Zero-field-cooled magnetization, magnetic relaxation, and a.c. susceptibility measurements have been performed on a Mn12-Ac oriented powdered sample as a function of temperature, field, and orientation. Magnetization jumps and susceptibility peaks have been observed about magnetic-field values Bn = nB1, where B1 ≈ 5 kG. These anomalies are due to the existence of relaxation rate maxima near Bn. From these experimental results we infer the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation.

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We report experimental evidence of macroscopic quantum tunneling in antiferromagnetic particles. The zero-field maximum in the dependence of the magnetic relaxation rate on the magnetic field in ferritin has been observed. This observation provides a new insight into the origin of the nonmonotonic field dependence of the blocking temperature reported recently by Gider et al. [J. Appl. Phys. 79, 5324 (1996)] and by Friedman et al. (unpublished). We argue that the effect comes from the resonant spin tunneling between matching spin levels and cannot be due to the interparticle interactions. Our findings are in agreement with the tunneling interpretation of earlier experiments in ferritin by Awschalom et al. [Phys. Rev. Lett. 68, 3092 (1992) Quantum dynamics of the magnetization in ferritin has received much attention since Awschalom and co-workers reported the existence of a low temperature resonance in the absorption spectrum of horse spleen ferritin, which they attributed to the macroscopic quantum coherence [1]. Ferritin is an iron storage protein. It has a spherical cage of about 8 nm in diameter that contains the mineral ferrihydrite combined with a phosphate. Its core is equivalent to a small antiferromagnetic particle. The size of the core in natural ferritin ranges from 3 to 7.5 nm. The fully packed ferritin contains 4500 Fe 31 ions. A small magnetic moment of the particle arises from the noncompensation of collinear spin sublattices due to the finite size and irregular shape of the core. The spin of the sublattice S is of the order of 5000, while the noncompensated spin s is below 100. This noncompensated spin looks in one of two directions along the anisotropy axis of the particle. Above 13 K the particles are superparamagnetic [2]; that is, their moments frequently jump between the two orientations over the anisotropy barrier. Well below 13 K thermal processes are essentially frozen and the transition from s to 2s can occur only due to quantum tunneling: in the absence of the magnetic field and in the limit of weak dissipation, sublattices should interchange in a coherent manner. It has been demonstrated [3] that quantum tunneling of the magnetization in antiferromagnets is much stronger than in ferromagnets [4][5][6]. Achieving a narrow distribution of the magnetic core sizes could then enable one to observe a resonance in the ac susceptibility and noise spectrum at the frequency of the ac field that coincides with the frequency of the coherent oscillations of the antiferromagnetic sublattices [1,7]. The additional, though indirect, evidence of spin tunneling in natural ferritin came from the observation of the temperature independent magnetic relaxation below 2.4 K [2]. There existed, however, a significant inconsistency, pointed out in Ref. [8], between high and low temperature behavior of narrowly distributed particles studied in Ref. [7]. While high temperature measurements indicated that magnetic moments of the particles were essentially frozen below a few kelvin, resonance experiments in the mil...

We report controlled ignition of magnetization reversal avalanches by surface acoustic waves in a single crystal of Mn 12 acetate. Our data show that the speed of the avalanche exhibits maxima on the magnetic field at the tunneling resonances of Mn 12 . Combined with the evidence of magnetic deflagration in Mn 12 acetate [Y. Suzuki et al., Phys. Rev. Lett. 95, 147201 (2005)], this suggests a novel physical phenomenon: deflagration assisted by quantum tunneling. DOI: 10.1103/PhysRevLett.95.217205 PACS numbers: 75.50.Xx, 45.70.Ht Magnetic properties of Mn 12 acetate have been intensively studied after the magnetic bi-stability of this molecular cluster below 3.5 K was demonstrated [1]. The bistability is caused by a large spin of the cluster, S 10, and by strong uniaxial magnetic anisotropy that provides a 65 K energy barrier between spin-up and spin-down states. At low temperature a magnetized Mn 12 crystal exhibits two modes of magnetic relaxation. The first mode is a slow one. It manifests itself in a staircase hysteresis curve which is due to thermally assisted quantum tunneling of the magnetization [2]. The second relaxation mode, exhibited by sufficiently large crystals, is a much more rapid magnetization reversal that typically lasts less than 1 ms. It was initially studied by Paulsen and Park [3] and attributed to a thermal runaway or avalanche [see also Ref. [4]]. In the avalanche, the initial relaxation of the magnetization towards the direction of the field results in the release of heat that further accelerates the magnetic relaxation. Recent local magnetic measurements of Mn 12 crystals [5] have demonstrated that during an avalanche the magnetization reversal occurs inside a narrow interface that propagates through a crystal at a constant speed of a few meters per second. It has been argued that this process is analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. The conventional theory of deflagration, in the first approximation, yields the following expression for the velocity of the flame front [5-7]:Here U, 0 , and T f are the energy barrier, the attempt frequency, and the temperature of the ''flame'' in the expression 0 expU=k B T f for the ''chemical reaction'' time, and is thermal diffusivity. In the case of Mn 12 , 10 ÿ5 m 2 =s, 0 10 ÿ7 s, and the field dependence of the energy barrier, UH, is well known.In a flammable chemical substance the potential barrier is a constant determined by the nature of the chemical reaction that transforms a metastable chemical into a stable chemical (e.g., a mixture of hydrogen and oxygen transforms into water). On the contrary, in molecular magnets the energy barrier, as well as the released energy, can be controlled by the magnetic field. At certain values of the magnetic field the spin levels on the two sides of the energy barrier come to resonance and thermally assisted quantum spin tunneling under the barrier takes place; see Fig. 1. Therefore, the effect of the tunneling is roughly equivalent to cutting ...

The magnetic ac susceptibility of oriented Mn 12 Ac crystallites has been measured as a function of temperature, field, and frequency. The field has been applied at different values of the angle with respect to the sample easy axis. For Tϭ5 K, the isothermal and adiabatic limits have been determined as a function of field. For ϭ0°and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field values H n ϭnH 1 with nϭ0, 1, and 2 where H 1 ϭ4.1 kOe. As increases these maxima shift to higher fields, that satisfy H n cosϭconst, and decrease in amplitude. The relaxation time 1 follows Arrhenius' law with respect to temperature and decreases sharply at HϭH n . The observed phenomenology unambiguously proves the existence of field-tuned tunneling between excited magnetic states which are thermally populated. At 5 K, the effective activation energy and the spin states involved in the tunneling process have been obtained.

We have measured the dc magnetization at low temperatures of tetragonal crystals of Mn 12 acetate complex ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔, a material composed of a large ͑Avogadro's͒ number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy ͑ϳ60 K͒ gives rise to a doubly degenerate ground state corresponding to spin projections of Ϯ10 along the easy axis ͑c axis͒; hysteretic behavior is found below a blocking temperature T b ϳ3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results. ͓S0163-1829͑97͒00709-1͔

The paper reports a detailed experimental study on magnetic relaxation of natural horse-spleen ferritin. ac susceptibility measurements performed on three samples of different concentration show that dipole-dipole interactions between uncompensated moments play no significant role. Furthermore, the distribution of relaxation times in these samples has been obtained from a scaling of experimental Љ data, obtained at different frequencies. The average uncompensated magnetic moment per protein is compatible with a disordered arrangement of atomic spins throughout the core, rather than with surface disorder. The observed field dependence of the blocking temperature suggests that magnetic relaxation is faster at zero field than at intermediate field values. This is confirmed by the fact that the magnetic viscosity peaks at zero field, too. Using the distribution of relaxation times obtained independently, we show that these results cannot be explained in terms of classical relaxation theory. The most plausible explanation of these results is the existence, near zero field, of resonant magnetic tunneling between magnetic states of opposite orientation, which are thermally populated.

Context. Yebes 40 m radio telescope is the main and largest observing instrument at Yebes Observatory and is devoted to very long baseline interferometry (VLBI) and single-dish observations since 2010. It has been covering frequency bands between 2 GHz and 90 GHz in discontinuous and narrow windows in most cases in order to match the current needs of the European VLBI Network (EVN) and the Global Millimeter VLBI Array (GMVA). Aims. The Nanocosmos project, a European Union-funded synergy grant, has enabled an increase in the instantaneous frequency coverage of the Yebes 40 m radio telescope, making it possible to observe many molecular transitions with single tunings in singledish mode. This reduces the observing time and maximises the output from the telescope. Methods. We present technical specifications of the recently installed 31.5 − 50 GHz (Q band) and 72 − 90.5 GHz (W band) receivers along with the main characteristics of the telescope at these frequency ranges. We observed IRC+10216, CRL 2688, and CRL 618, which harbour a rich molecular chemistry, to demonstrate the capabilities of the new instrumentation for spectral observations in single-dish mode. Results. Our results show the high sensitivity of the telescope in the Q band. The spectrum of IRC+10126 offers an unprecedented signal-to-noise ratio for this source in this band. On the other hand, the spectrum normalised by the continuum flux towards CRL 618 in the W band demonstrates that the 40 m radio telescope produces comparable results to those from the IRAM 30 m radio telescope, although with a lower sensitivity. The new receivers fulfil one of the main goals of Nanocosmos and open up the possibility to study the spectrum of different astrophysical media with unprecedented sensitivity.