitution of Co in Ni2 xCoxMn1.4Ga0.6 alloys results in enhanced magnetic anisotropy. For Co concentrations x r0.2, all alloys exhibit EB under
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Fig.4. Magnetization as a function of field, M(H), of Ni2 xCoxMn1.4Ga0.6 obtained at 5 K under FC condition.
Fig. 5. (a) Exchange bias field, HEB, as a function of Co concentration for Ni2 xCox Mn1.4Ga0.6 The inset shows the saturation moment, MS, as a function of x. The measurements were performed at 5 K under ZFC and FC conditions.
FC condition as well. Fig. 5 shows the HEB, evaluated from the ZFC and FC M(H) loops of Ni2 xCoxMn1.4Ga0.6, as a function of Co concentration x. As shown in the figure, for both ZFC and FC conditions, HEB initially
increases in the negative direction with increasing Co concentration. As x exceeds 0.1, the magnitude of HEB decreases and eventually becomes nearly zero for the alloy with x¼ 0.3. For ZFC condition, HEB increases from 422 Oe (x ¼0) to 585 Oe (x ¼0.1). On the other hand, HEB obtained from the FC data increases from 130 Oe (x ¼0) to 290 Oe (x ¼0.1). The inset of Fig. 5 shows the saturation magnetization (MS) as a function of Co concentration. For both ZFC and FC conditions, MS decreases with increasing x. However, for x o0.2, MS obtained from the ZFC M(H) data is slightly lower than those obtained from the FC data. The decrease of MS and TC (Fig. 2c) with increasing Co concentration signifies the reduction of FM interactions in Ni2 xCoxMn1.4Ga0.6. We have performed ac susceptibility measurements on the Ni2 xCoxMn1.4Ga0.6 samples in order to explore possible explanations behind the observed EB phenomena in the system. [8,18] In Fig. 6, we show the temperature dependence of the real components (χ′(T)) of the ac susceptibility data of Ni2 xCoxMn1.4Ga0.6 (x ¼.05 and 0.3) samples measured in an ac field of amplitude 10 Oe and frequencies (f) of 10 Hz, 100 Hz, 1000 Hz, and 10,000 Hz. For the sample with x ¼0.05 the susceptibility increases with increasing temperature until peaking at Tp ∼150 K (f¼ 10 Hz) and then decreases to zero. All samples with x r0.3 show similar behavior with the exception of exhibiting relatively sharper peaks (see Fig. 6b) in the vicinity of Tp. A strong frequency dependence is observed in the vicinity of TP for all alloys (see inset of Fig. 6). The
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
Fig. 6. Temperature dependence of the real component of the ac magnetic susceptibility of Ni2 xCoxMn1.4Ga0.6 measured in an ac field of 10 Oe and frequencies from 10 Hz to 10,000 Hz.
101
The empirical relation, Φ = ΔTp [Tp Δlog10f ]−1, is a simple but useful criterion to determine whether the observed frequency dependence in the ac susceptibility data is associated with metallic spin glass or not [18–20]. Here, Φ represents the relative shift of Tp with f. For metallic spin-glass systems Φ typically lies between 0.005 and 0.01. A value greater than 0.01 and up to 0.04 would typically signify a SSG state. Fig. 7 shows the values of Φ for Ni2 xCoxMn1.4Ga0.6 alloys as a function of x. It is interesting to note that Φ increases rapidly with increasing Co concentration from ∼0.016 (x¼ 0) to ∼0.047 (x ¼0.3). This behavior indicates that with increasing Co concentration the low temperature SG state in Ni2 xCoxMn1.4Ga0.6 transforms to a SSG state. The magnetic properties of Ni2 xCoxMn1.4Ga0.6 described above can be explained by considering the occupancy of the Mn atoms and the corresponding magnetic interactions as discussed earlier [15]. In case of Ni2Mn1.4Ga0.6 (with only Mn/Mn and Mn/Ga atoms) the magnetic properties, including the EB effects, are governed by the FM interactions between the regular Mn/Mn atoms and AFM interactions between the Mn/Mn and Mn/Ga atoms. Apparently, the addition of Co atoms in Ni2 xCoxMn1.4Ga0.6 do not change the amount of Mn/Mn and Mn/Ga atoms. However, the interatomic distances between the Mn atoms on the different sites changes due to the larger radius of Co (1.252 Å) when compared to Ni (1.246 Å). This change of interatomic distances results in the reduction of FM interactions while increasing the AFM interactions, which is clearly demonstrated in the magnetization and ac susceptibility data. As discussed above, the experimental results show that TC, TP, and MS decrease (Fig. 2 and inset of Fig.5) while Φ increases (Fig. 7) monotonically with increasing Cr concentration. However, as shown in Fig. 5, HEB (for both ZFC and FC condition) shows a non-monotonic dependence on Co concentration. This behavior can be explained by considering the fact the EB arises mainly due to the co-existence of FM and AFM (or SG) interactions in the system. Therefore, it may be expected that in a given system HEB will maximize for certain concentrations of FM and AFM interactions. It appears that in case of the Ni2 xCoxMn1.4Ga0.6, the optimized FM and SG interactions exist for Co concentration x ¼0.1, and hence the HEB peaks at this concentration. In conclusion, we have investigated the magnetic properties and exchange bias phenomena in Ni2 xCoxMn1.4Ga0.6 Heusler alloys. For xr 0.2, all the samples exhibit EB effects in both ZFC and FC condition. The experimental results show that Co doping results in the reduction of the FM interactions and increases the AFM interaction in the system. These variations in the FM and AFM interactions changes the low temperature magnetic state from SG to SSG, which is revealed in the ac susceptibility data. The observed behavior is attributed to the difference in the atomic radii of Co and Ni.
References [1] [2] [3] [4] [5] Fig. 7. The empirical parameter, Φ , as a function of Co concentration x for Ni2 xCox Mn1.4Ga0.6.
behavior observed in the ac susceptibility data of Ni2 xCox Mn1.4Ga0.6 indicates towards a spin glass type frustrated nature of the ground state [18]. However, frequency dependence in magnetic systems may also arise from magnetic blocking.
[6] [7] [8] [9] [10] [11]
W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. J. Nogués, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. Manuel Bibes, Javier E. Villegas, Agnès Barthélémy, Adv. Phys. 60 (2011) 5. B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J.S. Muñoz, M.D. Baró, Phys. Rep. 422 (2005) 65. M. Khan, I. Dubenko, S. Stadler, N. Ali, Appl. Phys. Lett. 91 (2007) 072510. B.M. Wang, Y. Liu, P. Ren, B. Xia, K.B. Ruan, J.B. Yi, J. Ding, X.G. Li, L. Wang, Phys. Rev. Lett. 106 (2011) 07720. Z.D. Han, B. Qian, D.H. Wang, P. Zhang, X.F. Jiang, C.L. Zhang, Y.W. Du, Appl. Phys. Lett. 103 (2013) 172403. A.K. Pathak, D. Paudyal, W.T. Jayasekara, S. Calder, A. Kreyssig, A.I. Goldman, K. A. Gschneidner Jr., V.K. Pecharsky, Phys. Rev. B 89 (2014) 224411. M. Khan, I. Dubenko, S. Stadler, N. Ali, J. Appl. Phys. 102 (2007) 113914. A.K. Nayak, M. Nicklas, S. Chadov, C. Shekhar, Y. Skourski, J. Winterlik, C. Felser,
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Phys. Rev. Lett. 110 (2013) 127204. [12] J. Enkovaara, O. Heczko, A. Ayuela, R.M. Nieminen, Phys. Rev. B 67 (2003) 212405. [13] S. Aksoy, M. Acet, P.P. Deen, L. Mañosa, A. Planes, Phys. Rev. B 79 (2009) 212401. [14] M. Khan, R. Chapai, J. Alloy. Compd. 647 (2015) 935. [15] P. Lázpita, J.M. Barandiarán, J. Feuchtwanger, J. Gutiérrez, I. Rodriguez, V. A. Chernenko, A. Stunault, C. Mondelli, J. Phys.: Conf. Ser. 325 (2011) 012016.
[16] 〈http://www.bam.de/de/service/publikationen/powder_cell.htm〉. [17] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, WileyInterscience, New-York (1972), p. 146–148. [18] R.Y. Umetsu, A. Fujita, W. Ito, T. Kanomata, R. Kainuma, J. Phys.: Condens. Matter 23 (2011) 326001. [19] S. Bedanta, W. Kleemann, J. Phys. D.: Appl. Phys. 42 (2009) 013001. [20] P. Liao, C. Jing, X.L. Wang, Y.J. Yang, D. Zheng, Z. Li, B.J. Kang, D.M. Deng, S. X. Cao, J.C. Zhang, B. Lu, Appl. Phys. Lett. 104 (2014) 092410 5.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetism and associated exchange bias in Ni2 xCoxMn1.4Ga0.6 Ramakanta Chapai, Mahmud Khan n Department of Physics, Miami University, Oxford, Ohio 45056, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 18 October 2015 Received in revised form 26 November 2015 Accepted 27 November 2015 Available online 28 November 2015
A series of Ni2 xCoxMn1.4Ga0.6 Heusler alloys have been systematically investigated by x-ray diffraction, dc magnetization, and ac susceptibility measurements. For all Co concentration, the alloys exhibit the L10 martensitic structure at room temperature. Interestingly, Co doping simultaneously causes a reduction in the ferromagnetic exchange interaction and enhancement of magnetic anisotropy in Ni2 xCoxMn1.4Ga0.6. Exchange bias effects under both zero field cooled and field cooled condition have been observed in all alloys for x o 0.3. The ac susceptibility data show frequency dependence that changes with increasing Co concentration, indicating a change of ground state from spin glass to super spin glass. The experimental results are explained considering the atomic radii of Ni and Co and the fundamental magnetic interactions in Heusler alloys. & 2015 Elsevier B.V. All rights reserved.
1. Introduction The exchange bias (EB) effect, discovered in 1956 by Meiklejohn and Bean [1], is of great interest both due to its fundamental aspects [2] and its application in technologies including spin valves, magnetic recording read heads, and giant magnetoresistive sensors [3,4]. The phenomenon is traditionally observed in layered or composite materials with ferromagnetic (FM) and antiferromagnetic (AFM) interfaces, and is described as a shift of the magnetic hysteresis loop along the magnetic field axis [5]. The reason of such shift is generally attributed to a unidirectional exchange anisotropy that is induced in the FM layer of the FM/AFM interface. Although the EB effect was initially observed in nanoparticles and thin films, EB has also been reported in several bulk polycrystalline materials including several Mn rich Ni2Mn1 þ xZ1 x (Z ¼Ga, In, Sn, Sb) Heusler alloys. [6,7,8–11] The EB in the Mn-rich Huesler alloys is observed at low temperatures where the Mn–Mn exchange interaction within the regular Mn sublattices is ferromagnetic (FM), while the excess Mn atoms occupying the Z sites couple antiferromagnetically with the Mn atoms on the regular sites [12,13]. Such co-existence of FM and AFM interactions in the alloys often creates magnetic disorder in the system, giving rise to spin-glass (SG) phase at low temperatures. Conventionally, the EB effect is observed after the material is cooled in the presence of a magnetic field to temperatures below the Neel temperature of the AFM layer. Interestingly, the EB effect after zero magnetic field cooling (ZFC) has been reported in several n
Corresponding author. E-mail address: [email protected] (M. Khan).
http://dx.doi.org/10.1016/j.jmmm.2015.11.086 0304-8853/& 2015 Elsevier B.V. All rights reserved.
Heusler alloys [7,11]. EB under both ZFC and FC conditions has been reported recently in Ni2Mn1.4Ga0.6 and a series of Ni2 xMn1.4 þ xGa0.6 alloys [8,14]. The observation of EB in these alloys were explained in terms of a model suggested by Lázpita et al [15]. According to the model, the Mn atoms may occupy the regular Mn sites, Ni sites, and Ga sites. The respective Mn atoms are referred to as Mn/Mn (for regular site), Mn/Ni (for Ni site) and Mn/Ga (for Ga site). In the absence of the Mn/Ni atoms, the coupling between the Mn/Mn atoms and the Mn/Ga atoms is AFM, which was apparently the case for Ni2Mn1.4Ga0.6. When Ni is partially replaced with Mn in Ni2 xMn1.4 þ xGa0.6, the excess Mn atoms occupy the Ni sites. In this case, the Mn/Ni atoms become the closest neighbor to the Mn/ Mn atoms, and thus the stronger AFM coupling between Mn/Ni and Mn/Mn atoms overcome the AFM coupling between the Mn/ Mn and Mn/Ga atoms, resulting in FM coupling between the Mn/ Mn and the Mn/Ga atoms. This results in an enhancement of overall ferromagnetic exchange interactions in Ni2 xMn1.4 þ xGa0.6 [14]. Considering the observation of EB in Ni2Mn1.4Ga0.6 and Ni2 xMn1.4 þ xGa0.6 alloys, it is interesting to explore the magnetic properties of Ni2 xCoxMn1.4Ga0.6, where Ni is partially replaced with Co. The substitution of Ni by Co will apparently keep the amount of Mn/Mn and Mn/Ga atoms same while changing the interatomic distances between the atoms. Since the magnetic interactions between the Mn atoms at different crystalline sites are strongly dependent on the interatomic distances, it is interesting to explore, how the magnetic properties and EB will be effected due to Co substitution in Ni2 xCoxMn1.4Ga0.6. Therefore, in this paper we report an experimental study on a series of Ni2 xCox Mn1.4Ga0.6 Heusler alloys. The idea is to explore the change in the
98
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
magnetic interactions and the associated EB phenomena in the alloys caused by the systematic substitution of Ni by Co.
Table 1 Lattice parameters and c/a ratios of Ni2 xCoxMn1.4Ga0.6 as evaluated from the XRD data. Co concentration (x)
a (Å)
c (Å)
c/a
0.0 0.05 0.10 0.2 0.3
7.6328 7.6393 7.6319 7.6459 7.6468
6.7836 6.7750 6.7734 6.7902 6.7872
0.88874 0.88686 0.88751 0.88808 0.88759
2. Experimental techniques The Ni2 xCoxMn1.4Ga0.6 (0 rxr 0.3) alloys were prepared by arc melting under argon atmosphere using high-purity Ni, Mn, Co, and Ga metals. For homogenization, the as-prepared sample were sealed in partially evacuated (partially filled with inert gas) vycor tubes and annealed at 1123 K for 72 h. After annealing, the samples were rapidly cooled down to room temperature by quenching in cold water. To determine the phase purity and crystal structure of the alloys, x-ray diffraction (XRD) measurements were performed on a Scintag Pad X-ray powder diffractometer with Cu-Kα radiation. The diffraction patterns were indexed using a computer program known as PowderCell [16]. Magnetization (M) measurements as a function of temperature and applied magnetic field were conducted on a Physical Property Measurement System (PPMS) made by Quantum Design Inc. Before obtaining the zero field cooled (ZFC) temperature dependence of magnetization, M(T), data, the samples were cooled from 300 K to 5 K in a zero magnetic field. When the temperature stabilized at 5 K, a magnetic field of 1000 Oe was applied, and the ZFC M (T) data was recorded as a function of increasing temperature. The field cooled cooling (FCC) M(T) data were obtained while sweeping down the temperature from 300 K to 5 K in the presence of 1000 Oe field. Prior to each ZFC magnetization versus field, M(H), measurement, the samples were cooled from 300 K to 5 K in zero magnetic field. Before the field cooled FC M(H) measurements, each sample was cooled down to 5 K in the presence of 50 kOe magnetic field. The ac magnetic susceptibility measurements at an ac field of 10 Oe and frequencies of 10 Hz, 100 Hz, 1000 Hz, and 10,000 Hz were also conducted on the PPMS.
3. Results and discussion The XRD patterns of Ni2 xCoxMn1.4Ga0.6 (0.05 rx r 0.3) alloys obtained at room temperature are shown in Fig. 1. The alloys are single phase and exhibit the L10 non-modulated tetragonal martensitic structure at room temperature. The major peaks observed in the XRD data are labeled in Fig. 1. In the vicinity of 49° (next to the 400 peak) and 74° (next to the 044 peak), minor humps are
Fig. 2. The temperature dependence of magnetization of Ni2 xCoxMn1.4Ga0.6 (a) x¼ 0 and (b) x ¼0.3, measured in a magnetic field of 1000 Oe. (c) TC and Tp of Ni2 xCoxMn1.4Ga0.6 as a function of Co concentration x.
Fig.1. X-ray diffraction patterns temperature.
of
Ni2 xCoxMn1.4Ga0.6
obtained
at
room
observed in the XRD data of some samples. These humps are most likely related to the hkl miller indices 213 (49°) and 600 (74°), respectively. The lattice parameters and c/a ratio of all samples are listed in Table 1. The roughly estimated error in the lattice parameters vary from 70.003 to 70.011. The alloy with x ¼0 has the
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
99
Fig. 3. Magnetization as a function of field, M(H), of Ni2 xCoxMn1.4Ga0.6 obtained at 5 K under ZFC condition.
following lattice parameters at room temperature: a ¼b¼ 7.6328 Å and c ¼6.7836 Å. With increasing Co concentration, the lattice parameters changes marginally. The lattice parameters for the alloy with x ¼0.3 are, a ¼b¼ 7.6468 Å and c¼6.7872 Å. The small change in the lattice parameters of the Ni2 xCoxMn1.4Ga0.6 alloys may be attributed to the small difference in the atomic radii of Ni (1.246 Å) and Co (1.252 Å) considering the coordination number, CN¼ 12 [17]. The ZFC and FCC M(T) data of Ni2 xCoxMn1.4Ga0.6 (x ¼0 and 0.3) samples measured in a magnetic field of 1 kOe are shown in Fig. 2a and 2b. As shown in Fig. 2a, the ZFC magnetization of Ni2 xCoxMn1.4Ga0.6 (x¼ 0) initially increases with increasing temperature until it peaks at TP ¼97 K. Beyond this temperature the magnetization decreases with increasing temperature due to the ferromagnetic–paramagnetic transition. From the derivative (dM/ dT) plot of the M(T) data, the ferromagnetic transition temperature, TC, of Ni2 xCoxMn1.4Ga0.6 (x¼ 0) has been determined to be ∼165 K. The FCC M(T) data exhibit similar behavior with the exception of the thermomagnetic irreversibility below ∼130 K. Such an irreversibility is usually observed in these alloys due to the pinning of the ferromagnetic domains below the freezing temperature of the SG phase. The M(T) data of the alloy with x ¼0.3 exhibit similar behavior with the exception that TC and TP are observed at lower temperatures. This decrease of TC and TP with increasing Co concentration is systematically observed in Ni2 xCox
Mn1.4Ga0.6, as shown in Fig. 2c. The decrease of TC demonstrates a reduction in the ferromagnetic exchange interactions in the system with increasing Co concentration. Fig. 3 shows the M(H) data of Ni2 xCoxMn1.4Ga0.6 (x ¼0, 0.05, 0.1, and 0.2) measured at 5 K under ZFC conditions. The hysteresis loops were measured in magnetic fields from 50 kOe to 50 kOe. For clearer visualization of the lower field region, only the magnetization curves from 2 kOe to 2 kOe are shown. As shown in Fig. 3a, the ZFC M(H) data of the Ni2 xCoxMn1.4Ga0.6 (x ¼0) exhibit a non-symmetric frustrated feature in the vicinity of the center of the hysteresis loop, which is shifted towards the negative field direction demonstrating an EB effect with an exchange bias field, HEB ¼ 422 Oe. The observed behavior is consistent with existing literature [8]. As shown in Fig. 3b, 3c and 3d, the alloys with Co exhibit more symmetric M(H) loops and higher coercive field. For all alloys with Co concentration xr0.2, the ZFC M(H) data demonstrates EB. As shown in Fig. 4, unlike the ZFC data, for all Co concentration the M(H) loops of the Ni2 xCoxMn1.4Ga0.6 alloys, obtained under FC condition, exhibit normal symmetric features. It is interesting to note that when compared to the alloy with x¼ 0, the squareness of the FC M(H) loops is enhanced in the Co containing alloys. This improvement of the squareness indicates that the substitution of Co in Ni2 xCoxMn1.4Ga0.6 alloys results in enhanced magnetic anisotropy. For Co concentrations x r0.2, all alloys exhibit EB under
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Fig.4. Magnetization as a function of field, M(H), of Ni2 xCoxMn1.4Ga0.6 obtained at 5 K under FC condition.
Fig. 5. (a) Exchange bias field, HEB, as a function of Co concentration for Ni2 xCox Mn1.4Ga0.6 The inset shows the saturation moment, MS, as a function of x. The measurements were performed at 5 K under ZFC and FC conditions.
FC condition as well. Fig. 5 shows the HEB, evaluated from the ZFC and FC M(H) loops of Ni2 xCoxMn1.4Ga0.6, as a function of Co concentration x. As shown in the figure, for both ZFC and FC conditions, HEB initially
increases in the negative direction with increasing Co concentration. As x exceeds 0.1, the magnitude of HEB decreases and eventually becomes nearly zero for the alloy with x¼ 0.3. For ZFC condition, HEB increases from 422 Oe (x ¼0) to 585 Oe (x ¼0.1). On the other hand, HEB obtained from the FC data increases from 130 Oe (x ¼0) to 290 Oe (x ¼0.1). The inset of Fig. 5 shows the saturation magnetization (MS) as a function of Co concentration. For both ZFC and FC conditions, MS decreases with increasing x. However, for x o0.2, MS obtained from the ZFC M(H) data is slightly lower than those obtained from the FC data. The decrease of MS and TC (Fig. 2c) with increasing Co concentration signifies the reduction of FM interactions in Ni2 xCoxMn1.4Ga0.6. We have performed ac susceptibility measurements on the Ni2 xCoxMn1.4Ga0.6 samples in order to explore possible explanations behind the observed EB phenomena in the system. [8,18] In Fig. 6, we show the temperature dependence of the real components (χ′(T)) of the ac susceptibility data of Ni2 xCoxMn1.4Ga0.6 (x ¼.05 and 0.3) samples measured in an ac field of amplitude 10 Oe and frequencies (f) of 10 Hz, 100 Hz, 1000 Hz, and 10,000 Hz. For the sample with x ¼0.05 the susceptibility increases with increasing temperature until peaking at Tp ∼150 K (f¼ 10 Hz) and then decreases to zero. All samples with x r0.3 show similar behavior with the exception of exhibiting relatively sharper peaks (see Fig. 6b) in the vicinity of Tp. A strong frequency dependence is observed in the vicinity of TP for all alloys (see inset of Fig. 6). The
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
Fig. 6. Temperature dependence of the real component of the ac magnetic susceptibility of Ni2 xCoxMn1.4Ga0.6 measured in an ac field of 10 Oe and frequencies from 10 Hz to 10,000 Hz.
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The empirical relation, Φ = ΔTp [Tp Δlog10f ]−1, is a simple but useful criterion to determine whether the observed frequency dependence in the ac susceptibility data is associated with metallic spin glass or not [18–20]. Here, Φ represents the relative shift of Tp with f. For metallic spin-glass systems Φ typically lies between 0.005 and 0.01. A value greater than 0.01 and up to 0.04 would typically signify a SSG state. Fig. 7 shows the values of Φ for Ni2 xCoxMn1.4Ga0.6 alloys as a function of x. It is interesting to note that Φ increases rapidly with increasing Co concentration from ∼0.016 (x¼ 0) to ∼0.047 (x ¼0.3). This behavior indicates that with increasing Co concentration the low temperature SG state in Ni2 xCoxMn1.4Ga0.6 transforms to a SSG state. The magnetic properties of Ni2 xCoxMn1.4Ga0.6 described above can be explained by considering the occupancy of the Mn atoms and the corresponding magnetic interactions as discussed earlier [15]. In case of Ni2Mn1.4Ga0.6 (with only Mn/Mn and Mn/Ga atoms) the magnetic properties, including the EB effects, are governed by the FM interactions between the regular Mn/Mn atoms and AFM interactions between the Mn/Mn and Mn/Ga atoms. Apparently, the addition of Co atoms in Ni2 xCoxMn1.4Ga0.6 do not change the amount of Mn/Mn and Mn/Ga atoms. However, the interatomic distances between the Mn atoms on the different sites changes due to the larger radius of Co (1.252 Å) when compared to Ni (1.246 Å). This change of interatomic distances results in the reduction of FM interactions while increasing the AFM interactions, which is clearly demonstrated in the magnetization and ac susceptibility data. As discussed above, the experimental results show that TC, TP, and MS decrease (Fig. 2 and inset of Fig.5) while Φ increases (Fig. 7) monotonically with increasing Cr concentration. However, as shown in Fig. 5, HEB (for both ZFC and FC condition) shows a non-monotonic dependence on Co concentration. This behavior can be explained by considering the fact the EB arises mainly due to the co-existence of FM and AFM (or SG) interactions in the system. Therefore, it may be expected that in a given system HEB will maximize for certain concentrations of FM and AFM interactions. It appears that in case of the Ni2 xCoxMn1.4Ga0.6, the optimized FM and SG interactions exist for Co concentration x ¼0.1, and hence the HEB peaks at this concentration. In conclusion, we have investigated the magnetic properties and exchange bias phenomena in Ni2 xCoxMn1.4Ga0.6 Heusler alloys. For xr 0.2, all the samples exhibit EB effects in both ZFC and FC condition. The experimental results show that Co doping results in the reduction of the FM interactions and increases the AFM interaction in the system. These variations in the FM and AFM interactions changes the low temperature magnetic state from SG to SSG, which is revealed in the ac susceptibility data. The observed behavior is attributed to the difference in the atomic radii of Co and Ni.
References [1] [2] [3] [4] [5] Fig. 7. The empirical parameter, Φ , as a function of Co concentration x for Ni2 xCox Mn1.4Ga0.6.
behavior observed in the ac susceptibility data of Ni2 xCox Mn1.4Ga0.6 indicates towards a spin glass type frustrated nature of the ground state [18]. However, frequency dependence in magnetic systems may also arise from magnetic blocking.
[6] [7] [8] [9] [10] [11]
W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. J. Nogués, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. Manuel Bibes, Javier E. Villegas, Agnès Barthélémy, Adv. Phys. 60 (2011) 5. B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J.S. Muñoz, M.D. Baró, Phys. Rep. 422 (2005) 65. M. Khan, I. Dubenko, S. Stadler, N. Ali, Appl. Phys. Lett. 91 (2007) 072510. B.M. Wang, Y. Liu, P. Ren, B. Xia, K.B. Ruan, J.B. Yi, J. Ding, X.G. Li, L. Wang, Phys. Rev. Lett. 106 (2011) 07720. Z.D. Han, B. Qian, D.H. Wang, P. Zhang, X.F. Jiang, C.L. Zhang, Y.W. Du, Appl. Phys. Lett. 103 (2013) 172403. A.K. Pathak, D. Paudyal, W.T. Jayasekara, S. Calder, A. Kreyssig, A.I. Goldman, K. A. Gschneidner Jr., V.K. Pecharsky, Phys. Rev. B 89 (2014) 224411. M. Khan, I. Dubenko, S. Stadler, N. Ali, J. Appl. Phys. 102 (2007) 113914. A.K. Nayak, M. Nicklas, S. Chadov, C. Shekhar, Y. Skourski, J. Winterlik, C. Felser,
102
R. Chapai, M. Khan / Journal of Magnetism and Magnetic Materials 403 (2016) 97–102
Phys. Rev. Lett. 110 (2013) 127204. [12] J. Enkovaara, O. Heczko, A. Ayuela, R.M. Nieminen, Phys. Rev. B 67 (2003) 212405. [13] S. Aksoy, M. Acet, P.P. Deen, L. Mañosa, A. Planes, Phys. Rev. B 79 (2009) 212401. [14] M. Khan, R. Chapai, J. Alloy. Compd. 647 (2015) 935. [15] P. Lázpita, J.M. Barandiarán, J. Feuchtwanger, J. Gutiérrez, I. Rodriguez, V. A. Chernenko, A. Stunault, C. Mondelli, J. Phys.: Conf. Ser. 325 (2011) 012016.
[16] 〈http://www.bam.de/de/service/publikationen/powder_cell.htm〉. [17] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, WileyInterscience, New-York (1972), p. 146–148. [18] R.Y. Umetsu, A. Fujita, W. Ito, T. Kanomata, R. Kainuma, J. Phys.: Condens. Matter 23 (2011) 326001. [19] S. Bedanta, W. Kleemann, J. Phys. D.: Appl. Phys. 42 (2009) 013001. [20] P. Liao, C. Jing, X.L. Wang, Y.J. Yang, D. Zheng, Z. Li, B.J. Kang, D.M. Deng, S. X. Cao, J.C. Zhang, B. Lu, Appl. Phys. Lett. 104 (2014) 092410 5.