ni/fef2’bilayers:’model’exchange’bias’system examples_files/eb_presentation.pdf ·...
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![Page 1: Ni/FeF2’bilayers:’model’exchange’bias’system Examples_files/EB_presentation.pdf · Sample’preparaon’ FeF 2:’e9beam’evaporaon’onto’a’ ’(110)’MgF 2 ’single’crystal’](https://reader033.vdocument.in/reader033/viewer/2022052001/60138efc845e846f924fec1e/html5/thumbnails/1.jpg)
• Ni/FeF2 shows coexistence of nega3ve and posi3ve EB domains
Ni/FeF2 bilayers: model exchange bias system
Bi-‐domain state
Ni
FeF2 NEB PEB
frac;on of posi;ve/nega;ve EB domains can be tuned by the strength of HFC and/or paAerning
Roshchin et al, EPL 71, 297 (2005) Petracic et al., APL 87, 222509 (2005) Kovylina et al, APL 95, 152507 (2009)
Li et al, APL 94, 142503 (2009)
Morales et al, APL 104, 032401 (2014) US Patent Number 7,764,454
FeF2 (70 nm)/Ni (20 nm)/Al (4 nm)
• bi-‐domain state can be used in mul;-‐state magne;c recording
• each of the remanence states defines a digit
EB domains: AF regions for which the pinned uncompensated AF spins induce posi;ve or nega;ve unidirec;onal anisotropy in the FM.
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Sample prepara;on
FeF2: e-‐beam evapora;on onto a (110) MgF2 single crystal
sample (110)
Al Ni
FeF2
MgF2
FM
AF
[001] crystal domains
2 nm 0 -‐ 11 nm
70 nm
AF easy axis
polycrystalline
single crystal
single crystal
30-‐80 nm
AF HA
FM H’A
MgF2
FeF2
5 nm
FeF2
Ni
Cross-‐sec;on HRTEM S. Estradé, F. Peiró, UB
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Imaging of Magne;c Domains: XMCD-‐PEEM
70 nm
(110)
Al Ni FeF2
MgF2
4 nm 2 nm
Magne;za;on direc;on
FeF2 easy axis
T= 29 K
[001]
ZFC
• Fe signal disappears above TN • pinned Fe spins induce local reversal of the overlying Ni spins when cooling through TN
• Coexistence of EB domains with opposite orienta3ons
• Fe uncompensated domains replicate inverted Ni paAern
PEEM measurements at PEEM3, ALS
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Tunable configura;ons of EB domains
• ZFC from a Ni saturated state Measurements at H=0, T=30 K
• Fe XMCD: Fe uncompensated spins
Fe XMCD • Ni XMCD: dark: ini;al saturated state bright: inverted domains ader cooling through TN of FeF2
• Domain paeerns are: - stochas;c - stable - reproducible over cycling
FeF2 easy axis Magne;za;on direc;on
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Frac;on of Ni inverted domains vs Ni thickness
• bare FeF2 layer: Equal probability of domains with opposite orienta;on is consistent with the absence of local exchange interac;on with the Ni overlayer
• FM layer thickness has a large influence on the AF uncompensated domain configura;on
sta;s;cal error bars
AF
FM
δDW
tFM
Energy balance:
lateral domain walls of inverted FM domains (propor;onal to domain perimeter: scales with FM thickness)
vs
forma;on of small uncompensated AF domains to reduce both the magnetosta;c energy and the frustra;on at boundaries between crystal domains
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Focused ion beam lithography of square arrays of 200 nm square an;dots with an;dot densi;es (AD)= 9%, 12%, and 24%
Kovylina et al, Nanotechnology 21 175301 (2010)
Lateral confinement through an;dot paeerning
6 nm
70 nm
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AD= 24%: Remarkable increase of the frac;on of inverted Ni domains
! FM order gets destabilized
AD≤12%: constant and comparable to con;nuous areas
Lateral confinement by an;dot paeerning ZFC
H=0, T=30 K
Magne;za;on direc;on AF easy axis
[001]
Ni XMCD
Ni XMCD
AD 12% FC (HFC=50 Oe) H=0, T=30 K
increase in the frac;on of inverted Ni domains ader FC
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• an;dot separa;on imposes a maximum threshold for the average size of the inverted domains
• Under FC the frac;on of inverted domains is always enhanced:
- Zeeman energy of pinned uncompensated Fe spins promotes addi;onal inverted domains along the field direc;on
Lateral confinement by an;dot paeerning
• progressive reduc;on of the correla;on length of inverted domains with increasing an;dot density
• laterally constraining the FM/AF heterostructure by paeerning may be an effec;ve way to fine tune the EB cri;cal size
• Radial distribu;on func;on analysis: - lateral correla;on length of the Ni domains
sta;s;cal error bars
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Temperature dependence across the TN of FeF2
66 K 65 K 64.5 K 64 K 67 K
5 μm
Ni XMCD (cooling)
• Temperature-‐dependent data recorded “on the fly” - short image acquisi;on ;me and no T stabiliza;on
• Purpose-‐built algorithm enabling automa;c processing and analysis of large stacks of images with low SNR - dis;nguish nuclea;on, expansion, and coalescence
events
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• Ni cycle 1 • Ni cycle 2 • Fe cycle 3
Temperature dependence across the TN of FeF2 • The inverted Ni domains appear right ader
the AF transi;on is crossed
• the Fe uncompensated domains reproduce well the Ni domain fill frac;on
• this frac;on reaches satura;on within only a few degrees below TN
• Ni reversal mechanism involves nuclea;on and expansion
6 nm Ni
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Summary: Ni/FeF2 nanostructures
• The spa;al confinement of a FM ac;vely controls the AF uncompensated domain structure in exchange biased systems, either through thickness varia;on or laterally via paeerning.
• The compe;ng FM and AF interac;ons lead to tunable configura;ons of coexis;ng posi;ve and nega;ve EB domains in Ni/FeF2 bilayers for Ni thicknesses below 10 nm.
• An;dot paeerning of the whole Ni/FeF2 heterostructure creates addi;onal posi;ve EB domains below a cri;cal an;dot interspace of the order of a few FeF2 crystal domains.
Ongoing:
• finish a growth model of inverted Ni domains / uncompensated Fe domains across TN: nuclea;on, expansion and coalescence events vs T (sta;s;cs)