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Pressure effects on single chain magnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1118–1119
Pressure effects on single chain magnets M. Mitoa,, N. Shindoa, T. Tajiria, H. Deguchia, S. Takagia, H. Miyasakab, M. Yamashitac, R. Cle! racd, C. Coulond a
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan Graduate School of Science, Tokyo Metropolitan University & PRESTO (JST), Tokyo 192-0397, Japan c Graduate School of Science, Tokyo Metropolitan University & CREST (JST), Tokyo 192-0397, Japan d Centre de Recherche Paul Pascal, CNRS UPR 8641, 33600 Pessac, France
b
Abstract Pressure effects on a single chain magnet [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (saltmen2=N,N0 -(1,1,2,2tetramethylethylene)bis(salicylideneiminate), and pao=pyridine-2-aldoximate) have been investigated through AC magnetic measurements under pressure (P). The slow relaxation of the magnetization depends on pressure. Both the blocking temperature (TB ) and energy barrier (D) increase by pressurization, and those enhancements saturate at around P ¼ 7 kbar. r 2004 Elsevier B.V. All rights reserved. PACS: 75.10.Hk; 75.50.Xx; 75.40.Gb; 62.50.+p Keywords: Single chain magnet; Pressure effects; Glauber theory; Slow dynamics
1. Introduction The slow relaxation of the magnetization in the paramagnetic phase, predicted by Glauber for the Ising chain system in 1963 [1], has been recently observed successively in two one-dimensional (1D) materials: (1) [CoII(hfac)2 (NITPhOMe)] (hfac=hexafluoroacetylacetonate, NITPhOMe=40 -methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) [2], (2) [Mn2(saltmen)2 Ni(pao)2 (py)2](ClO4)2 (saltmen2=N; N 0 -(1,1,2,2-tetramethylethylene)bis(salicylideneiminate), and pao= pyridine-2-aldoximate) [3]. The above system requires a strong uniaxial anisotropy (D) and high one-dimensionality. The energy barrier (D) connected with D and the intrachain interaction makes the magnetic moment freeze below a so-called ‘‘blocking temperature’’ (TB ), below which a very slow relaxation and magnetic hysteresis are observed. The former material is comprised of alternating Co2+ and organic radical spins antiferromagnetically coupled to give a 1D ferrimagnet with a helical structure. The E-mail address: [email protected] (M. Mito).
Corresponding author. Tel./fax: +81-93-884-3286I
latter is an ideal 1D system with the repeating unit of [Mn(O)2MnONNiNO]. The anisotropy parallel to the chain originates from the Jahn–Teller distortion around Mn3+ ion. Between Ni2+ (S=1) and Mn3+ (S=2), antiferromagnetic interaction of J=kB ¼ 21 K works, and the magnetic property can be described by the S=3 ferromagnetic Ising chain model, in which the [Mn3+–Ni2+–Mn3+] trimers are connected through a ferromagnetic Mn3+–Mn3+ interaction (J 0 =kB ¼ 1 K). This material is the first heterometallic chain showing a characteristic of ‘‘single chain magnet’’. In the present study, pressure effects on the slow dynamics of [Mn2(saltmen)2Ni(pao)2(py)2] (ClO4)2 are investigated through AC magnetic measurements. Applying pressure may give some influence to the crystal structure, leading to the changes of D and TB :
2. Experimental The sample of [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 was snythesized according to the procedure described in
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.046
ARTICLE IN PRESS M. Mito et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1118–1119
Fig. 2. Pressure dependence of TB at f ¼ 1 Hz Pp13:1 kbar. The solid line is a guide for the eye.
1119
for
Fig. 1. Frequency dependence of the out-of-phase of the AC magnetic susceptibility for P ¼ 0 and 13.1 kbar. The solid curves are guides for the eye.
Ref. [3]. Pressure up to 13.1 kbar was applied using a CuBe pressure cell (CR-PSC-KY05-1, Kyowa-Seisakusho Co. ltd.), in which the polycrystalline sample (16.7 mg) and an Apiezon-J oil as the pressure transmitting oil were held with a few pieces of Pb. The pressure at liquid helium temperature was estimated from the superconducting transition temperature of Pb [4]. The AC magnetic susceptibility was measured at various frequencies between 1 and 500 Hz with an AC field of 2 Oe. It has been confirmed that the changes of the magnetic property of [Mn2(saltmen)2Ni(pao)2 (py)2] (ClO4)2 are reversible against the increase and decrease of pressure.
3. Results and discussion Fig. 1 shows the frequency dependence of the out-ofphase of AC magnetic susceptibility (w00 ) for P ¼ 0 and 13.1 kbar. The temperature, at which w00 has the maximum, corresponds to TB : With increasing pressure, the peak for each frequency shifts toward high temperatures. For Pp13:1 kbar, no anomaly due to the magnetic order, etc. was observed. The pressure dependence of TB at f ¼ 1 Hz is shown in Fig. 2. TB is enhanced linearly against pressure for Po6 kbar, at around which the enhancement tends to saturate. The increase of TB is 1.8% of TB (P ¼ 0).
Fig. 3. Pressure dependence of D=kB for Pp13:1 kbar. The solid line is a guide for the eye.
By analyzing TB versus lnð2pf Þ using the Arrhenius equation, 1=TB ¼ ðkB =DÞflnð2pf Þ þ lnðt0 Þg; the value of D is estimated as shown in Fig. 3. Similar to the pressure dependence of TB ; D increases linearly against pressure up to about 7 kbar, above which it has the almost constant value of 1:08 D (P ¼ 0). The present result indicates that the slow relaxation depends on pressure for Po7 kbar. It is supposed that the structure gradually changes by pressurization for Po627 kbar, above which the Van der Waals force may hamper the molecular shrinkage and/or intramolecular deformation. The quantitative analyses of J; J 0 and D are necessary to elucidate the origin of the pressure effects on TB and D: The structural analysis under pressure is undergoing for the detailed discussion.
References [1] [2] [3] [4]
R.J. Glauber, J. Math. Phys. 4 (1963) 294. A. Caneschi, et al., Angew. Chem. Int. Ed. 40 (2001) 1760. R. Clerac, et al., J. Am. Chem. Soc. 124 (2002) 12837. A. Eiling, J.S. Schilling, J. Phys. F 11 (1981) 623.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1118–1119
Pressure effects on single chain magnets M. Mitoa,, N. Shindoa, T. Tajiria, H. Deguchia, S. Takagia, H. Miyasakab, M. Yamashitac, R. Cle! racd, C. Coulond a
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan Graduate School of Science, Tokyo Metropolitan University & PRESTO (JST), Tokyo 192-0397, Japan c Graduate School of Science, Tokyo Metropolitan University & CREST (JST), Tokyo 192-0397, Japan d Centre de Recherche Paul Pascal, CNRS UPR 8641, 33600 Pessac, France
b
Abstract Pressure effects on a single chain magnet [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (saltmen2=N,N0 -(1,1,2,2tetramethylethylene)bis(salicylideneiminate), and pao=pyridine-2-aldoximate) have been investigated through AC magnetic measurements under pressure (P). The slow relaxation of the magnetization depends on pressure. Both the blocking temperature (TB ) and energy barrier (D) increase by pressurization, and those enhancements saturate at around P ¼ 7 kbar. r 2004 Elsevier B.V. All rights reserved. PACS: 75.10.Hk; 75.50.Xx; 75.40.Gb; 62.50.+p Keywords: Single chain magnet; Pressure effects; Glauber theory; Slow dynamics
1. Introduction The slow relaxation of the magnetization in the paramagnetic phase, predicted by Glauber for the Ising chain system in 1963 [1], has been recently observed successively in two one-dimensional (1D) materials: (1) [CoII(hfac)2 (NITPhOMe)] (hfac=hexafluoroacetylacetonate, NITPhOMe=40 -methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) [2], (2) [Mn2(saltmen)2 Ni(pao)2 (py)2](ClO4)2 (saltmen2=N; N 0 -(1,1,2,2-tetramethylethylene)bis(salicylideneiminate), and pao= pyridine-2-aldoximate) [3]. The above system requires a strong uniaxial anisotropy (D) and high one-dimensionality. The energy barrier (D) connected with D and the intrachain interaction makes the magnetic moment freeze below a so-called ‘‘blocking temperature’’ (TB ), below which a very slow relaxation and magnetic hysteresis are observed. The former material is comprised of alternating Co2+ and organic radical spins antiferromagnetically coupled to give a 1D ferrimagnet with a helical structure. The E-mail address: [email protected] (M. Mito).
Corresponding author. Tel./fax: +81-93-884-3286I
latter is an ideal 1D system with the repeating unit of [Mn(O)2MnONNiNO]. The anisotropy parallel to the chain originates from the Jahn–Teller distortion around Mn3+ ion. Between Ni2+ (S=1) and Mn3+ (S=2), antiferromagnetic interaction of J=kB ¼ 21 K works, and the magnetic property can be described by the S=3 ferromagnetic Ising chain model, in which the [Mn3+–Ni2+–Mn3+] trimers are connected through a ferromagnetic Mn3+–Mn3+ interaction (J 0 =kB ¼ 1 K). This material is the first heterometallic chain showing a characteristic of ‘‘single chain magnet’’. In the present study, pressure effects on the slow dynamics of [Mn2(saltmen)2Ni(pao)2(py)2] (ClO4)2 are investigated through AC magnetic measurements. Applying pressure may give some influence to the crystal structure, leading to the changes of D and TB :
2. Experimental The sample of [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 was snythesized according to the procedure described in
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.046
ARTICLE IN PRESS M. Mito et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1118–1119
Fig. 2. Pressure dependence of TB at f ¼ 1 Hz Pp13:1 kbar. The solid line is a guide for the eye.
1119
for
Fig. 1. Frequency dependence of the out-of-phase of the AC magnetic susceptibility for P ¼ 0 and 13.1 kbar. The solid curves are guides for the eye.
Ref. [3]. Pressure up to 13.1 kbar was applied using a CuBe pressure cell (CR-PSC-KY05-1, Kyowa-Seisakusho Co. ltd.), in which the polycrystalline sample (16.7 mg) and an Apiezon-J oil as the pressure transmitting oil were held with a few pieces of Pb. The pressure at liquid helium temperature was estimated from the superconducting transition temperature of Pb [4]. The AC magnetic susceptibility was measured at various frequencies between 1 and 500 Hz with an AC field of 2 Oe. It has been confirmed that the changes of the magnetic property of [Mn2(saltmen)2Ni(pao)2 (py)2] (ClO4)2 are reversible against the increase and decrease of pressure.
3. Results and discussion Fig. 1 shows the frequency dependence of the out-ofphase of AC magnetic susceptibility (w00 ) for P ¼ 0 and 13.1 kbar. The temperature, at which w00 has the maximum, corresponds to TB : With increasing pressure, the peak for each frequency shifts toward high temperatures. For Pp13:1 kbar, no anomaly due to the magnetic order, etc. was observed. The pressure dependence of TB at f ¼ 1 Hz is shown in Fig. 2. TB is enhanced linearly against pressure for Po6 kbar, at around which the enhancement tends to saturate. The increase of TB is 1.8% of TB (P ¼ 0).
Fig. 3. Pressure dependence of D=kB for Pp13:1 kbar. The solid line is a guide for the eye.
By analyzing TB versus lnð2pf Þ using the Arrhenius equation, 1=TB ¼ ðkB =DÞflnð2pf Þ þ lnðt0 Þg; the value of D is estimated as shown in Fig. 3. Similar to the pressure dependence of TB ; D increases linearly against pressure up to about 7 kbar, above which it has the almost constant value of 1:08 D (P ¼ 0). The present result indicates that the slow relaxation depends on pressure for Po7 kbar. It is supposed that the structure gradually changes by pressurization for Po627 kbar, above which the Van der Waals force may hamper the molecular shrinkage and/or intramolecular deformation. The quantitative analyses of J; J 0 and D are necessary to elucidate the origin of the pressure effects on TB and D: The structural analysis under pressure is undergoing for the detailed discussion.
References [1] [2] [3] [4]
R.J. Glauber, J. Math. Phys. 4 (1963) 294. A. Caneschi, et al., Angew. Chem. Int. Ed. 40 (2001) 1760. R. Clerac, et al., J. Am. Chem. Soc. 124 (2002) 12837. A. Eiling, J.S. Schilling, J. Phys. F 11 (1981) 623.