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A New Geometrically Frustrated Cluster Spin-Glass LiZn2V3O8: A new geometrically frustrated cluster spin-glass S. Kundu,1, ∗ T. Dey,2 A. V. Mahajan,1 and N. B¨uttgen3 1Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India 2Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany 3Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany (Dated: January 1, 2020) Abstract We have investigated the structural and magnetic properties of a new cubic spinel LiZn2V3O8 (LZVO) through x-ray diffraction, dc and ac susceptibility, magnetic relaxation, aging, memory effect, heat capacity and 7Li nuclear magnetic resonance (NMR) measurements. A Curie-Weiss fit of the dc susceptibility χdc(T ) yields a Curie-Weiss temperature θCW = -185 K. This suggests strong antiferromagnetic (AFM) interactions among the magnetic vanadium ions. The dc and ac susceptibility data indicate the spin-glass behavior below a freezing temperature Tf ' 3 K. The frequency dependence of the Tf is characterized by the Vogel-Fulcher law and critical dynamic scaling behavior or power law. 6 From both fitting, we obtained the value of the characteristic angular frequency !0 ≈ 3.56×10 Hz, the dynamic −6 exponent zv ≈ 2.65, and the critical time constant τ0 ≈ 1.82×10 s, which falls in the conventional range for typical cluster spin-glass (CSG) systems. The value of relative shift in freezing temperature δTf ' 0.039 supports a CSG ground states. We also found aging phenomena and memory effects in LZVO. The asymmetric response of the magnetic relaxation below Tf supports the hierarchical model. Heat capacity data show no long-range or short-range ordering down to 2 K. Only about 25% magnetic entropy change (∆Sm) signifies the presence of strong frustration in the system. The 7Li NMR spectra show a shift and broadening with decreasing temperature. The spin-lattice and spin-spin relaxation rates show anomalies due to spin freezing around 3 K as the bulk magnetization. PACS numbers: 75.50.Lk, 75.40.Cx, 76.60.-k I. INTRODUCTION tem. With a frustrated geometry, it exhibits a resonating valence-bond condensed state [14]. The strong interplay between the spin, charge, lattice and orbital degrees of Spinel oxides with the general formula AB2O4 have freedom in these transition metal oxides (TMO) are the provided an excellent arena for studying the effects of source of these novel magnetic properties. Motivated by geometrical frustration [1,2] and have seen a surge of these exotic behaviors of the cubic spinels, we decided to interest in the past decade due to a series of excit- investigate the mixed valent system LiZn2V3O8 (LZVO). ing experimental observations. In the cubic spinel, the Only the structural and electrical properties of LZVO B-sites form a corner shared, 3D tetrahedral network were reported long back in 1972 by B. Reuter and G. like the pyrochlore lattice which is geometrically frus- Colsmann [15]. The magnetic properties of LZVO have trated. Geometrical frustration arises when magnetic not been reported so far. As zinc prefers the A-site in moments occupying the B-sites interact antiferromagnet- spinel like in ZnCr2O4 [16, 17] and ZnV2O4 [18, 19], we ically with each other. With the intention of unraveling expect the site occupancy of LZVO to be represented as novel magnetic properties arising due to geometric frus- [Zn1:0(Li0:25V0:75)2O4]2 - where the octahedral B-site is tration, we were in the quest for new B-site cubic spinel occupied by Li and V in the 1:3 ratio. It will be inter- compounds. In this category, 3d-transition metal oxide 3+ 4+ esting to observe how the 25% dilution via non-magnetic LiV2O4 [3–5] (V :V = 1:1) is a well-studied system. lithium affects the magnetic properties of LZVO. Also, It shows heavy fermion behavior [6]. But with non- this system is amenable to nuclear magnetic resonance magnetic impurity (Zn/Ti) doping, it shows spin-glass (NMR) local probe measurement of 7Li nuclei. From behavior [7–9]. Recently studied mixed valent spinel 3+ stoichiometry, LZVO is a mixed valent spinel as in this system Zn3V3O8 or [Zn1:0(Zn0:25V0:75)2O4]2 with V : 11 4+ compound the average vanadium valence is + 3 : So it V = 2:1 [10] has been suggested to possess a cluster will be interesting to see whether this leads to tetrava- spin-glass ground state. Similarly, Li2ZnV3O8 (better lent and trivalent V in a 2:1 ratio. written as [Zn0:5Li0:5(Li0:25V0:75)2O4]2) containing only V4+ (S = 1=2) magnetic ions shows spin-glass behav- The motivation behind studying the system arXiv:1807.05612v3 [cond-mat.str-el] 31 Dec 2019 ior at low temperatures[11]. LiZn2Mo3O8 [12, 13] is an- LiZn2V3O8 is to contrast its properties with those other system stoichiometrically similar to our probed sys- of the resonating valence bond solid and cluster magnet compound LiZn2Mo3O8, whereas Zn3V3O8 (ZVO) was motivated by Majumdar-Ghosh chain system Ba3V3O8. Also, in LZVO, the effective spin value is close to S ∗Electronic address: [email protected] = 1/2 whereas in ZVO the effective spin is close to 1 ˚ S = 1. As quantum fluctuations are more prominent temperature (RT) with Cu Kα radiation (λ = 1:54182 A) in low spin systems, quantum effects in LZVO might on a PANalytical X’Pert PRO diffractometer. Magneti- play a dominant role compared to classical ZVO. Also, zation measurements were carried out in the temperature 3+ 4+ Zn3V3O8 (ZVO) has the ratio of V :V = 2:1 whereas range 1:8 − 400 K and the field range 0 − 70 kOe using for LZVO it is 1:2. In contrast, Li2ZnV3O8 contains a Quantum Design SQUID VSM. For the low-field mag- only S = 1=2 V4+. The strength of the exchange netization measurements, the reset magnet mode option couplings reflected in the Curie-Weiss temperature θCW of the SQUID VSM was used to set the field to zero. is about -370 K for ZVO while for Li2ZnV3O8 it is about Heat capacity measurements were performed in the tem- -210 K. For both ZVO and Li2ZnV3O8 the irreversible perature range 1:8 − 295 K and in the field range 0 − 90 temperature is Tirr ' 6.0 K and freezing temperatures kOe using the heat capacity option of a Quantum De- (Tf ) is around 3.75 K and 3.50 K respectively. sign PPMS. The NMR measurements were carried out In this work, we report sample preparation, structural using a phase-coherent pulse spectrometer on 7Li nuclei analysis, bulk magnetic properties, heat capacity and 7Li in a temperature range 1.8 - 200 K. We have measured NMR measurements on LZVO. The system crystallizes field sweep 7Li NMR spectra, spin-spin relaxation rate Fd¯3m 1 1 in the centrosymmetric space group with a site ( T2 ) and spin-lattice relaxation rate ( T1 ) at two different sharing between the lithium and the vanadium atoms at fixed radio frequencies (rf) of about 95 MHz and 30 MHz, the crystallographic 16c sites. Our magnetization data respectively. The spectra were measured using a con- π show no long-range ordering down to 2 K but we found ventional spin-echo sequence ( 2 − τecho − π). We have a splitting between the zero field cooled (ZFC) and field used the spin-echo pulse sequence with a variable delay cooled (FC) data at 3 K in low fields. A large Curie- time τD to measure T2 and a saturation pulse sequence θ = π Weiss temperature, CW - 185 K indicates strong an- ( 2 − τD − π) is used after a waiting time or last delay of tiferromagnetic (AFM) interactions between the mag- three to five times of T1 between each saturation pulse netic ions. Our frequency dependent ac susceptibility to measure T1. results indicate a spin-glass ground state. To know the spin-dynamics of the glassy phase in detail, we have car- ried out further magnetization measurements below the III. RESULTS AND DISCUSSION freezing temperature (Tf ) and observed clear signature of magnetic relaxation, aging effect and memory phe- nomena which are thought to be typical characteristics A. Crystal structure of spin-glasses. In memory effects, we observed that a To check for phase purity, we have measured the XRD small heating cycle erases its previous memory and reini- pattern of polycrystalline LZVO. The Rietveld refine- tializes the relaxation process. This type of asymmetric ment of LZVO (shown in Fig.1) revealed that it crys- response favors by the hierarchical model [20, 21]. The tallizes in the centrosymmetric cubic spinel Fd¯3m (227) inferred magnetic heat capacity has a broad maximum at space group. The two phase refinement of LZVO in- T ∼ 7 K but no sharp anomaly indicative of long-range dicates the presence of non-magnetic impurity phase ordering (LRO) is observed. The field swept 7Li NMR Li3VO4 (less than 2%) together with the main phase. spectra down to 1.8 K indicate the presence of an NMR The atomic positions obtained after Rietveld refinement line shift and gradual broadening of the spectra as T de- (using Fullprof suite [22]) are given in TableI. After re- creases. The spin-lattice and spin-spin relaxation rates finement, we obtained the lattice parameters as : a = b both show anomalies at a freezing temperature of 3 K. = c = 8.364 Å, α = β = γ = 900. The goodness of the Rietveld refinement is inferred from the following param- 2 eters χ : 1.37; Rp: 21.8%; Rwp: 10.7%; Rexp: 9.14%. II. EXPERIMENTAL DETAIL Note that from Rietveld refinement we can rewrite the chemical formula of LiZn2V3O8 as The polycrystalline LiZn2V3O8 sample was prepared [Zn1:0(Li0:241V0:759)2O4]2 like the cubic spinel gen- by conventional solid-state reaction techniques using high eral formula AB2O4.
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