The Concentration Metamagnetic Transition in Tm 1 x Tb x Co 2 Compounds

Transcription

1 ISSN , Physics of the Solid State,, Vol. 8, No. 7, pp Pleiades Publishing, Inc.,. Original Russian Text E.A. Sherstobitova, A.F. Gubkin, A.A. Ermakov, A.V. Zakharov, N.V. Baranov, Yu.A. Dorofeev, A.N. Pirogov, A.A. Podlesnyak, V.Yu. Pomyakushin,, published in Fizika Tverdogo Tela,, Vol. 8, No. 7, pp MAGNETISM AND FERROELECTRICITY The Concentration Metamagnetic Transition in Tm 1 x Tb x Co Compounds E. A. Sherstobitova a, A. F. Gubkin a, b, A. A. Ermakov a, b, A. V. Zakharov b, N. V. Baranov a, b, Yu. A. Dorofeev a, A. N. Pirogov a, A. A. Podlesnyak c, and V. Yu. Pomyakushin c a Institute of Metal Physics, Ural Division, Russian Academy of Sciences, ul. S. Kovalevskoœ 18, Yekaterinburg, 19 Russia sherl@imp.uran.ru b Ural State University, pr. Lenina 51, Yekaterinburg, 83 Russia c Laboratory for Neutron Scattering, ETH Zürich & Paul Scherrer Institute, Villingen PSI, CH-53 Switzerland Received July 7, 5 Abstract The Tm 1 x Tb x Co ( x 1) system was studied by measuring the magnetic susceptibility, electrical resistance, and neutron diffraction. In the compounds with < x.15, an inhomogeneous magnetic state characterized by the existence of large regions (up to 1 Å in size) with short-range ferrimagnetic order was found to occur. The maximum of the residual electrical resistance observed in the compound with x =.1 at the magnetic ordering temperature was established to be due to the scattering of conduction electrons by localized spin fluctuations associated with f d exchange fluctuations caused by the substitution of terbium for thulium. The increase in the terbium concentration to x.15 leads to a sharp increase in the Co sublattice magnetization and the establishment of a long-range ferromagnetic order, which indicates a concentration metamagnetic transition in the band subsystem. PACS numbers: 75.3.Hx, y, 7.8. r, 1.1. q DOI: 1.113/S INTRODUCTION Cubic rare-earth Laves phases of the RCo type consist of two magnetic subsystems: one subsystem is formed by the localized f electrons of R ions, and the other consists mainly of the 3d-band electrons of cobalt. The behavior of the band subsystem of these compounds has been attracting the attention of researchers for more than twenty years, because the dependence of the Co sublattice magnetization on the effective magnetic field is metamagnetic in character. As the effective field reaches a critical value H c 7 T, the d band splits and the subsystem of itinerant d electrons changes suddenly from the paramagnetic to ferromagnetic state; i.e., a band metamagnetic transition (BMT) occurs [1]. Recently, a great magnetocaloric effect was revealed in RCo compounds [], which makes them promising materials for magnetic refrigerators. This discovery has stimulated a new upsurge of investigations of RCo - type systems. It is of interest that, among these systems, the magnetocaloric effect is the strongest in the compounds that undergo a BMT [3]. In terms of their magnetic properties, the RCo compounds can be divided into two groups. One group includes the compounds with R = Y and Lu, in which the rare-earth and cobalt ions do not exhibit an intrinsic magnetic moment. These compounds are exchangeenhanced Pauli paramagnets. The other group includes the compounds characterized by a long-range magnetic order with a ferromagnetic (for light R ions) or ferrimagnetic (for heavy R ions, except Tm) arrangement of the magnetizations of the rare-earth and cobalt sublattices. Depending on the R ion, the Co sublattice magnetization varies within the limits (.7 1.)µ B per Co atom. For example, the magnetic moment of a cobalt atom in TbCo is µ Co = 1.µ B []. The appearance of a ferrimagnetic order in TbO below the Curie temperature (T C = K) is accompanied by a transition from the cubic to rhombohedral structure. In this case, both the structural and magnetic transitions are of second order. The TmCo compound belongs neither to the first nor second group. In accordance with neutron diffraction data [5], the cobalt atoms in TmCo do not have a magnetic moment and the magnetic order in the Tm sublattice depends on the method of sample synthesis and on the purity of the initial components. In various TmCo samples at temperatures below T C K, a variety of magnetic states have been observed, ranging from incomplete ordering of the Tm magnetic moments to a collinear arrangement, including helicoidal structures. The magnetic moments of the Co atoms in TmCo can be zero because the effective field generated by the Tm f electrons is lower than the critical field H c necessary to split the 3d band. It has been established that a 131

2 13 SHERSTOBITOVA et al. T C, K 3 1 χ, arb. units 8 x = T, K χ, arb. units 3 1 x =.15 T, K TmCo x TbCo ρ, µω cm x = ρ, µω cm TmCo x TbCo 1 3 T, K Fig. 1. Concentration dependence of the magnetic ordering temperature for Tm 1 x Tb x Co. The insets show the temperature dependences of the ac magnetic susceptibility for x =.5 and.15. Fig.. Temperature dependences of the electrical resistivity of the Tm 1 x Tb x Co compounds for x.3. The inset shows the concentration dependence of the residual resistivity. partial replacement of Tm ions with R ions with a larger spin (R = Gd [] and Er [7]) results in the appearance of a magnetization in the Co sublattice. Therefore, we may expect that an increase in the Tb content in the Tm 1 x Tb x Co compounds to a certain critical value x c will lead to the appearance of a nonzero magnetic moment of the Co atoms; i.e., a concentration BMT will occur. It is of interest to study how this transition occurs and to determine the value of x c and the magnetizations of the R and Co sublattices. Note that systematic investigations of these compounds have not yet been performed. In this work, we studied in detail the structural and magnetic states of the Tm 1 x Tb x Co compounds by measuring the susceptibility, magnetization, electrical resistance, and neutron diffraction.. EXPERIMENTAL Polycrystalline samples of the Tm 1 x Tb x Co compounds ( x 1) were prepared by induction melting followed by homogenizing annealing at 85 C for 5 h. The samples were characterized using metallographic, x-ray diffraction, and neutron diffraction analyses. In all of the samples, the RCo phase with the MgCu structure was the basic phase. The content of impurity phases (RCo 3, R O 3 ) was lower than 5%. The magnetic susceptibility and magnetization of the samples were measured, using an MPMS SQUID magnetometer (Quantum Design, United States), in magnetic fields up to 5 T at temperatures of to 3 K at the Magnetometry Center of the Institute of Metal Physics (Ural Division, Russian Academy of Sciences). The temperature dependences of the electrical resistivity were measured by the four-probe potentiometric method on samples ~1 1 mm in size. Neutron diffraction studies were carried out using a D-3 diffractometer (IVV-M reactor, Zarechnyœ) at a wavelength λ =.3 Å and DMC (λ = 3.8 Å) and HRPT (λ = 1.9 Å) diffractometers (Paul Scherrer Institute). Neutron diffraction patterns were calculated using the FullProf software [8]. 3. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 shows the concentration dependence of the magnetic ordering temperature T C (x) for the Tm 1 x Tb x Co compounds. The temperature T C was determined from the position of the maximum for the temperature dependences of the magnetic susceptibility χ(t). For x =.5 and.15, these dependences are shown in the insets to Fig. 1. Two ranges can be distinguished in the T C (x) dependence. At < x.15, a partial replacement of thulium with terbium leads to a weak increase in T C. A further increase in the concentration x (.15 x 1.) leads to a sharper rise of T C. The change in the slope of the T C (x) dependence near x =.15 can be related to a change in the magnetic state of the compounds. Note that an analogous change in T C was observed in the Y 1 x Gd x Co system as the gadolinium concentration increased [9]. This behavior was assumed in [9] to be due to the transition from the cluster glass state at x <.15 (where the compound consists of a paramagnetic matrix and clusters with short-range ferrimagnetic order) to a long-range ferrimagnetic ordering of the Gd and Co magnetic moments at a higher Gd concentration. The measurements of the electrical resistance also indicate a change in the magnetic state of the Tm 1 x Tb x Co compounds as the terbium concentration increases to x.3. Figure shows the temperature

3 THE CONCENTRATION METAMAGNETIC TRANSITION 133 Space group of the Tm 1 x Tb x Co compounds above and below the ordering temperature T > T C (Fd3m) T < T C (R-3m) atom positions atom positions R (Tm, Tb) 8a (1/8 1/8 1/8), (7/8 7/8 7/8) R (Tm, Tb) c (x xx) ( x x x) Co 1d (1/ 1/ 1/) Co 1b (1/ 1/ 1/) (1/ 1/ 1/) (1/ 1/ 1/) Co 3e ( 1/ 1/) (1/ 1/ 1/) (1/ 1/) (1/ 1/ ) dependences of the electrical resistivity ρ(t) for Tm 1 x Tb x Co (x.3). In the paramagnetic region, all of the curves show a tendency toward saturation of the electrical resistivity as the temperature increases, which is characteristic of the RCo compounds. The magnetic contribution to the electrical resistivity of RCo with magnetoactive R ions is due to scattering of the conduction electrons by the localized f electrons and thermal spin fluctuations in the band subsystem, with the latter mechanism being predominant [1]. It is the nonmonotonic variation in the spin-fluctuation contribution with temperature that is responsible in these compounds for the tendency of the resistivity toward saturation with increasing temperature. In contrast to the paramagnetic region, where substituting terbium for thulium in the Tm 1 x Tb x Co system does not change the ρ(t) dependence, the behavior of the electrical resistivity with variations in the concentration x changes qualitatively at temperatures T < T C. As seen from Fig. (inset), the increase in the Tb concentration to x =.1 causes a significant increase in the residual resistivity ρ and the ρ(t) dependences exhibit a pronounced minimum, which is absent for the compounds with x >.15. The minimum in the ρ(t) curves was also observed in quasi-binary Y 1 x R x Co compounds with R = Gd [9], Tb [1], Ho, and Er [11] at concentrations near the critical value at which the magnetic moment of the Co atoms becomes nonzero in these systems. The existence of this minimum in Y 1 x R x Co is explained by a superposition of contributions to the resistivity that have opposite tendencies with variations in temperature. On cooling a sample, the contribution from scattering by phonons and thermal spin fluctuations to the total resistivity decreases, whereas the contribution from scattering by localized spin fluctuations in the 3delectron subsystem caused by fluctuations in the f d exchange due to an inhomogeneous substitution and correlations of the short-range order in the R sublattice increases []. The latter is supported by the neutron diffraction data for Y 1 x R x Co compounds at x < x c, which revealed a correlation between the resistivity and the intensity of low-angle neutron scattering [11]. As seen from Fig., in the Tm 1 x Tb x Co compounds, ρ decreases significantly as the Tb content increases above x =.1. In this case, the ρ(t) dependence becomes similar to that observed in the ErCo and HoCo compounds undergoing a first-order phase transition at T C. This transition to an ordered magnetic state is accompanied by a sharp decrease in the electrical resistivity, which is related to the suppression of the contribution from thermal spin fluctuations. The nonmonotonic variation in the residual electrical resistivity with a sharp maximum at a concentration slightly lower than the critical value we observed in the Tm 1 x Tb x Co system was detected earlier in the Y 1 x R x Co compounds in [9]. As was shown in [9], the contribution from scattering by localized spin fluctuations to the residual resistivity at x < x c depends on the spin of the substituted R ion. The above results of the study of the magnetic susceptibility and resistivity of the Tm 1 x Tb x Co compounds, in combination with the available published data for Y 1 x R x Co, indicate that the magnetic state undergoes a qualitative change near the critical concentration x c.15. Direct evidence of this change was obtained from neutron diffraction studies (see below). At T > T C, neutron diffraction patterns were obtained for all of the Tm 1 x Tb x Co samples. As an example, Fig. 3 shows the neutron diffraction pattern of the Tm.9 Tb.1 Co compound recorded at 5 K. An analysis of this pattern showed that the sample is single-phase and has the cubic MgCu structure (space group Fd3m). On replacing thulium ions with terbium, the cubic structure of the samples is retained but the lattice parameters somewhat increase. Under cooling below T C, the samples underwent a transition from the cubic to rhombohedral structure (R- 3m). On the rhombohedral lattice, the R ions occupy the position c and the Co atoms occupy two nonequivalent positions, 1b and 3e (see table). At 11 K, the neutron diffraction pattern for Tm.9 Tb.1 Co (Fig. 3) contains nuclear Bragg reflections (with an instrumental width) and broad maxima of

4 13 SHERSTOBITOVA et al. 1 8 Intensity, 1 3 counts T =. K 11 K 5 K 1 1 θ, deg Fig. 3. Neutron diffraction patterns for the Tm.9 Tb.1 Co at., 11, and 5 K (wavelength λ = 1.9 Å). The points are experimental data, and the lines are calculated data. The bars at the bottom indicate the positions of the nuclear and magnetic reflections. the magnetic scattering. At. K, the intensity of the Bragg reflections increases sharply due to an increase in the magnetic contribution caused by an enhanced magnetic order. The wave vector of the magnetic structure is k =. The R-sublattice magnetization is µ R = 1.3(1)µ B. Unfortunately, we failed to reliably establish the value of the Co-sublattice magnetization. We can only assert that it is less than.3µ B at x =.1. The broad maxima of the magnetic scattering observed in the neutron diffraction patterns for the Tm.9 Tb.1 Co compound at T < T C can be explained in terms of the inhomogeneous distribution of Tb ions over the lattice. As shown in [1], the field H eff in TmCo is T, which is lower than the critical field H c for splitting of the 3d band. Conceivably, the substitution of a Tb ion having spin S = 3 for a Tm ion whose spin is S = 1 leads to a nonzero magnetic moment of the nearest neighbor Co atoms and the occurrence of a localized spin-density fluctuation in the 3d-electron subsystem. The exchange-interaction mechanism responsible for this effect involves the intraatomic f 5d exchange (which causes a spin polarization of the Tb 5d electrons) and the 5d 3d exchange. If there is a partial 5d 3d hybridization, then the 5d 3d exchange causes an inverse polarization of the Co 3d electrons. As a result, antiparallel ordering of the spins of the f ions and Co atoms arises in a region near a Tb ion. Estimation of the size of these regions from the half-width of the magnetic-scattering peaks for Tm.9 Tb.1 Co gives a value of about 1 Å. It should be noted that this value substantially exceeds the size of magnetic clusters detected near the critical concentration in the Y 1 x R x Co compounds. An analogous estimation from the magnetic diffuse scattering data for the Y 1 x Er x Co and Y 1 x Ho x Co systems yields ~3 Å [11]. The difference in the sizes of these regions is related, in our opinion, to the fact that the Tm ions, unlike the Y ions, have a localized magnetic moment. Therefore, the Co 3d Tm f exchange interaction causes the correlation in the magnetic-moment arrangement in the vicinity of a Tb ion in Tm.9 Tb.1 Co to extend to great distances. As the Tb concentration in Tm 1 x Tb x Co increases above x =.1, a long-range ferrimagnetic order arises in the sample and the fraction of clusters with a shortrange order decreases. As a result, there appears a contribution from magnetic scattering to the Bragg reflections and the intensity of the diffuse maxima decreases. The evolution of the magnetic scattering with increasing x can be seen from the neutron diffraction patterns shown in Fig. for the compounds with x =,.3,., and 1. at. K.

5 THE CONCENTRATION METAMAGNETIC TRANSITION (111) () Intensity, 1 3 counts 1 (311) () () x = 1.. (331) θ, deg Fig.. Neutron diffraction patterns for Tm 1 x Tb x Co with x =,.3,., and 1. at. K (wavelength λ =.3 Å). The designations are the same as in Fig. 3. Calculations of the neutron diffraction patterns show that the magnetizations of the rare-earth and Co sublattices are oriented antiparallel in the samples studied (except in the samples with x = ). The magnetic moments of the Co atoms in the 1b and 3e positions differ, if at all, by less than.1µ B, which agrees with the experimental data for TbCo from []. It was assumed that, in the intermediate compositions, the easy-magnetization axis is [111] (in terms of the rhombohedral unit cell), as is the case in the extreme compositions. Figure 5 shows the concentration dependences of the average magnetic moments of the Tb and Co ions. These dependences can be interpreted as follows. The increase in the Tb concentration to x c.15 leads to an increase in the average exchange field of the rare-earth subsystem to a value higher than H c, and the 3d-band subsystem undergoes a transition to an ordered magnetic state, which is a first-order phase transition (BMT). Figure shows the field dependences of the spontaneous magnetization measured in fields of up to 5 T for the Tm 1 x Tb x Co compounds. The concentration dependence of the total magnetic moment obtained from these data is shown in Fig. 7. For the sake of comparison, Fig. 7 also shows an analogous dependence obtained from the neutron diffraction data (in the absence of an external field). The discrepancy between µ Co, µ B µ R, µ B (a) (b) Fig. 5. Concentration dependences of the magnetization of (a) the cobalt and (b) rare-earth sublattices.

6 13 SHERSTOBITOVA et al. µ, µ B /f. u x = Magnetic field, T Fig.. Field dependences of the magnetization of the Tm 1 x Tb x Co compounds at K. µ, µ B /f. u. Magnetization data Neutron data x Fig. 7. Concentration dependences of the magnetic moment (per formula unit) for the Tm 1 x Tb x Co compounds obtained from magnetic and neutron-diffraction measurements. the values of the total magnetic moment is the largest for the compositions with x.15, which is due to the strong effect of the external field on the magnetic state of the compounds. The external field (5 T) induces a BMT in the Co sublattice, and, owing to the R Co exchange, the magnetization of the R sublattice likewise increases. As a result, the total magnetic moment of the compound increases significantly. The discrepancy between the data obtained from the magnetic measurements and from analyzing the neutron diffraction patterns for compounds with x. is due to the high magnetocrystalline anisotropy of Tm 1 x Tb x Co. According to the measurements performed on TbCo single crystals [13], the magnetization does not reach its saturation value even in a field of 13 T, which far exceeds the magnetic fields used in this work. Moreover, the situation is complicated by the fact that the anisotropy energy seems to vary with concentration.. CONCLUSIONS We have performed complex investigations of the Tm 1 x Tb x Co intermetallic compounds and have shown that the replacement of thulium ions with terbium to a critical concentration x c.15 leads to a band metamagnetic transition; as a result, a long-range magnetic order occurs in the Co 3d-electron subsystem and the magnetic moment per Co ion reaches a value of ~1 µ B. This behavior is a consequence of an increase in the effective field exerted on the Co 3d subsystem by the rare-earth sublattice, since the Tb ion has a substantially larger spin (S = 3) than the Tm ion has (S = 1). In the compounds with x <.15, the effective field is less than the critical field (H c 7 T) above which a longrange magnetic order is possible in the Co sublattice. In the compositions with x.15, the exchange field exceeds H c ; as a result, the cobalt sublattice is in an ordered magnetic state. Furthermore, the cobalt sublattice increases the magnetic order in the Tm sublattice. In the concentration range < x.15, the magnetic state of the Tm 1 x Tb x Co compounds is nonuniform and characterized by the existence of large regions (up to 1 Å in size) of a short-range ferrimagnetic order in the vicinity of the Tb ions. The variation in the magnetic state of the Tm 1 x Tb x Co compounds as x increases to x c.15 results in an increase in the slope of the T C (x) dependence and in a qualitative change in the behavior of the temperature dependence of the electrical resistivity. The minimum in the temperature dependence of the resistivity at < x <.15 and the nonmonotonic concentration dependence of the residual resistivity with a sharp maximum at x =.1 are related to the existence of localized spin fluctuations in the Co 3d-electron subsystem caused by the fluctuations in the f d exchange due to a partial substitution of terbium ions for thulium. ACKNOWLEDGMENTS This work was supported in part by the Swiss National Scientific Foundation (SCOPES grant no. 7 IP 5598), the Department of Physical Sciences of the Russian Academy of Sciences (program Neutron Studies on Material Structure and the Fundamental Properties of Matter, project no. 1, Ural Division, RAS, contract no. 11/5), and the Russian Foundation for Basic Research (project Ural no ). REFERENCES 1. E. Gratz and A. S. Markosyan, J. Phys.: Condens. Matter 13 (3), R385 (1).

7 THE CONCENTRATION METAMAGNETIC TRANSITION 137. N. H. Duc and D. T. Kim Anh, J. Magn. Magn. Mater. 5 Part, 873 (). 3. N. H. Duc, D. T. Kim Anh, and P. E. Brommer, Physica B (Amsterdam) 319 (1), 1 ().. Z. W. Ouyang, F. W. Wang, Q. Hang, W. F. Liu, G. Y. Lin, J. W. Lynn, and J. K. Liang, J. Alloys Compd. 39 (1 ), 1 (5). 5. I. V. Golosovsky, B. E. Kviatkovsky, S. V. Sharygin, I. S. Dubenko, R. Z. Levitin, A. S. Markosyan, E. Gratz, I. Mirebeau, I. N. Goncharenko, and F. Bouree, J. Magn. Magn. Mater. 19 (), 13 (1997).. E. Gratz, R. Hauser, A. Lindbaum, M. Maikis, R. Resel, G. Schaudy, R. Z. Levitin, A. S. Markosyan, I. S. Dubenko, A. Yu. Sokolov, and S. W. Zochowski, J. Phys.: Condens. Matter 7 (3), 597 (1995). 7. R. Hauser, E. Bauer, E. Gratz, H. Muller, M. Rotter, H. Michor, and G. Hilscher, Phys. Rev. B: Condens. Matter (), 1198 (). 8. J. Rodriguez-Carvajal, Physica B (Amsterdam) 19 (1 ), 55 (1993). 9. N. V. Baranov, A. A. Yermakov, and A. A. Podlesnyak, J. Phys.: Condens. Matter 15 (31), 5371 (3). 1. N. V. Baranov, A. A. Yermakov, A. N. Pirogov, A. E. Teplykh, K. Inoue, and Yu. Hosokoshi, Physica B (Amsterdam) 9 (3), 8 (1999). 11. N. V. Baranov and A. N. Pirogov, J. Alloys Compd. 17 (1), 31 (1995). 1. P. E. Brommer, I. S. Dubenko, J. J. M. Franse, R. Z. Levitin, A. S. Markosyan, R. J. Radwanski, V. V. Snegirev, and A. Yu. Sokolov, Physica B (Amsterdam) 183 (), 33 (1993). 13. D. Gignoux, F. Givord, R. Perrier, and F. Sayetat, J. Phys. F: Met. Phys. 9 (5), 73 (1979). Translated by Yu. Ryzhkov