Showing papers on "Magnetic domain published in 2002"

Observation of coupled magnetic and electric domains

Abstract: Ferroelectromagnets are an interesting group of compounds that complement purely (anti-)ferroelectric or (anti-)ferromagnetic materials--they display simultaneous electric and magnetic order. With this coexistence they supplement materials in which magnetization can be induced by an electric field and electrical polarization by a magnetic field, a property which is termed the magnetoelectric effect. Aside from its fundamental importance, the mutual control of electric and magnetic properties is of significant interest for applications in magnetic storage media and 'spintronics'. The coupled electric and magnetic ordering in ferroelectromagnets is accompanied by the formation of domains and domain walls. However, such a cross-correlation between magnetic and electric domains has so far not been observed. Here we report spatial maps of coupled antiferromagnetic and ferroelectric domains in YMnO3, obtained by imaging with optical second harmonic generation. The coupling originates from an interaction between magnetic and electric domain walls, which leads to a configuration that is dominated by the ferroelectromagnetic product of the order parameters.

2.5-inch disk patterned media prepared by an artificially assisted self-assembling method

Abstract: Circumferential magnetic patterned media were prepared on a 2.5-inch-diameter glass plate and on a 3-in-diameter silicon plate. A Ni master disk possessing spiral patterns with 60-250-nm-width lands and a 400-nm-width groove was pressed into a resist film on a CoPt or CoCrPt film to transfer the spiral patterns. A diblock copolymer solution was cast into the obtained grooves and then annealed to prepare self-assembling dot structures aligned along the grooves. According to the dot patterns, the underlying magnetic films were patterned by ion milling to yield patterned media with a 40-nm diameter. Coercive forces and squareness ratios of the patterned media increased compared to those of the continuous media, probably due to the decrement of a demagnetizing field. Single magnetic domains with an almost perpendicular orientation were confirmed in each magnetic dot.

Direct observation of percolation in a manganite thin film

Abstract: Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3start to grow and merge at the metal-insulator transition temperatureTP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance 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.

Magnetic characterization of nanocrystalline Ni-Zn ferrite powder prepared by the glyoxylate precursor method

Abstract: Nanocrystalline Ni0.35Zn0.65Fe2O4 mixed ferrite was obtained from the Fe2(Ni0.35,Zn0.65)(OH)4(C2H2O4)2 · H2O complex combination that corresponds to the atomic ratio Ni(II) : Zn(II) : Fe(III) = 0.35 : 0.65 : 2; the complex combination was decomposed at 325uC and the resulting oxides mixture was annealed in the temperature range of 400–1000uC for 2 h. The thermal analysis of the synthesized complex combination was done by TG–DTA techniques. It has been shown by means of x-ray diffraction that even at 400uC Ni–Zn mixed spinel ferrite is formed with an face-centred cubic structure and a lattice parameter that is in agreement with the reported value. Thus, the formation temperature of ferrite was drastically reduced (by cca. 900uC) compared to that of the conventional ceramic method. The magnetic measurements showed the increase of the saturation magnetization σs and a maximum of the coercivity Hc of the nanocrystalline system with the increase of the annealing temperature. These changes can be attributed to the increase of the average diameter of the nano-sized crystallites from 14.6 to 46.3 nm when the temperature increases from 400uC to 1000uC. The nanocrystallites are single-domain up to ∼28 nm; above this value they have an incipient structure of Weiss domains, a result that is in agreement with the critical diameter of the single-domain deduced from theoretical calculation.

Spin-polarized scanning tunneling microscopy with antiferromagnetic probe tips.

Abstract: We have performed low temperature spin-polarized scanning tunneling microscopy (SP-STM) of two monolayers Fe on W(110) using tungsten tips coated with different magnetic materials. We observe stripe domains with a magnetic period of 50 +/- 5 nm. Employing Cr as a coating material we recorded SP-STM images with an antiferromagnetic probe tip. The advantage of its vanishing dipole field is most apparent in external magnetic fields. This new approach resolves the problem of the disturbing influence of a ferromagnetic tip in the investigation of soft magnetic materials and superparamagnetic particles.

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.

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.

Magnetoresistive element and magnetic head

Abstract: A magnetic transducing element in which a ferromagnetic tunneling junction film, including first and second ferromagnetic layers and an insulating layer are enclosed between the ferromagnetic layers, and a nonmagnetic metal thin film is inserted between the second ferromagnetic layer and the insulating layer, is formed on a substrate.

Magnetic Ordering of Rare Earth Ions and Magnetic-Electric Interaction of Hexagonal RMnO3 (R=Ho, Er, Yb or Lu)

Abstract: From magnetic, dielectric and magnetoelectric measurement, it is concluded that hexagonal HoMnO 3 , ErMnO 3 and YbMnO 3 undergo a magnetic phase transition at a low temperature where a magnetic lon...

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.

Magnetic field induced entropy change and magnetoelasticity in Ni-Mn-Ga alloys

Abstract: The magnetocaloric effect that originates from the martensitic transition in the ferromagnetic Ni-Mn-Ga shape-memory alloy is studied. We show that this effect is controlled by the magnetostructural coupling at both the martensitic variant and magnetic domain length scales. A large entropy change induced by moderate magnetic fields is obtained for alloys in which the magnetic moment of the two structural phases is not very different. We also show that this entropy change is not associated with the entropy difference between the martensitic and the parent phase---arising from the change in the crystallographic structure---which has been found to be independent of the magnetic field within this range of fields.

Influence of stress and texture on soft magnetic properties of thin films

Abstract: This paper presents the relationship among stress, crystallographic texture, and soft magnetic properties of thin films. The magnetic properties considered are those affected by the formation of stripe domains and by the formation of magnetization ripple. In practice, one of these two undesirable domain structures is almost always the impediment to improving the soft magnetic properties. The theoretical analysis accounts for the contribution from stress through magnetostrictive anisotropy, and calculates the resulting total, stress-dependent anisotropy of the film. This anisotropy is then analyzed to yield the effective perpendicular and local in-plane anisotropy constants. These constants allow the calculation of the stripe domain onset thickness through Murayama's stripe domain theory, and the ripple coercivity through Hoffmann's ripple theory. The influence of stress and texture on the stripe domain onset thickness and ripple coercivity is theoretically calculated and experimentally verified for the examples of sputtered CoFe and FeAlN films, two of the most promising and widely studied materials for high-density recording head poles. The results indicate that the interactions between stress and soft magnetic properties depend on the details of composition, growth texture, growth morphology (whether the film is grown in columnar fashion or as equiaxial crystallites), grain size, and thickness of the film under consideration. On the basis of these results, the paper offers a systematic approach to choosing the appropriate composition, texture, and thickness so that low stresses simultaneously promote magnetic states in which stripe domains are suppressed and coercivities are low. The calculations presented here can be easily extended to other aspects of soft magnetic properties addressed by the ripple theory, such as initial permeability.

Fully relativistic calculation of magnetic properties of Fe, Co, and Ni adclusters on Ag(100)

Abstract: We present first-principles calculations of the magnetic moments and magnetic anisotropy energies of small Fe, Co, and Ni clusters on top of a Ag(100) surface as well as the exchange-coupling energy between two single adatoms of Fe or Co on Ag(100). The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method. The magnetic anisotropy and the exchange-coupling energies are calculated by means of the force theorem. In the case of adatoms and dimers of iron and cobalt we obtain enhanced spin moments and, especially, unusually large orbital moments, while for nickel our calculations predict a complete absence of magnetism. For larger clusters, the magnitudes of the local moments of the atoms in the center of the cluster are very close to those calculated for the corresponding monolayers. Similar to the orbital moments, the contributions of the individual atoms to the magnetic anisotropy energy strongly depend on the position, hence, on the local environment of a particular atom within a given cluster. We find strong ferromagnetic coupling between two neighboring Fe or Co atoms and a rapid, oscillatory decay of the exchange-coupling energy with increasing distance between these two adatoms.

Magnetic switching element and a magnetic memory

Abstract: A magnetic switching element includes: a ferromagnetic layer which is substantially pinned in magnetization in one direction; and a magnetic semiconductor layer provided within a range where a magnetic field from the ferromagnetic layer reaches, where the magnetic semiconductor layer changes its state from a paramagnetic state to a ferromagnetic state by applying a voltage thereto, and a magnetization corresponding to the magnetization of the ferromagnetic layer is induced in the magnetic semiconductor layer by applying a voltage to the magnetic semiconductor layer.

Transition from single-domain to vortex state in soft magnetic cylindrical nanodots.

Abstract: The authors have investigated the magnetic properties of submicron soft magnetic cylindrical nanodots using an analytical model as well as three dimensional numerical finite element simulations. A detailed comparison of the magnetic vortex state shows the differences between these two models. It appears that the magnetic surface charges play a crucial role in the equilibrium magnetization distribution especially for shifted vortices. In addition magnetic volume charges, which arise from a radial component of the magnetization, have been found. Finally, the magnetic phase diagram for soft magnetic particles with varying aspect ratio is presented.

Magnetoresistance random access memory

Abstract: A magnetoresistive tunneling junction memory cell comprises a magnetoresistive tunneling barrier (16), a bit magnetic region (15), a reference magnetic region (17), and current lines (20, 30) for inducing an applied magnetic field in the bit and reference magnetic regions. The bit magnetic region has a bit magnetic moment (43, 40,1425, 1625, 1950, 2315) that has a polarity in a bit easy axis (59, 1435) when there is no applied magnetic field. The tunneling barrier and the bit and reference magnetic regions form a magnetoresistive tunneling junction device (10, 72, 73, 74, 75, 76). In some implementations (73, 74, 75), the reference magnetic region has a reference magnetic moment (40, 1430, 1440, 1920, 1925) that is non-parallel to the bit easy axis. In other implementations (76), the reference magnetic region has a magnetization vortex (2310) with a net reference magnetic moment that is essentially zero. An applied magnetic field changes the magnetic state of the reference magnetic region such that the magnetic state of the bit magnetic region can be determined by a magnetoresistive measurement.

Multi-state magnetoresistance random access cell with improved memory storage density

Abstract: A multi-state magnetoresistive random access memory device having a pinned ferromagnetic region with a magnetic moment vector fixed in a preferred direction in the absence of an applied magnetic field, a non-ferromagnetic spacer layer positioned on the pinned ferromagnetic region, and a free ferromagnetic region with an anisotropy designed to provide a free magnetic moment vector within the free ferromagnetic region with N stable positions, wherein N is a whole number greater than two, positioned on the non-ferromagnetic spacer layer. The number N of stable positions can be induced by a shape anisotropy of the free ferromagnetic region wherein each N stable position has a unique resistance value.

Tunable Quantum Tunneling of Magnetic Domain Walls

Abstract: Magnetic domain wall tunneling is observed in the disordered Ising ferromagnet LiHo_{0.44}Y_{0.56}F_4. The tunneling can be tuned by applying a magnetic field transverse to the Ising axis, and is readily modeled with a WKB formalism.

Controlled magnetic switching in single narrow rings probed by magnetoresistance measurements

Abstract: We present anisotropic magnetoresistance measurements of magnetic switching processes in narrow ferromagnetic permalloy rings fabricated with six nonmagnetic electrical contacts. We demonstrate that measuring the resistance between different contacts allows the determination of the domain wall positions. Furthermore, these measurements also yield the possibility of determining the local switching fields of different parts of the ring. This provides a very useful tool to explore the complete switching process of a single ring. We show that, by using notches of suitable size and by applying fields along appropriate directions, it is possible to select the circulation direction of the vortex state using a uniform field only.

Magnetocrystalline anisotropy in single-crystal Co-Ni-Al ferromagnetic shape-memory alloy

Abstract: The magnetocrystalline anisotropy in a single-crystal Co37Ni34Al29 ferromagnetic shape-memory alloy has been investigated. The prestrain was applied to the parent phase in order to nucleate the specific variant in the sample cooled down through the martensitic transformation temperature. The applied magnetic field facilitates the growth of variants parallel to the applied magnetic field in analogy with the prestrain. From these results of selective nucleation of variants, the magnetocrystalline anisotropy energy in the single crystal Co37Ni34Al29 β′ martensite phase is estimated to be 3.9×106 erg/cm3. In the single crystal, the observed magnitude of the reversible magnetic-field-induced strains is 0.06%.

X-ray Microdiffraction Images of Antiferromagnetic Domain Evolution in Chromium

Abstract: Magnetic x-ray diffraction combined with x-ray focusing optics was used to image individual antiferromagnetic spin density wave domains in a chromium single crystal at the micron scale. The cross section for nonresonant magnetic x-ray scattering depends on the antiferromagnetic modulation vector and spin polarization direction and allows these quantities to be extracted independently. The technique was used to show that the broadening of the nominally first-order “spin-flip” transition at 123 kelvin, at which the spins rotate by 90°, originates at the walls between domains with orthogonal modulation vectors. During cooling, the transition begins at these walls and progresses inward. The modulation vector domains are themselves unchanged.

Lattice symmetry and magnetization reversal in micron-size antidot arrays in Permalloy film

Abstract: The magnetization reversal in four arrays of micron-size circular holes (antidots) in a Permalloy film has been studied by means of quantitative magneto-optic Kerr vector magnetometry and magnetic force microscopy. The primitive antidot meshes of the arrays investigated here can be classified as square, rectangular, hexagonal, and oblique. The vector magnetometry data show that the hole arrays induce a magnetic anisotropy completely different from that of the unpatterned film, with new hard axes along the directions connecting nearest neighboring holes. Also the coercive field is strongly affected by the pattern. The results of the vector magnetometry analysis indicate that the reversal process takes place through a collective and periodic domain nucleation and expansion process. The domain structure in the remanent state has been investigated by magnetic force microscopy imaging. The images display well-defined domain structures, which are periodic and commensurate with the holes array.

Magnetic correlations induced by magnetic field and temperature in Gd5Ge4

Abstract: Several unexpected and intriguing magnetic phenomena have been observed in the Gd 5 Ge 4 compound. First, below ∼130 K, Gd 5 Ge 4 is antiferromagnetic in a zero magnetic field, but it can be transformed into the ferromagnetic slate both irreversibly (below 10 K) and reversibly (above 20 K) depending on the magnitude of the applied magnetic field, the temperature, and the direction of their changes. Second, the irreversible antiferromagnetic→ferromagnetic transformation at 4.3 K is abrupt in magnetic fields exceeding 18 kOe, but it is sluggish in lower (∼17 kOe) magnetic fields. Third, both the antiferromagnetic and ferromagnetic Gd 5 Ge 4 phases may coexist indefinitely under certain combinations of the magnetic field and temperature. It is likely that the unusual magnetic correlations in Gd 5 Ge 4 arise due to strongly anisotropic exchange interactions as a result of variations in the chemical bonding in this naturally layered, and therefore, low-dimensional magnetic system.

Magnetic force microscopy observation of antivortex core with perpendicular magnetization in patterned thin film of permalloy

Abstract: The cross-tie wall is a kind of magnetic domain wall composed of a main straight wall and crossing subwalls and observed in magnetic thin films. This wall contains two kinds of magnetic vortex structures: “circular vortex” and “antivortex.” At the cores of both vortices, the existence of a spot with perpendicular magnetization has been theoretically predicted. We have detected the perpendicular magnetization spots at each vortex core and identified the direction of it by applying magnetic force microscopy imaging to cross-tie walls in patterned rectangular thin permalloy (Ni80Fe20) films. We also fabricated magnetic structures that contain only antivortex by engineering the shape of thin films.

Laser-Induced Ultrafast Demagnetization: Femtomagnetism, a New Frontier?

Abstract: The conventional demagnetization process (spin precession, magnetic domain motion and rotation) is governed mainly by spin—lattice, magnetic dipole and Zeeman, and spin-spin interactions. It occurs on a timescale of nanoseconds. Technologically, much faster magnetization changes are always in great demand to improve data processing speed. Unfortunately, the present speed of magnetic devices is already at the limit of the conventional mechanism with little room left. Fortunately and unprecedentedly, recent experimental investigations have evidenced much faster magnetization dynamics which occurs on a femtosecond time scale: femtomagnetism. This novel spin dynamics has not been well-understood until now. This article reviews the current status of ultrafast spin dynamics and presents a perspective for future experimental and theoretical investigations.