Size-dependent magnetic ordering and spin dynamics in DyPO4and GdPO4nanoparticles

Abstract

Low-temperature magnetic susceptibility and heat-capacity measurements on nanoparticles (d ≈ 2.6 nm) of the antiferromagnetic compounds DyPO 4 (T N = 3.4 K) and GdPO 4 (T N = 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 DyPO 4 superparamagnetic blocking is found in the ac susceptibility at 1 K, those of the XY -type GdPO 4 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 Dy 3+ and Gd 3+ spin dynamics, which were studied by zero-field muon spin rotation (μSR) and high-field 31 P-nuclear magnetic resonance ( 31 P-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 GdPO 4 , the muon and 31 P-NMR rates are for T 5 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 31 P are, however, much reduced in the nanoparticles. For the DyPO 4 analogs the high-temperature rates appear to be fully determined by electron spin-lattice relaxation processes.

Separating the ferromagnetic and glassy behavior within the metal-organic magnetNi(TCNQ)2

“…The dynamic component persists down to 40 mK and is best described by a single exponential relaxation where below 0.7 K bulk magnetic ordering is observed. The main difference is that there is another peak in λ at a lower temperature where the parameter increases at 0.4 K which Evangelisti et al have attributed to a spin freezing effect where the nanoparticulate system moves into a quasistatic regime from interparticle interactions [63]. GdVO 4 nanoparticles were also synthesized where similar behavior was observed, however the spin freezing peak in λ was much less pronounced which is suggested to be due to more magnetic disorder within this quasistatic regime [63].…”

“…The main difference is that there is another peak in λ at a lower temperature where the parameter increases at 0.4 K which Evangelisti et al have attributed to a spin freezing effect where the nanoparticulate system moves into a quasistatic regime from interparticle interactions [63]. GdVO 4 nanoparticles were also synthesized where similar behavior was observed, however the spin freezing peak in λ was much less pronounced which is suggested to be due to more magnetic disorder within this quasistatic regime [63]. This may support the similar behavior seen within the Ni(TCNQ − D 4 ) 2 system where the increase in the ZF λ 1 parameter is attributed to a spin freezing temperature or the material entering a quasistatic regime.…”

Magnetic structure and magnetocalorics ofGdPO4

“…6 Jkg −1 K −1 = 416 Jcm −3 K −1 , suggests that its increase on demagnetization might be very high. This compound has been studied by heat capacity and magnetic susceptibility in the form of nanoparticles and in bulk, both for powder samples and single crystals [5,6]. Also Monte Carlo simulations were made [6].…”

“…This compound has been studied by heat capacity and magnetic susceptibility in the form of nanoparticles and in bulk, both for powder samples and single crystals [5,6]. Also Monte Carlo simulations were made [6]. The experiments indicated a compensated antiferromagnetic structure below T N , interpreted as resulting from a competition between the dipolar, exchange, and anisotropy energies.…”

Efficient multiphoton upconversion and synthesis route dependent emission tunability in GdPO4:Ho3+, Yb3+ nanocrystals

“…However, it has been shown recently that the intensity of up-converted luminescence, excited by sequential absorption of n photons, has a dependence on absorbed power P, which may range between limits of P n and P 1 . 16 Laser pump power vs. UC emission intensity plots indicate the values of slopes for the green emission at 547 nm, and the red emission at 657 nm in Fig. 4(a and b) indicates that two photon absorption is involved in RE ion excitation process from ground state to various excited levels of Ho 3+ ions and different multiphonon relaxation paths lead to different population of the emitting level giving rise to UC emission predominantly in either green or red.…”