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Heat capacity anomalies in Fe8 single crystal
LETTER TO THE EDITOR
Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Letter to the Editor
Heat capacity anomalies in Fe single crystal F. Fominaya , P. Gandit *, G. Gaudin , J. Chaussy , R. Sessoli, C. Sangregorio CRTBT-CNRS, 25 av. des Martyrs, BP166, F-38042 Grenoble Cedex 9, France Department of Chemistry, University of Florence, Via Maragliano 77, I-550144 Firenze, Italy Received 29 October 1998
Abstract A preliminary heat capacity study of Fe molecules as a function of applied magnetic "eld, temperature and measuring frequency is presented. The heat capacity versus temperature curve shows in zero "eld several peaks between 2 and 3 K. The anomalies shift to a lower temperature when a magnetic "eld is applied. With increasing "eld the peaks are smeared. No frequency dependence could be established in the measured range of u "217}4440 Hz. We have also performed heat capacity measurements as a function of magnetic "eld at "xed temperature. Between 2 and 3 K a pattern of well developed peaks at discrete magnetic "eld values is observed. The pattern strongly changes in the small temperature range between 2.3 and 2.5 K. 1999 Elsevier Science B.V. All rights reserved. PACS: 07.20.Fw; 75.45.#j; 61.46.#w Keywords: Heat capacity; Magnetic molecule; Tunneling of magnetization
Big magnetic molecules can form crystals of identical, iso-oriented, high spin nanomagnets. They can show a variety of interesting features such as a quantum tunneling of magnetization and a very long relaxation time at low temperature [1]. Fe crystals (chemical formula [(tacn) Fe O (OH) ]>, where tacn is triazacy clononane) are one of the most interesting molecules of this class and they are being intensively studied (see Ref. [2] and references therein). Fe clusters have a spin ground state S"10 and Ising* Corresponding author. E-mail address: [email protected] (P. Gandit) Associated with the university Joseph Fourier, Grenoble, France.
type magnetic anisotropy. The main di!erence to the better known Mn clusters is a lower barrier for reversal of magnetization (Fe : 22 K, Mn : 61 K) and a second-order anisotropy term in Fe , that does not exist in Mn . In principle, the heat capacity of Fe was expected to show a similar behavior to Mn -acetate, which is now qualita tively quite well understood [3}6]. However, from the beginning strong di!erences were remarked. We have performed the heat capacity measurements with a highly sensitive nanocalorimeter [7] on a 9.6 lg Fe single crystal. The equilibrium heat capacity as a function of temperature of the sample was recorded in zero "eld (Fig. 1). The measurements were performed as reported in Refs. [3,7]. Four main peaks were observed between 1.9 and
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 0 3 7 - 2
LETTER TO THE EDITOR
L254
F. Fominaya et al. / Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Fig. 3. Heat capacity in di!erent applied magnetic "elds. Fig. 1. Zero "eld heat capacity of an Fe single crystal of 9.6 lg.
Fig. 2. Heat capacity versus temperature at di!erent frequencies.
2.9 K. The peaks are independent of the history of the sample. Quite surprisingly the change of the measuring frequency does not alter the peaks, at least in the accessible range of frequencies: Fig. 2 shows a zoom of the main peaks at di!erent frequencies. This feature seems to exclude a mechanism of sudden reduction of the relaxation time of magnetization in the crystal as the source of peaks. The smearing of the peaks at the lower frequencies is due to the greater amplitude of the temperature oscillation when frequency is decreased: A greater amplitude implies, for a constant dissipated power, an average over a slightly larger temperature range. Fig. 2 shows a di!erence with Fig. 1: a new peak appears at 2.5 K. Both curves have been recorded in similar conditions, the only di!erence being the dissipated power P. In one case P"82 nW and in
the other P"13 nW. A higher power results in a greater amplitude d¹ of the temperature oscillation (remember we are using the AC-steady state method). In principle, however, the amplitude d¹ should not a!ect a measurement. Fig. 3 shows the heat capacity versus temperature curve at di!erent magnetic "elds. The peaks smear and shift towards lower temperatures with increasing "eld. Note that in Mn no such anomalies were observed. Globally, the heat capacity of the sample rises when the "eld is increased. This supplementary contribution is due to the Zeeman splitting introduced by the magnetic "eld. A closer look at curves 2 and 3 shows that the temperature at which a peak appears slightly changes from one "gure to the other. This is caused by the spatial distance between the sample and the thermometer of the device. We always admit that the crystal temperature and the temperature measured by the thermometer are the same. This is actually not always the case, in fact a small error (less than 200 mK) is done in the determination of the absolute temperature. The error depends on the power that is dissipated on the sample-holder. All the measurements shown in Fig. 3 were performed using the same power. We have recorded heat capacity versus magnetic "eld at constant temperature following the same procedure reported in Ref. [3]. In Mn we found a pattern of peaks where the position of peaks was always at the same discrete "eld values. A temperature change could alter the shape of peaks but not their position. In Fe a completely di!erent
LETTER TO THE EDITOR
F. Fominaya et al. / Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Fig. 4. Heat capacity versus magnetic "eld at constant temperature.
behavior is found: Fig. 4 shows the C(H) curves in the most interesting temperature range, i.e. between 2.3 and 2.6 K. Interpreting our results the way it was done for Mn [3] will give us peaks regularly spaced of *H"D/gk +0.2 T (with D"0.27 K [2]). This behavior cannot be clearly observed in our measurements. However, the surprising feature is the strong change of the pattern in a relatively small temperature range of 200 mK. So, a slight increase of temperature from 2.37 to 2.45 K strongly enhances the peak at 0.5 T. The anomaly at 1.25 T disappears and a further peak at 1.5 T is
L255
created. Below 2.3 K, the lower the temperature the higher the magnetic "elds at which peaks appear. Above 2.5 K, the peaks tend to smear over a large "eld range. Finally, above 3 K no more peaks are found. In conclusion an unexpected behavior has been observed in heat capacity measurements of an Fe single crystal. No plausible explanation can be advanced except a possible interplay between the spin tunneling and the temperature-dependent secondorder crystal anisotropy. A further study of this highly interesting molecular magnet will certainly allow to understand the phenomenon.
References [1] B. Schwarzschild, Phys. Today 50 (1) (1997) 17. [2] C. Sangregorio, T. Ohm, C. Paulsen, R. Sessoli, D. Gatteschi, Phys. Rev. Lett. 78 (1997) 4645. [3] F. Fominaya, J. Villain, P. Gandit, J. Chaussy, A. Caneschi, Phys. Rev. Lett. 79 (1997) 1126. [4] F. Fominaya, J. Villain, T. Fournier, P. Gandit, J. Chaussy, A. Fort, A. Caneschi, Phys. Rev. B 59 (1999) 519. [5] A.M. Gomes, M.A. Novak, R. Sessoli, A. Caneschi, D. Gatteschi, Phys. Rev. B 57 (1998) 5021. [6] J.F. FernaH ndez, F. Luis, J. BartolomeH , Phys. Rev. Lett. 80 (1999) 5659. [7] F. Fominaya, T. Fournier, P. Gandit, J. Chaussy, Rev. Sci. Instr. 68 (1997) 4191.
Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Letter to the Editor
Heat capacity anomalies in Fe single crystal F. Fominaya , P. Gandit *, G. Gaudin , J. Chaussy , R. Sessoli, C. Sangregorio CRTBT-CNRS, 25 av. des Martyrs, BP166, F-38042 Grenoble Cedex 9, France Department of Chemistry, University of Florence, Via Maragliano 77, I-550144 Firenze, Italy Received 29 October 1998
Abstract A preliminary heat capacity study of Fe molecules as a function of applied magnetic "eld, temperature and measuring frequency is presented. The heat capacity versus temperature curve shows in zero "eld several peaks between 2 and 3 K. The anomalies shift to a lower temperature when a magnetic "eld is applied. With increasing "eld the peaks are smeared. No frequency dependence could be established in the measured range of u "217}4440 Hz. We have also performed heat capacity measurements as a function of magnetic "eld at "xed temperature. Between 2 and 3 K a pattern of well developed peaks at discrete magnetic "eld values is observed. The pattern strongly changes in the small temperature range between 2.3 and 2.5 K. 1999 Elsevier Science B.V. All rights reserved. PACS: 07.20.Fw; 75.45.#j; 61.46.#w Keywords: Heat capacity; Magnetic molecule; Tunneling of magnetization
Big magnetic molecules can form crystals of identical, iso-oriented, high spin nanomagnets. They can show a variety of interesting features such as a quantum tunneling of magnetization and a very long relaxation time at low temperature [1]. Fe crystals (chemical formula [(tacn) Fe O (OH) ]>, where tacn is triazacy clononane) are one of the most interesting molecules of this class and they are being intensively studied (see Ref. [2] and references therein). Fe clusters have a spin ground state S"10 and Ising* Corresponding author. E-mail address: [email protected] (P. Gandit) Associated with the university Joseph Fourier, Grenoble, France.
type magnetic anisotropy. The main di!erence to the better known Mn clusters is a lower barrier for reversal of magnetization (Fe : 22 K, Mn : 61 K) and a second-order anisotropy term in Fe , that does not exist in Mn . In principle, the heat capacity of Fe was expected to show a similar behavior to Mn -acetate, which is now qualita tively quite well understood [3}6]. However, from the beginning strong di!erences were remarked. We have performed the heat capacity measurements with a highly sensitive nanocalorimeter [7] on a 9.6 lg Fe single crystal. The equilibrium heat capacity as a function of temperature of the sample was recorded in zero "eld (Fig. 1). The measurements were performed as reported in Refs. [3,7]. Four main peaks were observed between 1.9 and
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 0 3 7 - 2
LETTER TO THE EDITOR
L254
F. Fominaya et al. / Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Fig. 3. Heat capacity in di!erent applied magnetic "elds. Fig. 1. Zero "eld heat capacity of an Fe single crystal of 9.6 lg.
Fig. 2. Heat capacity versus temperature at di!erent frequencies.
2.9 K. The peaks are independent of the history of the sample. Quite surprisingly the change of the measuring frequency does not alter the peaks, at least in the accessible range of frequencies: Fig. 2 shows a zoom of the main peaks at di!erent frequencies. This feature seems to exclude a mechanism of sudden reduction of the relaxation time of magnetization in the crystal as the source of peaks. The smearing of the peaks at the lower frequencies is due to the greater amplitude of the temperature oscillation when frequency is decreased: A greater amplitude implies, for a constant dissipated power, an average over a slightly larger temperature range. Fig. 2 shows a di!erence with Fig. 1: a new peak appears at 2.5 K. Both curves have been recorded in similar conditions, the only di!erence being the dissipated power P. In one case P"82 nW and in
the other P"13 nW. A higher power results in a greater amplitude d¹ of the temperature oscillation (remember we are using the AC-steady state method). In principle, however, the amplitude d¹ should not a!ect a measurement. Fig. 3 shows the heat capacity versus temperature curve at di!erent magnetic "elds. The peaks smear and shift towards lower temperatures with increasing "eld. Note that in Mn no such anomalies were observed. Globally, the heat capacity of the sample rises when the "eld is increased. This supplementary contribution is due to the Zeeman splitting introduced by the magnetic "eld. A closer look at curves 2 and 3 shows that the temperature at which a peak appears slightly changes from one "gure to the other. This is caused by the spatial distance between the sample and the thermometer of the device. We always admit that the crystal temperature and the temperature measured by the thermometer are the same. This is actually not always the case, in fact a small error (less than 200 mK) is done in the determination of the absolute temperature. The error depends on the power that is dissipated on the sample-holder. All the measurements shown in Fig. 3 were performed using the same power. We have recorded heat capacity versus magnetic "eld at constant temperature following the same procedure reported in Ref. [3]. In Mn we found a pattern of peaks where the position of peaks was always at the same discrete "eld values. A temperature change could alter the shape of peaks but not their position. In Fe a completely di!erent
LETTER TO THE EDITOR
F. Fominaya et al. / Journal of Magnetism and Magnetic Materials 195 (1999) L253}L255
Fig. 4. Heat capacity versus magnetic "eld at constant temperature.
behavior is found: Fig. 4 shows the C(H) curves in the most interesting temperature range, i.e. between 2.3 and 2.6 K. Interpreting our results the way it was done for Mn [3] will give us peaks regularly spaced of *H"D/gk +0.2 T (with D"0.27 K [2]). This behavior cannot be clearly observed in our measurements. However, the surprising feature is the strong change of the pattern in a relatively small temperature range of 200 mK. So, a slight increase of temperature from 2.37 to 2.45 K strongly enhances the peak at 0.5 T. The anomaly at 1.25 T disappears and a further peak at 1.5 T is
L255
created. Below 2.3 K, the lower the temperature the higher the magnetic "elds at which peaks appear. Above 2.5 K, the peaks tend to smear over a large "eld range. Finally, above 3 K no more peaks are found. In conclusion an unexpected behavior has been observed in heat capacity measurements of an Fe single crystal. No plausible explanation can be advanced except a possible interplay between the spin tunneling and the temperature-dependent secondorder crystal anisotropy. A further study of this highly interesting molecular magnet will certainly allow to understand the phenomenon.
References [1] B. Schwarzschild, Phys. Today 50 (1) (1997) 17. [2] C. Sangregorio, T. Ohm, C. Paulsen, R. Sessoli, D. Gatteschi, Phys. Rev. Lett. 78 (1997) 4645. [3] F. Fominaya, J. Villain, P. Gandit, J. Chaussy, A. Caneschi, Phys. Rev. Lett. 79 (1997) 1126. [4] F. Fominaya, J. Villain, T. Fournier, P. Gandit, J. Chaussy, A. Fort, A. Caneschi, Phys. Rev. B 59 (1999) 519. [5] A.M. Gomes, M.A. Novak, R. Sessoli, A. Caneschi, D. Gatteschi, Phys. Rev. B 57 (1998) 5021. [6] J.F. FernaH ndez, F. Luis, J. BartolomeH , Phys. Rev. Lett. 80 (1999) 5659. [7] F. Fominaya, T. Fournier, P. Gandit, J. Chaussy, Rev. Sci. Instr. 68 (1997) 4191.