study of heusler alloys for spintronic application · study of heusler alloys for spintronic...

International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 4, Issue-10, Oct.-2016

Study of Heusler Alloys For Spintronic Application

128

STUDY OF HEUSLER ALLOYS FOR SPINTRONIC APPLICATION

1AMBILY SNAIR, 2Y. VENKATESWARA, 3K.G SURESH

1,2Magnetic materials lab, Indian Institute of Technology Bombay, Mumbai, India E-mail: [email protected],[email protected], [email protected]

Abstract- This study is focused on certain Heusler alloys to search for half metallicity. The alloy CoMnCrGa was prepared by arc melting technique and its crystal structure was studied using X-ray diffraction. The disorder in the alloy was also analyzed in detail. The X-Ray refinement of the diffraction pattern was done using Fullprof suite in order to establish the phase purity of the alloy. From the Rietveld refinement, lattice constant for CoMnCrGa, is found to be 5.84 ± 0.01 Å. The alloy crystallizes in LiMgPdSn prototype structure with space group F4̄3m (#216). The magnetization study of the alloy was done in a Vibrating Sample Magnetometer. Both Zero Field Cooled measurement (ZFC) and Field Cooled measurements (FC) reveal that the alloy has a Curie temperature above 400 K. Theoretical studies were carried out using Quantum ESPRESSO. It predicts that the alloy has a nearly half metallic character. Besides, the theoretical calculations for alloys CoFeScSb and CoFeYSb have also shown half-metallic character for the minimum energy configuration. Hence, theoretically both these alloys are useful candidates for spintronic application. Keywords- Half metallic ferromagnet, Heusler alloy, Quantum ESPRESSO, Spintronics. I. INTRODUCTION Spintronics is a very important area of research now. It is believed that in the near future it could be more revolutionary than any other technology. It is a new paradigm of electronics, based on the spin degree of the electron. Spintronic devices have the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption and also less heat dissipation. Using 100% spin-polarized ferromagnets, called half-metallic ferromagnet, one can optimize the performance of these devices. Until recently, the spin of the electron was ignored in mainstream charge-based electronics. A new technology called Spintronics (spin transport electronics or spin-based electronics), has emerged where it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices which will be smaller, more versatile and more robust than present day electronic devices. Adding the spin degree of freedom to conventional semiconductor charge-based electronics or using the spin degree of freedom alone will add substantially more capability and performance to electronic products. The advantages of these new devices would be non-volatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.1, 2 1.1. Half-Metallic Heusler Alloys Half metals are solids that are metals with a Fermi surface in one spin channel, but for opposite spin there is a gap in the spin polarized density of states. The gap may occur either in the majority or minority carrier. So the half-metals are the special case of ferromagnetic materials3,4. Among different types of half metals, Heusler alloys gained attention for Spintronic applications due to several advantages

over other half-metals. Some of them are i) Crystal structure is compatible with that of semiconductor (Silicon, Germanium) and lattice constant of many Heusler alloys are from 5.4 Å to 6.3 Å. These lattice constants are in range that of semiconductors, so can be grown on them easily. So there is no need of replacement of existing fabrication technology. ii) Most of the Heusler alloys, especially Co2based Heusler alloys, have transition temperature above the room temperature and have as high as 1100 K, making them reliable for Spintronic applications. This is because spin polarization falls more rapidly than magnetization of alloys. So to retain spin polarization at room temperature, the falling of magnetization with temperature should be as low as possible and transition temperature should be as much as possible higher than the room temperature. iii) The Fermi level and band gaps of Heusler alloys can be tuned by substituting other transition elements in parent alloys. In general, Half or Semi – Heusler materials XYZ can be understood as compounds consisting of superposition of covalent and ionic bonds. The X and Y atoms have a distinct cationic character, whereas Z can be seen as the anionic counterpart. Full Heusler alloys have the general formula X2YZ, where X and Y are transition metals and Z is a main group element. In this study, we focus on the equiatomic alloys of the type XX’YZ. 5, 6

II. DETAILS EXPERIMENTAL 2.1. Materials and Procedures The sample CoMnCrGa was synthesized by arc melting. The purity of sample was at least 99.9%. The ingot was re-melted 4-5 times by flipping back and forth to ensure good homogeneity. The final weight loss in all the cases has been found to be less than 1%.The samples were annealed under high vacuum for 7 days at 800 K, followed by quenching in ice water mixture to further increase the



129

homogeneity. The crystal structure was found out from the powder X-ray diffraction (XRD) data. The X-ray refinement of the diffraction pattern was done using Fullprof suite in order to establish the phase purity of alloys. The Fullprof program has mainly been developed for Rietveld analysis. The lattice parameters were calculated by least square fit method. In Rietveld method the least square refinement were carried out until the best fit was obtained between the observed diffraction patterns and calculated. The magnetization (M) measurements both under zero field cooled (ZFC) and field-cooled (FC) conditions, in the temperature (T) range of 5–400 K and up to a maximum field (H) of 500Oe, were performed by using a vibrating sample magnetometer (VSM)attached to a Physical Property Measurement System(PPMS, Quantum Design). In the ZFC mode, the sample was cooled in the absence of any field and then a field was applied on warming, during which the magnetization was measured. In the FC mode, the sample was cooled in presence of a field, which was retained during warming, for measuring the magnetization. The theoretical calculations were carried out using Quantum ESPRESSO.7, 8

III. RESULTS AND DISCUSSION 3.1. XRD Analysis of CoMnCrGa From the Rietveld refinement of CoMnCrGa, lattice constant is found to be 5.84 ± 0.01 Å. This alloy also crystallizes in LiMgPdSn prototype structure with space group F4̄3m (#216). Here the super-lattice peaks (1 11) and (2 22) are not clearly resolved in the presence of huge background in the XRD data. It can be clearly observed from the Table 1 that these peaks are more than 100 times less intense than (2 2 0) peak intensity. This table shows the refinement results done by taking constituent elements Ga at 4a(0,0,0), Mn at 4b(0.5,0.5,0.5), Co at 4c(0.25,0.25,0.25),Cr at 4d(0.75,0.75,0.75). From refinement, the super lattice peaks also shows very low intensity. This is because the constituent elements are from the same period and hence their atomic scattering factor is almost same. Due to this, here it can’t be concluded whether there is perfect ordering or not using XRD.9 Even though two other configurations of constituent elements were tried; the change in 2value is negligible. Of course this is obvious. The Fig 1 shows the Rietveld refinement for this alloy.

Fig.1. Rietveld refinement of CoMnCrGa

Table 1: Rietveld refinement results for CoMnCrGa alloy.

3.2. Magnetization Measurement of CoMnCrGa Magnetization measurement was done for the CoMnCrGa alloy.Figure.2 shows the plot of Magnetization vs. Temperature at constant applied magnetic field H=500 Oe. From the graph it is clear that for both ZFC and FC magnetic moment stays constant up to 400 K. This means the alloy CoMnCrGa is having high Curie temperature above 400 K. The alloys whose transition temperature is above room temperature are useful for Spintronic application.Figure.3 shows the plot of Magnetization vs. Magnetic Field at T=3 K, T=100 K and T=300 K. It is observed that the falling of magnetic moment with temperature is small. The decrease in saturation magnetic moment from 3 K to 100 K is 0.96 % and from 3 K to 300 K is 5.10%. The saturation magnetic moment is found to be 1.25 µB /f.u.at 3 K from the M H plot. Theoretical calculation shows the magnetic moment to be 1.41 µB /f.u. and from Slater - Pauling rule magnetic moment is NV-24 where NV for CoMnCrGa is 25, hence 25-24 = 1 µB /f.u.

Fig.2. Magnetization - Temperature graph

Fig.3. Magnetization - Magnetic Field graph

-50 0 50 100 150 200 250 300 350 400

6

8

10

12

14

16

18

20

22

-50 0 50 100 150 200 250 300 350 400

6

8

10

12

14

16

18

20

22

Mag

netiz

atio

n (e

mu/

g)

Temperature (K)

FC ZFC

H=500 Oe

CoMnCrGa

®

-60 -40 -20 0 20 40 60-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5-60 -40 -20 0 20 40 60

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Mom

ent (m B

/f.u)

Field (kOe)

3 K 100 K 300 K

®

CoMnCrGa



130

3.3 Theoretical calculations For CoFeScSb alloy, the spin resolved density of states vs energy plot is shown in Fig. 4 for the configuration Sb at 4a (0,0,0), Sc at 4b (0.5,0.5,0.5) , Fe at 4c(0.25,0.25,0.25) and Co at 4d(0.75,0.75,0.75). For this configuration, the number density for down spin electron vanishes at Fermi level while for spin up electrons it is non-zero10. So this is a half-metallic ferromagnet. This configuration is also showing the minimum energy among three different configurations in Table 2. The alloy CoFeYSb shows half metallic character for the configuration Sb 4a (0,0,0), Y 4b (0.5,0.5,0.5), Fe 4c(0.25,0.25,0.25) and Co 4d(0.75,0.75,0.75) as shown in Fig.5.Here the number density of downspin electrons vanishes at Fermi-level and only spin up electrons are present at Fermi level which gives 100% polarization. This configuration possesses the lowest total energy. Other two configurations have higher energies and do not show any half metallic character that can be seen from the Table.2. For, CoMnCrGa the plot of density of states vs energy shows nearly half-metallic character for configuration Ga at 4a(0,0,0), Mn at 4b(0.5,0.5,0.5), Co at 4c(0.25,0.25,0.25),Cr at 4d(0.75,0.75,0.75) shown in Fig.6.Other configurations possess high energy as given in Table.2.

Fig.4. Spin resolved density of states vs energy of states for

CoFeScSb


CoFeYSb


CoMnCrGa

Table 2: Theoretical Calculation for different alloys using

Quantum ESPRESSO CONCLUSIONS Crystal structure, magnetic properties and disorder in certain equiatomic Heusler alloys were analyzed in detail. Both experimental and theoretical studies were carried. Theoretical calculation predicts that the alloy CoMnCrGa shows nearly half-metallic character for the minimum energy configuration. The theoretical calculations for alloys CoFeScSb and CoFeYSb also show half-metallic character for the minimum energy configuration. Detailed experimental studies regarding the spin polarization11 is essential to verify the theoretical results. ACKNOWLEDGMENTS The author would like to thank all members of Magnetic Materials lab, Indian Institute of Technology Bombay for all their help throughout the work. REFERENCES [1]. R. Srinivasan. Spintronics-devices and materials. Resonance,

10:8–17, 2005. [2]. R. Srinivasan. Spintronics-principles. Resonance, 10:53–62,

2005. [3]. S. Blundell. Magnetism in Condensed Matter. Oxford Master

Series in Condensed Matter PhysicsOUP Oxford, 2001. [4]. Bernard Dennis Cullity and Chad D Graham. Introduction to

magnetic materials. John Wiley &Sons, 2011. [5]. Tanja Graf, Claudia Felser, and Stuart S.P. Parkin. Simple

rules for the understanding of heusler compounds. Progress in Solid State Chemistry, 39(1): 1 – 50, 2011

[6]. H. Kronmüller and S. Parkin. Handbook of Magnetism and

-6 -4 -2 0 2

-6 -4 -2 0 2

n (E

)

E-EF (eV)

DOS Up DOS Down

n ¯(E

)

¯

®

CoFeScSb

-6 -4 -2 0 2

-6 -4 -2 0 2

E-EF(eV)

DOS Up DOS Down

CoFeYSb

n (E)

n ¯(E

)

¯

®

-6 -4 -2 0 2 4

-6 -4 -2 0 2 4

E-EF(eV)

DOS Up DOS Down

®

CoMnCrGa

n (E

)

n̄(E

)

¯



131

Advanced Magnetic Materials. Numberv. 3 in Handbook of Magnetism and Advanced Magnetic Materials. Wiley, 2007.

[7]. DM Raj Kumar. Microstructural, magnetic and magnetocaloric studies in Gd-Si-Ge and Ni-Mn-In based alloys. PhD thesis, IIT Bombay, 2013.

[8]. Roshneesahoo. Structural, magnetic and related properties in certain NiMnSb based full Heusler alloys in bulk, fine particle and ribbon forms. PhD thesis, IIT Bombay, 2013.

[9]. ChallapalliSuryanarayana and M Grant Norton. X-ray diffraction: a practical approach, volume207. Cambridge Univ Press, 1998.

[10]. Paolo Giannozziet .al. Quantum espresso: a modular and open-source software project for quantum simulations of materials. Journal of Physics: Condensed Matter, 21(39): 395502 (19pp), 2009.

[11]. R. J. Soulen, J. M. Byers, M. S. Osofsky, B. Nadgorny, T. Ambrose, S. F. Cheng, P. R. Broussard,C. T. Tanaka, J. Nowak, J. S. Moodera, A. Barry, and J. M. D. Coey. Measuring the spin polarization of a metal with a superconducting point contact. Science, 282(5386):85–88, 1998.

study of heusler alloys for spintronic application · study of heusler alloys for spintronic...

Documents