Thermoremanent Magnetization Research Articles

RESET: A Method to Monitor Thermoremanent Alteration in Thellier‐Series Paleointensity Experiments

AbstractAccurately estimating the paleomagnetic field intensity recorded in terrestrial and planetary materials is the key to understanding dynamo processes. Thellier‐series stepwise‐heating methods with partial thermoremanent magnetization (pTRM) checks for alteration are considered the most reliable technique even though pTRM checks are shown to be incapable of detecting the entirety of thermal alteration. We utilize a recently developed multidomain‐correction experiment (or RESET) to monitor thermally induced magnetic property alteration that may happen to the very specimen used in Thellier‐series experiments. We also use rock magnetic property changes to track the physicochemical alteration of fresh companion specimens of Galapagos lavas that were previously used for paleointensity determinations. Our results show that laboratory heating induced thermal alteration of a typical Galapagos lava sample starts to occur at around 500°C. It escaped detection by pTRM checks but was caught by our RESET method.

The impact of solvent tanδ on the magnetic characteristics of nanostructured NiZn-ferrite film deposited by microwave-assisted solvothermal technique

Nanostructured Ni-Zn ferrite (NZF) thin films, deposited by a CMOS-compatible microwave-assisted solvothermal deposition (MAS-D) process, exhibit ‘far-from-equilibrium’ distribution of cations in the spinel structure, and thus, exciting magnetic properties. Solvents with different dielectric loss-tangents, such as ethanol (tanδ≈0.94), 1-decanol (tanδ≈0.1), and benzyl alcohol (tanδ≈0.67), were mixed in five judiciously-chosen proportions – ED35 (ethanol:1-decanol=3:5; tanδ≈0.51), ED33 (tanδ≈0.63), EB25 (tanδ≈0.66), EB35 (tanδ≈0.70), and EB33 (tanδ≈0.74) – to study the impact of effective tanδ on cation occupancy and the nature of magnetization in the resulting films. The maximum temperature attained during microwave irradiation (Tmax) and the heating rate (∆T/trise) increase as the effective tanδ of the solvent mixture increases, which in effect leads the Ni atoms to migrate towards equilibrium lattice sites. Unlike bulk NZF, where all Ni atoms occupy octahedral sites (B-sites), the films reported here exhibit just 28% of Ni atoms in B-sites at best when deposited from the precursor solution EB25. At room temperature all films are superparamagnetic, while the maximum moment (MS=100 emu/cc) is observed in the film with the highest % of Ni atoms in B-sites. Thermoremanent magnetization (TRM) of the samples is studied and compared. Very high effective anisotropy constants (Keff=140 kJ/m3), and two-orders-of magnitude-higher inter-particle dipole moment (Edipole≈8×10-20 J @300 K) are observed in samples prepared from benzyl alcohol rather than 1-decanol – signifying the potential for tailoring magnetic properties by the choice of solvents in the MAS-D process.

Exchange bias in BiFeO3 and Bi0.9La0.1FeO3 nanoparticles

The exchange bias (EB) effects of BiFeO3 (BFO) and Bi0.9La0.1FeO3 nanopowders are investigated. An EB field of 152 Oe and 902 Oe at 60 K are observed for BFO and Bi0.9La0.1FeO3, respectively, when cooled in 20 kOe magnetic field. The enhancement of EB values in the case of Bi0.9La0.1FeO3 is explained based on the combination of Malozemoff’s and domain state models. Training effect measurements data fitted with Binek’s model suggested that the origin of EB lies in the interaction between the antiferromagnetic (AFM) core and the soft magnetic shell. Memory effect measurements, thermoremanent and isoremanent magnetization studies, and time decay of thermoremanent magnetization studies are done to understand the nature of the shell of both the samples. Time decay of thermoremanent magnetization of BFO is fitted with a stretched exponential based on Kohlrausch–Williams–Watt model, the obtained shape parameter value lies in the range of Heisenberg type spin-glasses. From thermoremanent and isoremanent magnetization plots, a 2D-diluted AFM shell is observed for Bi0.9La0.1FeO3. The field cooling and the temperature dependence of EB are investigated and the results are explained based on the available models of EB.

Geomagnetic field intensity changes in the Central Mediterranean between 1500 BCE and 150 CE: Implications for the Levantine Iron Age Anomaly evolution

The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.

Synthesis and magnetic properties of solid solutions of a diluted magnetic semiconductor (Zn1-xFex)3As2

The magnetic properties of the new semimagnetic semiconductor (Zn1-xFex)3As2 with x < 0.04 have been investigated over a wide temperature range. Single crystals of ZFA were grown using the modified Bridgeman technique. The results of X-ray analyses and the magnetometry data show that the border of solid solutions in this system lay near x = 0.015. Further increasing of x leads to the growing of the second phase, which was identified as Fe2As. A notable feature of our magnetometry results is the spin-glass-type freezing of magnetic moments in the temperature range of 10 - 50 K. The value of the freezing temperature decreases when the magnetic field increases and in 2 kOe magnetic field the effect is no longer observable. It is likely that the observed freezing phenomena are due to the cluster nature of magnetic properties of ZFA. The thermoremanent magnetization measurements give evidence for two types of clusters in the magnetic system of ZFA.

Evidence of two dimensional dilute antiferromagnet mediated exchange bias field and vertical shift in LaFeO3-NiO nanocomposite

Low dimensional antiferromagnetic systems, such as nanoparticles, are known to exhibit weak ferromagnetic behavior due to the uncompensated surface spins. In this report, we present a detailed magnetic study of LaFeO3/NiO nanocomposite, synthesized by propylene glycol gel method. The X-ray diffraction (XRD) based analyses, in combination with transmission electron microscopy (TEM), confirms 50:50 ratio composite with ~35–40 nm size particles. Owing to the high Néel temperatures (TN) of LaFeO3 and NiO, significant exchange bias (EB) and vertical magnetization shift (VMS) have been observed in the nanocomposite at room temperature. These features are attributed to the exchange interaction at the interface between the core and shell of the nanoparticles that are homogeneously dispersed in the nanocomposite. In our studies, the EB field and the VMS attains a value as high as 2050 Oe & 1.8 × 10-1 emu/g at 5 K and 75 Oe & 1.7 × 10-2 emu/g at 300 K, respectively. The thermoremanent magnetization (TRM) and isothermoremanent magnetization (IRM) analyses are done to identify the two-dimensional (2D) diluted antiferromagnet (DAFF) behavior of the surface shell of nanoparticles. We propose a phenomenological antiferromagnetic-(2D-DAFF)-antiferromagnetic magnetic structure model, to explain all the rich features and the observed magnetic properties of this nanocomposite.

Spin-glass magnetism of the non-equiatomic CoCrFeMnNi high-entropy alloy

The CoCrFeMnNi high-entropy alloy (HEA) is a magnetically concentrated crystalline system with all lattice sites magnetic, containing randomness (five different types of spins are randomly positioned on the lattice) and frustration (a consequence of mixed ferromagnetic and antiferromagnetic interactions). The sample material was prepared as a non-equiatomic, fully random solid solution of the five magnetic elements and we have studied experimentally the nature of the magnetic state. Upon cooling, no long-range magnetic ordering takes place, but the spin system undergoes a kinetic freezing transition to a spin glass phase, where below the spin freezing temperature Tf≈ 20 K, ergodicity of the system is broken. The observed broken-ergodicity phenomena include zero-field-cooled – field-cooled magnetization splitting in low magnetic fields, a frequency-dependent cusp in the ac susceptibility, an ultraslow time-decay of the thermoremanent magnetization and the memory effect, where the state of the spin system reached upon isothermal aging at a certain temperature can be retrieved after a reverse temperature cycle. All these phenomena are associated with the out-of-equilibrium dynamics of a nonergodic, frustrated system of coupled spins that approach thermal equilibrium, but can never reach it on a finite experimental time scale, so that we are observing only transient effects of partial equilibration within localized spin domains.

New archeomagnetic secular variation data from Central Europe, II: Intensities

Variations of the geomagnetic field during the past millennia can be obtained by investigating archeological material. The spatial distribution of the available data across Europe is rather uneven and only about 15% of the sites provide a full vector record of the geomagnetic field. Here we report 39 new reliable paleointensity data for sites from Austria, Germany and Switzerland, with ages ranging between 1500 BCE and 1900 CE. For 31 of them, directional information is available, too. Paleointensities were determined with Thellier type techniques, with the multiple-specimen domain-state corrected paleointensity protocol or with both techniques. Corrections for anisotropy of thermoremanent magnetization were carried out for the structures, showing small variations of only a few per cent in most cases. Cooling rate dependence was tested for 16 structures. Often strong alteration during the experiments was observed. Seven successful cooling rate corrections were applied, which lowered the paleointensity values by about 10%. Values obtained from the multiple-specimen technique are generally somewhat lower than those from the Thellier experiments. Rock magnetic measurements revealed magnetite-type minerals mostly in pseudo single domain range as main magnetic carrier, but also high coercive mineral components like hematite or epsilon iron oxide are present. The new data are mostly in good agreement with published paleointensities. They support the presence of strong intensity variations around 800 CE and in the millennium BCE. The new full vector data reveal that the deep paleointensity minimum observed from German data around 850 BCE is accompanied by large variations in direction with declinations up to 50°.

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.

Experimental test of the cooling rate effect on blocking temperatures in stepwise thermal demagnetization

SUMMARYUpon cooling, most rocks acquire a thermoremanent magnetization (TRM); the cooling rate at which this happens not only affects palaeointensity estimates, but also their unblocking temperatures in stepwise thermal demagnetization experiments, which is important, for example, to estimate volcanic emplacement temperatures. Traditional single-domain (SD) theory of magnetic remanence relates relaxation times to blocking temperatures— the blocking temperature is the temperature at which the relaxation time becomes shorter than the experimental timescale—and therefore strictly only applies to remanence acquisition mechanisms at constant temperatures (i.e. viscous remanent magnetizations, VRMs). A theoretical framework to relate (constant) blocking temperatures to (time-varying) cooling rates exists, but this theory has very limited experimental verification—partly due to the difficulty of accurately knowing the cooling rates of geological materials. Here we present an experimental test of this ‘cooling rate effect on blocking temperatures’ through a series of demagnetization experiments of laboratory-induced TRMs with controlled cooling rates. The tested cooling rates span about 1 order of magnitude and are made possible through (1) extremely accurate demagnetization experiments using a low-temperature magnetic properties measurement system (MPMS) and (2) the use of a ‘1-step-only’ stepwise thermal demagnetization protocol where the relaxation process is measured over time. In this way the relaxation time corresponding to the blocking temperature is measured, which can be done to much higher accuracy than measuring the blocking temperature directly as done in traditional stepwise thermal demagnetization experiments. Our experiments confirm that the cooling rate relationship holds to high accuracy for ideal magnetic recorders, as shown for a synthetic weakly interacting SD magnetoferritin sample. A SD-dominated low-Ti titanomagnetite Tiva Canyon Tuff sample, however, showed that natural samples are unlikely to be sufficiently ‘ideal’ to meet the theoretical predictions to high accuracy—the experimental data agrees only approximately with the theoretical predictions, which may potentially affect blocking temperature estimates in stepwise thermal demagnetization experiments. Moreover, we find a strongly enhanced cooling rate effect on palaeointensities for even marginally non-ideal samples (up to 43 per cent increase in pTRM for a halving of the cooling rate).

Record-dynamics description of spin-glass thermoremanent magnetization: A numerical and analytical study.

Thermoremanent magnetization data for the three-dimensional Edwards-Anderson (EA) spin glass are generated using the waiting time method as a simulational tool and interpreted using record dynamics. We verify that clusters of contiguous spins are overturned by quakes, nonequilibrium events linked to record-sized energy fluctuations, and we show that quaking is a log-Poisson process, i.e., a Poisson process whose average depends on the logarithm of the system age, counted from the initial quench. Our findings compare favorably with experimental thermoremanent magnetization findings and with the spontaneous fluctuation dynamics of the EA model. The logarithmic growth of the size of overturned clusters is related to similar experimental results and to the growing length scale of the spin-spin spatial correlation function. The analysis buttresses the applicability of the waiting time method as a simulational tool, and of record dynamics as a coarse-graining method for aging dynamics.

Strong magnetic coupling between antiferromagnetic and ferromagnetic phases in polycrystalline hollow nanoparticles composed of spinel solid solution

Exchange bias is an interfacial phenomenon that is used to increase the magnetic anisotropy in order to surpass the superparamagnetic limit. Exchange bias has been effectively manipulated through variation of local arrangement of ferromagnetic and antiferromagnetic nanocrystals in an unusual morphology and for very specific compositions. For this purpose, polycrystalline hollow nanoparticles with spinel structure composed of Co3-δFeδO4 (with δ > 0) and Co1+γFe2-γO4 (γ > 0) crystallites, were prepared using = CoFe metal nanoparticles as precursors. These particles were oxidized to obtain the desired mixed-oxide nanoparticles with various compositions using the well-known Kirkendall effect. Spinel nanoparticles were composed of an antiferromagnetic phase, which was predominantly Co3-δFeδO4, and a ferromagnetic phase, Co1+γFe2-γO4. The antiferromagnetic phase showed a relative increase in its concentration with the increase in Co content. The magnetic response includes high field thermoremanent magnetization in antiferromagnetic phase, ferromagnetic crystal size, increased anisotropy due to antiferromagnetic and surface spin glass phases, etc. Antiferromagnetic phase has been demonstrated to be robust in comparison with the surface spin glass phase to generate exchange bias. Finally, a phase diagram of the magnetic phases as a function of Co concentration is constructed to identify ferromagnetic, antiferromagnetic and spin glass regimes in this system.

New high precision full-vector archaeomagnetic data from a roman kiln in Mérida (Spain)

This study presents new high precision age and full-vector archaeomagnetic data from a kiln excavated in the Roman archaeological site of Mitreo's house (Mérida, Badajoz, Spain). The age of the kiln was obtained by the stratigraphic method and by the ceramological study of the italic and sigillata pottery found with a very precise age date of 55 ± 15 years AD. Rock-magnetic experiments pointed towards magnetite as the main carrier of remanence and highly reversible thermomagnetic curves suggested that the studied samples were suitable for archaeoeointensity determinations.Paleomagnetic experiments including thermal and stepwise alternating field demagnetization yielded the following mean direction for the kiln: declination D = 0.6°; inclination I = 54.0°; (k = 481; α95 = 2.5°). Archaeointensityexperiments with the Thellier – Coe protocol on 28 samples yielded successful determinations in 27 cases. Anisotropy factors between 0.90 and 1.04 were obtained from anisotropy of thermoremanent magnetization (ATRM) experiments. A mean anisotropy-corrected archaeointensity value F = 56.3 ± 5.5 μT was obtained.The geomagnetic model SHA.DIF.14 k was used for an archaeomagnetic dating yielding a chronological interval between 40 BCE and 150 CE. This low age resolution when compared with the pottery-based age data is related to the behavior of the geomagnetic field in the Iberian Peninsula during the Roman period, which does not allow to differentiate well between results corresponding to those centuries. However, it is also related to the fact that the Iberian archaeomagnetic dataset in the analysed time range is highly scattered. A high quality full-vector data with a very precise age have been obtained which contribute to improve the Iberian secular variation curve and geomagnetic databases.

Single Phase Oxidation of Ferrimagnetic Grains as a Cause of L-Shaped Arai–Nagata Diagrams

Abstract—The experiments on the determination of paleointensity Banc of the Earth’s magnetic field by Thellier method on basalts of the Berd (Jurassic) and Paravakar (Cretaceous) collections sampled in the northeastern Armenia in 2006 revealed an anomalous behavior of the Arai–Nagata diagrams. The anomaly manifests itself by a sharp drop in the intensity of natural remanent magnetization (NRM) under heating of samples to about 400°C, which is accompanied by a very weak acquisition of partial thermoremanent magnetizations (pTRMs) in this temperature interval. The further increase in temperature leads to the opposite phenomenon—an unexpectedly sharp rise in pTRMs intensity with almost no decrease in NRM, which creates the L-shape of the Arai–Nagata diagram. A similar phenomenon of a steep decline during thermal demagnetization was also established for remanent saturation magnetization Mrs(T). We carried out a number of magnetomineralogical experiments from which we conclude that, consistent with the hypothesis suggested by (Kosterov and Prevot, 1998), the sharp drop in Mrs(T) and NRM(T) curves is caused by the transitions of the domain state from a metastable configuration to a more stable one. These transitions are initiated by the processes of single- and heterophase oxidations of primary titanomagnetites under the laboratory heating of samples to moderate temperatures.

Experimental determination of the critical spin-glass correlation length in single-crystal CuMn

Using high resolution superconducting quantum interference device magnetometry we have made detailed measurements of the waiting time effect of the thermoremanent magnetization (TRM) decays on a single-crystal $\mathrm{Cu}\mathrm{Mn}(6%)$ spin-glass sample near ${T}_{g}$. We have systematically mapped the rapid decrease of the characteristic timescale ${\mathrm{tw}}_{\mathrm{eff}}$, approaching ${T}_{g}$ from below, for waiting times ranging from 100 to 100 000 s. Using ${\mathrm{tw}}_{\mathrm{eff}}$ to determine the length scale of the growth of correlations during the waiting time, ${\ensuremath{\xi}}_{\mathrm{TRM}}$ (observed in both numerical studies and experiment), we observe both growth of ${\ensuremath{\xi}}_{\mathrm{TRM}}$ in the spin-glass phase and then a rapid reduction very close to ${T}_{g}$. We interpret this reduction in ${\ensuremath{\xi}}_{\mathrm{TRM}}$, for all waiting times, as being governed by the critical correlation length scale ${\ensuremath{\xi}}_{\mathrm{crit}}=a{(T\ensuremath{-}{T}_{c})}^{\ensuremath{-}\ensuremath{\nu}}$.

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.

Determination of the Formation Field of Artificial CRM and pTRM by the Thellier Method at Different Oxidation Stages of Natural Titanomagnetite

Artificial chemical remanent magnetization (CRM) created on a basalt rock from the Red Sea rift zone, originally containing titanomagnetite (TM) with a Curie temperature of 203–208°С, is studied at various oxidation stages. CRM was formed by annealing the samples at temperature T = 355°C during 4.5–350 h in the magnetic field with induction of 100 μT. The thermomagnetic analysis has shown that with the increase of the annealing time, a single-phase oxidation process takes place, the degree of the oxidation increases and reaches its maximum after annealing during 110 hours. It is established that the subsequent heating of the samples to 600°C in argon leads to the homogenization of the single-phase-oxidized titanomaghemite. When the annealing time increases to 350 h, oxi-exsolution of titanomaghemite begins. CRM as well as the sum of CRM and partial thermoremanent magnetization (pTRM) have been investigated by the Thellier–Coe procedure. The magnetic field Bcalc calculated in the temperature interval 355–475°C from chemical magnetization formed at the annealing times of 4.5, 16.5, and 40.5 h (CRM4.5, CRM16.5 and CRM40.5) proved to be underestimated by 55, 48–52, and 16–34%, respectively. The difference between the calculated and true field decreases with the growth in the degree of single-phase oxidation. The Bcalc values calculated in the temperature interval 475–580°С from CRM40.5 and CRM110 corresponding to the high degree of single-phase oxidation turned out to be underestimated by 69–72 and 62–63%. The last result is caused by the mineral transformations of titanomaghemite at heatings in the Thellier–Coe experiments. During paleointensity determination, it is proposed to control mineral transformations not only using the check-point procedure but also based on the difference in the removal of remanent magnetization in the Thellier experiments and by thermal demagnetization.

Strong anisotropy in the mixed antiferromagnetic system Mn1−xFexPSe3

We report the magnetic phase diagram of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$, which represents a random magnet system of two antiferromagnetic systems with mixed spin, mixed spin anisotropies, mixed nearest-neighbor magnetic interactions, and mixed periodicities in their respective antiferromagnetic structure. Bulk samples of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$ have been prepared and characterized phase pure by powder x-ray and neutron diffraction and x-ray fluorescence. Nature and extent of magnetically ordered state has been established using powder neutron diffraction, dc magnetic susceptibility, and heat capacity. Long-range magnetic ordering exists between $x=0.0$ and 0.25 (${\mathrm{MnPSe}}_{3}$ type) and between $x=0.875$ and 1 (${\mathrm{FePSe}}_{3}$ type). A short-range magnetic order with the existence of both ${\mathrm{MnPSe}}_{3}$- and ${\mathrm{FePSe}}_{3}$-type nanoclusters has been established between $x=0.25$ and 0.875. Irreversibility in dc magnetization measurements, also characterized by isothermal and thermoremanent magnetization measurements, suggest similarities to magnetic nanoparticles where uncompensated surface spins result in diverging thermoremanent and isothermal remanent magnetization responses, further reinforcing existence of magnetic nanoclusters or domains. A spin-glass state, observed in analogous ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PS}}_{3}$, has been ruled out, and formation of nanoclusters exhibiting both ordering types results from unusually high anisotropy values. The effect of ligand contributions to the spin-orbit interactions has been suggested as a possible explanation for high $D$ values in these compounds.

Synthesis and magnetic properties of Ba2Fe14O22 Y-type hexaferrite

Using the solid state reaction method, Y-type hexaferrite Ba2Fe2+2Fe3+12O22 (Fe2Y) polycrystalline ceramics were fabricated by calcining and sintering in a nitrogen atmosphere, and their magnetic properties were systematically investigated. The kink occurs at around 130 K in the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves is independent of measuring frequencies, which is probably derived by the temperature-induced spin reorientation transitions of Fe2Y phase. While the kink at 50 K in the ZFC magnetization and the irreversibility between ZFC and FC curves were observed and ascribed to the spin-glass-like transition in the low temperature region. Both ac susceptibility and thermoremanent magnetization (TRM) measurements confirmed this assumption.

Spin glass and exchange bias behavior in magnetically frustrated Ni1−xMgxCr2O4 (x = 0.0–0.50)

Single-phase polycrystalline samples of Ni1−xMgxCr2O4 (x = 0.0–0.50) were prepared by sol gel route and their structural and magnetic properties were studied. Structural transformation from tetragonal (I41/amd) to cubic (Fd3-m) phase is observed at room temperature due to Mg substitution. Mg substitution gives rise to reduction in ferrimagnetic and antiferromagnetic transition temperatures along with a signature of spin glass like phase in samples of intermediate compositions, i.e., for x = 0.10, 0.20 and 0.30. The observed stretched exponential type relaxation of thermoremanent magnetization and the highly frustrated magnetic behavior confirm the glassy magnetic phase. Significant increase in exchange bias field under field cooled condition for the Mg substituted samples is observed. The origin of exchange bias in theses samples is explained by considering exchange anisotropy between the ferrimagnetic and antiferromagnetic components of canted spin. The training effect of exchange bias field is also observed.