Antiferromagnetic PtMn Research Articles

Shape anisotropy induced field-free switching and enhancement of dampinglike field in Pt/Co/PtMn heterostructures with a wedged ultrathin antiferromagnetic PtMn layer

Shape anisotropy induced field-free switching and enhancement of dampinglike field in Pt/Co/PtMn heterostructures with a wedged ultrathin antiferromagnetic PtMn layer

Magnetic Transition State Searching: Beyond the Static Ion Approximation

The effect of structural relaxations on the magnetocrystalline anisotropy energy (MAE) was investigated by using density functional theory (DFT). The theory of the impact of magnetostructural coupling on the MAE was discussed, including the effects on attempt frequency. The MAE for ferromagnetic FePt (3.45 meV/formula unit) and antiferromagnetic PtMn (0.41 meV/formula unit) were calculated within the local density approximation (LDA). The effects of the structural relaxation were calculated and found to give a <0.5% reduction to the MAE for the ferromagnet and ∼20% for the antiferromagnet.

Electrical manipulation of the magnetic order in antiferromagnetic PtMn pillars

Antiferromagnets are magnetically ordered materials without a macroscopic magnetization. As a result, they could be of use in the development of memory devices because data cannot be erased by external magnetic fields. However, this also makes it difficult to electrically control their magnetic order (Neel vector). Here, we show that pillars of antiferromagnetic PtMn, which are grown on a heavy-metal layer and have diameters down to 800 nm, can be reversibly switched between different magnetic states by electric currents. The devices are based on materials that are typically used in the magnetic memory industry, and we observe switching down to a current density of ~2 MA cm−2. Furthermore, by varying the amplitude of the writing current, multilevel memory characteristics can be achieved. Micromagnetic simulations suggest that the different magnetic states may consist of domains separated by domain walls with vortex and anti-vortex textures that move in response to current, modifying the average Neel vector. Pillars of antiferromagnetic PtMn, grown on a heavy-metal layer, can be reversibly switched between different magnetic states by electric currents, illustrating the potential of silicon-compatible antiferromagnetic materials in the development of memory and computing devices.

Composition dependence of exchange anisotropy in PtxMn1−x/Co70Fe30 films

We systematically investigated the exchange anisotropy for ferromagnetic Co70Fe30 and antiferromagnetic Pt-Mn bilayer films. We focused on the relevance between the exchange bias and the composition of the PtxMn1−x (14 < x < 22 and 45 < x < 56 at%) films, and we successfully optimized the composition. The crystal structure of the PtxMn1−x films was FCC for 14 < x < 22 at% and FCT for 45 < x < 56 at% after annealing at 370 °C for 6 h. The unidirectional anisotropy constant (Jk) for fcc-Pt15Mn85 (20 nm) and fct-Pt48Mn52 (20 nm) prepared under optimum conditions in composition were 0.16 and 0.20 erg cm−2, respectively. Both Pt15Mn85 and Pt48Mn52 films showed a larger unidirectional anisotropy constant (Jk) than in other reports for the bilayer system that uses Pt-Mn antiferromagnetic layer. They also showed a flatter surface than that of other antiferromagnetic/ferromagnetic materials, which is suitable to avoid unexpected interlayer coupling in spin-valve structure. The obtained Pt-Mn films with a large exchange anisotropy and slight roughness are useful as an antiferromagnetic layer in spintronic applications.

Angle dependent magnetoresistance in heterostructures with antiferromagnetic and non-magnetic metals

We report on the magnetoresistive effects in a metallic antiferromagnet (AFM)/nonmagnet (NM) PtMn/Pt heterostructure with varying thickness of PtMn (1 nm ≤ tPtMn ≤ 10 nm). Longitudinal magnetoresistive effects are measured for magnetic field rotations along three mutually perpendicular directions. The results show a transformation in the functional dependencies with the increase in tPtMn and are attributed to the spin Hall magnetoresistive effects arising from the interaction of spin currents generated by Pt with the antiferromagnetic PtMn. The present findings with appreciable magnetoresistive effects in an AFM/NM metallic heterostructure add yet another option of the material system for antiferromagnetic spintronics.

59Co NMR analysis of CoFeB-MgO based magnetic tunnel junctions

59Co Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) has been used to investigate annealing effects on crystallographic structure and magnetic properties of ferromagnetic CoFeB layer in pseudo spin valves (P-SV): Ta/CoFeB/MgO/CoFeB and in an exchange bias spin valves (EB-SV): PtMn/CoFe/Ru/CoFeB/MgO/CoFeB magnetic tunnel junctions. It was found, that the as-deposited CoFeB layers in the P-SV and EB-SV are amorphous. The annealing treatment at 340 °C and 360 °C induces in all studied systems progressive phase segregation in CoFeB layer to areas of bcc CoFe regions displaying different degree of B2 order and to areas of an amorphous magnetic reminder with a higher B content. In the CoFeB layers grown on Ru (EB-SV) and Ta (P-SV) and covered by MgO in the upper interface, after annealing at 500 °C a state of substantially full phase segregation to B2 ordered bcc CoFe phase has been observed. The annealing at 500 °C of the CoFeB layers grown on MgO and covered by Ru in P-SV and EB-SV triggers extensive thermal diffusion and induces mixing at the CoFeB/Ru interface which transforms the CoFeB into a non-magnetic layer. Moreover, it has been found that CoFeB crystallizes better into the bcc CoFe (100) texture when deposited on Ta than on a Ru layer. XRD profiles show that while CoFeB layer is growing on Ru it crystallizes into bcc CoFe (110) oriented texture, and the one grown on Ta crystallizes into bcc CoFe (200) when annealed at 500 °C. 59Co NMR restoring field analysis (Hrest - magnetic stiffness) showed that the insertion of the antiferromagnetic PtMn layer into the EB-SV system increases the magnetic hardness of the CoFeB layer by an order of the magnitude. However, adding CoFeB/MgO layers between PtMn/CoFe/Ru and CoFeB layers in the EB-SV decouples the CoFeB from the hard-magnetic system making it magnetically soft.

Stack Structure Dependence of Magnetic Properties of PtMn/[Co/Ni] Films for Spin-Orbit Torque Switching Device

We investigate the stack structure dependence of magnetic properties on thin films that consist of an antiferromagnetic PtMn and a ferromagnetic Co/Ni multilayer for field-free spin-orbit torque-induced magnetization switching devices. Magnetic parameters, such as the spontaneous magnetization, effective and interfacial magnetic anisotropies, and exchange bias field are quantified as a function of stack structure. Engineering of the stack allows the improvement of current-induced magnetization switching characteristics compared with a previous work, which is confirmed using patterned Hall cross devices.

Spin-orbit torques and Dzyaloshinskii-Moriya interaction in PtMn/[Co/Ni] heterostructures

Antiferromagnet (AFM)/ferromagnet (FM) heterostructures with broken inversion symmetry are perceived to open new opportunities for nonvolatile spintronic devices. Previous studies of such systems have demonstrated an emergence of spin-orbit torques (SOTs) in the heterostructures which are strong enough to bring about magnetization reversal. The impact of broken inversion symmetry and spin-orbit coupling also leads to an emergence of the Dzyaloshinskii-Moriya interaction (DMI) which governs the magnetic configuration and magnetization reversal. In this work, we study the SOT-induced effective fields and DMI in a heterostructure with an antiferromagnetic PtMn layer and a ferromagnetic [Co/Ni] multilayer and compare the results with a reference Pt/[Co/Ni] system. Magnetotransport measurements reveal the same sign and similar magnitude of SOT-induced effective fields for the two systems while current-induced domain wall motion measurements under in-plane fields reveal the opposite sign and smaller magnitude of DMI at the PtMn/[Co/Ni] interface compared to the Pt/[Co/Ni]. The obtained results offer in-depth information concerning the manifestations of spin-orbit interactions in AFM/FM systems, which is key to understanding of static magnetic configuration and magnetization reversal for their possible applications in antiferromagnetic spintronics.

Field-free spin-orbit torque switching of composite perpendicular CoFeB/Gd/CoFeB layers utilized for three-terminal magnetic tunnel junctions

Spin-orbit torque (SOT) induced magnetization switching has become a research focus in spintronics because it enables energy-efficient switching. There have been several experiments realizing field-free SOT-induced magnetization switching of materials with perpendicular magnetic anisotropy (PMA) in a bilayer system, either using thin Co(Fe) and CoFeB layers with interfacial PMA or using Co/Ni multilayers. All of these stacks are ferromagnets with large saturation magnetization (MS). Here, we demonstrate SOT switching in a multilayer stack of CoFeB/Gd/CoFeB. This stack shows a good PMA and a low MS (370 ± 20 emu/cm3), where CoFeB and Gd layers are antiferromagnetically exchange-coupled with each other. SOT induced magnetization switching has been demonstrated in this stack at zero magnetic field with a switching current density of ∼9.6 × 106 A/cm2 by using antiferromagnetic PtMn as the spin Hall channel material. The spin Hall angle of PtMn was also determined to be ∼0.084 ± 0.005 by performing a second harmonic Hall measurement. This layer structure is compatible with perpendicular magnetic tunnel junctions (p-MTJs), which could enable field-free three-terminal p-MTJs and lead to memory and logic devices based on SOT.

Device-size dependence of field-free spin-orbit torque induced magnetization switching in antiferromagnet/ferromagnet structures

We study spin-orbit torque induced magnetization switching in devices consisting of an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with sizes ranging from 5 μm to 50 nm. As the size decreases, switching behavior changes from analogue-like to stepwise with several intermediate levels. The number of intermediate levels decreases with the decreasing size and finally evolves into a binary mode below a certain threshold. The results are found to be explained by a unique reversal process of this system, where ferromagnetic domains comprising a number of polycrystalline grains reverse individually and among the domains both out-of-plane and in-plane components of exchange bias vary.

(001) FePt graded media with PtMn underlayers

(001)-oriented FePt graded media are obtained by using PtMn underlayers. The PtMn underlayer not only behaves as the (001) structural template but provides the diffusion source of Mn. The diffusion of Mn into FePt reduces its anisotropy but, on the other hand, the exchange coupling between antiferromagnetic PtMn and ferromagnetic FePt enhances the anisotropy. Hysteresis loops taken from x-ray magnetic circular dichroism confirm the competition between these two effects, leading to the lowest anisotropy in the middle of FePt.

Enhanced exchange anisotropy between antiferromagnetic PtMn and ferromagnetic Co system by AuCu underlayer

The introduction of a Au50Cu50 (in at. %) seed and dynamic-stress-induced Pt50Mn50 ordered-assisting layer into PtMn/Co exchange-coupled films resulted in a unidirectional anisotropy constant Ke≒0.37 erg/cm2, approximately 20% larger than the previous reported results and in a strong PtMn(111) crystalline orientation even after thermal annealing. It was considered that the former was due to the increase in exchange-coupling density with the strong PtMn(111) orientation and to the increase in lattice distortion of the PtMn film with excessive dynamic-stress-induced PtMn ordered-assisting effect of the AuCu film. The cause of the latter was that the AuCu film effectively functioned as a seed layer of the PtMn film.

Molecular beam epitaxy growth of exchange-biased PtMn∕NiFe bilayers with a spontaneously ordered PtMn layer

We report the direct epitaxial growth of equiatomic ordered antiferromagnetic PtMn layers by molecular beam epitaxy. Such layers are used in giant magnetoresistance spin valve sensors as the antiferromagnetic pinning layer. Structural characterization and phase identification confirmed the spontaneous formation of the chemically ordered face-centered-tetragonal (L10) phase of PtMn with about 87.4% ordering. Based on the antiferromagnetic PtMn layer, we prepared exchange-biased PtMn∕NiFe bilayers with various PtMn thicknesses. The exchange anisotropy field of the bilayer with NiFe grown on PtMn stabilizes at about 50Oe beyond a PtMn thickness of 15nm. Although the exchange anisotropy field is small compared to that of the polycrystalline system, the antiferromagnetic domain structure is stable over repetitive external magnetic field cycling and no training effect is observed.

Antiferromagnetic relaxation and induced anisotropy inFe∕twinned-PtMnbilayers

We report the observation of unusual magnetic anisotropies in epitaxially grown Fe layers on PtMn films. The antiferromagnetic PtMn film has a twinned structure with crystallites of 8 nm. Each single PtMn grain is chemically ordered and exhibits a twofold magnetocrystalline anisotropy with an uncompensated surface magnetization. The bilayers display hysteresis with a complex dependence on applied field angle. A three vector model predicts the angular dependence of the hysteresis loops (shift and shape) of the Fe film and gives a detailed understanding of the Fe and antiferromagnet spin orientations during reversal. The interface exchange coupling between the Fe and the uncoupled PtMn twins accounts for the asymmetric magnetization reversal and high order anisotropies.

XMCD measurements of exchange biased PtMn/Co bilayers

The magnetic properties of a series of PtMn (t A ̊ )/ Co (20 A ̊ ) (t=50, 100, 150, 200 A ̊ ) exchange biased films are investigated using bulk magnetometry and X-ray magnetic circular dichroism (XMCD). From the bulk magnetometry measurements, the exchange field and coercivity is observed to increase with PtMn thickness. XMCD spectra are used to probe the interface region between the ferromagnetic Co and antiferromagnetic PtMn. The observed Mn XMCD is indicative of uncompensated Mn at the interface. These uncompensated Mn spins may be partially responsible for the increased coercivity observed in this exchange biased system.

Transmission electron microscopy analysis of crystallographic transition from fcc to fct on PtMn spin valves

The structural changes during the crystallographic transition from fcc to fct of antiferromagnetic PtMn layers with postdeposition annealing, in spin-valve multilayers constructed by Ta/PtMn/CoFe/Cu/CoFe/NiFe/Ta/Si, were investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. We determined that the crystallographic orientation of the PtMn layers after postdeposition annealing depends on the initial size of the columnar grains in as-deposited spin valves. For spin valves with an initial grain size of 10 nm, the fcc→fct transition of PtMn layers was completed with the crystallographic orientation on the (111) texture. In contrast, spin valves with an initial grain size of 50 nm brought about a crystallographic reconstruction of the PtMn layer due to the tensile stress from neighboring grains during the fcc→fct transition. This results in a transformation from a fcc(111) to a fct(101) texture and a recombination of grains to form giant grains of 300–500 nm in the PtMn layer. This dynamic reconstruction promotes the interlayer diffusion of constituent atoms in spin-valve multilayers, leading to an increase in the interlayer coupling field between the pinned and free layers (Hin) and a promotion of thermal degradation of magnetoresistance (ΔR).

Exchange coupling and stability of PtMn-based spin valves

The exchange coupling and stability of spin valves with antiferromagnetic (AF) PtMn films were investigated through the correlation between the magnetic properties and their microstructures. The exchange coupling field (H/sub ex/) decreases during iterative MR measurements and this decrease associates closely with the changes in the rotational hysteresis loss (W/sub r/), exchange anisotropy energy (K/sub e/) and blocking temperature (T/sub b/) distribution. The ordering of PtMn films, featuring an antiphase configuration, is found to play a significant role in determining the exchange coupling properties and stability of these spin valves.

PtMn 反強磁性膜と強磁性膜の交換結合

PtMn 反強磁性膜と強磁性膜の交換結合

Dual-Spin-Valve Elements Using PtMn Films

PtMn-dual-spin-valve and PtMn-single-spin-valve unshielded elements using NiFe/Cu/NiFe films with longitudinal hard bias were tested by measuring the transfer curves in various transverse magnetic fields. Transfer curves measured at ±2kOe had steep slopes, showing the rotation of the magnetization direction in the free layer from parallel to antiparallel relation to the pinned layer in low magnetic fields. After magnetization of the CoPt hard-magnetic-bias films in a longitudinal direction, the transfer curve measured at ±200 Oe showed no hysteresis and no Barkhausen-jump. The output of the dual-spin- valve elements increased in proportion to the increase in the sense current, and did not depend on the polarity of the sense current. Further, the asymmetry of the output at ±40 Oe was found to be flat over a wide range of sense currents from -10 mA to +10 m A. The output normalized by the sense current density of the dual-spin-valve element was approximately 1.4 times as large as that of the single-spin-valve element. We have thus demonstrated some advantages of dual-spin-valve elements using antiferromagnetic PtMn films.