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Magnetic properties such as the magnetization, internal energy and specific heat for Heisenberg ferromagnet with spin - 1/2 on honeycomb lattice are have been calculated using Oguchi’s method. We have found that the magnetic specific heat exhibits two peaks.
We present a magneto-optical technique to spatially amplify and image fringe fields from single ferromagnetic nanorods. The fringe fields nucleate magnetic domains in a low-coercivity, perpendicularly magnetized indicator film, which are expanded by an applied out-of-plane field from the nanoscale to the microscale for measurement with polar Kerr microscopy. The nucleation location and therefore magnetic orientation of the sample nanorod are detected as spatially dependent field biases in locally measured hysteresis loops of the indicator film. We first discuss our method to fabricate the high-sensitivity indicator film with low energy argon ion irradiation. We then present a map of the amplified signal produced from a single nanorod as measured by the indicator film and compare it with a simultaneously obtained, unamplified fringe field map. The comparison demonstrates the advantage of the amplification mechanism and the capability of the technique to be performed with single-spot magneto-optical Kerr effect magnetometers. Our signal-to-noise ratio determines a minimum measureable particle diameter of tens of nanometers for typical transition metals. We finally use our method to obtain hysteresis loops from multiple nanorods in parallel. Our technique is unperturbed by applied in-plane fields for magnetic manipulation of nanoparticles, is robust against many common noise sources, and is applicable in a variety of test environments. We conclude with a discussion of the future optimization and application of our indicator film technique.
The effects of inhomogeneous spin dynamics on magnetic damping in ferromagnetic metals are studied. On the basis of linear response theory, we derive the microscopic expression for the Gilbert damping term in a two-dimensional electron gas interacting with the magnetization via exchange coupling in the presence of Rashba spin-orbit coupling (SOC). In the spin wave propagating with the wave vector, q, the behavior of q-dependent damping can be explained in terms of both inter- and intra-band spin excitations. The spatially dependent damping torques originating from Rashba SOC that cancel out in a uniform precession system distort the circular orbit of a magnetization-precession trajectory in the presence of inhomogeneous spin dynamics.
High resolution imaging and electron diffraction confirm that in the as-quenched state the structure of Fe{sub 79.7−x}Cr{sub x}Nb{sub 0.3}B{sub 20} (x = 11–13 at. %) melt-spun ribbons is completely amorphous, independent of the Cr content. Energy-dispersive X-ray spectroscopy mapping emphasizes clearly the presence of Fe and Cr clusters varying from approximately 1 to 2–3 nm in size with the increase of Cr content from 11 to 13 at. %. The Fe and Cr atoms segregate the atomic scale to form nanometer sized clusters, influencing strongly the macroscopic magnetic behavior. The Curie temperature of the system, T{sub C}{sup system}, confirmed by the magnetic susceptibility versus temperature measurements, gives the strength of the magnetic interactions between clusters. The inter-cluster interactions are much stronger for lower contents of Cr, the microstructure is less uniform, and T{sub C}{sup system} increases from 290 K for 13 at. % Cr to 330 K for 11.5 at. % Cr. The whole system transforms to a ferromagnetic state through interactions between the clusters. Zero-field cooling and field cooling curves confirm the cluster behavior with a blocking temperature, T{sub b}, of about 250 K. Above T{sub b}, the ribbons behave as a superferromagnetic system, whilst below the blocking temperature a classical ferromagnetic behavior is observed.
We report crystallographic and magnetic properties of layered iron oxypnictide CeFe{sub 1−x}Cr{sub x}PO (x = 0.000–0.692). Interlayer distances between Ce{sub 2}O{sub 2} and (Fe{sub 1−x}Cr{sub x}){sub 2}P{sub 2} layers increase as a function of x, suggesting suppression of Kondo coupling among hybridized conducting orbitals and localized Ce 4f orbitals. CeFe{sub 1−x}Cr{sub x}PO (x = 0.100–0.384) exhibits finite ferromagnetic transition temperatures (T{sub curie}) obtained by Arrott plots, although {sup 57}Fe Mössbauer spectra reveal paramagnetic Fe sublattice at T ≥ 4.2 K. These results indicate that the ferromagnetic phase transitions of samples are mainly due to Ce sublattice. For the samples with x ≥ 0.500, no ferromagnetic order is observed down to 2 K. These results verify that ferromagnetic quantum critical points of CeFe{sub 1−x}Cr{sub x}PO appear at 0.045 ≤ x ≤ 0.100 and 0.384 ≤ x ≤ 0.500.
The phase transformation and magnetostriction of bulk Fe{sub 73}Ga{sub 27} and Fe{sub 73}Ga{sub 18}Zn{sub 9} (at. %) ferromagnetic shape memory alloys (FSMs) were investigated by transmission electron microscopy (TEM), x-ray diffraction (XRD), and a magnetostrictive-meter setup. For the Fe{sub 73}Ga{sub 27} FSM alloy solution treated at 1100 °C for 4 h and quenched in ice brine, the antiphase boundary segments of the D0{sub 3} domain were observed in the A2 (disordered) matrix, and the Fe{sub 73}Ga{sub 27} FSM alloy had an optimal magnetostriction (λ{sub ‖}{sup s }= 71 × 10{sup −6} and λ{sub ⊥}{sup s }= −31 × 10{sup −6}). In Fe{sub 73}Ga{sub 27} FSM alloy as-quenched, aged at 700 °C for 24 h, and furnace cooled, D0{sub 3} nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., L1{sub 0}-like martensite) via Bain distortion, and finally L1{sub 2} (Fe{sub 3}Ga) structures precipitated, as observed by TEM and XRD. The L1{sub 0}-like martensite and L1{sub 2} phases in the aged Fe{sub 73}Ga{sub 27} FSM alloy drastically decreased the magnetostriction from positive to negative (λ{sub ‖}{sup s }= −20 × 10{sup −6} and λ{sub ⊥}{sup s }= −8 × 10{sup −6}). However, in Fe{sub 73}Ga{sub 18}Zn{sub 9} FSM alloy as-quenched and aged, the phase transformation of D0{sub 3} to an intermediate tetragonal martensite phase and precipitation of L1{sub 2} structures were not found. The results indicate that the aged Fe{sub 73}Ga{sub 18}Zn{sub 9} FSM alloy maintained stable magnetostriction (λ{sub ‖}{sup s }= 36 × 10{sup −6} and λ{sub ⊥}{sup s }= −31 × 10{sup −6}). Adding Zn can improve the ferromagnetic shape memory effect of aged Fe{sub 73}Ga{sub 18}Zn{sub 9} alloy, which may be useful in application of the alloy in high temperature environments.
The structural, electronic and elastic properties of alkali- metal nitride (KN) is investigated by the first principles calculations based on density functional theory as implemented in Vienna ab-initio simulation package. At ambient pressure KN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that the KN is half metallic ferromagnet at normal pressure. A pressure-induced structural phase transition from NaCl to ZB phase is observed in KN. Half metallicity and ferromagnetism is maintained at all pressures.
The structural, electronic, elastic and magnetic properties of the perovskite structure of RuFe{sub 3}N, and OsFe{sub 3}N have been reported using the VASP within the gradient generalized approximation. Total energy calculations are performed using both spin and non-spin polarized calculations and it is found that, at ambient pressure both RuFe{sub 3}N and OsFe{sub 3}N are stable in ferromagnetic phase. The electronic structure reveals that both RuFe{sub 3}N and OsFe{sub 3}N are metallic in nature at ambient pressure.
We report on critical behavior in La{sub 1-x}Bi{sub x}MnO{sub 3} (x=0.4 & 0.6) perovskite manganite oxide by studying AC susceptibility, χ, measured at randomly chosen low magnetic fields (H=9.45 Oe to 0.95 Oe) at fixed frequency of f=131 Hz. The samples exhibit non-uniformity in their magnetic spin interactions around paramagnetic-to-ferromagnetic transition temperature (T{sub C}). By non-universal low-field magnetic scaling of AC χ given as, χ{sup −1}-χ{sup −1}(T{sub C}) ∼ (T/T{sub C}−1){sup γ} ≡ τ{sup γ} the magnetic interactions in the samples around T{sub C} has been studied. The linear fit to ln (χ{sup −1} - χ{sub C}{sup −1}) vs. ln (T/T{sub C}-1) gives the values of critical exponent ‘γ’ under applied magnetic fields. The values of γ is found to be ∼1.48±0.007 for the sample with x=0.4 and ∼1.36±0.004 for x=0.6 at 0.95 Oe lie in the context of 3D Heisenberg type spin interactions. The profound 3D Heisenberg type spin interactions arises due to the phase separation effect for x=0.4 and breaking of ferromagnetic order and induced Bi{sup 3+} (6S{sup 2}) − O{sup 2−} (2P{sup 6}) lattice distortions for x=0.6 as discussed in manuscript.
First five harmonics of the ac magnetic response were measuredas functions of temperature at different frequencies and with or without superposed dc magnetic field on Cr{sub 70}Fe{sub 30} thin films. An elaborate analysis of the linear susceptibility data reveals that (i) a well-defined paramagnetic (PM) to ferromagnetic (FM) phase transition, characterized by three-dimensional Heisenberg critical exponents, occurs for film thickness down to t≃ 90 nm, and (ii) the film with t≃40 nm exhibits experimental signatures in the critical region that are characteristic of both ferromagnets and spin glasses while the spin glass behavior dominates the FM one when t≃ 20 nm.
In this paper, I have presented the spin-dependent momentum density of ferromagnetic dysprosium using spin polarized relativistic Korringa-Kohn-Rostoker method. A fully relativistic approach has been used to determine the magnetic Compton profile. The density of state in term of majority-spin and minority-spin of Dy also calculated using SPR-KKR. The magnetic Compton profile discussed in term of 4f and diffused electrons.
Using first principles density functional theory (DFT) calculations within GGA and GGA+U approximations we studied both ferromagnetic (FM) and antiferromagnetic (AFM) phases of zircon type RCrO{sub 4} (R= Dy, Ho, Gd) oxides. We estimated and compared the 3d-4f exchange interaction strengths J between the nearest neighbor R{sup 3+} and Cr{sup 5+} ions for R=Dy, Gd. Our results predict that DyCrO{sub 4}, GdCrO{sub 4} and HoCrO{sub 4} have ferromagnetic ground state which is consistent with experimental observations.
Polycrystalline CdCr{sub 2}Se{sub 4} spinels with non-magnetic indium substitution were investigated with an aim to study the influence of non-magnetic substitution at the Cd site on its structural and magnetic properties. We carried out powder XRD and magnetic measurements on three compositions: Cd{sub 1-x}In{sub x}Cr{sub 2}Se{sub 4} (x=0, 0.05, 0.1). The XRD patterns were analyzed by carrying out Rietveld analysis and structural parameters were estimated. Phase purity of the compounds using reitveld refinement technique where confirming the indium distribution at tetrahedral site of the cubic close-packed sublattice formed by the selenium ions. The chemical compositions of these compounds were determined by energy dispersive x-ray analysis. Compared to parent compound, CdCr{sub 2}Se{sub 4}, the ferromagnetic Curie temperature T{sub C} was found to decrease from 130K to 123K and 114K as x varied from 0 to 0.05 and 0.1 respectively, whereas the saturation magnetization increases in Cd{sub 0.95}In{sub 0.05}Cr{sub 2}Se{sub 4} and again decreases in Cd{sub 0.9}In{sub 0.1}Cr{sub 2}Se{sub 4}.
The Exchange bias phenomenon and methods to manipulate the bias field in a controlled manner are thrust areas in magnetism due to its sophisticated theoretical concepts as well as advanced technological utility in the field of spintronics. The Exchange bias effect is observed as a result of ferromagnetic-antiferromagnetic (FM-AFM) exchange interaction, usually observed as a loop shift on field cooling below the Neel temperature of AFM. In the present study, we have chosen zinc ferrite which is a well known antiferromagnet, and FeNiMoB based metallic glass as the ferromagnet. The films were prepared by RF sputtering technique. The thickness and composition was obtained by RBS. The magnetic studies using SQUID VSM indicate exchange bias effect in the system. The effect of thermal annealing on exchange bias effect was studied. The observed exchange bias in the zinc ferrite-FeNiMoB system is not due to FM-AFM coupling but due to spin glass-ferromagnetic interaction.
We report the influence of magnetic field on I-V characteristics of α-Fe{sub 1.64}Ga{sub 0.36}O{sub 3} sample. Synchrotron X-ray diffraction pattern and Raman Spectroscopy have confirmed rhombohedral structure with space group R3C in the sample. The sample exhibits ferromagnetic feature at room temperature and non saturation of magnetization up to 7Tesla suggests the effect of non-collinear structure (canting) of the spins on the ferromagnetic properties. We have recorded I-V characteristics of the sample under magnetic field to study the effect of non-collinear spin structure on the electrical properties. Space charge limited current mechanism controlled the nature of non-linear I-V curves and the curves are significantly affected by magnetic field.
We consider a thermally stable, metallic nanoscale ferromagnet (FM) subject to spin-polarized current injection and exchange coupling from the spin-helically locked surface states of a topological insulator (TI) to evaluate possible non-volatile memory applications. We consider parallel transport in the TI and the metallic FM, and focus on the efficiency of magnetization switching as a function of transport between the TI and the FM. Transport is modeled as diffusive in the TI beneath the FM, consistent with the mobility in the TI at room temperature, and in the FM, which essentially serves as a constant potential region albeit spin-dependent except in the low conductivity, diffusive limit. Thus, it can be captured by drift-diffusion simulation, which allows for ready interpretation of the results. We calculate switching time and energy consumed per write operation using self-consistent transport, spin-transfer-torque (STT), and magnetization dynamics calculations. Calculated switching energies and times compare favorably to conventional spin-torque memory schemes for substantial interlayer conductivity. Nevertheless, we find that shunting of current from the TI to a metallic nanomagnet can substantially limit efficiency. Exacerbating the problem, STT from the TI effectively increases the TI resistivity. We show that for optimum performance, the sheet resistivity of the FM layer should be comparable to or larger than that of the TI surface layer. Thus, the effective conductivity of the FM layer becomes a critical design consideration for TI-based non-volatile memory.
We report ferromagnetic-like properties and exchange bias effect in akaganeite (β-FeOOH) nanorods. They exhibit a Néel temperature T{sub N} = 259 K and ferromagnetic-like hysteresis behavior both below and above T{sub N}. An exchange bias effect is observed below T{sub N} and represents an interesting behavior for akaganeite nanorods. These results are explained on the basis of a core-shell structure in which the core has bulk akaganeite magnetic properties (i.e., antiferromagnetic ordering) while the shell exhibits a disordered spin state. Thus, the nanorods show ferromagnetic properties and an exchange bias effect at the same time, increasing their potential for use in practical applications.
Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.