- Email: [email protected]
59Co NMR study of (U,Nd)Co2Ge2
Physica B 312–313 (2002) 280–282
59
Co NMR study of (U,Nd)Co2Ge2
E.N. Caspia,b,*, M. Melamudc, H. Shakedb, A.I. Shamesb, S.D. Gorenb b
a Nuclear Research Centre–Negev, POBox 9001, 84190 Beer-Sheva, Israel Physics Department, Ben-Gurion University of the Negev, POBox 653, 84105 Beer-Sheva, Israel c Israel Atomic Energy Commission, POBox 7061, 61070 Tel-Aviv, Israel
Abstract 59
Co NMR of (U1xNdx)Co2Ge2 (x=0, 0.25, 0.5, 0.75, 1) was studied in an external field of 8.0196 T in the temperature range of 5–300 K. Three well distinguished regions, corresponding to the paramagnetic, intermediate, and antiferromagnetically ordered states of these compounds, were observed, in a good agreement with neutron diffraction, AC-susceptibility, and SQUID magnetization measurements. Using the results for paramagnetic region, the coefficients of indirect exchange interaction between conduction and f-electrons in these compounds were determined as 0.06(3) and 0.16(9) eV, for U and Nd, respectively. r 2002 Elsevier Science B.V. All rights reserved. Keywords: NMR; RKKY interaction; Exchange interactions; Uranium compounds
Compounds of the AM2X2 system [A=U, lanthanide (Ln); M=Co,Ni,Cu; X=Si,Ge] crystallize in the bodycentered tetragonal ThCr2Si2-type structure, and order magnetically in a variety of magnetic structures, in which only A atoms (f-electrons) have ordered magnetic moments [1–3]. The type of the magnetic ordering of the A atoms is determined by the M atoms (d-electrons). Mostly, the magnetic order consists of ferromagnetic A planes stacked antiferromagnetically with different sequences [1,3]. We have previously studied the magnetic and electronic nature of the U atom, and the d2f itinerant magnetic interaction, using an 1D-isotropic RKKY model. The most important conclusion of this study is that U atoms behave magnetically like light Ln atoms in this system [4,5]. As a complementary study of the magnetic interactions, governing the AM2X2 system, we performed 59Co NMR measurements on previously prepared samples of (U1xNdx)Co2Ge2 (x=0, 0.25, 0.5, 0.75, 1) [3,4] and LaCo2Ge2 compounds the latter being a non-magnetic Ln reference. The 59Co NMR spectra of polycrystalline samples were obtained using Tecmag Libra Solid State Pulsed *Corresponding author. Tel.: +1-630-252-9760; fax: +1630-252-7777. E-mail address: [email protected] (E.N. Caspi).
NMR Spectrometer using a superconducting magnet (H0 ¼ 8:0196 T). NMR measurements were obtained using a variable frequency probe within the temperature ranges of 5–300 K (RT) for (U,Nd)Co2Ge2 compounds, and 70 K–RT for LaCo2Ge2. Each point in the spectra (e.g. Fig. 1) is an accumulate of 10/100 K spin-echo signals collected using the 2.5/8/5 ms 16 phase cycled Hahn echo sequence with a repetition rate of 1.5 ms. The RT NMR spectra consist of 7 lines (e.g. Fig. 1), attributed to the quadrupole splitting of the 59Co nuclear spin I ¼ 7=2 with tetragonal site symmetry, having a quadrupole splitting of Dnq B1 MHz. This main spectrum is observed for all compounds. An additional set of 7 lines with a quadrupole splitting of Dnq B0:5 MHz, and an integrated intensity of 40% of the entire spectrum is observed for x ¼ 0: Due to the crystallographic phase separation observed in the x ¼ 0:25 sample [6] into x ¼ 0:1 and 0.4 solid solutions, its spectrum is more complicated and may be deconvoluted into two spectra: the x ¼ 0 type double spectrum from the x ¼ 0:1 phase, and the single 7 line set spectrum from the x ¼ 0:4 phase. The RT NMR lines of the solid solutions (0oxo1) are broader than the corresponding lines of the x ¼ 0; 1 and LaCo2Ge2 compounds. LaCo2Ge2 shows no significant change throughout the whole temperature range from 70 K to RT (Fig. 2). For all other compounds, in the paramagnetic region
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 5 2 2 - 8
E.N. Caspi et al. / Physica B 312–313 (2002) 280–282
3.5x105
Echo amplitude (a.u.)
3.0x105 2.5x105
UCo2 Ge2 H0 = 8.0196 T 5K
2.0x105 1.5x105 1.0x105 RT
5.0x104 0.0 78
79
80
81
82
83
84
85
86
Frequency (MHz) Fig. 1. 59Co NMR spectra of UCo2Ge2 at RT and 5 K. The solid lines are best fit of two sets of seven Lorentzians each, one for RT, and another for 5 K.
Fig. 2. Central line frequency, fcenter ; vs. 1=T: The solid lines are best linear fits of the data.
below RT, the resonance frequencies of the NMR central lines of the main spectrum increase linearly with 1=T (e.g. Fig. 2). For x=0, 0.25, 0.5, 0.75 the NMR lines of the main spectrum become smeared out in the vicinity of their corresponding magnetic transition temperatures, as obtained by neutron diffraction and susceptibility measurements [3,6]. Such a smearing may be attributed to fast fluctuations of the magnetic moments in the vicinity of the magnetic transition whose frequencies are comparable with the quadrupole splitting (B1 MHz). Below their magnetic transition temperatures, for x=0.25, 0.5, 0.75, the 7 lines are unresolved, while for x ¼ 0 the spectrum with Dnq B0:5 MHz reappears (Fig 1). For x ¼ 1 we were unable to observe the NMR echo signal below 75 K.
281
The existence of two separate sets of 7 lines in UCo2Ge2 at RT indicates the existence of two types of Co atoms. Yet, there is only one non-equivalent Co site in UCo2Ge2, and no Co impurity bigger than 5 w/o was detected by X-ray and neutron diffraction [3]. Thus, we propose that the existence of regions with Co2+ ions and other regions with Co3+ ions throughout the sample cause the double spectrum in UCo2Ge2 at RT. This could be due to the ill-defined valence state of U atoms which affects the valence state of the Co atoms [8]. Correspondingly, for NdCo2Ge2 and LaCo2Ge2, only one set of 7 lines is observed in agreement with the single valence state of Nd and La. The double spectrum phenomenon in UCo2Ge2, as well as its temperature dependence is presently under study. Unlike Ref. [7], our results for LaCo2Ge2 show temperature-dependent magnetic bahavior in AC-susceptibility and SQUID measurements (to be published), exhibiting a paramagnetic moment of B1mB on Co atoms, similar to its value for NdCo2Ge2 [4]. Yet, NMR measurements on LaCo2Ge2 show no such temperaturedependent behavior (Fig. 2). Thus, we conclude that the linear behavior of central line frequency vs. 1=T depicted in Fig. 2 is solely characteristic of an indirect exchange interaction between conduction ðdÞ and localized ðfÞ electrons in these compounds [9]. From the positive slope of the best fit lines (Fig. 2) and our previous result that the U behaves like a light Ln [4], we conclude that the sign of exchange coefficient between the conduction and f-electrons, Jcf ; is negative [9] for all compounds. In addition to the linear central line frequency dependence on 1=T (Fig. 2) a universal linear dependence of fcenter on the measured magnetic susceptibility, wM ; is observed for all compounds in the paramagnetic state: fcenter ¼ 81:32ð3Þ þ 0:134ð4ÞwM : Combining this dependence with both fcenter ð1=TÞ (Fig. 2) and NMR results of LaCo2Ge2 (Fig. 2), one can estimate the following values for U and Nd exchange coefficients: Jcf ðUÞ ¼ 0:06ð3Þ eV, Jcf ðNdÞ ¼ 0:16ð9Þ eV [9]. The Jcf ðNdÞ value in NdCo2Ge2 is close to its value in NdAl2 [9]. The result that Jcf2 ðNdÞ >Jcf2 ðUÞ; whereas TN (NdCo2Ge2)oTN (UCo2Ge2) [4], as well as a negative Jcf value is a characteristic feature of the Kondo effect shown to exist in similar compounds [10]. This effect should be taken into consideration [10] in future RKKY treatment of these compounds at high temperatures.
References [1] H. Pinto, et al., Phys. Rev. B 31 (1985) 508. [2] A. Szytu"a, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991, p. 85. [3] M. Kuznietz, et al., Phys. Rev. B 40 (1989) 7328.
282 [4] [5] [6] [7]
E.N. Caspi et al. / Physica B 312–313 (2002) 280–282 E. Caspi, et al., Phys. Rev. B 57 (1998) 449. E.N. Caspi, et al., Phys. Rev. B 62 (2000) 9418. E.N. Caspi, et al., Physica B 276–278 (2000) 604. W.H. McCall, et al., J. Appl. Phys. 44 (1973) 4724.
[8] B.R. Cooper, et al., J. Alloys Compounds 192 (1993) 223. [9] V. Jaccarino, et al., Phys. Rev. Lett. 5 (1960) 251. [10] T. Endstra, et al., J. Appl. Phys. 69 (1991) 4810.
59
Co NMR study of (U,Nd)Co2Ge2
E.N. Caspia,b,*, M. Melamudc, H. Shakedb, A.I. Shamesb, S.D. Gorenb b
a Nuclear Research Centre–Negev, POBox 9001, 84190 Beer-Sheva, Israel Physics Department, Ben-Gurion University of the Negev, POBox 653, 84105 Beer-Sheva, Israel c Israel Atomic Energy Commission, POBox 7061, 61070 Tel-Aviv, Israel
Abstract 59
Co NMR of (U1xNdx)Co2Ge2 (x=0, 0.25, 0.5, 0.75, 1) was studied in an external field of 8.0196 T in the temperature range of 5–300 K. Three well distinguished regions, corresponding to the paramagnetic, intermediate, and antiferromagnetically ordered states of these compounds, were observed, in a good agreement with neutron diffraction, AC-susceptibility, and SQUID magnetization measurements. Using the results for paramagnetic region, the coefficients of indirect exchange interaction between conduction and f-electrons in these compounds were determined as 0.06(3) and 0.16(9) eV, for U and Nd, respectively. r 2002 Elsevier Science B.V. All rights reserved. Keywords: NMR; RKKY interaction; Exchange interactions; Uranium compounds
Compounds of the AM2X2 system [A=U, lanthanide (Ln); M=Co,Ni,Cu; X=Si,Ge] crystallize in the bodycentered tetragonal ThCr2Si2-type structure, and order magnetically in a variety of magnetic structures, in which only A atoms (f-electrons) have ordered magnetic moments [1–3]. The type of the magnetic ordering of the A atoms is determined by the M atoms (d-electrons). Mostly, the magnetic order consists of ferromagnetic A planes stacked antiferromagnetically with different sequences [1,3]. We have previously studied the magnetic and electronic nature of the U atom, and the d2f itinerant magnetic interaction, using an 1D-isotropic RKKY model. The most important conclusion of this study is that U atoms behave magnetically like light Ln atoms in this system [4,5]. As a complementary study of the magnetic interactions, governing the AM2X2 system, we performed 59Co NMR measurements on previously prepared samples of (U1xNdx)Co2Ge2 (x=0, 0.25, 0.5, 0.75, 1) [3,4] and LaCo2Ge2 compounds the latter being a non-magnetic Ln reference. The 59Co NMR spectra of polycrystalline samples were obtained using Tecmag Libra Solid State Pulsed *Corresponding author. Tel.: +1-630-252-9760; fax: +1630-252-7777. E-mail address: [email protected] (E.N. Caspi).
NMR Spectrometer using a superconducting magnet (H0 ¼ 8:0196 T). NMR measurements were obtained using a variable frequency probe within the temperature ranges of 5–300 K (RT) for (U,Nd)Co2Ge2 compounds, and 70 K–RT for LaCo2Ge2. Each point in the spectra (e.g. Fig. 1) is an accumulate of 10/100 K spin-echo signals collected using the 2.5/8/5 ms 16 phase cycled Hahn echo sequence with a repetition rate of 1.5 ms. The RT NMR spectra consist of 7 lines (e.g. Fig. 1), attributed to the quadrupole splitting of the 59Co nuclear spin I ¼ 7=2 with tetragonal site symmetry, having a quadrupole splitting of Dnq B1 MHz. This main spectrum is observed for all compounds. An additional set of 7 lines with a quadrupole splitting of Dnq B0:5 MHz, and an integrated intensity of 40% of the entire spectrum is observed for x ¼ 0: Due to the crystallographic phase separation observed in the x ¼ 0:25 sample [6] into x ¼ 0:1 and 0.4 solid solutions, its spectrum is more complicated and may be deconvoluted into two spectra: the x ¼ 0 type double spectrum from the x ¼ 0:1 phase, and the single 7 line set spectrum from the x ¼ 0:4 phase. The RT NMR lines of the solid solutions (0oxo1) are broader than the corresponding lines of the x ¼ 0; 1 and LaCo2Ge2 compounds. LaCo2Ge2 shows no significant change throughout the whole temperature range from 70 K to RT (Fig. 2). For all other compounds, in the paramagnetic region
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 5 2 2 - 8
E.N. Caspi et al. / Physica B 312–313 (2002) 280–282
3.5x105
Echo amplitude (a.u.)
3.0x105 2.5x105
UCo2 Ge2 H0 = 8.0196 T 5K
2.0x105 1.5x105 1.0x105 RT
5.0x104 0.0 78
79
80
81
82
83
84
85
86
Frequency (MHz) Fig. 1. 59Co NMR spectra of UCo2Ge2 at RT and 5 K. The solid lines are best fit of two sets of seven Lorentzians each, one for RT, and another for 5 K.
Fig. 2. Central line frequency, fcenter ; vs. 1=T: The solid lines are best linear fits of the data.
below RT, the resonance frequencies of the NMR central lines of the main spectrum increase linearly with 1=T (e.g. Fig. 2). For x=0, 0.25, 0.5, 0.75 the NMR lines of the main spectrum become smeared out in the vicinity of their corresponding magnetic transition temperatures, as obtained by neutron diffraction and susceptibility measurements [3,6]. Such a smearing may be attributed to fast fluctuations of the magnetic moments in the vicinity of the magnetic transition whose frequencies are comparable with the quadrupole splitting (B1 MHz). Below their magnetic transition temperatures, for x=0.25, 0.5, 0.75, the 7 lines are unresolved, while for x ¼ 0 the spectrum with Dnq B0:5 MHz reappears (Fig 1). For x ¼ 1 we were unable to observe the NMR echo signal below 75 K.
281
The existence of two separate sets of 7 lines in UCo2Ge2 at RT indicates the existence of two types of Co atoms. Yet, there is only one non-equivalent Co site in UCo2Ge2, and no Co impurity bigger than 5 w/o was detected by X-ray and neutron diffraction [3]. Thus, we propose that the existence of regions with Co2+ ions and other regions with Co3+ ions throughout the sample cause the double spectrum in UCo2Ge2 at RT. This could be due to the ill-defined valence state of U atoms which affects the valence state of the Co atoms [8]. Correspondingly, for NdCo2Ge2 and LaCo2Ge2, only one set of 7 lines is observed in agreement with the single valence state of Nd and La. The double spectrum phenomenon in UCo2Ge2, as well as its temperature dependence is presently under study. Unlike Ref. [7], our results for LaCo2Ge2 show temperature-dependent magnetic bahavior in AC-susceptibility and SQUID measurements (to be published), exhibiting a paramagnetic moment of B1mB on Co atoms, similar to its value for NdCo2Ge2 [4]. Yet, NMR measurements on LaCo2Ge2 show no such temperaturedependent behavior (Fig. 2). Thus, we conclude that the linear behavior of central line frequency vs. 1=T depicted in Fig. 2 is solely characteristic of an indirect exchange interaction between conduction ðdÞ and localized ðfÞ electrons in these compounds [9]. From the positive slope of the best fit lines (Fig. 2) and our previous result that the U behaves like a light Ln [4], we conclude that the sign of exchange coefficient between the conduction and f-electrons, Jcf ; is negative [9] for all compounds. In addition to the linear central line frequency dependence on 1=T (Fig. 2) a universal linear dependence of fcenter on the measured magnetic susceptibility, wM ; is observed for all compounds in the paramagnetic state: fcenter ¼ 81:32ð3Þ þ 0:134ð4ÞwM : Combining this dependence with both fcenter ð1=TÞ (Fig. 2) and NMR results of LaCo2Ge2 (Fig. 2), one can estimate the following values for U and Nd exchange coefficients: Jcf ðUÞ ¼ 0:06ð3Þ eV, Jcf ðNdÞ ¼ 0:16ð9Þ eV [9]. The Jcf ðNdÞ value in NdCo2Ge2 is close to its value in NdAl2 [9]. The result that Jcf2 ðNdÞ >Jcf2 ðUÞ; whereas TN (NdCo2Ge2)oTN (UCo2Ge2) [4], as well as a negative Jcf value is a characteristic feature of the Kondo effect shown to exist in similar compounds [10]. This effect should be taken into consideration [10] in future RKKY treatment of these compounds at high temperatures.
References [1] H. Pinto, et al., Phys. Rev. B 31 (1985) 508. [2] A. Szytu"a, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991, p. 85. [3] M. Kuznietz, et al., Phys. Rev. B 40 (1989) 7328.
282 [4] [5] [6] [7]
E.N. Caspi et al. / Physica B 312–313 (2002) 280–282 E. Caspi, et al., Phys. Rev. B 57 (1998) 449. E.N. Caspi, et al., Phys. Rev. B 62 (2000) 9418. E.N. Caspi, et al., Physica B 276–278 (2000) 604. W.H. McCall, et al., J. Appl. Phys. 44 (1973) 4724.
[8] B.R. Cooper, et al., J. Alloys Compounds 192 (1993) 223. [9] V. Jaccarino, et al., Phys. Rev. Lett. 5 (1960) 251. [10] T. Endstra, et al., J. Appl. Phys. 69 (1991) 4810.