Effects of Cu doping on the electronic structure and magnetic properties of MnCo₂O₄nanostructures

Abstract

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. 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]) [Formula: see text] [Formula: see text] [Formula: see text]] [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 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 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 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 and remanence [Formula: see text]. Since the results reported here in these nanostructures are significantly different from those in bulk [...

Mn substituted MnxZn1−xCo2O4 oxides synthesized by co-precipitation; effect of doping on the structural, electronic and magnetic properties

“…For this sample weak hysteresis is still observed, thus indicating the antiferromagnetic nature of this sample at 300 K. 43,49 This type of behavior is observed for Cu substituted spinel MnCo 2Àx Cu x O 4 (0 # x # 0.2) nanostructures with the samples x $ 0.1 having the coercivity H c and remanence M r close to 0. 50 In the Mn x Zn 1Àx Co 2 O 4 the non-magnetic divalent Zn 2+ ions tend to occupy tetrahedral (A) sites and thereby force the incoming Mn ions to octahedral (B) sites due to their favoritism by polarization effect. 30 In general, spinel type cobalt-based oxides contain high spin Co 2+ (S ¼ 3/2) in the tetrahedral site and low spin diamagnetic Co 3+ (S ¼ 0) in the octahedral site; magnetic ions in the sub lattices are arranged antiparallel to their four neighboring ions resulting in antiferromagnetic behavior of the material.…”

Cationic distribution, exchange interactions, and relaxation dynamics in Zn-diluted MnCo2O4 nanostructures

“…20 Recent studies reveal that MnCo 2 O 4 nanoparticles exhibit memory and aging effects, spinglass like characteristics below the freezing point T F ¼ 144:8 K (with zν ¼ 6:01 and τ o 6:18 Â 10 À15 s) together with a finite size scaling of T C (T C (d) ¼ T C (1)[1 À (ξ 0 =d) λ ], λ ¼ 0:75 + 0:15, and ξ 0 ¼ 1:4 + 0:3 nm). 17,19,21 Moreover, a significant reduction of T C (167 K) was noticed with the incorporation of Jahn-Teller active ion Cu at the octahedral Co(B) sites of MnCo 2 O 4 . 19 On the other hand, Bhowmik and Ranganathan reported that the substitution of 50 at.…”

“…17,19,21 Moreover, a significant reduction of T C (167 K) was noticed with the incorporation of Jahn-Teller active ion Cu at the octahedral Co(B) sites of MnCo 2 O 4 . 19 On the other hand, Bhowmik and Ranganathan reported that the substitution of 50 at. % of Ru at the Mn site leads to a further decrease of T C (100 K) together with the signatures of pinning effects in domain wall dynamics.…”

“…18 Also, the tetrahedrally coordinated Mn and Co atoms at (001) and (110) surfaces exhibit bulk like characteristics, whereas the electronic structure of octahedral Co atoms changes substantially from the bulk state into the surface state at the (001) surface of MnCo 2 O 4 . 18 Besides, MnCo 2 O 4 exhibits ferrimagnetic properties with ordering temperature T C 183 K for the bulk system 17,19 and reduced T C ( ¼ 130 K) values for nanometer sized particles (d 8 nm). 17 It has been shown in the literature that the T C values may vary markedly depending upon the oxygen stoichiometry δ as well as in MnCo 2 O 4+δ .…”

Emerging High Coercivity and Huge Exchange Bias Effect in Single Phased Mn1−xRuxCo2O4 Compounds

“…Even though the authors owed it to domain wall pinning effect, it was not confirmed by further evidences. In the counterpart Mn 1 − x Cu x Co 2 O 4 , two magnetic phases can be identified from the hysteresis loops . Therefore, the observed high coercivity in Mn 0.5 Ru 0.5 Co 2 O 4 may not be solely due to the domain wall pinning effect, but the interaction among different magnetic phases.…”