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Induced magnetic moment on Rh in the (FexRh1−x)100−yBy amorphous system
Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
Induced magnetic moment on Rh in the (FexRh1x)100yBy amorphous system Sarbari Bhattacharya, P.L. Paulose* Department of Condensed Matter Physics and MS, Tata Institute of Fundamental Research, Homi Bhabha Road, 400 005 Mumbai, India Received 14 March 2002; received in revised form 25 July 2002
Abstract The magnetic properties of the (FeRh)100y By ðy ¼ 20; 25Þ amorphous system have been investigated by AC . susceptibility, DC magnetization and 57 Fe Mossbauer spectroscopy. The experimental data give strong evidence for the presence of a sizable moment on Rh. Our data suggest that the moment on Rh evolves discontinuously, existing only when it has at least four Fe nearest neighbours. While the condition for the appearance of the moment on Rh is identical in both the a-(FeRh)75 B25 and a-(FeRh)80 B20 systems, the magnitude of the induced moment decreases despite an enhancement in the average ferromagnetic exchange in the Boron-rich system. We show that the induced Rh moment is correlated to the Fe–Rh exchange and there are indications of induced moment on Rh even in the paramagnetic state. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Cr; 75.50.Bb; 75.50.Lk; 76.80.+y . Keywords: Ferromagnets; Spin glasses; Intermetallic compounds; Magnetic ordering; Mossbauer spectroscopy
1. Introduction The formation of magnetic moment on 4d ions in metallic systems has been the subject of intense research both experimentally and theoretically [1–12]. The 4d element Rh is non-magnetic but exhibits a large paramagnetic susceptibility. It is isoelectronic to Co and is expected to be nearly magnetic. Neutron diffraction experiments have revealed that there is a moment of about 1 mB on Rh in the ferromagnetic phase of crystalline *Corresponding author. Tel.: +91-215-2971x2436; fax: +91215-2110. E-mail address: [email protected] (P.L. Paulose).
Fex Rh1x for 0.68XxX0.52. [3] It is conjectured that this magnetic moment on Rh is induced by the exchange field produced at the Rh sites by the ferromagnetically aligned Fe moments [5]. We showed for the first time the existence of a sizable induced moment on Rh in an amorphous matrix from the magnetic studies of (FeRh)80 B20 glasses [9]. The presence of a magnetic moment on Rh in this system reveals that neither the non-crystalline matrix nor the presence of B which is suggested to act as an electron donor disrupts the polarization process. It has been shown that a moment of B1:1 mB appears discontinuously on Rh when it has at least four Fe nearest neighbours. However, effects due to the local atomic arrangement in the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 6 9 3 - 5
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
non-crystalline matrix, the strength of the exchange interaction or the influence of Boron on the moment formation process on Rh are not known. The local atomic arrangements of amorphous Fe100y By in the vicinity of y ¼ 20 are based on a nearly 12–fold coordinated dense random packing structure [13,14]. Rhodium has a FCC structure and hence the amorphous state offers a unique opportunity of studying magnetism of the FCC phase of Rh in an extended range compared to the related crystalline phases. It has been reported that a-Fe75 B25 (prefix a- is used for amorphous henceforth) has a higher estimated Curie temperature (TC ) of 735 K compared to the TC of 647 K for a-Fe80 B20 [14]. The study of moment formation on Rh in the (FeRh)100y By amorphous alloys thereby gives us an opportunity to shed light on the effect of increased exchange interaction and B content on the polarization process.
335
and Rh. In Ref. [9], some of the TC ’s in the range T > 450 K were in error by about 5% due to temperature gradients in the measurement set-up. The present paper gives the corrected J values for (FeRh)80 B20 glassy system. The CPA analysis shows a sharp onset of the critical concentration (Xc ) for the appearance of ferromagnetic ordering in the (FeRh)75 B25 and (FeRh)80 B20 glassy systems, the respective values being 0.2 and 0.3. The detailed parameters are given in Table 1. The Xc in these systems is clearly above the percolation value of about 17 at% that is expected in a close packed structure with first near neighbour interactions
2. Experimental Amorphous alloys of (Fex Rh1x )100y By (y= 20 and 25) were prepared by melt spinning in an atmosphere of argon and their glassy nature was ascertained by X-ray diffraction. The magnetic behaviour of the materials were investigated by AC w in very low fields ð1 mTÞ using mutual . induction method, 57 Fe Mossbauer spectroscopy using 57 Co source in Rh matrix and DC magnetization by a SQUID magnetometer.
3. Results and discussion AC susceptibility studies carried out on the (Fex Rh1x )75 B25 glasses show that these are ferromagnetic at room temperature for xX0:33; the TC dropping gradually with the Rh substitution. The variation of TC over the entire concentration range for the a-(FeRh)80 B20 and a-(FeRh)75 B25 is shown in Fig. 1. The solid line represents a coherent potential approximation (CPA) fit for disordered alloys [9,15] which yields a sizable Fe–Rh exchange (denoted by JFe2Rh ) indicative of strong overlap of the d bands of Fe
Fig. 1. Plot of Curie temperature as a function of Fe concentration x in amorphous (Fex Rh1x )100y By alloys. The solid line is fit to CPA.
Table 1 CPA fit parameters and the critical concentrations obtained for a-(FeRh)80 B20 and a-(FeRh)75 B25 :
JFe2Fe JFe2Rh JRh2Rh Xc
a-(FeRh)80 B20
a-(FeRh)75 B25
639 823 123 0.3
727 797 41 0.2
J is given in units of K. The estimated error is about 73 K in the J value
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
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and we find that it is strongly dependent on the Boron content. . The 57 Fe Mossbauer spectra of the a(Fex Rh1x )75 B25 alloys at 4.2 K are shown in Fig. 2. The spectra are composed of well-defined but broadened lines characteristic of amorphous alloys and were analysed using the Window method [16] to get the hyperfine field distribution and the average hyperfine field ðHhf Þ: The probability distribution pðHÞ is given by the Fourier series N X pðHÞ ¼ an fn ðHÞ; ð1Þ n¼1
npðH Hmin Þ fn ðHÞ ¼ cos ð1Þn ; Hmax Hmin
ð2Þ
where Hmin and Hmax are the appropriate cutoffs for the hyperfine field, typically about 15 and 40 T, respectively. N; the number of terms, was chosen as 8 as it gives an optimum fit without generating artificial oscillations due to truncation. The other free parameters were the single linewidth of Fe (G), the intensity ratio (b) and the average isomer shift
Fig. 2.
57
(IS). The best fit was obtained with G of about 0.36 mm/s for all the alloys, which is 20% above the natural Fe width of 0.3 mm/s in our set-up. It is evident that for xX0:33 in this glassy system, with increasing Rh substitution, hyperfine field distribution gets narrowed and shifts towards higher fields. Although the Hhf increases gradually, the overall change is marginal in the whole concentration range (Fig. 3). It is to be noted that even though x = 0.2 is close to the percolation limit of xB0:17; the Fe moment and hyperfine field remain strong down to this concentration. Hence, it can be inferred that in this system the moment on Fe remains stable down to very low concentrations of Fe and is possibly even strengthened by Rh substitution despite a rapid weakening of average ferromagnetic exchange. The average isomer shift at 4.2 K with respect to the natural Fe is found to increase initially with inreasing Rh substitution (see Fig. 3). This can be due to either an increase in d electron density at Fe or a decrease in the 4s electrons density. Fig. 4 shows the variation of DC magnetization with applied field at 5 K for the a-(Fex Rh1x )75 B25
. Fe Mossbauer spectra and hyperfine field distributions for the a-(Fex Rh1x )75 B25 alloys at 4.2 K.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
alloys. The magnetic momet shown in Fig. 4 is calculated assuming Fe to be the only magnetic moment bearing atom. The glasses corresponding to xX0:33 behave like good ferromagnets achieving near saturation of magnetization in applied fields as small as 50 mT. The glass corresponding to x ¼ 0:2 has a large high field slope, showing a substantial increase in magnetization of about 1 mB per Fe atom beyond the technical saturation. This behaviour is similar to that shown by reentrant Au–Fe alloys and is consistent with the low-field
337
magnetization measurements which shows spin glass like features at low temperatures for x ¼ 0:2 [17]. If the entire magnetization is due to Fe ions only, the Fe moment is found to increase from the value of 2 mB in a-Fe75 B25 to a maximum of 3.3 mB for x ¼ 0:33 alloy. The highest moment reported at the Fe site in crystalline FeRh alloys from neutron diffraction studies [3] is 3.1 mB : The inclusion of glass formers is known to decrease the Fe moment in crystalline and amorphous alloys [18]. So it is highly unlikely that the increase in moment in this glassy system is due to the increase in moment on Fe atoms alone. Alternatively one could ascribe a moment on Rh. Assuming a linear relation between the average moment on Fe and the average 57 Fe hyperfine field (13 T/mB ), as has been found to be the case for a large number of Fe-based amorphous alloys [19], we have an independent estimate of the moment on Fe. The moment on Fe thus estimated lies between 1.98 and 2.16 mB ; an overall variation of about 10%: Using this, the average moment on the Rh atoms (mRh ) is calculated from the bulk magnetization. The variation of mRh with x has been given in Fig. 5. If the appearance of the moment on Rh is due to polarization effects by Fe
Fig. 3. Variation of 57 Fe average hyperfine field and isomer shift at 4.2 K with Fe concentration in a-(Fex Rh1x )75 B25 (closed symbols) and in a-(Fex Rh1x )80 B20 (open symbols).
Fig. 4. Variation of DC magnetization with applied field at 5 K for a-(Fex Rh1x )75 B25 alloys. The magnetic moment is calculated attributing the entire moment to only the Fe atoms.
Fig. 5. Variation of the estimated average moment per Rh atom in the a-(Fex Rh1x )75 B25 alloys, normalized to the value m0 = 0.85 mB ; with Fe concentration. Superposed on this data is the curve which gives the probability that a Rh atom will have at least four Fe nearest neighbours.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
338
near neighbours, depending on local environment, the moment evolution on Rh could be continuous or discontinuous. We have considered the latter possibility where the Rh atom develops the full moment only when it has at least m Fe near neighbours and remains unmagnetized otherwise. This is the Jaccarino–Walker model [20]. The probability Pm of there being at least m Fe nearest neighbours for a Rh atom is given by Pm ¼
Z X
Z! xn ð1 xÞZn ; n!ðZ nÞ! n¼m
ð3Þ
where Z is the local coordination. Z is taken as 12, assuming a close packed structure. We find that the curve simulated for the probability that a Rh atom has a fully developed moment of 0.85 mB when it has at least four Fe near neighbours (given by the continuous line in Fig. 5) coincides best with the variation of the estimated mRh with x. The analysis of the data on a-(Fex Rh1x )80 B20 system also leads to a similar results but with a higher moment of 1.1 mB on Rh atom. This may indicate that the Rh moment gets stabilized in a close packed local environment. Several striking features emerge when one considers the results of the magnetic studies on the a-(FeRh)100y By systems for y = 20 and 25. The polarization effects which lead to the formation of a moment on Rh are present irrespective of the B content in these glasses. We find that in both the systems the condition for the appearance of a moment on Rh is identical. However, the magnitude of the induced moment on Rh decreases from 1.1 mB for y ¼ 20 to 0.85 mB for y ¼ 25: We know that a-(FeRh)75 B25 has a larger average ferromagnetic exchange compared to that in a-(FeRh)80 B20 as can be inferred from a comparison of their Curie temperatures (Fig. 1). So it is clear that the Rh moment is not influenced by the average ferromagnetic exchange of the system. A careful examination of the CPA results (see Table 1) reveals that in the Boron-rich system (y ¼ 25), the JFe2Fe is increased while the JFe2Rh is decreased compared to the y ¼ 20 system. This gives direct experimental evidence that the moment induced on Rh is correlated to the Fe–Rh exchange.
Moruzzi et al. show that the Rh metal undergo first-order transition from nonmagnetic to magnetic behaviour at expanded volumes with Rh acquiring a moment of about 1.3 mB [8]. With increasing volume, they find a depletion of s and p states and a corresponding increase of d states towards the 4d9 configuration. The decrease in the 4s electron density or a related possible increase in 3d states of Fe will lead to an increase in the isomer shift. The isomer shift data show an increasing trend over a wide range of concentration with Rh substitution in Fe–B glassy system (see Fig. 3). In Fig. 6 we show the inverse susceptibility of a(Fe0:25 Rh0:75 )80 B20 and a-(Fe0:2 Rh0:8 )75 B25 : The high-temperature fit gives an effective paramagnetic moment of 5.85 and 5.4 mB per Fe atom, respectively, for these alloys. This is much above the value of 4.9 mB expected for high spin d6 configuration. This may indicate a polarized moment on Rh even in the paramagnetic state. Also note that the alloy with 25% B has a significantly lower moment compared to the alloy with 20% B and it may imply a reduced Rh moment for the former. This demonstrates
Fig. 6. The inverse susceptibility of a-(Fe0:25 Rh0:75 )80 B20 and a(Fe0:2 Rh0:8 )75 B25 : The straight lines are a Curie-Weiss fit to high temperature region between 240 and 310 K extrapolated to axes.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
experimentally that there is a direct correspondence of induced moment on Rh in the ferromagnetic and in the paramagnetic state. In conclusion, we have carried out investigations on the moment formation on Rh atoms due to polarization in (FeRh)75 B25 amorphous alloys. We show that a Jaccarino–Walker model where there is a discontinuous evolution of the moment on Rh, a Rh atom requiring at least four Fe nearest neighbours to polarize it, best describes the evolution of the moment on Rh with Fe concentration in this system. However, while the condition for the appearance of a moment on Rh is identical with that for a-(FeRh)80 B20 ; the magnitude of the induced moment reduces as the B content increases. We have shown that the induced moment is correlated to the Fe3d–Rh4d interactions. This correlation may exist even in the paramagnetic state.
Acknowledgements We would like to thank R. Nagarajan for his . help with the Mossbauer spectroscopy.
References [1] J.S. Kouvel, C.C. Hartelius, J. Appl. Phys. 33 (Suppl.) (1962) 1343.
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[2] G. Shirane, C.W. Chen, P.A. Flinn, R. Nathans, Phys. Rev. 131 (1963) 183. [3] G. Shirane, R. Nathans, C.W. Chen, Phys. Rev. 134 (1964) A1547. [4] P.P. Craig, B. Mozer, R. Segnan, Phys. Rev. Lett. 14 (1965) 895. [5] J.S. Kouvel, J. Appl. Phys. 37 (1966) 1257. [6] A.P. Murani, B.R. Coles, J. Phys. C (2) (1970) S159. [7] Hideo Ohno, J. Phys. Soc. Jpn. 31 (1971) 92. [8] V.L. Moruzzi, P.M. Markus, Phys. Rev. B 39 (1989) 471 and references therein. [9] P.L. Paulose, V. Nagarajan, R. Nagarajan, R. Vijayaraghavan, Phys. Rev. B 42 (1990) 1070. [10] V.V. Krishnamurthy, S.N. Mishra, P.L. Paulose, S.H. Devare, H.G. Devare, Phys. Rev. B 49 (1994) 6726. [11] G.R. Harp, S.S.P. Parkin, W.L. O’Brien, B.P. Tonner, Phys. Rev. B 51 (1995) 12,037. [12] J.-J. Gallet, J.-M. Mariot, L. Journel, C.F. Hague, J.-P. Kappler, G. Schmerber, D.J. Singh, G. Krill, J. Goulon, A. Rogalev, Phys. Rev. B 57 (1998) 7835. [13] J. Dixmier, J.F. Sadoc, in: J.J. Gilman, H.J. Leamy (Eds.), Proceedings of ASM Symposium on Metallic Glasses, ASM, Metals Park, OH, 1977. [14] R. Hasegawa, R. Ray, J. Appl. Phys. 49 (1978) 4174. [15] E-Ni Foo, Der-Hsueh Wu, Phys. Rev. B 5 (1972) 98. [16] B. Window, J. Phys. E 4 (1971) 401. [17] P.L. Paulose, et al., to be published. [18] K. Moorjani, J.M.D. Coey, Magnetic Glasses, Elsevier, Amsterdam, 1984, p. 92. [19] P. Panissod, J. Durand, J.I. Budnick, Nucl. Instrum. Methods 199 (1982) 99. [20] V. Jaccarino, L.R. Walker, Phys. Rev. Lett. 15 (1965) 258.
Induced magnetic moment on Rh in the (FexRh1x)100yBy amorphous system Sarbari Bhattacharya, P.L. Paulose* Department of Condensed Matter Physics and MS, Tata Institute of Fundamental Research, Homi Bhabha Road, 400 005 Mumbai, India Received 14 March 2002; received in revised form 25 July 2002
Abstract The magnetic properties of the (FeRh)100y By ðy ¼ 20; 25Þ amorphous system have been investigated by AC . susceptibility, DC magnetization and 57 Fe Mossbauer spectroscopy. The experimental data give strong evidence for the presence of a sizable moment on Rh. Our data suggest that the moment on Rh evolves discontinuously, existing only when it has at least four Fe nearest neighbours. While the condition for the appearance of the moment on Rh is identical in both the a-(FeRh)75 B25 and a-(FeRh)80 B20 systems, the magnitude of the induced moment decreases despite an enhancement in the average ferromagnetic exchange in the Boron-rich system. We show that the induced Rh moment is correlated to the Fe–Rh exchange and there are indications of induced moment on Rh even in the paramagnetic state. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Cr; 75.50.Bb; 75.50.Lk; 76.80.+y . Keywords: Ferromagnets; Spin glasses; Intermetallic compounds; Magnetic ordering; Mossbauer spectroscopy
1. Introduction The formation of magnetic moment on 4d ions in metallic systems has been the subject of intense research both experimentally and theoretically [1–12]. The 4d element Rh is non-magnetic but exhibits a large paramagnetic susceptibility. It is isoelectronic to Co and is expected to be nearly magnetic. Neutron diffraction experiments have revealed that there is a moment of about 1 mB on Rh in the ferromagnetic phase of crystalline *Corresponding author. Tel.: +91-215-2971x2436; fax: +91215-2110. E-mail address: [email protected] (P.L. Paulose).
Fex Rh1x for 0.68XxX0.52. [3] It is conjectured that this magnetic moment on Rh is induced by the exchange field produced at the Rh sites by the ferromagnetically aligned Fe moments [5]. We showed for the first time the existence of a sizable induced moment on Rh in an amorphous matrix from the magnetic studies of (FeRh)80 B20 glasses [9]. The presence of a magnetic moment on Rh in this system reveals that neither the non-crystalline matrix nor the presence of B which is suggested to act as an electron donor disrupts the polarization process. It has been shown that a moment of B1:1 mB appears discontinuously on Rh when it has at least four Fe nearest neighbours. However, effects due to the local atomic arrangement in the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 6 9 3 - 5
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
non-crystalline matrix, the strength of the exchange interaction or the influence of Boron on the moment formation process on Rh are not known. The local atomic arrangements of amorphous Fe100y By in the vicinity of y ¼ 20 are based on a nearly 12–fold coordinated dense random packing structure [13,14]. Rhodium has a FCC structure and hence the amorphous state offers a unique opportunity of studying magnetism of the FCC phase of Rh in an extended range compared to the related crystalline phases. It has been reported that a-Fe75 B25 (prefix a- is used for amorphous henceforth) has a higher estimated Curie temperature (TC ) of 735 K compared to the TC of 647 K for a-Fe80 B20 [14]. The study of moment formation on Rh in the (FeRh)100y By amorphous alloys thereby gives us an opportunity to shed light on the effect of increased exchange interaction and B content on the polarization process.
335
and Rh. In Ref. [9], some of the TC ’s in the range T > 450 K were in error by about 5% due to temperature gradients in the measurement set-up. The present paper gives the corrected J values for (FeRh)80 B20 glassy system. The CPA analysis shows a sharp onset of the critical concentration (Xc ) for the appearance of ferromagnetic ordering in the (FeRh)75 B25 and (FeRh)80 B20 glassy systems, the respective values being 0.2 and 0.3. The detailed parameters are given in Table 1. The Xc in these systems is clearly above the percolation value of about 17 at% that is expected in a close packed structure with first near neighbour interactions
2. Experimental Amorphous alloys of (Fex Rh1x )100y By (y= 20 and 25) were prepared by melt spinning in an atmosphere of argon and their glassy nature was ascertained by X-ray diffraction. The magnetic behaviour of the materials were investigated by AC w in very low fields ð1 mTÞ using mutual . induction method, 57 Fe Mossbauer spectroscopy using 57 Co source in Rh matrix and DC magnetization by a SQUID magnetometer.
3. Results and discussion AC susceptibility studies carried out on the (Fex Rh1x )75 B25 glasses show that these are ferromagnetic at room temperature for xX0:33; the TC dropping gradually with the Rh substitution. The variation of TC over the entire concentration range for the a-(FeRh)80 B20 and a-(FeRh)75 B25 is shown in Fig. 1. The solid line represents a coherent potential approximation (CPA) fit for disordered alloys [9,15] which yields a sizable Fe–Rh exchange (denoted by JFe2Rh ) indicative of strong overlap of the d bands of Fe
Fig. 1. Plot of Curie temperature as a function of Fe concentration x in amorphous (Fex Rh1x )100y By alloys. The solid line is fit to CPA.
Table 1 CPA fit parameters and the critical concentrations obtained for a-(FeRh)80 B20 and a-(FeRh)75 B25 :
JFe2Fe JFe2Rh JRh2Rh Xc
a-(FeRh)80 B20
a-(FeRh)75 B25
639 823 123 0.3
727 797 41 0.2
J is given in units of K. The estimated error is about 73 K in the J value
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
336
and we find that it is strongly dependent on the Boron content. . The 57 Fe Mossbauer spectra of the a(Fex Rh1x )75 B25 alloys at 4.2 K are shown in Fig. 2. The spectra are composed of well-defined but broadened lines characteristic of amorphous alloys and were analysed using the Window method [16] to get the hyperfine field distribution and the average hyperfine field ðHhf Þ: The probability distribution pðHÞ is given by the Fourier series N X pðHÞ ¼ an fn ðHÞ; ð1Þ n¼1
npðH Hmin Þ fn ðHÞ ¼ cos ð1Þn ; Hmax Hmin
ð2Þ
where Hmin and Hmax are the appropriate cutoffs for the hyperfine field, typically about 15 and 40 T, respectively. N; the number of terms, was chosen as 8 as it gives an optimum fit without generating artificial oscillations due to truncation. The other free parameters were the single linewidth of Fe (G), the intensity ratio (b) and the average isomer shift
Fig. 2.
57
(IS). The best fit was obtained with G of about 0.36 mm/s for all the alloys, which is 20% above the natural Fe width of 0.3 mm/s in our set-up. It is evident that for xX0:33 in this glassy system, with increasing Rh substitution, hyperfine field distribution gets narrowed and shifts towards higher fields. Although the Hhf increases gradually, the overall change is marginal in the whole concentration range (Fig. 3). It is to be noted that even though x = 0.2 is close to the percolation limit of xB0:17; the Fe moment and hyperfine field remain strong down to this concentration. Hence, it can be inferred that in this system the moment on Fe remains stable down to very low concentrations of Fe and is possibly even strengthened by Rh substitution despite a rapid weakening of average ferromagnetic exchange. The average isomer shift at 4.2 K with respect to the natural Fe is found to increase initially with inreasing Rh substitution (see Fig. 3). This can be due to either an increase in d electron density at Fe or a decrease in the 4s electrons density. Fig. 4 shows the variation of DC magnetization with applied field at 5 K for the a-(Fex Rh1x )75 B25
. Fe Mossbauer spectra and hyperfine field distributions for the a-(Fex Rh1x )75 B25 alloys at 4.2 K.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
alloys. The magnetic momet shown in Fig. 4 is calculated assuming Fe to be the only magnetic moment bearing atom. The glasses corresponding to xX0:33 behave like good ferromagnets achieving near saturation of magnetization in applied fields as small as 50 mT. The glass corresponding to x ¼ 0:2 has a large high field slope, showing a substantial increase in magnetization of about 1 mB per Fe atom beyond the technical saturation. This behaviour is similar to that shown by reentrant Au–Fe alloys and is consistent with the low-field
337
magnetization measurements which shows spin glass like features at low temperatures for x ¼ 0:2 [17]. If the entire magnetization is due to Fe ions only, the Fe moment is found to increase from the value of 2 mB in a-Fe75 B25 to a maximum of 3.3 mB for x ¼ 0:33 alloy. The highest moment reported at the Fe site in crystalline FeRh alloys from neutron diffraction studies [3] is 3.1 mB : The inclusion of glass formers is known to decrease the Fe moment in crystalline and amorphous alloys [18]. So it is highly unlikely that the increase in moment in this glassy system is due to the increase in moment on Fe atoms alone. Alternatively one could ascribe a moment on Rh. Assuming a linear relation between the average moment on Fe and the average 57 Fe hyperfine field (13 T/mB ), as has been found to be the case for a large number of Fe-based amorphous alloys [19], we have an independent estimate of the moment on Fe. The moment on Fe thus estimated lies between 1.98 and 2.16 mB ; an overall variation of about 10%: Using this, the average moment on the Rh atoms (mRh ) is calculated from the bulk magnetization. The variation of mRh with x has been given in Fig. 5. If the appearance of the moment on Rh is due to polarization effects by Fe
Fig. 3. Variation of 57 Fe average hyperfine field and isomer shift at 4.2 K with Fe concentration in a-(Fex Rh1x )75 B25 (closed symbols) and in a-(Fex Rh1x )80 B20 (open symbols).
Fig. 4. Variation of DC magnetization with applied field at 5 K for a-(Fex Rh1x )75 B25 alloys. The magnetic moment is calculated attributing the entire moment to only the Fe atoms.
Fig. 5. Variation of the estimated average moment per Rh atom in the a-(Fex Rh1x )75 B25 alloys, normalized to the value m0 = 0.85 mB ; with Fe concentration. Superposed on this data is the curve which gives the probability that a Rh atom will have at least four Fe nearest neighbours.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
338
near neighbours, depending on local environment, the moment evolution on Rh could be continuous or discontinuous. We have considered the latter possibility where the Rh atom develops the full moment only when it has at least m Fe near neighbours and remains unmagnetized otherwise. This is the Jaccarino–Walker model [20]. The probability Pm of there being at least m Fe nearest neighbours for a Rh atom is given by Pm ¼
Z X
Z! xn ð1 xÞZn ; n!ðZ nÞ! n¼m
ð3Þ
where Z is the local coordination. Z is taken as 12, assuming a close packed structure. We find that the curve simulated for the probability that a Rh atom has a fully developed moment of 0.85 mB when it has at least four Fe near neighbours (given by the continuous line in Fig. 5) coincides best with the variation of the estimated mRh with x. The analysis of the data on a-(Fex Rh1x )80 B20 system also leads to a similar results but with a higher moment of 1.1 mB on Rh atom. This may indicate that the Rh moment gets stabilized in a close packed local environment. Several striking features emerge when one considers the results of the magnetic studies on the a-(FeRh)100y By systems for y = 20 and 25. The polarization effects which lead to the formation of a moment on Rh are present irrespective of the B content in these glasses. We find that in both the systems the condition for the appearance of a moment on Rh is identical. However, the magnitude of the induced moment on Rh decreases from 1.1 mB for y ¼ 20 to 0.85 mB for y ¼ 25: We know that a-(FeRh)75 B25 has a larger average ferromagnetic exchange compared to that in a-(FeRh)80 B20 as can be inferred from a comparison of their Curie temperatures (Fig. 1). So it is clear that the Rh moment is not influenced by the average ferromagnetic exchange of the system. A careful examination of the CPA results (see Table 1) reveals that in the Boron-rich system (y ¼ 25), the JFe2Fe is increased while the JFe2Rh is decreased compared to the y ¼ 20 system. This gives direct experimental evidence that the moment induced on Rh is correlated to the Fe–Rh exchange.
Moruzzi et al. show that the Rh metal undergo first-order transition from nonmagnetic to magnetic behaviour at expanded volumes with Rh acquiring a moment of about 1.3 mB [8]. With increasing volume, they find a depletion of s and p states and a corresponding increase of d states towards the 4d9 configuration. The decrease in the 4s electron density or a related possible increase in 3d states of Fe will lead to an increase in the isomer shift. The isomer shift data show an increasing trend over a wide range of concentration with Rh substitution in Fe–B glassy system (see Fig. 3). In Fig. 6 we show the inverse susceptibility of a(Fe0:25 Rh0:75 )80 B20 and a-(Fe0:2 Rh0:8 )75 B25 : The high-temperature fit gives an effective paramagnetic moment of 5.85 and 5.4 mB per Fe atom, respectively, for these alloys. This is much above the value of 4.9 mB expected for high spin d6 configuration. This may indicate a polarized moment on Rh even in the paramagnetic state. Also note that the alloy with 25% B has a significantly lower moment compared to the alloy with 20% B and it may imply a reduced Rh moment for the former. This demonstrates
Fig. 6. The inverse susceptibility of a-(Fe0:25 Rh0:75 )80 B20 and a(Fe0:2 Rh0:8 )75 B25 : The straight lines are a Curie-Weiss fit to high temperature region between 240 and 310 K extrapolated to axes.
S. Bhattacharya, P.L. Paulose / Journal of Magnetism and Magnetic Materials 256 (2003) 334–339
experimentally that there is a direct correspondence of induced moment on Rh in the ferromagnetic and in the paramagnetic state. In conclusion, we have carried out investigations on the moment formation on Rh atoms due to polarization in (FeRh)75 B25 amorphous alloys. We show that a Jaccarino–Walker model where there is a discontinuous evolution of the moment on Rh, a Rh atom requiring at least four Fe nearest neighbours to polarize it, best describes the evolution of the moment on Rh with Fe concentration in this system. However, while the condition for the appearance of a moment on Rh is identical with that for a-(FeRh)80 B20 ; the magnitude of the induced moment reduces as the B content increases. We have shown that the induced moment is correlated to the Fe3d–Rh4d interactions. This correlation may exist even in the paramagnetic state.
Acknowledgements We would like to thank R. Nagarajan for his . help with the Mossbauer spectroscopy.
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