Rhodes-Wohlfarth Ratio Research Articles

Spin stiffness and spin excitation gap of van der Waals ferromagnetic Fe3+δGeTe2

Fe3+δGeTe2 (FGT) has proved to be an interesting van der Waals (vdW) ferromagnetic compound with a tunable Curie temperature ( TC ). However, the underlying mechanism for varying TC remains elusive. Here, we systematically investigate and compare low-temperature magnetic properties of single crystalline FGT samples that exhibit TC s ranging from 160 K to 205 K. Spin stiffness (D) and spin excitation gap (Δ) are extracted using Bloch’s theory for crystals with varying Fe content. Compared to Cr-based vdW ferromagnets, FGT compounds have higher spin stiffness values but lower spin wave excitation gaps. We discuss the implication of these relationships in Fe–Fe ion magnetic interactions in FGT unit cells. The itinerancy of magnetic electrons is measured and discussed under the Rhodes–Wohlfarth ratio (RWR) and the Takahashi theory.

Structural trends and itinerant magnetism of the new cage-structured compound HfMn2Zn20

A new cage-structured compound—HfMn2Zn20—belonging to the AB 2 C 20 (A, B = transition or rare earth metals, and C = Al, Zn, or Cd) family of structures has been synthesized via the self-flux method. The new compound crystallizes in the space group Fd3¯m with lattice parameter a ≈ 14.0543(2) Å (Z = 8) and exhibits non-stoichiometry due to Mn/Zn mixing on the Mn-site and an underoccupied Hf-site. The structure refines to Hf0.93Mn1.63Zn20.37 and follows lattice size trends when compared to other HfM 2Zn20 (M = Fe, Co, and Ni) structures. The magnetic measurements show that this compound displays a modified Curie-Weiss behavior with a transition temperature around 22 K. The magnetization shows no saturation, a small magnetic moment, and near negligible hysteresis, all signs of itinerant magnetism. The Rhodes–Wohlfarth ratio and the spin fluctuation parameters ratio both confirm the itinerant nature of the magnetism in HfMn2Zn20.

Experimental studies on noncentrosymmetric Nd3Se4: Strongly correlated noncollinear ferromagnet

We have synthesized polycrystalline Nd3Se4 by the solid state sealed tube reaction route followed by structural characterization and magnetic property measurements. The temperature-dependent magnetization studies reveal the existence of competing magnetic interactions below the ferromagnetic ordering (TC ≈ 48.4 K). The observation of hysteresis in the isothermal magnetization M(H) data with the coercive field of ≈ 0.25 T at 2 K indicates dominant ferromagnetic interactions in the applied field. The nonsaturating behavior of M(H) data at higher applied field and kinks in corresponding dM/dHvs. H plots suggest the coexistence of multiple magnetic interactions at low temperatures. Using the M(T,H=2 T) data, the Rhodes-Wohlfarth ratio (qc/qs) has been estimated to be ≈ 3.6, which supports the itinerant magnetism in Nd3Se4. These competing interactions have been further investigated by AC-susceptibility measurements at various excitation frequencies. The χ′ reduces drastically with an increase in the applied DC field, as observed from isothermal field-dependent AC-χ studies. Additionally, the strong correlation between delocalized charge and localized magnetic moments is evident from ρ(T) measurements at H = 0 and 5 T. The ρ(T,H=0 T) data shows a clear drop having quadratic temperature dependence, below TC. Furthermore, the inter-dependence between electrical transport and magnetism has been established firmly through negative magnetoresistance, in which the maxima correspond to the coercive field obtained from M(H) data.

Evidence of weak itinerant ferromagnetism and Griffiths like phase in MnFeGe

Realizing the itinerant type of magnetic exchange in Mn-based alloys is quite unusual because of the weak hybridization between Mn-moments and conduction electrons. However, in the current study, we found MnFeGe to exhibit weak itinerant type magnetic character possibly arising due to the hybridization between Mn, Fe atoms with Ge atoms. Here, we present a comprehensive structural and magnetic study on polycrystalline MnFeGe. Rietveld refinement of the XRD pattern confirms that Mn and Fe atoms randomly (atomic disorder) occupy the 2a and 2d sites in MnFeGe. From the magnetic measurements, Curie temperature and saturation moment values are found to be 162 K and 1.58 at 2 K, respectively. The relative low values of Curie temperature, saturation magnetic moment, and a large value of Rhodes–Wohlfarth ratio (∼3.4) indicate the weak itinerant ferromagnetic character of the system. The Takahashi spin fluctuation theory analysis further supports the weak itinerant ferromagnetism in MnFeGe. Besides, we also observed Griffiths phase (GP) like behavior above the Curie temperature, which is attributed to the presence of atomic disorder in the system. The presence of GP is further confirmed by the zero spontaneous magnetization. Our findings demonstrate that MnFeGe is one of the very few Mn-based systems with weak itinerant magnetic behavior.

Weak itinerant magnetic behaviour in Al substituted Ni 92 Cr 8 alloys

In the present work we report low temperature magnetic properties of Ni92−xAlxCr8 alloys. It is found that the Curie temperature and spontaneous magnetization (MSP) get suppressed around xC ≈ 7. From the analysis of the temperature and field dependent magnetization data we have obtained high value of Rhodes–Wohlfarth ratio, which indicates the weak itinerant ferromagnetic behavior for samples below xC. This is further supported by analysing the temperature variation of MSP in the framework of spin fluctuations, SCR theory. Furthermore, the Ginzburg-Landau formalism-based on spin fluctuation theory revealed that the contribution of spin fluctuations at low temperature increases as we approach xC.

Magnetic properties and critical behavior of the perovskite manganite La0.825Sr0.175MnO3

Polycrystalline La0.825Sr0.175MnO3 was prepared by solid state reaction method. It exhibits a ferromagnetic–paramagnetic phase transition at TC∼286 K. The intrinsic magnetic properties were clarified by the scaling theory and the isothermal magnetization measured around TC. Based on the modified Arrott plot, Kouvel–Fisher method and Widom relation, the critical exponents are determined to be β=0.308(7), γ=1.065(8), δ=4.457(17). Normalization of the critical exponents shows that the extended exchange interaction decays with J∼r−4.861, consistent with Tricritical mean field model. Rhodes–Wohlfarth ratio PC/MS>1 suggests that La0.825Sr0.175MnO3 is completely itinerant ferromagnet.

Fe-Rich Ferromagnetic Cleavable Van der Waals Telluride Fe5AsTe2

Transition metal-based layered compounds with van der Waals gaps between the structural layers are a rich source of magnetic materials for spintronic applications. Bulk crystals can be cleaved, providing high-quality two-dimensional nanomaterials, which are promising for the manipulation of spins in spintronic devices and low power quantum logic interfaces. The layered van der Waals telluride Fe5AsTe2 can be synthesized by the high-temperature reaction of elements. In the crystal structure, Fe-rich structural layers with the composition of Fe4.58(4)AsTe2 are separated by the van der Waals gaps with no atoms in the interstitial region. Crystal growth employing chemical vapor transport reactions yields bulk cleavable crystals, which exhibit weak inherent ferromagnetism below the Curie temperature of TC = 48 K. In the ordered state, the magnetization shows a dual-slope behavior in low magnetic fields, indicating the compensated or canted nature of magnetism. Magnetic susceptibility and magnetization measurements reveal perpendicular magnetic anisotropy. The large Rhodes-Wohlfarth ratio of 4.6 indicates the itinerant nature of ferromagnetism in Fe5AsTe2.

Magnetic and Critical Properties of Cr1/3NbS1.86 with TC = 56 K

The magnetic and critical properties of a TeBr‐transported CrNbS single crystal using magnetic susceptibility χ(T) and isothermal magnetization M(H) measurements are studied. A paramagnetic‐to‐ferromagnetic phase transition temperature (T C) is determined to be 56 K from χ(T) and an Arrott–Noakes’ plot. At high temperatures (T ≥ 85 K), χ(T) is well captured by the Curie–Weiss law and the effective magnetic moment is derived as P eff = 3.77 μ B per Cr. The signature of helical and chiral soliton spin structures is absent, as inferred from the (H, 2 K) data. The spontaneous magnetic moment P = 1.66 μ B per Cr compared with P eff, leading to the Rhodes–Wohlfarth ratio to be 1.75, is rather small, suggesting a slightly itinerant magnetism. However, the spin‐wave (SW) model describes the temperature‐dependent magnetization better than the single‐particle (SP) model. Further, analysis of the critical exponent yields β = 0.3852(1) and γ = 1.3575(4), which are close to the theoretical prediction of the 3D Heisenberg model and therefore suggests dominant short‐range ferromagnetic interactions. Finally, the deviation of the critical exponents from the theoretical model and the violation of Widom scaling law is discussed.

Non-Fermi liquid behavior and signature of Griffiths phase in Ni–Cr binary alloy

Detailed magnetic, electrical, and thermal property measurements have been carried out on Ni100−xCrx binary alloys, mainly to study the effect of Cr. The following points emerge from this study: with the increase in Cr concentration, magnetic moment and Curie temperature linearly decreased and the ferromagnetic order is completely suppressed at the critical concentration (xcr ≈ 12.16 ± 0.03). The Rhodes–Wohlfarth ratio increases as the concentration approaches xcr, suggesting a weak itinerant ferromagnetic character of NiCr compositions (x < xcr). Analysis of low-temperature electrical resistivity and specific heat data suggests that the spin fluctuation’s contribution increases and the Fermi-liquid behavior breaks down as the concentration approaches xcr. For x ∼ xcr, the dc susceptibility χ(T) deviates from the Curie–Weiss law reminiscent to that of the Griffiths phase. The low-temperature magnetic isotherms of Ni–Cr follow power law, M(H)=M0+dλHλ, and the non-universal exponent (λ) shows a minimum at xcr ∼ 12. Further, temperature dependence of magnetization studies also support the presence of the quantum Griffiths phase, similar to that reported in the Ni–V alloy system. The temperature dependencies of the electrical resistivity, magnetization, and specific heat follow the theoretical predictions of a quantum critical point within experimental uncertainties.

Magnetism and magnetocaloric effect in TlCo2−xNixS2 (0 x 0.05)

We have investigated the magnetism and magnetocaloric effect of TlCo2−xNixS2 (0 x 0.05). TlCo2S2 and TlCo1.97Ni0.03S2 are ferromagnetically ordered below 153 K and 142 K with saturated moment 0.82 /Co and 0.80 /Co, respectively, while TlCo1.95Ni0.05S2 shows ferromagnetic transition at 132 K. The magnetism displays slightly itinerant character, as inferred from the Rhodes-Wohlfarth ratio. An unusual historical dependence of magnetization in TlCo1.95Ni0.05S2 is consistently observed by scanning both the temperature and magnetic field. The isothermal magnetic entropy change ( S) of TlCo2S2 and TlCo1.95Ni0.05S2 are calculated by Maxwell relation and a temperature averaged entropy change is adopted to estimate the figure of merit for the compounds.

Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr(Co1−xNix)2As2 single crystals

Electron-doped Sr(Co{1-x}Ni{x})2As2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD), magnetic susceptibility chi(H,T), isothermal magnetization M(H,T), heat capacity Cp(H,T), and electrical resistivity ho(H,T) measurements versus applied magnetic field H and temperature T. The chi(T) data show that the crystals exhibit an antiferromagnetic (AFM) ground state almost immediately upon Ni doping on the Co site. Ab-initio electronic-structure calculations for x = 0 and x = 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The Curie-Weiss-like T dependence of \chi in the paramagnetic (PM) state indicates dominant ferromagnetic (FM) interactions. The small ordered moments ~0.1 muB per transition metal atom and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The Cp(T) at low T exhibits Fermi-liquid behavior for 0 < x < 0.15 whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low- T rho(H = 0,T) data show a T^2 dependence for 0 < x < 0.20 and a power-law dependence with n < 2 for x = 0.20 and 0.30. These low-T NFL behaviors observed in the Cp and rho measurements are most evident near the quantum-critical concentration x ~ 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.

“Entropics”: Science and engineering of caloric phenomena related to itinerant-electron magnetism and spin fluctuations

In this paper, the relationship between giant caloric phenomena and itinerant-electron magnetism is examined in order to construct of a newly proposed concept “Entropics”, which is a fusion of science and technology with the objective of solving and controlling entropic phenomena. An anomalous hall resistivity is present in the paramagnetic state of the La(Fe0.88Si0.12)13 magnetocaloric compound. Further, its coefficient exhibits a Curie-Weiss type temperature dependence, indicating the existence of disordered local moment, even though the Rhodes-Wohlfarth (RW) ratio reveals that the magnetic feature in the system is an itinerant-electron type. In addition, the correlation between the magnitude of the transition entropy change of the itinerant-electron metamagnetic transition and the RW ratio was observed. In the Mn3GaN barocaloric compound, the transition entropy of the first-order antiferromagnetic-paramagnetic phase transition marginally depends on the external pressure, in contrast to the data for Gd5Ge2Si2. The origin of this tendency is phase stability against the pressure, as opposed to large volume change at the transition temperature, which results in an enhancement of the barocaloric effect. The influence of topological frustration is also distinguished by comparing it with that of other Mn-based antiperovskite compounds.

Heisenberg-like ferromagnetism in3d−4fintermetallicLa0.75Pr0.25Co2P2with localized Co moments

The critical behavior near the continuous paramagnetic to ferromagnetic transition in a single crystal of ${\mathrm{La}}_{0.75}{\mathrm{Pr}}_{0.25}{\mathrm{Co}}_{2}{\mathrm{P}}_{2}$ has been determined based on high-resolution bulk magnetization data near ${T}_{C}$ \ensuremath{\sim} 167 K, where long-range order is established in the Co sublattice. Scaling equation of state analysis and the Kouvel-Fisher method under a moderate applied magnetic field yielded critical exponents ($\ensuremath{\beta}=0.3685\ifmmode\pm\else\textpm\fi{}0.0017$, $\ensuremath{\gamma}=1.3361\ifmmode\pm\else\textpm\fi{}0.0083$), consistent with the $d$ = 3, $n$ = 3 Heisenberg model of short-range interactions. Calculation of the Rhodes-Wohlfarth ratio confirmed that a localized rather than itinerant description of the $3d$ Co moments is appropriate in the ferromagnetic region of the sample. The critical susceptibility exponent \ensuremath{\gamma} was found to decrease systematically from the Heisenberg model value toward the mean-field model value as the maximum applied magnetic field considered in the analysis was increased above 2 T. The phenomenon is discussed in terms of mixed exchange mechanisms due to the coexistence of $3d$ and $4f$ magnetic sublattices and ordered clusters in the paramagnetic region.

Effect of Cr substitution on the magnetic and magnetic-transport properties of Fe2Mn1-xCrxSi alloys

Fe2Mn1-xCrxSi (x = 0, 0.1, and 0.2) alloys were investigated for their magnetic and transport properties in view of the expected half metallicity. It is found that Cr substitution suppresses the antiferromagnetic phase present in parent Fe2MnSi, which completely disappears for x = 0.2. Curie temperature of the alloys increases from 230 K to 299 K as x is increased from 0 to 0.2. The value of the Rhodes-Wohlfarth ratio indicates that the system shows iterant magnetism. Resistivity measurements also show absence of antiferromagnetic phase for x = 0.2. Resistivity data have been fitted by considering the electron-phonon and electron-magnon scattering contributions, which indicates the presence of half metallicity in these compounds. Temperature dependence of resistivity data shows magnetoresistance of −3% and −2.5% at Curie temperature with applied field of 50 kOe for x = 0.1 and 0.2, respectively.

Magnetic Properties of Layered Itinerant Electron Ferromagnet Fe3GeTe2

We have successfully synthesized a single-crystal sample of the layered structure ferromagnet Fe3GeTe2 and present the results of its complete characterization using powder X-ray diffraction (XRD), magnetic susceptibility χ(T), isotherm magnetization \(M(H)\), specific heat \(C(T)\), and electrical resistivity ρ(T) measurements. The rietveld refinement of the powder XRD data confirms that Fe3GeTe2 crystallizes in the space group \(p6_{3}/mmc\). The χ(T), \(C(T)\), and ρ(T) data indicate bulk ferromagnetic ordering at \(T_{\text{C}} = 220\) K with a saturation moment of 1.625 µB/Fe at zero temperature. The magnetic measurements of the external magnetic field \(H\) applied along the \(c\)-axis and \(ab\)-plane show a large anisotropy in this material. We found that the modified Arrott plot (\(M^{4}\)–\(H/M\)) rather than the \(M^{2}\)–\(H/M\) plot is obeyed in this material. The generalized Rhodes–Wohlfarth ratio indicates an itinerant magnetism in this material. The spin fluctuation parameters are estimate...

Magnetic behavior of layered perovskite Sr2CoO4 thin film

We have studied the magnetic properties of Sr2CoO4 (SCO) thin film. Magnetic hysteresis loops of the film divulge two-phase-like magnetic behavior consisting of hard and soft magnetic phases. Temperature dependent magnetization measurements and time evolution of thermo-remanent magnetization relaxation indicate the possibility of spin-glass dynamics in SCO. The Arrott plots (M2 vs. H/M) exhibit the convex behavior, while the M4 vs. H/M plots show the linear relation near Curie temperature signifying itinerant ferromagnetism in the material, consistent with the calculated Rhodes-Wohlfarth ratio for the film. It is suggested that SCO has electrons with localized character as well as itinerant character.

Magnetic properties in layeredACo2Se2(A =K, Rb, Cs) with the ThCr2Si2-type structure

The magnetic properties of ThCr${}_{2}$Si${}_{2}$-type single crystals $A$Co${}_{2}$Se${}_{2}$ ($A=\text{K}$, Rb, and Cs) were investigated by means of magnetic susceptibilities and isothermal magnetizations measurements. The ferromagnetic phase transition temperatures are $\ensuremath{\approx}$74 and 76 K for $A=\text{K}$ and Rb, respectively. A possible antiferromagnetic phase transition occurs around 80 K in susceptibility and a metamagnetismlike behavior is first discovered at a field of $\ensuremath{\approx}$3.5 T for $H\ensuremath{\parallel}ab$ plane in CsCo${}_{2}$Se${}_{2}$ below 62 K. The susceptibilities in the paramagnetic state obey the modified Curie-Weiss law quite well in all samples. Moreover, the derived effective magnetic moments of the Co atom are about 2.21, 2.04, and 2.04 ${\ensuremath{\mu}}_{B}$/Co for $A=\text{K}$, Rb, and Cs, respectively. The generalized Rhodes-Wohlfarth ratio indicates an itinerant magnetism in this series. Finally, we discussed the magnetic properties in this series within the frameworks of the self-consistent renormalization and Takahashi's theory of spin fluctuations.

Investigation of weak itinerant ferromagnetism and critical behavior of Y2Ni7

The weak itinerant ferromagnetic behavior of Y$_2$Ni$_7$ is investigated through magnetic, transport and calorimetric measurements. The low value of saturation moment, large Rhodes-Wohlfarth ratio, large value of the linear term in heat capacity and Fermi liquid like resistivity behavior with enhanced electron-electron scattering contribution establish firmly the weak itinerant ferromagetic nature of the sample. The critical exponents associated with the paramagnetic to ferromagnetic transition are also investigated from magnetization isotherms using modified Arrott plot, Kouvel-Fisher plot and critical isotherm technique. The more accurate Kouvel-Fisher plot provides the critical exponents to be $\beta$ = 0.306, $\gamma$ = 1.401 and $\delta$ = 5.578. These values are markedly different from the mean field values and correspond closely to the two dimensional Ising spin system with long range spin spin interaction.

Electrical and magnetic properties of the complete solid solution series between SrRuO3and LaRhO3: Fillingt2gversus tilting

A complete solid solution series between the t2g^4 perovskite ferromagnet SrRuO3 and the diamagnetic t2g^6 perovskite LaRhO3 has been prepared. The evolution with composition x in (SrRuO3)(1-x)(LaRhO3)(x) of the crystal structure and electrical and magnetic properties has been studied and is reported here. As x increases, the octahedral tilt angle gradually increases, along with the pseudocubic lattice parameter and unit cell volume. Electrical resistivity measurements reveal a compositionally driven metal to insulator transition between x = 0.1 and 0.2. Ferromagnetic ordering gives over to glassy magnetism for x > 0.3 and no magnetic ordering is found above 2 K for x > 0.5. M_sat and Theta_CW decrease with increasing x and remain constant after x = 0.5. The magnetism appears poised between localized and itinerant behavior, and becomes more localized with increasing x as evidenced by the evolution of the Rhodes-Wohlfarth ratio. mu_eff per Ru is equal to the quenched spin-only S value across the entire solid solution. Comparisons with Sr(1-x)Ca(x)RuO3 reinforce the important role of structural distortions in determining magnetic ground state. It is suggested that electrical transport and magnetic properties are not strongly coupled in this system.

Synthesis and Magnetic Properties of the Layered Compound Fe3GeTe2

Fe3GeTe2 was synthesized by solid state reaction and chemical transport methods. Our magnetic measurements show that it is a ferromagnet with the Curie temperature of 226 K. The Rhodes-Wohlfarth ratio Pc/Ps∼6.6 indicates that it is an itinerant ferromagnet. On the other hand, we found the single crystal sample with the Curie temperature of 146 K shows the large anisotropy in the temperature dependence of magnetic susceptibilities and the magnetization curves when the external magnetic fields are applied along the c axis and within the ab plane.