Paramagnetic Ferrimagnetic Transition Research Articles

Low-temperature cluster spin glass transition in the single-domain NiCr2O4 nanoparticles

NiCr2O4 nanoparticles with average particle size ∼15 nm, a single-domain size maintains the bulk canted antiferromagnetic ground state, were synthesized by a microwave combustion method. The magnetic behavior was carefully investigated by static and dynamic magnetic susceptibility measurements. In addition to a spin-glass-like behavior below paramagnetic-ferrimagnetic transition at T C, the NiCr2O4 nanoparticles demonstrate a low-temperature cluster spin glass transition below the spin canting transition T S, which manifests itself as a magnetic anomaly peak around ∼12 K (at 100 Oe) in the zero-field cooled magnetization with a relatively stronger field dependence in a ‘de Almeida-Thouless’ line for spin glasses. The AC susceptibility analyses in different approaches demonstrate a larger relative peak temperature variation per frequency decade and a longer characteristic relaxation time in the order of 0.04 and 10−7 s, against 0.01 and 10−9 s for the high-temperature blocking, indicating the slow spin dynamics for the low-temperature cluster glassy phase. A field-temperature magnetic phase diagram is proposed for the single-domain NiCr2O4 nanoparticles.

Critical behavior at paramagnetic to ferrimagnetic phase transition in A-site ordered perovskite CaCu3Cr2Nb2O12

A new perovskite-type oxide, CaCu3Cr2Nb2O12, has been synthesized at 10 GPa and 1100 °C. The compound crystallizes in space group Im3¯ with 1:3 ordered Ca and Cu cations at the A sites and disordered Cr and Nb cations at the octahedral B sites. CaCu3Cr2Nb2O12 exhibits a paramagnetic–ferrimagnetic transition at around 192.3 K and the Arrott plots manifest this transition is a second-order transition. We investigate the critical behavior of CaCu3Cr2Nb2O12 using the modified Arrott plots, Kouvel–Fisher plot, and the scale theory. The estimated critical exponents β, γ and δ, which yield β = 0.465 ± 0.006, γ = 1.312 ± 0.019 and δ = 3.833 ± 0.019 with TC = 197 K, fall within the predicted values from the mean-field model and three-dimensional Heisenberg model. The renormalization group analysis suggests that the exchange interaction decays as J(r)∼ r−4.871, indicating the magnetic interactions may be long-range coupling in CaCu3Cr2Nb2O12.

Magnetocaloric effect and magnetic properties of Dy2CoMnO6: Monte Carlo study

ABSTRACT We studied the magnetic properties and magnetocaloric effect of Dy2CoMnO6 (DCMO) by using the Monte Carlo study. The magnetisation vs. the temperature was found. The ferrimagnetic-paramagnetic transition was obtained around the transition temperature T C = 82 K. The value of T C was found. dM/dT and internal energy vs. the temperature have been given for several magnetic fields. The magnetic entropy changes vs. temperature have been established. The maximum of magnetic entropy changes increases with increasing the magnetic field from 2.53, 3.43, 4.19, 4.93 and 5.63 J/kg. K with h = 1, 2, 3, 4 and 5 T, respectively. Relative cooling power was found and increases with increasing the value of the magnetic field from 18 (at 1 T) to 50 J/kg (at 5 T). The thermal adiabatic temperature was also determined. The magnetisation vs. the magnetic field has been deduced for several temperatures. Our results offer comprehensive information for understanding the roles of mixed valent magnetic ions and rare earth magnetic moments on the magnetic and magnetocaloric properties.

Magnetic and dielectric properties of single-crystalline ErFeMnO4

We report on the growth of single-crystal ErFeMnO4 and its magnetization, resistivity, and dielectric dispersion behaviors for varying temperature and electric field. A clear bifurcation between the in-plane and out-of-plane M(T) curves is observed due to a paramagnetic-ferrimagnetic transition with a transition temperature of ∼301 K, which is higher than for other RFe2O4 or substituted compounds. Over a similar temperature range, sudden changes in both the relative permittivity and resistivity were also observed, indicating a strong connection between the magnetization, resistivity and dielectric properties. It can be concluded that the electron transfer between the Fe-site ions plays an important role in these behaviors. Finally, a large relative tunability of 75.9% for relative permittivity εr and a net polarization P of about ∼408.7 μC/m2 were attained for a DC biased electric field E = 25 kV/m at 390 K, which are promising for the application of tunable ferroelectric devices.

Structural and Magnetic Properties of the New SrLa2FeCuSbO9 Perovskite Phase

We report on the synthesis and structural and magnetic properties of the new SrLa2FeCuSbO9 perovskite. The compound was prepared by the solid-state reaction method. The crystalline structure was studied by X-ray diffraction experiments using the Rietveld refinement analysis. SrLa2FeCuSbO9 crystallizes in a monoclinic system (P21/n space group). Its unit cell parameters are a = 5.6101(2) A, b = 5.6125(2) A, c = 7.9358(3) A, and β = 90.094(2)°. Sr2+ and La3+, in SrLa2FeCuSbO9, are statistically distributed within the A-perovskite sites. The obtained cationic distribution can be illustrated by the [SrLa2]A[(SbCu0.5)2c(FeCu0.5)2d]BO9 crystallographic formula. SrLa2FeCuSbO9 was found to exhibit a paramagnetic–ferrimagnetic transition around TC = 215 K. At high temperature, the magnetic susceptibility was described by the Curie–Weiss law. The experimental effective moments were explained by only spin contribution for Cu2+ and Fe3+ ions. At low temperature, the compound presents a ferrimagnetic state with signs of frustration.

Substantially enhanced ferroelectricity in JT ion Cu2+-doped Co1–xCuxCr2O4 (0≤x≤0.4)

We investigate the effect of Jahn-Teller (JT) active Cu2+ ions on the structural, magnetic, and ferroelectric properties of Co1–xCuxCr2O4 (0 ≤x≤ 0.4). All the samples can be indexed with a cubic lattice with space group Fd3¯m at room temperature. All the samples undergo a paramagnetic-ferrimagnetic transition at TC and spiral ferrimagnetic ordering at TS. The magnetization measurements manifest that magnetization reversal occurs in the x = 0.2 and 0.3 samples. The x = 0.3 sample possesses a rather large polarization of 14 μC/m2, which is 7 times in magnitude compared with the CoCr2O4 single crystal. The analysis of the infrared active phonon modes implies that two T1u(IR) optical phonon modes are gradually split into a doublet at around 40 K, implying the lowering of lattice symmetry. The presence of a low-symmetry tetragonal phase becomes polar and enhances the electric polarization of Co0.7Cu0.3Cr2O4.

Structural and magnetic studies of Co1−xNixCr2O4 (0 ≤ x ≤ 1)

We have investigated the structural, magnetic, and thermal transport properties of Co1−xNixCr2O4 (0≤x≤1) to check the evolution of structural and magnetic properties upon the introduction of the Jahn–Teller active ion Ni2+. The x≤0.95 samples can be indexed with a cubic lattice with the space group Fd3m, whereas the x=0.98 sample undergoes a cubic-tetragonal structural transition, and the x>0.98 samples belong to a tetragonal lattice with the space group I41-amd at room temperature. All these samples undergo a paramagnetic–ferrimagnetic transition at TC. A spiral ferrimagnetic ordering at TS is suppressed when the amount of Ni is increased, and the abnormal magnetization at TS for the x=0.6 and 0.8 samples is not pronounced. The measurements of magnetic hysteresis reveal that the x=0.4 and 0.6 samples possess a rather large coercivity HC compared with the x=0 and 1 samples. Specifically, the x=0.4 and 0.6 samples undergo a short-range antiferromagnetic transition above TC. The magnetic properties of the Ni-doped samples can be understood in terms of the magnetic structure model consisting of longitudinal and transverse magnetic sublattices.

Order Parameter Profiles near Surfaces of Super-weak Ferrimagnetic Materials Exhibiting a Paramagnetic-Ferrimagnetic Transition

The aim of this paper is to determine the full order parameter profiles close to the surface for a system made of two strongly coupled paramagnetic sublattices of respective moments and . The material exhibits a para-ferrimagnetic transition at some critical temperature Tc greater than the room temperature. The free energy describing the physics of the system is of Landau type, and involves, beside quadratic and quartic terms in both and , a lowest-order coupling, -Co where CoL, and the free energy is then a sum of bulk and surface contributions expanded in terms of the local order parameters. The magnetization at the surface are s and s. As in a recent paper (whom the present work is an extension), we first reduce the model to an effective 4 -theory written in terms of the overall magnetization =+ and the associated fraction of magnetization a=/(+). For the ordinary transition where the extrapolation length is positive s>0, we determine the order parameter profile near surfaces. We show, in particular, that the magnetization behavior is governed, in addition of s and the bulk correlation length b-, by a new length La. We have interpreted this latter as the width of the ordered ferrimagnetic layer close to the surface. Finally, we examine the extraordinary (ss=) and surface (at Tcs>Tc) transitions.

Critical Behavior at Surfaces of Strong Coupling Paramagnetic Systems Exhibiting a Paramagnetic-Ferrimagnetic Transition

The purpose of this work is the investigation of the critical surface effects of two strongly coupled paramagnetic sublattices exhibiting a para-ferrimagnetic transition. The model is of Landau-Ginzburg type, whose bulk free energy is a functional of two kind of order parameters (local magnetizations) and This free energy involves, beside quadratic and quartic terms in both and , a lowest-order coupling, where Co<0 is the coupling constant measuring the interaction between the two sublattices. Two terms H and are also introduced, to describe the interaction within an external magnetic field H. We introduce a surface free energy expanded in terms of the local order parameters. The magnetization at the surface are s and We show, in particular, that the model can be reduced to an effective theory written in terms of the overall magnetization and the associated fraction of magnetization . This formulation leads us to define an effective extrapolation length s. We then derive all the critical properties of the system close to the critical temperature Tc. In particular, we determine the critical behavior of the overall surface magnetization , in terms of b above and below Tc. The variations of s with the magnetic field H, and when a surface field Hs is applied actually at Tc, are derived. We determine, also, the associated susceptibilities at the surface s and s.s. The determination of the full profile of the magnetization close to the surface will be the subject of a future communication.

Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles

Effect of silica (SiO2) coating concentration on structural and magnetic properties of multiferroic cobalt chromite (CoCr2O4) nanoparticles have been studied. The nanoparticles with average crystallite size in the range 19 to 28 nm were synthesised by sol-gel method. X-ray diffraction (XRD) analysis has verified the composition of single-phase cubic normal spinel structure of CoCr2O4 nanoparticles. The average crystallite size and cell parameter decreased with increasing SiO2 concentration. TEM image revealed that the shape of nanoparticles was non-spherical. Zero field cooled/field cooled (ZFC/FC) curves revealed that nanoparticles underwent a transition from paramagnetic (PM) state to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition temperature decreased from 101 to 95 K with increasing SiO2 concentration or decreasing crystallite size. A conical spin state at Ts = 27 K was also observed for all the samples which decreased with decreasing average crystallite size. Low temperature lock-in transition was also observed in these nanoparticles at 12 K for uncoated nanoparticles which slightly shifted towards low temperature with decreasing average crystallite size. Saturation magnetization (Ms) showed decreasing trend with increasing SiO2 concentration, which was due to decrease in average crystallite size of nanoparticles and enhanced surface disorder in smaller nanoparticles. The temperature dependent AC-susceptibility also showed the decrease in the transition temperature (Tc), broadening of the Tc peak and decrease in magnetization with increasing SiO2 concentration or decreasing average crystallite size. In summary, the concentration of SiO2 has significantly affected the structural and magnetic properties of CoCr2O4 nanoparticles.

Structural, magnetic and magnetodielectric properties of the Mn3O4 thin films epitaxially grown on SrTiO3 (001) substrates

The (001)-orientated Mn3O4 thin films were fabricated on Nb-doped SrTiO3 (001) substrates by a pulsed laser deposition method, and the structural, magnetic, and magnetodielectric properties were investigated. We found that the as-prepared thin film is tetragonal with biaxial compressive strain in the film plane. The orientation relationship in the film plane is Mn3O4 [100] || SrTiO3 [110]. Compared with the single crystals, the paramagnetic–ferrimagnetic transition temperature TN is greatly increased due to the in-plane compressive strain. At TN, a sharp drop in the dielectric constant was observed. Under a magnetic field, the dielectric constant shows a negative field-direction dependent response around TN. Our experiments indicated the strong coupling between the strain, lattice, spin, and orbital degrees of freedom in the epitaxial Mn3O4 thin films.

Evolution of film orientation in Y2MnCrO6 epitaxial films prepared by a two-step growth method

Epitaxial thin films of Y2MnCrO6 (YMCO) have been fabricated on various substrates via a two-step process combining magnetron sputtering technique and ex-situ annealing method. X-ray diffraction observations revealed that film orientation and quality are dominantly governed by the lattice mismatch between film and substrate. Moreover, orientation of the films grown on SrTiO3 can be tuned to some extent by adjusting substrate temperature and post-annealing process. The YMCO films were found to undergo a paramagnetic–ferrimagnetic transition at ~70K, which is consistent with the bulk sample. Furthermore, the hysteresis loops indicate that the films may own a uniaxial anisotropy with the easy axis along the c-direction.

Temperature Dependent EXAFS Study of Chromium-Doped GaFeO3 at Gallium and Iron Edges

We report the results of EXAFS measurements on undoped and Cr-doped GaFeO3 samples both at Ga and Fe K edges at various temperatures in the regime of 25–300 K, to explore the changes in local structure surrounding the cations (viz., Ga and Fe) in GaFeO3 due to Cr substitution and across its magnetic transition temperature. The EXAFS results along with neutron diffraction studies and theoretical simulations establish the mechanism of Cr substitution in the GaFeO3 lattice in a microscopic scale. The decrease in the paramagnetic–ferrimagnetic transition temperature (TC) and the saturation magnetization observed in the samples with increase in Cr doping concentration, has been attributed to the decrease of Fe and concomitant increase of Cr in Ga2 site. The bond lengths at both Fe and Ga sites exhibit changes at the magnetic transition temperatures, indicating enhanced cationic site disorder, which also explains the reported dielectric anomalies observed in these samples at the magnetic transition temperatures.

Size dependence of structure and magnetic properties of CoCr2O4 nanoparticles synthesized by hydrothermal technique

The size dependence of structure and magnetic behavior has been investigated on spinel chromite CoCr2O4 nanoparticles (NPs) with different particles from ~2.8 to ~19.7 nm, synthesized by a hydrothermal technique. The microstructure analysis shows that the lattice parameter increases monotonically with reduced particle sizes. Compared with the bulk sample, all synthesized NPs exhibit lower Curie temperature (TC~87 K) from paramagnetic (PM) to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition is gradually degraded with decreased particle sizes and nearly vanishes for ~2.8 nm NPs. No magnetic transition from short-range FiM order to long-range spiral spin structure is found at low temperatures contrast to the bulk ones. Instead, a cluster spin-glass (SG) transition appears for NPs with D≤5.4 nm. The dynamic behavior of the cluster SG can be described by a power law with the spin relaxation time in the range of 10−10–10−8 s and critical exponent zv′=9.9±0.6, in accordance with a three-dimensional Ising SG behavior. The glass transition temperature (Tg) decreases from 16.3 K for ~5.4 nm to 12.8 K for ~2.8 nm NPs.

Magnetic phase diagram of Al-doped spinel MnV2O4

We present the structure, magnetization, and resistivity results of the spinel Al-doped MnV2O4. When small amount of Al ions substitutes V ones, the structural transition accompanying orbital order of MnV2O4 disappears. With further Al doping, the paramagnetic–ferrimagnetic transition temperature TC increased firstly and then decreased. The magnetic phase diagram is discussed in terms of the competition among the lattice shrinkage and the non-magnetic substitution aroused by Al3+ ions doping and the different degree of freedoms of lattice, spin and orbit in Mn(V1−xAlx)2O4. We also study the resistivity of these samples and find that the doping of Al ions does not change the electric transport mechanism, but increases the difficulty of electron hopping as shown in the obvious increasing of the activation energy.

Structural, magnetic, multiferroic and electronic properties of Sr2ZrMnO6 double perovskite

We report structural analysis, magnetic ordering, ferroelectric behavior and electronic structure of Sr2ZrMnO6 complex perovskite. Samples were produced by the solid state reaction recipe. Crystallographic analysis was performed by Rietveld refinement of experimental X-ray diffraction patterns. Results show that this material crystallizes in a cubic perovskite structure which corresponds to the space group Fm-3m. From measurements of susceptibility as a function of temperature, we determine the occurrence of magnetic ordering for a critical temperature of 53.2K. From the fitting with the Molecular Field Theory of Ferrimagnetism we establish that the ordering corresponds to a paramagnetic–ferrimagnetic transition. We obtained an effective magnetic moment of 3.3μB. A ferromagnetic–hysteretic behavior was observed in results of magnetization as a function of applied field at T=4.0K. The curve of polarization as a function of applied voltage evidences a ferroelectric character. From the saturation polarization values we determined the dielectric permittivity to be 330. Calculations of density of states and band structure for this manganite-like material were carried out by means of the Density Functional Theory implemented in the Quantum EXPRESSO code, for both spin up and down orientations. Results of total and partial density of states reveal the semiconductor character of this material with effective magnetic moment of 2.84μB.

Structural, dielectric, magnetic, and nuclear magnetic resonance studies of multiferroic Y-type hexaferrites

The effect of strontium substitution on structural, magnetic, and dielectric properties of a multiferroic Y-type hexaferrite (chemical formula Ba2−xSrxMg2Fe12O22 with 0 ≤ x ≤ 2) was investigated. Y-type hexaferrite phase formation was not affected by strontium substitution for barium, in the range 0 ≤ x ≤ 1.5, confirmed by x-ray diffraction and Raman spectroscopy measured at room temperature. Two intermediate magnetic spin phase transitions (at tempertures TI and TII) and a ferrimagnetic-paramagnetic transition (at Curie temperature TC) were identified from the temperature dependence of the magnetic susceptibility. Magnetic transition temperatures (TI, TII, and TC) increased with increasing strontium content. Magnetic hysteresis measurements indicated that by increasing strontium concentration, the coercivity increases, while the saturation magnetization decreases. The 57Fe NMR spectrum of the Y-type hexaferrite measured at 5 K and in zero magnetic field showed remarkable differences compared to that of other hexaferrites due to their different number of tetrahedral and octahedral iron sites. The temperature and frequency dependence of the dielectric permittivity evidenced broad peaks with frequency dispersion in correspondence of the Curie temperature.

Local spin reversal and associated magnetic responses in Ga-substituted Pb-hexaferrites

We have studied magnetic structure and properties of Ga-substituted Pb-hexaferrites having the stoichiometry of PbFe12−xGaxO19 with x=6 (i.e., Fe:Ga=1:1). According to the neutron diffraction results, this compound is characterized by a collinear spin structure below its Curie temperature (∼325K). Analysis of the neutron diffraction patterns further indicates that the magnetic-moment direction of Fe3+ ions located at the octahedral 2a sublattice is downward while that of the unsubstituted PbFe12O19 is upward at room temperature. With decreasing temperature, the Fe3+ magnetic moment at the octahedral 2a sublattice undergoes a reorientation to the upward direction while that of the unsubstituted PbFe12O19 remains upward down to 5K. This selective local spin reversal at the 2a sublattice of PbFe6Ga6O19 was attributed to the weakening of the superexchange interaction between the octahedral 2a site and the tetrahedral 4fIV site upon the preferential substitution of Ga ions for Fe ions at these two neighboring sites. Comparison of the neutron diffraction results with dc magnetization responses and ac susceptibilities further indicates that the paramagnetic–ferrimagnetic transition at ∼325K (Tc) is followed by the local spin reversal at lower temperatures.

Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter 〈ϕ〉 = 15 nm have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ′) and out of phase (χ″) ac-susceptibility. The studies were performed in non-interacting and interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 and 17.6%. The ac-susceptibility of the non-interacting system, measured with HDC = 0 Oe, presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)–ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system shows two anomalies. One frequency independent peak associated to TC and a second low temperature maximum was observed in χ″, located at TP. The position of the TP maximum shifts to higher temperature when the frequency increases. The relation between the relaxation time and TP was well described by the Vogel–Fulcher law. When the susceptibility was measured with HDC = 20 kOe, the PM–FiM transition was observed in both systems. Remarkably, in the non-interacting system, the low temperature anomaly is evidenced by the magnetic field. This anomaly is present as a well-defined maximum, which shifts to higher temperature when the frequency increases.

The observation of a positive magnetoresistance and close correlation among lattice, spin, and charge around TC in antipervoskite SnCMn3

The temperature dependences of magnetization, electrical transport, and thermal transport properties of antiperovskite compound SnCMn3 have been investigated systematically. A positive magnetoresistance (∼11%) is observed around the ferrimagnetic-paramagnetic transition (TC∼280 K) in the field of 50 kOe, which can be attributed to the field-induced magnetic phase transition. The abnormalities of resistivity, Seebeck coefficient, normal Hall effect, and thermal conductivity near TC are suggested to be associated with an abrupt reconstruction of electronic structure. Further, our results indicate an essential interaction among lattice, spin, and charge degrees of freedom around TC. Such an interaction among various degrees of freedom associated with sudden phase transition is suggested to be characteristic of Mn-based antiperovskite compounds.