Relation between the weak itinerant magnetism in A2A_2Ni7_7 compounds (AA = Y, La) and their stacked crystal structures

The weak itinerant magnetic properties of $A_2$Ni$_7$ compounds with $A$ = {Y, La} have been investigated using electronic band structure calculations in the relation with their polymorphic crystal structures. These compounds crystallizes in two structures resulting from the stacking of two and three blocks of [$A_2$Ni$_4$ + 2 $A$Ni$_5$] units for hexagonal $2H$-La$_2$Ni$_7$ (Ce$_2$Ni$_7$ type) and rhombohedral $3R$-Y$_2$Ni$_7$ (Gd$_2$Co$_7$ type) respectively. Experimentally, $2H$-La$_2$Ni$_7$ is a weak itinerant antiferromagnet whereas $3R$-Y$_2$Ni$_7$ is a weak itinerant ferromagnet. From the present first principles calculation within non-spin polarized state, both compounds present an electronic density of state with a sharp and narrow peak centered at the Fermi level corresponding to flat bands from $3d$-Ni. This induces a magnetic instability and both compounds are more stable in a ferromagnetic (FM) order compared to a paramagnetic state ($\Delta E \simeq$ -35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with their positions relative to the [$A_2$Ni$_4$] and [$A$Ni$_5$] units: they are minimum in the [$A_2$Ni$_4$] unit and maximum at the interface between two [$A$Ni$_5$] units. For $2H$-La$_2$Ni$_7$, an antiferromagnetic (AFM) structure has been proposed and found with an energy comparable to that of the FM state. This AFM structure is described by two FM unit blocks of opposite Ni spin sign separated by a non-magnetic layer at z = 0 and $\frac12$. The Ni ($2a$) atoms belonging to this intermediate layer are located in the [La$_2$Ni$_4$] unit and are at a center of symmetry of the hexagonal cell ($P6_3/mmc$) where the resultant molecular field is cancelled. Further non-collinear spin calculations have been performed to determine the Ni moment orientations which are found preferentially parallel to the $c$ axis for both FM and AFM structures.Comment: 19 pages, 7 figures, 2 table

On the origin of the giant isotopic effect of hydrogen on the magnetic properties of YFe2A4.2 (A5H, D): A high pressure study

International audienceIn order to identify the origin of the giant isotope effect observed on the magnetic properties of YFe2A4.2 compounds (A¼H, D) and probe the role of the cell volume difference, we have compared the effect of hydrostatic pressure up to 1.0 GPa on the ferro-antiferromagnetic transition temperatures TF-AF and spontaneous magnetization of YFe2D4.2 and YFe2H4.2. Using compressibility value of 0.013 GPa 1 and the remarkably different negative pressure slopes of TF-AF, the existence of critical volume where the ferromagnetism is suppressed at 0K, VC¼501.760.3 ˚A3, was demonstrated. This consequently established the crucial role of volume on the huge isotope effect observed on the magnetic properties

Correlations between stacked structures and weak itinerant magnetic properties of La2−x_{2-x} Yx_x Ni7_7 compounds

Hexagonal La$_2$Ni$_7$ and rhombohedral Y$_2$Ni$_7$ are weak itinerant antiferromagnet (wAFM) and ferromagnet (wFM), respectively. The crystal structure and magnetic properties of $A_2B_7$ intermetallic compounds ($A$ = La, Y, $B$ = Ni) have been investigated combining X-ray powder diffraction and magnetic measurements. The La$_{2-x}$Y$_x$Ni$_7$ intermetallic compounds with $0 \leq x \leq 1$ crystallize in the Ce$_{2}$Ni$_7$-type hexagonal structure with Y preferentially located in the [$AB_2$] units. The compounds with larger Y content ($1.2 \leq x < 2$) crystallize in both hexagonal and rhombohedral (Gd$_2$Co$_7$-type) structures with a progressive substitution of Y for La in the $A$ sites belonging to the [$AB_5$] units. Y$_2$Ni$_7$ crystallizes in the rhombohedral structure only. The average cell volume decreases linearly versus Y content, whereas the c/a ratio presents a minimum at $x = 1$ due to geometric constrains. The magnetic properties are strongly dependent on the structure type and the Y content. La$_{2}$Ni$_7$ displays a complex metamagnetic behavior with split AFM peaks. Compounds with x = 0.25 and 0.5 display a wAFM ground state and two metamagnetic transitions, the first one towards an intermediate wAFM state and the second one towards a FM state.T$_N$ and the second critical field increase with the Y content, indicating a stabilization of the AFM state. LaYNi$_7$, which is as the boundary between the two structure types, presents a very wFM state at low field and an AFM state as the applied field increases. All the compounds with $x > 1$ and containing a rhombohedral phase are wFM with $T_C$ = 53(2) K. In addition to the experimental studies, first principles calculations using spin polarization have been performed to interpret the evolution of both structural phase stability and magnetic ordering for $0 \leq x < 2$.Comment: 26 pages (7 for supplementary material), 4 tables, 9 main figures and 8 figures in supplementary materia

Étude des propriétés structurales et magnétiques des composés Y1-yRyFe2H(D)x(y) (R=Tb, Er, Lu) (effet isotopique sur la transition magnétovolumique, transition ordre-désordre et thermodésorption)

L absorption de l hydrogène dans les phases de Laves RFe2 modifie fortement leurs propriétés magnétiques. Pour une concentration x proche de 4.2, il a été observé que le deutérure YFe2Dx de structure monoclinique (phase g) présentait une transition magnétovolumique du premier ordre à TM0 = 84 K, liée à un comportement métamagnétique des électrons itinérants. De plus, cette transition magnétique présente un effet isotopique très important avec une forte augmentation de TM0 quand on substitue D par H. Un des objectifs de ce travail de thèse était de déterminer plus précisément les paramètres influençant cette transition métamagnétique et cet effet isotopique. Des mesures de diffraction de neutrons sous pression sur le deutérure YFe2Dx(g) ont montré que la diminution du volume entraînait une diminution rapide de la température de transition métamagnétique TM0, qui disparaissait pour des pressions supérieures à 10 kbar. La substitution de l Y par le Lu et l Er a confirmé que la diminution de volume de maille entrainait une diminution de TM0 et montré que l effet isotopique sur TM0 se décomposait en un effet volumique et un effet quantique. La substitution de l Y par le Tb, qui entraine un accroissement important de TM0, a montré qu aux effets précédents s ajoutait l influence du champ moléculaire du Tb. L étude de la transition ordre-désordre du deutérium à température proche de l ambiante, a mis en évidence un abaissement progressif de la symétrie, d une structure cubique vers une structure rhomboédrique puis monoclinique L étude à plus haute température de la thermodésorption du deutérium dans les composés RFe2Dx(g) (R=Y, Tb) a montré l existence de plusieurs pics de désorption liés à des phases de stabilités différentes.Hydrogen absorption in RFe2 Laves phases modifies strongly their magnetic properties. For a concentration x close to 4.2, it was observed that the monoclinic deuteride YFe2Dx (g phase) undergoes a first order magnetovolumic transition at TM0 =84 K which is linked to an itinerant electron metamagnetic effect. Moreover, a very large isotopic effect has been observed on this transition, with a strong increase of TM0 upon H for D substitution. One of the thesis objectives was to determine more precisely the parameters influencing this metamagnetic transition and this isotope effect. Neutron diffraction measurements on the deuteride YFe2Dx(g) under pressure have shown that the volume decrease led to a lowering of TM0 until complete disappearance above P= 10 kbar . The Lu or Er for Y substitution has confirmed that the volume decrease lowers TM0 and shown that the isotopic effect can be decomposed into a volume and a quantum effects. The Tb for Y substitution, which leads to a strong increase of TM0, has shown that the influence of the Tb molecular field can be added to the two previous effects. The deuterium order-disorder transition near room leads to a symmetry lowering from cubic to a rhombohedral and then a monoclinic structure. The deuterium thermal desorption study, which occurs at high temperature, of RFe2Dx(g) (R=Y, Tb) has shown the existence of several desorption peaks linked to several phases with different stabilities.PARIS12-CRETEIL BU Multidisc. (940282102) / SudocSudocFranceF

Tuning the Magnetocaloric Properties of the La(Fe,Si)13 Compounds by Chemical Substitution and Light Element Insertion

International audienc

Ab initio approach of the hydrogen insertion effect on the magnetic properties of YFe2

Based on experimental crystallographic and magnetic results on the family of hydrides YFe2Hx (

Étude des propriétés des composés LaNi3.55Mn0.4Al0.3Co0.75-xFex (0 = x = 0.75) (application aux accumulateurs Ni-MH)

Nous avons étudié l'influence de la substitution du cobalt par le fer sur le comportement thermodynamique et électrochimique des hydrures des composés intermétalliques polysubstitués LaNi3.55Mn0.4Al0.3Co0.75-xFex (x = 0, 0.35, 0.75). La substitution partielle du cobalt par le fer permet de conserver une bonne durée de vie des électrodes, par contre la substitution totale entraîne une décroissance rapide de la capacité. Cette décroissance provient de la corrosion de l'alliage qui est attaqué par la potasse au cours du cyclage électrochimique. La décrépitation de l'alliage a été suivie par des mesures de microscopie électronique à balayage. Nous avons aussi établi une méthode originale de caractérisation quantitative de la décomposition de l'alliage en utilisant l'évolution des propriétés magnétiques de ces composés avant et après cyclage électrochimique. Cette méthode nous a permis de calculer le taux de décomposition de l'alliage, le pourcentage, la taille et la nature des particules de métaux de transition (Ni, Co, Fe) ségrégées en surface. Nous avons pu estimer l'épaisseur de la couche de corrosion. Un modèle pour le mécanisme de la corrosion basé sur ces résultats expérimentaux, est proposé afin d'expliquer les différences de comportement observés pour ces matériaux d'électrodes.We have studied the influence of iron substitution for cobalt on the thermodynamic and electrochemical behavior of the hydrides of polysubstituted LaNi 3.55Mn0.4Al0.3Co0.75-x(X = 0, 0.35, 0.75) alloys. Partial iron for cobalt substitution allows to maintain a good cycle life, whereas complete substitution leads to a rapid loss of the capacity. This decrease is due to KOH corrosion during the electrochemical cycling. The decrepitation of the alloys was followed by measurements of electronic scan microscopy. We also established an original method of quantitative characterization of the decomposition of the alloy by using the evolution of the magnetic properties of these compounds before and afier electrochemical cycling. This method enabled us to calculate the rate of the alloy decomposition, the percentage, the size and the nature of the transition metal (Ni,Co,Fe) particles segregated on the surface. We have also estimated the thickness of the corrosion layer. A model for the corrosion mechanism, which is based on these results, is proposed to explain the differences in behavior observed for these electrode materials.PARIS12-CRETEIL BU Multidisc. (940282102) / SudocSudocFranceTunisiaFRT

On the magnetic properties of pseudo-Laves phases RE1-yYyNi4-xAlxMg with RE = La, Ce and Gd prepared by both melting and ball milling

Magneticproperties of RE1−yYyNi4−xAlxMg (RE = La, Ce and Gd) are reported. LaNi4Mg displays a weak magnetization indicating that Ni is non magnetic as often observed in RENi2 compounds. The magnetization of CeNi4Mg compounds shows a Curie Weiss behaviour with an effective paramagnetic moment μeff = 2.2 μB. The magnetization of Gd compounds is dominated by the contribution of Gd moment with a paramagnetic effective moment close to 7.7 μB/Gd for all studied compounds. The Curie temperature, which is 75 K for GdNi2, decreases almost linearly with the number of Gd neighbours when more than half Gd is replaced by Y. The decrease of crystallinity of GdNi4Mg, which is monitored by ball milling and heat treatment, strongly influences the magneticproperties and a relationship between the transition temperature and the crystallites size is reported