Spin Glass-like Phase Research Articles

Investigation of structural phase stability, modified magnetic spin order and low temperature spin glass-like phase transitions in Sc-doped M-type BaFe12O19 hexaferrite

This work used two approaches of mechanical alloying before high temperature solid state reaction to synthesize BaFe12-xScxO19 (x = 0.5 to 6.0) system. Rietveld refinement of X-ray diffraction patterns showed the formation of extra phases at higher Sc doping, along with the main magnetoplumbite structure. Raman spectroscopy suggested random distribution of Sc ions at the Fe-sites of hexaferrite structure. X-ray photoelectron spectroscopy (XPS) confirmed the +3 charge state at the Fe/Sc-sites. The Fe 3s band suggested dilution of ferromagnetic 3s-3d spin exchange interactions. The neutron diffraction study re-confirmed random occupancy of the non-magnetic Sc3+ ions at Fe3+ sites of the Sc doped Ba hexaferrite system. The transformation of magnetic spin order from collinear to non-collinear state has shown spin glass-like features at low temperature regime in the dc magnetization curves. This is an effect of dilution of Fe–O–Fe ferrimagnetic super exchange interactions and spin frustration in Sc-doped samples.

Griffiths phase and spontaneous exchange bias in La1.5Sr0.5CoMn0.5Fe0.5O6

La1.5Sr0.5CoMn0.5Fe0.5O6 (LSCMFO) compound was prepared by solid state reaction and its structural, electronic and magnetic properties were investigated. The material forms in rhombohedral structure, and the presence of distinct magnetic interactions leads to the formation of a Griffiths phase above its FM transition temperature (150 K), possibly related to the nucleation of small short-ranged ferromagnetic clusters. At low temperatures, a spin glass-like phase emerges and the system exhibits both the conventional and the spontaneous exchange bias (EB) effects. These results resemble those reported for La1.5Sr0.5CoMnO6 but are discrepant to those found when Fe partially substitutes Co in La1.5Sr0.5(Co1−x Fe x )MnO6, for which the EB effect is observed in a much broader temperature range. The unidirectional anisotropy observed for LSCMFO is discussed and compared with those of resembling double-perovskite compounds, being plausibly explained in terms of its structural and electronic properties.

Influence of spin glass-like magnetic relaxation on the zero-field-cooled exchange bias effect

The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon recently reported for some reentrant spin glass-like compounds. In this work, the time-evolution of magnetization is thoroughly investigated for two ZEB materials in order to figure out the role played by the spin glass-like phase on such effect. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ and La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ were chosen as representative samples of ZEB systems, since the former compound presents the largest ZEB reported so far, while the second has a much smaller effect, despite being structurally/chemically similar. Comprehensive magnetic measurements were carried on both samples, and the results are discussed in terms of the amount and time-evolution of the spin glass-like phase under the influence of a varying field. We also propose a phenomenological model, based on the pinning of spin glass-like moments and on the dynamics of their magnetic relaxation, to explain the asymmetry observed in the hysteresis loops. The good agreement between the simulated and experimental results confirms our hypothesis that the spin glass-like phase is key to the ZEB effect.

Magnetic, magnetoelectric, magnetodielectric and magnetoresistance studies on CuO doped Sr2Bi4Ti5O18 lead free ferroelectric ceramics

As an effort to develop new multiferroic materials at room temperature, we have interstitially doped (x mol %) low dimensional CuO with strong superexchange interactions into lead-free ferroelectric Sr2Bi4Ti5O18 (SBT) ceramics. Upon increase of CuO doping concentration (x = 0, 0.25, 0.5, 0.75 & 1) in SBT + x mol % compound a small and noticeable, but not appreciable, SrTiO3 secondary phase segregates in the SBT. Thus, CuO doping induces SrTiO3 phase and induced volume increase with “x”. The undoped SBT shows weak magnetization (μemu/g) at room temperature and magnetization enhances three-fold (memu/g) with CuO doping in SBT. The thermomagnetic studies of all the samples under zero field cooled, and field cooled modes show typical paramagnetic nature below room temperature and no spin glass-like phases. The Curie–Weiss law analysis suggest that antiferromagnetic interactions are predominant at low temperature in all the CuO doped samples except x = 0.75, which show ferromagnetic interactions. In the direct test for magnetoelectric properties, all the samples show similar magnetoelectric coupling coefficient (α). Among all, the x = 1 sample shows relatively low “α” indicating the magnetoelectric coupling weakens at high CuO concentration. Further, in an indirect test (magnetocapacitance) for magnetoelectric coupling shows similar to magnetoelectric behavior. As reported, the magnetocapacitance may originate from magnetoresistance (MR) and Maxwell–Wagner polarization effect. Our low field MR study on all samples suggests that large magnetocapacitance in x = 1 sample is not due to the extrinsic phenomenon. However further studies are essential to confirm our present inference.

Phase separation and exchange bias effect in Ca doped EuCrO3

The rare-earth chromites have attracted increasing interests in recent years, as a member of a few single-phase multiferroic materials. We studied the structure and magnetic property of a series of Ca-doped EuCrO3 samples by using X-ray powder diffraction and Physical Property Measurement System. Phase separation, rotation of magnetization in M(T) curve and exchange bias effect have been identified. The Eu0.7Ca0.3CrO3 polycrystalline sample may be intrinsically phase-separated, with Cr3+-rich, Cr4+-rich canted antiferromagnetic regions surrounded by spin glass-like frustrated phase, resulting in several magnetic features including: (1) a broad and slow increase of M(T) curve with the decrease of temperature; (2) rotation of magnetization with increasing cooling field; (3) exchange bias and glassy magnetism. The rotation of magnetization is ascribed to the rotation of the moment of Cr4+-rich regions, arising from the competition between exchange coupling energy and magnetostatic energy. The exchange bias effect suggests the formation of weak ferromagnetic unidirectional anisotropy during field cooling, due to the exchange coupling among weak ferromagnetic domains and surrounding spin glass-like regions. This result helps understanding the interaction among different magnetic domains and phases in a complex system.

Multistage ordering and critical singularities inCo1−xZnxAl2O4(0≤x≤1): Dilution and pressure effects in a magnetically frustrated system

We report comprehensive studies of the crystallographic,magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system,Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x < 0.6. A multicritical point was apparent at T = 3.4 K and x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x = 0.6. Around x = 0.6, magnetic susceptibility and specific heat were described by temperature power laws, chi ~ C/T ~ T-d, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an x-temperature phase diagram in the range 0 < x < 1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.

Giant exchange bias behavior and training effect in spin-glass-like NiCr2O4/NiO ceramics

NiCr2O4/NiO ceramic has been synthesized successfully through co-precipitation method to study the magnetic properties. Field cooling (FC) magnetic hysteresis loops measured at low temperature show significant shift in both coercive field and remnant magnetization. It has been plotted that the exchange bias (EB) field can be as large as 11.86 kOe at 10 K followed with an enhanced coercive field. The variation of EB field and shift of remnant magnetization after FC processes shows a regular tendency with increasing temperature and cooling field. Known as training effect, the EB behavior also presents a strong dependence on loop cycle. The cycle dependence of large EB field, vertical shift of magnetization, and coercive field at low temperature can be ascribed to the pinned spins in spin-glass-like (SGL) phase of the system interface. The SGL phase has been proved by Almeida–Thouless line and high field relaxation effect. The disorder structure is responsible for the frustration interface and the formation of SGL phase in the interface.

Effects of coexisting spin disorder and antiferromagnetism on the magnetic behavior of nanostructured (Fe79Mn21)1−xCux alloys

We report a magnetic study on nanostructured (Fe79Mn21)1−xCux (0.00 ≤ x ≤ 0.30) alloys using static magnetic measurements. The alloys are mainly composed by an antiferromagnetic fcc phase and a disordered region that displays a spin-glass-like behavior. The interplay between the antiferromagnetic and magnetically disordered phases establishes an exchange anisotropy that gives rise to a loop shift at temperatures below the freezing temperature of moments belonging to the disordered region. The loop shift is more noticeable as the Cu content increases, which also enhances the spin-glass-like features. Further, in the x = 0.30 alloy the alignment imposed by applied magnetic fields higher than 4 kOe prevail over the configuration determined by the frustration mechanism that characterizes the spin glass-like phase.

Size-dependent scaling of exchange bias in NiFe2O4/NiO nanogranular systems synthesized by a phase separation method

Exchange bias (EB) effect has been studied in a series of nanogranular systems of ferrimagnetic (FiM) NiFe2O4 nanoparticles embedded into antiferromagnetic (AFM) NiO matrix, synthesized by a phase pprecipitation method from diluted Ni(1−x)FexO3 (x = 0.09) oxides. For these systems, the crystalline size (DNFO) of NiFe2O4 ranging from ∼3 nm to ∼55 nm has been obtained with thermal treated at different temperatures from 550 °C to 1000 °C. Magnetization measurement shows that both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be exhibited below 250 K after field cooling procedure. The HEB and MShift decrease monotonically with crystalline size, and their behavior strongly depend on the crystalline size of NiFe2O4 nanoparticles. Linear relationship between HEB and MShift is observed for systems with smaller sizes (DNFO ≤ 8 nm), reveals a straightforward correlation between them. This phenomenon is ascribed to the interfacial exchange coupling between FiM NiFe2O4 clusters and spin-glass-like (SGL) phases, where the frozen uncompensated spins in SGL phases play critical role of inducing EB effect. As DNFO is above 12 nm, the dependence of HEB on MShift deviates from the linear relationship, which is discussed in terms of the superimposed contribution from the exchange coupling between FiM NiFe2O4 core with the SGL phase, and the exchange coupling between FiM NiFe2O4 core and AFM NiO phases at the interfaces.

Signature of exchange bias and spin-glass like phenomena in Fe/CoO nanocomposite

Nanocomposite of Fe/CoO having 10 at. wt. % of α-Fe has been prepared by the high energy ball-milling method. The average particle size was estimated to be 15 nm from the results obtained by the rietveld refinement of x-ray diffraction pattern. The presence of exchange coupling at the interface of ferromagnetic and antiferromagnetic components was confirmed from the shift observed in field-cooled (FC) magnetic hysteresis loop. Irreversibility in temperature dependent zero field cooled (ZFC)-FC magnetization at different applied fields indicates the presence of spin-glass like (SGL) phase. Memory effect observed in the temperature dependent dc-magnetization at FC condition confirmed the existence of SGL state. It is argued that polydispersity and defects at the interfaces of nanoparticles are the possible causes of the observed SGL phase.

Metamagnetic transition in Tb2MnCoO6

The magnetic properties of the double perovskite Tb2MnCoO6 have been studied. The refinement of neutron pattern reveals antisite defects in the ordered array of Mn4+ and Co2+ cations. The temperature dependence of dc magnetization exhibits a magnetic transition at ~100 K with a strong irreversibility between ZFC and FC conditions. The ac magnetic susceptibility curve shows a strong peak at the same temperature whose position and intensity slightly depends on the field frequency. These features are typical of a spin-glasslike phase but neutron diffraction shows the onset of a ferromagnetic contribution at the same temperature. Ferromagnetism in this sample is associated to superexchange interaction between Mn4+ and Co2+ cations. The most striking property of Tb2MnCoO6 is the presence of field induced transitions. This is observed in the magnetic hysteresis loops below 100K. The metamagnetic transition was studied by powder neutron diffraction at different temperatures and magnetic fields. At low temperature, the magnetic field induces a long range magnetic ordering of Tb3+ moments in the ab-plane. The magnetic peaks of Tb moments at 30 kOe vanish at 60 K. Above this temperature, the metamagnetic transition is ascribed to the field induced transition from short to long range ferromagnetic ordering in the Mn-Co sublattice.

Spin-Glass Behavior and the Magnetic Relaxation Effects in Nd&lt;sub&gt;0.90&lt;/sub&gt;Sr&lt;sub&gt;0.10&lt;/sub&gt;CoO&lt;sub&gt;3&lt;/sub&gt;

The magnetization, ac susceptibility and magnetic relaxation of Nd0.90Sr0.10CoO3polycrystalline sample were systematically investigated in this paper. The experimental studies of susceptibility and magnetic relaxation evidence the existence of a low-temperature spin-glass. A dynamic analysis of ac susceptibility implies a spin-glass transition temperature TSG =12.17 K and the dynamical exponent zv=8. Moreover, low-temperature zero-field cooling and field cooling magnetic relaxation show perfectly mirror symmetry, and field cooling processes relaxation obeys a stretched exponential form. Therefore, our study confirms that the phase separation in Nd0.90Sr0.10CoO3originates from both the ferromagnetic clusters interaction and the spin glasslike phase at low temperature.

Coulomb-driven cluster-glass behavior in Mn-intercalated Ti1+yS2

We have investigated the low-temperature spin-glasslike phase in the intercalated transition-metal dichalcogenide Mn${}_{0.09}$Ti${}_{1.1}$S${}_{2}$. A departure from Curie--Weiss behavior in the paramagnetic regime indicated the formation of small ferromagnetically correlated clusters. The Vogel--Fulcher law provided an excellent description of relaxation times in the vicinity of the transition, showing that the glasslike phase occurs due to interaction between the clusters. Cole--Cole plots for data close to the transition were linear, which is consistent with a simple exponential distribution of cluster sizes. A Monte Carlo simulation of the dichalcogenide system, including excess self-intercalated Ti ions, gave an exponential cluster-size distribution for a relatively narrow range of concentration values of Mn and Ti ions, values that were consistent with those of the Mn${}_{0.09}$Ti${}_{1.1}$S${}_{2}$ sample. Strong commonality in the relaxation behavior with certain ferroelectric relaxor systems suggests underlying similarity in the microscopic structure of the clusters in both systems, which may be chainlike or quasi-one-dimensional.

Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition.

Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden–Popper structure La2Sr2Mn3O10 in epitaxial thin film form by sequentially layering La1–xSrxMnO3 and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr2+ predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature Tg ≈ 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.

Exchange bias effect in multiferroic CoCr2O4/Cr2O3 nanogranular system synthesized through a phase segregation route

A nanogranular system of multiferroic CoCr2O4 nanoparticles embedded in an antiferromagnetic Cr2O3 matrix has been synthesized through a high-temperature phase segregation route from a Co-doped Cr2O3 matrix. Magnetic studies show that exchange bias fields (HEB) accompanying vertical magnetization shifts (MShift) are observed at low temperatures after field cooled from 350 K. The corresponding exchange bias field can be as large as 1420 Oe, and the vertical magnetization shift reaches 0.116 emu/g at 10 K. The exchange bias field decreases with temperature increasing and disappears at T ≈ 70 K, while the coercive field (HC) initially increases with the temperature up to 40 K, and thereafter, it decreases to zero at 100 K. This exchange bias behavior is discussed in terms of the existence of exchange coupling between the ferrimagnetic CoCr2O4 core and spin glass-like phase at the interfaces.

Superspin glass originating from dipolar interaction with controlled interparticle distance amongγ-Fe2O3nanoparticles with silica shells

\ensuremath{\gamma}-Fe${}_{2}$O${}_{3}$/SiO${}_{2}$ core-shell nanoparticles with different shell thicknesses were prepared to elucidate the condition for superspin-glass (SSG) dynamics. As the shell thickness decreases, the contribution of interparticle dipolar interaction becomes apparent in the magnetic dynamics of nanoparticle assembly. The frequency dependence of peaks in ac-magnetic susceptibility in samples with strong interactions slows down, which is characterized as the emergence of a spin-glasslike phase. Aging in magnetization relaxation is found in a strongly interacting sample with an interparticle distance of $L\ensuremath{\leqslant}14$ nm but is scarce in a sample with $L$ $=$ 18 nm. Scaling analysis reveals an increase in superparamagnetic properties with an increase in $L$. Therefore the critical interparticle distance necessary for SSG transition is 15--18 nm with 11-nm $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$ nanoparticles. This corresponds to the ratio of interparticle-interaction energy to the magnetic-anisotropy energy ${E}_{\mathrm{dip}}$/${E}_{\mathrm{a}}$ of 6--12%.

Two-dimensional magnetism and spin-size effect in theS = 1 triangularantiferromagnet NiGa2S4

The triangular antiferromagnet is one of the most fundamental systems of geometrically frustrated magnets.NiGa2S4 is alayered chalcogenide compound with an equilateral triangular lattice, and it is a prime candidate foran S = 1 triangular antiferromagnet. Here we focus on low temperature magnetism inNiGa2S4,where quasi-static spins develop a spin-wave-like mode without forming any long-range ordering.We have studied low temperature magnetism of both polycrystalline samples and single crystals ofNi1 − xAxGa2S4 (A = Mn, Fe, Co, and Zn). A scaling law with a single energy scale of the Weisstemperature is found as an impurity effect and a hydrostatic pressure effect,providing evidence that it is in-plane interactions in the two-dimensionalNiS2 plane that drive the critical slowing down to the viscous spin liquid state atT* = 8.5 K and the spin-wave-likeexcitations of NiGa2S4 that emerge below T ∼ 3 K. Furthermore, we find spin-size dependent impurity effects in the temperature dependence of the specific heat ofNi1 − xAxGa2S4. Even with a highimpurity content, Zn2 + (S = 0) andFe2 + (S = 2) substitutedsystems with weak XY anisotropy and integral spins retain the quadratic temperature dependence of the magnetic specific heatlike pure NiGa2S4. A spin glass-like phase, on the other hand, emerges at low temperatureswith the substitution of magnetic impurities with half-odd integer spins: IsingCo2 + (S = 3⁄2) and weak XY Mn2 + S = 5⁄2) spins. This indicates that an integer size of spins is important for stabilizing thetwo-dimensional spin-wave-like behavior, and the unconventional spin state ofNiGa2S4 at low temperatures is distinct from a canonical spin glass.

Elemental analysis and magnetism of hydronium jarosites—model kagome antiferromagnetsand topological spin glasses

The jarosites are the most studied examples of kagome antiferromagnets. Research into themhas inspired new directions in magnetism, such as the role of the Dzyaloshinsky–Moriyainteraction in symmetry breaking, kagome spin ice, and whether spin glass-like phases canexist in the disorder-free limit. This last point is based around the observationof unconventional thermodynamic and kinetic responses in hydronium jarosite,H3OFe3(SO4)2(OH)6, that have led to its classification as a ‘topological’ spin glass, reflecting the defining rolethat the underlying geometry of the kagome lattice plays in the formation of the spin glassstate.In this paper we explore one of the fundamental questions concerning the frustratedmagnetism in hydronium jarosite: whether the spin glass phase is the result of chemicaldisorder and concomitant randomness in the exchange interactions. Confirming previouscrystallographic studies, we use elemental analysis to show that the nature of the lowtemperature magnetic state is not a simple function of chemical disorder andprovide evidence to support the hypothesis that anisotropies drive the spin glasstransition.

Magnetization of Nd1-xSrxCoO3 (0.1&lt;=x&lt;=0.5)

In this paper, the rare earth cobaltite Nd1-xSrxCoO3 (0.1&amp;lt;=x&amp;lt;=0.5) samples were prepared using conventional solid&lt;br /&gt;state reaction method. Dc magnetization, ac susceptibility, magnetic relaxation of polycrystalline Nd1-xSrxCoO3&lt;br /&gt;is investigated in detail. The presence of the typical features of spin/cluster glass state in x&amp;lt;+0.2 samples was well&lt;br /&gt;revealed by ac susceptibility. Therefore, our study confirms that the phase separation in Nd1-xSrxCoO3 originates&lt;br /&gt;from both the ferromagnetic clusters interaction and the spin glasslike phase at low temperature.

Ferrimagnetism in Mn1−xAl1−yCux+y alloys in the cubic κ (B2) phase

In this study we report results of X-ray diffraction and dc magnetization measurements in Mn 1-x Al 1-y Cu x+y alloys in the κ phase which has a B2 (CsCl-type) structure. Increasing Cu doping not only weakens the ferromagnetic order and strongly affects Curie temperature (T c ) but also imposes a competition mechanism between ferromagnetic and ferromagnetic spin correlations which induces the appearance of cluster spin-glasslike phase at low temperatures.