Strong Perpendicular Magnetocrystalline Anisotropy Research Articles

Magnetoresistance and magnetic field-induced phase transition in two-dimensional antiferromagnet Fe1/3NbS2

We perform systematic studies on the magnetic and magnetotransport properties of Fe1/3NbS2 single crystal. Results show that Fe1/3NbS2 is an antiferromagnet with strong perpendicular magnetocrystalline anisotropy even in the paramagnetic state. The Fe1/3NbS2 nanoplate exhibits exotic magnetotransport characteristics with variation of temperature and magnetic field in the vicinity of Néel temperature. The temperature dependence of resistivity indicates that the magnetic field has a substantial effect on the Néel order of Fe1/3NbS2, and an intermediate phase occurs before the sample enters into the antiferromagnetic state. It is the field- or temperature-induced antiferromagnetic–intermediate phase and intermediate phase–paramagnetic phase transitions that lead to the novel magnetotransport characteristics in the vicinity of Néel temperature. The findings in this work will promote future studies based on two-dimensional antiferromagnetic spintronics.

Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy

Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

Transverse Rashba effect and unconventional magnetocrystalline anisotropy in graphene-nanoribbon-based centrosymmetric antiferromagnet

The transverse Rashba effect in a centrosymmetric antiferromagnet materialized by double-Gd-adsorbed graphene nanoribbon is investigated via the first-principle calculations. The Rashba effect is associated with not only the local transverse dipole fields induced by the off-center adsorption of the Gd adatoms but also the local magnetic moments. It is perfectly realized that the transverse Rashba effect (Rashba parameter αx reaches to 2.205 eV Å) and strong perpendicular magnetocrystalline anisotropy energy (MAE) coexist in antiferromagnetic (AFM) ground-state structure. However, the global Rashba effect disappears in ferromagnetic structure. The origin of the strong MAE is elaborated in k − space. It is intriguing that the first-order perturbation of the orbit and spin angular momentum coupling is the major source of the MAE due to the specific one-dimensional band structure. The transverse Rashba effect and the strong perpendicular magnetization hosted simultaneously by the proposed centrosymmetric antiferromagnet lock the up- (or down-) spin quantization direction (SQD) to the backward (or forward) movement, and the SQD is related to the chirality of the AFM structure. This finding offers a magnetic approach to a high coherency spin propagation in one-dimensionality, and open a new door to manipulating spin transportation in graphene nanoribbon-based spintronics.

Ultrathin Scale Tailoring of Anisotropic Magnetic Coupling and Anomalous Magnetoresistance in SrRuO3-PrMnO3 Superlattices.

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO3(17 uc (unit cell))/PrMnO3( n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO3 layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 106 to 1.60 × 106 erg/cm3. The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO3 layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO3 layer. Tunneling-like transport at thicker spacer layer in the SrRuO3-PrMnO3 superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

Effects of overlayer capping and lattice strain on perpendicular magnetic anisotropy of TM|FePt|MgO heterostructures

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing the strong perpendicular magnetocrystalline anisotropy (PMA) are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips. To date, it is an urgent and critical issue to continuously promote the PMAs through feasible modifications such as the substitution of ferromagnetic layers as well as the overlayer coating on them. Here, we perform the relativistic first principles calculations of TM|L10-FePt|MgO sandwich systems, and demonstrate that the changes in PMAs by capping TM layers are always giant and positive, e.g., PMA of Fe|FePt|MgO, the largest one among all our studied systems, is about 2 times larger than that of FePt|MgO. The interfacial PMAs at TM|FePt and FePt|MgO interfaces are extracted to be 3.31~9.40 meV and 3.32 meV, respectively, which are at least 3 times larger than 0.93 meV/ML of interior FePt layer. We illustratively verify that PMAs of TM|FePt|MgO can be turned in a large range by varying the TM layer and in-plane strain. Our results and model analyses provide useful insights for how these magnetic quantities are linked, and pave a way for the promotion of PMAs of FePt-based heterostructures via contact with TM overlayers.

페로브스카이트 구조를 가지는 CoFeX3(X = O, F, S, Cl) 합금의 자성과 전자구조에 대한 제일원리계산

스핀전달토크(Spin-Transfer Torque: STT) MRAM의 상용화를 위해서는 낮은 반전전류와 높은 열적 안정성을 동시에 만족해야 하고, 이를 위해서는 큰 스핀 분극, 강한 수직자기이방성 에너지을 가지는 물질이 요구된다. 본 연구에서는 STT-MRAM에 적합한 물질로 알려진 B2 CoFe 면심에 X(O, F, S, Cl) 원자가 위치한 <TEX>$CoFeX_3$</TEX> 합금의 전자구조와 자기결정이방성(Magnetocrystalline anisotropy: MCA) 에너지를 계산하였다. X 원자가 F나 Cl일 때는 페르미 준위에서의 스핀 분극율이 각각 97 %, 96 %로, 반쪽 금속에 근접한 전자구조를 가짐을 확인할 수 있었다. 뿐만 아니라 표면이 Co 원자로 끝나는 5층 박막은 모든 X에 대해 수직 자기이방성를 가졌으며, 특히 <TEX>$CoFeCl_3$</TEX>의 자기이방성 에너지는 약 1.0 meV/cell로 상당히 컸다. 따라서 6, 7 족 원소를 잘 활용하면 높은 스핀 분극율과 강한 수직 자기이방성를 동시에 가지는 물질을 제조할 수 있게 되어 STT-MRAM의 상용화에 기여를 할 수 있을 것으로 기대한다. For an industrial spin-transfer torque (STT) MRAM, low switching current and high thermal stability are required, simultaneously. For this point of view, it is essential to find magnetic materials which satisfy high spin polarization and strong perpendicular magnetocrystalline anisotropy (MCA). In this paper, we investigate electronic structures and MCA energies of perovskite <TEX>$CoFeX_3$</TEX> (X = O, F, S, Cl). For X = F and Cl, spin polarization at the Fermi level are 97 % and 96 %, respectively, which are close to a half metal. Furthermore, Co-terminated 5-monolayer (ML) <TEX>$CoFeX_3$</TEX> (X = O, F, S, Cl) films show perpendicular MCA. In particular, the MCA energy of the Co-terminated <TEX>$CoFeCl_3$</TEX> is about 1.0 meV/cell which is three times larger than that of a 5-ML CoFe film. Therefore, we expect to realize a magnetic material with high spin polarization and strong perpendicular MCA energy by utilizing group 6 and 7 elements in the periodic table, and to contribute to commercializing of the STT-MRAM.

Structural and Magnetic Properties of Sputter-Deposited Mn–Fe–Ga Thin Films

We investigated structural, magnetic and electrical properties of sputter deposited Mn-Fe-Ga compounds. The crystallinity of the Mn-Fe-Ga thin films was confirmed using x-ray diffraction. X-ray reflection and atomic force microscopy measurements were utilized to investigate the surface properties, roughness, thickness and density of the deposited Mn-Fe-Ga. Depending on the stoichiometry, as well as the used substrates (SrTiO3 (001) and MgO (001)) or buffer layer (TiN) the Mn-Fe-Ga crystallizes in the cubic or the tetragonally distorted phase. Anomalous Hall effect and alternating gradient magnetometry measurements confirmed strong perpendicular magnetocrystalline anisotropy. Low saturation magnetization and hard magnetic behavior was reached by tuning the composition. Temperature dependent anomalous Hall effect measurements in a closed cycle He-cryostat showed a slight increase in coercivity with decreasing temperature (300K to 2K). TiN buffered Mn2.7Fe0.3Ga revealed sharper switching of the magnetization compared to the unbuffered layers.

Ni/Fe(001)의 자성과 자기이방성에 대한 제일원리계산

Recent theoretical calculations predicted that a system composed exclusively of 3d transition metals without 4d/5d transition metals or rare earth metals can have strong perpendicular magnetocrystalline anisotropy (MCA) if Fe and Ni layers are arranged appropriately. They considered only Fe-terminated surfaces, noting that Fe/MgO(001) and CoFeB/MgO(001) show strong perpendicular MCA. In this paper, we investigate magnetism and MCA of Ni/Fe(001) surface where Ni layer is positioned at the surface, by using complementarily the first principles calculational methods of Vienna Ab-initio Simulation Package (VASP) and Fullpotential Linearized Augmented Plane Wave (FLAPW) method. Comparing results of magnetism and MCA obtained by VASP with the results by FLAPW method, we find the VASP results do not show big difference from results by FLAPW method. Magnetic moments of Fe and Ni are enhanced due to strong hybridization between Fe and Ni bands. MCA of Ni/Fe(001) is parallel to the surface, which implies the surface termination plays a crucial role in determining MCA of a system.

Strong perpendicular magnetocrystalline anisotropy of bulk and the (001) surface of DO22 Mn3Ga: a density functional study

Strong perpendicular magnetocrystalline anisotropy (MCA) and low saturation magnetization are found in DO22 Mn3Ga using the full-potential linearized augmented plane wave (FLAPW) method. The ferrimagnetism in the bulk is well preserved in the surfaces of Mn3Ga for two possible terminations, where the perpendicular MCA in the (001) direction is greatly enhanced over the bulk, consistent with experiments. Furthermore, the robustness of MCA with respect to lattice strain and a good lattice match with popular substrates suggest that Mn3Ga can be a good candidate for strain-resistance spintronics applications.

Magnetization dynamics in exchange-coupled spring systems with perpendicular anisotropy

Magnetisation dynamics in exchange spring magnets have been studied using simulations of the FePt/Fe bilayer system. The FePt hard layer exhibits strong perpendicular magnetocrystalline anisotropy, while the soft (Fe) layer has negligible magnetocrystalline anisotropy. The variation of the local spin orientation in the Fe layer is determined by the competition of the exchange coupling interaction with the hard layer and the magnetostatic energy which favours in-plane magnetisation. Dynamics were studied by monitoring the response of the Fe layer magnetisation after the abrupt application of a magnetic field which causes the systems to realign via precessional motion. This precessional motion allows us to obtain the frequency spectrum and hence examine the dynamical magnetisation motion. Since the rotation of the spins in the soft layer does not have a well defined magnetic anisotropy, the system does not present the usual frequency field characteristics for a thin film. Additionally we obtain multi-peaked resonance spectra for the application of magnetic fields perpendicular to the film plane, though we discount the existence of spin wave modes and propose that this arises due to variations in the local effective field across the Fe layer. The dynamic response is only considered in the Fe layer, with the FePt layer held fixed in the perpendicular orientation.

Domain wall motion in ferromagnetic systems with perpendicular magnetization

Although we lack clear experimental evidence, apparently out-of-plane magnetized systems are better suited for spintronic applications than the in-plane magnetized ones, mainly due to the smaller current densities required for achieving domain wall motion. [Co/Pt] multilayers belong to the first category of materials, the out-of-plane magnetization orientation arising from the strong perpendicular magnetocrystalline anisotropy. If the magnetization arranges itself out-of-plane narrow Bloch walls occur. In the present paper, both field and current-driven domain wall motion have been investigated for this system, using micromagnetic simulations. Three types of geometries have been taken into account: bulk, thin film and wire, and for all of them a full comparison is done between the effect of the applied field and injected current. The reduction of the system's dimension induces the decrease of the critical field and the critical current, but it does not influence the domain wall displacement mechanism.

Spin injection from perpendicular magnetized ferromagnetic δ-MnGa into (Al,Ga)As heterostructures

Electrical spin injection from ferromagnetic δ-MnGa into an (Al,Ga)As p-i-n light-emitting diode (LED) is demonstrated. The δ-MnGa layers show strong perpendicular magnetocrystalline anisotropy, enabling detection of spin injection at remanence, without an applied magnetic field. The bias and temperature dependence of the spin injection are found to be qualitatively similar to Fe-based spin LED devices. A Hanle effect is observed and demonstrates complete depolarization of spins in the semiconductor in a transverse magnetic field.

Magnetocrystalline anisotropy in (111)CoPt3thin films probed by x-ray magnetic circular dichroism

Angle-dependent x-ray magnetic circular dichroism experiments have been performed at both the Co and Pt ${L}_{2,3}$ edges in two epitaxial (111) ${\mathrm{CoPt}}_{3}$ thin films grown at 690 and 800 K. The analysis of the angular variations of the $3d$ orbital magnetic moment shows two different magnetic behaviors: a strong perpendicular magnetocrystalline anisotropy (PMA) for the film grown at 690 K and an almost isotropic behavior for the film grown at higher temperature. The same analysis at the Pt ${L}_{2,3}$ edges suggests that the $5d$ electrons play an important role in the PMA. Our results correlate the appearance of PMA with the existence of anisotropic structural effects induced during the codeposition process.