TbFeCo Films Research Articles

Effects of inserted ultrathin Pt buffer on perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures

We investigated effects of inserted ultrathin Pt buffer on the perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures. For Tb-rich TbFeCo films buffered with Ta and Ta/Pt a compensation thickness is observed at ∼ 6 nm, where the net saturation magnetization vanishes. The perpendicular magnetic anisotropy properties of two structures are found to be governed by a competition of a perpendicular bulk anisotropy and an in-plane interface anisotropy. Compared with Ta-buffered TbFeCo films, the counterparts with insertion of Pt buffer exhibits a larger negative interface anisotropy. A large variation of saturation magnetization and perpendicular magnetic anisotropy with increasing inserted thickness of Pt buffer is observed in the Ta/Pt/TbFeCo/Ta structure. This can be ascribed to the interface-driven variation of exchange interaction between Tb and FeCo at the interface, confirmed by thermomagnetic measurements. A clear correlation between the interface-driven variation of exchange interaction and perpendicular magnetic properties of the TbFeCo films is identified. Our results show the interface plays an important role in the perpendicular magnetic properties of TbFeCo films, which is critical for the application in spintronic devices.

Field-free transformations of topological spin textures in ferrimagnetic TbFeCo films

The generation of topological spin textures under ultrafast laser pulse excitations and field-free manipulation of topological transitions between different spin textures has attracted enormous interest from the perspective of spintronic applications. Here, we utilize ultrafast electron microscopy to showcase the femtosecond laser pulses excitation on magnetic materials and have generated multiple topological spin textures in an amorphous ferrimagnetic TbFeCo film. Furthermore, the following field-free topological transitions between skyrmions and bubbles with diversified topology are identified via in situ heating the sample in Lorentz transmission electron microscopy. The critical role of uniaxial anisotropy variation on changing the magnetic textures during the heating process is confirmed by micromagnetic simulations. Our results provide a perspective on the generation and transformation of topological spin textures.

MgAl2O4 capping effects on the magnetic properties of TbFeCo films

We investigate MgAl2O4 (MAO) capping effects on the magnetic properties of TbFeCo films. Compared with TbFeCo films, a shift of compensation point to lower temperatures together with significantly reduced saturation magnetization is found in the case of MAO capping. An antiferromagnetic exchange coupling is observed in TbFeCo/MAO structures. This can be well explained by a double layer model with opposite phases. An exchange energy density of ∼ 0.5 erg/cm2 is obtained, which is comparable but lower than that of TbFeCo bilayers with opposite phases. Our findings may be useful for developing ultrathin TbFeCo/MAO heterostructure-based spintronic devices.

Actuation of microstructures with spin-current volume effect

Microactuators are essential for advances in micro-optics, ultrasonic transducers and microsensors, and there is a growing demand for miniaturization and improved power. Here we demonstrate the actuation of micromechanical structures based on spin-current volume effect using an amorphous magnetic film of TbFeCo with volume magnetostriction. A 2 mm-diameter circular polyimide diaphragm coated with thin TbFeCo/non-magnetic metal films is prepared as the micromechanical structure. When an alternating charge current flows through the TbFeCo/non-magnetic metal films on the diaphragm under an external magnetic field orthogonal to the charge current, an alternating spin-current flows in the non-magnetic metal film due to the spin-Hall effect. In the spin-current volume effect, the spin-current transports angular momentum from the non-magnetic metal to TbFeCo film, and the spin-transfer torque modulates the magnetization fluctuation of the TbFeCo film, causing the diaphragm to vibrate due to spin-lattice coupling. The power density of the TbFeCo/Pt films actuator is larger than 1.17 × 106 W m−3 at 20 mA charge current under 7.2 kOe magnetic field. This value is much higher than that of various existing film-type microactuators. This spin-current volume effect is effective as a new actuation mechanism for microactuators used in micro-optical systems, acoustic diagnostic equipment, and micro-fluidic systems etc.

Ultrafast switching to zero field topological spin textures in ferrimagnetic TbFeCo films.

The capability of femtosecond (fs) laser pulses to manipulate topological spin textures on a very short time scale is sparking considerable interest. This article presents the creation of high density zero field topological spin textures by fs laser excitation in ferrimagnetic TbFeCo amorphous films. The topological spin textures are demonstrated to emerge under fs laser pulse excitation through a unique ultrafast nucleation mechanism, rather than thermal effects. Notably, large intrinsic uniaxial anisotropy could substitute the external magnetic field for the creation and stabilization of topological spin textures, which is further verified by the corresponding micromagnetic simulation. The ultrafast switching between topological trivial and nontrivial magnetic states is realized at an optimum magnitude of magnetic field and laser fluence. Our results would broaden the options to generate zero-field topological spin textures from versatile magnetic states and provides a new perspective for ultrafast switching of 0/1 magnetic states in spintronic devices.

Magnetostriction of electroplated TbFeCo thin films

In this work, amorphous TbFeCo films were deposited by an electroplating method. The cantilevered bilayer structures are used to investigate the magnetostriction performance of the TbFeCo films with 0.3–8.1 at% Tb. The electroplated TbFeCo films with Tb composition below 1.5 at% exhibits positive magnetostriction performance, and the films with Tb composition higher than 1.5 at% exhibits negative magnetostriction performance. The largest positive and negative magnetostriction coefficients are 52 and −148 ppm, respectively, at 7490 Oe of the external magnetic field. In addition, magnetization measurements are conducted for the positive and negative magnetostriction samples under strain. A measurable change in the magnetization curves is observed for the negative magnetostriction films. These results suggest that the negative magnetoelastic coupling of Tb-Fe and Tb-Co bonds can cause the dimensions of the films to decrease by the magnetic field, thus, the addition of the Tb element to the electroplated FeCo film results in negative magnetostriction.

Magnetic properties and thermal stability of ultrathin TbFeCo films encapsulated by heavy metals Pt and W

Amorphous rare earth (RE)-transition metal (TM) ferrimagnetic alloy films have been intensively studied recently in spintronics and ultrafast information storage due to the large perpendicular magnetic anisotropy (PMA), ultrafast magnetization switching, and the presence of magnetization compensation and angular momentum compensation. In this work, we fabricate <i>X</i>/Tb<sub><i>x</i></sub>(Fe<sub>0.75</sub>Co<sub>0.25</sub>)<sub>1–<i>x</i></sub>/<i>X</i> (0.13 ≤ <i>x</i> ≤ 0.32, <i>X</i> = SiO<sub>2</sub>, Pt and W) trilayers by magnetron sputtering, and systematically investigate the magnetic properties and thermal stabilities of the ultrathin TbFeCo films encapsulated by heavy metals Pt and W at room temperature. The 5–50-nm-thick TbFeCo films sandwiched by SiO<sub>2</sub> exhibit PMA with magnetic compensation occurring in Tb concentration <i>x</i> between 0.21 with 0.24. For 3-nm- and 5- nm-thick TbFeCo ultrathin films encapsulated by Pt, however, there is no magnetic compensation observed throughout the composition range 0.13 ≤ <i>x</i> ≤ 0.32 with the film magnetization dominated by the FeCo moment. Nevertheless, the weakened PMA for the Pt/ultrathin TbFeCo/Pt trilayers is completely destroyed after annealing at 250 ℃. When the buffer layer and capping layer of Pt are replaced by W, the ultrathin TbFeCo films show magnetic compensation at 0.21 < <i>x</i> < 0.24, so do the thick TbFeCo films. The effective PMA field (<i>H</i><sub>K</sub>) exceeds 11.5 T for the W/ultrathin TbFeCo/W films near the compensation composition, and remarkably, the <i>H</i><sub>K</sub> decreases slowly on annealing, with PMA maintained even after annealing at 350–400℃. We further prepare [Pt/TbFeCo]<sub>5</sub>/Pt and [W/TbFeCo]<sub>5</sub>/W multilayers to clarify the origin of the huge difference between Pt/ultrathin TbFeCo/Pt and the W counterpart. It is found that there are partial recrystallization and phase separation for TbFeCo layer around the Pt/TbFeCo interface, leading to the disappearance of magnetic compensation and the deterioration of the PMA in the Pt/ultrathin TbFeCo/Pt films. With large PMA, W/ultrathin TbFeCo/W films show the presence of magnetic compensation, and excellent thermal robustness. The present study provides a promising heavy metal/RE-TM heterostructure for spintronic applications.

Laser induced spin injection to [GeTe/Sb2Te3] superlattice through a TbFeCo film

A [GeTe/Sb2Te3] superlattice is known as a topological insulator and it shows magnetic responses such as magneto-optical effect although it does not contain any magnetic element. We reported the superlattice has topologically protected spin diffusion length more than 100 μm at room temperature. In this paper, we show a laser induced spin injection to the superlattice using a TbFeCo film. We got spin injection signals between the TbFeCo and the Pt bars through the superlattice without electric current. The signals showed the maximum value of 0.06 μV around laser power of 1.0 mW which was focused to a spot diameter of 0.7 μm. The laser induced spin injection method will be one of the useful methods for future low-energy spintronics devices.

Ferrimagnetic compensation and its thickness dependence in TbFeCo alloy thin films

Rare earth (RE)–transition metal (TM) ferrimagnetic thin films have attracted significant interest for their broad range of functionalities associated with the reduced net magnetization. Here, we study the magnetic properties of Tb-based RE–TM ferrimagnetic [Tbx(FeCo)1−x] thin films. We find that the Tb concentration at which the magnetic moments of the RE- and TM- sublattices compensate increases with decreasing film thickness when the films are grown on Pt underlayers. For the thinnest TbFeCo films (1.5 nm-thick), the magnetic compensation is not observed at room temperature, suggesting that the Tb atoms do not contribute to the magnetization. In these thin films, the perpendicular magnetic anisotropy (PMA) increases when a thin Co layer is inserted, while it drops to zero when a thin Tb layer is inserted. Such contrasting behavior reveals that the PMA originates from the Pt/Co interface. The Tb concentration at which magnetic compensation occurs decreases when the underlayer is changed from Pt to Ta. We infer that Tb becomes magnetically inactive due to intermixing with the Pt underlayer, causing the reduction in PMA.

Fabrication of TbFeCo alloy films with tunable perpendicular coercivity evaluated by extraordinary Hall effect measurements

Rare-earth transition metal (RE-TM) TbFeCo alloy films with adjustable perpendicular switching field and anisotropy are desirable for the applications in high-density spintronic devices. A series of TbFeCo alloy films are fabricated by sputtering a composite target method. The perpendicular magnetic properties of the ferrimagnetic TbFeCo films are investigated by Extraordinary Hall effect (EHE) measurements. With increasing either sputtering power or gas flow rate film composition shifting from FeCo-rich to Tb-rich side is observed and the orientation of magnetic easy axis changing from in-plane, tilted to out-of-plane direction is witnessed. Pronounced change in magnetic properties particularly the coercivity of TbFeCo films is demonstrated, in line with the increased Tb-to-FeCo ratio of the alloy films. It shows TbFeCo films with desirable coercivity can be achieved by proper combination of sputtering power and flow rate. Our work provides an efficient way in tuning the PMA of the sputtered TbFeCo films during the growth and may be used as guidance for designing TbFeCo-based spintronic devices.

Electric field-induced non-volatile 90° rotation of in-plane magnetic anisotropy in TbFeCo film grown on PMN-PT(0 1 1) substrate

A TbFeCo alloy film with in-plane uniaxial magnetic anisotropy was fabricated by pulsed laser deposition on a (0 1 1)-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) substrate. By applying electric field along the surface normal, the magnetic easy axis was rotated 90° from the [0 1 ] to the [1 0 0] direction by strain-mediated magnetoelectric coupling. The modulation effect was further confirmed microscopically from variation of the magnetic domain configuration. Importantly, the magnetic easy axis remains along the [1 0 0] direction after removal of the electric field. We speculate that this non-volatile modulation was triggered by an irreversible rhombohedral to orthorhombic phase transition of PMN-PT.

The effects of annealing temperature and heating rate on Ta/TbFeCo bilayers

Among various perpendicularly anisotropic magnetic materials, amorphous rare earth-transition metal RE-TM alloys have attracted much interest due to the advantages of tunable magnetic properties and suitable perpendicular magnetic anisotropy (PMA) strength. In this study, the magnetic properties of the Ta/TbFeCo/MgO structure with various Tb-contents are investigated under different annealing conditions like heating rate and annealing temperature. The samples were annealed at heating rate from 10 °C/min to 50 °C/min and holding time was varied from 10 min to 30 min with annealing temperatures ranging from 100 to 150 °C. We found that fast heating rate and low holding temperature help to improve PMA and significantly increase the saturation magnetization and squareness of the TbFeCo films.

Ultrafast dynamics of 4f electron spins in TbFeCo film driven by inter-atomic 3d–5d–4f exchange coupling

The ultrafast demagnetization dynamics of 3d and 4f spins, respectively, in FeCo and Tb of TbFeCo alloy film are studied independently by employing a dual-color time-resolved magneto-optical Kerr spectroscopy. The demagnetization dynamics of 3d and 4f spins are independently probed, respectively, by 800 and 400 nm light. Two-step demagnetization dynamics are observed for both the 3d and 4f spins under the excitation of 800 nm laser. In particular, the onset of 4f spin dynamics presents a delayed time with respect to the one of 3d spin dynamics. Those results clearly reveal a strong inter-atomic 3d–5d–4f exchange coupling which drives the first-step subpicosecond ultrafast demagnetization process of 4f spins, and a spin(4f)-lattice coupling which drives the second-step slower demagnetization process of 4f spins. A numerical calculation based on four temperature model reproduces the coupling characteristics in the demagnetization dynamics, and reveals the energy evolution dynamics among the different subsystems. These results provide a direct demonstration of strong coupling dynamics between the two spin subsystems in rare earth-transition metal alloy occurring within subpicosecond timescale, and show a new approach for ultrafast control of 4f spins via an indirect excitation.

Strain-induced switching of heat current direction generated by magneto-thermoelectric effects

Since the charge current plays a major role in information processing and Joule heating is inevitable in electronic devices, thermal management, i.e., designing heat flows, is required. Here, we report that strain application can change a direction of a heat current generated by magneto-thermoelectric effects. For demonstration, we used metallic magnets in a thin-film form, wherein the anomalous Ettingshausen effect mainly determines the direction of the heat flow. Strain application can alter the magnetization direction owing to the magnetoelastic effect. As a result, the heat current, which is in the direction of the cross product of the charge current and the magnetization vector, can be switched or rotated simply by applying a tensile strain to the metallic magnets. We demonstrate 180° switching and 90° rotation of the heat currents in an in-plane magnetized Ni sample on a rigid sapphire substrate and a perpendicularly magnetized TbFeCo film on a flexible substrate, respectively. An active thermography technique was used to capture the strain-induced change in the heat current direction. The method presented here provides a novel method for controlling thermal energy in electronic devices.

Correlation between the Effective Amounts of Elements in TbFeCo Thin Films and Their Magnetic Properties

In this study, TbFeCo thin films were prepared using DC-magnetron sputtering technique utilizing a conventional batch-type sputtering machine. The properties such as magnetic hysteresis loops and magneto-optical properties of the obtained films were measured. Furthermore, we precisely evaluated the “effective” amounts of elements, i.e., the amounts of constituents released by oxidation, in TbFeCo thin films. We also attempted to control the amounts of effective elements by changing the pre-sputtering condition before film preparation. This is because the pre-sputtering process greatly affected the effective amounts of elements in the prepared TbFeCo thin films, such that the magnetic properties were drastically different from sample to sample by altering the pre-sputtering condition. Insufficient pre-sputtering led to a relatively high oxygen content in the films in comparison with those prepared with sufficient pre-sputtering. Outstanding perpendicular magnetic anisotropy in the out-of-plane direction was observed in a film prepared after 60 min of pre-sputtering, which exhibited a coercivity (Hc⊥) value of 6.4 kOe from magnetic hysteresis measurements and 8.2 kOe from magneto-optical polar Kerr hysteresis measurements at an incident light of wavelength 700 nm. The saturation value of the polar Kerr rotation angle (θK) of this film was approximately 0.3°, which is comparable to the theoretically optimized value. Therefore, we demonstrated that high-quality TbFeCo films can be obtained with high reproducibility by using a simple batch-type sputtering machine and that there is a strong correlation between effective amounts of elements in TbFeCo thin films and their magnetic properties.

Thickness and substrate effects on the perpendicular magnetic properties of ultra-thin TbFeCo films

Rare-earth transition metal ultra-thin TbFeCo amorphous films in the thickness range from 3.5 to 35 nm are radio frequency (RF)-sputtered based on a composition target method. Perpendicular magnetic anisotropy (PMA) is found to be strongly related to the thickness of sputtered TbFeCo film in this thickness range. No PMA can be observed in the 3.5 nm film, which may be too short to form local intrinsic single ion anisotropy. Strong PMA develops with the increasing film thickness. A critical compensation thickness of ~14 nm is found, where both the largest coercivity and lowest saturation magnetization are demonstrated. Across this critical thickness the change in polarity of the EHE loops is observed, corresponding to a transition from a FeCo-dominant to a Tb-dominant configuration. Additionally, the magnetic properties of the alloy films are shown to be significantly affected by the substrates used for deposition, which can be mainly attributed to preferential oxidization of the rare-earth Tb element in the films at the interfaces between the film and the substrates. Our results provide a simple method to manipulate magnetic properties of ultra-thin TbFeCo alloy films for the applications in spintronic devices.

A simple method for tuning perpendicular magnetic properties of ultra-thin TbFeCo films

Ultra-thin amorphous TbFeCo films with perpendicular magnetic anisotropy (PMA) are sputtered. The PMA of the alloy films is found to be strongly related to the thickness. A critical thickness is observed to exist, where a transition from a FeCo-dominant to a Tb-dominant configuration occurs. Large variation of magnetic properties is demonstrated for the film in the thickness range across this critical thickness. Our results show the critical thickness may be used as an alternative way to tune the perpendicular magnetic properties of the ultra-thin TbFeCo films.

Roles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown—the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal.

Novel behaviors of anomalous Hall effect in TbFeCo ferrimagnetic thin films

We investigate the temperature dependence and the thickness dependence of anomalous Hall effect (AHE) of TbFeCo ultra-thin films under high magnetic field. The sign change on temperature dependence of AHE in 20nm-thick TbFeCo film with rare-earth (RE) rich composition was observed. The AHE sign at low temperature is negative while it gradually becomes positive as the temperature increases. Moreover, the AHE sign for 5nm-thick TbFeCo film remains positive while that for 50nm-thick TbFeCo film remains negative at temperature in the range from 5 K to 400 K. The similar thickness dependence of AHE in TM-rich samples was also observed. From the mean-field approximation, the sign change temperature in AHE is related to the compensation temperature and the existence of interfacial region, which has the TM-rich composition and the weak anisotropy. Therefore, We clarified that the novel behavior of AHE sign changes in TbFeCo thin films with different thickness can be explained by the interfacial layer with weak anisotropy and two phase model.

Role of the Heat Sink Layer Ta for Ultrafast Spin Dynamic Process in Amorphous TbFeCo Thin Films

The ultrafast demagnetization processes (UDP) in Ta ([Formula: see text] nm)/TbFeCo (20 nm) films have been studied using the time-resolved magneto-optical Kerr effect (TRMOKE). With a fixed pump fluence of 2 mJ/cm2, for the sample without a Ta underlayer ([Formula: see text][Formula: see text]nm), we observed the UDP showing a two-step decay behavior, with a relatively longer decay time ([Formula: see text] around 3.0 ps in the second step due to the equilibrium of spin-lattice relaxation following the 4[Formula: see text] occupation. As a 10[Formula: see text]nm Ta layer is deposited, the two-step demagnetization still exists while [Formula: see text] decreases to [Formula: see text]1.9[Formula: see text]ps. Nevertheless, the second-step decay ([Formula: see text]) disappears as the Ta layer thickness is increased up to 20 nm, only the first-step UDP occurs within 500 fs, followed by a fast recovery process. The rapid magnetization recovery rate strongly depends on the pump fluence. We infer that the Ta layer provides conduction electrons involving the thermal equilibrium of spin-lattice interaction and serves as heat bath taking away energy from spins of TbFeCo alloy film in UDP.