Ac Susceptibility Studies Research Articles

Structure and magnetism of DyIII2 based triple-stranded helicates derived from dinucleating Schiff base ligands

We report herein the synthesis of three dinuclear DyIII helicates, [DyIII2(H2LR)3](NO3)3·xH2O (1–3, R = Me, Cl, Br), by reacting Dy(NO3)3·5H2O and three closely related Schiff bases derived from 3-Hydroxy-2-naphthohydrazide and three different 2,6-diformylphenols. Single crystal X-ray structural characterization revealed that all these complexes are isostructural and composed of complex cations of general formula [DyIII2(H2LR)3]3+, in which three monoanionic Schiff base ligands are wrapped around two DyIII centers, leading to triple-stranded helical structures. Notably, the configuration of monoanionic H2LR ligands imparts a coordination-induced chirality on individual complex cation, and the presence of equivalent amounts of enantiopure Λ and Δ complex cations made the solid products as racemic mixtures, crystalizing in the centrosymmetrical space groups. Variable-temperature magnetic susceptibility measurements have been performed in the temperature range 2 − 300 K for a representative complex 2, and AC susceptibility studies further disclose the slow relaxation of magnetization in 2, characteristics for single-molecule magnets.

Spin glass state in Co doped NdMnO3

The temperature-dependent ac susceptibility studies of in-phase component (χ′ac) and out of phase component (χac″) was done on a single phase polycrystalline NdMn0.6Co0.4O3 sample. The measurements were conducted in 4 Oe ac field and 0 Oe dc field at frequencies of 111, 311, 1011, and 1311 Hz. As the frequency increases, the magnitude of χ′ac decreases, and its peak position, corresponding to Tf, moves towards higher temperatures. This strongly suggests the presence of a spin glass (SG) state and the absence of long-range ferromagnetic ordering. The value of relative shift in freezing temperature (δTf) is found to be δTf = 0.0089, which suggests the existence of canonical SG state in NdMn0.6Co0.4O3 sample. To confirm it the ac susceptibility data was further analyzed using Arrhenius, Vogel Fulcher, and Power–law.

Study of AC Susceptibility of La0.8Sr0.2MnO3 at Different Frequencies

In the present work, the structural and ac magnetic susceptibility as a function of temperature at different frequencies of perovskite type La08Sr0.2MnO3 (LSMO) compound have been discussed. Conventional solid-state route was used to synthesize LSMO compound. Orthorhombic crystal structure with Pnma space group of the compound was witnessed from the analysis of X-ray diffractograms by Rietveld refinement. The real (χꞌ) and imaginary (χꞌꞌ) part of the ac susceptibility of the compound have been studied. The real part of magnetic susceptibility (χꞌ) of La0.8Sr0.2MnO3 sample increases as temperature decrease. This indicates that the paramagnetic-ferromagnetic phase transition occurs. At different frequencies, i.e. 100Hz and 500Hz the Curie temperature Tc are 326K and 336K respectively. Peaks observed in the imaginary part χꞌꞌ) of magnetic susceptibility plot indicate the presence of sample inhomogeneity.

Ac-Susceptibility Studies of the Energy Barrier to Magnetization Reversal in Frozen Magnetic Nanofluids of Different Concentrations

We used ac-susceptibility to measure the blocking temperature, TB, and energy barrier to the magnetization reversal, EB, of nanomagnetic fluids of different concentrations, c. We collected data on five samples synthesized by dispersing Fe3O4 nanoparticles of average diameter ⟨D⟩ = 8 nm in different volumes of carrier fluid (hexane). We found that TB increases with the increase in c, a behavior predicted by the Dormann–Bessais–Fiorani (DBF) theory. In addition, our observed TB vs. c dependence is excellently described by a power law TB = A∙cγ, with A = 64 K and γ = 0.056. Our data also show that a Néel–Brown activation law τT=τ0exp⁡EBkBT describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, Ead, is needed at higher concentrations, according to the DBF model: τT=τrexp⁡EB+EadkBT. We found EB/kB = 366 K and additional energy barriers Ead/kB that increase linearly with the common logarithm of the volume concentration, from 138 K at c = 8.3 × 10−4% to 745 K at c = 4 × 10−2%. These results add to our understanding of the contributions by different factors to the superspin dynamics. In addition, the quantitative relations that we established between the TB, Ead, and c support the current efforts towards the rational design of functional nanomaterials.

Effect of Sr doping on structural, magnetic and transport properties of La1-ySryMn0.5Co0·5O3±δ

Structural, magnetic and magneto transport properties of La1-ySryMn0.5Co0·5O3±δ perovskites prepared by aqueous solution combustion synthesis were studied in detail. The effect of Sr doping on the properties of these perovskites was investigated. As-prepared perovskites were characterized using X-ray diffraction for single phase formation. Magnetic study was carried out for in depth analysis of the magnetic properties. AC susceptibility study shows a change in the behavior of undoped and doped compounds in low temperature region. Ferromagnetic interactions was found to increase in the low temperature region in the doped compounds compared to that in the undoped compound. The measurements on resistivity showed reduction with Sr doping up to y = 0.5. This is attributed to the high spin stabilized Co3+ ions which enhance the ferromagnetic interactions and the hopping probability of eg charge carriers.

Chiral-fluctuations mediated helical to paramagnetic phase transition and scaling study in β-Mn type Co8Zn8Mn4 chiral magnet

We report detailed magnetization and ac susceptibility studies on a chiral cubic β-Mn type Co8Zn8Mn4 alloy near the onset of magnetic ordering temperature. The Co8Zn8Mn4 alloy undergoes a complex series of temperature-driven transition from paramagnetic (PM) to helimagnetic phase through an intermediate inhomogeneous chiral fluctuations phase. The Gaussian shaped peak in ( = 0 Oe) at 343.19 K is analogous to that of the zero-field transition to long-period modulated helical state. Moreover, a pronounced field-induced anomaly in (T, 500 Oe) rises quickly with field above like a fingerprint, evident of a field-induced crossover into a precursor region. The in-field critical behavior characterizing a ferromagnetic-PM phase transition in the vicinity of ordering temperature confirms that Co8Zn8Mn4 chiral magnet belongs to the 3D-Heisenberg universality class. The existence of precursor region and the sizable expansion in the transition temperature, can be explained by the Landau equation.

Non-equilibrium magnetic response in concentrated spin-glass Au0.89Fe0.11 alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline Au0.89Fe0.11 alloy. The temperature variation of ZFC and FC dc magnetization at low fields shows a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. Further down the temperature at Tsh<Tir, the ZFC and FC magnetization curves exhibit strong thermomagnetic irreversibility. The ac susceptibility recorded after cooling in presence of field shows significant dispersion below Tf, and also, the FC state of Au0.89Fe0.11 alloy exhibits a pronounced memory effect in the dc magnetization which clearly underline that the FC state is not an equilibrium state. In contrast to the general perception obtained through the mean-field theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of Au0.89Fe0.11 alloy is a nontrivial one.

Structures and magnetic relaxation properties of cyclopentadienyl/β-diketonate/trispyrazolylborate hybridized dysprosium single-molecule magnets

The combination of cyclopentadiene, β-diketonate and tripyrazoylborate ligands with dysprosium ion afforded five mononuclear compounds: [(Cp)2Dy(Tp*)] (1Dy), [(Cp)Dy(Tp*)Cl(THF)] (2Dy), [(Cp)Dy(Tp)Cl(THF)] (3Dy), [(DBM)Dy(Tp)Cl(THF)] (4Dy), [{(Tp)Dy(DBM)2(H2O)}·THF] (5Dy) (Cp = cyclopentadiene; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; Tp = hydrotris(1-pyrazolyl)borate; DBM = dibenzoylmethanoate). Magnetic study revealed that 1Dy and 3Dy exhibited typical butterfly-type hysteresis. AC susceptibility study combined with ab initio calculations indicated that the magnetic relaxation behaviors of 1Dy–4Dy were governed by the Orbach and Raman processes under applied DC field. Moreover, 3Dy showed two-step magnetic relaxation, which was attributed to the static disordering of the coordinated THF molecule. Magnetic anisotropy analysis indicated that it was the relative strength of the interactions between DyIII and surrounding ligands that determined the orientation of the magnetic easy axis.

Migration of carbon atoms in irradiated and non-irradiated alpha-iron studied by magnetic after-effect

Magnetic After-Effect (MAE) and magnetic AC susceptibility studies have been performed on high-purity non-irradiated and Fe-ion irradiated, α-Fe foil samples in the high-temperature range, which became experimentally accessible recently. The pronounced difference in the temperature profile of MAE between non-irradiated and irradiated samples has been identified and ascribed to the trapping of carbon in the irradiated sample into structures that are reluctant to decompose by standard temperature cycling. The accurate background of MAE relaxations at 430 and 610 K in non-irradiated α-Fe samples has been scrutinized by the annealing-type studies in temperature and time domains to conclude that it relies on the formation, decomposition, and mutual transformations of carbide nanoprecipitates nucleated in dislocations, grain boundaries, and in the bcc matrix. Long-term trapping of migrating carbon into carbides and in the 100 nm thick surface layer has also been shown to take place.

Lanthanide Luminescence Thermometry and Slow Magnetic Relaxation in 3-D Polycyanidometallate-Based Materials.

Two three-dimensional (3-D) polycyanidometallate-based luminescent thermometers with the general formula {Ln4Co4(CN)24(4-benpyo)17(H2O)·7H2O}n Ln = (Dy(III)(1), Eu(III)(2)), based on the red-emissive diamagnetic linker [Co(CN)6]3- and the bulky pyridine derivative that possesses the N-oxide moiety, 4-benzyloxy-pyridine N-oxide (benpyo), were prepared for the first time. The structure of compound 1 has been determined by single-crystal X-ray crystallography while the purity and structure of 2 have been confirmed by CHN, Fourier transform infrared spectroscopy (FT-IR), and powder X-ray diffraction (PXRD) analysis. Magnetic AC susceptibility measurements at zero field show no single-molecule magnet (SMM) behavior indicating fast relaxation operating in 1. Upon application of an optimal field of 2 kOe, the SMM character of compound 1 is revealed while the τ(Τ) can be reproduced solely considering the Raman process τ-1 = CTn with C = 7.0901(3) s-1 K-n and n = 3.58(1), indicating that a high density of low-lying states and optical as well as acoustic phonons play a major role in the relaxation mechanism. Micron-sized superconducting quantum interference device (μ-SQUID) loops show a very narrow opening in agreement with the AC susceptibility studies and complete active space self-consistent field (CASSCF) calculations. The interaction operating between the Dy(III) ions was quantified from CASSCF calculations. Good agreement is found by fitting the experimental DC χMΤ(Τ) and M(H), employing the Lines model, with JLines = -0.087 cm-1 (-0.125 K). The excitation spectra of compound 2 are used for temperature sensing in the 25-325 nm range with a maximum relative thermal sensitivity, Sr = 0.6% K-1 at 325 K, whereas compound 1 operates as a luminescent thermometer based on its emission features in the temperature range of 16-350 K with Sr ≈ 2.3% K-1 at 240 K.

Observation of structural change-driven Griffiths to non-Griffiths-like phase transformation in Pr2-xSrxCoFeO6 (x = 0 to 1)

The study of crystal structure, electronic structure, transport, and magnetic properties of heterovalent Sr2+ doped Pr2-xSrxCoFeO6 (x = 0.0 to 1.0) system have been done. Crystal structure study reveals an occurrence of structural change from orthorhombic (Pnma) to tetragonal (I4/m) phase above x = 0.6. A sudden transformation of the Griffiths-like to non-Griffiths-like magnetic phase is observed as the system changes its crystal structure from Pnma to I4/m. The X-ray photoemission spectroscopy (XPS) study suggests for the existence of mixed oxidation states of the B-site ions viz., Co3+/Co4+ and Fe3+/Fe4+, and it also indicates an increase in the mean oxidation states owing to the hole substitution (Sr2+). The temperature variation of the electrical resistivity of the studied systems follows two different transport mechanisms, such as the variable range hopping (VRH) (in the lower temperature region) and small polaron hoping (SPH) (in the higher temperature region) models. Dc magnetization study shows that a local competing ferromagnetic (FM) exchange interaction increases with Sr doping. Finally, the ac susceptibility study reveals breaking of the long-range-ordering in the system x = 1.0, which appears to be related to the structural change and enhanced spin frustration due to increased competing local FM exchange interactions. In addition, electronic density of states (DOS) calculations of PrSrCoFeO6 (i.e. x = 1.0) using the density functional theory (DFT) have been performed for various Co/Fe atomic distributions. For most of the Co/Fe atomic distributions studied, the calculations show that the total energy of the system with FM coupling among spins has slightly lower energy than that for antiferromagnetic (AFM) coupling.

Structures and Magnetic Properties of Carbonato-Bridged Hexanuclear NiII4LnIII2 (Ln = Gd, Tb, Dy) Complexes Formed by Atmospheric Carbon Dioxide Fixation in the Absence of an External Base

The reaction of the mononuclear Ni(II) building block [NiL] of an asymmetrically dicondensed N2O3 donor Schiff base ligand, N-salicylidene-N′-3-methoxysalicylidene-1,3-propanediamine (H2L), with LnCl3·6H2O (Ln = Gd, Tb, Dy) in a 2:1 molar ratio leads to the formation of three hexanuclear carbonato-bridged Ni4Ln2 complexes formulated as [Ni4Ln2(CO3)2Cl2(L)2(L′)2(CH3CN)2]·4CH3CN·2H2O (Ln = Tb (2), Dy (3)), where H2L′ = N,N′-bis(salicylidene)-1,3-propanediamine. In complex 1, the unsymmetrical nature of the Schiff base remains intact; however, in 2 and 3, between the two metalloligands coordinated to each Ln, one is the precursor [NiL] of the unsymmetrical ligand whereas the other [NiL′] of the symmetrical ligand, formed by an in situ aldehyde-exchange reaction during complex formation with Ln salts. The Ln(III) centers are octacoordinated with a biaugmented-trigonal-prismatic geometry in all three complexes. In complex 1, half of the Ni(II) centers possess a distorted octahedral geometry while the other half has a square-pyramidal geometry; all of the Ni(II) centers in complexes 2 and 3 have a distorted-octahedral geometry. A magnetic study clarifies that the Ni–Ln magnetic exchange couplings are ferromagnetic. Though the complexes consist of a hexanuclear [Ln2Ni4] system, the magnetic properties have been analyzed on a trinuclear Ni–Ln–Ni basis. The exchange parameter 2JGd–Ni/kB in 1 was estimated to be +0.61 to +0.84 K across the double-oxide bridges. Complexes 2 and 3 exhibited an indication of single-molecule magnets in an ac susceptibility study.

Spin glass behavior and critical analysis across the magnetic phase transition in Gd2CoMnO6: dc magnetization and ac susceptibility study

We report here on critical analysis across magnetic phase transition and spin dynamics in Gd2CoMnO6. We found that this material behaves differently below and above the applied magnetic field of 20 kOe. The magnetic phase transition switches from nearly mean-field type to unusual class and Tc shifts towards the high temperature above 20 kOe field. The nature of the magnetic phase transition is explored by carrying out critical analysis at low as well as at high magnetic field. The critical exponents obtained at low field using Kouvel-Fisher method are β = 0.65 (2) γ = 0.90 (2), δ = 2.43 and Tc = 120 K. Apparently, these values of critical exponents appear close to mean-field model. For high field the critical exponents are β = 1.24 (2) γ = 0.64 (5), δ = 1.51 (3) and Tc = 128 K. The critical exponents show significant deviation from any universal class. This switchover in the nature of the magnetic phase transition is unique and not seen in many compounds. The formation of non-Griffiths-like clusters in this compound can be a reason for such unique behavior. Further, ac susceptibility has been measured to understand the spin dynamics in detail. The dispersion of frequency-dependent χac below Tc confirms a spin glass state in this material. The observed value of τo and To indicate the slow dynamic spin which is caused by co-existence of Co/Mn spin magnetic moments. The magneto-caloric effect is also presented for Gd2CoMnO6 in this study. The magnetic study and critical analysis across the phase transition reveal a switchover in the nature of phase transition in this material. A non-Griffiths like cluster formation above Tc is found and dynamic susceptibility study reveals a spin glass state below Tc in Gd2CoMnO6.

Robust evidence for the stabilization of the premartensite phase in Ni-Mn-In magnetic shape memory alloys by chemical pressure

The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 K to around 5 K in a magnetic shape memory alloy (MSMA) Ni50Mn34In16 using magnetic susceptibility, synchrotron X-ray powder diffraction (SXRPD) studies and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for around 5% Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at and below 300 K without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as around 3% Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion/symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state.

AC Susceptibility Studies of Magnetic Relaxation in Mn12-Stearate SMMs on the Spherical Silica Surface

The study of magnetic relaxations in Mn12-stearate single-molecule magnets deposited on the surface of spherical silica nanoparticles was performed. For such a purpose, the investigation of AC magnetic susceptibility dependence on the frequency and temperature was performed. Based on the Argand plots obtained for different temperatures and temperature dependencies of susceptibility, obtained for different frequencies of AC field, the corresponding relaxation times were derived. Fitting to the Arrhenius law revealed the values of an effective energy barrier and a mean relaxation time, which were consistent for both measuring techniques (Ueff/kB∼ 50 K and τ0∼ 10−7 s) and similar to the corresponding values for the analogous bulk compounds. Additionally, the obtained relaxation parameters for the Mn12-stearate molecules on the spherical silica surface were compared with corresponding values for the Mn12-based single-molecule magnets deposited upon other types of nanostructured silica surface.

Competing magnetic interactions and emergent phase diagrams in double perovskite Y2NixCo1−xMnO6

We present a comparative study of double perovskites Y2NixCo1−xMnO6 for x = 1, 0.5, and 0. The polycrystalline samples of Y2NixCo1−xMnO6 with space group P21/n were synthesized via sol-gel technique. X-ray photoelectron spectroscopy (XPS) confirms the presence of majority Ni2+/Mn4+ in Y2NiMnO6 (YNMO) and a mixed valence state Ni2+/3+/Co2+/3+/Mn4+/3+ in co-doped Y2Ni0.5Co0.5MnO6 (YNCMO). The temperature dependent magnetization results suggest homogeneous substitution of Ni ions with Co ions with minimal contribution of 3 + cationic magnetic interactions in YNCMO. The superexchange ferromagnetic (FM) interaction has a major contribution to the magnetism of YNMO. The strength of antiferromagnetic (AFM) coupling resulting from the antiphase boundaries in Y2NixCo1−xMnO6 increases with the increase in the Co concentration. The AC susceptibility (AC-χ) study further validates the presence of higher density of antiphase boundaries, which is reflected by the dynamics of domain walls in YNCMO. The isothermal magnetic entropy change (ΔSM) as functions of temperature and magnetic field is exploited to assess the stabilization of different magnetic phases. The findings of ΔSM (T,µ0H) lead to the proposed new magnetic phase diagrams for Y2NixCo1−xMnO6 for x = 1 and 0.5, in comparison with the previously established phase diagram of Y2CoMnO6.

YBCO superconductor added with one-dimensional TiO2 nanostructures: Frequency dependencies of AC susceptibility, FC-ZFC magnetization, and pseudo-gap studies

This article reports the role of adding one-dimensional titanium dioxide nanowires (TiO2-NWs, having 30 nm in diameter and some micrometers in length) on FC-ZFC magnetization, frequency-dependent AC susceptibility, and electrical measurements of the YBCO bulk system. The non-added (A0) and added TiO2-NWs samples (A1) were prepared by solid-state reaction route. TiO2-NWs did not affect the crystalline structure of YBCO. SEM examinations revealed that TiO2-NWs preferred to localize at the grain boundaries filling the pores existing between YBCO grains. FC-ZFC magnetization and frequency-dependent AC susceptibility analysis showed an enhancement of flux pinning properties with TiO2-NWs addition. TiO2-NWs addition affected also the electrical and pseudo-gap analysis of the samples.

Nickel(II) salicylaldiminates: Re-visiting a classic

A series of Ni(II) trinuclear complexes, [Ni3(sala-X)6] (sala-X = o-[(p-X-phenylimino)methyl]phenol; X = Br 1-Br, Me 1-Me and OMe 1-OMe), have been prepared and fully characterised. X-ray crystallographic studies reveal that the complexes are composed of three face-sharing octahedral Ni(II) metal ions bridged by the phenoxide oxygens of the sala-X ligands. Magnetic studies indicate that the magnetic Ni(II) centres are ferromagnetically coupled with the substituent group having only a minor impact on the magnitude of coupling. AC susceptibility studies show no evidence of single-molecule magnet behaviour. DFT calculations also support ferromagnetic coupling with smaller Ni-O-Ni angles leading to slightly larger coupling constants in line with previous studies.

Enhancing the barrier height for magnetization reversal in 4d/4f RuIII2LnIII2 "butterfly" single molecule magnets (Ln = Gd, Dy) via targeted structural alterations.

A series of 4d-4f {RuIII2DyIII2} and {RuIII2GdIII2} 'butterfly' (rhombohedral) complexes have been synthesized and characterized and their magnetic properties investigated. Earlier, we have reported the first 4d/4f SMM - [RuIII2DyIII2(OMe)2(O2CPh)4(mdea)2(NO3)2] (1Dy) with a Ueff value of 10.7 cm-1. As the structural distortion around the DyIII centres and the RuIII⋯DyIII exchange interactions are key to enhancing the anisotropy, in this work we have synthesised three more {Ru2Dy2} butterfly complexes where structural alteration around the DyIII centres and alterations to the bridging groups are performed with an aim to improve the magnetic properties. The new complexes reported here are [Ru2Dy2(OMe)2(O2C(4-Me-Ph)4(mdea)2(MeOH)4], 2Dy, [Ru2Dy2(OMe)2(O2C(2-Cl,4,5-F-Ph)4(mdea)2(NO3)2], 3Dy, and an acac derivative [Ru2Dy2(OMe)2(acac)4(NO3)2(edea)2], 4Dy, where acac- = acetylacetonate, edea2- = N-ethyldiethanolamine dianion. Complex 2Dy describes alteration in the DyIII centers, while complexes 3Dy and 4Dy are aimed to alter the RuIII⋯DyIII exchange pathways. To ascertain the 4d-4f exchange, the Gd-analogues of 1Dy and 4Dy were synthesised [Ru2Gd2(OMe)2(O2CPh)4(mdea)2(NO3)2], 1Gd, [Ru2Gd2(OMe)2(acac)4(NO3)2(edea)2], 4Gd. Both ac and dc susceptibility studies were performed on all these complexes, and out-of-phase signals were observed for 3Dy in zero-field while 2Dy and 4Dy show out-of-phase signals in the presence of an applied field. Complex 3Dy reveals a barrier height Ueff of 45 K. To understand the difference in the magnetic dynamic behavior compared to our earlier reported {RuIII2DyIII2} analogue, detailed theoretical calculations based on ab initio CASSCF/RASSI-SO calculations have been performed. Calculations reveal that the JRu⋯Dy value varies from -1.8 cm-1 (4Dy) to -2.4 cm-1 (3Dy). These values are also affirmed by DFT calculations performed on the corresponding GdIII analogues. The origin of the largest barrier and observation of slow magnetic relaxation in 3Dy is routed back to the stronger single-ion anisotropy and stronger JRu⋯Dy exchange which quenches the QTM effects more efficiently. This study thus paves the way forward to tune local structure around the LnIII center and the exchange pathway to enhance the SMM characteristics in other {3d-4f}/{4d-4f} SMMs.

Magnetic ordering of Ho and its role in the magnetization reversal and coercivity double peaks in the Ho3Fe5O12 garnet

Rare-earth based garnet system, Ho3Fe5O12 shows magnetization compensation phenomenon at Tcomp = 138 K and a magnetic transition at 50 K. In the present work, we reveal the physics of these low temperature magnetic transitions by carrying out detailed dc magnetization, ac susceptibility, neutron depolarization, and neutron diffraction studies. Interestingly, our dc magnetization study under low (≲ 50 Oe) magnetic fields has shown a previously unknown sign reversal of magnetization (below the Tcomp) that persists down to 5 K, the lowest measured temperature. Magnetic measurements under moderate fields between 50 Oe and 1 kOe reveal two compensation temperatures (in the range of 105 – 150 K depending upon the applied field value) leading to double magnetization reversals. Interestingly, for magnetic fields ≥ 1 kOe, these two compensation temperatures merge to a single Tcomp (=138 K). A modified Stoner-Wohlfarth model has been used to explain the asymmetrical nature of double peak around the Tcomp in the temperature dependent coercivity data. The ac susceptibility study supports the existence of both the transitions with an evidence of deep minimum at the Tcomp (138 K) and a broad peak at 50 K. Mesoscopic neutron depolarization study has revealed a zero domain-magnetization state at the Tcomp. Temperature dependent neutron diffraction study has revealed that Ho carries an induced moment above the Tcomp due to polarizing effect under the internal magnetic field of the two magnetically ordered Fe sublattices at 567 K, whereas a single umbrella type ordering of Ho sublattice moments appears below the Tcomp, resulting in the distortion of magnetic unit cell from cubic to rhombohedral symmetry. However, below 50 K, Ho3+ sites are divided into two inequivalent magnetic sublattices, having different moments and canting angles, and it leads to double umbrella type magnetic structure. Neutron diffraction study has revealed an asymmetric variation of the tetrahedral, octahedral, and dodecahedral sublattice moments resulting in a magnetization reversal at ~ 138 K which is very well supported by the mean field theory calculations. In the present study, we have resolved the ambiguity of magnetic ordering of the rare-earth (Ho3+) sublattice and its role in multiple magnetic transitions. The utility of such compensated ferrimagnetic materials in spin polarizers/analyzers and fast switching memory devices has been emphasized.