Engineered Magnetic Shape Anisotropy in BiFeO3-CoFe2O4 Self-Assembled Thin Films

xed in CFO on(111)STO due to the triangular shape of the ferromagnetic phase magnetic anisotropy with speci?c shape and dimensions of ordered magnetic arrays.The patterned ferromagnetic oxide arrays with tunable shape,aspect ratio,and elastic

magnetic shape anisotropy

single domain nanoparticles,the mag-

can randomly?ip direction under

uence of temperature(i.e.,super-

(i)a top-down lithography;14(ii)a template-

deposition;3and(iii)a precipitation of immis-phases that would form speci?c patterns through spontaneous phase separation.4With regards to the lithographic technique,the minimal size of patterned magnets is con?ned by the refractory and che-

inertness of CFO compared with traditional

substrates.Nanoporous anodic aluminum oxide

membranes as stencil masks have been em-fabricate CFO nanodot arrays.3However,it is

to obtain CFO nanostructures with a high

ratio using these methods,due to the growth mechanism and the subsequent lift-o?from the mem-Moreover,the shape of the dot arrays is con-

the AAO holes.On the other hand,both the direction.16,17STO has a cubic structure with parameter of a c=3.91?.BFOàCFO self-assembled heterostructures have vertically aligned interfaces which induce a compressive strain on the CFO

rays due to the crystal lattice mismatch between BFO and CFO phases(8.39>3.96?2).Perpendicular magnetic anisotropy has been reported and attributed to the compressive strain at the BFOàCFO However,TEM analysis of a similar BiFeO3àsystem has shown a fully relaxed interface with interfacial lattice https://www.360docs.net/doc/de12388289.html,ttice constant

ments in the out-of-plane direction revealed a

less than0.1%,whereas the lattice mismatch

and CFO was5.6%.In addition,BFO,CFO,and

di?erent thermal expansion coe?cients,thus

(a)Crystal structure of converse spinel CFO with the(001),(110)and(111)planes entitle.(b)Schematic architectures for di?erently oriented BFOàCFO thin?lms on STO substrates.(c)SEM of BFOàCFO with square,stripe, CFO features.

Figure2.(aàc)Out-of-plane X-ray di?raction line scan BFOàCFO on(111),(110),and(100)STO substrates. rhombohedrally distorted perovskite one.The large crystal lattice mismatch(7.4%)between CFO and STO induces a big surface energy di?erence,and a lack wetting”on STO drives CFO to form nanopillars embedded in a coherent BFO matrix which has a close lattice constant to that of STO.Scanning electron microscopy(SEM)images in panel c reveal feature morphologies of square,stripe,and triangular nano-

thin?lm had the highest remnance value

the OP direction.For a300nm BFOàCFO

with R=3:1,the easy axis angle isθ= 71or109°.Accordingly,the largest

values observed in the MàH loops oc-

=70and110°.These results unambigu-demonstrate that the shape anisotropy is a predominant factor for BFOàCFO self-assembled nanostructures on(001)STO,where the easy axis can be rotated by tuning of the aspect ratio

nanopillars.

phase has a large saturation magnetization,

the magnetic anisotropy should be sensitive conditions of the CFO nanostructure.We

strain relaxation as a function of the?lm

as given in Figure4a.The200nm CFO thin

STO showed a(400)peak at43.166°,which

stable as the thickness changed.This corre-crystal lattice parameter a c=8.3815?, slightly smaller than that of CFO bulk.In BFOàCFO self-assembled thin?lms,CFO

by a compressive strain from the BFO matrix. will relax as the thin?lm thickness increases. BFOàCFO thickness increased from200to CFO di?raction peaks shifted from43.215 indicating a strain relaxation from0.079to strain anisotropy energy density was then with the shape-induced ones for BFOàCFO

di?erent thicknesses,as given in Figure4c. induced anisotropy energy was larger for with small aspect ratios.However,as the?increased,the shape anisotropy will become more important.We also measured the coercive?eld as a function of the thickness,

in Figure4c.With increasing thickness,the from the BFO matrix was relaxed,thus the H c should decrease if the strain e?ect is the

of the magnetic anisotropy in BFOàCFO.

H c increased from1.2to2.53and then to the BFOàCFO thickness increased from300

àc)MFM images of single domain CFO nanopillars after applying di?erent magnetic?elds.(dàf)M measurement angles of CFO nanopillars with di?erent aspect ratios.(g)Remnant magnetization as a function out-of-plane rotation angle.

nm due to the existence of shape aniso-

summarizes the magnetic properties of

(110)STO substrates,where the morpho-features were aligned along the in-plane direc-deposition temperature was set to be700°C requirement of well-separated phase distribu-related to the growth thermal dynamics.Both growth rate and deposition temperature a?ect the feature size of CFO nanostripes:

rate and higher deposition temperature

CFO nanostripes with larger width and Detailed CFO nanostructure dimension control found in our previous results.25The CFO

as nanostripes with R≈5:1that were oriented plane direction,with lengths of~300nm

Figure4.(a)X-ray line scan of pure CFO and BFOàCFO thin?lms with di?erent thickness.(b)MàH loops for900nm BFOàCFO IP and OP directions.(c)Comparison of strain-and shape-induced magnetic anisotropy energy density(K)and magnetic coercive?eld(H c)as a function of BFOàCFO thickness.

AFM and MFM images of BFOàCFO on(110)STO(AFM error bar:50nm).(c)Schematic of magnetic

direction.(d)MàH loops of(110)BFOàCFO in IP and OP directions.(e)MàH loops of BFOàCFO on in-plane directions.(f)Remnant magnetization as a function of in-plane rotation angleθfor BFOàCFO

a gives an atomic force microscopy topography which shows that the CFO nano-slightly higher than the BFO matrix,and

corresponding MFM image where a strong response was evident in the CFO phase but matrix.The dimensions of CFO nanos-

uniform,and a stand deviation on the

shown in panel c.The CFO phase has a magnetocrystalline anisotropy of K1=2?106 the crystalline anisotropy energy di?er-

the?001?and?110?directions isΔE=

=5?105erg/cm3,21which is close to anisotropy energy for a nanostructure with 5:1.However,it has been reported that

magnetocrystalline anisotropy energy

dependent on the size of the magnetic

K e?will decrease as the particle size

The MàH loops for BFOàCFO on(001)

exhibited similar ferromagnetic behav-referenced to their in-plane?001?and?110?crystal lattice mismatch between CFO

7.7%,while that of CFO and BFO is6.06%.OP directions are shown in panel e.The sample was200nm,thus the aspect ratio

and OP was3:2.IP1is the easy axis with

M r,which also had a larger H c compared direction.The value of M r was then function of the in-plane rotation angle

panel g.A sine-like wave function for

with a large M r in the IP1direction(0.79)

along IP2(0.33).Again,the largest M

in the directions around OP(80and100 limited R value.These results demonstrate anisotropy for BFOàCFO layers.This

o?ers an e?ective method to solve

of the superparamagnetic limit,where tence of a strong easy axis may provide tolerance to track misregistration and

for longitudinal recording.29Although

the CFO nanostructures in BFOàCFO

thin?lms(>100nm)are far from magnetic limit(<10nm),30the conclusions BFOàCFO can be applied to all magnetic nanoarrays,such as self-assembled

Schematic of two di?erent strain e?ects in the BFOàCFO interfaces along di?erent directions.

?erent in-plane rotation angles from IP1direction.(c)Magnetic remnance as a function of

direction,the e?ect of strain1and strain2can be added together as they both act along the same direction,whereas the strain e?ects act separately along the IP1and IP2directions.Thus,the strain along the three di?erent directions can be roughly estimated to be S op:S ip1:S ip2=6:5:1.The CFO crystal structures along(110)and(110)directions are identical,thus there is no magnetocrystalline anisotropy between IP1and OP.We measured the MàH loops for200nm

BFOàCFO thin?lm with di?erent IP1to OP rotation angles,as shown in Figure6b,c.IP1is the length direction of the CFO nanostripes;thus,it is the mag-netic easy axis,which had a magnetic remnance of 0.55.The OP direction is expected to be the magnetic hard axis as a result of the magnetic shape anisotropy. However,smaller remnance values(~0.35)were found forθ=60and120°relative to the value(0.43)atθ=90°along the OP direction:this is due to the existence of a compressive strain along the OP direction,which induces an out-of-plane magnetic anisotropy.The total magnetic anisotropy is the competing e?ect of the IP1 shape anisotropy and the OP strain anisotropy,where shape anisotropy is more signi?cant,resulting in a uniaxial in-plane magnetic easy axis along the IP1 direction.

Figure7shows the phase architecture and magnetic response of a BFOàCFO?lm on(111)STO.Although it has been reported that CFO has a smaller surface energy mismatch with STO compared with the BFO phase,5we found that CFO formed as segregated triangular cylinders embedded in a coherent BFO matrix.A BFOàCFO thin?lm with a thickness of 500nm was selected due to a comparatively large out-of-plane magnetic anisotropy(compared with thinner?lms)which favors MFM measurements.

Figure7.(aàc)AFM and MFM images of500nm BFOàCFO?lms on(111)STO with di?erent magnetic con?gurations(AFM error bar:125nm).(d)MàH loops for500nm BFOàCFO on(111)STO and a schematic of demagnetization?eld(inset).(e)Top-view SEM of released CFO prism arrays.(f)Comparison of XRD results for BFOàCFO,released CFO,and pure CFO thin?lms.

(g)MàH loops of BFOàCFO and released CFO.(h)MàH loops for BFOàCFO with di?erent in-plane rotation angle.

(i)Magnetization remnance and H c as a function of in-plane rotation angleθ.

https://www.360docs.net/doc/de12388289.html,parison of Coercive Field(H c)and Remnant

Magnetization(M r)for BFOàCFO and Released CFO

Nanoarray Structures in Both In-Plane(ip)and Out-of-

Plane(op)Directions

structure H c?ip(Oe)H c?op(Oe)H c?ip/H c?op M r?ip(emu/cc)M r?op(emu/cc)M r?ip/M r?op

BFOàCFO1235889 1.3911566 1.74

released CFO450320 1.408938 2.34

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