Strong Unidirectional Anisotropy Research Articles

Dynamic unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling

Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. Such anisotropy is not present in static magnetometry measurements. B20 FeGe exhibits inherent Dzyaloshinskii-Moriya interaction, resulting in a nonreciprocal spin-wave dispersion. Bulk and micron sized samples were produced and characterized. By X-band ferromagnetic resonance spectroscopy at 276 K ± 1 K, near the Curie temperature, a distribution of resonance modes was observed in accordance with the cubic anisotropy of FeGe. This distribution exhibits a unidirectional anisotropy, i.e. shift of the resonance field under field inversion, of KUD = 960 J/m3 ± 10 J/m3, previously unknown in bulk ferromagnets. Additionally, more than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. These modes also exhibit unidirectional anisotropy. This effect, only dynamically measurable and not detectable in static magnetometry measurements, may open new possibilities for directed spin transport in chiral magnetic systems.

Unidirectional and uniaxial anisotropies in the MnN/CoFeB exchange bias system

Abstract We report on broadband and in-plane angle dependent ferromagnetic resonance measurements of MnN (30 nm)/CoFeB ( t CoFeB ) exchange bias bilayer systems with CoFeB thicknesses ranging from 5 to 20 nm. We find that the strong unidirectional anisotropy in this system is accompanied by a strong uniaxial in-plane anisotropy and a small interfacial perpendicular anisotropy ( K ⊥ , i = 0.043 ± 0.001 erg / cm 2 ). The strength of the interfacial exchange bias coupling in this system Δ σ eb = 0.193 ± 0.005 erg / cm 2 is only 2.5 times larger than the interfacial uniaxial coupling strength Δ σ u = 0.076 ± 0.003 erg / cm 2 . The relaxation in this system also shows a strong unidirectional and uniaxial in-plane anisotropy. While the observed thickness dependence of the uniaxial anisotropy of the relaxation is consistent with an interfacial two-magnon scattering contribution, we find that this is not the case for the unidirectional relaxation.

Analysis of the crystallographic and magnetic structures of the Tb0.1Pr0.9Al2 and Tb0.25Pr0.75Al2 magnetocaloric compounds by means of neutron scattering

Neutron powder diffraction and inelastic neutron scattering data were used to simulate and understand the magnetization and heat capacity curves of the pseudobinary Tb x Pr1−x Al2, with x = 0.10 and 0.25, as a function of temperature. From the Rietveld analysis, we concluded that no crystallographic transition occurs in these samples, and the high symmetry of the magnetic structure was confirmed. Moreover, the different contributions from the reflection planes could be related to the known exchange bias-like effect characteristic for the x = 0.25 sample, also suggesting the existence of some rearrangement of the magnetic moments or even the presence of spin frustration in this system. Finally, the obtained set of theoretical parameters using the mean field approach for the two systems consisting of two sublattices allowed the experimental data to be described and to explain their physical behaviors. The ensemble of our results leads us to affirm that the quadrupolar interactions as well as an existence of some rearrangement of the magnetic moments or a frustration play an important role in the strong unidirectional anisotropy and the exchange bias-like effect observed in this pseudobinary system.

Unidirectional anisotropy in the spin pumping voltage in yttrium iron garnet/platinum bilayers

Detailed measurements of the dc voltage generated in a thin Pt layer deposited on films of yttrium iron garnet (YIG) have been carried out to study the spin pumping effect produced by magnetostatic (MS) modes excited by a microwave field. In relatively thick YIG films the modes are far apart so that one can identify clearly the spin pumping voltage in VSP produced by each MS mode. We have discovered that when the sputter deposition of the thin Pt layer is made on the YIG film magnetized by a static magnetic field, VSP exhibits a strong unidirectional anisotropy.

Exchange coupling in a half-metallic and hard-magnetic Fe3O4–antiferromagnetic CrMnPt system, and unidirectional anisotropy enhancement with the induction of stress-induced anisotropy in the CrMnPt film

Exchange coupling between half-metallic Fe3O4 and antiferromagnetic CrMnPt films was investigated, with the goal of inducing unidirectional anisotropy in the Fe3O4 film having hard magnetic properties. We succeeded in obtaining a large unidirectional anisotropy constant of ∼0.15 erg/cm2, a high blocking temperature of ∼320 °C, and an almost unidirectionally shifted M–H curve. The unidirectional anisotropy constant of the Fe3O4–CrMnPt system increased to ∼0.18 erg/cm2, while the high blocking temperature and the nearly unidirectionally shifted M–H curve were retained, when a NiO film was deposited by sputtering onto the CrMnPt film, and the resulting system was annealed at 230 °C for 3 h. This strong unidirectional anisotropy constant is thought to have been caused by large stress relief in the NiO film resulting from the thermal annealing that acted to increase the c/a ratio in the CrMnPt film (the a and c are lattice constants), inducing so-called stress-induced anisotropy in the CrMnPt film.

ＣｒＭｎＰｔ反強磁性膜の応力誘導磁気異方性

The unidirectional anisotropy of exchange-coupled Ni81Fe19/Co90Fe10/Cr45.5Mn45.5Pt9 films was found to amount to ∼0.345 erg/cm2 when they were subjected to the following process: (a) an NiO-capped layer was sputter-deposited onto the CrMnPt film, (b) subsequently the films were annealed at 230°C for 3 h, and (c) finally the NiO-capped layer was eliminated by Ar ion-milling. The above anisotropy value was comparable with that of ferromagnet/Pt50Mn50 films. Furthermore, the strong unidirectional anisotropy is thought to be caused by the facts that large stress relief of the NiO-capped layer through thermal annealing acts to enlarge the c/a ratio within CrMnPt (a and c are lattice parameters), inducing a so-called stress-induced anisotropy within CrMnPt.

Strong unidirectional anisotropy in mechanically alloyed spinel ferrites

Cluster glass and relatively high coercivity at low temperatures were found in disordered ultrafine nickel ferrite powders. High-energy mechanical milling of spinel NiFe2O4 led to formation of a wüstitelike structure. Our investigation suggested that ferrimagnetic clusters formed in an antiferromagnetic matrix. The strong ferri/antiferromagnetic exchange coupling resulted in a strong unidirectional anisotropy and a coercivity of over 10 kOe at 4.2 K.

Strong uni-directional anisotropy in disordered NiFe 2O 4

High-energy mechanical milling of spinel NiFe 2O 4 leads to the formation of a disordered wustite-like structure. Cluster glass behavior was found in the Mössbauer study. The investigation suggested ferrimagnetic clusters in an antiferromagnetic matrix. The ferrimagnetic and antiferromagnetic exchange coupling results in a strong uni-directional anisotropy and a coercivity of over 10 kOe after magnetic cooling.

Strong unidirectional anisotropy in spin-density-wave (Cr+6.5%Co)1−xVx alloys

The magnetization, M(T), and magnetic susceptibility, χ(T) = M(T)/H, of the (Cr+6.5% Co)1−xVx (0⩽×⩽5% V) spin-density-wave (SDW) alloy system, have been measured in the presence of a magnetic field, H=100 Oe–5 T, in the field-cooled and zero field-cooled states, over the temperature range, 2⩽T⩽400 K. The thermal hysteresis corresponds to an irreversibility temperature Ti above the Néel temperature TN. M(H) measured at T=2 K in the FC (H=5 T) state exhibits magnetic hysteresis corresponding to a strong unidirectional anisotropy. The remanent magnetization MRem(T) decreases rapidly with increasing temperature, disappearing at Ti. The behavior is explained in terms of spin frustration of Cr moments close to the Co impurity atoms, with a component also due to Co clusters, whose moments are frozen in the SDW phase.

Ferromagnetic resonance in a magnetic trilayer with biquadratic exchange and unidirectional anisotropy

Stable states in a trilayer with a bilinear and biquadratic exchange interaction between ferromagnetically ordered layers and with strong unidirectional anisotropy are studied theoretically. It is shown that, in spin valves of this type, there may be a number of phase transitions of first or second order, and in the former case the width of the hysteresis loop is determined by biquadratic exchange. In the vicinity of the critical field the magnetic dynamics of the layer is shown to be approximately independent of that of other layers.

Impulsive bursts of energetic particles in the high‐latitude duskside magnetosphere of Jupiter

Recent studies using the entire set of proton and electron measurements of the cosmic and solar particle investigations (COSPIN) on Ulysses revealed that the impulsive, sometimes quasi‐periodic bursts discovered during the Ulysses' pass through Jupiter's duskside magnetosphere consisted not only of relativistic electrons and radio wave emissions but also protons and helium nuclei of energies from ∼0.7 to ∼10 MeV/nucleon. In this paper we present a detailed analysis of observations of the proton and helium bursts and of their correlation with relativistic electron bursts. Using an automated computer algorithm, we found 37 nucleonic bursts, all of which occurred in the duskside magnetosphere. Typically, the nucleonic bursts lasted only ∼20 s to ∼2 min and had a very rapid onset and shutoff, showing a spike‐like profile. The flux during the bursts often increased by a factor >10 above the background and had a very strong unidirectional anisotropy directed along the magnetic fields outward from the southern polar region of the planet. The half width of the pitch angle distributions for protons in the bursts was usually less than a few degrees. In the bursts, particles with highest energies were detected first, followed by lower‐energy particles, consistent with the velocity dispersion expected for particles arriving from a source at a significant distance. From this velocity dispersion relation and the O4CS magnetic field model we found that the source of the bursts was located a few Jovian radii above the surface over the southern high‐latitude polar region. Of the 37 nucleonic bursts, about half were associated with an electron burst originating at the same source and at the same time. Energy spectra for protons in the bursts often showed a peak at ∼1 MeV, and above this energy there was spectral hardening relative to preburst and postburst background spectra. These observations suggest that particle acceleration occurs in the high‐latitude polar inner magnetosphere of Jupiter and may be similar to those phenomena observed in the auroral region of Earth's magnetosphere. If the same acceleration process operates on both Earth and Jupiter, these observations imply that Jovian aurorae are probably associated with the high‐latitude boundaries of the polar cap rather than with the Io torus.