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MAGNETIC SYSTEMS 2009Magnetic properties of Y 0.7 Er 0.3 Fe 2 (H, D) 4.2 compounds up to 35 TRFe 2 Laves phases can absorb hydrogen or deuterium upto 5H(D)/mol and this absorption modifies significantly thestructural, magnetic and electronic properties. The YFe 2 D xdeuterides are ferromagnetic with an increase of the meanFe moment and a decrease of T C for x ≤ 3.5 D/mol. Forx = 4.2, the monoclinic compound is ferromagnetic at lowtemperature, then undergoes a sharp first order magnetovolumictransition towards an antiferromagnetic structure at84 K. Surprisingly this transition is very sensitive to the Hfor D substitution, which increases the mean Fe moment at4.2 K and shifts the transition temperature extrapolated atzero field T M0 to 131 K (by 50%). Since the cell volumeof the hydride is 0.78% larger than the deuteride, this giantisotope effect has been related to the strong dependence ofthe itinerant electron metamagnetic behavior (IEM) behavioron the volume of one of the Fe sites among eight whichhas a different number of H(D) neighbors.Thermomagnetization curves deduced from cooling underlow magnetic field (300 Oe) for Y 0.7 Er 0.3 Fe 2 H 4.2 andY 0.7 Er 0.3 Fe 2 D 4.2 respectively show sharp transitions atT MO = (107±2) K and (61±1) K respectively. These transitiontemperatures were found equal to 131 and 84 K inthe YFe 2 hydride and deuteride. The higher T MO valuefor the ferromagnetic YFe 2 H 4.2 hydride is attributed toa larger cell volume. However, in Y 0.7 Er 0.3 Fe 2 H 4.2 andY 0.7 Er 0.3 Fe 2 D 4.2 the exchange couplings between the differentmagnetic atoms may also influence the transitiontemperatures. This is confirmed by the thermomagnetizationcurves (figures 126 and 127) where a large increaseof T MO is observed together with a metamagnetic behavior.The transition field (HTR) is determined from themaximum of (dM T /dH). For Y 0.7 Er 0.3 Fe 2 H 4.2 the transitionis observed in the 4.2 − 40 K and 125 − 175 K temperaturesranges On the contrary the transition exists inthe 4.2 − 55 K and 80 − 175 K temperature ranges forY 0.7 Er 0.3 Fe 2 D 4.2 . In the 4.2 − 40 K range the (HTR) valuesare identical. However, when the temperature increasesa large isotopic effect is revealed by the temperature variationof HTR (figure 128).The transition field increases linearlyversus temperature but with a smaller HTR/T slopefor the deuteride (2.48 kOe/K) compared to the hydride(2.58 kOe/K). These values are nevertheless larger than forYFe 2 H 4.2 (1.34 kOe/K) and YFe 2 D 4.2 (1.37 kOe/K), showingan additional influence of the Er substitution. This suggeststhat the Y for Er substitution modifies the structuraland magnetic properties of the hydrides and deuterides: (i)The cell volume of the hydride is 0.8% larger than the correspondingdeuteride; (ii) A spin reorientation of Er momentis induced at low temperature, independently to the H/Disotope effect; (iii) The values of T M0 are smaller than inthe YFe 2 H 4.2 and YFe2D 4.2 compounds, but remain sensitiveto the isotope influence. The sensitivity of T M0 to thechange of volume confirms the magnetovolumic characterof the transition. (iv) The B/T slopes are different for thehydride and deuterides, whereas there were similar in thenon substituted compounds.Figure 126: Thermomagnetization of Y 0.7 Er 0.3 Fe 2 (H, D) 4.2 .Figure 127:D) 4.2 .Figure 128:Isothermal magnetization of Y 0.7 Er 0.3 Fe 2 (H,Transition field versus temperature.M. GuillotV. Paul-Boncour, T. Leblond (CMTR, ICMPE, CNRS and Univ. Paris XII, Thiais)90