MAGNETIC SHORT-RANGE ORDER AND MAGNETIC-MOMENTS OF CO-MN AND NI-MN ALLOYS

The magnetic behavior of Co-Mn and Ni-Mn alloys is typical of competin g interaction systems with regions of ferromagnetism and antiferromagn etism enclosing an intermediate concentration region in which only mag netic short-range order (MSRO) occurs. Neutron diffuse scattering meas urements were made on alloys from this intermediate region to characte rize the MSRO and to determine the associated magnetic moments. Diffus e peaks indicative of antiferromagnetic short-range order were observe d and average sublattice moments were determined by comparison of the integrated diffuse peak intensities with the cross section for Bragg s cattering from the alloys with assumed antiferromagnetic long-range or der. The average sublattice moment of the Co-Mn alloys is appreciably smaller than that for the Ni-Mn alloys. Recent local density calculati ons indicate vanishing moments for Ni and Co atoms in fcc antiferromag nets with large wave vectors. If this applies to these alloys, then al l of the moment resides at the Mn sites and corresponds to about 1mu(B )/Mn for the Co-Mn alloys and 4mu(B)/Mn for Ni-Mn alloys. This moment difference for these otherwise similar systems is attributed to a mome nt-volume instability of the type more commonly associated with fcc Fe .