Search for ferromagnetism in Hf2Co7: An investigation by perturbed angular correlation (PAC) spectroscopy

Journal Pre-proofs Search for ferromagnetism in Hf2Co7; an investigation by perturbed angular correlation (PAC) spectroscopy R. Sewak, C.C. Dey PII: DOI: Reference:

S0304-8853(19)33126-9 https://doi.org/10.1016/j.jmmm.2019.166105 MAGMA 166105

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Journal of Magnetism and Magnetic Materials

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5 September 2019 4 November 2019

Please cite this article as: R. Sewak, C.C. Dey, Search for ferromagnetism in Hf2Co7; an investigation by perturbed angular correlation (PAC) spectroscopy, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/ 10.1016/j.jmmm.2019.166105

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Search for ferromagnetism in Hf2 Co7 ; an investigation by perturbed angular correlation (PAC) spectroscopy R. Sewak1,2 , C.C. Dey1,2∗ 1 Saha

Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700064, India

2 Homi

Bhabha National Institute, Anushaktinagar, Mumbai-400094, India

Abstract Perturbed angular correlation (PAC) measurements in Hf2 Co7 have been performed in the temperature range 77-973 K using the 181 Hf probe to observe ferromagnetism in this intermetallic alloy. From present measurements, no magnetic interaction is observed at any temperature in the above temperature range. Only two quadrupole interaction frequencies have been found. At room temperature, values of quadrupole frequency and asymmetry parameter are found to be ωQ = 15.7(4) Mrad/s, η = 0, δ = 0 for site 1 (∼ 71%) and ωQ = 46(1) Mrad/s, η = 0.74(5), δ = 5(3)% for site 2 (∼29%). Present results contradict with the earlier reported results where a room temperature ferromagnetism was found with a Curie temperature of ∼400 K. X-ray diffraction measurement in Hf2 Co7 has also been performed. From the measured XRD pattern, an almost pure phase of Hf2 Co7 was observed. Keywords: Intermetallics; mechanical alloying; hyperfine interactions; ferromagnetism; perturbed angular correlations, PAC; X-ray diffraction.

1.

Introduction

The intermetallic compounds of Hf-Co and Zr-Co have important application in the preparation of rare-earth free permanent magnets with high Curie temperature. The alloys with compositions ranging from MCo3.5 -MCo7 (M= Hf, Zr) were reported to have Curie temperatures around 400-800 K [1–7]. These magnetic materials have high Curie temperature as well as moderate values of BHmax which is the quantity often described to quantify the strength of a permanent magnet. The BHmax is the maximum absolute value of the product of the magnetic induction B and the applied magnetic field H realized in the second quadrant of the magnetic hysteresis loop. The rare earth based magnet Nd2 Fe14 B is known to have the highest BHmax (∼55 MGOe) [8, 9] followed by SmCo5 with a BHmax ∼ 30 MGOe [10]. The rare earth free permanent magnet AlNiCo5 [11] has also a high value of BHmax up to 10 MGOe [12]. To search for new permanent magnets which do not contain rare earth elements and have high Curie temperatures, cobalt rich alloys with Zr and Hf have attracted recent attention [5, 13]. The properties of nanostructured Co rich transition metal alloys, Co100−x TM x (TM = Hf, Zr and 10 ≤ x ≤ 18) were studied by Balamurugan et al. [5]. It was found that highanisotropy structures of HfCo7 and Zr2 Co11 were formed for a broader composition region compared to the equilibrium bulk phase diagrams and exhibit high curie temperatures of above 750 K. For cluster deposited HfCo7 nanoparticle film, these authors [5] reported a maximum energy product of about 12.6 MGOe which is comparable to those of alnico. The magnetism in the low Co rich compounds of ∗ corresponding

author Email addresses: [email protected] (R. Sewak1,2 ), [email protected] (C.C. Dey1,2 ) Preprint submitted to Elsevier

Hf2 Co7 [6, 7] and Hf6 Co23 [7] were studied earlier by perturbed angular correlation (PAC) technique. In Hf2 Co7 [6], a ferromagnetism was found at room temperature and a Curie temperature around 390 K was reported earlier. In Hf6 Co23 [7], on the other hand, a relatively high Curie temperature (∼750 K) was reported. No further studies by PAC in these compounds are available in literature. However, from a recent study by Lu et al. [14], no phase of Hf2 Co7 was reported for Hf-Co system. From experimental studies by Xray diffraction, transmission electron microscopy, scanning electron microscopy, electron probe micro-analysis and differential scanning calorimetry, these authors [14] reported a revised phase diagram of the Hf-Co system where no phase of Hf2 Co7 is shown. In a subsequent study, Li et al. [15] reported a minor phase of Hf2 Co7 in the samples with the composition ranges HfCo6 to HfCo8 . In a recent report by Singh et al. [16], results of magnetization measurements of three samples of Hf-Co system viz. Hf6 Co23 , Hf2 Co11 and HfCo7 were reported. But, there was no report of magnetization or phase existence of Hf2 Co7 by these authors [16]. The perturbed angular correlation [17, 18] is an useful nuclear technique where the local environments of the probe can be studied in materials with Hf/Zr as one constituent element (by using 181 Hf probe) for the determination of magnetic field strength in a ferromagnetic material or for the determination of electric field gradient (EFG) in a non-cubic crystalline medium. Here, the electromagnetic moments (magnetic/quadrupole) of the probe nucleus interact with the surrounding electromagnetic fields (magnetic/electric field gradient) that perturb the angular correlation of a γ-γ cascade of the probe nucleus. From the measured interaction frequencies (ωL for magnetic and ωQ for quadrupole), the strengths of magnetic field and electric field gradient are obtained by knowing the values of magnetic moment and quadrupole moment of the intermediate level of the November 8, 2019

the PAC sample. The active 181 Hf was produced in a natural Hf wire by capturing thermal neutron at Dhruba reactor (Mumbai), India with a flux ∼1013 /cm2 /s. The remaining two-third part of sample was powdered for X-ray diffraction (XRD) measurement. The XRD measurement was carried out with Rigaku X-ray diffractometer TTRAX-III using Cu Kα radiation (λ = 1.54056 Å) and Ni filter. For PAC studies, a four detector BaF2 -BaF2 setup (two BaF2 crystals of size 38×25.4 mm2 and two BaF2 crystals of size 50.8×50.8 mm2 ) was used to acquire four coincidence spectra in slow-fast coincidence assemblies [21]. A prompt time resolution (FWHM) of ∼900 ps was found with 22 Na source for 181 Hf energy window settings.

probe nucleus. From temperature dependent PAC studies, the phase transition from a ferromagnetic to paramagnetic in case of a ferromagnetic material or any structural phase change in a non-magnetic material can be observed. In a ferromagnetic material with non-cubic crystal structure both magnetic and electric quadrupole hyperfine interaction are expected below Curie temperature (Tc ) and only pure quadrupole interaction above Tc . But, in a ferromagnetic material with cubic crystal structure only magnetic interaction is expected below Tc and no interactions (magnetic or electric quadrupole) above Tc . Thus, a ferromagnetic material with cubic crystalline steucture should give an unperturbed angular correlation above Curie tempearture. The crystal structure of Hf2 Co7 is reported as to be orthorhombic of type Zr2 Ni7 with lattice constants a = 0.4444 nm, b = 0.8191 nm, c = 1.214 nm [19]. Therefore, if Hf2 Co7 is ferromagnetic, PAC measurement should give a combined magnetic and electric quadrupole interaction below Curie temperature and a pure quadrupole interaction above Tc . Since, no phase existence or a small phase component of Hf2 Co7 was reported from recent measurements [14, 15] while a magnetic interaction (at ≤400 K) and a quadrupole interaction (above 400 K) in this compound was reported from previous PAC measurements [6, 7], we have reinvestigated Hf2 Co7 to confirm the existence of phase component and its ferromagnetic property. XRD measurement in Hf2 Co7 has also been carried out to determine the phase composition in the prepared sample. Present PAC measurements have been carried out using the 133-482 keV γ- γ cascade of 181 Ta (in the β− decay of 181 Hf) which passes through the 482 keV intermediate level and having a large angular anisotropy value (A22 = −0.288, A44 = −0.076) [17]. This level has a spin angular momentum I =5/2+ , T1/2 = 10.8 ns and have large values of electric quadrupole moment (Q = 2.35 b) and magnetic dipole moment (µ = 3.25 µN )[20]. These values are highly suitable for using 181 Ta as a probe for PAC measurements, particularly, for compounds with Hf/Zr/Ti. Ideally, the daughter and mother of the probe should have same crystal symmetry i.e. there should not be any transformation of element accompanies the decay (isomeric decay). But, in our case, there is a transformation of element accompanying the decay. Here, the local probe site symmetry will be broken and the value of EFG at the probe site will be different from that when there is no transformation of element in the decay. However, since the probe concentration is very low (∼1 at% or lower) in the sample, the probe should not influence the sample properties.

2. 2.1

2.2

Data analysis

The angular correlation of two successive γ rays of a γ-γ cascade is unperturbed if there is no strong perturbing interaction or the intermediate level lifetime is very small. This, however, can be perturbed by the hyperfine interaction (HFI) due to surrounding electric field gradient (EFG) and/or magnetic field present in crystalline environment of the probe. This HFI can be expressed by time dependent perturbation function Gkk (t) which depend upon symmetry of HFI interaction, orientation of HFI symmetry with direction of γ detection and nuclear spin of probe [22]. For a static electric quadrupole interaction in a polycrystalline sample, the perturbation function G22 (t) can be expressed as [17] 3 X h i G22 (t) = S 20 (η) + S 2i (η)cos(ωi t)exp(−δωi t) .

(1)

i=1

The frequencies ωi and their amplitudes S 2i are related to the hyperfine splitting of the intermediate nuclear level and δ characterizes the frequency distribution of EFG due to crystal imperfection. The S-coefficients are defined as given in reference [21]. Using principal axis system convention ( |V xx | ≤ |Vyy | ≤ |Vzz | ) and ∇2 V = 0, the EFG tensor can be expressed with only largest diagonal component Vzz and asymmetry parameter (η) defined as [23] η=

(V xx − Vyy ) , Vzz

0≤η≤1

(2)

For I = 5/2+ and η = 0 (axial symmetry), the three observable frequencies ω1 , ω2 and ω3 are related to the nuclear quadrupole frequency ωQ as ωQ = ω1 /6 = ω2 /12 = ω3 /18

(3)

and this ωQ is related to Vzz by

Experimental details

ωQ =

Sample preparation and equipment

The Hf2 Co7 sample was prepared by arc melting in argon atmosphere with constituent elements taken in stochiometric ratio. The sample was remelted to mix homogeneously and this was found in a shiny globule form. No appreciable loss of sample mass was found during sample preparation in arc furnace. After that, one-third part of this sample (∼75 mg) was separated out and remelted, in a similar condition, with a tiny piece (∼0.8 wt%) of active 181 Hf wire to produce

eQVzz 4I(2I − 1)~

(4)

When the probe nuclei occupy the cubic site in the ferromagnetic phase, it produces a pure magnetic HFI. This perturbation can be characterized by the Larmor frequency, ωL = gµN Bh f /~. In this case, the perturbation function can be expressed as G22 (t) = 2

1 2 X + cos(nωL t), 5 5 n=1,2

(5)

at 1275 ◦ C . The compounds HfCo2 and HfCo in the HfCo system are formed congruently from liquid at 1670 and 1640 ◦ C. It was reported that HfCo7 and Hf6 Co23 are stable only above 1050 and 950 ◦ C, respectively while Hf2 Co7 , HfCo2 , HfCo and Hf2 Co are stable down to room temperature [26, 27]. However, the present sample is found to be produced in an almost pure phase of Hf2 Co7 . It can be mentioned here is that powder XRD measurement in Hf-Co system is difficult because of its hardness and it is very difficult to make it powder. The PAC spectrum in Hf2 Co7 at room temperature is shown in figure 2. The spectrum can be best fitted by considering a pure electric quadrupole interaction only. For analysis of spectra, we have used WINFIT program [28]. Two quadropole frequency components are found from our PAC measurement at room temperature. The predominant component (∼71%) gives ωQ = 15.1(5) Mrad/s, η = 0, δ = 0 while the second quadrupole frequency component (∼29%) gives ωQ = 42.5(9) Mrad/s, η = 0.74(3), δ = 0. For fitting of data, no magnetic interaction is required to be considered. From previous PAC measurement also [6], two quadrupole frequency components were found due to two different nonequivalent crystallographic sites of Hf. From our PAC measurements in the temperature range 77-973 K, same two quadrupole frequency components have been found (table 1). The two decomposed components for the PAC spectrum at 873 K are shown in figure 3. In the above temperature range, results are found to be similar but, the relative fractions for the two components are found to change with temperature as shown in figure 4. Variations of ωQ for the two components and η for the asymmetric component with temperature are also shown in that plot. These are found to vary slowly with temperature indicating that crystal structure of Hf2 Co7 is not influenced much with temperature. To observe any ferromagnetism at lower temperature, we have carried out PAC measurement at 77 K also. But, at this temperature again, only same two quadrupole frequency components were found. No magnetic interaction is required to fit the data at 77 K. After measurement at 973 K, the PAC measurement was repeated at room temperature and the results were found to be reversible. From previous PAC measurement [6] also, two quadrupole frequency components in Hf2 Co7 were reported. A frequency component with η ∼ 0 was found earlier. The corresponding value of ωQ was reported to be ∼19 Mrad/s at a temperature of 1023 K. This is similar to the present measured value for the lower frequency component. But, from the previous measurement, a large value of δ (∼30%) was reported. For the higher frequency component, however, both results of ωQ and η disagree with the values found from present measurement. From present measurement, a value of η ∼ 0.75 is found instead of η ∼ 0.48 reported earlier. However, a ferromagnetism in Hf2 Co7 with a Curie temperature of ∼400 K was reported earlier [6, 7]. But, present results do not support the previous findings. The low Co concentrated compound of HfCo2 which crystallizes in the C15 type Laves phase structure also showed no ferromagnetism [29]. Based on the reported XRD data and results from our present PAC measurements, a model structure of Hf2 Co7 showing the two non-equivalent Hf sites is presented in figure 5. From the previous PAC measurements in Hf2 Co7 [6],

while for a combined magnetic dipole and electric quadrupole hyperfine interaction, the theoretical perturbation function depends on five interaction parameters and on the nuclear spin I [24]. In this case, the interaction parameters are the magnetic frequency ωL , the quadrupole frequency ωQ , the asymmetry parameter η, the Euler angles θ and φ which describe the relative orientation of the magnetic hyperfine field and the EFG tensor. The PAC spectrum i.e. A22 G22 (t) vs t has been obtained from the ratio of coincidence counts at 180◦ and 90◦ [17]. From the measured PAC spectrum, the nature of interaction (electric quadrupole, magnetic or combined interaction) can be determined by least squres fitting of data. 3.

Results and Discussion Table 1: Results of PAC measurements in Hf2 Co7

†

Temperature (K)

Component

ωQ (Mrad/s)

η

δ (%)

f (%)

Assignment

77

1 2

16(2) 40(2)

0 0.8(fixed)

0 0

75(3) 25(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

273

1 2

15.1(5) 42.5(9)

0 0.74(3)

0 0

71(3) 29(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

373

1 2

15(1) 40.8(7)

0 0.78(6)

0 0

70(3) 30(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

423

1 2

14.6(9) 38.6(6)

0 0.81(6)

0 0

71(3) 29(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

473

1 2

13.9(5) 41.2(8)

0 0.70(3)

0 0

68(3) 32(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

573

1 2

13.9(4) 40.9(7)

0 0.75(2)

0 0

65(3) 34(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

673

1 2

14.3(9) 40.7(9)

0 0.71(4)

0 0

63(3) 37(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

773

1 2

12.6(7) 37.9(5)

0 0.69(3)

0 0

61(3) 39(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

873

1 2

14(1) 38.8(4)

0 0.73(4)

0 0

59(3) 41(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

973

1 2

13(2) 37.6(5)

0 0.69(6)

0 0

56(3) 44(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

298†

1 2

15.2(8) 42.1(6)

0 0.74(3)

0 0

71(3) 29(2)

Hf2 Co(1) 7 Hf2 Co(2) 7

after measurement at 973 K.

The powder XRD pattern of Hf2 Co7 is shown in figure 1. The prominent peaks in the spectrum have been assigned to Hf2 Co7 . The peaks have been identified by ICDD2019 database. The different peaks are assigned by using ICDD PDF No.04-002-9849 for Hf2 Co7 and ICDD PDF No.04-002-9850 for Hf6 Co23 . Apart from Hf2 Co7 , a very small phase due to Hf6 Co23 and a small trace of monoclinic HfO2 (space group P21 /m) [25] have been found. The existence of monoclinic Hf2 Co7 phase with space group C2/m is thus confirmed from present XRD measurement. The Hf6 Co23 is present in Hf2 Co7 due to the fact that it is produced by the peritectic reaction L + Hf2 Co7 ↔ Hf6 Co23 3

Figure 1: Powder X-ray diffraction pattern of Hf2 Co7 in as prepared sample with the peaks identified for different phases as shown.

Figure 2: TDPAC spectra of Hf2 Co7 at different temperatures. Left panel shows anisotropy vs time (t) while right panel shows corresponding

Fourier transforms.

however, a room temperature ferromagnetism was reported. The PAC spectrum at room temperature reported in reference [6] was found to be highly damped and it was interpreted to be due to combined electric quadrupole and magnetic interaction. But, the magnetic interaction here should not be so strong to influence the quadrupole perturbation. Rather, the spectrum was damped due to the strong low frequency symmetric component (η = 0) compared to the relatively high frequency asymmetric component (η , 0). From present measurement, the low frequency symmetric component is found to be ∼70% at room temperature and its strength is found to decrease with temperature at the expense of the asymmetric component (fig. 4). Therefore, at higher temperatures (∼400 K and above), the damping of the PAC spectra were reduced [6] and the spectra above 400 K were interpreted with no magnetism. This was actually due to change in relative fraction for the two quadrupole interactions. However, from present measurements, the PAC spectra found at 298 K and 973 K are not very different compared to the spectra shown by Saitovitch et al. [6] at 298 K and 463 K. The large changes in PAC spectra at these temperatures shown in reference [6] could be due to presence of other contaminating phases in the sample. In the other previous report of ferromagnetism in Hf2 Co7 by Bedi et al. [7], a strong quadrupole interaction frequency with values of νQ = 400 MHz (ωQ = 62.8 Mrad/s), η = 0 at 700 K was reported. But, this component disappeared at ∼400 K and therefrom Bedi et al. [7] interpreted that there was a magnetic ordering at ∼400 K where, due to combined electric quadrupole and magnetic interaction, this fraction disappeared. Our present data do not support this earlier results also, particularly, the report of only one strong quadrupole interaction above 400 K with a value of ωQ ∼63 Mrad/s, η = 0 and the ferromagnetic ordering at ∼400 K.

phase in the sample at higher temperatures. 4.

Conclusion

Ferromagnetism in Hf2 Co7 is not found in the temperature range 77-973 K. Present results contradict with the earlier results where a room temperature ferromagnetism was reported. The most likely interpretation of present PAC data gives no magnetism in Hf2 Co7 down to 77 K but, the data can be interpreted with two frequency components originating from two non-equivalent Hf sites in the crystal structure. Present XRD result at room temperature clearly shows the existence of Hf2 Co7 and, from our PAC results, it is found to be stable up to 1000 K. The strength of symmetric component decreases with temperature at the expense of the asymmetric component. Our future outlook is to investigate the compound Hf6 Co23 which is a slightly higher Co-rich than Hf2 Co7 and has a cubic crystal structure to observe whether it has really any ferromagnetism at any temperature.

Acknowledgement: We would like to thank A. Karmahapatra of Saha Institute of Nuclear Physics, Kolkata for his help in XRD measurement. The present work is supported by the Department of Atomic Energy, Goverment of India through the Grant no. 12-R&D-SIN-5.02-0102. References [1] T. Ishikawa and K. Ohmori, IEEE Transactions on Magnetics 26 (5) (1990) 1370 - 1372. [2] A. M. Gabay, Y. Zhang, G. C. Hadjipanayis, J. Magnetism and Magnetic Materials 236(2001) 37-41. [3] M. A. McGuire, O. Rios, N. J. Ghimire, M. Koehler, Appl. Phys. Lett. 101 (2012) 202401. [4] H. W. Chang, Y. H. Lin, C. W. Shih, W. C. Chang, C. C. Shaw, J. Appl. Phys. 115 (2014) 17A724. [5] B. Balamurugan, B. Das, W. Y. Zhang, R. Skomski and D. J. Sellmyer, J. Phys.: Condens. Matter 26 (2014) 064204 (8pp). [6] H. Saitovitch, P. R. J. Silva, M. Marszalek, Hyperfine Interactions 133 (2001) 6570. [7] S. C. Bedi and M. Forker, Phys. Rev. B 47 (1993) 14948. [8] J. F. Herbst,Rev. Mod. Phys. 63 (1991) 819. [9] Y. Matsuura, IEEE Transactions on Appl. Superconductivity 10 (2000) 883-886. [10] K. J. Strnat, R. M. W. Strnat, J. Magnetism and Magnetic Materials 100 (1991) 38-56. [11] A. S. Rao, Proc. of IEEE conference on EEIC/ICWA, Chicago (1993) 373. [12] R. C. O’Handley, Modern Magnetic Materials:Principles and Applications, 1st ed. (John Wiley and Sons, Inc., New York,2000) [13] H. W. Chang, M. C. Liao, C. W. Shih, W. C. Chang, C. C. Yang, C. H. Hsiao, H. Ouyang, Appl. Phys. Lett. 105 (2014) 192404. [14] X. Lu, K. Cheng, S. Liu, K. Li, F. Zheng, Y. Du, J. Alloys and Comp. 627 (2015) 251-260. [15] X. Z. Li, Y. L. Jin, M. Y. Wang, J. E. Shield, R. Skomski, David J Sellmyer, Intermetallics, 75 (2016) 54-61.

The point to note is that Bedi et al. [7] started with a Co sample for PAC measurements which was prepared by alloying with 181 Hf metal (∼0.2 at%) used as a probe. They prepared the sample by high vacuum electron gun melting of metallic Co (50 mg) with a tiny piece of active Hf metal (∼0.25 mg). At room temperature, the PAC spectrum was interpreted due to combined electric quadrupole and magnetic interaction. But, at a temperature of 700 K, they analyzed the PAC spectrum with a quadrupole frequency νQ = 400 MHz, η = 0 which they interpreted as due to Hf2 Co7 from the consideration of phase diagram. But, this was quite unlikely to be produced as the Hf content in the sample was too small for producing the Hf2 Co7 intermetallic compound. If it was assumed to be produced, only a minor fraction of this was expected to be present. The predominant fraction should be due to elemental Co. Probably, their interpretation that the PAC spectrum at 700 K is due to Hf2 Co7 was not correct. In fact, they reported ferromagnetism in Hf2 Co7 from an indirect consideration of this 4

Figure 3: (A) is the TDPAC spectrum of Hf2 Co7 at 873 K while (B) shows its decompose spectra. Left panel shows anisotropy vs time (t)

while right panel shows corresponding Fourier transforms.

Figure 4: Variation of quadrupole frequency (ωQ ), asymmetry parameter (η) and component fraction (f ) with temperature for Hf2 Co7 .

Figure 5: Model crystal structure of Hf2 Co7 illustrated by Vesta software [30]. The two non-equivalent Hf atoms are represented by Hf1 and Hf2.

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