Size-dependent magnetic ordering and spin dynamics in DyPO4 and GdPO4 nanoparticles

Marco Evangelisti, Tibi G. Sorop, Oleg N. Bakharev, Dirk Visser, Adrian D. Hillier, Juan J. Alonso, Markus Haase, Lynn A. Boatner, and L. Jos de Jongh
Phys. Rev. B 84, 094408 – Published 13 September 2011

Abstract

Low-temperature magnetic susceptibility and heat-capacity measurements on nanoparticles (d 2.6 nm) of the antiferromagnetic compounds DyPO4 (TN=3.4 K) and GdPO4 (TN=0.77 K) provide clear demonstrations of finite-size effects, which limit the divergence of the magnetic correlation lengths, thereby suppressing the bulk long-range magnetic ordering transitions. Instead, the incomplete antiferromagnetic order inside the particles leads to the formation of net magnetic moments on the particles. For the nanoparticles of Ising-type DyPO4 superparamagnetic blocking is found in the ac susceptibility at 1 K, those of the XY-type GdPO4 analog show a dipolar spin-glass transition at 0.2 K. Monte Carlo simulations for the magnetic heat capacities of both bulk and nanoparticle samples are in agreement with the experimental data. Strong size effects are also apparent in the Dy3+ and Gd3+ spin dynamics, which were studied by zero-field muon spin rotation (μSR) and high-field 31P-nuclear magnetic resonance (31P-NMR) nuclear relaxation measurements. The freezing transitions observed in the ac susceptibility of the nanoparticles also appear as peaks in the temperature dependence of the zero-field μSR rates, but at slightly higher temperatures, as to be expected from the higher frequency of the muon probe. For both bulk and nanoparticles of GdPO4, the muon and 31P-NMR rates are for T5 K dominated by exchange-narrowed hyperfine broadening arising from the electron spin-spin interactions inside the particles. The dipolar hyperfine interactions acting on the muons and the 31P are, however, much reduced in the nanoparticles. For the DyPO4 analogs the high-temperature rates appear to be fully determined by electron spin-lattice relaxation processes.

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  • Received 4 July 2011

DOI:https://doi.org/10.1103/PhysRevB.84.094408

©2011 American Physical Society

Authors & Affiliations

Marco Evangelisti1,*, Tibi G. Sorop2, Oleg N. Bakharev2, Dirk Visser3,4, Adrian D. Hillier3, Juan J. Alonso5, Markus Haase6, Lynn A. Boatner7, and L. Jos de Jongh2,†

  • 1Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC-Universidad de Zaragoza, Departamento de Física de la Materia Condensada, E-50009 Zaragoza, Spain
  • 2Kamerlingh Onnes Laboratory, Leiden University, NL-2300 RA Leiden, The Netherlands
  • 3ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom
  • 4Department Radiation, Radionuclides & Reactors, Section FAME, Delft University of Technology, NL-2629 JB Delft, The Netherlands
  • 5Departamento de Física Aplicada I, Universidad de Málaga, E-29071 Málaga, Spain
  • 6Institut für Chemie, Universität Osnabrück, Barbarastrasse 7, D-49076 Osnabrück, Germany
  • 7Center for Radiation Detection Materials and Systems, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056, USA

  • *http://molchip.unizar.es/
  • jongh@physics.leidenuniv.nl

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Issue

Vol. 84, Iss. 9 — 1 September 2011

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