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59Co NMR study in Y(Co,Ti)12
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1807–1808 www.elsevier.com/locate/jmmm
59
Co NMR study in Y(Co,Ti)12
Yasushi Amakoa, Yuto Kashua, Takeshi Shinoharab, Hiroyuki Nagaia a
Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
b
Available online 16 November 2006
Abstract The nuclear magnetic resonance (NMR) of 59Co nuclei for YCo12xTix has been measured in zero external field at 4.2 K. The absolute values of the hyperfine field at Co atoms are estimated to be 209, 174 and 131 kOe for YCo11Ti and 143, 116 and 89 kOe for YCo10Ti2. The averaged Co moments for these compounds at 4 K are 1.38 and 0.86 mB/Co, respectively. These results show that the moments of each Co site decrease almost uniformly. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Cc; 76.60.Jx Keywords: YCo12x Tix; NMR; Hyperfine field
1. Introduction Intermetallic compounds with the composition RTl2xMx, where R ¼ rare earth, T ¼ Fe or Co and M ¼ Ti, V, Cr, Mo, W or Si have potential permanent magnet applications owing to their relatively high Curie temperature, saturation magnetization and favourable magnetocrystalline anisotropy [1]. They crystallize in the tetragonal ThMn12-type structure with a space group of I4/mmm which contains three inequivalent transition metal sites, i.e., the 8f, 8i and 8j sites. Unfortunately, compounds with nominal composition RT12 having the ThMn12-type structure do not exist but can be stabilized by a partial substitution of T by M. In YCo12xTix, the Ti atoms occupy the 8i sites preferentially [2,3] and it was found that both the Curie temperature and the Co moments strongly depend on the Ti concentration [4]. In this work, nuclear magnetic resonance (NMR) measurements have been performed to investigate from a microscopic view point to the drastic reduction of Co-magnetic moment observed in YCo12xTix compounds. 2. Experimental Samples were prepared by melting the constituents of Y, Co and Ti of 99.9% purity in an induction furnace. The 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.724
obtained ingots were annealed in vacuum at 1050 1C for 3 days. X-ray diffraction analysis showed that all of these compounds have the crystal structure of ThMn12-type. Magnetization measurements in fields up to 8 T at 4 K were carried out by using vibrating sample magnetometer (VSM) method. Spin-echo NMR measurements at 4.2 K were performed using a conventional incoherent spectrometer with a pulse width of about 7 ms to detect the correct signals from 59Co nuclei in the magnetic domains. 3. Results and discussion Fig. 1 shows the NMR spectra in the zero magnetic field at 4.2 K for YCo11Ti and YCo10Ti2. The value of averaged Co moments for these compounds obtained from the magnetization measurements at 4 K were 1.38 and 0.86 mB/ Co, respectively, which correspond to the results of measurements by Brabers et al. [4]. Corresponding to the number of sites, three resonance lines which had peaks of 211, 176 and 132 MHz were observed for YCo11Ti. We can estimate the absolute value of the hyperfine field by using 59 g value ( ¼ 1.01 kHz/Oe) as 209, 174 and 131 kOe, respectively. According to the time-of-flight neutron diffraction at 293 K (TC ¼ 1050 K [5]), the refined Co moments at 8i, 8j and 8f sites are 1.66, 1.42 and 1.47 mB, respectively [3]. Though it is difficult to assign the
ARTICLE IN PRESS Y. Amako et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1807–1808
1808
59Co
where a and b are the hyperfine coupling constants due to spin and orbital moments, respectively. Hd was calculated as a classical dipole sum, which is negligibly small. In order to estimate the Co magnetic moment, we will assume as follows: (i) the hyperfine coupling constants a and b do not depend on crystallographic sites and we employed the hyperfine coupling constants a ¼ 160 and b ¼ þ490 kOe=mB [6], (ii) the hyperfine field is negative and (iii) the absolute value of the hyperfine field depends on the magnitude of magnetization at each site. Then the orbital magnetic moments of Co atoms in YCo11Ti are estimated as 0.09, 0.15 and 0.09 mB for the 8i, 8j and 8f sites, respectively. The absolute values of the hyperfine field at Co atoms are estimated as 143, 116 and 89 kOe for YCo10Ti2. The absolute values of the hyperfine field for each site are almost in proportion to the averaged Co moments, which shows that the moments of each Co site decrease almost uniformly.
in YCo12-xTix
spin echo intensity (a.u.)
4.2K
x=1.0
x=2.0 50
100
150 frequency (MHz)
200
250
Fig. 1. The NMR spectra in the zero magnetic field at 4.2 K for YCo12xTix with x ¼ 1:0 and 2.0.
resonance lines because intensity ratios are not accurate with influences such as relaxation time, we have tried to estimate the Co magnetic moment (spin and orbital moment separately) for each site in YCo11Ti on the basis of an empirical relation between the 59Co hyperfine fields and Co moments given by [6], H hf ¼ a mCo ðspinÞ þ bmCo ðorbÞ þ H d ,
References [1] H.S. Li, J.M.D. Coey, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North Holland, Amsterdam, 1993, p. 1. [2] Y. Yang, L. Kong, H. Sun, J. Yang, Y. Ding, B. Zhang, C. Ye, L. Jin, J. Appl. Phys. 69 (1990) 4632. [3] O. Moze, L. Pareti, K.H.J. Buschow, J. Phys.: Condens. Matter 7 (1995) 9255. [4] J.H.V.J. Brabers, D.C. Zeng, N. Tang, F.R. de Boer, K.H.J. Buschow, J. Magn. Magn. Mater. 140–144 (1995) 1087. [5] K. Ohashi, H. Ido, K. Konno, Y. Yoneda, J. Appl. Phys. 70 (1991) 5986. [6] H. Yoshie, W. Takahashi, H. Ishizone, Y. Amako, H. Nagai, M. Shiga, H. Wada, K. Adachi, J. Magn. Magn. Mater. 177–181 (1998) 481.
Journal of Magnetism and Magnetic Materials 310 (2007) 1807–1808 www.elsevier.com/locate/jmmm
59
Co NMR study in Y(Co,Ti)12
Yasushi Amakoa, Yuto Kashua, Takeshi Shinoharab, Hiroyuki Nagaia a
Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
b
Available online 16 November 2006
Abstract The nuclear magnetic resonance (NMR) of 59Co nuclei for YCo12xTix has been measured in zero external field at 4.2 K. The absolute values of the hyperfine field at Co atoms are estimated to be 209, 174 and 131 kOe for YCo11Ti and 143, 116 and 89 kOe for YCo10Ti2. The averaged Co moments for these compounds at 4 K are 1.38 and 0.86 mB/Co, respectively. These results show that the moments of each Co site decrease almost uniformly. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Cc; 76.60.Jx Keywords: YCo12x Tix; NMR; Hyperfine field
1. Introduction Intermetallic compounds with the composition RTl2xMx, where R ¼ rare earth, T ¼ Fe or Co and M ¼ Ti, V, Cr, Mo, W or Si have potential permanent magnet applications owing to their relatively high Curie temperature, saturation magnetization and favourable magnetocrystalline anisotropy [1]. They crystallize in the tetragonal ThMn12-type structure with a space group of I4/mmm which contains three inequivalent transition metal sites, i.e., the 8f, 8i and 8j sites. Unfortunately, compounds with nominal composition RT12 having the ThMn12-type structure do not exist but can be stabilized by a partial substitution of T by M. In YCo12xTix, the Ti atoms occupy the 8i sites preferentially [2,3] and it was found that both the Curie temperature and the Co moments strongly depend on the Ti concentration [4]. In this work, nuclear magnetic resonance (NMR) measurements have been performed to investigate from a microscopic view point to the drastic reduction of Co-magnetic moment observed in YCo12xTix compounds. 2. Experimental Samples were prepared by melting the constituents of Y, Co and Ti of 99.9% purity in an induction furnace. The 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.724
obtained ingots were annealed in vacuum at 1050 1C for 3 days. X-ray diffraction analysis showed that all of these compounds have the crystal structure of ThMn12-type. Magnetization measurements in fields up to 8 T at 4 K were carried out by using vibrating sample magnetometer (VSM) method. Spin-echo NMR measurements at 4.2 K were performed using a conventional incoherent spectrometer with a pulse width of about 7 ms to detect the correct signals from 59Co nuclei in the magnetic domains. 3. Results and discussion Fig. 1 shows the NMR spectra in the zero magnetic field at 4.2 K for YCo11Ti and YCo10Ti2. The value of averaged Co moments for these compounds obtained from the magnetization measurements at 4 K were 1.38 and 0.86 mB/ Co, respectively, which correspond to the results of measurements by Brabers et al. [4]. Corresponding to the number of sites, three resonance lines which had peaks of 211, 176 and 132 MHz were observed for YCo11Ti. We can estimate the absolute value of the hyperfine field by using 59 g value ( ¼ 1.01 kHz/Oe) as 209, 174 and 131 kOe, respectively. According to the time-of-flight neutron diffraction at 293 K (TC ¼ 1050 K [5]), the refined Co moments at 8i, 8j and 8f sites are 1.66, 1.42 and 1.47 mB, respectively [3]. Though it is difficult to assign the
ARTICLE IN PRESS Y. Amako et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1807–1808
1808
59Co
where a and b are the hyperfine coupling constants due to spin and orbital moments, respectively. Hd was calculated as a classical dipole sum, which is negligibly small. In order to estimate the Co magnetic moment, we will assume as follows: (i) the hyperfine coupling constants a and b do not depend on crystallographic sites and we employed the hyperfine coupling constants a ¼ 160 and b ¼ þ490 kOe=mB [6], (ii) the hyperfine field is negative and (iii) the absolute value of the hyperfine field depends on the magnitude of magnetization at each site. Then the orbital magnetic moments of Co atoms in YCo11Ti are estimated as 0.09, 0.15 and 0.09 mB for the 8i, 8j and 8f sites, respectively. The absolute values of the hyperfine field at Co atoms are estimated as 143, 116 and 89 kOe for YCo10Ti2. The absolute values of the hyperfine field for each site are almost in proportion to the averaged Co moments, which shows that the moments of each Co site decrease almost uniformly.
in YCo12-xTix
spin echo intensity (a.u.)
4.2K
x=1.0
x=2.0 50
100
150 frequency (MHz)
200
250
Fig. 1. The NMR spectra in the zero magnetic field at 4.2 K for YCo12xTix with x ¼ 1:0 and 2.0.
resonance lines because intensity ratios are not accurate with influences such as relaxation time, we have tried to estimate the Co magnetic moment (spin and orbital moment separately) for each site in YCo11Ti on the basis of an empirical relation between the 59Co hyperfine fields and Co moments given by [6], H hf ¼ a mCo ðspinÞ þ bmCo ðorbÞ þ H d ,
References [1] H.S. Li, J.M.D. Coey, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North Holland, Amsterdam, 1993, p. 1. [2] Y. Yang, L. Kong, H. Sun, J. Yang, Y. Ding, B. Zhang, C. Ye, L. Jin, J. Appl. Phys. 69 (1990) 4632. [3] O. Moze, L. Pareti, K.H.J. Buschow, J. Phys.: Condens. Matter 7 (1995) 9255. [4] J.H.V.J. Brabers, D.C. Zeng, N. Tang, F.R. de Boer, K.H.J. Buschow, J. Magn. Magn. Mater. 140–144 (1995) 1087. [5] K. Ohashi, H. Ido, K. Konno, Y. Yoneda, J. Appl. Phys. 70 (1991) 5986. [6] H. Yoshie, W. Takahashi, H. Ishizone, Y. Amako, H. Nagai, M. Shiga, H. Wada, K. Adachi, J. Magn. Magn. Mater. 177–181 (1998) 481.