J. Phys. IV France 125 (2005) 173-175  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125040 About the photoacoustic signal of magnetic materials D. Acosta-Avalos 1 , P.R. Barja 1 and M.D. Silva 2 1 Instituto de Pesquisa e Desenvolvimento (IP&D), Universidade do Vale do Paraiba, Av. Shishima Hifumi 2911, CEP 12244-000, São José dos Campos, SP, Brasil 2 Instituto de Pesquisas Espaciais (INPE), Av. dos Astronautas, São José dos Campos, SP, Brasil Abstract. The photoacoustic signal of magnets was measured in the presence of external magnetic fields, through the presence of intense magnets. It was observed that the intense magnetic field at the surface of a rare earth magnet can change the photoacoustic signal of a weak ferrite magnet. A possible explanation for the PA signal change is discussed. 1. INTRODUCTION The photoacoustic signal is the result of the conversion of electromagnetic energy of modulated amplitude in modulated thermal energy, through non-radiative decays in the absorbing matter. It depends on the thermal properties of the sample, as the thermal diffusivity, effusivity and conductivity. This enables photoacoustic and photothermal techniques to study optical and thermal properties of solid matter. The photoacoustic phenomena has been used to study magnetic materials, as microwave absorption in ferromagnetic resonance [1], the photoacoustic spectroscopy of magnetic semiconductors [2] and magnetic fluids [3], the absorption of X rays by magnetic materials [4], and the monitoring of magnetite formation in heated iron (III) hydroxide acetate [5]. It is observed that none of those studies explores the possible existence of magnetic contributions to the photoacoustic signal and its detection through photoacoustic techniques. A recent theoretical report by Kabychenkov [6] admits that light pulses can modify sublattice magnetizations and effective magnetic fields in magnets. This means that the non-radiative decays that are the energy source to the photoacoustic signal can also contribute to local magnetization changes in magnets, changing the photoacoustic signal in these materials. In this report, the photoacoustic signal of a magnet was monitored during its interaction with another magnet. 2. MATERIAL AND METHODS The PA setup was composed by a Tungsten arc lamp (150 W), a mechanical chopper, a PA cell and a lock-in amplifier. Light was chopped at 17 Hz. As figure 1 shows, the PA cell was one with two faces, where one face was closed with a glass window and the other was closed with the sample. Light incidence was frontal. The magnet used in this study was a disk of ferrite (1.6 cm diameter and 2 mm thick) and it interacted with another disk of rare earth magnet, that produces surface magnetic fields of about 0.1 T. The photoacoustic signal before and after the interaction was monitored for 5 minutes. At 2.5 minutes, two situations were analyzed: the proximity of the rare earth magnet and the contact between them. In the first case, the ferrite magnet senses the dipolar magnetic field of the rare earth magnet. In the second case, the contact between the magnets must change the magnetic domain distribution in the ferrite disk, because of the high magnetic field of the rare earth magnetic disk.