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1.
Inorg Chem ; 63(15): 6616-6625, 2024 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-38569100

RESUMO

Four new compositionally complex perovskites with multiple (four or more) cations on the B site of the perovskites have been studied. The materials have the general formula La0.5Sr2.5(M)2O7-δ (M = Ti, Mn, Fe, Co, and Ni) and have been synthesized via conventional solid-state synthesis. The compounds are the first reported examples of compositionally complex n = 2 Ruddlesden-Popper perovskites. The structure and properties of the materials have been determined using powder X-ray diffraction, neutron diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and magnetometry. The materials are isostructural and adopt the archetypal I4/mmm space group with the following unit cell parameters: a ∼ 3.84 Å, and c ∼ 20.1 Å. The measured compositions from energy dispersive X-ray spectroscopy were La0.51(2)Sr2.57(7)Ti0.41(2)Mn0.41(2)Fe0.39(2)Co0.38(1)Ni0.34(1)O7-δ, La0.59(4)Sr2.29(23)Mn0.58(5)Fe0.56(6)Co0.55(6)Ni0.42(4)O7-δ, La0.54(2)Sr2.49(13)Mn0.41(2)Fe0.81(5)Co0.39(3)Ni0.36(3)O7-δ, and La0.53(4)Sr2.55(19)Mn0.67(6)Fe0.64(5)Co0.31(2)Ni0.30(3)O7-δ. No magnetic contribution is observed in the neutron diffraction data, and magnetometry indicates a spin glass transition at low temperatures.

2.
J Phys Condens Matter ; 34(27)2022 May 06.
Artigo em Inglês | MEDLINE | ID: mdl-35439746

RESUMO

The nature of magnetism in the doubly-diluted spinel ZnTiCoO4= (Zn2+)A[Ti4+Co2+]BO4is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (χ), ac susceptibilities (χ' andχ″), and heat capacity (Cp) measurements. Whereas antiferromagnetic (AFM) Néel temperatureTN= 13.9 K is determined from the peak in the ∂(χT)/∂TvsTplot, the fit of the relaxation timeτ(determined from the peak in theχ″ vsTdata at different frequencies) to the Power law:τ=τ0[(T-TSG)/TSG]-zνyields the spin glass freezing temperatureTSG= 12.9 K,zν∼ 11.75, andτ0∼ 10-12s. Since the magnitudes ofτ0andzνdepend on the magnitude ofTSG, a procedure is developed to find the optimum value ofTSG= 12.9 K. A similar procedure is used to determine the optimumT0= 10.9 K in the Vogel-Fulcher law:τ=τ0 exp[Ea/kB(T-T0)] yieldingEa/kB= 95 K, andτ0= 1.6 × 10-13s. It is argued that the comparatively large magnitude of the Mydosh parameter Ω = 0.026 andkBT0/Ea= 0.115 (≪1) suggests cluster spin-glass state in ZnTiCoO4below TSG. In theCpvsTdata from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence ofλ-type anomaly nearTNorTSGcombined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence ofTSGshows slight departure (ϕ∼ 4.0) from the non-mean-field Almeida-Thouless lineTSG(H) =TSG(0) (1 -AH2/ϕ). Strong temperature dependence of magnetic viscositySand coercivityHCwithout exchange bias, both tending to zero on approach toTSGfrom below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+and Ti4+ions leading to magnetic frustration. Magnetic phase diagram in theH-Tplane is established using the high-field magnetization dataM(H,T) forTTN, the data ofχvsTare fit to the modified Curie-Weiss law,χ=χ0+C/(T+θ), withχ0= 3.2 × 10-4emu mol-1Oe-1yieldingθ= 4 K andC= 2.70 emu K mol-1Oe-1. This magnitude ofCyields effective magnetic moment = 4.65µBfor Co2+, characteristic of Co2+ions with some contribution from spin-orbit coupling. Molecular field theory with effective spinS= 3/2 of Co2+is used to determine the nearest-neighbor exchange constantJ1/kB= 2.39 K AFM and next-nearest-neighbor exchange constantJ2/kB= -0.66 K (ferromagnetic).

3.
J Phys Condens Matter ; 32(48): 485806, 2020 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-32903218

RESUMO

Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+)[Formula: see text] are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at T N ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (T SG) near 14.2 K. It is argued that T N is mainly governed by the ordering of the spins of Co2+ ions occupying the A-site, whereas the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site triggers the spin-glass phase, Co3+ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < T N are used to construct the H-T phase diagram showing that T SG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas T N is nearly H-independent. For T > T N, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie-Weiss law, χ = χ 0 + C/(T + θ), with χ 0 = 0.0015 emu mol-1Oe-1 yielding θ = 53 K and C = 2.16 emu-K mol-1Oe-1, the later yielding an effective magnetic moment µ eff = 4.16 µ B comparable to the expected value of µ eff = 4.24 µ B per Co2RuO4. Using T N, θ and high temperature series for χ, dominant exchange constant J 1/k B ∼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near T SG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ 0 ∼ 1.16 × 10-10 sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M-H loop asymmetry at low temperatures with giant exchange bias effect (H EB ∼ 1.8 kOe) and coercivity (H C ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below T N is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > T N to inverse MCE for T < T N observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

4.
J Phys Condens Matter ; 29(42): 425803, 2017 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-28767047

RESUMO

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping ([Formula: see text]) on the electronic structure and magnetic properties of the spinel [Formula: see text]O4. A detailed comparison is given for the [Formula: see text] and [Formula: see text] cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ([Formula: see text])A [Formula: see text] [Formula: see text] [Formula: see text]]B [Formula: see text] i.e. [Formula: see text] substitutes for [Formula: see text] on the octahedral B-sites. For the bulk samples, the ferrimagnetic [Formula: see text] K for [Formula: see text] is lowered to [Formula: see text] K for the [Formula: see text] sample, this decrease being due to the effect of Cu doping. For the nanosize [Formula: see text] ([Formula: see text]) sample, the lower [Formula: see text] K ([Formula: see text] K) is observed using [Formula: see text] analysis, this lowering being due to finite size effects. For [Formula: see text], fits of dc paramagnetic susceptibility data of [Formula: see text] versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: [Formula: see text] K (4.1 K), [Formula: see text] K (16.3 K) and [Formula: see text] K (13.8 K) for [Formula: see text]. The temperature dependence of ac susceptibilities [Formula: see text] and [Formula: see text] at different frequencies shows that in bulk samples of [Formula: see text] and [Formula: see text], the transition at T C is the normal second order transition. But for the nanosize [Formula: see text] and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at T C is followed by a re-entrant spin-glass transition at lower temperatures [Formula: see text] K (138 K) for [Formula: see text] ([Formula: see text]). Analysis of the ac susceptibilities, [Formula: see text] and [Formula: see text], versus T data is done in terms of two scaling laws: (i) Vogel-Fulcher law [Formula: see text] [Formula: see text]; and (ii) power law of critical slowing-down [Formula: see text]. These fits confirm the existence of glassy behavior below T SG with the parameters [Formula: see text] (8.91), [Formula: see text] (9.6 × 10[Formula: see text]) and [Formula: see text] K (∼138 K) for the samples [Formula: see text] (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in [Formula: see text] versus [Formula: see text] (AT-line) further supports the existence of glassy states in nanosize samples. For [Formula: see text], the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with x ⩾ 0.1 have the coercivity H C and remanence [Formula: see text]. Since the results reported here in these nanostructures are significantly different from those in bulk [Formula: see text] [Formula: see text], further investigations of their magnetic structures using neutron diffraction are warranted.

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