College Park, Maryland      June 6 - 10 , 2004

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TP58: Ferromagnetic quantum critical fluctuations in Zr_1-xNb_xZn₂ (x=0.05)

D. Sokolov, M. C. Aronson (The University of Michigan, Ann Arbor), J.W. Lynn, B. Hammouda (NIST Center for Neutron Research), Z. Fisk (Florida State University)

ZrZn₂ is a weak itinerant ferromagnet with the Curie temperature T_C=18 K. Magnetization measurements showed that doping with Nb acts like a chemical pressure suppressing both T_C and the spontaneous moment to zero at 5% of Nb. We have studied the development of long range critical fluctuations above T_C in both compositions, and below T_C in ZrZn₂ using the NG3 SANS instrument at the NIST Center for Neutron Research.

In undoped ZrZn₂ above T_C, a suppression of scattering for 0.002 < q < 0.008 Å^-1 was observed, suggesting the development of extremely long wavelength correlations with lowered temperature. Surprisingly, the magnetic scattering is still temperature dependent at the smallest wave vectors at temperatures as large as 100 K. It was also observed that the small angle scattering increases below T_C in undoped ZrZn₂, implying the development of a new scattering mechanism involving ferromagnetic domains. For the quantum critical composition Zr_1-xNb_xZn₂ (x=0.05), magnetic scattering develops suddenly below 1 K. The scattering is enchanced at small (q < 0.004 Å^-1) wave vectors and suppressed at intermediate (q > 0.006 Å^-1) wave vectors indicating transfer of intensity from intermediate (q > 0.006 Å^-1) to small (q < 0.004 Å^-1) wave vectors at lower temperatures, consistent with incipient ferromagnetic correlations, albeit with relatively shorter ranges than those found in undoped ZrZn₂.

The work at the University of Michigan was carried out under NSF-DMR-9977300 grant.

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