Le phénomène de superparamagnétisme

Abstract

2014 Une revue sur le magnétisme des très petites particules, le superparamagnétisme, est effectuée. Dans l'introduction, un bref historique des mesures mettant en évidence le phénomène et une rapide revue des domaines où son étude peut apporter des renseignements importants sont faits. La première partie est consacrée à la théorie du phénomène. Il est examiné en détail les modèles de Néel et de Brown et comparé les valeurs asymptotiques du temps de relaxation aux valeurs numériques calculées par Aharoni. Des développements particuliers sont consacrés aux énergies d'anisotropie et leurs effets sur les énergies de barrières et les temps de relaxation, aux particules de plus grand diamètre où le mode de rotation des spins devient complexe et aux particules antiferromagnétiques. La deuxième partie de cette revue porte sur les mesures expérimentales. Après un exposé des problèmes concernant l'ensemble des mesures, telles les notions de temps de mesure et de température de blocage, tels les effets d'une répartition des volumes des particules..., il est examiné en détail, aussi bien du point de vue théorique qu'expérimental, les conséquences du phénomène du superparamagnétisme sur l'aimantation, l'aimantation rémanente, le cycle d'hystérésis, la susceptibilité en continu et en alternatif, la magnétorésistance, sur les mesures de spectroscopie Môssbauer et de résonance ferromagnétique. Abstract. 2014 Superparamagnetism phenomenon are reviewed. A short historical account of the first significant measurements and a short review of the topics where the study of the superparamagnetism bring pertinent results are done in the introduction. The theory of the phenomenon is studied in the first part. The Néel's model and the Brown's model are discussed in detail. The asymptotic values of the relaxation time are compared with the numerical solutions of Aharoni. Several chapters discuss the anisotropy energies and their effects on the barrier energy, the rotation mode of the spin particles and the antiferromagnetic particles. The different measurements are studied in the second part. At once, problems of general interest are analysed, i.e. : the measuring time, the blocking temperature, the effects of a diameter distribution... Afterwards the different measurements are reviewed from both theoretical and experimental points of view : magnetization, remanent magnetization, hysteresis loop, susceptibility, a.c. susceptibility, magnetoresistance, Mössbauer effect and FRM. Some examples are given.

Magnetic Fluids: Fabrication, Magnetic Properties, and Organization of Nanocrystals

“…[79] The relaxation time of the magnetization markedly decreases with decreasing particle size. The magnetization follows the direction of the external field when present, and the coercive force appears to be negligible in contrast to that of permanent magnets.…”

“…This process implies the crossing of an anisotropy energy barrier, and thus the thermal variation of the relaxation time, s, has an activation-like dependence. Following Neel, most authors have assumed a simple exponential temperature dependence for s. For an axial magnetocrystalline anisotropy the following dependence was found: [80m] s = s 0 exp(KV/kT) (6) whereas for a cubic magnetocrystalline anisotropy, [79] assuming a [100] easy magnetization axis and rotation restricted to (100) planes, the following relation has been proposed:…”

Magnetic interactions in transition metal clusters embedded in matrices prepared by LECBD technique

“…From T B measurements for both bcc-Fe and fcc-Co samples embedded in silver matrix which have different radius (2R centered around 4.5 and 2.8 nm, respectively), we can underline that the ratio T B (Fe)/T B (Co) equal to 2.6 is well related to the square particles radius ratio (R 2 (Fe)/R 2 (Co)), certainly because the surface grain anisotropy governs the magnetic local order in such nanoparticles [5].…”

“…The transition metal nanoparticles reveal a particular magnetic state namely the superparamagnetism [5] which can be easely described.…”

Magnetic Behaviors of a Silicate Glass System Containing Deposited Super Fine Particles of Iron Oxides at the Superparamagnetic State

“…[1][2][3] In our study on 2CaO-2Si(h-Fe203_5, one of slug materials, the clnsters have been found to be magnetite or maghemite of nano meter size scale, and thus bring superparamagnetism [4,5] into the system at least around room temperature. [3,6] In this case, magnetic Fe cations are present in purely amorphous region surrounding the clusters as well as in the crystallographic clusters.…”