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Spin freezing in maghemite nanoparticle systems
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1942}1944
Spin freezing in maghemite nanoparticle systems D. Fiorani *, A.M. Testa , E. Tronc, F. Lucari, F. D'Orazio, M. NogueH s ICMAT-CNR, Area della Ricerca di Roma, C.P. 10,00016 Monterotondo Stazione, Rome, Italy LCMC, UMR 7574 CNRS-Univ. Pierre et Marie Curie, 75252 Paris Cedex 05, France Dip.to di Fisica, Universita& di L+Aquila and INFM, 67010 Coppito, L+Aquila, Italy UMR 8634 CNRS-Universite% de Versailles-Saint Quentin, 78035 Versailles Cedex, France
Abstract The static and dynamical properties of two powder samples of -Fe O nanoparticles, with average diameter D"2.7 and 4.6 nm have been investigated by AC and DC susceptibility measurements. The results provide evidence of a collective dynamical freezing due to dipole interactions, at a size-dependent temperature (T"95 K for D"2.7 nm; 130 K for D"4.6 nm ). Between 50 and 60 K, a change of magnetic regime occurs for both samples induced by increasing surface e!ects with decreasing temperature. 2001 Elsevier Science B.V. All rights reserved. Keywords: Fine particles; Maghemite; Spin freezing
1. Introduction Recent studies, by a variety of techniques, have shown the great complexity of the magnetic state of real assemblies of "ne particles [1]. Such state has been found to be strongly dependent on the particle volume distribution, particle surface spin state and strength and type of interparticle interactions (dipole}dipole and exchange, if the particles are in close contact). Moreover, intraparticle ( e.g. volume and surface anisotropy, exchange anisotropy [2]) and interparticle phenomena [3] are so closely related that they can hardly be disentangled. Both surface spin disorder and frustration, intrinsic in dipole}dipole interactions, lead to a multi-state energy system, with a broad distribution of energy barriers, like in spinglass-type systems [4]. Our previous investigations [5] on the static and dynamic magnetic properties of many series of -Fe O particles of di!erent size ( between 3 and 10 nm ) dispersed in polyvinylic alcohol (PVA), with di!erent interparticle distance, have given evidence of the major role of the surface in the particle magnetic anisotropy and of the
* Corresponding author. Tel.: #39-06-90672553; fax: #3906-90672552. E-mail address: "[email protected] (D. Fiorani).
e!ect of dipole}dipole interparticle interactions on the blocking process of particle moments. Interactions have been found to increase the e!ective anisotropy energy barriers of individual particles, which become interdependent, as predicted by a modi"ed superparamagnetic model accounting for interparticle interactions e!ects [6] and by Monte Carlo simulations [7]. Investigations on a powder sample, where interparticle interactions are much stronger, gave evidence of collective e!ects, and glassy features ( critical divergence of the relaxation time, waiting time e!ect in magnetic relaxation ) [8]. Aiming at a better understanding of the nature of the collective magnetic state and of the role played by particle surface e!ects, in this paper we report the results of DC and AC susceptibility measurements on two powder samples of -Fe O with average diameter D"2.7 and 4.6 nm.
2. Results and discussion -Fe O particles were prepared by a chemical method according to Ref. [9]. The X-ray di!raction patterns are typical of the cubic ( Fd3 m) structure with lattice constant a"(0.835$0.001) nm. TEM measurements showed that the particles are roughly spheroidal
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 8 0 - 6
D. Fiorani et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1942}1944
Fig. 1. Temperature dependence of ZFC and FC susceptibility (H "20 Oe) for the sample of average particle diameter D"2.7 nm.
and ellipsoidal and that the particle size distribution is lognormal in shape with an average diameter, de"ned as D"(6</) , equal to 2.7 nm (51 As sample) and 4.6 nm (36 As sample ). The Fe O content determined by thermal analysis is 90 wt% (sample 51 As ) and 95 wt% (sample 36 As ), with 10 and 5 wt%, respectively, of physisorbed and chemisorbed water making the interparticle coupling by superexchange possible in principle, through one or more terminal hydroxyl groups, if particles are in close contact. DC susceptibility measurements were performed by a commercial SQUID magnetometer; AC susceptibility measurements were performed at di!erent frequencies (5((10 Hz), with an amplitude h"1 Oe, by a commercial susceptometer. The temperature dependence of the low-"eld susceptibility, measured according to the usual zero-"eld-cooling (ZFC) and "eld-cooling (FC) procedures, is reported in Fig. 1 for the sample with D"2.7 nm. The ZFC magnetization shows a well-de"ned maximum at T "95 K,
below which the FC magnetization splits from the ZFC curve and becomes almost temperature independent. Such features are similar to those exhibited by spin-glass systems, suggesting a collective blocking of particle moments. The same behavior has been found for the powder sample with D"4.6 nm, where T "130 K [8].
The temperature dependence of the in phase and out of phase components of the "rst harmonic of the AC susceptibility is reported in Fig. 2 for the sample with D"4.6 nm. shows a sharp maximum at a frequencydependent temperature ¹ (), e.g. at "5 Hz,
T "133 K.
The same behavior was observed for the sample with D"2.7 nm ( T "97 K at "5 Hz). shows a com plex structure for both samples: with decreasing temperature it increases rapidly in the proximity of ¹ (v), then
at low temperature, between 50 and 60 K for both samples, it decreases very rapidly and becomes almost frequency independent. At higher temperatures, increases with frequency, as found in strongly dipole}dipole interacting nanoparticle systems, unlike in non-interacting
1943
Fig. 2. AC susceptibility for the sample of average particle diameter D"4.6 nm.
ones. This suggests that both samples enter a di!erent magnetic regime below 50}60 K, characterized by a different distribution of relaxation times.This is supported by changes observed in the features of the hysteresis cycles below 50 K (not shown) [10]: the virgin curve lies below the remagnetizing one and remanence and coercive "eld strongly increase. On the other hand, there are no evidences of change of magnetic regime for the samples of dipole}dipole interacting particles dispersed in PVA. ¹ () changes with particle size, increasing with it,
whereas the feature occurs at comparable temperature for the two powder samples, being absent for the samples of dispersed dipole}dipole interacting particles. This suggests that the collective blocking at T is mainly deter mined by dipole}dipole interactions and that the particle surface e!ects play a signi"cant role in inducing the low-temperature magnetic state. This could be established through exchange or super-exchange interactions between some particles in close contact, if the geometrical requirements for the overlap of orbitals are satis"ed, and/or through strong short-range dipole}dipole interactions possibly leading to local variations of surface spins orientations. In such a case, the resulting low temperature magnetic state should consist in a collection of "nite size domains of particle moments. In conclusion, the results show that both interparticle interactions and surface e!ects play a dominant role in determining the observed collective e!ects between particle moments with decreasing temperature.
References [1] J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997) 283. [2] A.E. Berkowitz, Kentano Takano, J. Magn. Magn. Mater. 200 (1999) 552.
1944
D. Fiorani et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1942}1944
[3] T. Jonsonn, P. Svedlindh, M.F. Hansen, Phys. Rev. Lett. 81 (1998) 3796. [4] T. Jonsonn, P. Nordlab, P. Svedlindh, Phys. Rev. B 57 (1998) 497. [5] J.L. Dormann, F. D'Orazio, F. Lucari, E. Tronc, P. Prene`, J.P. Jolivet, D. Fiorani, R. Cherkaoui, M. Nogue`s, Phys. Rev. B 53 (1996) 14291. [6] J.L. Dormann, L. Bessais, D. Fiorani, J. Phys. C 21 (1998) 2015.
[7] R.W. Chantrell, N.S. Walmsley, J. Gore, M. Maylin, J. Magn. Magn. Mater. 196}197 (1999) 118. [8] D. Fiorani, J.L. Dormann, R. Cherkaoui, E. Tronc, F. Lucari, F. D'Orazio, L. Spinu, M. Nogue`s, A. Garcia, A.M. Testa, J. Magn. Magn. Mater. 196}197 (1999) 143. [9] J.P. Jolivet, C. Chaneac, P. PreneH , L. Vayssie`res, E. Tronc, J. Phys. IV France 7 (1997) C1}573. [10] D. Fiorani et al., unpublished.
Spin freezing in maghemite nanoparticle systems D. Fiorani *, A.M. Testa , E. Tronc, F. Lucari, F. D'Orazio, M. NogueH s ICMAT-CNR, Area della Ricerca di Roma, C.P. 10,00016 Monterotondo Stazione, Rome, Italy LCMC, UMR 7574 CNRS-Univ. Pierre et Marie Curie, 75252 Paris Cedex 05, France Dip.to di Fisica, Universita& di L+Aquila and INFM, 67010 Coppito, L+Aquila, Italy UMR 8634 CNRS-Universite% de Versailles-Saint Quentin, 78035 Versailles Cedex, France
Abstract The static and dynamical properties of two powder samples of -Fe O nanoparticles, with average diameter D"2.7 and 4.6 nm have been investigated by AC and DC susceptibility measurements. The results provide evidence of a collective dynamical freezing due to dipole interactions, at a size-dependent temperature (T"95 K for D"2.7 nm; 130 K for D"4.6 nm ). Between 50 and 60 K, a change of magnetic regime occurs for both samples induced by increasing surface e!ects with decreasing temperature. 2001 Elsevier Science B.V. All rights reserved. Keywords: Fine particles; Maghemite; Spin freezing
1. Introduction Recent studies, by a variety of techniques, have shown the great complexity of the magnetic state of real assemblies of "ne particles [1]. Such state has been found to be strongly dependent on the particle volume distribution, particle surface spin state and strength and type of interparticle interactions (dipole}dipole and exchange, if the particles are in close contact). Moreover, intraparticle ( e.g. volume and surface anisotropy, exchange anisotropy [2]) and interparticle phenomena [3] are so closely related that they can hardly be disentangled. Both surface spin disorder and frustration, intrinsic in dipole}dipole interactions, lead to a multi-state energy system, with a broad distribution of energy barriers, like in spinglass-type systems [4]. Our previous investigations [5] on the static and dynamic magnetic properties of many series of -Fe O particles of di!erent size ( between 3 and 10 nm ) dispersed in polyvinylic alcohol (PVA), with di!erent interparticle distance, have given evidence of the major role of the surface in the particle magnetic anisotropy and of the
* Corresponding author. Tel.: #39-06-90672553; fax: #3906-90672552. E-mail address: "[email protected] (D. Fiorani).
e!ect of dipole}dipole interparticle interactions on the blocking process of particle moments. Interactions have been found to increase the e!ective anisotropy energy barriers of individual particles, which become interdependent, as predicted by a modi"ed superparamagnetic model accounting for interparticle interactions e!ects [6] and by Monte Carlo simulations [7]. Investigations on a powder sample, where interparticle interactions are much stronger, gave evidence of collective e!ects, and glassy features ( critical divergence of the relaxation time, waiting time e!ect in magnetic relaxation ) [8]. Aiming at a better understanding of the nature of the collective magnetic state and of the role played by particle surface e!ects, in this paper we report the results of DC and AC susceptibility measurements on two powder samples of -Fe O with average diameter D"2.7 and 4.6 nm.
2. Results and discussion -Fe O particles were prepared by a chemical method according to Ref. [9]. The X-ray di!raction patterns are typical of the cubic ( Fd3 m) structure with lattice constant a"(0.835$0.001) nm. TEM measurements showed that the particles are roughly spheroidal
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 8 0 - 6
D. Fiorani et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1942}1944
Fig. 1. Temperature dependence of ZFC and FC susceptibility (H "20 Oe) for the sample of average particle diameter D"2.7 nm.
and ellipsoidal and that the particle size distribution is lognormal in shape with an average diameter, de"ned as D"(6</) , equal to 2.7 nm (51 As sample) and 4.6 nm (36 As sample ). The Fe O content determined by thermal analysis is 90 wt% (sample 51 As ) and 95 wt% (sample 36 As ), with 10 and 5 wt%, respectively, of physisorbed and chemisorbed water making the interparticle coupling by superexchange possible in principle, through one or more terminal hydroxyl groups, if particles are in close contact. DC susceptibility measurements were performed by a commercial SQUID magnetometer; AC susceptibility measurements were performed at di!erent frequencies (5((10 Hz), with an amplitude h"1 Oe, by a commercial susceptometer. The temperature dependence of the low-"eld susceptibility, measured according to the usual zero-"eld-cooling (ZFC) and "eld-cooling (FC) procedures, is reported in Fig. 1 for the sample with D"2.7 nm. The ZFC magnetization shows a well-de"ned maximum at T "95 K,
below which the FC magnetization splits from the ZFC curve and becomes almost temperature independent. Such features are similar to those exhibited by spin-glass systems, suggesting a collective blocking of particle moments. The same behavior has been found for the powder sample with D"4.6 nm, where T "130 K [8].
The temperature dependence of the in phase and out of phase components of the "rst harmonic of the AC susceptibility is reported in Fig. 2 for the sample with D"4.6 nm. shows a sharp maximum at a frequencydependent temperature ¹ (), e.g. at "5 Hz,
T "133 K.
The same behavior was observed for the sample with D"2.7 nm ( T "97 K at "5 Hz). shows a com plex structure for both samples: with decreasing temperature it increases rapidly in the proximity of ¹ (v), then
at low temperature, between 50 and 60 K for both samples, it decreases very rapidly and becomes almost frequency independent. At higher temperatures, increases with frequency, as found in strongly dipole}dipole interacting nanoparticle systems, unlike in non-interacting
1943
Fig. 2. AC susceptibility for the sample of average particle diameter D"4.6 nm.
ones. This suggests that both samples enter a di!erent magnetic regime below 50}60 K, characterized by a different distribution of relaxation times.This is supported by changes observed in the features of the hysteresis cycles below 50 K (not shown) [10]: the virgin curve lies below the remagnetizing one and remanence and coercive "eld strongly increase. On the other hand, there are no evidences of change of magnetic regime for the samples of dipole}dipole interacting particles dispersed in PVA. ¹ () changes with particle size, increasing with it,
whereas the feature occurs at comparable temperature for the two powder samples, being absent for the samples of dispersed dipole}dipole interacting particles. This suggests that the collective blocking at T is mainly deter mined by dipole}dipole interactions and that the particle surface e!ects play a signi"cant role in inducing the low-temperature magnetic state. This could be established through exchange or super-exchange interactions between some particles in close contact, if the geometrical requirements for the overlap of orbitals are satis"ed, and/or through strong short-range dipole}dipole interactions possibly leading to local variations of surface spins orientations. In such a case, the resulting low temperature magnetic state should consist in a collection of "nite size domains of particle moments. In conclusion, the results show that both interparticle interactions and surface e!ects play a dominant role in determining the observed collective e!ects between particle moments with decreasing temperature.
References [1] J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997) 283. [2] A.E. Berkowitz, Kentano Takano, J. Magn. Magn. Mater. 200 (1999) 552.
1944
D. Fiorani et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1942}1944
[3] T. Jonsonn, P. Svedlindh, M.F. Hansen, Phys. Rev. Lett. 81 (1998) 3796. [4] T. Jonsonn, P. Nordlab, P. Svedlindh, Phys. Rev. B 57 (1998) 497. [5] J.L. Dormann, F. D'Orazio, F. Lucari, E. Tronc, P. Prene`, J.P. Jolivet, D. Fiorani, R. Cherkaoui, M. Nogue`s, Phys. Rev. B 53 (1996) 14291. [6] J.L. Dormann, L. Bessais, D. Fiorani, J. Phys. C 21 (1998) 2015.
[7] R.W. Chantrell, N.S. Walmsley, J. Gore, M. Maylin, J. Magn. Magn. Mater. 196}197 (1999) 118. [8] D. Fiorani, J.L. Dormann, R. Cherkaoui, E. Tronc, F. Lucari, F. D'Orazio, L. Spinu, M. Nogue`s, A. Garcia, A.M. Testa, J. Magn. Magn. Mater. 196}197 (1999) 143. [9] J.P. Jolivet, C. Chaneac, P. PreneH , L. Vayssie`res, E. Tronc, J. Phys. IV France 7 (1997) C1}573. [10] D. Fiorani et al., unpublished.