Investigation of interlayer exchange coupling in ferro-, antiferro-, ferromagnetic trilayers [Elektronische Ressource] / Christian Schanzer

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• Physik

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2006
• Nombre de lectures: 68
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

Lehrstuhl fur¨ Experimentalphysik E21
Investigation of interlayer exchange
coupling in ferro-/antiferro-/ferromagnetic
trilayers
Christian Schanzer
Vollst¨andiger Abdruck der von der Fakult¨at fur¨ Physik der Technischen Universit¨at
Munc¨ hen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. M. Kleber
Prufer¨ der Dissertation:
1. Univ.-Prof. Dr. P.B¨oni
2. Univ.-Prof. Dr. R. Gross
Die Dissertation wurde am 20.10.2005 bei der Technischen Universit¨at Munc¨ hen ein-
gereicht und durch die Fakult¨at fur¨ Physik am 28.03.2006 angenommen.3
Thesis Outline
Interfaces between ferromagnetic and antiferromagnetic layers have shown a variety
of intriguing features like unidirectional anisotropy leading to exchange bias, enhance-
ment of coercivity, rotational hysteresis etc.. When an antiferromagnet is sandwiched
between two ferromagnetic layers a nonvanishing exchange interaction between the
two ferromagnets has been observed. Theoretically, the existence of a non-collinear
magnetic structure in the antiferromagnetic spacer has been proposed. Previous inves-
tigations deduced a linear dependence of the turn-angle between the magnetization of
the ferromagnetic layers on the thickness of the antiferromagnet from bulk magneti-
zation measurements. These experiments provided indirect evidence for a spiraling of
the moments within the antiferromagnetic spacer.
The aim of the present work is to investigate the role of an antiferromagnet in me-
diating exchange interaction between two adjacent ferromagnetic layers as a function
of the thickness of the antiferromagnetic spacer layer. In contrast to previous work,
we probed directly the magnetization reversal of individual ferromagnetic layers using
polarizedneutronreflectometrywithpolarizationanalysisinordertoobtainaninsight
into the mechanism of interlayer coupling.
For this purpose, we prepared samples of FeCoV (20 nm)/NiO (t )/FeCoV (20 nm)NiO
trilayers with FeCoV as the ferromagnet and NiO as the antiferromagnet. The tri-
layer series covers a range of NiO thickness from 1.5 to 100 nm. Additionally, NiO
and FeCoV single layers and FeCoV/NiO and NiO/FeCoV bilayers were produced to
investigate systematically the individual properties of the layers and interfaces, which
constitute the trilayer samples. All samples were deposited using DC magnetron sput-
tering while a reactive Ar:O atmosphere was used to deposit NiO from a Ni metal2
target. In the series of NiO layers the composition of the sputter atmosphere was var-
ied to find optimum conditions to obtain stoichiometric NiO with (111) out-of-plane
texture. The stoichiometry and the texture were determined from the critical angle of
X-ray total reflection and the X-ray diffraction pattern, respectively. Based on these
results, NiO-FeCoV bilayers and FeCoV/NiO/FeCoV trilayers were prepared.
X-ray diffraction and reflectometry were applied to characterize the structure of the
samples. TheX-raydiffractionmeasurementsshowthattheout-of-planetextureofthe
NiOlayersdependsontheunderlyingmaterial. Predominant(111)textureisfoundfor
the NiO layers deposited on glass substrates as expected from the chosen preparation
conditions. WhenNiOisgrownontopofFeCoV,grainswith(200)texturearepresent
in addition. The out-of-plane texture of FeCoV layers is always (110). X-ray reflectiv-
ity measurements were employed to probe the chemical depth profiles of the multilayer
samples. A detailed layer structure of the samples was deduced from the refinement
of models of the depth profile. The models were developed systematically from FeCoV
single layers to trilayers via NiO-FeCoV bilayers. Due to this systematic procedure,
finer details about layer thickness, interface roughness, interfacial layers and surface
oxidation were unraveled. The structural characterization confirms the consistent and4
high quality of the samples prepared by DC magnetron sputtering.
Bulk magnetic properties of FeCoV single layers, bilayers and trilayers were obtained
from DC magnetization measurements. Hysteresis loops were measured along differ-
ent directions in the plane of the samples in order to determine coercive fields and
to obtain any net magnetic in-plane anisotropy. In addition to experiments at room
temperature, measurements were performed at temperatures T = 2, 400 and 530 K
as well, for the investigation of the temperature dependence of magnetization reversal.
At T = 530 K (> T ), in the paramagnetic state of NiO, the magnetic properties ofN
ferromagnetic layers in bilayers and trilayers were obtained without the contribution
of interfacial exchange from antiferromagnetic NiO.
The single layers of FeCoV provide intrinsic magnetic properties of free FeCoV layers
isolated from other magnetic layers. They show isotropic magnetic properties in the
plane of the films with a relatively high coercivity compared to bulk. It is inferred that
themagneticpropertiesareaconsequenceofarandomdistributionoflocalstresscaus-
ing high local magnetic anisotropies because of the large magnetostriction of FeCoV.
In addition, FeCoV layers with different layer thicknesses were investigated to test the
sensitivity of the magnetic properties on the layer thickness. A variation of coerciv-
ityisfound,whichcanbeproperlyexplainedintermsoftherandomanisotropymodel.
Separate information about the magnetic properties of the bottom FeCoV layer in the
trilayers including the influence of antiferromagnetic NiO on top was obtained from
the series of FeCoV/NiO (t ) bilayers. At room temperature, the hysteresis loopsNiO
of the bilayers are very similar to that of the FeCoV single layers indicating that the
magneticpropertiesoftheFeCoV/NiObilayersaregovernedbytheintrinsicproperties
of the ferromagnetic layer. Only a weak influence of the antiferromagnet is observed
in the case of a thick NiO spacer layer, which manifests itself as small exchange bias.
The influence of NiO becomes prominent at low temperature enhancing the exchange
bias significantly. It is found that the direction of pinning is defined by the orientation
of the magnetization of the ferromagnet during cooling. This result suggests that the
ferromagnet causes a reorientation of the antiferromagnetic spins, which are stabilized
when samples are cooled to low temperature.
NiO (t )/FeCoV bilayers represent the upper part of the trilayer motif. These sam-NiO
ples show a distinct magnetic anisotropy and rather low coercivity at room tempera-
ture, which is significantly different to FeCoV single layers and FeCoV/NiO bilayers.
Weak exchange bias is observed for thick NiO layers. At low temperatures, significant
exchange bias was measured for all thicknesses of the NiO layer. The unidirectional
anisotropy could be tuned along either direction of the anisotropy axis dependent on
the orientation of the ferromagnet during cooling, similar to the series of FeCoV/NiO
bilayers. ForparamagneticNiO(T >523K)theeasyaxisofmagnetizationisobserved
perpendicular to the direction of the easy axis for antiferromagnetic NiO suggesting
thattheanisotropybelowT isinducedbytheNiOlayerandtransferredtotheFeCoVN
layer via exchange coupling across their common interface. From these results, it is
inferred that at room temperature the antiferromagnetic spins are rotated between the
two possible orientations of their uniaxial anisotropy axis initiated by the reversal of5
the exchange coupled ferromagnet. At low temperature, the antiferromagnetic spins
become stabilized inducing exchange bias.
The MH-loops of FeCoV/NiO (t )/FeCoV trilayers show a strong dependence ofNiO
themagnetizationreversalonthethicknessoftheNiOspacerlayer. Fort ≤10nm,NiO
the reversal occurs via a single gradual transition whereas for t ≥20 nm, the MH-NiO
loops exhibit two steps during the magnetization reversal. The first step shifts towards
lower applied fields for increasing thickness of the NiO spacer layer. The second step
remains constant at an applied field similar to the coercive field of the FeCoV single
layer. For t ≥40 nm, the intermediate plateau in between the two steps has almostNiO
zero net magnetization. The two step process is present unless NiO becomes param-
agnetic. At T = 530 K the magnetization reversal for t ≥ 40 nm occurs also in aNiO
single gradual process. At T = 2 K, trilayers with t ≥ 40 nm shows exchange biasNiO
similar to the NiO-FeCoV bilayers whereas for trilayers with t ≤ 10 nm almost noNiO
exchange bias is observed.
In order to resolve the magnetization reversal of individual ferromagnetic layers of the
FeCoV/NiO/FeCoV trilayers, polarized neutron reflectometry with polarization anal-
ysis was employed. Measurements were performed on selected samples representing
the different regimes observed in bulk magnetic measurements. The obtained reflec-
tivity profiles were modeled, based on the detailed chemical layer structure deduced
from X-ray reflectivity data and adjusting the magnetization vectors of each FeCoV
layer. First measurements were performed at magnetic saturation of the samples pro-
vid