Showing papers on "Magnetic anisotropy published in 2002"

Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase

Abstract: Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[110]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.

Direct Observation of Internal Spin Structure of Magnetic Vortex Cores

Abstract: Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal spin structure of magnetic vortex cores. Using tips coated with a layer of antiferromagnetic chromium, we obtained images of the curling in-plane magnetization around and of the out-of-plane magnetization inside the core region. The experimental data are compared with micromagnetic simulations. The results confirm theoretical predictions that the size and the shape of the vortex core as well as its magnetic field dependence are governed by only two material parameters, the exchange stiffness and the saturation magnetization that determines the stray field energy.

Ferromagnetism in one-dimensional monatomic metal chains.

Abstract: Two-dimensional systems, such as ultrathin epitaxial films and superlattices, display magnetic properties distinct from bulk materials. A challenging aim of current research in magnetism is to explore structures of still lower dimensionality. As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important. Spin lattice models predict that an infinite one-dimensional linear chain with short-range magnetic interactions spontaneously breaks up into segments with different orientation of the magnetization, thereby prohibiting long-range ferromagnetic order at a finite temperature. These models, however, do not take into account kinetic barriers to reaching equilibrium or interactions with the substrates that support the one-dimensional nanostructures. Here we demonstrate the existence of both short- and long-range ferromagnetic order for one-dimensional monatomic chains of Co constructed on a Pt substrate. We find evidence that the monatomic chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which, below a threshold temperature, evolve into a ferromagnetic long-range-ordered state owing to the presence of anisotropy barriers. The Co chains are characterized by large localized orbital moments and correspondingly large magnetic anisotropy energies compared to two-dimensional films and bulk Co.

Ferromagnetic semiconductors

Abstract: The current status and prospects of research on ferromagnetism in semiconductors are reviewed The question of the origin of ferromagnetism in europium chalcogenides, chromium spinels and, particularly, in diluted magnetic semiconductors is addressed The nature of electronic states derived from 3d of magnetic impurities is discussed in some details Results of a quantitative comparison between experimental and theoretical results, notably for Mn-based III-V and II-VI compounds, are presented This comparison demonstrates that the current theory of the exchange interactions mediated by holes in the valence band describes correctly the values of Curie temperatures T_C magnetic anisotropy, domain structure, and magnetic circular dichroism On this basis, chemical trends are examined and show to lead to the prediction of semiconductor systems with T_C that may exceed room temperature, an expectation that are being confirmed by recent findings Results for materials containing magnetic ions other than Mn are also presented emphasizing that the double exchange involving hoping through d states may operate in those systems

All-optical probe of coherent spin waves.

Abstract: A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.

Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory

Abstract: We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% w...

A Cyano-Bridged Single-Molecule Magnet: Slow Magnetic Relaxation in a Trigonal Prismatic MnMo6(CN)18 Cluster

Abstract: We report the synthesis of the first well-documented example of a cyano-bridged single-molecule magnet. An assembly reaction parallel to that employed in producing the trigonal prismatic [(Me3tacn)6MnCr6(CN)18]2+ (Me3tacn = N,N‘,N‘‘-trimethyl-1,4,7-triazacyclononane) cluster affords K[(Me3tacn)6MnMo6(CN)18](ClO4)3 (1), containing an analogous molybdenum(III)-substituted cluster. Fits to the DC magnetic susceptibility and magnetization data for 1 show that the MnMo6 cluster possesses weak antiferromagnetic coupling (J = −6.7 cm-1), leading to an S = 13/2 ground state with significantly enhanced magnetic anisotropy (D = −0.33 cm-1 and E = −0.018 cm-1). Consistent with these results, AC magnetic susceptibility measurements show the molecule to exhibit slow magnetic relaxation indicative of a single-molecule magnet with an energy barrier of 10 cm-1 for spin reversal.

Atom Transfer Radical Polymerization Synthesis and Magnetic Characterization of MnFe2O4/Polystyrene Core/Shell Nanoparticles

Abstract: An atom transfer radical polymerization route is developed for the coating of MnFe2O4 nanoparticles with polystyrene yielding the core−shell nanoparticles with size <15 nm. Magnetic studies show a decrease in coercivity after the formation of polystyrene shell, which is considered due to the reduction of magnetic surface anisotropy upon polymer coating. The MnFe2O4 nanoparticles as the magnetic core were separately prepared by a reverse micelle microemulsion method. Polymerization initiators are chemically attached onto the surface of nanoparticles. The modified nanoparticles are then used as macro-initiators in the subsequent polymerization reaction. This approach provides great flexibility in the selection of magnetic core. Consequently, magnetic tunability is able to be introduced into these core/shell nanoparticulate systems to achieve the desired superparamagnetic response.

Magnetic-phase transitions and magnetocaloric effects

Abstract: We have studied the magnetocaloric properties of a variety of compounds, like Gd5(Si1−xGex)4 with x=0.576 and 0.5875, MnFeP1−xAsx with x between 0.25 and 0.65, RTiGe with R=Tb, Dy, Ho, Er and Tm, Ni53Mn22Ga25, Mn5Si3, and Mn1.95Cr0.05Sb. These compounds have in common that they exhibit either temperature- or field-induced first-order magnetic-phase transitions. Gd5(Si1−xGex)4 exhibits simultaneously a magnetic and a structural transition, which is accompanied by a huge magnetic-entropy change. A temperature-induced ferromagnetic (FM) to paramagnetic (PM) transition and a magnetic-field-induced PM to FM transition which are both of first order are observed in MnFeP1−xAsx compounds. Here the magnetic-phase transitions are not accompanied by structural transitions. Nevertheless, a large magnetic-entropy change, comparable with that in Gd5(Si1−xGex)4, is observed in the MnFeP1−xAsx compounds. In several of the RTiGe compounds, an applied magnetic field induces an antiferromagnetic (AF) to FM phase transition. Here, we observed a magnetic anisotropy dependence of the magnetic-entropy change. The Heusler alloy Ni53Mn22Ga25 exhibits a first-order martensitic transformation accompanied by a magnetic-phase transition around 220 K. The magnitude and the shape of the magnetic-entropy changes observed for this compound are quite different. Mn5Si3 compound exhibits two successive first-order magnetic-phase transitions and shows a different type of magnetocaloric effect (MCE). Mn1.95Cr0.05Sb exhibits an AF to FM phase transition and a negative MCE. The relationship between the magnetic-phase transitions and the MCE is discussed, based on the comparison of the observed MCEs and the exchange interactions in these materials.

Preparation and magnetic properties of highly coercive FePt films

Abstract: The magnetization processes of highly ordered FePt(001) films with large perpendicular magnetic anisotropy have been studied. The film morphology was controlled from isolated particles to continuous film by varying the nominal thickness (tN) of the FePt film sputter deposited directly on a MgO(001) substrate at an elevated temperature. A drastic change in the coercivity by one order of magnitude has been found at the critical thickness (tN=45 nm) where the film morphology changes from a particulate to a continuous state. A huge coercivity exceeding 40 kOe has been achieved in the film with tN=10 nm, which comprises single domain particles with an average lateral size of approximately 50 nm.

Low-temperature fabrication of L10 ordered FePt alloy by alternate monatomic layer deposition

Abstract: L10 ordered FePt alloy films with large magnetic anisotropy have been successfully prepared by alternating Fe(001) and Pt(001) monatomic layers on MgO (001) substrates at low temperatures below 230 °C. In addition to the fundamental (002) peak, (001) and (003) superlattice peaks have clearly been observed in the x-ray diffraction patterns for all the samples, indicating the formation of L10 ordered structure. The magnetization measurements show that all the samples are perpendicularly magnetized. Large uniaxial magnetic anisotropy (Ku=3.0×107 erg/cc) and high chemical ordering (long-range order parameter S=0.7±0.1) have been obtained even at the substrate temperature Ts=200 °C. The magnetization curves show good magnetic squareness (Mr/Ms∼0.9) for the samples grown at Ts⩾160 °C.

Temperature dependent magnetic properties of highly chemically ordered Fe55−xNixPt45L10 films

Abstract: Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, KU, combined with a thermal assist to overcome write field limitations have been proposed as one of the potential technologies to extend the areal density of magnetic disk recording beyond the limitations of current technology. Here we present an investigation on structural and temperature dependent magnetic properties of chemically ordered epitaxial Fe55−xNixPt45 thin films. Increasing Ni additions result in a steady reduction of magneto-crystalline anisotropy, saturation magnetization, and Curie temperature. The ability to control thermomagnetic properties over a wide range makes Fe55−xNixPt45 and similar FePt-based pseudo-binary alloys attractive base materials for media applications in thermally assisted magnetic recording.

Cobalt single-molecule magnet

Abstract: A cobalt molecule that functions as a single-molecule magnet, [Co4(hmp)4(MeOH)4Cl4], where hmp− is the anion of hydroxymethylpyridine, is reported. The core of the molecule consists of four Co(II) cations and four hmp− oxygen atom ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DSz2) at each cobalt ion are the origin of the negative magnetoanisotropy. A single crystal of the compound was studied by means of a micro-superconducting quantum interference device magnetometer in the range of 0.040–1.0 K. Hysteresis was found in the magnetization versus magnetic field response of this single crystal.

Structure and magnetic properties of ferromagnetic nanowires in self-assembled arrays

Abstract: Static and dynamic aspects of the magnetization reversal in nanowire arrays are investigated. The arrays have been produced by electrodeposition of ferromagnetic metals (Fe, Co, and Ni) into porous anodic alumina templates, with diameters as small as 5 nm. The crystal structures of the nanowires are bcc (Fe) and fcc (Ni) and a mixture of fcc and hcp (Co), with grain sizes of a few nanometers. Magnetic properties as a function of temperature are investigated. The temperature dependence of coercivity can be understood in terms of thermal activation over an energy barrier with a $\frac{3}{2}$-power dependence on the field. Coercivity as a function of diameter reveals a change of the magnetization reversal mechanism from localized quasicoherent nucleation for small diameters to a localized curlinglike nucleation as the diameter exceeds a critical value determined by the exchange length. The quasicoherent limit is described by a model that yields explicitly real-structure-dependent expressions for coercivity, localization length, and activation volume.

Empirical mapping of Ni–Mn–Ga properties with composition and valence electron concentration

Abstract: A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K

Superparamagnetism and magnetic properties of Ni nanoparticles embedded in SiO 2

Abstract: We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a SiO2 amorphous matrix A modified sol-gel method was employed which resulted in Ni particles with average radius ;3 nm, as inferred by TEM analysis Above the blocking temperature TB’20 K for the most diluted sample, magnetization data show the expected scaling of the M/ MS vs H/T curves for superparamagnetic particles The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer For T,TB the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation HC(T)5HC0@12(T/TB) 1/2 # below TB with HC0;780 Oe The obtained values of HC0, considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles

Crystal structures and magnetic anisotropy properties of Ni-Mn-Ga martensitic phases with giant magnetic-field-induced strain

Abstract: Summary form only given. Magnetic shape memory materials are expected to have potential for a variety of actuating devices and sensors. Magnetic-field-induced rearrangement of the crystallographic domains (twin variants) can produce a large strain similar to a stress-induced one. We have found a giant magnetic field-induced strain approximately 10% at ambient temperature in a magnetic field less then 1 T in NiMnGa seven-layered martensitic phase. The strain is contributed by twin boundary motion which was confirmed by different experimental methods. From the analysis of X-ray diffraction data it was found that crystal structure of this phase is nearly orthorhombic having lattice parameters at ambient temperature a=0.619 nm, b=0.580 nm and c=0.553 nm (in cubic parent phase coordinates). The magnetic anisotropy properties of this phase were determined on the single-variant constrained samples using the magnetization curves M(H) recorded along [100], [010] and [001] directions. We demonstrate that low twinning stresses and a high level of magnetic anisotropy energy are the critical factors for the observation of a giant magnetic field induced strain.

Field-dependent surface impedance tensor in amorphous wires with two types of magnetic anisotropy: helical and circumferential

Abstract: This paper concerns the theoretical and experimental investigation of the magneto-impedance (MI) effect in amorphous wires in terms of the surface impedance tensor. Physical concepts of MI and problems of significant practical importance are discussed using the results obtained. The theoretical analysis is based on employing the asymptotic-series expansion method of solving the Maxwell equations for a ferromagnetic wire with an ac permeability tensor of a general form associated with magnetisation rotation. The magnetic structure-dependent impedance tensor is calculated for any frequency and external magnetic field, and is not restricted to the case when only strong skin-effect is present. This approach allows us to develop a rigorous quantitative analysis of MI characteristics in wires, depending on the type of magnetic anisotropy, the magnitude of dc bias current, and an excitation method. The theoretical model has been tested by comparing the obtained results with experiment. For the sake of an adequate comparison, the full impedance tensor is measured in CoFeSiB and CoSiB amorphous wires having a circumferential and helical anisotropy, respectively, by determining the S21 parameter. In cases, when the rotational dynamics is responsible for the impedance behaviour, there is a reasonable agreement between the experimental and theoretical results. Such effects as the ac biased asymmetrical MI in wires with a circumferential anisotropy, the transformation in MI behaviour caused by a dc current (from that having a symmetric hysteresis to an asymmetric anhysteretic one) in wires with a helical anisotropy are discussed.

Crossover from in-plane to perpendicular anisotropy in Pt/CoFe/AlOx sandwiches as a function of Al oxidation: A very accurate control of the oxidation of tunnel barriers

Abstract: By measuring the extraordinary Hall effect on a series of naturally oxidized Pt3 nm/Co90Fe10 0.6 nm/Al tAl samples with 0

Outstanding magnetic properties of nematic suspensions of goethite (α-FeOOH) nanorods

Abstract: Aqueous suspensions of goethite (alpha-FeOOH) nanorods form a mineral lyotropic nematic phase that aligns in a very low magnetic field (20 mT for samples 20 microm thick). The particles orient along the field direction at intensities smaller than 350 mT, but they reorient perpendicular to the field beyond 350 mT. This outstanding behavior is also observed in the isotropic phase which has a very strong magnetic-field induced birefringence that could be interesting for applications. We interpret these magnetic effects as resulting from a competition between a nanorod remanent magnetic moment and a negative anisotropy of its magnetic susceptibility.

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: Influence of the surface and of interparticle interactions

Abstract: We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25--7000 atoms) prepared by sequential sputtering of Co and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant ${K}_{\mathrm{eff}}.$ We find that ${K}_{\mathrm{eff}}$ is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal process is shown to proceed classically down to the lowest temperature investigated (1.8 K).

Magnetic anisotropy of the spin-ice compound Dy2Ti2O7

Abstract: We report magnetization and ac susceptibility of single crystals of the spin-ice compound ${\mathrm{Dy}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$ Saturated moments at 1.8 K along the characteristic axes [100] and [110] agree with the expected values for an effective ferromagnetic nearest-neighbor Ising pyrochlore with local $〈111〉$ anisotropy, where each magnetic moment is constrained to obey the ``ice rule.'' At high enough magnetic fields along the [111] axis, the saturated moment exhibits a breaking of the ice rule; it agrees with the value expected for a three-in, one-out spin configuration. Assuming the realistic magnetic interaction between Dy ions given by the dipolar spin ice model, we completely reproduce the results at 2 K by Monte Carlo calculations. However, down to at least 60 mK, we have not found any experimental evidence of the long-range magnetic ordering predicted by this model to occur at around 180 mK. Instead, we confirm the spin freezing of the system below 0.5 K.

Magnetic anisotropy of the antiferromagnetic ring [Cr8F8Piv16].

Abstract: A new tetragonal (P4212) crystalline form of [Cr8F8Piv16] (HPiv=pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the CrIII ions is antiferromagnetic (J=12 cm−1) which results in a S=0 spin ground state. The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S=1 and S=2, respectively) are D1=1.59(3) cm−1 or 1.63 cm−1 (from CTM and HFEPR, respectively) and D2=0.37 cm−1 (from HFEPR). The dipolar contributions to the ZFS of the S=1 and S=2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1)° with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic CrIII rings.

Off-axis pinned layer magnetic element utilizing spin transfer and an MRAM device using the magnetic element

Abstract: A method and system for providing a magnetic element capable of being written in a reduced time using the spin-transfer effect while generating a high output signal and a magnetic memory using the magnetic element are disclosed. The magnetic element includes a ferromagnetic pinned layer, a nonmagnetic intermediate layer, and a ferromagnetic free layer. The pinned layer has a magnetization pinned in a first direction. The nonmagnetic intermediate layer resides between the pinned layer and the free layer. The free layer has a magnetization with an easy axis in a second direction. The first direction is in the same plane as the second direction and is oriented at an angle with respect to the second direction. This angle is different from zero and π radians. The magnetic element is also configured to allow the magnetization of the free layer to change direction due to spin transfer when a write current is passed through the magnetic element.

Effect of interdot magnetostatic interaction on magnetization reversal in circular dot arrays

Abstract: The effect of the interdot magnetostatic interaction on the magnetization reversal due to the ``nucleation'' and ``annihilation'' of magnetic vortices in arrays of ferromagnetic submicron circular dots has been investigated experimentally and theoretically. The magnetostatic interaction plays an important role in magnetization reversal for the arrays with a small interdot distance, leading to decreases in the vortex nucleation and annihilation fields, and an increase in initial susceptibility.

Large cyclic deformation of a Ni-Mn-Ga shape memory alloy induced by magnetic fields

Abstract: A single-crystalline ferromagnetic Ni-Mn-Ga shape memory specimen was deformed in the martensitic state by uniaxial compression to a total strain of 2%. The stress-strain curve displays a yield point at 6 MPa followed by jerky flow. After mechanical deformation, magnetomechanical tests in a rotating magnetic field of constant strength as well as experiments in a magnetic field of variable strength and constant direction were performed. Upon field rotation, a cyclic-field-induced strain of 1.8% is obtained repeatedly. Upon repeated field variation with constant field direction, a large field-induced strain occurs only in the first test after a change of the field direction. The force of a magnetic field on a twinning dislocation and the resulting magnetostress are discussed. For small fields, the magnetic force is proportional to the applied field. Above the saturation field, the magnetic stress is constant and equals the ratio of the magnetic anisotropy constant and the twinning shear. A microscopic model relating large field-induced strain to the motion of twinning dislocations is presented and applied to the experiments. It is concluded that cyclic magnetic-field-induced strain can be obtained only by changing the field direction because the magnetic force cannot be reversed otherwise.

Large surface magnetic contribution in amorphous ferromagnetic nanoparticles

Abstract: We present magnetization measurements on \ensuremath{\sim}3-nm $({\mathrm{Fe}}_{0.26}{\mathrm{Ni}}_{0.74}{)}_{50}{\mathrm{B}}_{50}$ ferromagnetic amorphous nanoparticles. Our results show evidence of the surface contribution to the magnetic properties in the $M(H),$ $M(T)$ and relaxation measurements. We observe a large increase in the magnetization with decreasing temperature, a nonsaturated component in the $M(H)$ curves, the presence of two maxima in the zero-field-cooling $M(T)$ data, and two viscosity regimes in magnetization relaxation data. The results have been interpreted by a simple model where we consider each single-domain particle as a core plus shell system with a uniaxial anisotropy acting on the core and a surface anisotropy acting on the shell. Monte Carlo simulations based in this core-shell model qualitatively reproduce all the observed phenomena.

Influence of in-plane crystalline quality of an antiferromagnet on perpendicular exchange coupling and exchange bias

Abstract: We have undertaken a systematic study of the influence of in-plane crystalline quality of the antiferromagnet on exchange bias. Polarized neutron reflectometry and magnetometry were used to determine the anisotropies of polycrystalline ferromagnetic (F) Fe thin films exchange coupled to antiferromagnetic (AF) untwinned single crystal (110) ${\mathrm{FeF}}_{2},$ twinned single crystal (110) ${\mathrm{FeF}}_{2}$ thin films and (110) textured polycrystalline ${\mathrm{FeF}}_{2}$ thin films. A correlation between the anisotropies of the AF and F thin films with exchange bias was identified. Specifically, when exchange coupling across the F-AF interface introduces an additional anisotropy axis in the F thin film---one perpendicular to the cooling field, the magnetization reversal mechanism is affected (as observed with neutron scattering) and exchange bias is significantly enhanced.

Synthesis, Structure, and Magnetic Properties of the Single-Molecule Magnet [Ni21(cit)12(OH)10(H2O)10]16-

Abstract: The preparation of two new compounds containing the cluster [Ni21(cit)12(OH)10(H2O)10]16- is presented, together with a detailed magnetic investigation of one of the compounds. We found that this cluster shows an unexpected stability and that it exists as different stereoisomers. Compound 1 contains the achiral cluster with a Δ−Λ configuration, and compound 2 contains a pair of enantiomeric clusters with the configurations Δ−Δ and Λ−Λ, respectively. Magnetic measurements of 1 in the millikelvin range were necessary to determine the spin ground state of S = 3, and they also revealed a magnetic anisotropy within the ground state. A frequency-dependent out-of-phase signal was found in alternating current susceptibility measurements at very low temperatures, which indicates a slow relaxation of the magnetization. Thus, individual molecules are acting as single magnetic units, which is a rare phenomenon for nickel clusters. The energy barrier exhibited by compound 1 has been calculated to be 2.9 K.