Current-Induced Magnetic Switching in an L1_(0) FePt Single Layer with Large Perpendicular Anisotropy Through Spin–Orbit Torque

In this study,current-induced partial magnetization-based switching was realized through the spin–orbit torque(SOT)in single-layer L1_(0) FePt with a perpendicular anisotropy(K_(u⊥))of 1.19×10^(7) erg·cm^(-3)(1 erg·cm^(-3)=0.1 J·m^(-3)),and its corresponding SOT efficiency(βDL)was 8×10^(-6) Oe·(A·cm^(-2))^(-1)(1 Oe=79.57747 A·m^(-1)),which is several times higher than that of the traditional Ta/CoFeB/MgO structure reported in past work.The SOT in the FePt films originated from the structural inversion asymmetry in the FePt films since the dislocations and defects were inhomogeneously distributed within the samples.Furthermore,the FePt grown on MgO with a granular structure had a larger effective SOT field and effi-ciency than that grown on SrTiO_(3)(STO)with a continuous structure.The SOT efficiency was found to be considerably dependent on not only the sputtering temperature-induced chemical ordering but also the lattice mismatch-induced evolution of the microstructure.Our findings can provide a useful means of efficiently electrically controlling a magnetic bit that is highly thermally stable via SOT.

THERMAL STABILITY AND PERPENDICULAR ANISOTROPY OF AMORPHOUS TbFeCo FILMS

The perpendicular anisotopy,coercivity and Kerr rotation angle of the amorphous Tb_(32)Fe_(54)Co_(14) films were studied.The X-ray,electron diffraction,XPS and AES profile con- firmed that no obvious change in their amorphous structure and oxygen concentration is ob- served after annealing at 100℃,while their properties alter evidently under bending stress.It seems to be believed that the perpendicular anisotropy in the Tb_(32)Fe_(54)Co_(14) films mainly arises from the induced stress during preparation and the magnetostriction coupling stress,as well as,the thermal instability of the film relates closely to the stress relaxation during annealing.

NANO-MULTILAYERS WITH HIGH PERPENDICULAR ANISOTROPY FOR MAGNETIC RECORDING

(FePt/Ag)n nano-multilayers were deposited on MgO (100) single crystal with laser ablation and then subjected to annealing. FePt L1o grains with (001) texture and thus a large perpendicular magnetic anisotropy constant Ku of the order of 106 J/m3 were formed. A thick Ag layer is found to be favorable for decreasing the dispersion of the easy axis for magnetization. The measurement of time decay of magnetization gave rise to a small activation volume of the order of 10-25m3, showing the promising of being the recording medium for future high density perpendicular recording.

Thickness-dependent magnetic properties in Pt/[Co/Ni]_(n) multilayers with perpendicular magnetic anisotropy

We systematically investigated the Ni and Co thickness-dependent perpendicular magnetic anisotropy(PMA)coefficient,magnetic domain structures,and magnetization dynamics of Pt(5 nm)/[Co(t_(Co))/Ni(t_(Ni))]_(5)/Pt(1 nm)multilayers by combining the four standard magnetic characterization techniques.The magnetic-related hysteresis loops obtained from the field-dependent magnetization M and anomalous Hall resistivity(AHR)ρxy showed that the two serial multilayers with t_(Co)=0.2 nm and 0.3 nm have the optimum PMA coefficient K_(U) as well as the highest coercivity H_(C) at the Ni thickness t_(Ni)=0.6 nm.Additionally,the magnetic domain structures obtained by magneto-optic Kerr effect(MOKE)microscopy also significantly depend on the thickness and K_(U) of the films.Furthermore,the thickness-dependent linewidth of ferromagnetic resonance is inversely proportional to K_(U) and H_(C),indicating that inhomogeneous magnetic properties dominate the linewidth.However,the intrinsic Gilbert damping constant determined by a linear fitting of the frequency-dependent linewidth does not depend on the Ni thickness and K_(U).Our results could help promote the PMA[Co/Ni]multilayer applications in various spintronic and spin-orbitronic devices.

Electric-field control of perpendicular magnetic anisotropy by resistive switching via electrochemical metallization

Electric-field control of perpendicular magnetic anisotropy(PMA) is a feasible way to manipulate perpendicular magnetization,which is of great importance for realizing energy-efficient spintronics.Here,we propose a novel approach to accomplish this task at room temperature by resistive switching(RS) via electrochemical metallization(ECM) in a device with the stack of Si/SiO_(2)/Ta/Pt/Ag/Mn-doped ZnO(MZO)/Pt/Co/Pt/ITO.By applying certain voltages,the device could be set at high-resistance-state(HRS) and low-resistance-state(LRS),accompanied with a larger and a smaller coercivity(H_(C)),respectively,which demonstrates a nonvolatile E-field control of PMA.Based on our previous studies and the present control experiments,the electric modulation of PMA can be briefly explained as follows.At LRS,the Ag conductive filaments form and pass through the entire MZO layer and finally reach the Pt/Co/Pt sandwich,leading to weakening of PMA and reduction of H_(C).In contrast,at HRS,most of the Ag filaments dissolve and leave away from the Pt/Co/Pt sandwich,causing partial recovery of PMA and an increase of H_(C).This work provides a new clue to designing low-power spintronic devices based on PMA films.

RF magnetron sputtering induced the perpendicular magnetic anisotropy modification in Pt/Co based multilayers

We demonstrate that radio frequency(RF)magnetron sputtering technique can modify the perpendicular magnetic anisotropy(PMA)of Pt/Co/normal metal(NM)thin films.Influence of ion irradiation during RF magnetron sputtering should not be neglected and it can weaken PMA of the deposited magnetic films.The magnitude of this influence can be controlled by tuning RF magnetron sputtering deposition conditions and the upper NM layer thickness.According to the stopping and range of ions in matter(SRIM)simulation results,defects such as displacement atoms and vacancies in the deposited film will increase after the RF magnetron sputtering,which can account for the weakness of PMA.The amplitude changes of the Hall resistance and the threshold current intensity of spin orbit torque(SOT)induced magnetization switching also can be modified.Our study could be useful for controlling magnetic properties of PMA films and designing new type of SOT-based spintronic devices.

Magnetic properties of a Pt/Co_2 FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co 2 FeAl（CFA）/MgO multilayers are studied to understand perpendicular magnetic anisotropy（PMA） of half-metallic full-Heusler alloy films.PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope.It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA.The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature.At the intersection points,the decreasing slope of the saturation magnetization（M s） changes because of the conversions.The dependence of M s on the annealing temperature and MgO thickness is also studied.

Nonmonotonic effects of perpendicular magnetic anisotropy on current-driven vortex wall motions in magnetic nanostripes

In a magnetic nanostripe, the effects of perpendicular magnetic anisotropy（PMA） on the current-driven horizontal motion of vortex wall along the stripe and the vertical motion of the vortex core are studied by micromagnetic simulations.The results show that the horizontal and vertical motion can generally be monotonously enhanced by PMA. However, when the current is small, a nonmonotonic phenomenon for the horizontal motion is found. Namely, the velocity of the horizontal motion firstly decreases and then increases with the increase of the PMA. We find that the reason for this is that the PMA can firstly increase and then decrease the confining force induced by the confining potential energy. In addition, the PMA always enhances the driving force induced by the current.

Optimized growth of compensated ferrimagnetic insulator Gd_(3)Fe_(5)O_(12)with a perpendicular magnetic anisotropy

Compensated ferrimagnetic insulators are particularly interesting for enabling functional spintronic,optical,and microwave devices.Among many different garnets,Gd_(3)Fe_(5)O_(12)(GdIG)is a representative compensated ferrimagnetic insulator.In this paper,we will study the evolution of the surface morphology,the magnetic properties,and the magnetization compensation through changing the following parameters:the annealing temperature,the growth temperature,the annealing duration,and the choice of different single crystalline garnet substrates.Our objective is to find the optimized growth condition of the GdIG films,for the purpose of achieving a strong perpendicular magnetic anisotropy(PMA)and a flat surface,together with a small effective damping parameter.Through our experiments,we have found that the surface roughness approaching 0.15 nm can be obtained by choosing the growth temperature around 700℃,together with an enhanced PMA.We have also found the modulation of magnetic anisotropy by choosing different single crystalline garnet substrates which change the tensile strain to the compressive strain.A measure of the effective magnetic damping parameter(α_(eff)=0.04±0.01)through a spin pumping experiment in a GdIG/Pt bilayer is also made.Through optimizing the growth dynamics of GdIG films,our results could be useful for synthesizing garnet films with a PMA,which could be beneficial for the future development of ferrimagnetic spintronics.

The 50 nm-thick yttrium iron garnet films with perpendicular magnetic anisotropy

Yttrium iron garnet(YIG) films possessing both perpendicular magnetic anisotropy(PMA) and low damping would serve as ideal candidates for high-speed energy-efficient spintronic and magnonic devices.However,it is still challenging to achieve PMA in YIG films thicker than 20 nm,which is a major bottleneck for their development.In this work,we demonstrate that this problem can be solved by using substrates with moderate lattice mismatch with YIG so as to suppress the excessive strain-induced stress release as increasing the YIG thickness.After carefully optimizing the growth and annealing conditions,we have achieved out-of-plane spontaneous magnetization in YIG films grown on sGGG substrates,even when they are as thick as 50 nm.Furthermore,ferromagnetic resonance and spin pumping induced inverse spin Hall effect measurements further verify the good spin transparency at the surface of our YIG films.

Effects of MgO Thickness and Roughness on Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta Multilayers

The dependence of perpendicular magnetic anisotropy （PMA） on the barrier layer MgO thickness in MgO/CoFeB /Ta multilayers is investigated. The results show that the strongest PMA occurs in a small window of about 2 4nm with the increase of MgO thickness from 1-1Onto. The crystalline degree of MgO and the change of interatomic distance along the out-of-plane direction may be the main reasons for the change of PMA in these multilayers. Moreover, the roughnesses of 2- and 4-nm-thick MgO samples are 3.163 and 1.8 nm, respectively, and both the samples show PMA. These results could be used to tune the magnetic characteristic of the ultra thin CoFeB film for future applications in perpendicular magnetic devices.

Dependence of switching process on the perpendicular magnetic anisotropy constant in P-MTJ

We investigate the dependence of the switching process on the perpendicular magnetic anisotropy （PMA） constant in perpendicular spin transfer torque magnetic tunnel junctions （P-MTJs） using micromagnetic simulations. It is found that the final stable states of the magnetization distribution of the free layer after switching can be divided into three different states based on different PMA constants： vortex, uniform, and steady. Different magnetic states can be attributed to a trade-off among demagnetization, exchange, and PMA energies. The generation of the vortex state is also related to the non-uniform stray field from the polarizer, and the final stable magnetization is sensitive to the PMA constant. The vortex and uniform states have different switching processes, and the switching time of the vortex state is longer than that of the uniform state due to hindrance by the vortex.

Role of TbFe on Perpendicular Magnetic Anisotropy and Giant Magnetoresistance Effect in [Co/Ni]_N-Based Spin Valves

The exchange-coupled [Co/Ni]N/Tb Fe nano-magnetic films can display strong perpendicular magnetic anisotropy(PMA) which depends on the Tb:Fe component ratio, Tb Fe layer thickness and the repetition number N of [Co/Ni]Nmultilayer. Perpendicular spin valves in the nano thickness scale, consisting of a [Co/Ni]3free and a [Co/Ni]5/Tb Fe reference multilayer, show high giant magnetoresistance(GMR) signal of 6.5 % and a large switching field difference over3 k Oe. However, unexpected slanting of the free layer magnetization, accompanied by a reduced GMR ratio, was found to be caused by the presence of a thick Fe-rich or even a thin but Tb-rich Tb Fe layer. We attribute this phenomenon to the large magnetostriction effect of Tb Fe which probably induces strong stress acting on the free layer and hence reduces its interfacial PMA.

Strong perpendicular magnetic anisotropy in Co_2FeAl_(0.5)Si_(0.5) film sandwiched by MgO layers

Co2FeA10.5Si0.5 （CFAS）-based multilayers sandwiched by MgO layers have been deposited and annealed at different temperatures. Perpendicular magnetic anisotropy （PMA） with the magnetic anisotropy energy density Ku ≈2.5x106 erg/cm3 （1 erg = 10-7 J） and the coercivity He = 363 Oe （10e = 79.9775 A.m-1） has been achieved in the Si/SiO2/MgO （1.5 nm）/CFAS （2.5 nm）/MgO （0.8 nm）/Pt （5 nm） film annealed at 300 ℃. The strong PMA is mainly due to the top MgO layer. The structure can be used as top magnetic electrodes in half-metallic perpendicular magnetic tunnel junctions.

Perpendicular magnetic anisotropy of Pd/Co_(2)MnSi/NiFe_(2)O_(4)/Pd multilayers on F-mica substrates

Pd/Co_(2)MnSi(CMS)/NiFe_(2)O_(4)(NFO)/Pd multilayers were fabricated on F-mica substrate by magnetron sputtering.The best PMA performance of the multilayer structure Pd(3 nm)/CMS(5 nm)/NFO(0.8 nm)/Pd(3 nm)was obtained by adjusting the thickness of the CMS and NFO layers.F-mica substrate has a flatter surface than glass and Si/SiO_(2) substrate.The magnetic anisotropy energy density(K_(eff))of the sample deposited on F-mica substrates is 0.6711 Merg/cm^(3)(1 erg=10^(-7) J),which is about 30%higher than that of the multilayer films deposited on glass(0.475 Merg/cm^(3))and Si/SiO_(2)(0.511 Merg/cm^(3))substrates,and the R_(Hall) and H_(C) are also significantly increased.In this study,the NFO layer prepared by sputtering in the high purity Ar environment was exposed to the high purity O_(2) atmosphere for 5 min,which can effectively eliminate the oxygen loss and oxygen vacancy in NFO,ensuring enough Co-O orbital hybridization at the interface of CMS/NFO,and thus effectively improve the sample PMA.

Perpendicular magnetic anisotropy of Co_(85)Cr_(50)/Pt multilayers

The CoCr/Pt bilayers and (CoCr/Pt)_(20) multilayers with Pt underlayer wereprepared by DC magnetron sputtering. The effects of prepared condition on perpendicular magneticanisotropy were investigated. The results show that the thickness of Pt under-layer has a greateffect on the microstructure and perpendicular magnetic anisotropy of CoCr/Pt bilayers and(CoCr/Pt)_(20) multilayers. When the thickness of Pt underlayer increases, Pt(lll) and CoCr(002)peaks of both CoCr/Pt bilayers and (CoCr/Pt)_(20) multilayers increase and the bilayer periodicityof the multilayers is improved. The effective magnetic anisotropy of (CoCr/Pt)_(20) multilayers withPt underlayer was much larger than that of CoCr/Pt bilayers. The (CoCr/Pt)_(20) multilayers has astronger perpendicular magnetic anisotropy than that of CoCr/Pt bilayers. This is ascribed to theinterface magnetic anisotropy of the multilayers.

Effect of Mo capping layers thickness on the perpendicular magnetic anisotropy in MgO/CoFeB based top magnetic tunnel junction structure

A detailed study of the magnetic characterizations of the top structure MgO/CoFeB/Mo is presented.The samples show strong perpendicular magnetic anisotropy（PMA） when the thickness of CoFeB is 0.9 nm and 1.1 nm.The saturation magnetic moment and interface anisotropy constant are 1566 emu/cm3 and 3.75 erg/cm2,respectively.The magnetic dead layer（MDL） is about 0.23 nm in this system.Furthermore,strong capping layer thickness dependence is also observed.The strong PMA of 1.1 nm CoFeB only exists in a Mo cap layer thickness window of 1.2-2 nm.To maintain PMA,the metal layer could not be too thin or thick in these multilayers.The oxidation and diffusion of the metal capping layer should be respectively responsibility for the degradation of PMA in these thin or thick metal capping layer samples.

High repetition granular Co/Pt multilayers with improved perpendicular remanent magnetization for high-density magnetic recording

Thanks to the strong perpendicular magnetic anisotropy(PMA), excellent processing compatibility as well as novel spintronic phenomenon, Co/Pt multilayers have been attracting massive attention and widely used in magnetic storage.However, reversed magnetic domains come into being with the increasing layer repetition ‘N’ to reduce magneto-static energy, resulting in the remarkable diminishment of the remanent magnetization(Mr). As a result, the product of Mrand thickness(i.e., the remanent moment-thickness product, Mrt), a key parameter in magnetic recording for reliable data storing and reading, also decreases dramatically. To overcome this issue, we deposit an ultra-thick granular [Co/Pt]80multilayer with a total thickness of 68 nm on granular SiNxbuffer layer. The Mrt value, Mrto saturation magnetization(Ms) ratio as well as out of plane(OOP) coercivity(Hcoop) are high up to 2.97 memu/cm^(2), 67%, and 1940 Oe(1 Oe = 79.5775 A·m^(-1)),respectively, which is remarkably improved compared with that of continuous [Co/Pt]80multilayers. That is because large amounts of grain boundaries in the granular multilayers can efficiently impede the propagation and expansion of reversed magnetic domains, which is verified by experimental investigations and micromagnetic simulation results. The simulation results also indicate that the value of Mrt, Mr/Msratio, and Hcoopcan be further improved through optimizing the granule size, which can be experimentally realized by manipulating the process parameter of SiNxbuffer layer. This work provides an alternative solution for achieving high Mrt value in ultra-thick Co/Pt multilayers, which is of unneglectable potential in applications of high-density magnetic recording.

Spin–orbit torque in perpendicularly magnetized[Pt/Ni]multilayers

The performance of spin–orbit torque(SOT)in heavy metal/ferromagnetic metal periodic multilayers has attracted widespread attention.In this paper,we have successfully fabricated a series of perpendicular magnetized[Pt(2-t)/Ni(t)]_4 multilayers,and studied the SOT in the multilayers by varying the thickness of Ni layer t.The current induced magnetization switching was achieved with a critical current density of 1×10^(7)A/cm^(2).The damping-like SOT efficiencyξ_(DL)was extracted from an extended harmonic Hall measurement.We demonstrated that theξ_(DL)can be effectively modulated by t_(Pt)/t_(Ni)ratio of Pt and Ni in the multilayers.The SOT investigation about the[Pt/Ni]N multilayers might provide new material candidates for practical perpendicular SOT-MRAM devices.

Perpendicular exchange bias of (Pt/Co)_n/FeMn multilayers

（Pt/Co）n/FeMn multilayers with perpendicular anisotropy （PA） were prepared by magnetron sputtering with Pt as the buffer layer and the capping layer. The dependence of perpendicular exchange bias （PEB）, Hex, on the thickness of the FeMn anfiferromagnet （AFM） layer is similar to that of in-plane exchange bias. The value of Hex for the （Pt/Co）3/FeMn multilayer reaches 22.3 kA/m. A thin Pt spacer was inserted between the Co/FeMn interface to enhance PEB. The PEB reaches the largest at 39.8 kA/m when the thickness of the Pt spacer is 0.4 nm.