- Email: [email protected]
Pulsed field studies on the quantum nanomagnet Mn12Ac
Physica B 294}295 (2001) 307}309
Pulsed "eld studies on the quantum nanomagnet Mn Ac
R. GroK ssinger *, A. Caneschi, M. DoK rr, D. Eckert, F. Fischer, A. Handstein, D. Hinz, R. Kratz, H. Krug, K.H. MuK ller, M.A. Novak, H. Siegel, P. Verges Institut fu( r Experimentalphysik, Techn. Univ. Wien, A-1040 Vienna, Austria Institut fu( r Angewandte Physik, Techn. Univ. Dresden, D-01062 Dresden, Germany Dipartamento di Chimica, Univ. de Firenze, Via Maragliano 77, I-50144, Italy Institut fu( r Festko( rper- und Werkstoworschung, P.O. Box 270016, D-01171 Dresden, Germany Forschungszentrum Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany Instituto de Fisica, Univ. Federale do Rio de Janeiro, CP 68528, RJ 21945, Brazil
Abstract We present preliminary high-"eld pulse magnetization measurements done in the new IFW Dresden facility on powder Mn Ac quantum nanomagnets. Measurements were done between 1.9 and 4.2 K; at di!erent sweep rates; as well as in static "elds in a SQUID magnetometer for comparison. The pulsed "eld technique opens new possibilities to study quantum tunneling in nanomagnets. 2001 Elsevier Science B.V. All rights reserved. Keywords: Quantum nanomagnet; Time dependence
1. Introduction Molecular crystals containing isolated magnetic clusters have got increasing attention due to their magnetic and relaxation properties. This type of compounds permits the macroscopic observation of single nanomagnetic particle properties, avoiding complications due to size and orientation distribution existing in traditional small magnetic particle systems. Spin clusters have got important attention in the last years as new quantum phenomena could be evidenced at the nanoscopic level. Mn Ac[Mn O (CH COO) (H O) ) ) CH COOH ) 4H O] shows hysteresis loops with steps * Corresponding author. E-mail address: [email protected] (R. GroK ssinger).
at "elds where the relaxation time is accelerated due to thermally activated quantum tunneling (see e.g. Refs. [1,2]). The results which are available on this material up to now can be explained by treating a cluster as a macrospin with S"10 in an uniaxial crystal "eld with a total anisotropy-energy barrier between the up and down ground state of about 62 K. Many experimental studies of this material were done with di!erent techniques, but up to now only one was carried out by magnetization measurements in pulsed high "elds [3]. In this work the measurements were done by using long pulse duration and only with positive "elds. The purpose was to explain the avalanches driven by thermally activated quantum tunneling. In this work, we report on the in#uence of a pulse "eld oscillating through zero on the magnetization behavior of Mn Ac, using di!erent sweeping rates.
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 6 5 - 7
308
R. Gro( ssinger et al. / Physica B 294}295 (2001) 307}309
The simplest model Hamiltonian to describe the system is H"!DS!g S ) B#H, (1) X where H represents the smaller terms that do not commute with H, thus providing some "nite tunneling probability. The last term was recently determined to be (AS#B(S #S )) from recent X > \ high-"eld EPR and also from inelastic neutronscattering measurements [4,5]. The energy spectrum of Hamiltonian (1) yields within the lowest S"10 multiplet 2S#1"21 states, labeled with the S eigenvalues, or m "$10,$9,20. The X 1 spectrum of H is not much di!erent from the one when H"0. In this case, several pairs of m with 1 opposite signs cross at some well-de"ned "eld along the easy axis B "nB "nD/g +0.44 T L [6], where n is an integer. At these "eld values, the magnetization sharply steps due to thermally activated quantum tunneling, and these incidents depend considerably on temperature and "eldsweeping rate. The small perturbation H creates a small gap , known as the tunnel splitting, which depends on the m values and on H. Due to the 1 tunnel splitting, the crossing levels become anticrossing [7], and the probability of tunneling depends on the sweeping rate. The use of high sweeping rates (of the order of several kT/s) opens new possibilities of studying the switching of the macrospin within the Landau-Zener model as recently proposed by Wernsdorfer et al. [8]. In brief, by measuring the tunneling probability under a sweeping "eld of constant rate through the level crossing, one can deduce the tunnel splitting between the levels.
#op of MnF which occurs at 9.3 T and the mag netization was calibrated using an isotropic Ba-ferrite sample. The calibration has an accuracy of $2%. The system has a sensitivity of about 0.1 emu. We have used a powder sample for these preliminary measurements. Di!erent pulses were used as shown in Fig. 1. Typical results are shown in Fig. 2, where the pulse-"eld measurements are recorded together with the very slow sweeping rate of SQUID-magnetometer measurements. As expected it is clear to recognize that the sweeping rate a!ects the results considerably. This means that the transition exhibits a considerable
Fig. 1. In#uence of the capacitor-bank circuit on the time dependence of a "eld pulse. The following notation was used: the numbers give the voltage in kV, the letters a and d denote the use of one module (a or d, resp.), v denotes that of the full bank.
2. Experimental The magnetization measurements were made with a highly precise pick-up coil system in the 50 T pulse "eld equipment of the IFW Dresden built up in a module technique of the capacitor bank with a maximum stored energy of 1 MJ [9,10]. Di!erent sweep rates can be realized by various circuits and charging of the four modules, as shown in Fig. 1. It is possible to use only one module or two to four together. The "eld was calibrated using the spin
Fig. 2. Magnetization loops obtained at 4.2 K with the di!erent pulses displayed in Fig. 1, together with the SQUID measurements.
R. Gro( ssinger et al. / Physica B 294}295 (2001) 307}309 Table 1 Energy-level-crossing "elds obtained from the magnetization curves at the maximum susceptibility positions and temperatures denoted in "rst column. H#, the positive "eld value and H}, the negative value. The pulse used is indicated in the 4th column T (K)
H# (T)
H! (T)
Pulse
1.9 1.9 1.9 1.9 1.9 2.2 2.2 2.4 2.5 2.5 3.1 3.1 4.2 4.2 4.2
2.35 2.32 2.32 2.25 2.17 2.16 2.07 2.07 1.94 1.94 1.56 1.51 2.00 1.96 1.50
1.33 1.21 1.33 1.17 1.20 1.17 1.35 1.31 1.25 1.25 1.09 0.95 1.50 1.48 1.17
2.5a 2.5a 1.1v 1.1v 2.5a 2.5a 2.5a 2.5a 2.5a 2.5a 2.5a 1.1v 2.0v 2.0v 2.0v
time dependence, which can be related to the activation energy of the process. The results obtained for the energy-level-crossing "elds B by several L sets of measurements are shown in Table 1. From these data, it is di$cult to "nd a general trend due to the small statistics. It is known that avalanches due to spin-phonon coupling occur in a non-reproducible way [2,11], but in our case it can be assumed that it is even more spread out due to the use of a powder sample. All the observed "eld positions of the transitions are given in Table 1. Note that the negative "elds are always smaller in absolute values than the positive ones, due to the smaller sweeping rates. Zero-"eld crossing happens only in the SQUID measurements and by the end of the high-"eld pulse near zero "eld. The other crossing-"eld values obtained from the SQUID
309
measurements give B"0.46, 0.84, 1.19 and 1.55 T at temperatures of 3.1, 2.4, 2.2 and 1.9 K, respectively; these values are close to the expected ones. In conclusion, the use of the high-"eld pulse technique opens new possibilities to study the switching process due to quantum tunneling of the magnetization, and the determination of the tunnel splitting in single-molecule nanomagnets. Measurements on single crystals of Mn and Fe are on the way and are expected to bring new results to this "eld. References [1] M.A. Novak, A.M. Gomes, W.S.D. Folly, R.E. Rapp, Mater. Sci. Forum 302}303 (1999) 334. [2] A. Caneschi, D. Gatteschi, C. Sangregorio, R. Sessoli, L. Sorace, A. Cornia, M.A. Novak, C. Paulsen, W. Wernsdorfer, J. Magn. Magn. Mater. 200 (1999) 182. [3] E. del Barco, J.M. Hernandez, M. Sales, J. Tejada, H. Rakoto, J.M. Broto, E.M. Chudnovsky, Phys. Rev. B 60 (1999) 11898. [4] A.L. Barra, D. Gatteschi, R. Sessoli, Phys. Rev. B 56 (1997) 1. [5] M. Hennion et al., Phys. Rev. B 56 (1997) 8819. [6] L. Thomas, F. Lionti, R. Ballou, D. Gatteschi, R. Sessoli, B. Barbara, Nature 383 (1996) 145. [7] L. Gunther, Europhys. Lett. 39 (1997) 1. [8] W. Wernsdorfer, R. Sessoli, A. Caneschi, D. Gatteschi, A. Cornia, D. Mailly, J. Appl. Phys. 87 (2000) 5481. [9] D. Eckert, R. GroK ssinger, M. Doerr, F. Fischer, A. Handstein, D. Hinz, H. Siegel, P. Verges, K.-H. MuK ller, in these Proceedings (RHMF 2000), Physica B 294}295 (2001). [10] H. Krug, M. Doerr, D. Eckert, H. Eschrig, F. Fischer, P. Fulde, R. GroK ssinger, A. Handstein, F. Herlach, D. Hinz, R. Kratz, M. Loewenhaupt, K.-H. MuK ller, F. Pobell, L. Schultz, H. Siegel, F. Steglich, P. Verges, in these Proceedings (RHMF 2000), Physica B 294}295 (2001). [11] C. Paulsen, J.G. Park, in: L. Gunther, B. Barbara (Eds.), Quantum Tunneling of Magnetization, Kluwer, Dordrecht, 1995.
Pulsed "eld studies on the quantum nanomagnet Mn Ac
R. GroK ssinger *, A. Caneschi, M. DoK rr, D. Eckert, F. Fischer, A. Handstein, D. Hinz, R. Kratz, H. Krug, K.H. MuK ller, M.A. Novak, H. Siegel, P. Verges Institut fu( r Experimentalphysik, Techn. Univ. Wien, A-1040 Vienna, Austria Institut fu( r Angewandte Physik, Techn. Univ. Dresden, D-01062 Dresden, Germany Dipartamento di Chimica, Univ. de Firenze, Via Maragliano 77, I-50144, Italy Institut fu( r Festko( rper- und Werkstoworschung, P.O. Box 270016, D-01171 Dresden, Germany Forschungszentrum Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany Instituto de Fisica, Univ. Federale do Rio de Janeiro, CP 68528, RJ 21945, Brazil
Abstract We present preliminary high-"eld pulse magnetization measurements done in the new IFW Dresden facility on powder Mn Ac quantum nanomagnets. Measurements were done between 1.9 and 4.2 K; at di!erent sweep rates; as well as in static "elds in a SQUID magnetometer for comparison. The pulsed "eld technique opens new possibilities to study quantum tunneling in nanomagnets. 2001 Elsevier Science B.V. All rights reserved. Keywords: Quantum nanomagnet; Time dependence
1. Introduction Molecular crystals containing isolated magnetic clusters have got increasing attention due to their magnetic and relaxation properties. This type of compounds permits the macroscopic observation of single nanomagnetic particle properties, avoiding complications due to size and orientation distribution existing in traditional small magnetic particle systems. Spin clusters have got important attention in the last years as new quantum phenomena could be evidenced at the nanoscopic level. Mn Ac[Mn O (CH COO) (H O) ) ) CH COOH ) 4H O] shows hysteresis loops with steps * Corresponding author. E-mail address: [email protected] (R. GroK ssinger).
at "elds where the relaxation time is accelerated due to thermally activated quantum tunneling (see e.g. Refs. [1,2]). The results which are available on this material up to now can be explained by treating a cluster as a macrospin with S"10 in an uniaxial crystal "eld with a total anisotropy-energy barrier between the up and down ground state of about 62 K. Many experimental studies of this material were done with di!erent techniques, but up to now only one was carried out by magnetization measurements in pulsed high "elds [3]. In this work the measurements were done by using long pulse duration and only with positive "elds. The purpose was to explain the avalanches driven by thermally activated quantum tunneling. In this work, we report on the in#uence of a pulse "eld oscillating through zero on the magnetization behavior of Mn Ac, using di!erent sweeping rates.
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 6 5 - 7
308
R. Gro( ssinger et al. / Physica B 294}295 (2001) 307}309
The simplest model Hamiltonian to describe the system is H"!DS!g S ) B#H, (1) X where H represents the smaller terms that do not commute with H, thus providing some "nite tunneling probability. The last term was recently determined to be (AS#B(S #S )) from recent X > \ high-"eld EPR and also from inelastic neutronscattering measurements [4,5]. The energy spectrum of Hamiltonian (1) yields within the lowest S"10 multiplet 2S#1"21 states, labeled with the S eigenvalues, or m "$10,$9,20. The X 1 spectrum of H is not much di!erent from the one when H"0. In this case, several pairs of m with 1 opposite signs cross at some well-de"ned "eld along the easy axis B "nB "nD/g +0.44 T L [6], where n is an integer. At these "eld values, the magnetization sharply steps due to thermally activated quantum tunneling, and these incidents depend considerably on temperature and "eldsweeping rate. The small perturbation H creates a small gap , known as the tunnel splitting, which depends on the m values and on H. Due to the 1 tunnel splitting, the crossing levels become anticrossing [7], and the probability of tunneling depends on the sweeping rate. The use of high sweeping rates (of the order of several kT/s) opens new possibilities of studying the switching of the macrospin within the Landau-Zener model as recently proposed by Wernsdorfer et al. [8]. In brief, by measuring the tunneling probability under a sweeping "eld of constant rate through the level crossing, one can deduce the tunnel splitting between the levels.
#op of MnF which occurs at 9.3 T and the mag netization was calibrated using an isotropic Ba-ferrite sample. The calibration has an accuracy of $2%. The system has a sensitivity of about 0.1 emu. We have used a powder sample for these preliminary measurements. Di!erent pulses were used as shown in Fig. 1. Typical results are shown in Fig. 2, where the pulse-"eld measurements are recorded together with the very slow sweeping rate of SQUID-magnetometer measurements. As expected it is clear to recognize that the sweeping rate a!ects the results considerably. This means that the transition exhibits a considerable
Fig. 1. In#uence of the capacitor-bank circuit on the time dependence of a "eld pulse. The following notation was used: the numbers give the voltage in kV, the letters a and d denote the use of one module (a or d, resp.), v denotes that of the full bank.
2. Experimental The magnetization measurements were made with a highly precise pick-up coil system in the 50 T pulse "eld equipment of the IFW Dresden built up in a module technique of the capacitor bank with a maximum stored energy of 1 MJ [9,10]. Di!erent sweep rates can be realized by various circuits and charging of the four modules, as shown in Fig. 1. It is possible to use only one module or two to four together. The "eld was calibrated using the spin
Fig. 2. Magnetization loops obtained at 4.2 K with the di!erent pulses displayed in Fig. 1, together with the SQUID measurements.
R. Gro( ssinger et al. / Physica B 294}295 (2001) 307}309 Table 1 Energy-level-crossing "elds obtained from the magnetization curves at the maximum susceptibility positions and temperatures denoted in "rst column. H#, the positive "eld value and H}, the negative value. The pulse used is indicated in the 4th column T (K)
H# (T)
H! (T)
Pulse
1.9 1.9 1.9 1.9 1.9 2.2 2.2 2.4 2.5 2.5 3.1 3.1 4.2 4.2 4.2
2.35 2.32 2.32 2.25 2.17 2.16 2.07 2.07 1.94 1.94 1.56 1.51 2.00 1.96 1.50
1.33 1.21 1.33 1.17 1.20 1.17 1.35 1.31 1.25 1.25 1.09 0.95 1.50 1.48 1.17
2.5a 2.5a 1.1v 1.1v 2.5a 2.5a 2.5a 2.5a 2.5a 2.5a 2.5a 1.1v 2.0v 2.0v 2.0v
time dependence, which can be related to the activation energy of the process. The results obtained for the energy-level-crossing "elds B by several L sets of measurements are shown in Table 1. From these data, it is di$cult to "nd a general trend due to the small statistics. It is known that avalanches due to spin-phonon coupling occur in a non-reproducible way [2,11], but in our case it can be assumed that it is even more spread out due to the use of a powder sample. All the observed "eld positions of the transitions are given in Table 1. Note that the negative "elds are always smaller in absolute values than the positive ones, due to the smaller sweeping rates. Zero-"eld crossing happens only in the SQUID measurements and by the end of the high-"eld pulse near zero "eld. The other crossing-"eld values obtained from the SQUID
309
measurements give B"0.46, 0.84, 1.19 and 1.55 T at temperatures of 3.1, 2.4, 2.2 and 1.9 K, respectively; these values are close to the expected ones. In conclusion, the use of the high-"eld pulse technique opens new possibilities to study the switching process due to quantum tunneling of the magnetization, and the determination of the tunnel splitting in single-molecule nanomagnets. Measurements on single crystals of Mn and Fe are on the way and are expected to bring new results to this "eld. References [1] M.A. Novak, A.M. Gomes, W.S.D. Folly, R.E. Rapp, Mater. Sci. Forum 302}303 (1999) 334. [2] A. Caneschi, D. Gatteschi, C. Sangregorio, R. Sessoli, L. Sorace, A. Cornia, M.A. Novak, C. Paulsen, W. Wernsdorfer, J. Magn. Magn. Mater. 200 (1999) 182. [3] E. del Barco, J.M. Hernandez, M. Sales, J. Tejada, H. Rakoto, J.M. Broto, E.M. Chudnovsky, Phys. Rev. B 60 (1999) 11898. [4] A.L. Barra, D. Gatteschi, R. Sessoli, Phys. Rev. B 56 (1997) 1. [5] M. Hennion et al., Phys. Rev. B 56 (1997) 8819. [6] L. Thomas, F. Lionti, R. Ballou, D. Gatteschi, R. Sessoli, B. Barbara, Nature 383 (1996) 145. [7] L. Gunther, Europhys. Lett. 39 (1997) 1. [8] W. Wernsdorfer, R. Sessoli, A. Caneschi, D. Gatteschi, A. Cornia, D. Mailly, J. Appl. Phys. 87 (2000) 5481. [9] D. Eckert, R. GroK ssinger, M. Doerr, F. Fischer, A. Handstein, D. Hinz, H. Siegel, P. Verges, K.-H. MuK ller, in these Proceedings (RHMF 2000), Physica B 294}295 (2001). [10] H. Krug, M. Doerr, D. Eckert, H. Eschrig, F. Fischer, P. Fulde, R. GroK ssinger, A. Handstein, F. Herlach, D. Hinz, R. Kratz, M. Loewenhaupt, K.-H. MuK ller, F. Pobell, L. Schultz, H. Siegel, F. Steglich, P. Verges, in these Proceedings (RHMF 2000), Physica B 294}295 (2001). [11] C. Paulsen, J.G. Park, in: L. Gunther, B. Barbara (Eds.), Quantum Tunneling of Magnetization, Kluwer, Dordrecht, 1995.