Spin glass and Invar effect for Fe(ZrB) amorphous alloys

Journal of Magnetism and Magnetic Materials 177 181 (1998) 125 126

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Journal Of magnetism and magnetic materials

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Spin glass and Invar effect for Fe(ZrB) amorphous alloys H. Tange*, T. Matsuyama, A. Chikazawa, K. Konishi, T. Kamimori FaculO, of Science, Ehime University, Matsuyama 790-77,Japan

Abstract Spin glass (SG) and Invar effect for amorphous Feg,(Zrl-xBx)9 Ix = 0-0.4] alloys have been investigated. Curie temperature, To, and spin freezing point, Tf, have been obtained from measurements of AC susceptibility )~AC.The value of Tc increases with increasing B content, but Tf decreases and vanishes at around x = 0.7. These behaviors are explained by the Sherington Kirkpatrick model. The pressure effect of T~, dT~/dp, has been obtained indirectly from measurements of forced volume magnetostriction do)/dH. The curve of d Tc/dp versus Tc indicates that the measured samples are in the magnetically inhomogeneous state (MIS) according to the Wagner-Wohlfarth discussion. The relation between SG and MIS of samples are discussed. © 1998 Elsevier Science B.V. All rights reserved.

Keywords." Amorphous systems - alloys; Susceptibility - AC; Re-entrant spin glass; Invar effect; Magnetostriction - forced volume; Curie temperature- pressure dependent

Spin glass (SG) and Invar effect (IE) are typical magnetic properties in Fe-rich amorphous (a-) alloys. a-Fe-Zr alloys show both re-entrant spin glass (RSG) and IE [1], but a-Fe B ones show only IE [2]. It is expected that in a-Fe91(Zr1 -xBx)9 alloys RSG will vanish in the middle range of B content. IE expected in the whole range of B content is worth being investigated through measurements of forced volume magnetostriction, dco/dH, and pressure effect on Curie temperature, dTc/dp, instead of a thermal expansion measurement. dTc/dp is obtained indirectly from the experiments of dm/dH and using thermodynamical relations. According to Wagner and Wohlfarth's discussion [-3], the magnetically inhomogeneous state (MIS) or homogeneous one (MHS) of samples can be discussed from results of dTc/dp. Moreover, the relation between MIS and RSG in a-Feg,(Zr1_xBx)9 is discussed in comparison to the resuits of dTc/dp and RSG for a-We, -~.Niy)9oZrlo [4, 5]. Samples of a-Fe91(Zr1 xBx)9 [x = 0, 0.1, 0.2, 0.3, 0.4] were prepared by a single-roller quenching technique in an argon atmosphere. The amorphous state was verified by X-ray diffraction. The shape of the ribbons was about 1-2 mm in width and 20 I.tm in thickness. Temperature dependence of AC susceptibility ZACwas measured from

4.2 to 300 K by a Hartshorn bridge method with a frequency of 85 Hz and an amplitude of 1.00e. The magnetization was measured by the vibrating sample method (VSM). Measurements of do~/dH were done by a threeterminal capacitance method in fields up to 18 kOe. Fig. 1 shows the heating curves ])f the real part ;( and the imaginary one Z" of XAC measured after zero-field cooling for a-Fe91(Zrl xBx)9.The arrows at Tc indicate '............. _ 2 _ ' . x - .. . .i

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*Corresponding author. Fax: + 81 89 927 9580; e-mail: [email protected].

a_l~e91(Z~l_xBx/9

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Fig. 1. Real part 7/ and imaginary part 7," of ZAC for a-Fe91(Zr1 xBx)9.

0304-8853/98/$19.00 (() 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 5 0 5 - 2

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t-L Tange et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 125-126

Curie temperature defined as the shoulder point. The arrows at Tf indicate the spin freezing point defined as the peak point. Fig. 2 shows the magnetic phase diagram as a function of B content for a-Fe91(Zr1_xBx)9. T~ increases with increasing B content, but T f decreases and vanishes at around x = 0.7 if extrapolated to 0 K. These specimens up to x = 0.7 show RSG. These behaviors might be explained by the Sherington-Kirkpatrick (SK) model [6]: The increase of B content means the increase of Jo/kJ, because k J in a-Fe-B is considered to be smaller than that in a - F e - Z r owing to the atomic size difference between B and Zr. This difference leads to the different distribution of exchange interaction, depending on atomic distance between Fe Fe atoms around B or Zr. Jo is hardly considered to change macroscopically. Here, J0 is the mean exchange interaction between Fe-Fe atoms and A J is the fluctuation from Jo. Fig. 3 shows d~o/dH for a-Fe91(Zrl - x B x ) 9 . The arrows at T~ indicate Curie temperature determined by ZAC,and also the arrows at Tr indicate the spin freezing point though there is a difference between Tf defined by ZAC and that defined by the shoulder of & o / d H due to the

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Fig. 4. T~ dependence of d In Tc/dp for Fe91(Zra ~Bx)9.

different magnitude of magnetic fields. From these results, pressure effects on magnetic moment per gram a, at 0 K, d In ao/dp, and dTc/dp can be estimated indirectly using thermodynamical relations: dm/dH = - p(das/dp) and doo/dH = pa,[T(d In ~rs/dT)(d In Tc/dp) - d In ao/dp], where p is density. In this paper, we deal with only dTc/dp. Fig. 4 shows the Tc dependence of d In To/dp for a-Fe9 l ( Z r l - xBx)9. The value of d In To~alp increases with increasing To. According to Wagner and Wohlfarth's discussion [3], dTo/dp's are expressed as dTo/dp = - aTe + bT~ for MIS and as dTo/dp = aTo - b/T~ for MHS. This figure suggests that the present samples are in MIS. Here, MIS means that magnetic atoms have various magnetic moments depending on the bond, topological and chemical disorder. Finally, we consider the relation between SG and MIS of the measured samples of a-Fe91(Zrl-xBx)9, together with results for a-(Fel yNir)90Zrl0 [4, 5]. In a-(Fex-yNiy)9oZrlo the behaviour of dTffdp versus T~ changes from the MIS to M H S types and RSG disappears around y = 0.1 [4, 5]. These results suggest a strong correlation between R S G and MIS: MIS and RSG originate from the distribution of magnetic moments depending on the bond, topological and chemical disorder. We are further investigating IE and RSG for a - F e 9 , ( Z r i - x B x ) 9 with x > 0.4. References

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[l] H. Tange, Y. Tanaka, M. Goto, K. Fukamichi, J. Magn. Magn. Mater. 81 (1989)L243. [2] J. Kamarad, Z. Arnold, J. Schneider, S. Krupicka, J. Magn. Magn. Mater. 15-18 (1980) 1409. [3] D. Wagner, E.P. Wohlfarth, J. Phys. F 11 (1981)2417. [4] H. Tange, Y. Tanaka, K. Shirakawa, J. Phys. 49 (1988) C8-1281. [5] H. Tange, T. Kamimori, M. Goto, K. Fukamichi, J. Magn. Magn. Mater. 90&91 (1990) 335. [6] D. Sherington, S. Kirkpatrick, Phys. Rev. Lett. 35 (1975) 1792.