Spin-glass Transition Temperature Research Articles

Investigation of experimental signatures of spin glass transition temperature

Investigation of experimental signatures of spin glass transition temperature

Exploring the critical behavior of the anomalous spin-glass transition in Ga1−xMnxS

Single-crystalline Ga1−xMnxS is a quasi-two-dimensional system that exhibits an anomalous spin-glass transition temperature compared with the other well-known spin-glass systems. In contrast to the other known spin-glasses that all have three-dimensional structures, our host chalcogenide GaS system is quasi-two-dimensional. Recent interest in utilizing spin-glass materials for applications in short-term, low-energy memory and processing power make this new 2-D system important for further exploration. We report on the critical behavior of the anomalous spin-glass transition in a single-crystalline Ga0.91Mn0.09S system. Using the scaling equation of state describing the spin-glass transition in Ga1−xMnxS, we obtained the relation χnl = C1H2/δ and extract the value δ = 5.5 ± 0.5 for this critical exponent as well as a value of ϕ = 4.8 for another critical exponent. We find this value of delta for the critical temperature Tc = 11.2 K, combined with the other critical exponents γ = 4.0 and β = 0.8 form a self-consistent description of the spin-glass transition in this unusual 2-D spin-glass system. Interestingly, these results represent convincing evidence that, despite Ga1−xMnxS having a quasi-two-dimensional structure, Ga1−xMnxS undergoes a true spin-glass transition and is related to the class of semiconducting spin-glass materials with short-range interactions. The spin-glass transition in Ga1−xMnxS is characterized by critical exponents similar to the three-dimensional spin-glass systems.

Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl2−x Ga x O4

We report the properties of an A-site spinel magnet, CoAl2−x Ga x O4, and analyze its anomalous, low-temperature magnetic behavior, which is derived from inherent, magnetically frustrated interactions. Rietveld analysis of the x-ray diffraction profile for CoAl2−x Ga x O4 revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameter η could be controlled by varying the gallium (Ga) composition in the range 0.055 ⩽ η ⩽ 0.664. The composition-induced Néel-to-spin-glass (NSG) transition occurred between 0.05 ⩽ η ⩽ 0.08 and was verified by measurements of DC-AC susceptibilities χ and thermoremanent magnetization (TRM) below the Néel transition temperature T N. The relaxation rate and derivative with respect to temperature of TRM increased at both T N and the spin glass (SG) transition temperature T SG. The TRM decayed rapidly above and below these transitions. TRM was highly sensitive to macroscopic magnetic transitions that occurred in both the Néel and SG phases of CoAl2−x Ga x O4. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate at T max < T N. With increasing inversion η(x), the anomaly at T max merged with that of the Néel transition at a tricritical point (η tc, T tc) = (0.08, 4.0 K), where the paramagnetic, Néel, and SG states met. We successfully extracted the relaxation time τ and other characteristic parameters from the TRM isothermal temporal evolution based on the Weron function derived for a purely stochastic process. To distinguish the magnetic states, we compared our results with previously studied inversion-free A-site spinel, CoRh2O4, and CoGa2O4 cluster glass. We generated an inversion-temperature phase diagram based on the comprehensive measurements of DC and AC susceptibilities, TRM, and specific heat in the range 0.055 ⩽ η ⩽ 0.664 for CoAl2−x Ga x O4. Based on this phase diagram, we speculate that a NSG quantum critical phase transition occurred at η = 0.050(6). Our findings are consistent with suppression of the long-range order antiferromagnetic state in CoAl2O4 revealed through neutron diffraction studies, even at T << T N.

Cluster spin glass in off-stoichiometric Ni2.01Mn1.58Sn0.41 alloy

The Ni2.01Mn1.58Sn0.41 alloy is a member of the Heusler material family which undergoes diffusionless structural transition from cubic austenite to orthorhombic martensite during its cooling. The transition was observed at temperature 398 K from paramagnetic austenite to martensite with complex magnetic structure. The measurement of AC magnetic susceptibility of the alloy confirmed spin-glass state at temperatures below 155 K and from the frequency dependence of the cups in AC susceptibility, a presence of clusters (11 nm3 in size) with strong interaction is deduced. The alloy shows anomalous Hall effect (AHE) of the order of 0.051 μΩ cm at 2 K in the spin-glass state of the alloy and its sign changes at around 140 K. The ordinary Hall coefficient of the metallic alloy is strikingly positive in the whole studied temperature region, and it points to complex magnetic and electronic structure of the alloy. The electrical resistance of the sample shows a flat maximum of 220 μΩ cm at spin-glass transition temperature. The magnetoresistance is of the order of 1% and is negative in the whole temperature region which points to a standard scattering of charge carriers on magnetic fluctuations. Quantum-mechanical calculations of supercells with compositions Ni32Mn25Sn7, Ni32Mn26Sn6 and Ni32Mn24Sn8 simulated different arrangement of Mn and Sn atoms. Ni32Mn25Sn7 composition is thermodynamically the most stable, although its magnetic moment is 1.67 μB/f.u., twice higher than the measured value. The combination of the latter two states, with additional Mn–Sn swaps, provides the magnetic moments matching the experimental value at a very small energy cost. Theoretical results then indicate that a scenario of local chemical variations and clustering is likely in the studied alloy.

Mg-Doped CoCr2O4 Nanoparticles: Implications for Magnetic Memory and Magnetocaloric Effect

Using the co-precipitate method, Mg-doped CoCr2O4 nanoparticles (NPs) with a particle size ranging from 11 to 22 nm were synthesized, and their structural, surface, vibrational, and magnetic properties were examined through experimental and theoretical approaches. The inherent magnetic inhomogeneity of the NPs was found to be the cause of the observed exchange bias effect. The magnetic memory effect was studied below the spin glass-transition temperature using time- and temperature-dependent magnetization measurements, and the magnetocaloric effect was investigated across the Curie temperature. These NPs show promising potential for magnetic memory and magnetocaloric effect fields.

3d and 5d electronic structures and orbital hybridization in Ba- and Ca-doped La2CoIrO6 double perovskites

Here we present a detailed investigation of the Co and Ir local electronic structures in ${\mathrm{La}}_{1.5}{A}_{0.5}{\mathrm{CoIrO}}_{6}$ ($A=\mathrm{Ba}$, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir ${L}_{2,3}$-edge and Co $K$-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for $A=\mathrm{Ca}$ below $\ensuremath{\sim}96\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the competing magnetic phases, and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the $A=\mathrm{Ba}$, on the other hand, there is no evidence of long-range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds are discussed in terms of the structural distortion that alters the strength of the Co-Ir couplings, with a relevant role played by the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}--\mathrm{Ir} 5d\phantom{\rule{4pt}{0ex}}{j}_{\mathrm{eff}}=1/2$ hybridization.

Antisite disorder mediated exchange bias effect and spin-glass state in La[formula omitted]Sm[formula omitted]NiMnO[formula omitted

We report the synthesis of La2−xSmxNiMnO6 (x = 0, 0.1, 0.2) polycrystalline compounds through the sol–gel process using all-aqueous precursors. The influence of Sm-substitution on magnetic properties of La2−xSmxNiMnO6 (LSNMO) compounds are investigated through X-ray diffraction (XRD), dc-magnetization (Mdc), ac-magnetic susceptibility measurements, and Raman spectroscopy. XRD analysis confirms that the phase stability of LSNMO is improved significantly upon Sm-substitution as evident from a structural phase transformation of the compound from a biphasic (monoclinic + rhombohedral) into an aphasic (monoclinic) structure. The magnetic measurements indicate a formation of a metastable magnetic state with random ferromagnetic (FM) and antiferromagnetic (AFM) interactions below 100 K. The metastable spin configuration exhibits critical slow-down dynamics and memory effect that signifies a spin-glass (SG) state. The influence of Sm-doping reduces the Curie temperature (Tc) from 284 K → 245 K and enhances saturation magnetization (Ms). These findings are attributed to the combined effect of a reduced degree of antisite disorder upon the addition of Sm. A profound variation (45.5 → 72.9 K) in SG transition temperature (TSG) is also reported. Magnetization loops M(H) confirm the presence of an exchange bias (EB) effect that value suppressed prominently from 120 → 28 Oe upon Sm-substitution. The temperature-dependent magnetization M(T) and frequency-dependent ac-susceptibility χ[ω, T] are used to study the antisite-driven SG phase, spin relaxation dynamics, magnetic memory, and rejuvenation effect. Raman spectroscopy is employed to probe the degree of antisites disorder that reduces after Sm-addition. The phase stability, lower Tc, tunable Ms, and EB in the single-phase compound make LSNMO a potential candidate for its applications in realizing energy-efficient devices.

Cationic redistribution induced spin-glass and cluster-glass states in spinel ferrite

The effect of the cationic redistribution on the complex spinel structure and magnetic properties were investigated in ${\mathrm{Zn}}_{0.7}{\mathrm{Cu}}_{0.3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ ferrite. X-ray photoelectron spectroscopy and x-ray diffraction studies revealed that the system exhibits a mixed spinel structure with ${\mathrm{Fe}}^{3+}, {\mathrm{Zn}}^{2+}$, and ${\mathrm{Cu}}^{2+}$ occupying both tetrahedral and octahedral sublattices. The DC magnetization results revealed the absence of long-range magnetic order in the system. Furthermore, the AC susceptibility data analysis using dynamic scaling laws suggests that the system exhibits magnetic relaxation below two different temperatures: (i) a spin-glass--like transition at low temperature ($\ensuremath{\sim}49.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) with critical exponent 10.3 and spin-flip time $\ensuremath{\sim}{10}^{\ensuremath{-}11}\phantom{\rule{0.28em}{0ex}}\mathrm{s}$, and (ii) a cluster-glass--like transition at higher temperature ($\ensuremath{\sim}317\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) with critical exponent 4.6 and spin-flip time $\ensuremath{\sim}{10}^{\ensuremath{-}10}\phantom{\rule{0.28em}{0ex}}\mathrm{s}$. The existence of glassy behavior and magnetic memory effects below the spin-glass transition temperature proves that the system is in nonequilibrium dynamical state. The coexistence of spin-glass and cluster-glass along with the thermal hysteresis between these two transitions could widen the technological applications of these systems.

Slow spin dynamics in a CoM2O4 A-site spinel (M=Al, Ga, and Rh)

We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl2O4, CoRh2O4, and cluster glass CoGa2O4 to comprehensively show the proximity effects of the Néel-to-spin-spiral (NSS) transition, which is predicted theoretically to occur in a magnetically frustrated A-site spinel antiferromagnet. The low-temperature magnetic phase remains controversial since the magnetic state of CoAl2O4 is in the vicinity of the NSS transition derived by the inherent magnetic frustrated interaction for an A-site spinel magnet and is also considerably sensitive to crystallographic disorder. The antiferromagnetic and spin-glass transitions were detected by measuring the direct-current and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Néel point (T N) and spin-glass transition temperature (T SG). The relaxation rate and the temperature derivative of TRM were both enhanced at T N and T SG, and they decayed rapidly above and below the transitions. We succeeded in extracting the relaxation time τ and other characteristic parameters from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form formulated by Weron based on purely stochastic theory in order to describe the originally dielectric relaxation. For the typical CoRh2O4 normal spinel (inversion-free A site) antiferromagnet and the CoGa2O4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh2O4 and CoGa2O4, an enhancement of the relaxation rate of TRM for CoAl2O4 is indicated at low temperatures, which is probably related to the suppression of long-range antiferromagnetic order revealed by neutron diffraction studies.

Enhancement of the spin-glass transition temperature through pd -orbital hybridization in Zn1−xMnxTe

To gain insight into the spin-glass state of diluted magnetic semiconductors, we have examined the magnetic and electronic properties of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ using density-functional theory as well as performed magnetization measurements on the $x=0.43$ and 0.55 systems to demonstrate a clear spin-glass transition consistent with previous literature. Using a generalized gradient approximation, we investigate the electronic and magnetic properties for $x=0$, 0.075, 0.15, 0.25, and 0.50 doping levels using the magnetic moment of ${\mathrm{Mn}}^{2+}$ as guide for the dependence of the Hubbard onsite potential on the electronic structure. Simulations on both ferromagnetic (FM) and antiferromagnetic (AFM) configurations yield a distinct AFM ground-state preference, which is consistent with a zero-magnetic-moment spin-glass state. Here an onsite potential of up to 8 eV on the Mn $3d$ orbitals is needed to harden the magnetic moment toward $S=5/2$. From our analysis of the electronic structure evolution with doping and onsite potential, we confirm the semiconducting state of the Mn-doped ZnTe as well as show that the presence of Mn incorporated into the ZnTe matrix at the Zn lattice site produces magnetic interactions through the Te ions with a distinct Te-Mn $pd$-orbital hybridization. Furthermore, we show that this hybridization is activated with the Mn doping above 0.25 concentration, which corresponds to the doping level in which the spin-glass transition begins to rise. Therefore, it is likely that the coupling of $pd$-orbital hybridization of the Mn and Te $p$ orbitals is a precursor to the enhancement of the spin-glass transition temperature.

Nontrivial maturation metastate-average state in a one-dimensional long-range Ising spin glass: Above and below the upper critical range.

Understanding the low-temperature pure state structure of spin glasses remains an open problem in the field of statistical mechanics of disordered systems. Here we study Monte Carlo dynamics, performing simulations of the growth of correlations following a quench from infinite temperature to a temperature well below the spin-glass transition temperature T_{c} for a one-dimensional Ising spin-glass model with diluted long-range interactions. In this model, the probability P_{ij} that an edge {i,j} has nonvanishing interaction falls as a power law with chord distance, P_{ij}∝1/R_{ij}^{2σ}, and we study a range of values of σ with 1/2<σ<1. We consider a correlation function C_{4}(r,t). A dynamic correlation length that shows power-law growth with time ξ(t)∝t^{1/z} can be identified in the data and, for large time t,C_{4}(r,t) decays as a power law r^{-α_{d}} with distance r when r≪ξ(t). The calculation can be interpreted in terms of the maturation metastate averaged Gibbs state, or MMAS, and the decay exponent α_{d} differentiates between a trivial MMAS (α_{d}=0), as expected in the droplet picture of spin glasses, and a nontrivial MMAS (α_{d}≠0), as in the replica-symmetry-breaking (RSB) or chaotic pairs pictures. We find nonzero α_{d} even in the regime σ>2/3 which corresponds to short-range systems below six dimensions. For σ<2/3, the decay exponent α_{d} follows the RSB prediction for the decay exponent α_{s}=3-4σ of the static metastate, consistent with a conjectured statics-dynamics relation, while it approaches α_{d}=1-σ in the regime 2/3<σ<1; however, it deviates from both lines in the vicinity of σ=2/3.

Magnetic ground state and crystal structure of the giant dielectric constant material Ba(Fe1/2Nb1/2)O3

The room temperature crystal structure as well as the magnetic ground state of Ba(Fe1/2Nb1/2)O3 (BFN) are controversial. The results of Rietveld refinements using high-resolution synchrotron x-ray powder diffraction (SXRPD) data are presented to show that BFN belongs to cubic perovskite structure in the Pm3¯m space group and not in the cubic Fm3¯m or monoclinic symmetries, proposed by some earlier workers based on the analysis of the laboratory source XRD data. The temperature dependent specific heat measurements in the 1.8 to 300 K range on well characterized BFN samples are presented to show the absence of any long-range ordered (LRO) antiferromagnetic (AFM) transition, proposed by some earlier workers based on the observation of a peak in the temperature dependence of zero-field cooled (ZFC) DC magnetization around 25 to 32 K. Results of temperature dependent magnetization measurements during warming cycle on ZFC and field cooled (FC) BFN samples reveal history dependent bifurcation with a peak in the ZFC M (T) curve at Tf ~ 29 K, in agreement with some previous results where it was attributed to a spin-glass transition even though such an irreversibility can result from superparamagnetic (SPM) blocking as well. The appearance of a short plateau in our FC M (T) curve below Tf suggests a cluster spin-glass phase of BFN below Tf. This was confirmed by us through the ac susceptibility χ' (T, ω) studies which reveal divergence of spin dynamics at the spin-glass transition temperature TSG ~ (28 ± 1) K with a large characteristic spin relaxation time τ0 ~ 10−6 s following Vogel-Fulcher as well as power-law type critical dynamics. The existence of the magnetic glassy state in BFN is further supported by the observation of (1) magnetic field dependent shift of Tf with increasing magnetic field along the de Almeida-Thouless line in the T-H plane with a characteristic exponent of m = 2/3, (2) weak remanent magnetization Mr below Tf with exponentially decreasing Mr on approaching Tf, (3) extremely slow decay of the thermoremanent magnetization below Tf with time as per a stretched exponential function, and (4) characteristic aging, rejuvenation and memory effects below Tf. These results provide the first unambiguous confirmation of the cluster spin-glass state of BFN and settle the existing controversies related to its magnetic ground state.

Spin-glass dynamics in the presence of a magnetic field: exploration of microscopic properties

The synergy between experiment, theory, and simulations enables a microscopic analysis of spin-glass dynamics in a magnetic field in the vicinity of and below the spin-glass transition temperature T g. The spin-glass correlation length, ξ(t, t w; T), is analysed both in experiments and in simulations in terms of the waiting time t w after the spin glass has been cooled down to a stabilised measuring temperature T < T g and of the time t after the magnetic field is changed. This correlation length is extracted experimentally for a CuMn 6 at. % single crystal, as well as for simulations on the Janus II special-purpose supercomputer, the latter with time and length scales comparable to experiment. The non-linear magnetic susceptibility is reported from experiment and simulations, using ξ(t, t w; T) as the scaling variable. Previous experiments are reanalysed, and disagreements about the nature of the Zeeman energy are resolved. The growth of the spin-glass magnetisation in zero-field magnetisation experiments, M ZFC(t, t w; T), is measured from simulations, verifying the scaling relationships in the dynamical or non-equilibrium regime. Our preliminary search for the de Almeida–Thouless line in D = 3 is discussed.

Aggregation vs Surface Segregation: Antagonism over the Magnetic Behavior of NiCr Nanoparticles.

Annealing is a valuable method for fine-tuning the ultrasmall magnetic properties of alloy nanoparticles (NPs) by controlling their sizes, modifying their surfaces, and affecting their magnetic interactions. Herein, we study the effect of moderate annealing (450 °C) on strongly interacting NiCr nanoparticle assemblies (0 ≤ atom % Cr ≤ 15) immediately after deposition. Concurrent temperature-dependent electron microscopy and magnetization data demonstrate the interplay of two competing factors, namely, enhanced particle aggregation and element-specific surface segregation, on the magnetic properties, with the former boosting and the latter suppressing them. Strong interparticle interactions can lead to a magnetic response different from that of superparamagnetic particles, namely, from canonical spin-glass (0 atom % Cr) to correlated spin-glass (5–15 atom % Cr) behavior below higher spin-glass transition temperatures Tg (20–350 K). The observation of “high-field susceptibility” below cryogenic temperatures (≤20 K) is ascribed to the presence of inhomogeneity/defects caused by Cr segregation. This work emphasizes the necessity of taking into account the delicate balance of such competing factors to understand the magnetic properties of nanoparticulate samples.

On the magnetic ground state of lead-free complex perovskite Sr(Fe1/2Nb1/2)O3

We present here the results of dc magnetization (M) measurements as a function of temperature (T), field (H) and time (t) with different histories and ac susceptibility measurements as a function of temperature and frequency to understand the magnetic ground state of Sr(Fe1/2Nb1/2)O3 (SFN). We show that the zero-field cooled (ZFC) and field cooled (FC) magnetization curves (M(T)) not only bifurcate but also exhibit a short plateau with a dip in the FC M(T) curve suggesting that the observed history dependent irreversibility may be due to cluster spin-glass (CSG) freezing and not superparamagnetic (SPM) blocking. This is confirmed by ac susceptibility χ(ω, T) studies which reveal critical slowing down of the spin dynamics as per Vogel Fulcher and power laws with spin-glass transition temperature TSG ∼(24 ± 1) K. The attempt time obtained by these laws (τ0 ∼10−6 s) is found to be consistent with CSG picture. Further, the field dependent shift of Tf follows de Almeida-Thouless line in the T-H plane with a characteristic critical exponent m = 2/3. Further support for the existence of the spin-glass phase is obtained by the observation of non-exponential decay of the thermoremanent magnetization as well as aging, rejuvenation and memory effects below Tf. The non-zero value of remanent magnetization Mr without saturation below Tf is shown to vanish exponentially with increasing temperature as T → TSG in agreement with what is observed in spin-glass systems. We also present a critical comparison of the magnetic ground states of SFN with CaCu3Ti4O12 (CCTO) and other complex niobate perovskites in terms of the strength of the nearest neighbour exchange interaction parameter J1, the coordination polyhedral, the disorder/dilution level of the magnetic sublattices, the presence or absence of octahedral tilts and the possibility of alternative superexchange pathways in these compounds.

Non-equilibrium magnetic response of canonical spin glass and magnetic glass

Time and history dependent magnetization has been observed in a wide variety of materials, which are collectively termed as the glassy magnetic systems. However, such systems showing similar non-equilibrium magnetic response can be microscopically very different and they can be distinguished by carefully looking into the details of the observed metastable magnetic behavior. Canonical spin glass (SG) is the most well studied member of this class and has been extensively investigated both experimentally and theoretically over the last five decades. In canonical SGs, the low temperature magnetic state obtained by cooling across the SG transition temperature in presence of an applied magnetic field is known as the field cooled (FC) state. This FC state in canonical SG is widely believed as an equilibrium state arising out of a thermodynamic second order phase transition. Here, we show that the FC state in canonical SG is not really an equilibrium state of the system. We report careful dc magnetization and ac susceptibility measurements on two canonical SG systems, AuMn (1.8%) and AgMn (1.1%). The dc magnetization in the FC state shows clear temperature dependence. In addition, the magnetization shows a distinct thermal hysteresis in the temperature regime below the SG transition temperature. On the other hand, the temperature dependence of ac susceptibility has clear frequency dispersion below SG transition in the FC state prepared by cooling the sample in the presence of a dc-bias field. We further distinguish the metastable response of the FC state of canonical SG from the metastable response of the FC state in an entirely different class of glassy magnetic system namely magnetic glass, where the non-equilibrium behavior is associated with the kinetic-arrest of a first order magnetic phase transition.

Weak ferromagnetism and exchange bias effect in Co1+yAl2-yO4/SiO2 nanocomposites

Nanocomposites made of magnetic spinel oxide Co1+yAl2-yO4 nanoparticles (NPs) with y = 0.93, 1.20 and 1.34 dispersed in an amorphous SiO2 matrix were prepared by a sol-gel method. The average crystallite size DXRD of about 25 nm determined by X-ray diffraction (XRD) agrees well with the average particle size observed in transmission electron microscopy (TEM). Dynamic magnetic susceptibility measurements indicate a spin-glass-like behavior of the nanoparticles below the spin-glass (SG) transition temperature TC < 10 K. Field dependent magnetization exhibits weak ferromagnetism and shift of the magnetic hysteresis loop. On the basis of the observed magnetic properties, we propose a model of the core-shell structure of the Co1+yAl2-yO4 nanoparticles with the spin-glass phase in the core and weak ferromagnetism in the shell. The exchange bias field disappears above the spin-glass transition temperature TC, whereas the weak ferromagnetism persists also at temperatures above TC. The component of weak ferromagnetism is associated with the presence of Co vacancies in the nanoparticle spinel crystal lattice.

Study of magnetic relaxation, memory and rejuvenation effects in the cluster spin-glass phase of B-site disordered Ca(Fe1/2Nb1/2)O3 perovskite: Experimental evidence for hierarchical model

We explore the characteristics of the spin-glass phase of the lead-free complex perovskite Ca(Fe1/2Nb1/2)O3 (CFN) below the spin-glass transition temperature TSG ~24 K. The temperature dependence of the coercive field (Hc) and remanent magnetization (Mr) follow exponential behaviour as expected for the glassy phase. Detailed magnetic relaxation measurements as a function of time (t) with temperature (T) cycling on zero-field cooled (ZFC) and field cooled (FC) samples reveal that the cluster spin-glass phase of CFN exhibits a very slow non-exponential decay of thermoremanent magnetization with time as well as memory and rejuvenation effects. Our results demonstrate that the observed memory and rejuvenation effects in the cluster spin-glass phase of CFN can be explained by hierarchical model only and not by the droplet model.

Probing the Griffiths like phase, unconventional dual glassy states, giant exchange bias effects and its correlation with its electronic structure in Pr2−xSrxCoMnO6

Crystal, electronic structure, dc and ac magnetization properties of the hole substituted (Sr2+) and partially B-site disordered double perovskite Pr2−xSrxCoMnO6 system have been investigated. The XRD pattern analysis showed a systematic decrease in the lattice parameters owing to the enhanced oxidation states of the Co/Mn ions. The electronic structure study by XPS measurements suggested the presence of mixed valence states of the B-site ions (Co2+ /Co3+ and Mn3+ /Mn4+) with significant enhancement of the average oxidation states due to hole doping. The mere absence of electronic states near the Fermi level in the valence band (VB) spectra for both pure (x = 0.0) and Sr doped (x = 0.5) systems indicated the insulating nature of the samples. Sr substitution is observed to increase the spectral weight near the Fermi level suggesting for an enhanced conductivity of the hole doped system. The dc magnetization data divulged a Griffiths like phase above the long-range ordering temperature. A typical re-entrant spin glass like phase driven by the inherent anti-site disorder (ASD) has been recognized by ac susceptibility study for both the pure and doped systems. Most interestingly, the emergence of a new cluster glass like phase (immediately below the magnetic ordering temperature and above the spin-glass transition temperature) solely driven by the Sr substitution has been unravelled by ac magnetization dynamics study. Observation of these dual glassy states in a single system is scarce and hence placed the present system amongst the rare materials. The isothermal magnetization measurements further probed the exhibition of the giant exchange bias effect originated from the interfacial exchange interactions due to existence of low temperature antiferromagnetic clusters embedded in the glassy matrix.

Effect of Mn-doping on the low temperature magnetic phase transitions of BiFeO3

In the present work, we have carried out a comparative study of the low temperature phase transitions using dc magnetization (M (T)) and ac susceptibility (χ (ω, T)) measurements on undoped and 0.3 wt% MnO2 doped BiFeO3 samples in the 2–300 K temperature range. It is shown that MnO2 doping increases the resistivity and decreases the dielectric loss as a result of reduced oxygen vacancy concentration as confirmed by iodometry and x-ray photoelectron spectroscopy (XPS) studies. A comparative study of the dc M (T) and ac χ (ω, T) results on two types of samples reveal that the transitions around 25 K, 110 to 150 K and 260 K are intrinsic to BiFeO3. The widely reported transition at 50 K is argued to be defect induced, as it is absent in the doped samples. We also show that the spin-glass transition temperature (TSG) ∼20 K, determined from an analysis of χ (ω, T), is less than the spin-glass freezing temperature (Tf) ∼25 K in marked contrast to TSG > Tf reported by earlier workers.