Vemuru Krishnamurthy

Magnetic transitions in lattice-matched, ordered FePt3 based antiferromagnetic/ferromagnetic films

Measurements of lattice-matched antiferromagnetic/ferromagnetic films which are ideal layered systems to study exchange bias are reported. Epitaxial films of FePt3 have two kinds of antiferromagnetic ordering. The spin ordering phase with wave vector Q1=(12120) has a Neel temperature TN=160K and that with wave vector Q2=(1200) has TN=100K. Neutron diffraction confirmed the presence of Q2=(1200) antiferromagnetic ordering in 200nm Fe25Pt75 grown on MgO(100). The loop shift and coercivity of a trilayer film of CoPt3∕FePt3∕CoPt3 decrease with increasing temperature, consistent with the observed Neel temperature of FePt3. The x-ray diffraction rocking curve widths of films grown on MgO(100) and Al2O3(1120) are compared and related to the loop shifts that are observed in the films with lattice-matched antiferromagnetic/ferromagnetic interfaces.

Sintering behavior of spin-coated FePt and FePtAu nanoparticles

FePt and [FePt]95Au5 nanoparticles with an average size of about 4nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250to500°C for 30min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt]95Au5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0nm. The ratio of c∕a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss...

μ+SR Study of Ordering Phenomena in the Heavy-Fermion State of Ce3Pd20Ge6

Magnetic and quadrupolar ordering phenomena in a Ce3Pd20Ge6 single crystal have been investigated by muon spin rotation/relaxation (μ+SR) spectroscopy. We have observed spontaneous precession of muons in zero-field below TN=0.7 K in the antiferromagnetic state. The precession frequency follows the power law: ν(T)=ν(0)(1−T/TN)n. The exponent n=0.43(2) is close to the mean-field value of 0.5. The muon longitudinal spin relaxation rate 1/T1 is found to be nearly independent of temperature in the range of 0.3 to 2 K, i.e., across either TN or TQ=1.2 K, the quadrupolar ordering temperature. Two likely mechanisms for the temperature independent behavior of 1/T1 are suggested.