Transcript Ultrathin Films and Some Cross Effect

Ultrathin Films and Some Cross
Effect
(1) Some properties in ultra thin films
(a) surface and interface anisotropy
(b) fcc Fe and fcc Co
(c) surface magnetism
(d) M(T) in ultra thin film
(2) Some cross effect
(a) magneto-optical effect
(b) dilute magnetic semiconductor (DMS)
Interesting aspects:
(1) For studying new phases of materials
such as, fcc cobalt and fcc iron grown on Cu (001)
(2) Two dimensional features which not encountered in bulk
specimens.
(3) Whether any dead layer appears ?
Perpendicular anisotropy
Spin polarization, p (in%), of photoelectrons from Fe(100) on
Ag(100), versus magnetic field along the surface normal (Stampanoni et al.,PRL 59(1987)2483).
P=(N - N )/ ( N + N )
Temperature dependence of the saturation polarization of a 3.5 ML
thick epitaxial bcc Fe film on Ag(001) and a 3 ML fcc Fe on Cu(001).
Insert: thickness dependence of the Cuire temperature of the bcc Fe
films.
From the above two figures, the following facts are evident:
(1) Films of 1ML or thicker of bcc Fe/Ag(001) are ferromagnetic
(2) At T=30K, the remanence magnetization of 3.5 ML film is
dirrected along the surface normal. The remanence equals
practically the full saturation magnetization showing the
film to be essentially single domain
(3) The Curie temperature of films thicker than 5 ML equals the
bulk bcc Fe
Magnetisation loop of 2.2 nm thick Ni(111) on Cu(111), coated by
Cu(111), measured by TOM. The films show a perpendicular anisotropy (PA) between 1.0 – 2.5 nm (Gradmann, Ann. Phys.17(1966)91).
Magnetisation loops of Pd/Co multilayers, taken at 300 K, with the
field in the film plane (dashed curves) or along the surface normal
(full lines) (Carcia APL 47(1985)178).
200 C
50 C
Total anisotropy Kt for evaporated (111) texturized polycrystalline
Co/Pd multilayers versus thickness t of Co films.
Co/Pt Multilatersa
Magnetic hysteresis loops at 20 oC.
(Si substrate)
Effective anisotropy times Co thickness versus cobalt
thickness for [Co/Pt] multilayers (Engle PRL 67(1990)1910).
The effective anisotropy energy measured for a film of
thickness d may be described as
,
(1)
or writing as
(2)
=
Keff d = 2ks + (kV -2πMs2)d
(3)
Surface Magnetic Anisotropy ?
• The reduced symmetry at the surface
(Neel 1954);
• The ratio of Lz2 / (Lx2 + Ly2) is increased
near the surface
• Interface anisotropy (LS coupling)
[1] J.G.Gay and Roy Richter, PRL 56(1986)2728, [2] G.H.O. Daalderop et al.,
PRB 41(1990)11919, [3] D.S.Wang et al., PRL 70(1993)869.
Fcc Co on Cu(001)
Normalized polarization P/Po as a function of the externally applied field
perpendicular to the film plane. Data are given for five film thickness at
T=300 K.
Temperature dependence of the spin polarization for a 1 ML film
measured in saturation. Applied field 15 KOe.
Fcc Fe on (001)Cu
Polarization P(H) of a 5 ML fcc Fe film on Cu(001), (a) for sample
temperature T=215, 267, and 375 K, (b) at T=30 K, H is perpendicular to the film plan.
Polarization P(H) measured at T=30 K, (a) for 3 ML fcc Fe on Cu
(001), (b) for 1 ML. H is perpendicular to the film plane.
Temperature dependence of the reduced polarization P/Po for 1, 3,
and 5 ML films of fcc Fe on Cu(001). Po is the saturation polarization
at low temperature. The Curie temperature are 230K for 1ML film,
390 K for 3 and 5 ML films.
Conclution:
(1) 1ML of Co on Cu(001) is ferromagnetic; The
magnetization being in plane; Tc (for 1 ML) >
room temperature.
(2) Fcc Fe (1 ML)stabilized at the lattice constant of
Cu (001) has been found to have a ferromagnetic
ground state; Anisotropy and Curie temperature
is dependent on the temperature and film thickness
[1] D. Pescia et al., PRL 58(1987)933, [2] D.Pescia et al., PRL 58(1987)2126
Surface Magnetisation
Perpendicular Ms
Ferromagnetism in fcc Fe(111) on CuAu (111). Magnetic moment
µFe in the Fe film versus the mean lattice parameter aCuAu
or Au concentration cAu in the substrate.
Variation of magnetic moment calculated by layer in an 8 ML Ni/Cu
(001) film. The calculated bulk and surface moments are 0.56 µB/Ni
and 0.74 µB/Ni (bulk moment 0.6 µB/Ni) (Tersoff PRB 26(1982)6186).
The interior, bulklike layers
(layers 3-6 from Cu)
The surface-like layers
(layers 7 and 8 from Cu)
Spin-resoled density of state for 8 ML Ni(001) film on Cu(001)
Ms (T) behaviour
48Ni/52Fe (111) films
on Cu(111)
Curie temperature of 48Ni/52Fe(111) versus number of atomic
layers DM. The experiments is from Gradmann (Phys. Status Solidi
27(1968)313. Green-function theory from Brodkorb 16(1966)225.)
Domain in ultrathin films
Calculated spin distribution in a thinn sample containing
A 180O domain wall.
(a) Domain pattern as measured by
MFM above the surface of an eiptaix
Cu/200nmNi/Cu(100) film.
(b) Vibrating sample magnetometry
M-H loop of the sample
Magneto-optical Effect
1876 John Kerr
The three types of geometries of the Kerr effect
The arrangement of the magnetization M and wave vector
k in the local coodination employed in the derivation of the pMOKE equation for Normal incidence.
The dielectric tensor has the following form
(1)
The normal model solution to the Fresnel Eq.
(2)
and the corresponding electric field model are
(3)
The definition of Kerr rotation and
Kerr ellipticity
Kerr (Faraday) rotation and ellipticity are expressed by the
component of conductivity sensor
Petros N. Argyres, Theory of the Faraday and Kerr effect in ferromagnets, PR 97
(1955)334,
P.M. Oppeneer, Magneto-optical Kerr spectra in Handerbook of Magnetic Materials,
Edited by Buschow (Vol.13), Physical Review B, 45(1992)10924.
Diluted Magnetic Semiconductors
• The charge of electrons in Semiconductor
(Integrated circuits, devices);
• Spin of electrons in data storage (hard
disc, tapes, magneto-optical disks)
May we be able to use the capability of mass storage and
processing of information at the same time ?
If both the charge and spin of electrons can be used to further
enhance the performance of devices.
Three types of semiconductors: (A) a magnetic semiconductor, (B)
a diluted magnetic semiconductor, an alloy between nonmagnetic
semiconductor and magnetic element; and (c) a nonmagnetic semiconductor.
GaMnAs
Lattice constant a vs Mn composition x in (Ga1-x, Mnx)As films.
a was determined by XRD at room temperature (Ohno et al.,
APL 69(1996)363.
Magnetic field dependence of magnetization M at 5K for a
(Ga, Mn)As film with xMn=0.035. The field was applied parallel
to the sample surface (Ohno et al., APL 69(1996)363).
MnAs/ZnSe
GaAs(001)/200nmZnSe/170nmMnAs
Room temperature longitudinal MOKE responses for ferromagnetic
MnAs on ZnSe: (a) a single phase MnAs/ZnSe (b) a dual phase
MnAs/ZnAs heterostructure (Berry et al., APL 77(2000)3812).
ZnCoAl
XRD patterns and VSM curves of the thin films
deposited at 400 oC at oxygen pressure 5x10-5 Pa
(Yan et al., JAP 96(2004)508).
Co doped TiO2
Matsumoto et al., Science 291(2001)854
An XRD pattern of a Co doped TiO2 film
(x=0.08) showing (004) and (008) peaks
of anatase without any impurity peaks.
Atomic reslution TEM image. No
segregation of impurity phase was
observed.
A series of scanning SQUID microscope images
200 µm x 200 µm
Images taken at 3K for anatase thin films with different Co contents on a
combinatorial chip. (a) x=0, (b) 0.02, (c) 0.03, (d) 0.06. Magnetic domain
were observed in all doped film.
(a) an M-H curve of an x=0.07 film on SrTiO3 taken at room temperature.
(b) M-T curve in a field of 20 mT parallel to the surface. Tc > 400K.
Source ?
(1) RKKY interaction (H.Ohno Science 281(1998)951);
(2) Forming resonant states (J.Inoue et al., PRL 85(2000)
4610;
(3) Clusters of Co in Co-doped anatase TiO2 thin film (J.K.
Kim et al., PRL 90(2003)017401.
RKKY Theory
Origin of Ferromagnetism
In the absence of holes, the magnetic interaction
among Mn has been shown to be antiferromagnetic in
n-type (In,Mn)As and in fully carrier compensated
(Ga.Mn)As. This results show that the ferromagnetic
interaction is hole induced.
GaMnAs
Comparison between the experimental Tc (open circles).
The error bars for the calculated Tc represent the erro
involved in determining exchange and carrier concentration.
(F.Matsukura et al., PRB 57(1998)R2037)
The RKKY exchange Hamiltonian between the Mn spins at
sites i and J is expressed by H = -Ji j si•sj, where Ji j is given by
Here ri j is the distance between I and j, and F(2kFri j) is the
ordinary RKKY oscilation term, and l is the mean free path
of carries. The exp(-ri j/l) represants the effect of a finite l
following de Gennes. Form the ab ove equation, Tc is given
by,
Where zr is the number of r-th nearest group-III sites.