Ll. Balcells

Magnetization and Mössbauer studies of ultrafine Fe-C particles

Abstract The magnetic properties of ultrafine amorphous Fe 1- x C x particles have been studied as a function of temperature and applied field. Magnetization and Mossbauer spectroscopy investigations were combined in order to study superparamagnetic relaxation phenomena. The particles were prepared by thermal decomposition of Fe(CO) 5 . This preparation method is known to produce almost mono-size particles. The mean particle diameter of the Fe-C particles in the present study was about 3.1 nm. The small size resulted in finite size effects, which, e.g. for the temperature dependence of the saturation magnetization gave a significant deviation from the T 3 2 law. The deviation is in accord with recent theoretical calculations of the behaviour of spin waves in ultrafine particles. From the superparamagnetic relaxation studies the minimum relaxation time was estimated to be about 2 × 10 −12 s.

Nonthermal viscosity in magnets: Quantum tunneling of the magnetization (invited)

In this article we present experimental results on the magnetic relaxation in different systems (single domain particles, magnetic grains, and random magnets). The existence of two relaxation regimes is demonstrated. At high temperatures, the magnetic viscosity S≡1/M0∂M/∂u2009ln(t) is proportional to temperature in accordance with theoretical expectation for thermally activated processes. At low temperatures, the viscosity is independent of temperature, providing evidence to quantum tunneling of magnetization. Qualitative agreement between theory and experiment is found.In this article we present experimental results on the magnetic relaxation in different systems (single domain particles, magnetic grains, and random magnets). The existence of two relaxation regimes is demonstrated. At high temperatures, the magnetic viscosity S≡1/M0∂M/∂u2009ln(t) is proportional to temperature in accordance with theoretical expectation for thermally activated processes. At low temperatures, the viscosity is independent of temperature, providing evidence to quantum tunneling of magnetization. Qualitative agreement between theory and experiment is found.

Time dependent phenomena at low temperatures in SmCo multilayers: quantum nucleation phenomena

Abstract In this paper we report on the first observation of both the quantum nucleation of a ferromagnet and the space-time instantons in condensed matter physics. Our conclusions are based on the observance of non-thermally activated time dependent phenomena at temperatures below 3 K in the 2D random magnet SmCo multilayer.

Quantum tunneling of magnetization in single domain particles

We present a study of the magnetic relaxation of several ferrofluids composed of particles of about 40 A in diameter (Fe3O4FeC, CoFe2O4). Our key observation is a nonthermal character of the relaxation below 3 K for the CoFe2O4 ferrofluid and below 1 K for the FeC ferrofluid. The crossover temperature from thermal to nonthermal (quantum) regime is in accordance with theoretical suggestions of macroscopic quantum tunneling of magnetization in single domain particles.

T ln(tτ0) scaling in small-particle systems: low-temperature behaviour

An alternative method for analyzing the magnetic relaxation data in small-particle systems based on the T ln(tτ0) scaling is presented. Two experimental systems have been studied using this procedure and the resulting master curves have been fitted using different functions for the energy barrier distribution to reproduce the low-temperature relaxation behaviour.

Quantum tunneling of magnetization in multilayer systems

Abstract The observation of the quantum tunneling of magnetization has been accomplished in different multilayer systems of transition and rare earth metals. We have measured the time dependence of the thermoremanence magnetization up to temperaturesof 1.7 K. The tendency of the magnetic viscosity to have finite intercept at T = 0 and/or the plateau in the S(T) values are the experimental manifestations of this new and fascinating effect.

Quantum tunnelling effects in Fe/Sm multilayers

Magnetic relaxation investigations of Cu/Fe4 Sm/Cu multilayers from 20 K to 1.6 K at low and high fields have been performed. The constancy of both the magnetic viscosity and the coercive field below 2.5 K suggests that this temperature corresponds to the crossover between thermal and quantum regimes.

Quantum tunneling of magnetization in metallic FeC ferrofluids

We have achieved the observance of the quantum tunneling of magnetization in a ferrofluid system constituted by small single domain particles of FeC in a nonmagnetic carrier fluid. The constancy of the coercive field below 1 K indicates that this temperature corresponds to the crossover from the thermal to the quantum regime.

Non-Thermal Viscosity in the Magnetic Relaxation of 2d Random Magnets

2d random magnetic behaviour has been observed in several rare-earth-transition-metal multilayers. The magnetic relaxation in these systems is independent of temperature at low temperatures. This effect is discussed in terms of current theories for quantum nucleation in ferromagnets.

Dependence of magnetic viscosity on temperature and external magnetic fields in very thin iron films (5 Å) deposited onto crystalline Cu(111)

Abstract Magnetic relaxation measurements in constant fields ( H 2 = 10, 100, 156, 200 and 233 Oe ) in the temperature range between 1.8 and 16 K have been recorded. Our results show that the low-temperature magnetic viscosity has a non-thermal contribution that depends on the value of the applied magnetic field. The most plausible explanation for this non-thermal viscosity is the occurrence of single-domain tunneling in grains.