Measuring Lateral Magnetic Structure in Thin Films Using Polarized Neutron Reflectometry

Polarized neutron reflectometry (PNR) has long been applied to measure the magnetic depth profile of thin films. In recent years, interest has increased in observing lateral magnetic structures in a film. While magnetic arrays patterned by lithography and submicron-sized magnetic domains in thin films often give rise to off-specular reflections, micron-sized ferromagnetic domains on a thin film produce few off-specular reflections and the domain distribution information is contained within the specular reflection. In this paper, we will first present some preliminary results of off-specular reflectivity from arrays of micron-sized permalloy rectangular bars. We will then use specular reflections to study the domain dispersion of an exchange-biased Co/CoO bilayer at different locations of the hysteresis loop.Comment: 10 pages, 3 figurres, PNCMI 2002, Juelich, German

Hyper-domains in exchange bias micro-stripe pattern

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements

ON THE POLARIZATION APPEARING IN NEUTRON SCATTERING IN UNMAGNETIZED SAMPLES

L'apparition d'une polarisation non nulle des neutrons est prévue lors de leur diffusion par les corps magnétiques ne présentant pas d'aimantation spontanée (ferromagnétiques au-dessus de Tc ; au-dessous de Tc, subdivisés en domaines ; verres de spin en l'absence de champ extérieur, antiferromagnétiques). Cette polarisation qui est due aux corrélations dynamiques entre spins impliquant trois atomes, résulte du phénomène d'interférence entre les termes du premier et du deuxième ordre dans la théorie des perturbations pour l'amplitude de diffusion magnétique. La direction de cette polarisation est parallèle à la normale au plan de diffusion, [MATH] = [[MATH]] où [MATH] et [MATH] sont les impulsions des neutrons incidents et diffusés respectivement. Cet effet est extrêmement sensible à la dynamique du système de spins, dans un phénomène de diffusion élastique il n'apparaît pas. Sa valeur est déterminée par le rayon des corrélations de spins et par l'amplitude du processus de désintégration des fluctuations paires de spin en deux fluctuations (ou de la fusion de deux fluctuations en une seule). D'après le calcul [1], lors de la diffusion dans les ferromagnétiques au-dessus de Tc, pour τ = (T-Tc)/T de l'ordre de 10-3, une polarisation de l'ordre de 10-3 à 10-4 est attendue. Une telle valeur est en accord avec les résultats des mesures pour le fer [2]. Des mesures semblables dans les phases verre de spin, au voisinage des transitions de ces phases vers l'état paramagnétique, dans les phases aspéromagnétiques semblent présenter le plus grand intérêt.Polarisation of neutrons in their scattering from magnets which do not have a macroscopic moment is predicted. (This refers to ferromagnets above Tc, or below Tc in the multidomain state, spin glasses in the absence of external field, and antiferromagnets). The polarization which is due to threepoint dynamic spin correlations originates from the interference of contributions in the first and second orders of the perturbation theory for the magnetic scattering amplitude. It is directed parallel to the normal to the scattering plane, [MATH] = [[MATH]], where [MATH] and [MATH] are the incident and scattered neutron momenta, respectively. This effect is extremely sensitive to the spin system dynamics and is absent in elastic scattering. Its magnitude is determined by the spin correlation length and by the amplitude of the pair spin fluctuation decay (or of the merging of two fluctuations). Estimates [1] of the scattering in ferromagnets above Tc at τ = (T - Tc)x T-1c ~ 10-3 suggest the appearance of polarization at a level of 10-3 ÷ 10-4. The magnitude of the effect agrees with the results of measurements carried out on iron [2]. Similar experiments for the spin glass phase and near the transitions from the latter to the paramagnetic and asperomagnetic phases seem to be of most interest

2D reflectometry for the investigation of polymer interfaces: Off-specular neutron scattering

Specular and off-specular neutron reflectometry have been used in a combined approach to study thin polymer films. Our goal in this work is to illustrate the power of the off-specular scattering technique to probe the properties of the buried interface of immiscible polymer bilayers of deuterated polystyrene and protonated poly(methyl methacrylate) (h-PMMA). The diffuse scattering stemming from these systems is discussed in relation to thermal fluctuations at the polymer/polymer interface, providing a means to extract in-plane correlation lengths from buried interfaces. In addition the onset of hole formation in the top layer is evidenced by the diffuse scattering, not easily detectable by specular reflection alone.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

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Specular and off-specular polarized neutron reflectometry of canted magnetic domains in loose spin coupled CuMn/Co multilayers

Specular and off-specular polarized neutron reflectometry of strong biquadratic coupling in a Cu_0.94Mn_0.06/Co multilayer is presented. Detailed analysis of the reflectivity via the distorted-wave Born approximation reveals a formation of on average 〈±30^∘〉 canted lateral magnetic domains. Domains with mean width of 0.43 μm extend throughout the multilayer structure and magnetization alternates in the sign of the canting with subsequent layers. Temperature and field dependence of the magnetic structure are evaluated and included in the magnetic model, which is used to deduce bilinear and biquadratic interlayer exchange coupling energies