Laboratoire Léon Brillouin

UMR12 CEA-CNRS, Bât. 563 CEA Saclay

91191 Gif sur Yvette Cedex, France


Let's scatter neutrons

Antiferromagnetism on the surface and volume: NiO(111)
Antiferromagnetism on the surface and volume:  NiO(111)

(a)Magnetic configuration of surface diffraction: alpha is the incidence angle, 2-theta the twice of the Bragg angle. (b)Stacking of Ni and O planes perpendicular to the surface. (c)Scan in reciprocal space along the direction (11L) in the direct channel (surface diffraction) and in the channel which analyzes only the photons having undergone a rotation of polarization (magnetic diffraction).

In an antiferromagnetic material the magnetizations carried by the atoms are gradually tail; the antiferromagnetic order is established on twice lattices (at least in a direction) compared to the structural lattices (figure 1b). It results a zero global magnetization from it and consequently a great difficulty for the measurement of the magnetic properties, in particular the classical methods of magnetometry are unsuited. Moreover, if one carries the interest on the surface (some nanometers) the situation becomes even more complicate because of the reduction of the probed matter; within this limit even the experiments based on the interaction with the neutrons become difficult. However, the antiferromagnetic substrates play a significant part in the magnetic sensors (read head of hard disks, permanent magnetic memories ...) because they ensure the magnetic hardening of one of the ferromagnetic layers of the sensor; the comprehension of its properties is a significant goal as well from a fundamental point of view as in an approach directed towards the applications of spin-electronic. In order to overcome these difficulties and to apprehend the antiferromagnetism on a material like NiO(111) [ A.Barbier and coll, Physical Review Letters 84 (2000) 2897 and Physical Review B 62 (2000) 16056 ], one proposed a new experimental approach based on the synchrotron radiation. Indeed, this one is strongly and naturally polarized in the plan of the orbit of the particles circulating in the synchrotron ring and only magnetization can, while interacting with the photons, to cause a modification (phase rotation) of polarization. The experimental configuration (figure 1a) is inspired at the same time by the surface diffraction for which the incidence angle is maintained fixes and of magnetic diffraction where a analyzer crystal makes it possible to identify the polarization of the diffracted photons. The experiments were carried out except resonance with a PG(006) analyzer crystal (photons energy of 7981 eV). The incidence angle makes it possible to choose the probed depth and polarization analyzes it makes possible the separation of the structural signal from the magnetic signal (figure 1c) which much weaker (minimum of diffusion due to surface is approximately 100 times more intense than the maximum of magnetic diffraction).  
If one considers diffraction peaks families with the same transfer moment, this kind of measurement allows to obtain the magnetization of each sublattice (figure 2). One thus could show that the Néel temperature (TN) (transition from the ordered antiferromagnetic state at the disordered paramagnetic state) is significantly higher in surface than in volume and than the transition is described in term of first order transition. Moreover one discovered that the transition takes place in two stages, at temperature TSN (figure 2) the antiferromagnetic order at long distance is lost (the antiferromagnetic domains become equiprobable) and at TN the local order is lost. One finally could deduce the magnetization profile starting from surface and according to the temperature: the antiferromagnetic order is established surface towards volume, which can at least partially explain a certain number of properties observed when a ferromagnetic material is in contact with NiO(111). For more information one will be able to consult the article: A. Barbier and al, Physical Review Letters 93 (2004) 257208. 
Antiferromagnetism on the surface and volume:  NiO(111)

(Triangles, lozanges) Individual peaks intensities of the family (1,1,1.5). Evolution of the sublattices magnetization in volume (squares) and surfaces (circles) according to the temperature. (Continuous and dotted Lines) Better adjustments for first order transitions.

#502 - Last update : 07/20 2005


Retour en haut