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Spinels ( AB 2 O 4 ) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn 0.8 Cu 0.2 FeMnO 4 , the tetragonality resulting from the Jahn-Teller active Mn 3+ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn0.82+Cu0.22+) A [Fe0.42+Fe0.63+Mn3+] B O 4‒ δ . Analysis of the temperature dependence of magnetization ( M ), ac magnetic susceptibilities ( χ' and χ'' ), dc susceptibility ( χ ), heat capacity C p , and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of χ vs. T fits the Curie-Weiss law: χ = C /( T ‒ θ ) from T = 250 K to 400 K with θ ≃ 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J / k B = 17 K, and C = 3.29 emu K mol ‒1 Oe ‒1 yielding an effective magnetic moment µ eff = 5.13 µ B resulting from the high-spin states of Cu 2+ ( A site) and Fe 2+ ( B site), while the B site trivalent ions Mn 3+ and Fe 3+ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu 2+ ↓) A [Fe 2+ ↑, Fe 3+ ↓, Mn 3+ ↑] B leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d( χT)/ d T shows the onset of ferrimagnetism below ∼100 K and peaks near 47 K and 24 K. The relaxation time τ obtained from temperature and frequency dependence of χ″ when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperatureTSGHfollows the equation:TSGH=TSG01-AH2/ϕwith T SG (0) = 46.6 K, A = 8.6 × 10 ‒3 Oe ‒0.593 andϕ= 3.37. The temperature dependence of hysteresis loops yields coercivity H C ∼ 3.8 kOe at 2 K without exchange-bias, but H C decreases with increase in T becoming zero above 24 K, the T SG ( H ) for H = 800 Oe. Variations of C p vs. T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, C p varies as T 2 ; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization M TRM ( t ) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn 0.8 Cu 0.2 FeMnO 4 , ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at T SG (0) = 46.6 K, followed by SL behavior below 9 K., (© 2023 IOP Publishing Ltd.)

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]O 4 . 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.

In the spinel Co2SnO4, coexistence of ferrimagnetic ordering below T(N) ≃ 41 K followed by a spin glass state below T(SG) ≃ 39 K was proposed recently based on the temperature dependence of magnetization M(T) data. Here new measurements of the temperature dependence of the specific heat C(P)(T), ac-susceptibilities χ'(T) and χ″(T) measured at frequencies between 0.51 and 1.2 kHz, and the hysteresis loop parameters (coercivity H(C)(T) and remanence M(R)(T)) in two differently prepared samples of Co2SnO4 are reported. The presence of the Co(2+) and Sn(4+) states is confirmed by x-ray photoelectron spectroscopy (XPS) yielding the structure: Co2SnO4 = [Co(2+)][Co(2+)Sn(4+)]O4. The data of C(P) versus T shows only an inflection near 39 K characteristic of spin-glass ordering. The analysis of the frequency dependence of ac-magnetic susceptibility data near 39 K using the Vogel-Fulcher law and the power-law of the critical slowing-down suggests the presence of spin clusters in the system which is close to a spin-glass state. With a decrease in temperature below 39 K, the temperature dependence of the coercivity H(C) and remanence M(R) for the zero-field cooled samples show both H(C) and M(R) reaching their peak magnitudes near 25 K, then decreasing with decreasing T and becoming negligible below 15 K. The plot of C(P)/T versus T also yields a weak inflection near 15 K. This temperature dependence of H(C) and remanence M(R) is likely associated with the different magnitudes of the magnetic moments of Co(2+) ions on the 'A' and 'B' sites and their different temperature dependence.

The nature of magnetic ordering in quasi-2D layered hydroxide of Ni (LH-Ni-DS) with hexagonal structure and synthesized by intercalating dodecyl sulfate (DS) ligand, (C12H25OSO3)(-), between the layers using a hydrothermal technique is investigated. The observation of (00l) peaks up to l = 8 in x-ray diffraction on the sample yields an interlayer spacing c ≃ 30.5 Å and a crystallite size ≃ 16 nm. Assignment of the lines observed in the FTIR spectra to the various groups of the DS ligand confirms the intercalation. From the analysis of detailed investigations of the temperature dependence of the magnetization M at different magnetic fields, ac susceptibilities at frequencies from 0.1 to 1 kHz, and electron magnetic resonance spectra at 9.28 GHz, it is concluded that LH-Ni-DS orders ferromagnetically at TC ≃ 23 K. This TC is about 45% higher than TC ≃ 16 K reported for LH-Ni-Ac with c ≃ 8.6 Å obtained by intercalating an acetate ligand between the layers. The roles of the interlayer dipolar interaction, magnetic anisotropy and exchange interactions in determining TC in LH-Ni-L systems for several ligands L yielding different c-axes are discussed.

The two layered hexagonal hydroxides of Ni are β-Ni(OH)(2) and α-Ni(OH)(2); β-Ni(OH)(2) is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)(2) is not yet well established. Here, the magnetic properties of α-Ni(OH)(2) with lattice parameters a = 3.02 Å and c = 8.6 Å, and flower-like morphology with petal thickness of approximately equal to 50 Å are reported. Temperature (2-300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1-1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T(C) is approximately equal to 16 K. In addition, a nanosize related blocking temperature T(B) = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J(1)/k(B) = 4.38 K (J(2)/k(B) = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni(2+) ions.

Investigations of spin dynamics in Ni nanoparticles (NPs) with diameters D = 3.8, 11.7, 15 and 21 nm embedded in an amorphous SiO(2) matrix of composition 15/85 (Ni/SiO(2)) are reported using EMR (electron magnetic resonance) spectroscopy at 9.28 GHz. Three resonance lines are observed whose EMR parameters, namely linewidth ΔH, resonance field H(r) and intensity I(o), are measured from 5 to 300 K. Line 1 with temperature-independent ΔH = 50 Oe and g≈2, and intensity varying as 1/T, is shown to result from paramagnetic defects in the SiO(2) matrix. Lines 2 and 3, with g≈2.2 and 8, respectively, and temperature-dependent EMR parameters are assigned to Ni NPs. While line 2 with g≈2.2 is due to the majority of Ni NPs, the source of line 3 is discussed in terms of two possibilities: (i) large clusters of blocked Ni NPs and (ii) the inherent part of the composite asymmetric line made up of lines 2 and 3 predicted by the Raikher-Stepanov (RS) model for dispersed ferromagnets. The temperature dependence of ΔH (full width at half-maximum) of the composite line obtained by integration of the EMR spectra decreases with the increase in temperature, reaching a minimum near 300 K in agreement with the RS model. The observed decreasing asymmetry of the composite absorption spectra with increasing temperature is also in agreement with the predictions of the RS model, thus providing a satisfactory explanation for the observed temperature dependence of the EMR spectra of Ni NPs. Large clusters of blocked Ni NPs as the source of line 3 are ruled out and additional tests for the RS model are proposed. The decreasing magnetization with decreasing particle size of Ni NPs observed here is discussed in terms of the possible roles of the surface disordered spins and Ni-SiO(2) interaction.