Spin glass transitions in the absence of chemical disorder for the pyrochlores A2Sb2O7 (A=Mn, Co, Ni)

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Abstract

The pyrochlores in the series A2Sb2O7 have been synthesized and characterized as exhibiting spin glass transitions at TSG=41, 4.5, and 2.6 K (for A=Mn2+ S=52, Co2+ S=32 and Ni2+ S=1, respectively) despite the lack of chemical disorder. Since the Curie–Weiss temperature remains essentially constant for all members in the series (θ40K), the frustration index for these materials increases significantly as the moment size is reduced from f=|θ|/TSG=1.1 (Mn2+), to 9.3 (Co2+) to 14.6 (Ni2+). There is also a corresponding change in the spin dynamics measured by the shift in the AC susceptibility signal as a function of frequency. These new materials provide an avenue to investigate the effect of quantum fluctuations on the Heisenberg pyrochlore lattice in the low spin limit, and show there is a dramatic change in the spin dynamics as the quantum regime is approached.

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.

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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 S=52 (Mn2+) to S=32 (Co2+) to S=1 (Ni2+) with little change in the Weiss temperature θ40K. 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 f=|θ|/TSG increases by an order of magnitude as the moment size decreases from S=52 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, Mn(Ac)2·4H2O, Co(Ac)2·4H2O, Ni(Ac)2·2H2O, and antimonic acid (Sb2O5·xH2O) in the appropriate stoichiometric ratios were ground and calcined in air at 150C for 12 h, then reground and finally calcined in air at 450C for 12 h. All samples were single-phase as determined by Powder X-ray diffraction (XRD) with a Cu Kα1

Results

Fig. 1 shows the Rietveld refinement for the A2Sb2O7 XRD pattern with Rp9, Rwp13 and χ23.0 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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