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Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic (F) Fe films exchange coupled to twinned (110) MnF2 antiferromagnetic (AF) layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a S=5/2 Brillouin function - an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant √K1 of bulk MnF2. In contrast, for a rough F-AF interface the magnetization reversal process (and exchange bias) showed little temperature dependence up to temperatures approaching the AF Néel point - a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.

In this article, we show how neutron reflectometry (NR) can provide deep insight into the absorption and desorption properties of commercially promising hydrogen storage materials. NR benefits from the large negative scattering length of hydrogen atoms, which changes the reflectivity curve substantially, so that NR can determine not only the total amount of stored hydrogen but also the hydrogen distribution along the film normal, with nanometer resolution. To use NR, the samples must have smooth surfaces, and the film thickness should range between 10 and 200 nm. We performed a systematic study on thin [Mg.sub.1-x][Al.sub.x] alloy films (x = 0.2, 0.3, 0.4, 0.67) capped with a Pd catalyst layer. Our NR experiments showed that [Mg.sub.0.7][Al.sub.0.3] is the optimum alloy composition with the highest amount of stored hydrogen and the lowest desorption temperature. All the thin films expand by about 20% because of hydrogen absorption, and the hydrogen is stored only in the MgAl layer with no hydrogen content in the Pd layer. PACS Nos: 61.05.fj, 82.40 Nous montrons ici comment la Reflectomeetrie Neutronique (NR) peut permettre une etude plus approfondie des proprietes d'absorption et de desorption de mateeriaux de stockage d'hydrogene commercialement prometteurs. NR utilise la grande longueur de diffusion negative des atomes d'hydrogene qui change substantiellement la courbe de reflectivite de telle sorte que NR peut determiner non seulement la quantite totale d'hydrogene stockee, mais egalement la distribution de l'hydrogene selon la normale au film, avec une precision du nanometre. Pour pouvoir utiliser NR, les films doivent avoir une surface lisse et leur eepaisseur doit se trouver entre 10 et 200 nm. Nous avons etudie systematiquement des films minces d'alliage [Mg.sub.1-x][Al.sub.x] (x = 0,2, 0,3, 0,4, 0,67) recouverts d'une couche de Pd comme catalyseur. Nos mesures montrent que [Mg.sub.0.7][Al.sub.0,3] est l'alliage optimum qui permet le plus haut stockage d'hydrogene et la plus basse temperature de desorption. Tous les films minces gonflent d'environ 20 % suivant l'absorption d'hydrogene qui est stocke; uniquement dans la couche de MgAl, sans hydrogene dans le Pd. [Traduit par la Redaction], Introduction A modern vehicle would need about 5 kg of hydrogen in combination with a fuel cell to achieve a travel range of about 500 km. Intense research efforts have [...]