Spin glass transitions in the absence of chemical disorder for the pyrochlores A2Sb2O7 (A=Mn, Co, Ni)
A comparison of the spin glass ordering temperature, TSG, and the frustration index f as a function of the spin for the pyrochlore series A2Sb2O7. In the limit of low spin, the frustration index increases by an order of magnitude.
Introduction
Geometrically frustrated magnetism has proven to be a fruitful area of study for solid state chemists over the last few decades. Many theorists are still struggling to understand the wide variety of ground state observed in these systems, such as spin glassiness in the absence of chemical disorder, spin ice freezing in systems with large dipolar interactions, and even spin liquid candidates which remain quantum disordered down to zero Kelvin [1], [2]. Remarkably, all of these phases have been observed in the cubic pyrochlore lattice A2B2O7, a network of corner-shared tetrahedra which has become a model system for the testing of exotic ideas in condensed matter physics. One of the more troubling questions which has arose out of this research is the nature of the spin glass state in systems such as Y2Mo2O7, which have little or no bond disorder [3]. Although the freezing of spin fluctuations at TSG=22 K has been demonstrated through neutron scattering measurements, these systems have some properties which deviate from the canonical spin glasses and merit future research to understand the nature of the phase transition [4].
In this paper we report the synthesis and characterization of the pyrochlores in the series A2Sb2O7 (A=Mn, Co, Ni), and show that these systems also have glassy transitions at TSG=41, 4.5, and 2.6 K in the apparent absence of bond disorder, much like Y2Mo2O7. In these new pyrochlores, the value of the spin can be tuned from (Mn2+) to (Co2+) to S=1 (Ni2+) with little change in the Weiss temperature . This provides a route to investigating spin dynamics of this glassy state as the quantum regime is approached in the limit of low spin. Surprisingly, the frustration index, as defined by Ramirez as increases by an order of magnitude as the moment size decreases from to 1 [2]. As well, there is a corresponding change in spin dynamics as measured through the Mydosh parameter for the shift in the AC susceptibility peak at TSG as a function of driving frequency [5].
Section snippets
Experimental
Polycrystalline samples of A2Sb2O7 (A= Mn, Co, Ni) were prepared through a technique that was outlined by Brisse and refined to give a pure product [6], [7]. Mixtures of acetates, , , , and antimonic acid () in the appropriate stoichiometric ratios were ground and calcined in air at for 12 h, then reground and finally calcined in air at for 12 h. All samples were single-phase as determined by Powder X-ray diffraction (XRD) with a Cu
Results
Fig. 1 shows the Rietveld refinement for the A2Sb2O7 XRD pattern with , and using the program FullProf. A2Sb2O7 (A=Mn, Co, Ni) all have the cubic pyrochlore structure with space group Fd-3m. The lattice parameters are a=10.1664(3) Å, 10.0489(6) Å, and 9.9685(7) Å, for Mn2Sb2O7, Co2Sb2O7, and Ni2Sb2O7, respectively. With decreasing radius of A2+, a decreases (Fig. 2). The eight-fold coordinated magnetic ions, A2+, occupy the corner shared tetrahedral sublattice (inset of Fig. 1
Discussion
The neutron diffraction measurements show that there is no magnetic ordering nor any detectable distortion of the lattice for Mn2Sb2O7 below TSG [7]. The spin glass transition in Mn2Sb2O7 without apparent chemical disorder and structural distortion is reminiscent of other frustrated systems that show glassy behavior such as Y2Mo2O7 [9] or Sr2MgReO6 [10]. The glassiness of the system implies some kind of disorder in the exchange integral, whose origin might be a deformation at the local ionic
Acknowledgments
This work utilized facilities supported in part by the NSF Grant DMR-0504769. A portion of this work was made possible by the NHMFL IHRP, the EIEG Program, and the State of Florida.
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