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Anomalous magnetoresistance behavior of CoFe nano-oxide spin valves at low temperatures
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arXiv:cond-mat/0507423v1 [cond-mat.mtrl-sci] 18 Jul 2005
Anomalous magnetoresistan e b ehavior of CoFe nano-oxide spin valves at low
temp eratures
J. Ventura,∗ J. B. Sousa, and M. A. Salgueiro da Silva
IFIMUP and Fa ulty of S ien es U. Porto, Rua do Campo Alegre, 678, 4169-007, Porto, Portugal
P. P. Freitas and A. Veloso
INESC, Solid State Te hni al Group, R. Alves Redol, 9-1, Lisbon, 1000, Portugal
We rep ort magnetoresistan e urves of CoFe nano-oxide sp e ular spin valves of MnIr/CoFe/nano-
oxidized CoFe/CoFe/Cu/CoFe/nano-oxidized CoFe/Ta at dierent temp eratures from 300 to 20 K.
We extend the Stoner-Wolfarth mo del of a ommon spin valve to a sp e ular spin valve, intro du ing
the separation of the pinned layer into two sublayers and their magneti oupling a ross the nano-
oxide. We study the ee t of dierent oupling/ex hange (b etween the antiferromagneti layer and
the b ottom sublayer) eld ratios on the magnetization and magnetoresistan e, orresp onding with
the exp erimentally observed anomalous bumps in low temp erature magnetoresistan e urves.
I. INTRODUCTION
A simple spin valve1 (SV) is a nanostru ture with
two ferromagneti (FM) layers separated by a su iently
thi k nonmagneti (NM) spa er. An adja ent antifer-
romagneti (AFM) material xes the magnetization of
one of the FM layers, the so- alled pinned layer. The
other FM layer, alled the free layer, is only weakly ou-
pled (magneti ally) to the pinned layer. Spin up and
spin down ele trons are dierently s attered (either in
the bulk or at the interfa es), usually pro du ing a large
magnetoresistan e (MR) when a small applied magneti
eld H reverses the free layer magnetization with resp e t
to that of the pinned layer.
Re ent rep orts on spin valves with the pinned and/or
free layer partially oxidized,2,3,4 showed great MR en-
han ement over the onventional (nonoxidized) spin
valves (CSV). Ele trons are b elieved to re e t sp e u-
larly at the nano-oxide layer (NOL)/FM interfa es, thus
yielding higher MR ratios. Low temp erature and tem-
p erature dep enden e studies of the transp ort and mag-
neti prop erties of NOL SVs an provide valuable infor-
mation on the oxide layer prop erties, also assisting in
the optimization of NOL SVs and orresp onding physi-
al understanding.3,5
We previously rep orted5 the temp erature dep en-
den e of the magnetoresistan e MR(H) urves for
NOL SVs with the stru ture seed/MnIr/CoFe/oxidation
(NOL1)/CoFe/Cu/CoFe/ oxidation (NOL2). The rst
FM dep osited layer (CoFe on MnIr) will b e alled the
b elow-NOL1 pinned layer (FMb) and its upp er part is
oxidized over an adequate thi kness to form the NOL1
layer. The FM layer dep osited after this oxidation will b e
alled the ab ove-NOL1 pinned layer (FMa). The pinned
layer thus onsists of b oth FMb and FMa sublayers, sep-
∗Ele troni address: joventurf .up.pt
JNM
JNOL1
Jexch
t
ta
tb
Ma a
Mb
b
AFM
NOL2
NOL1
pinned layer FMb
pinned layer FMa
NM spacer
free layer
M e. a.
H
FIG. 1: Stru ture of a NOL SV.
arated by the NOL1 oxide layer. Our study5 in luded
the temp erature dep enden e of the M(H) and MR(H)
urves for su h NOL SVs, revealing the app earan e of
several anomalous features at low temp erature, namely
anomalous bumps in MR(H) at intermediate elds and
the absen e of omplete MR saturation up to large p osi-
tive elds. In this arti le we fo us on the physi al origin
of su h features. For this we present a mo del based on
the total energy6 of a NOL SV to des rib e the magnetiza-
tion orientation in ea h of the three FM layers under an
external magneti eld and the resulting MR(H) b ehav-
ior. We assume that the FMb and FMa layers are ferro-
magneti ally oupled a ross NOL1 (Fig. 1),3 and study
the ee t of su h oupling strength on the magnetiza-
tion M(H) and MR(H) urves. The mo del a ounts for
the observed anomalous MR(H) bumps,5 relating them
to the M reversal in the pinned layer.
Anomalous magnetoresistan e b ehavior of CoFe nano-oxide spin valves at low
temp eratures
J. Ventura,∗ J. B. Sousa, and M. A. Salgueiro da Silva
IFIMUP and Fa ulty of S ien es U. Porto, Rua do Campo Alegre, 678, 4169-007, Porto, Portugal
P. P. Freitas and A. Veloso
INESC, Solid State Te hni al Group, R. Alves Redol, 9-1, Lisbon, 1000, Portugal
We rep ort magnetoresistan e urves of CoFe nano-oxide sp e ular spin valves of MnIr/CoFe/nano-
oxidized CoFe/CoFe/Cu/CoFe/nano-oxidized CoFe/Ta at dierent temp eratures from 300 to 20 K.
We extend the Stoner-Wolfarth mo del of a ommon spin valve to a sp e ular spin valve, intro du ing
the separation of the pinned layer into two sublayers and their magneti oupling a ross the nano-
oxide. We study the ee t of dierent oupling/ex hange (b etween the antiferromagneti layer and
the b ottom sublayer) eld ratios on the magnetization and magnetoresistan e, orresp onding with
the exp erimentally observed anomalous bumps in low temp erature magnetoresistan e urves.
I. INTRODUCTION
A simple spin valve1 (SV) is a nanostru ture with
two ferromagneti (FM) layers separated by a su iently
thi k nonmagneti (NM) spa er. An adja ent antifer-
romagneti (AFM) material xes the magnetization of
one of the FM layers, the so- alled pinned layer. The
other FM layer, alled the free layer, is only weakly ou-
pled (magneti ally) to the pinned layer. Spin up and
spin down ele trons are dierently s attered (either in
the bulk or at the interfa es), usually pro du ing a large
magnetoresistan e (MR) when a small applied magneti
eld H reverses the free layer magnetization with resp e t
to that of the pinned layer.
Re ent rep orts on spin valves with the pinned and/or
free layer partially oxidized,2,3,4 showed great MR en-
han ement over the onventional (nonoxidized) spin
valves (CSV). Ele trons are b elieved to re e t sp e u-
larly at the nano-oxide layer (NOL)/FM interfa es, thus
yielding higher MR ratios. Low temp erature and tem-
p erature dep enden e studies of the transp ort and mag-
neti prop erties of NOL SVs an provide valuable infor-
mation on the oxide layer prop erties, also assisting in
the optimization of NOL SVs and orresp onding physi-
al understanding.3,5
We previously rep orted5 the temp erature dep en-
den e of the magnetoresistan e MR(H) urves for
NOL SVs with the stru ture seed/MnIr/CoFe/oxidation
(NOL1)/CoFe/Cu/CoFe/ oxidation (NOL2). The rst
FM dep osited layer (CoFe on MnIr) will b e alled the
b elow-NOL1 pinned layer (FMb) and its upp er part is
oxidized over an adequate thi kness to form the NOL1
layer. The FM layer dep osited after this oxidation will b e
alled the ab ove-NOL1 pinned layer (FMa). The pinned
layer thus onsists of b oth FMb and FMa sublayers, sep-
∗Ele troni address: joventurf .up.pt
JNM
JNOL1
Jexch
t
ta
tb
Ma a
Mb
b
AFM
NOL2
NOL1
pinned layer FMb
pinned layer FMa
NM spacer
free layer
M e. a.
H
FIG. 1: Stru ture of a NOL SV.
arated by the NOL1 oxide layer. Our study5 in luded
the temp erature dep enden e of the M(H) and MR(H)
urves for su h NOL SVs, revealing the app earan e of
several anomalous features at low temp erature, namely
anomalous bumps in MR(H) at intermediate elds and
the absen e of omplete MR saturation up to large p osi-
tive elds. In this arti le we fo us on the physi al origin
of su h features. For this we present a mo del based on
the total energy6 of a NOL SV to des rib e the magnetiza-
tion orientation in ea h of the three FM layers under an
external magneti eld and the resulting MR(H) b ehav-
ior. We assume that the FMb and FMa layers are ferro-
magneti ally oupled a ross NOL1 (Fig. 1),3 and study
the ee t of su h oupling strength on the magnetiza-
tion M(H) and MR(H) urves. The mo del a ounts for
the observed anomalous MR(H) bumps,5 relating them
to the M reversal in the pinned layer.
2
The absen e of full alignment in large p ositive elds at
low temp erature is dis ussed in terms of the p ermanen e
of 360â—¦ domain walls; the underlying H asymmetry is
related to the d eld anneal. The key role played by the
nano-oxide CoFe sp e ular layers is also emphasized.
I I. THEORETICAL MODEL
We extend the simple mo del des ribing the total en-
ergy of a CSV6 to the more omplex stru ture of NOL
SVs, expli itly intro du ing the pinned layer partition into
the FMb and FMa sublayers separated by NOL1. The ex-
hange oupling b etween the FMb and AFM layers xes
the easy axis of the magnetization in the FMb layer and
(as a onsequen e of oupling intera tions) in the FMa
and free layers. We assume oherent rotation of the mag-
netization within ea h layer, under an external magneti
eld H. To simplify the treatment no anisotropy terms
are in luded. The total energy p er unit area E of a NOL
SV is thus written as the sum of Zeeman E
The absen e of full alignment in large p ositive elds at
low temp erature is dis ussed in terms of the p ermanen e
of 360â—¦ domain walls; the underlying H asymmetry is
related to the d eld anneal. The key role played by the
nano-oxide CoFe sp e ular layers is also emphasized.
I I. THEORETICAL MODEL
We extend the simple mo del des ribing the total en-
ergy of a CSV6 to the more omplex stru ture of NOL
SVs, expli itly intro du ing the pinned layer partition into
the FMb and FMa sublayers separated by NOL1. The ex-
hange oupling b etween the FMb and AFM layers xes
the easy axis of the magnetization in the FMb layer and
(as a onsequen e of oupling intera tions) in the FMa
and free layers. We assume oherent rotation of the mag-
netization within ea h layer, under an external magneti
eld H. To simplify the treatment no anisotropy terms
are in luded. The total energy p er unit area E of a NOL
SV is thus written as the sum of Zeeman E
100%
