Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅

Magnetic susceptibility χ of the isostructural Ce(Ni₁₋xCux)₅ alloys (0 ≤ x ≤ 0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K. A pronounced pressure effect on susceptibility is found to be negative in sign and nonmonotonously dependent on t...

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Дата:	2011
Автори:	Grechnev, G.E., Logosha, A.V., Panfilov, A.S., Svechkarev, I.V., Musil, O., Svoboda, P.
Формат:	Стаття
Мова:	English
Опубліковано:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2011
Назва видання:	Физика низких температур
Теми:	Электронные свойства многокомпонентных соединений
Онлайн доступ:	http://dspace.nbuv.gov.ua/handle/123456789/118772
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Цитувати:	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅ / G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, P. Svoboda // Физика низких температур. — 2011. — Т. 37, № 9-10. — С. 1062–1067. — Бібліогр.: 28 назв. — англ.

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spelling	irk-123456789-1187722017-06-01T03:03:48Z Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅ Grechnev, G.E. Logosha, A.V. Panfilov, A.S. Svechkarev, I.V. Musil, O. Svoboda, P. Электронные свойства многокомпонентных соединений Magnetic susceptibility χ of the isostructural Ce(Ni₁₋xCux)₅ alloys (0 ≤ x ≤ 0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K. A pronounced pressure effect on susceptibility is found to be negative in sign and nonmonotonously dependent on the Cu content, showing a sharp maximum at x 0.4. The experimental results are discussed in terms of the valence instability of Ce ion in the studied alloys. For the reference CeNi5 compound the main contributions to χ and their volume dependence are calculated ab initio within the local spin density approximation, and appeared to be in close agreement with experimental data. 2011 Article Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅ / G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, P. Svoboda // Физика низких температур. — 2011. — Т. 37, № 9-10. — С. 1062–1067. — Бібліогр.: 28 назв. — англ. 0132-6414 PACS: 71.20.Eh, 75.30.Mb, 75.80.+q http://dspace.nbuv.gov.ua/handle/123456789/118772 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	Электронные свойства многокомпонентных соединений Электронные свойства многокомпонентных соединений
spellingShingle	Электронные свойства многокомпонентных соединений Электронные свойства многокомпонентных соединений Grechnev, G.E. Logosha, A.V. Panfilov, A.S. Svechkarev, I.V. Musil, O. Svoboda, P. Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅ Физика низких температур
description	Magnetic susceptibility χ of the isostructural Ce(Ni₁₋xCux)₅ alloys (0 ≤ x ≤ 0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K. A pronounced pressure effect on susceptibility is found to be negative in sign and nonmonotonously dependent on the Cu content, showing a sharp maximum at x 0.4. The experimental results are discussed in terms of the valence instability of Ce ion in the studied alloys. For the reference CeNi5 compound the main contributions to χ and their volume dependence are calculated ab initio within the local spin density approximation, and appeared to be in close agreement with experimental data.
format	Article
author	Grechnev, G.E. Logosha, A.V. Panfilov, A.S. Svechkarev, I.V. Musil, O. Svoboda, P.
author_facet	Grechnev, G.E. Logosha, A.V. Panfilov, A.S. Svechkarev, I.V. Musil, O. Svoboda, P.
author_sort	Grechnev, G.E.
title	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅
title_short	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅
title_full	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅
title_fullStr	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅
title_full_unstemmed	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅
title_sort	pressure effect on magnetic properties of valence fluctuating system ce(ni₁₋xcux)₅
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2011
topic_facet	Электронные свойства многокомпонентных соединений
url	http://dspace.nbuv.gov.ua/handle/123456789/118772
citation_txt	Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅ / G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, P. Svoboda // Физика низких температур. — 2011. — Т. 37, № 9-10. — С. 1062–1067. — Бібліогр.: 28 назв. — англ.
series	Физика низких температур
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first_indexed	2025-07-08T14:37:29Z
last_indexed	2025-07-08T14:37:29Z
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fulltext	© G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, and P. Svoboda, 2011 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10, p. 1062–1067 Pressure effect on magnetic properties of valence fluctuating system Ce(Ni1–xCux)5 G.E. Grechnev, A.V. Logosha, A.S. Panfilov, and I.V. Svechkarev B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: grechnev@ilt.kharkov.ua O. Musil and P. Svoboda Charles University, Faculty of Mathematics and Physics, Department of Electronic Structures, Ke Karlovu 5, 121 16 Prague 2, The Czech Republic Received March 17, 2011 Magnetic susceptibility χ of the isostructural Ce(Ni1–xCux)5 alloys (0 ≤ x ≤ 0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K. A pronounced pressure effect on sus- ceptibility is found to be negative in sign and nonmonotonously dependent on the Cu content, showing a sharp maximum at x 0.4. The experimental results are discussed in terms of the valence instability of Ce ion in the studied alloys. For the reference CeNi5 compound the main contributions to χ and their volume dependence are calculated ab initio within the local spin density approximation, and appeared to be in close agreement with ex- perimental data. PACS: 71.20.Eh Rare earth metals and alloys; 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena; 75.80.+q Magnetomechanical effects, magnetostriction. Keywords: intermediate valence, magnetic susceptibility, pressure effect, electronic structure. 1. Introduction Many of Ce intermetallics are characterized by a strong hybridization of the magnetic 4f-electrons with the conduc- tion electron states resulted in delocalization of the 4f-level and a change of its occupancy, and hence the Ce valence. As is evident from measurements of x-ray absorption and lattice parameters [1], together with the magnetic [2,3], electric and thermoelectric properties [3], in the isostruc- tural Ce(Ni1–xCux)5 alloys, the Ce valence decreases con- sistently from Ce4+ to Ce3+ with increase of the Cu con- tent. Accordingly, the system undergoes a series of transitions from the nonmagnetic metal with the unoccu- pied 4f-level (x = 0) through the intermediate valence (IV) state combined with a nonmagnetic dense Kondo state (0.1 0.8)x≤ ≤ to the magnetic 4f-metal (0.9 1).x≤ ≤ Thus, the reference CeNi5 compound is expected to be the exchange-enhanced itinerant paramagnet [1,4,5] with the temperature dependent magnetic susceptibility exhibiting a broad maximum around 100 K, similar to those observed in YNi5, LaNi5 and LuNi5 [4,6,7]. On the other side, the CeCu5 compound is a Kondo lattice antiferromagnet with = 3.9NT K and = 2.2KT K [8]. The magnetic suscepti- bility in CeCu5 obeys a Curie–Weiss law at 50T ≥ K with the effective magnetic moment value close to that expected for Ce3+ state [8–10]. Due to a direct relation between magnetic properties and the rare earth (RE) valence state, and also the strong correlation between the valence itself and RE ionic volume, the RE compounds with unstable f shell exhibit a large magnetovolume effect. Therefore, a study of pressure effect on magnetic properties of the sys- tems with variable RE valence is of great interest to gain insight into a nature of the IV state. Here we report results of our investigation of the pres- sure effect on the magnetic susceptibility of Ce(Ni1–xCux)5 alloys in a wide range of Cu concentrations. The experi- mental results are supplemented by calculations of the magnetovolume effect value for the reference CeNi5 com- pound, using a modified relativistic full potential approach within linearized “muffin-tin” orbital method (FP–LMTO). Pressure effect on magnetic properties of valence fluctuating system Ce(Ni1–xCux)5 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10 1063 2. Experimental details and results The polycrystalline samples of Ce(Ni1–xCux)5 alloys (0 0.9)x≤ ≤ were prepared by arc-melting of a stoichi- ometric amount of initial elements in a water cooled cruci- ble under protective argon atmosphere. The study of x-ray powder diffraction at room temperature revealed that all samples crystallize in CaCu5-type hexagonal structure, and obtained data on their lattice parameters agree closely with that published in literature. Any other phases were not de- tected within the resolution of the x-ray technique. The pressure effect on the magnetic susceptibility χ was measured under helium gas pressure up to 2 kbar at two fixed temperatures, 77.3 and 300 K, using a pendulum magnetometer placed into the nonmagnetic pressure cell [11]. The relative errors of our measurements, performed in the magnetic field = 1.7H T, did not exceed 0.05% . In Fig. 1 the typical pressure dependencies of the mag- netic susceptibility for Ce(Ni1–xCux)5 alloys demonstrate a magnitude of the pressure effect and its linear behavior. For each temperature the values of χ at ambient pressure and their pressure derivatives, ln ( , ) / ,d x T dPχ are listed in Table 1. These values include corrections for a weak field dependence of χ caused by ferromagnetic impurities, which are less than 5%. The negative sign of the pressure effect is consistent with anticipation that high pressure has to increase the valence, since the Ce ion in the less magnet- ic higher valence state has a smaller volume. Table 1. The magnetic susceptibilities and their pressure de- rivatives for Ce(Ni1–xCux)5 alloys at 77.3 and 300 K x χ, 10–3 emu/mole dlnχ/dP, Mbar–1 T, K T, K 77.3 300 77.3 300 0.0 3.29 2.12 –2.72±0.3 –1.93±0.3 0.1 2.74 1.47 –3.41±0.4 –3.02±0.4 0.2 1.55 1.09 –4.55±0.4 –4.93±0.3 0.3 1.11 1.08 –13.0±0.5 –9.93±0.5 0.4 1.47 1.26 –17.1±1 –9.5±0.5 0.5 3.67 1.87 –11.8±0.5 –5.52±0.5 0.6 7.85 2.47 –6.63±0.5 –3.28±0.3 0.7 9.55 2.78 –3.8±0.3 –2.03±0.3 0.9 9.93 2.76 –1.42±0.2 –1.26±0.2 Of particular interest is a strong and nonmonotonous concentration dependence of the pressure effect which shows a sharp maximum in vicinity of 0.4x for both temperatures, 77.3 and 300 K (Fig. 2,a). A comparison be- tween the obtained experimental results and the data on con- centration dependence of the lattice parameter a and the effective Ce valence ν from Ref. 1 (Fig. 2,b) indicates that the maximum in ln ( , ) /d x T dPχ correlates with a drastic change of a (and )ν around 0.4x ( 3.5).ν Fig. 1. Pressure dependence of the magnetic susceptibility of Ce(Ni1–xCux)5 alloys at T = 77.3 K (a) and 300 K (b) normalized to its value at = 0.P 0.97 0.98 0.99 1.00 = 0x 0.3 0.4 0.5 b a 0 0.5 1.0 1.5 2.0 0.98 0.99 1.00 T = 77.3 K T = 300 K P, kbar � � ( )/ (0 ) P Fig. 2. Pressure derivative of the magnetic susceptibility ln /d dPχ in Ce(Ni1–xCux)5 alloys at 77.3 and 300 K (a). Devia- tion of the a lattice parameter, ,aΔ in Ce(Ni1–xCux)5 alloys from the ( )a x dependence for the Ce ion assumed to be in a trivalent state (left scale) and the Ce valence deduced from x-ray absorption studies (right scale) at room temperature versus Cu content x (according to the data of Ref. 1) (b). –20 –16 –12 –8 –4 0 a 77.3 K 300 K 0 0.2 0.4 0.6 0.8 1.0 –0.10 –0.08 –0.06 –0.04 –0.02 0 b �a x 4.0 3.8 3.6 3.4 3.2 3.0 Ce valence 0 0.2 0.4 0.6 0.8 1.0 C e va le nc e d d P ln / , M ba r � –1 � a , Å G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, and P. Svoboda 1064 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10 It should be noted that a similar peculiarity in ln /d dPχ versus valence was observed for various Yb compounds at room temperature [12]. As was shown, the relative change of χ with pressure appeared to be the most pronounced also for the half-integer value of valence, 2.5,ν but contrary to the Ce compounds, the pressure effect in Yb compounds has a positive sign. 3. Electronic structure and magnetic properties of CeNi5 Ab initio calculations of the electronic structure were carried out for the CeNi5 compound by employing a FP– LMTO method [13,14]. The exchange-correlation potential was treated in the LSDA approximation [15] of the density functional theory. In order to analyze the observed magne- to-volume effect value in CeNi5, the magnetic susceptibili- ty and its volume dependence were calculated within the modified method, wherein the external magnetic field H was taken into account by means of the Zeeman operator, ˆˆ(2s l).H + The latter was incorporated in FP–LMTO Ha- miltonian [16–18] for calculations of the field-induced spin and orbital magnetic moments. The corresponding contri- butions to magnetic susceptibility, spinχ and orb ,χ were derived from these field-induced moments, which have been calculated in an external magnetic field of 10 T. This field was applied both parallel and perpendicular to the c axis, providing the components of anisotropic magnetic susceptibility, χ and ,⊥χ respectively. The electronic structure calculations were performed for a number of lattice parameters a close to the experi- mental one. However, in doing so the /c a ratio was fixed at its experimental value 0.8226. The equilibrium lattice spacing th =a 8.96 a.u. and corresponding theoretical bulk modulus th =B 1.9 Mbar were determined from depen- dence of the total energy on the unit cell volume, ( ),E V by using the Murnaghan equation [13]: 1 0 0 coh 0 ( / ) ( ) = . 1 1 BBV V V V BE V E B B V B ′−⎛ ⎞′ + + −⎜ ⎟⎜ ⎟′ ′ ′− −⎝ ⎠ (1) By this way the bulk properties, such as the equilibrium volume 0 ,V the bulk modulus ,B and its first pressure derivative B′ are directly related to the equation (1). Here cohE is the cohesive energy and it is treated as an adjusta- ble parameter. The Murnaghan equation is based on the assumption that the pressure derivative B′ of the bulk modulus B is constant. By using the evaluated from the Murnaghan equation value of = 3.73,B′ we have esti- mated est th =B 1.45 Mbar, corresponding to the experimen- tal exp =a 9.2 a.u. [1]. This correction counterbalances the well known over-bonding tendency of the LSDA approach ([13,14]), and provides a nice agreement with the available experimental value, exp =B 1.43 Mbar [19]. The strongly volume dependent spin contribution to susceptibility spinχ originates predominantly from the 3d-states of Ni. Regarding the orbital contribution orb ,χ it comes mainly from conduction electrons in the atomic sphere of Ce and amounts to about 20% of total suscepti- bility. One would expect that the anisotropy of the suscep- tibility in the non-magnetic hexagonal CeNi 5 compound is predominantly due to the orbital Van Vleck-like contribu- tion orb.χ The calculated anisotropy is found to be small, 40.5 10−Δχ ⋅ emu/mole, in agreement with our data and that of Ref. 1. At the theoretical lattice parameter the averaged value of the calculated susceptibility, = ( 2 ) / 3 =⊥χ χ + χ 32.9 10−= ⋅ emu/mole, appeared to be very close to the experimental value of 33.0 10−⋅ emu/mole at = 0T K [2,4]. The calculated volume derivative of susceptibility, ln / lnd Vχ = 4.2, is in agreement with that resulted from our experimentally observed pressure derivative for CeNi5 at = 77.3T K, ln / ln 3.9 0.4.d d Vχ = ± Thus it is demon- strated, that LSDA provides an adequate description of the strongly exchange enhanced magnetic susceptibility of CeNi5 and its pressure dependence. 4. Magnetic properties of Ce(Ni1–xCux)5 alloys As is demonstrated, the LSDA allows to describe the magnetic susceptibility and its volume dependence in the reference CeNi5 compound. This gives grounds for future application of ab initio approaches to evaluate the itinerant background paramagnetism 0χ in Ce(Ni1–xCux)5 alloys, which is expected to decrease progressively with increas- ing of Cu content for 0.1,x ≥ according to the experimen- tal and calculated data on susceptibility in Y(Ni1–xCux)5 and La(Ni1–xCux)5 alloys [7]. At the present, however, we are unable to take into account in a rigorous way such im- portant effects in the susceptibility of alloys as disorder, crystal electric fields, and indirect interactions of the mo- ments. Therefore we restrict here our consideration of the experimental data in alloys within a phenomenological approach to examine effects of the localized magnetism. 4.1. Concentration dependence Assuming that pressure effect on magnetic susceptibili- ty arises mainly from the change of Ce valence ν , or the fractional occupation of the 4f 1-magnetic state 4fn 4( = 4 ),fnν − the pressure effect can be analyzed within a simple relation 4 4 ln ( ) ln ( ) ,f f dnd T T dP n dP χ ∂ χ ≈ ∂ (2) in terms of the pressure dependence of 4fn (or ).ν The most reliable results of such analysis would be expected in the Cu-rich alloys at low temperatures where the 4f-con- tribution 4fχ becomes dominant 4( ).fχ ≈ χ In Fig. 3,a the χ versus 4fn dependence is shown for Ce(Ni1–xCux)5 alloys (0.4 0.7)x≤ ≤ at 77.3 K, which was obtained by using the experimental ( )xχ values from Ta- ble 1 and the ( )xν data in Fig. 2,b. A substitution of the Pressure effect on magnetic properties of valence fluctuating system Ce(Ni1–xCux)5 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10 1065 resulted from Fig. 3,a derivatives 4ln / fn∂ χ ∂ and expe- rimental data on ln /d dPχ at 77.3 K into Eq. (2) gives the value of 4 /fdn dP which strongly depends on 4fn (Fig. 3,b). Based on the concentration behavior of valence ν and the effect of pressure in the susceptibility (Fig. 2), the maximum value of 4 /fdn dP is expected at 4 0.5fn ∼ ( 3.5)ν∼ and found by extrapolation of the data in Fig. 3,b to be about 6.5 1.5− ± Mbar–1. The corresponding estimates of the valence change under pressure, /d dPν = 4 / ,fdn dP= − are of the same order that those observed in other IV compounds, e.g., resulted from the study of the magnetovolume effect in SmB6 (2 Mbar–1 [20]) and from the measurements of resonant inelastic x-ray emission in YbAl2 under pressure ( 5∼ Mbar–1 [21]). 4.2. Temperature dependence In a simple empirical model [22] which includes inter- configuration fluctuations between 1-nf + and -levels ,nf the contribution of the 04 ( 0)f J = and 14 ( 5 / 2)f J = states of Ce to magnetic susceptibility is given by 2 4 4( ) = ( ) / 3 ( ).f A f fT N n T k T Tχ μ + (3) Here N is the Avogadro number, μ is effective magnetic moment of the 4f 1-state, fT is the characteristic tempera- ture (valence fluctuation temperature, or Kondo tempera- ture, or heavy-fermion bandwidth). It should be noted that a quantitative analysis of the 4 ( )f Tχ dependence using Eq. (3) requires the complete data on 4 ( )fn T (and proba- bly on ( )fT T as well) which are actually unavailable. Fur- thermore, in order to separate the 4 ( )f Tχ term from the experimental data on ( )Tχ one needs to know a back- ground contribution 0 ,χ which generally can not be neg- lected. A simplified analysis of the experimental data can be performed assuming 4 ,fn fT and 0χ to be tempera- ture independent. Then the magnetic susceptibility obeys a modified Curie–Weiss law, 0 4 0( ) = ( ) /( ) ,fT T C Tχ χ + χ ≡ χ + −Θ (4) with 2 4= / 3fC N n kμ and = .fTΘ − For the representa- tive Ce(Ni0.5Cu0.5)5 alloy, the best fit of Eq. (4) to the ex- perimental data [2] at 50T ≥ K (Fig. 4,a) is obtained with 3 0 = 0.6 10−χ ⋅ emu/mole, C = 0.48 K ⋅ emu/mole and Θ = = –79 K. It should be pointed out that the estimate 4 = 0.6,fn resulted from C , is in a reasonable agreement with the value of 0.8 that follows from the data in Fig. 2,b for = 0.5.x As is evident from Eq. (3), the pressure effect on the 4f-susceptibility is governed by changes of 4fn and fT with pressure, 4ln ( ) ln 1= ( ) f f f d T dTd C dP dP T T dP χ − ≡ + 4 4ln ( ) ,f f fd n T dT dP C dP χ ≡ − (5) being a linear function of (1 / ( )fT T+ or 4 ( ).f Tχ The data on 4ln /fd dPχ for the Ce(Ni0.5Cu0.5)5 alloy were derived from the measured effect, ln / ,d dPχ in the framework of Eq. (4) by using a value 0ln / ln 1d d Vχ ∼ as a rough estimate for the volume dependence of the un- enhanced background susceptibility [23], which is assumed to originate from the sp-3d(5d) hybridized itinerant elec- trons. The obtained values are plotted in Fig. 4,b as a func- tion of 4 ( ).f Tχ Its linear approximation in accordance with Eq. (5) gives 4 1lnln 3.2 0.7 Mbar ,fd nd C dP dP −= = − ± –11650 250 K Mbar .fdT dP = ± ⋅ (6) The resultant value 4 / 2.5 0.5fdn dP = − ± Mbar–1 is in line with the value 4 / 2.0 0.3fdn dP = − ± Mbar–1 obtained above for = 0.5x from analysis of the concentration de- pendence of the pressure effect within Eq. (2). From the Fig. 3. Values of lnχ at 77.3 K (a) and 4 /fdn dP (b) plotted against 4fn for Ce(Ni1–xCux)5 alloys. Symbols ( ) and ( )Δ denote the data obtained within Eq. (2) and Eq. (5), respectively. Points for = 0.45x are interpolation of the experimental data on concentration dependence of χ and ln / .d dPχ 0.4 0.6 0.8 1.0 0 1 2 0.45 0.5 x = 0.4 0.7 T = 77.3 K a Ce(Ni Cu )1– 5x x 0.6 0.4 0.6 0.8 1.0 0 2 4 6 b n4f = 4 – � – / , M ba r d n d P 4f –1 ln � G.E. Grechnev, A.V. Logosha, A.S. Panfilov, I.V. Svechkarev, O. Musil, and P. Svoboda 1066 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10 pressure dependence of fT the corresponding Grüneisen parameter, ,Ω is estimated to be ln ln 31 5 ln f f f d T d T B d V dP Ω ≡ − = = ± (7) using the experimental bulk modulus = 1.5B Mbar [24]. It should be noted that the Anderson impurity model provides the Kondo temperature and its pressure derivative to be mainly described in terms of 4fn [25–27]: 4 4 4 4 1 lnln 1, . 1 f fK K f f n d nd T T n dP n dP − ∝ = − − (8) Then, assuming f KT T∝ and using in Eq. (8) the values 4 / = 3.2 0.7fdn dP − ± Mbar–1 and 4 = 0.8fn evaluated above for the alloy with = 0.5x , one obtains ln = 24 5 ln K f K d T d V Ω Ω − = ± (9) in a reasonable agreement with the direct estimate (7). For Ce(Ni0.4Cu0.6)5 alloy, the analogous analysis in the framework of Eq. (4) and Eq. (5) yields the following Cu- rie–Weiss parameters: 0.806C K ⋅ emu/mole, 0 0,χ ∼ 26fT = −Θ = K, and their pressure derivatives: 4 1lnln 1.7 0.5 Mbar ,fd nd C dP dP −= = − ± –1620 100 K Mbar .fdT dP = ± ⋅ The latter results in the Grüneisen parameter = 35 6,fΩ ± assuming the bulk modulus value = 1.5B Mbar, as in the Ce(Ni0.5Cu0.5)5 alloy. Within the Anderson impurity mod- el, the similar estimate 36 10f KΩ Ω = ± follows from Eq. (8) with 4 0.93fn derived from the data in Fig. 2,b for = 0.6.x It should be noted that the evaluated values of KΩ are about half of that obtained for Ce(In1–xSnx)3 alloys, which exhibit similar regimes of localization of the f-states [28]. 5. Conclusions The pressure effect on magnetic susceptibility of Ce(Ni1–xCux)5 alloys was studied for the first time. This effect is negative in sign, and also strongly and nonmono- tonously dependent on the Cu content. For the reference CeNi5 compound, the pressure effect value is successfully described within LSDA approximation by using the mod- ified full potential relativistic FP–LMTO method. For Ce(Ni1–xCux)5 alloys the effects of pressure and alloying on the valence state of Ce ion are the most pronounced around 0.4x ∼ , which corresponds to the half-integer valence 3.5ν ∼ . In other words, the fractional occupation 4 0.5fn ∼ with the nearly degenerate f 0- and f 1-con- figurations of electronic states is favorable for the valence instability. It is also found that the main contributions to the pressure effect on magnetic susceptibility for the Cu- rich alloys are i) the decrease of the effective Curie con- stant and ii) the increase of the characteristic temperature .fT The latter exhibits a large and positive value of the Grüneisen parameter, which can be apparently described within the Anderson impurity model. Both of these contri- butions have their common origin in the change of the Ce valence state caused by depopulation of the f-state under pressure. However, only additional experimental and theo- retical studies could shed light on the relative contributions of two principle mechanisms of such depopulation, name- ly, the shift of the Ce 4f-energy level relative to the Fermi energy, or the broadening of this level. The authors dedicate this work to the 80th birthday an- niversary of V.G. Peschansky, who is one of pioneers in the field of magnetic properties studies in metallic systems. The work of P.S. and O.M. is a part of the research pro- gram MSM 0021620834 financed by the Ministry of Edu- cation of the Czech Republic. The authors thank V.A. Des- nenko for help in magnetic measurements. Fig. 4. Temperature dependence of the magnetic susceptibility χ (a) and pressure derivative 4ln /fd dPχ plotted against 4fχ (b) for Ce(Ni0.5Cu0.5)5 alloy. 0 50 100 150 200 250 300 0.2 0.4 0.6 0.8 T, K a 0 1 2 3 4 –16 –12 –8 –4 0 b , 10 emu/mol�4f –3 Ce(Ni Cu )0.5 0.5 5 – ln / , M ba r d d P � 4f –1 ( – ) , 1 0 m ol /e m u � �� 0 –1 3 Pressure effect on magnetic properties of valence fluctuating system Ce(Ni1–xCux)5 Low Temperature Physics/Fizika Nizkikh Temperatur, 2011, v. 37, Nos. 9/10 1067 1. D. Gignoux, F. Givord, R. Lemaire, H. Launois, and F. Sayetat, J. Phys. (Paris) 43, 173 (1982). 2. O. Musil, P. Svoboda, M. Diviš, and V. Sechovský, Czech. J. Phys. 51, D311 (2005). 3. N.B. Brandt, V.V. Moshchalkov, N.E. Sluchanko, E.M. Savitskii, and T.M. Shkatova, Solid State Phys. 26, 2110 (1984). 4. M. Coldea, D. Andreica, M. Bitu, and V. Crisan, J. Magn. Magn. Mater. 157/158, 627 (1996). 5. L. Nordström, M.S.S. Brooks, and B. Johansson, Phys. Rev. B46, 3458 (1992). 6. E. Burzo, V. Pop, and I. Costina, J. Magn. Magn. Mater. 157/158, 615 (1996). 7. G.E. Grechnev, A.V. Logosha, I.V. Svechkarev, A.G. Kuchin, P.A. Korzhavyi, and O. Eriksson, Fiz. Nizk. Temp. 32, 1498 (2006) [Low Temp. Phys. 32, 1140 (2006)]. 8. E. Bauer, M. Rotter, L. Keller, P. Fisher, M. Ellerby, and K.M. McEwen, J. Phys.: Condens. Matter 6, 5533 (1994). 9. I. Pop, R. Pop, and M. Coldea, J. Phys. Chem. Solids 43, 199 (1982). 10. I. Pop, E. Rus, M. Coldea, and O. Pop, J. Phys. Chem. Solids 40, 683 (1979). 11. A.S. Panfilov, Phys. Techn. High Press. 2, 61 (1992). 12. W. Zell, R. Pott, B. Roden, and D. Wohlleben, Solid State Commun. 40, 751 (1981). 13. J.M. Wills and O. Eriksson, Electronic Structure and Physical Properties of Solids, H. Dreysse (ed.), Springer, Berlin (2000), p. 247. 14. J.M. Wills, M. Alouani, P. Andersson, A. Delin, O. Eriksson, and A. Grechnev, Full-Potential Electronic Structure Method, Springer, Berlin (2010). 15. U. von Barth and L. Hedin, J. Phys. C5, 1629 (1972). 16. G.E. Grechnev, R. Ahuja, and O. Eriksson, Phys. Rev. B68, 64414 (2003). 17. G.E. Grechnev, Fiz. Nizk. Temp. 35, 812 (2009) [Low Temp. Phys. 35, 638 (2009)]. 18. А.В. Федорченко, Г.Е. Гречнев, А.С. Панфилов, А.В. Логоша, И.В. Свечкарев, В.Б. Филиппов, А.Б. Лященко, А.В. Евдокимова, ФНТ 35, 1106 (2009) [Low Temp. Phys. 35, 862 (2009)]. 19. B. Butler, D. Givord, F. Givord, and S.B. Palmer, J. Phys. C13, L743 (1980). 20. A.S. Panfilov, I.V. Svechkarev, Yu.B. Paderno, E.S. Konovalova, and V.I. Lazorenko, Phys. Techn. High Press. 20, 3 (1985). 21. C. Dallera, E. Annese, J.-P. Rueff, M. Grioni, G. Vanko, L. Braicovich, A. Barla, J.-P. Sanchez, R. Gusmeroli, A. Palenzona, L. Degiorgi, and G. Lapertot, J. Phys.: Condens. Matter 17, S849 (2005). 22. B.C. Sales and D. Wohlleben, Phys. Rev. Lett. 35, 1240 (1975). 23. V. Heine, Phys. Rev. 153, 673 (1967). 24. U. Staub, C. Schulze-Briese, P.A. Alekseev, M. Hanfland, S. Pascarelli, V. Honkimäki, and O.D. Chistyakov, J. Phys.: Condens. Matter 13, 11511 (2001). 25. O. Gunnarsson and K. Schönhammer, Phys. Rev. B28, 4315 (1983). 26. A.C. Hewson in: The Kondo Problem to Heavy Fermions, Cambridge University Press, Cambridge (1992). 27. J. Flouquet, A. Barla, R. Boursier, J. Derr, and G. Knebel, J. Phys. Soc. Jpn. 74, 178 (2005). 28. G.E. Grechnev, A.S. Panfilov, I.V. Svechkarev, A. Czopnik, W. Suski, and A. Hackemer, J. Phys.: Condens. Matter 9, 6921 (1997).

Pressure effect on magnetic properties of valence fluctuating system Ce(Ni₁₋xCux)₅

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