Biaxial Magnetocrystalline Anisotropy Research Articles

Magnetic Anisotropy of a Quasi Two-Dimensional Canted Antiferromagnet.

We report the control of the interplane magnetic exchange coupling in CaIrO3 perovskite thin films and superlattices with SrTiO3. By analyzing the anisotropic magneto-transport data, we demonstrate that a semimetallic paramagnetic CaIrO3 turns into a canted antiferromagnetic Mott insulator at reduced dimensions. The emergence of a biaxial magneto-crystalline anisotropy indicates the canted moment responding to the cubic symmetry. Extending to superlattices and probing oxygen octahedral rotation by half-integer X-ray Braggs diffraction, a more complete picture about the canted moment evolution with interplane coupling can be understood. Remarkably, a rotation of the canted moments' easy axes by 45° is also observed by a sign reversal of the in-plane strain. These results demonstrate the robustness of anisotropic magnetoresistance in revealing quasi two-dimensional canted antiferromagnets, as well as valuable insights about quadrupolar magnetoelastic coupling, relevant for designing future antiferromagnetic spintronic devices.

Magnetic anisotropy in Fe films deposited on SiO2/Si(001) and Si(001) substrates

The magnetic anisotropy of 10nm iron films deposited in an ultra high vacuum on the Si(001) surface and on the Si(001) over caped by 1.5nm layer of SiO2 was investigated. There is in-plane uniaxial magnetic anisotropy caused by oblique sputtering in the Fe films on a SiO2 buffer layer. The easy magnetization axis is always normal to the atomic flux direction but the value of the anisotropy field is different depending on the axial angle among sputtering direction and the substrate crystallographic axes. It is argued that the uniaxial magnetic anisotropy results from elongated surface roughness formation during film deposition. Several easy magnetization axes are found in Fe/Si(001) film without the SiO2 buffer layer. The mutual orientation of the main easy axes and Si crystallographic axes indicates that there is epitaxial growth of Fe/Si(001) film with the following orientation relative to the substrate: Fe[100] ∥Si[110]. The anisotropy energy of Fe/Si(001) film is estimated by simulation of angle dependence of remnant magnetization mr as the sum of the mr angle plot from uniaxial anisotropy (induced by oblique deposition) and the polar plot from biaxial magnetocrystalline anisotropy.

The total energy of exchange bias systems with biaxial anisotropy

Most of theoretical models describing the magnetic properties of ferromagnetic or anti-ferromagnetic thin films assume that they have uniaxial magneto-crystalline anisotropy. This is a convenient assumption for computational reasons, which also reproduces experimental results with reasonable accuracy. However, many magnetic materials have cubic symmetry with more than one easy axis. In this paper we examine and compare the energy profiles of an anti-ferromagnet with uniaxial and bi-axial magneto-crystalline anisotropy, when it is coupled to a ferromagnetic layer in a well-known exchange bias system. We show that the bi-axial symmetry case introduces an extra energy minimum corresponding to anti-ferromagnetic spins oriented transverse to the easy axis of the ferromagnet. The multiple symmetry of the anti-ferromagnet could be responsible for the irreversible change in the exchange bias systems, known as the “training effect”.

Energy-Efficient Bennett Clocking Scheme for Four-State Multiferroic Logic

Nanomagnets with biaxial magnetocrystalline anisotropy have four stable magnetization orientations that can encode 4-state logic bits (00), (01), (11) and (10). Recently, a 4-state NOR gate derived from three such nanomagnets, interacting via dipole interaction, was proposed. Here, we devise a Bennett clocking scheme to propagate 4-state logic bits unidirectionally between such gates. The nanomagnets are assumed to be made of 2-phase strain-coupled magnetostrictive/piezoelectric multiferroic elements, such as nickel and lead zirconate titanate (PZT). A small voltage of 200 mV applied across the piezoelectric layer can generate enough mechanical stress in the magnetostrictive layer to rotate its magnetization away from one of the four stable orientations and implement Bennett clocking. We show that a particular sequence of positive and negative voltages will propagate 4-state logic bits unidirectionally down a chain of such multiferroic nanomagnets for logic flow.

An Ultrafast Image Recovery and Recognition System Implemented With Nanomagnets Possessing Biaxial Magnetocrystalline Anisotropy

A circular magnetic disk with biaxial magnetocrystalline anisotropy has four stable magnetization states which can be used to encode a pixel’s shade in a black/gray/white image. By solving the Landau–Lifshitz–Gilbert equation, we show that if moderate noise deflects the magnetization slightly from a stable state, it always returns to the original state, thereby automatically denoising the corrupted image. The same system can compare a noisy input image with a stored image and make a matching decision using magnetic tunnel junctions. These tasks are executed at ultrahigh speeds ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 2 ns for a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$512 \times 512$</tex></formula> pixel image).

Four-state nanomagnetic logic using multiferroics

The authors theoretically demonstrate the implementation of a low-power 4-state universal logic gate (NOR) using a linear array of three dipole-coupled magnetostrictive-piezoelectric multiferroic nanomagnets (e.g. Ni/PZT) with biaxial magnetocrystalline anisotropy. The two peripheral nanomagnets in the array encode the 4-state input bits in their magnetization orientations and the central nanomagnet's magnetization orientation represents the output bit. Numerical simulations are performed to confirm that the 4-state output bit is the Boolean NOR function of the two 4-state inputs bits when the array reaches its ground state. A voltage pulse alternating between −0.2 and +0.2 V, applied to the piezoelectric layer of the central nanomagnet, generates alternating tensile and compressive stress in its magnetostrictive layer. This drives the array to the correct ground state where dipole interaction between the magnets ensures that the output is the NOR function of the input. For the system considered, the gate operation is executed while dissipating only ∼33 000 kT (0.138 fJ) of energy.

Asymmetric double-shifted characteristics in epitaxial (002) exchange-biased IrMn/CoFe bilayers

Symmetric and asymmetric double-shifted loops were observed at room temperature in epitaxial (002) Ir20Mn80/Co50Fe50 samples placed in a setting field applied along collinear and noncollinear directions with the deposition-field direction, respectively. A coherent rotation model with extra unidirectional exchange and uniaxial anisotropies induced by the noncollinear setting field is proposed to explain the asymmetric double-shifted loops. Due to the existence of the biaxial magnetocrystalline anisotropy, the noncollinear setting field results in two interfacial Ir20Mn80 spin orientations, where the major spins determine the direction to observe double-shifted loops and the minor spins determine the field-shift and the asymmetry of double-shifted loops.

Magnetoresistance tensor ofLa0.8Sr0.2MnO3

We measure the temperature dependence of the anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) in c-axis oriented epitaxial thin films of La(0.8)Sr(0.2)MnO(3), for different current directions relative to the crystal axes, and show that both AMR and PHE depend strongly on current orientation. We determine a magnetoresistance tensor, extracted to 4th order, which reflects the crystal symmetry and provides a comprehensive description of the data. We extend the applicability of the extracted tensor by determining the bi-axial magnetocrystalline anisotropy in our samples.

Magnetic anisotropy in GaP(001) epilayers containing MnP nanoclusters observed by angle dependent ferromagnetic resonance measurements

We report on the angular dependence of ferromagnetic resonance (FMR) of GaP(001) epilayers with embedded MnP nanoclusters (GaP:MnP) grown on a GaP(001) substrate using metal-organic vapor phase epitaxy (MOVPE). Angle dependent FMR spectra obtained at 292K show several peaks and indicate a strong magnetic anisotropy. The measured angular dependence of resonance fields can be modeled assuming that the MnP clusters possess a strong biaxial magnetocrystalline anisotropy whose axes are oriented along specific GaP crystallographic directions. Dominant populations of MnP clusters have their b-axis along GaP [111] and [111¯] directions, while a smaller population has the b-axis along the GaP [001] direction. In comparison, angle dependent FMR spectra of a polycrystalline MnP thin film grown on a GaP(001) substrate using MOVPE suggest that the crystals b-axis is oriented favorably along the GaP [001] direction.

Hysteresis in Fe particles with surface and magnetoelastic anisotropies: Experiment and micromagnetic modeling

We report on the correlation between the experimentally obtained saturation coercive force of highly elongated Fe nanoribbons and the results of micromagnetic simulations. To describe various realistic situations in our micromagnetic model of the ribbons we incorporated, in addition to a biaxial magnetocrystalline anisotropy, surface magnetocrysalline and magnetoelastic anisotropies and crystalline orientation distributions.

Thickness dependence of magnetic domain formation in La 0.6Sr 0.4MnO 3 epitaxial thin films studied by XMCD–PEEM

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La 0.6Sr 0.4MnO 3 samples that were epitaxially grown on stepped SrTiO 3 (0 0 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films.

Observation of step-induced magnetic domain formation in La1−xSrxMnO3 thin films by photoelectron emission microscopy

Magnetic domain formation in thin films of La1−xSrxMnO3 (LSMO) with x=0.2 and 0.4 epitaxially grown on stepped SrTiO3 (001) substrates has been investigated by photoelectron emission microscopy. The magnetic domains show a stripe structure elongated along the step directions, indicating uniaxial magnetic anisotropy induced by step structures. We have also found that the magnetization of the magnetic domains is slightly meandering at domain boundaries. The existence of the additional structures suggests that the magnetic domains in LSMO films are also influenced by biaxial magnetocrystalline anisotropy. The direct observation of the magnetic domain structures suggests that the competition between the two magnetic anisotropies may play an important role in magnetic properties in LSMO films.

Characterization of the magnetic anisotropy in thin films of La 1–x Sr x MnO 3 using the planar Hall effect

Thin films of the colossal magnetoresistance material La1–xSrxMnO3 (LSMO) grown on SrTiO3 substrates exhibit bi-axial magnetocrystalline anisotropy with easy axes along the [110] and [10] directions. We have recently discovered that the intrinsic biaxial magnetic anisotropy combined with a giant planar Hall effect lead to striking switching behavior in the transverse resistivity of LSMO films (Appl. Phys. Lett. 84, 2593 (2004)). Here we use this phenomenon as a sensitive tool for measuring in-plane magnetization in order to characterize the magnetic anisotropy.

Stoner–Wohlfarth model applied to bicrystal magnetoresistance hysteresis

We calculate numerically the magnetization direction as function of magnetic field in the Stoner–Wohlfarth theory and are able to reproduce the shape of the low-field magnetoresistance hysteresis observed in manganite grain boundary junctions. Moreover, we show that it is necessary to include biaxial magnetocrystalline anisotropy to fully describe the grain boundary magnetoresistance in La0.7Sr0.3MnO3/SrTiO3 bicrystal tunnel junctions.

Misorientation angle dependence of bicrystal magnetoresistance within the model ofcoherent rotation

Bicrystal magnetoresistance hysteresis is studied within the model of coherent rotation ofmagnetization. The magnetoresistance hysteresis is calculated numerically in the limit offully spin polarized current for symmetric bicrystal junctions with biaxial magneticanisotropy and of varying misorientation angles. The shape of the curves obtained fromthe model displays different characteristic features, depending on the angularrelationships, which are consistent with experimental data from the literature. Theresults show a magnetoresistance increasing with increasing misorientation angle.The influence of biaxial magnetocrystalline anisotropy is reflected in a humpin the maximum magnetoresistance curve at around a misorientation angle of25°. This structure is absent in the case of uniaxial anisotropy.

Sign reversal of the magnetic anisotropy in La 0.7A 0.3MnO 3 (A = Ca, Sr, Ba, □) films

The magnetic anisotropy of epitaxial manganite La 0.7A 0.3MnO 3 films (A = Ca, Sr, Ba, □=vacancy) is studied as a function of tolerance factor. Within the film plane a biaxial magnetocrystalline anisotropy was observed with an anisotropy constant changing sign as function of doping: whereas La 0.7Ca 0.3MnO 3 and La 1− x □ x MnO 3 have [1 0 0] easy axes, La 0.7Sr 0.3MnO 3 and La 0.7Ba 0.3MnO 3 films were found to have [1 1 0] easy axes. This is related to a symmetry change of structural distortions with increasing tolerance factor. Films strained by the substrate show a large uniaxial magnetoelastic anisotropy along the film normal; the saturation magnetostriction was determined to λ 100=(3–6)×10 −5.

イオンビームスパッタ法によって作製したＦｅＮｉエピタキシャル薄膜の結晶構造および磁気特性

We prepared epitaxially grown Fe100-xNix films on (1100) GaAs single-crystal substrates by means of ion-beam sputtering, and examined their fundamental properties such as crystal structure and magnetic anisotropy in detail. In epitaxially grown Fe100-xNix films, we found that single-crystalline bcc phase remains unless x exceeds 30 at% judging from streaky RHEED patterns. A decrease in magnetization was observed at x=40 at%, which means that the invar composition shifts by about 10 at% toward Ni-rich composition. Reflecting single-crystal bcc alloys, magnetization curves measured with a VSM and a torque magnetometer could be well reproduced by biaxial magnetocrystalline anisotropy and magnetoelastic anisotropy caused by the lattice misfit between film and GaAs substrate, where the internal stress is considered to be greatly reduced.

Anisotropic properties of molecular beam epitaxy-grown colossal magnetoresistance manganite thin films

We show that both the magnetoresistance and magnetism in tetragonal MBE-grown films of La1−xCaxMnO3 show anisotropic effects that depend on both temperature and magnetic field. We show that the “colossal” magnetoresistance depends on the angle between the magnetization and the transport current and that the size of this effect is temperature-dependent. Below the Curie temperature this results in an unusual upturn in the magnetoresistance for small magnetic fields normal to the plane of the film as the magnetization rotates out of the plane. Low-field hysteresis of the in-plane magnetoresistance is also observed, and also shows an anisotropy with respect to the current and magnetization directions. We also find an in-plane biaxial magnetocrystalline anisotropy with easy axes along the {100} (Mn–O) crystal directions, and evidence for c-axis magnetocrystalline anisotropy.

Low-field magnetoresistance in tetragonalLa1−xCaxMnO3sfilms

We have measured the low-field magnetoresistance of molecular-beam-epitaxy-grown tetragonal ${\mathrm{La}}_{0.7}$ ${\mathrm{Ca}}_{0.3}$ ${\mathrm{MnO}}_{3}$ films as a function of temperature, and both magnitude and direction of the applied magnetic field. We observed low-field anisotropic hysteresis that depends on the direction of the applied field in the plane of the film. The hysteretic effect can result in a sharp drop in resistance during magnetization reversal which is more than 10 times steeper than the already 'colossal' magnetoresistance. We also present evidence of biaxial magnetocrystalline anisotropy with easy axes along the Mn-O bond ([100],[010]) directions. We show that the low-field anisotropic hysteresis arises from the combined effects of magnetocrystalline anisotropy, anisotropic magnetoresistance, and 'colossal' magnetoresistance. Based on a comparison of the data with a simple phenomenological model for the magnetoresistance, we argue that magnetization reversal must proceed by a domain process.