GdFeCo Films Research Articles

Intrinsic subpicosecond magnetization reversal driven by femtosecond laser pulses in GdFeCo amorphous films

Ultrafast magnetization dynamics in GdFeCo films triggered by femtosecond laser pulses with and without an external field applied is studied experimentally for different excitation fluence. It is found that subpicosecond magnetization reversal occurs simultaneously in the ultrafast dynamics of both saturation and remnant magnetization states and almost identical within 13 ps, whereas relatively slow magnetization reversal across compensation point appears only in the dynamics of saturation magnetization state. It shows the subpicosecond magnetization reversal is external field independent, and originates from intrinsic magnetic evolution in ferrimagnetic system. The intrinsic subpicosecond reversal is qualitatively explained by linear reversal.

The study of ultrafast magnetization reversal across magnetization compensation temperature in GdFeCo film induced by femtosecond laser pulses

The subpicosecond magnetization reversal is observed by a femtosecond (fs) laser ultrafast heating of the GdFeCo film across magnetization compensation temperature, under the saturated magnetic field. Then the film is induced by fs laser without external magnetic field, and the curve of magnetization dynamics also shows magnetization reversal crossing zero magnetization. Such result shows that the ultrafast magnetization reversal in GdFeCo system induced by ultrafast heating the film across magnetization compensation temperature does not originate from the extrinsic magnetic field. To study the origin of the magnetization reversal and reveal the role of the extrinsic magnetic field and the fs-laser pump, the external field dependence and the pump fluence dependence of magnetization dynamics are measured. The external field dependence result shows that the curves of ultrafast magnetization reversal almost overlap in the initial several picoseconds (ps) and then the nucleation and growth of the reversed domains are accelerated with increasing external fields. And the pump fluence dependence result shows that the magnetization reversal occurs only when the pump fluence is intense enough and the magnetization reversal increases with the pump fluence. Therefore the ultrafast magnetization reversal during the initial several picoseconds may originate from the intense-enough ultrafast heating of fs laser, and the extrinsic magnetic field is responsible for the later domain formation and domain-wall motion.

The Study of the Delay Kerr Loops Induced by Femtosecond Laser Pulses in GdFeCo Film

To study the evolution of the coercivity and magnetization induced by femtosecond laser pulses in GdFeCo film at distinct pump–probe delays and reveal the laser-induced magnetization reversal process, the static delay Kerr loops were measured by the traditional time-resolved magneto-optical Kerr effect technique. The results showed that the loops contained memory effect from external field history and could not reflect the real change of coercivity and magnetization after laser pump. To eliminate the memory effect, a home-made experimental setup of photomagnetic synchronized time-resolved magneto-optical Kerr effect technology was used to measure the dynamic delay Kerr loops and reveal the real evolution of the coercivity and magnetization after laser pump, whose results are of great importance in real magneto-optical recording application.

Tunable perpendicular magnetic anisotropy in GdFeCo amorphous films

We report the compositional and temperature dependence of magnetic compensation in amorphous ferrimagnetic GdxFe93−xCo7 alloy films. Magnetic compensation is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal (TM) ions and ferromagnetic interaction between the TM ions. The low-Gd region of x between 20 and 34 was found to exhibit compensation phenomena characterized by a low saturation magnetization and perpendicular magnetic anisotropy (PMA) near the compensation temperature. Compensation temperature was not observed in previously unreported high-Gd region of x=52–59, in qualitative agreement with results from recent model calculations. However, low magnetization was achieved at room temperature, accompanied by a large PMA with coercivity reaching ~6.6kOe. The observed perpendicular magnetic anisotropy of amorphous GdFeCo films probably has a structural origin consistent with certain aspects of the atomic-scale anisotropy. Our findings have broadened the composition range of transition metal-rare earth alloys for designing PMA films, making it attractive for tunable magnetic anisotropy in nanoscale devices.

Contributions of electron and phonon transport to the thermal conductivity of GdFeCo and TbFeCo amorphous rare-earth transition-metal alloys

We experimentally investigate the electron and phonon contributions to the thermal conductivity of amorphous GdFeCo and TbFeCo thin films. These amorphous rare-earth transition-metal (RE-TM) alloys exhibit thermal conductivities that increase nearly linearly with temperature from 90 to 375 K. Electrical resistivity measurements show that this trend is due to an increase in the electron thermal conductivity over this temperature range and a relatively constant phonon contribution to thermal conductivity. We find that at low temperatures (∼90 K), the phonon systems in these amorphous RE-TM alloys contribute ∼70% to thermal conduction with a decreasing contribution as temperature is increased.

Field-dependent ultrafast dynamics and mechanism of magnetization reversal across ferrimagnetic compensation points in GdFeCo amorphous alloy films

Ultrafast dynamics of genuine magneto-optical recording across ferrimagnetic compensation points is demonstrated in GdFeCo films using time-resolved polar Kerr spectroscopy combined with a laser-synchronized sinusoidal alternating magnetic field which can reinitialize irreversible initial magnetization state to laser radiating. The external field dependence of magnetization reversal dynamics is measured and shows that reversal rate accelerates with increasing external fields. Analysis of the magnetization reversal dynamics with Bloch equation shows the magnetization reversal rate is linearly dependent on the external fields within experimental errors, which supports quantitatively that the mechanism of magneto-optical recording in rare earth-transition metal ferromagnetic films is related to the formation and growth of the reversed domains. It is also shown that nucleation field is obviously larger than hot coercivity shown in the anomalous hysteresis loop.

Origin of anomalous hysteresis loops induced by femtosecond laser pulses in GdFeCo amorphous films

A controllable pump-pulse-number magneto-optical Kerr technique combined with an initializing field scanning approach is developed to eliminate and identify memory and accumulation effects, respectively, from external field history and multiple pulse excitations. A series of anomalous loops of GdFeCo films are measured for different amount of pump pulses using this technique, revealing that serious memory and accumulation effects exist in continuous-pulse-pumped anomalous hysteresis loops which show illusory information of hot coercivity and degree of magnetization reversal. Single-pulse-induced anomalous loop reveals that the hot coercivity shown by continuous-pulse-pumped anomalous loops is not the minimum external field that drives real magneto-optical recording.

Physical origin of transient Kerr peak induced by circularly polarized laser in ferromagnetic films

The magnetization dynamics of in-plane anisotropic FePt and perpendicularly anisotropic GdFeCo films induced by circularly polarized laser excitation is investigated using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that a transient Kerr peak occurs near zero-time delay, respectively for each sample. The origin of the peaks is analyzed. It is pointed out that the transient Kerr peaks are not related to ferromagnetism and may originate from paramagnetic magnetization of free electrons in the samples, while the magnetic field that magnetizes the free electrons comes from inverse Faraday effect of the circularly polarized pump laser. The calculation results based on the paramagnetic magnetization model support such a viewpoint. Based on our points, the duration of magnetic pulses induced by inverse Faraday effect should be the same as that of the pump laser pulses.

Ultrathin (Gd, Tb)-FeCo Films With Perpendicular Magnetic Anisotropy

Perpendicular ultrathin RE-TM films with thickness 3 nm were investigated for potential use as free layers of perpendicular magnetic tunnel junctions. The free layer thickness should be as thin as possible to reduce spin-transfer switching current density while maintaining adequate thermal stability. Using Ti or Al as under- and over-layers, we found that both 3 nm GdFeCo and TbFeCo films had perpendicular magnetic anisotropy (PMA), but DyFeCo had PMA only in cases where thickness exceeded 5 nm. The effective PMA constants of TbFeCo films were of order of 105 erg/cm3, which provided sufficient thermal stability. By annealing at various temperatures, we found that the coercivity of TbFeCo films can be ranged from 120 to 1000 Oe. Therefore, insertions of under/overlayers together with suitable annealing treatment provide a way to fabricate RE-TM ultrathin layer with desired magnetic properties.

Compositional Dependence of g-Factor and Damping Constant of GdFeCo Amorphous Alloy Films

Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g-factor g eff and effective damping constant alphaeff of the GdFeCo films were estimated by using a numerical calculation of Landau-Lifshitz-Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition C M of the GdFeCo, while the estimated g eff and alphaeff increased around the angular momentum compensation composition C A. The compositional dependences of g eff and alphaeff were roughly described by a mean-field model. The g eff and alphaeff were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near C M. The FMR method was unable to excite the magnetization near C M because of the small net magnetization.

Excitation of Coherent Spin Waves in GdFeCo film

We present results of a time-resolved study of the ultrafast magnetic response of Gd23.1Fe71.9Co5.0 MAMMOS structures under conditions near actual read/write temperatures. An all-optical pump and probe method was used in which an intense (pump) light beam excited a medium due to ultrafast laser heating and a less intense (probe) beam monitored this photo-excited state through the magneto-optical Kerr effect. Our experiment clearly demonstrates that the photo-excitation effectively excites coherent spin waves in the magnetic material. Precession frequencies of several GHz and relaxation times in the nanosecond range were observed.

Capability of Tera bps Ultrafast Rewritable Storage by Thermomagnetic Switching

Ultrafast 100 femtosecond laser light was irradiated on an amorphous GdFeCo film under a DC magnetic field. Complete magnetization reversal and rewritable response were observed in a less than 1 picosecond time period. This experiment reveals the intrinsic capability of ultrafast high-speed rewitable storage under the thermomagnetic switching scheme. Current pure magnetic recording such as that in the case of hard disk storage has a serious spin reversal time problem limited by the ferromagnetic resonance (FMR) frequency. Such frequency is ordinarily 1 to 3 GHz, which means that the recording speed limit is of a similar order. A one-picosecond response enables a very attractive future information storage scheme realizing 10 to 1000 Gbps ultrafast devices.

Different mechanisms of spin reorientation in exchange coupled double rare earth-transition metal layers with in-plane and perpendicular magnetic anisotropy

The objective of this article is to clarify the spin reorientation effects in exchange coupled layers based on rare earth and transition metals with in-plane and perpendicular anisotropy. These systems are proposed for high density magneto-optical recording with super resolution readout. Two mechanisms are found to be responsible for this change of the magnetization direction with the temperature. The first mechanism is an intrinsic spin reorientation in the GdFeCo layer and the second is the exchange coupling force between the perpendicularly magnetized TbFeCo film and the in-plane GdFeCo film. With appropriate adjustment of the compensation temperatures and the thicknesses of the films, it is possible that the one or the other mechanism dominates. The experimental results are in agreement with a numerical calculation of the magnetization profiles in the coupled system.

Magnetization reorientation in ferrimagneticGd27.5Fe59Co13.5/Dy28Fe60Co12double layers

We present experimental results on magnetization reversal for double-layer (GdFeCo/DyFeCo) magneto-optical films. The structure of interest consists of a 40 nm DyFeCo layer with perpendicular magnetization and a 50 nm layer of GdFeCo with in-plane magnetization. A transition from in-plane to perpendicular orientation of the magnetization in GdFeCo film caused by the effect of exchange interaction with DyFeCo has been observed in just a small temperature range. For higher temperatures, the exchange effect becomes smaller and GdFeCo magnetization returns back to in-plane orientation. On the basis of a micromagnetic model, we predict a change in orientation of GdFeCo magnetization from in-plane to perpendicular when it is coupled to DyFeCo film in a certain temperature range. This transition is sensitive to the magnetic properties of each layer that are dependent on temperature and on the values of compensation and Curie temperatures of both GdFeCo and DyFeCo films.

Exchange coupling and switching fields of RE—TM double and triple layer stacks for direct overwrite

The recording properties of common single layer magneto-optical (MO) disks can be improved by introducing exchange coupled rare-earth (RE) transition-metal (TM) films, both from the point of view of capacity and data transfer rate. Direct overwrite (DOW) is a method to double the data transfer rate during writing, because the write process of new bits and the erase process of old bits is performed simultaneously. MO multilayer stacks must exhibit exchange coupling in order to be suitable for DOW. The switching fields of each layer depend on coercive energy, magnetic field energy and the energy of the interface wall between coupled layers. In this paper we discuss the dependence of switching fields on layer thickness and wall energy σw for several double and triple layer stacks. The memory and reference layers are TbFeCo and DyFeCo, respectively. Triple layer stacks have an intermediate layer (GdFe and GdFeCo) to adjust the wall energy. DOW presupposes a weak coupling at 300 K and a strong coupling at higher temperatures. This requires a very thick reference layer in double layer stacks, causing excessive heat capacities. However, for triple layers the DOW demands are met in much thinner stacks as the wall energy is adjusted through the intermediate layer. The wall energy of the intermediate layer is determined by its anisotropy. We studied the anisotropy of evaporated GdFe, GdCo and GdFeCo films as a function of composition. GdCo showed only in-plane anisotropy, while GdFe was perpendicularly magnetized for Fe contents up to 87 at%. Adding Co to GdFe leads to a layer with a temperature dependent easy axis. The carrier-to-noise ratio (CNR) of a series of magneto-optical disks, both double and triple layer stacks, was determined. The maximum CNR of 51.4 dB is comparable to results on a single layer disk with the same memory layer. On triple layer stacks a CNR > 45 dB could be written at a laser power below 9 mW.

ANISOTROPY OF AMORPHOUS Gd-TM FILMS AND ITS APPLICATION FOR DOW AND MSR RECORDING

The recording properties of common single layer magneto-optical (MO) disks can be improved by introducing exchange coupled rare-earth (RE) transition-metal (TM) films, both from the point of view of capacity and data transfer rate. MO multilayer stacks must exhibit exchange coupling in order to be suitable for direct overwrite (DOW) and magnetic superresolution (MSR). Thin DOW stacks require an interface layer to adjust the exchange coupling. Soft magnetic Gd-Fe and Gd-FeCo interface layers exhibit suitable magnetic properties to meet the DOW demands. The best solution would be a RE-TM interface layer with inplane anisotropy at room temperature and perpendicular anisotropy at elevated temperatures. Such a layer can also be used as a read-out layer in MSR stacks. Therefore we studied the anisotropy of evaporated and sputtered Gd-TM films as a function of composition. Gd-FeCo films with a Co/(Fe+Co) ratio less than 0.47 showed a transition from inplane to perpendicular anisotropy with increasing temperature. The transition temperature Ttrans depends on the Co/(Fe+Co) ratio and the Gd content and can be adjusted between 0 K and TC. The temperature range ΔTtrans, where the magnetization direction changes from totally inplane to totally perpendicular, was 70 ± 25 K for all evaporated samples while it depends on Ar pressure for sputtered samples. Triple layer DOW disks showed a maximum CNR of 51.4dB.

TEMPERATURE INDUCED FERRO-ANTIFERROMAGNETIC TRANSITIONS IN EXCHANGE-COUPLED DOUBLE LAYERS WITH MIXED ANISOTROPIES

The temperature dependence of the magnetization processes in mixed exchange-coupled double layers ECDL with a planar GdFeCo film 1 and a perpendicular anisotropy DyFeCo film 2 is analyzed from Faraday rotation measurements. During heating the magnetic state of the ECDL presents strong modifications depending not only on the external field applied along the normal to the layers, but mainly on the ECDL type which can change from a type 1 with an interlayer macroscopic antiferromagnetic (AF) coupling -A12M1.M2 to a type 2 with a ferromagnetic (F) coupling. From appropriate choice of the deposition parameters the films compositions were adjusted for either Tcp1<RT<Tcp2 or RT<Tcp2<Tcp1 where Tcp1,2 are the compensation temperatures of the layer 1 and 2 which gives the possibility to obtain at RT ECDL of type 1/AF or type 2/F respectively. Depending on the respective values of Tcp1 and Tcp2, it is observed by increasing the temperature, F→AF or AF→F type transitions at Tcp1 and Tcp2. The evolutions with the temperature of the magnetooptical (MO) hysteresis loops of the ECDL are explained at the first order from the parallel change of the individual loops of the layers 1 and 2. However very close to Tcp2 more complex MO loops are observed and can be explained from the rise of the hysteresis Hc1 in the planar layer 1 induced by a strong increase of the coercitivity Hc2 of the perpendicular layer 2 near its compensation temperature.

Ferro- and antiferromagnetic exchange coupling in magnetooptical bilayers with planar and perpendicular anisotropy

Macroscopic ferro- (F) and antiferromagnetic (AF) exchange couplings -A/sub 12/M/sub 1/.M/sub 2/ between a planar GdFeCo film 1 and a perpendicular anisotropy DyFeCo film 2 have been investigated from the magnetization processes of the bilayer as observed from Faraday rotation measurements. Whatever the F or AF coupling, the microscopic exchange interaction between the rare-earths and the transition metals at the interface induces mutual effects with a reduction of the coercivity H/sub C2/ and the squareness M/sub r2//M/sub s2/ of the "perpendicular" layer and a reduction of the saturation field H/sub s1/, but an increase of the coercivity H/sub C1/ of the planar layer. Experiments performed in single planar and perpendicular layers and in both pseudo F and AF bilayers are in agreement with our theoretical model which enables deduction of the macroscopic exchange constant A/sub 12/.

SUPER RESOLUTION READOUT OF A MAGNETO-OPTICAL DISK WITH AN IN-PLANE MAGNETIZATION LAYER

A new magneto-optical (MO) disk, which consists of magnetic double layers, for super resolution readout was developed. We developed a new characteristic GdFeCo film as a readout layer whose magnetization direction changes from in-plane to perpendicular as a temperature rises. Using such a GdFeCo film combined with a DyFeCo recording film, higher linear density and track density than those of the conventional MO disks were achieved without an external magnetic field for readout and with a conventional optical head.

Magnetostriction of amorphous GdFeCo and YFeCo thin films

The contribution of Gd and/or (Fe,Co) to the saturation magnetostriction constant λs of pseudobinary Gdx(Fe100−yCoy)100−x (9&amp;lt;x&amp;lt;65, y=100, 74,38, or 0) films has been examined by substituting nonmagnetic Y for Gd or nonmagnetic Cu for (Fe,Co) and comparing λs of these alloys at 300 and 80 K. The magnetostriction of Gdx(Fe,Co)100−x as well as Yx(Fe,Co)100−x films are positive and the composition (x) dependence of λs in Gd alloys is similar to Y alloys, but λs of Gd alloys are larger than Y alloys. It was further found that GdxCu100−x (35&amp;lt;x&amp;lt;60) shows positive λs at 80 K. These facts indicate positive contribution of Gd to λs in amorphous Gd-(Fe,Co) alloys.