skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Lowering of the L1{sub 0} ordering temperature of FePt nanoparticles by He{sup +} ion irradiation

Abstract

Arrays of FePt particles (diameter 7 nm) with mean interparticle distances of 60 nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L1{sub 0} phase is tracked for 350 kV He{sup +} ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He{sup +} ion exposure yields a significant reduction of the ordering temperature by more than 100 K.

Authors:
; ; ; ; ; ;  [1];  [2]
  1. Institut fuer Festkoerperphysik, Universitaet Ulm, Albert-Einstein-Strasse 11, 89069 Ulm (Germany)
  2. (Germany)
Publication Date:
OSTI Identifier:
20971816
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 90; Journal Issue: 6; Other Information: DOI: 10.1063/1.2472177; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; AGGLOMERATION; ANNEALING; HELIUM IONS; ION BEAMS; IRON ALLOYS; IRRADIATION; NANOSTRUCTURES; PARTICLES; PHASE TRANSFORMATIONS; PLATINUM ALLOYS; SUBSTRATES; TEMPERATURE DEPENDENCE; TEMPERATURE RANGE 0065-0273 K

Citation Formats

Wiedwald, U., Klimmer, A., Kern, B., Han, L., Boyen, H.-G., Ziemann, P., Fauth, K., and Max-Planck-Institut fuer Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart. Lowering of the L1{sub 0} ordering temperature of FePt nanoparticles by He{sup +} ion irradiation. United States: N. p., 2007. Web. doi:10.1063/1.2472177.
Wiedwald, U., Klimmer, A., Kern, B., Han, L., Boyen, H.-G., Ziemann, P., Fauth, K., & Max-Planck-Institut fuer Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart. Lowering of the L1{sub 0} ordering temperature of FePt nanoparticles by He{sup +} ion irradiation. United States. doi:10.1063/1.2472177.
Wiedwald, U., Klimmer, A., Kern, B., Han, L., Boyen, H.-G., Ziemann, P., Fauth, K., and Max-Planck-Institut fuer Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart. Mon . "Lowering of the L1{sub 0} ordering temperature of FePt nanoparticles by He{sup +} ion irradiation". United States. doi:10.1063/1.2472177.
@article{osti_20971816,
title = {Lowering of the L1{sub 0} ordering temperature of FePt nanoparticles by He{sup +} ion irradiation},
author = {Wiedwald, U. and Klimmer, A. and Kern, B. and Han, L. and Boyen, H.-G. and Ziemann, P. and Fauth, K. and Max-Planck-Institut fuer Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart},
abstractNote = {Arrays of FePt particles (diameter 7 nm) with mean interparticle distances of 60 nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L1{sub 0} phase is tracked for 350 kV He{sup +} ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He{sup +} ion exposure yields a significant reduction of the ordering temperature by more than 100 K.},
doi = {10.1063/1.2472177},
journal = {Applied Physics Letters},
number = 6,
volume = 90,
place = {United States},
year = {Mon Feb 05 00:00:00 EST 2007},
month = {Mon Feb 05 00:00:00 EST 2007}
}
  • First, second, and third nearest-neighbor mixing potentials for FePt alloys were calculated from first principles using the Connolly-Williams approach. Using the mixing potentials obtained in this manner, the dependency of equilibrium L1{sub 0} ordering on temperature was studied for bulk and for a spherical nanoparticle with a 3.5-nm diameter at equiatomic composition by use of Monte Carlo simulation and the analytical ring approximation. The calculated order-disorder temperature for bulk (1495-1514 K) was in relatively good agreement (4% error) with the experimental value (1572 K). For nanoparticles of finite size, the (long-range) order parameter changed continuously from unity to zero withmore » increasing temperature. Rather than a discontinuity indicative of a phase-transition we obtained an inflection point in the order as a function of temperature. This inflection point occurred at a temperature below the bulk phase-transition temperature and which decreased as the particle size decreased. Our calculations predict that 3.5-nm diameter particles in configurational equilibrium at 600 deg. C (a typical annealing temperature for promoting L1{sub 0} ordering) have an L1{sub 0} order parameter of 0.83 (compared to a maximum possible value equal to unity). According to our investigations, the experimental absence of a (relatively) high L1{sub 0} order in 3.5-nm diameter nanoparticles annealed at 600 deg. C or below is primarily a problem of kinetics rather than equilibrium.« less
  • The size dependence of the order-disorder transition in FePt nanoparticles with an L1{sub 0} structure is investigated by means of Monte Carlo simulations based on an analytic bond-order potential for FePt. A cross parametrization for the Fe-Pt interaction is proposed, which complements existing potentials for the constituents Fe and Pt. This FePt potential properly describes structural properties of ordered and disordered phases, surface energies, and the L1{sub 0} to A1 transition temperature in bulk FePt. The potential is applied for examining the ordering behavior in small particles. The observed lowering of the order-disorder transition temperature with decreasing particle size confirmsmore » previous lattice-based Monte Carlo simulations [M. Mueller and K. Albe, Phys. Rev. B 72, 094203 (2005)]. Although a distinctly higher amount of surface induced disorder is found in comparison to previous studies based on lattice-type Hamiltonians, the presence of lattice strain caused by the tetragonal distortion of the L1{sub 0} structure does not have a significant influence on the depression of the ordering temperature with decreasing particle size.« less
  • We investigated magnetic properties and L1{sub 0} phase formation of FePt films by rapid thermal annealing (RTA) and high current-density ion-beam irradiation. The sample prepared by RTA at 550 deg. C has (001) texture and strong magnetic perpendicular anisotropy with H{sub c} equal to 6 kOe. The sample irradiated at 5.04 {mu}A/cm{sup 2} has H{sub c} equal to 10 kOe but has isotropic magnetic properties due to the (111) texture. The magnetic correlation length of the ion-irradiated sample was about twice as large as that of the RTA sample. This may be due to the inhomogeneity of the L1{sub 0}more » phase formation in the ion-irradiated film.« less
  • The effect of ion irradiation (B{sup +},Cr{sup +},Ga{sup +}, and Nb{sup +}) on the crystalline structure and magnetic properties of L1{sub 0} (face-centered tetragonal structure) FePt films was investigated. Irradiating with Cr{sup +}, Ga{sup +}, and Nb{sup +} ions of less than 1 at. % (1.6x10{sup 15} ions/cm{sup 2}) dose yielded an almost ideal structural transition from the L1{sub 0} to A1 (face-centered cubic structure) phase withoutdamage to the surface of the film. This structural transition was accompanied by a change in magnetic properties from a hard magnet with a coercivity H{sub c} of {approx}7 kOe to a soft magnetmore » with H{sub c}<1 kOe. A two-dimensional pattern composed of hard magnetic L1{sub 0} and soft magnetic A1 phases was fabricated by using a focused Ga{sup +} ion beam.« less
  • FePt and (FePt){sub 91.2}Ag{sub 8.8} alloy films were deposited by magnetron sputtering. The average coercivity of (FePt){sub 91.2}Ag{sub 8.8} films reaches 8.51 × 10{sup 5} A/m, which is 0.63 × 10{sup 5} A/m higher than that of the corresponding FePt films. Ag addition effectively promotes the FePt L1{sub 0} ordering transition at a relatively low annealing temperature of 400 °C. The promotion mechanism was investigated by using in situ high-resolution transmission electron microscopy (HRTEM) and ex situ X-ray absorption fine structure (XAFS). The concurrence of ordering transition and Ag segregation in FePtAg alloy films was first observed by using in situ heating HRTEM. The time-resolved evolution revealsmore » more details on the role of Ag addition in FePt low-temperature ordering. Ex situ XAFS results further confirm that Ag replaces Fe sites in the as-deposited films and segregates from FePt-Ag solid solution phase through annealing at elevated temperatures. The segregation of Ag atoms leaves vacancies in the grain. The vacancy formation is believed to accelerate the diffusion of Fe and Pt atoms, which is critical for the L1{sub 0} ordering transition.« less