Value Of Exchange Field Research Articles

Influence of polarity compensation on exchange bias field in LaMnO<sub>3</sub>/LaNiO<sub>3</sub> superlattices

Perovskite superlattices have received enormous attention in recent years, for they possess several new phases of quantum matter. In particular, an unexpected exchange bias effect in (111)-oriented superlattices composed of ferromagnetic LaMnO<sub>3</sub> and paramagnetic LaNiO<sub>3</sub> is observed, which has aroused broad interest. In this work, three kinds of LaMnO<sub>3</sub>/LaNiO<sub>3</sub> superlattices with (001), (110), and (111) out-of-plane orientation are fabricated by pulsed laser deposition, and also studied systemically. It is found that the superlattices are epitaxially grown on the SrTiO<sub>3</sub> substrates without strain relaxation. The superlattices have a monolayer terraced structure with a surface roughness below 0.1 nm. Electrical transport measurements reveal a Mott conducting behavior with strong localization of electrons in the superlattices. All the superlattices with different orientations exhibit exchange bias phenomenon. The field cooling and zero field cooling curves indicate that there are two different magnetic components in the superlattice in a low temperature range. Further analysis of the values of exchange field reveals that the exchange bias field is related to the orientation and polarity of the superlattices. Different superlattices form different charged planes stacked along out-of-plane orientation, leading to a polarity match/mismatch at the interface between the superlattices and substrates. The surface reconstructions that act as compensating for the polar mismatch influence the exchange bias field of the superlattices. It is observed that the intensities of the exchange field of the polar-matched superlattices are higher than those of the polar-mismatched superlattices at different temperatures. These results are helpful in further understanding the magnetoelectric transport properties in the perovskite superlattices.

Exchange Bias Effect Concerned with Tunneling Magnetoresistance in ${\hbox{Sm}}_{0.35} {\hbox{Pr}}_{0.15}{\hbox{Sr}}_{0.5}{\hbox{MnO}}_3$ Phase Separated Manganites

In this paper, we report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</sub> Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> manganite. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis, a shift is also observed in the MR-H curve when the sample is cooled in a static dc magnetic field at 5 K. The values of exchange fields (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ), coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ), remanence asymmetry (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ) and magnetic coercivity (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) are found to strongly depend on cooling magnetic field and temperature. Magnetic training effect (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ) is supportive experimental evidence which describes the decrease of exchange bias field when sample is successively field-cycled at a particular temperature. We have also observed T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> in the shift of the MR-H curve which has been interpreted by the spin relaxation model. We have observed the time dependent resistivity below the spin freezing temperature. The unusual MR behavior is interpreted in terms of the intragranular interface effect between short-range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect, which is of special interest for potential application in multifunctional spintronic devices.

High Chern number quantum anomalous Hall phases in single-layer graphene with Haldane orbital coupling

We investigate possible phase transitions among the different quantum anomalous Hall (QAH) phases in a zigzag graphene ribbon under the influence of the exchange field. The effective tight-binding Hamiltonian for graphene is made up of the hopping term, the Kane-Mele and Rashba spin-orbit couplings as well as the Haldane orbital term. We find that the variation of the exchange field results in bulk gap-closing phenomena and phase transitions occur in the graphene system. If the Haldane orbital coupling is absent, the phase transition between the chiral (anti-chiral) edge state $\nu=+2$ ($\nu=-2$) and the pseudo-quantum spin Hall state ($\nu=0$) takes place. Surprisingly, when the Haldane orbital coupling is taken into account, an intermediate QSH phase with two additional edge modes appears in between phases $\nu=+2$ and $\nu=-2$. This intermediate phase is therefore either the hyper-chiral edge state of high Chern number $\nu=+4$ or anti-hyper-chiral edge state of $\nu=-4$ when the direction of exchange field is reversed. We present the band structures, edge state wave functions and current distributions of the different QAH phases in the system. We also report the critical exchange field values for the QAH phase transitions.

Surface spin glass and exchange bias effect in Sm0.5Ca0.5MnO3 manganites nano particles

In this letter, we report that the charge/orbital order state of bulk antiferromagnetic Sm0.5Ca0.5MnO3 is suppressed and confirms the appearance of weak ferromagnetism below 65 K followed by a low temperature spin glass like transition at 41 K in its nano metric counterpart. Exchange anisotropy effect has been observed in the nano manganites and can be tuned by the strength of the cooling magnetic field (Hcool). The values of exchange fields (HE), coercivity (HC), remanence asymmetry (ME) and magnetic coercivity (MC) are found to strongly depend on cooling magnetic field and temperature. HE increases with increasing Hcool but for larger Hcool, HE tends to decrease due to the growth of ferromagnetic cluster size. Magnetic training effect has also been observed and it has been analyzed thoroughly using spin relaxation model. A proposed phenomenological core-shell type model is attributed to an exchange coupling between the spin-glass like shell (surrounding) and antiferromagnetic core of Sm0.5Ca0.5MnO3 nano manganites mainly on the basis of uncompensated surface spins. Results suggest that the intrinsic phase inhomogeneity due to the surface effects of the nanostructured manganites may cause exchange anisotropy, which is of special interests for potential application in multifunctional spintronic devices.

Structural characterization of MnNi and MnPt antiferromagnetic materials for spintronic applications

Abstract MnNi and MnPt become antiferromagnetic (AFM) when annealed at high enough temperatures, for which a face-centered cubic (fcc) to face-centered tetragonal (fct) structural transition occurs. Thus, structural information is essential when optimizing exchange bias in these materials. We performed structural (X-ray diffraction), magnetic (Magneto-Optical Kerr effect) and magneto-transport (magnetoresistance) measurements on MnNi based spin valves subjected to different annealing procedures and MnPt/CoFe bilayers with different MnPt thicknesses. We show that high annealing temperatures can in fact lead to the fct antiferromagnetic phase in both materials, but the annealing procedure is also an important factor on the obtained exchange field values. Also, MnPt/CoFe bilayers allowed us to probe the influence of average AFM-grain size on exchange bias.

Evidence of intrinsic exchange bias and its origin in spin-glass-like disorderedL0.5Sr0.5MnO3manganites (L=Y,Y0.5Sm0.5, andY0.5La0.5)

Exchange-bias (EB) phenomena have been observed in the ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($L=\mathrm{Y}$, ${\mathrm{Y}}_{0.5}{\mathrm{Sm}}_{0.5}$, and ${\mathrm{Y}}_{0.5}{\mathrm{La}}_{0.5}$)-type manganites showing cluster-glass-like and spin-glass-like behavior. The field cooled magnetic hysteresis loops exhibit shifts both in the field and magnetization axes. The values of exchange field $({H}_{E})$, coercivity $({H}_{C})$, remanence asymmetry $({M}_{E})$, and magnetic coercivity $({M}_{C})$ of ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ are found to depend strongly on temperature, measuring field, as well as strength of cooling field ${H}_{\mathrm{cool}}$. ${H}_{E}$ increases sharply with the magnitude of ${H}_{\mathrm{cool}}$ $(\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, but for larger ${H}_{\mathrm{cool}}$ $(g3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, it decreases due to the growth of the ferromagnetic cluster size. This observed behavior has been explained in terms of interfacial exchange coupling between coexisting ferromagnetic cluster glass and the disordered spin-glass-like phases, where the spin configurations are strongly affected by the cooling field strength. Below the spin and/or cluster glass freezing temperature $({T}_{f})$, both ${H}_{E}$ and ${M}_{E}$ decrease exponentially with temperature. The value of ${H}_{C}$ increases almost exponentially with increasing magnetic-field step size $(\ensuremath{\Delta}H)$. In addition, the observed training effect of the EB behavior has been explained well in terms of the existing relaxation model developed for other classical EB systems. In view of the use of manganites in spintronics, the present observation of EB-like shift even in the mixed valent polycrystalline manganites such as ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ is of paramount importance.

Numerical simulation of magnetic interactions in polycrystalline YFeO 3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO 3 sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M( H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field ( H E=5590 kOe), anisotropy field ( H A=0.5 kOe) and Dzyaloshinsky–Moriya antisymmetric field ( H D=149 kOe) are in good agreement with previous reports on this system.

Measurement of the energy barrier distribution in the antiferromagnetic layer of exchange-biased materials

The value of exchange field of two FeMn-NiFeCo-based spin valves with varying thickness of the pinned ferromagnetic layer has been determined as a function of temperature. The complexities caused by thermal activation of the antiferromagnet during measurement have been overcome by the development of a measurement protocol. The values of the exchange field obtained provide a measure of the degree of order in the antiferromagnet. Thus it is possible to determine the distribution of energy barriers to reversal for the system. We find that for a 110-/spl Aring/-thick pinned NiFeCo layer a broad distribution exists, whereas for an 80-/spl Aring/ layer, the distribution is bimodal and has a component subject to thermal activation at temperatures down to 260 K.

Effects of structure and ion irradiation on the exchange field of NiFe/NiMn

NiFe/NiMn exchange field was manipulated by an underlayer NiFeCr or by C ion irradiation. Underlayer NiFeCr significantly promoted (1 1 1) texture growth, leading to a higher exchange field. However, the threshold annealing temperature required to reach a large value of exchange field was raised. This can be attributed to well aligned orientations between NiFe and NiMn at the interface, which stabilizes the FCC γ phase of NiMn and increases the nucleation energy. C ion irradiation with the dosage of 3.7×10 15/cm 2 prior to the annealing increases the exchange field by 21% without altering the blocking temperature (>400°C). The enhancement of the exchange field by irradiation can be related to the increase of defect sites in NiMn or (and) to the new phase NiMn(C) formation having c/a ratio significantly less than NiMn.

Crystal field analysis of magnetization curves for aligned (Pr,Nd) 2Fe 14B

The high-field magnetization curves for a series of aligned (Nd 1− x Pr x ) 2Fe 14B were calculated on the basis of the single-ion model. The values of exchange field and crystalline-electric-field parameters, B ex and A nm , in the calculations are obtained from the best fit of the calculations to the experiments for the single crystals of R 2Fe 14B (R=Nd and Pr). The magnetization curves of aligned (Nd 1− x Pr x ) 2Fe 14B ( x=0.75, 0.5 and 0.25) were measured. The calculations reproduce the above and other experiments well. The curve along the direction perpendicular to the aligned direction depends on A n0 ( n=2, 4 and 6) sensitively, which suggests that the values of the parameters could be obtained from fit of the magnetization curve for the aligned compound.

Study of exchange anisotropy in NiFe/NiMn and NiFe/IrMn exchange coupled films

Exchange anisotropy in NiFe/NiMn and NiFe/IrMn exchange coupled films was studied as a function of temperature using vibrating sample magnetometry. The exchange field was measured using three different methods: (1) as a shift of the hysteresis loop measured in an external field applied parallel to the exchange field direction; (2) calculated from the initial susceptibility in the field applied perpendicular to the exchange field; and (3) calculated from the shift of minor reversible hysteresis loops measured in external fields applied in a few different directions close to the perpendicular to the exchange field. The values of the exchange field in NiFe/NiMn samples measured using methods 2 and 3 were similar and approximately twice as high as the values measured using method 1. For the NiFe/IrMn samples methods 2 and 3 gave exchange field values slightly exceeding the values obtained using method 1. The results are explained using a model in which it is assumed that the interfacial interactions between antiferromagnetic and ferromagnetic layers induce unidirectional and uniaxial anisotropy in the ferromagnetic layer. The temperature dependence of induced interfacial uniaxial anisotropy was calculated from the experimental data. For the NiFe/NiMn samples, the temperature dependence of the induced uniaxial anisotropy was significantly different from that of the unidirectional anisotropy.

Ion beam sputtered spin-valve films with improved giant magnetoresistance response

A large value of giant magnetoresistance ΔR/R=9% with an exchange field Hex=350 Oe has been measured from simple NiFe/CoFe/Cu/CoFe/IrMn top spin-valve films prepared by ion beam deposition (IBD) techniques. The exchange biasing was greatly enhanced when a synthetic pinned layer, CoFe/Ru/CoFe, is used in the spin-valve structures. Apparent exchange field values in excess of 2000 Oe and ΔR/R values above 8% have been obtained in synthetic spin-valve films. These IBD spin-valve films show excellent thermal stability and they are suitable for the applications in high density magnetic recording heads.

A RKKY Model Fitting of Nuclear Magnetic Resonance Spectra of 63Cu in Fe/Cu Multilayers

In this letter, a theoretical fit of nuclear magnetic resonance spectra of 63Cu in Cu layers of Fe/Cu Multilayers is given based on RKKY theory. The values of exchange field and additional Knight shift are fitted, which agree approximately with the experimental data.

Exchange interaction and magnetic anisotropy in Nd 2(Fe 13M)B compounds (M = Ga, Si, Al)

Static and pulse magnetic field measurements were performed to determine the spin reorientation temperature ( T sr) and the magnetic anisotropy field ( H a) of Nd 2 (Fe 13M)B compounds (M = Ga, Si, Al). The changes in T sr in the wake of Fe-substitution is attributed to the changes of the exchange field which could be explained satisfactorily based on the CEF theory by taking account of CEF parameters up to sixth order. Estimation of the value of exchange field (2μ B H ex) was made by fitting calculated data with experimental ones of T sr on the assumption that CEF parameters remain unchanged with small amount of substitution for Fe in Nd 2Fe 14B. The easy c-axis stability energy of Nd ion was calculated based on second-order crystal field model. It is thought that all substitutions for Fe in Nd 2Fe 14B lead to lowering of its T sr due to the decrease of exchange field which in turn results in deterioration of intrinsic uniaxial ferromagnetic thermostability of substituted compounds.

Optical absorption spectra of MnCl2.2H2O. I

The relative intensities and the energies of the electric dipole absorption spectra of MnCl2.2H2O (V-lines in the 27000 cm-1 region) are investigated as functions of temperature with polarised light. Their magnetic field dependence is also studied both in paramagnetic and ordered state. It is confirmed from the angular dependence of Zeeman patterns in the b plane that the principal axes x', y' and z of g-values in the excited states are obtained by rotating the crystallographic axes c, a' and b by pi /4 around b(z). The preferred spin axis in the spin-flop region is also found to coincide with the a (or a') axis. The values of exchange fields in the ground and excited state are determined from the observed exchange splittings and g-values. Experimental results suggest that the observed line spectra are all magnon sidebands.