A particularly sensitive heat capacity measuring device has allowed us to measure the tunneling process of Mn 12 O 12 -acetate single crystals (mass: 1 and 20 mg) from the irreversible tunneling process below the blocking temperature T B to the reversible resonant tunneling process above T B . Above the T B (typically 3.5 K) we find specific heat anomalies at the magnetic field values that correspond to the crossing of spin up and spin down levels of different magnetic quantum numbers. Below T B , heat relaxation pulses at the crossing of crystal field levels are observed for fields applied antiparallel to the initial magnetization. These measurements give a new scope to Mn 12 O 12 -acetate investigations and show the great interest of nanocalorimetry for studies of big magnetic molecules. [S0031-9007(97)03794-0] PACS numbers: 75.45. + j, 61.46. + w, 07.20.Fw Crystals of Mn 12 O 12 -acetate clusters [1] are molecular spin systems that exhibit spectacular effects [2]. Twelve manganese ions (4 Mn 31 and 8 Mn 41 ) are coupled by ferromagnetic exchange to a S 10 macrospin. The Mn 12 O 12 clusters are embedded in an organic matrix and show no exchange coupling from one cluster to another.The crystals are regular parallelepipeds with a strong magnetocrystalline anisotropy (ϳ60 K) along the longitudinal axis. Once oriented, they present at low temperatures and zero magnetic field a very long relaxation time of the magnetization (two months at 2 K [3]). Recently, quantum tunneling of molecular spins through the anisotropy, at magnetic field values that correspond to the crossing of spin up and spin down levels of different magnetic quantum numbers, has been demonstrated [4][5][6].The anisotropy lifts the 2S 1 1 degeneracy of the magnetic levels in zero field, creating a double well configuration [5] as sketched in Fig. 1. The manganese system is superparamagnetic and can be described by a Hamiltonian of the form [7]where D 0.6 K is the anisotropy energy per cluster, H the magnetic field applied parallel to the easy axis of magnetization, g Ӎ 2 is the gyromagnetic factor, and S is the spin per cluster. H l is a term that does not commute with S z and is due to the demagnetizing field, dipole coupling, higher anisotropy terms, and/or hyperfine splitting. A great effort is underway to understand how it comes about [7,8]. Thermal measurements as a function of a magnetic field enlighten the interplay between the spin system and the lattice of the crystal, e.g., clarifying how the phonons influence the tunneling of the macrospins, therefore, giving a great deal of new information.We have performed two kinds of measurements on Mn 12 O 12 monocrystals, the heat capacity as a function of an applied static magnetic field (C[H]) and the temperature as a function of a slowly scanned magnetic field ͑T ͓H͔͒. The measure of very small single crystals promises a high quality of the sample and avoids the broadening found with powder samples due to the slightly different characteristics of every crystal. On the other side, it implies a gr...
An innovative nanocalorimeter has been developed for measuring specific heats of thin films, multilayers (typical thickness: 1000 Å) and single crystals (mass: 1 μg) in the temperature range of 1.5–20 K. The addenda of the device are as small as 3 nJ/K at 4 K (0.5 nJ/K at 1.5 K), thus samples with a heat capacity of the order of nJ/K at 4 K can be measured. Heat capacity differences as a function of temperature or an external magnetic field (5 T) were determined with a resolution of ΔC/C≃10−4. This way we have seen heat capacity variations of less than a pJ/K. We present as an example measurements on very small Mn12O12 acetate single crystals and a measurement of a thin superconducting Pb layer. In the latter measurement we could evidence via specific heat a finite size effect.
We analyze the specific heat variations as a function of an external magnetic field of a simple model of superlattice that includes (i) in-plane ferromagnetic exchange, (ii) interplane ferromagnetic exchange, (iii) dipolar interactions, (iv) magnetocristalline anisotropy. The calculations are carried out at the spin wave level. The interplay between the existence of a canting transition and the anisotropy of the structure generate non trivial behavior for the spin wave contribution to the low temperature specific heat as a function of an external magnetic field when dipolar interactions and magnetocristalline anisotropy are taken into account. *
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