Optical Response on the Colossal Magnetoresistance Effect in Manganites

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J Supercond Nov Magn
DOI 10.1007/s10948-012-1867-8
O R I G I N A L PA P E R
Optical Response on the Colossal Magnetoresistance Effect
in Manganites
A.V. Telegin · V.D. Bessonov · Y.P. Sukhorukov
Received: 7 November 2012 / Accepted: 30 November 2012
© Springer Science+Business Media New York 2012
Abstract The magnetorefractive effect in magnetoreflection and magnetotransmission modes in the optimally doped
manganite epitaxial films possessed the colossal magnetoresistance effect was in the wide spectral region studied. The
strict correlation between the magnetorefractive effect and
colossal magnetoresistance in the middle infrared region for
all samples was observed. It was shown that the magnetorefractive effect is an optical response to the colossal magnetoresistance in manganites. The magnetorefractive effect
can reach a few tens of percents in the field of 3 kOe near
the Curie point and can be explained by the change of ratio
of localized and delocalized charge carriers under the magnetic field. On the contrary, in the visible range no direct
correlations between magnetorefractive effect and colossal
magnetoresistance were detected. The magnetorefractive effect is related with the alteration of the optical density under
the magnetic field in the region of interband optical transitions. The magnitude of the effect is at least one order less in
comparison with one in the infrared, but exceeds the values
of traditional linear magnetooptical phenomena as the Kerr
effect. Finally, huge magnetorefractive effect in manganites
may be used for creation of various magnetic sensors and
light modulation.
Keywords Magnetorefractive effect · Magnetoreflection ·
Magnetotransmission · Thin film · Manganite ·
Magnetooptical elements · Infrared
A.V. Telegin () · V.D. Bessonov · Y.P. Sukhorukov
Institute of Metal Physics, UD of RAS, S. Kovalevskaya 18,
Yekaterinburg 620990, Russia
e-mail: [email protected]
V.D. Bessonov
Laboratory of Magnetism, University of Bialystok,
Bialystok 15-424, Poland
1 Introduction
The magnetic field controlled reflection or absorption of
light and creation of modulators and ultra-speed magnetic
memory devices have been intensively studying [1]. Magnetorefractive effect (MRE) is a magnetic-field-induced
change in the reflection (R) and transmission (t) of natural light in magnetic materials having large magnetoresistance (ρ/ρ = (ρH − ρ0 )/ρ0 ). MRE is most pronounced
in manganites because of colossal ρ/ρ (see for a review
[1–3] and references therein). MRE may appear due to the
influence of magnetic fields on electronic band structure,
polarization, parameters of interband or intraband optical
transitions, and due to the magnetoresistance. According to
the MRE theory, effect in manganites may appear due to
the influence of magnetic field on electronic band structure, parameters of interband or intraband optical transitions, due to the magnetoresistance, and can be defined
as
R/R = (RH − R0 )/R0 = −(1/2)(1 − R0 )ρ/ρ
and
t/t = (tH − t0 )/t0 = (1/2)t0 ρ/ρ,
where RH , tH , ρH , R0 , t0 , and ρ0 are the parameters in a
nonzero and zero external magnetic field.
In contrast to traditional magnetooptical phenomena, the
MRE is not attributed to the spin–orbit interaction; hence,
the magnetoreflection and magnetotransmission of light
due to magnetorefractive effect can reach giant for magnetooptics values and can be used for contactless measurements of magnetoresistance, modulators of light, and
so on.
J Supercond Nov Magn
Fig. 1 On the left—spectra of the magnetoreflection (a) in the inplane magnetic field of H = 3 kOe and magnetotransmission (b) in the
out-of-plane magnetic field of 8 kOe of the La0.7 Ca0.3 MnO3 films at
the temperatures of the maximum of effects. On the right—temperature
dependences: 1—magnetoreflection of light at λ = 12.5 µm in a magnetic field H = 3.5 kOe, 2—magnetotransmission of light at λ = 6 µm,
and 3—magnetoresistance in H = 8 kOe for La0.7 Ca0.3 Mn0.3 film
In this paper, we studied the MRE in magnetoreflection
and magnetotransmission modes in the wide spectral region
for La0.7 Ca0.3 MnO3 manganite films, which possessed the
colossal magnetoresistance effect. It is shown that the MRE
in manganites is an optical response to the colossal ρ/ρ
in the middle infrared (IR) region and can reach tens of
percent near the Curie temperature. In the visible range,
the obtained results indicate that magnetotransmission and
magnetoreflection in manganites are mainly related with the
alternation of density of electronic states under magnetization.
and colossal ρ/ρ have a similar field and spectral behavior and reach maximum values near the Curie temperature
(Fig. 1).
The maximum of R/R(λ) in the films reaches up to
20 %. The magnetoreflection is enhanced in the films by
several times as compared to that in crystals [4] due to the
additional contribution of light reflected from the substrate.
The spectra of R/R(λ) show the presence of a resonancelike contribution against the background of weakly varying positive magnetoreflection. The nature of resonance-like
contribution connected with change in the frequency of the
minimum of the light reflectance in the magnetic field near
the TC . The value of this resonance-like contribution depends on the films inhomogeneity and differs for films with
different thickness. A giant t/t effect of about 10 % in
the 50 nm film demonstrates the possibility of increasing
the optical efficiency of manganite films by decreasing their
thickness. MRE in the IR spectral range is connected with
a contribution from free charge carriers, which is characterized by a maximum at a radiation frequency of ω ∼ τ −1 ,
where τ is the time of relaxation. The contribution from delocalized electrons to MRE in the region of interband transitions near λ ≤ 1 µm is small; consequently, the MRE is
almost absent. The field dependences of R/R, t/t, and
ρ/ρ of films exhibit a linear behavior and no saturation
of effects as well as hysteresis (are not presented). Such a
correlation is direct evidence of the relation of R/R(λ)
and t/t in La0.7 Ca0.3 MnO3 samples to the magnetorefractive effect. The observed features of spectral and temperature dependences of MRE in magnetoreflection and magnetotransmission modes in manganites can be consistently
explained in the framework of the developed MRE theory [3].
2 Experimental Set-up
The epitaxial La0.7 Ca0.3 MnO3 films with different thickness were grown on a perovskite-type substrate (LaAlO3 )
by a method of chemical deposition from vapor of metalloorganic compounds (MOCVD). The magnetoreflection
and magnetotransmission spectra of samples were investigated for natural light in the mid-IR and a visible spectral range (0.4 ≤ λ ≤ 27 µm) near the normal light incident in the magnetic field applied in-plane and out-of-plane
to the sample surface. All field and spectral dependences
were measured near the Curie temperature (TC ) of the samples.
3 Results
3.1 Infrared Range
In the middle IR region, the giant values of the negative
magnetotransmission and positive magnetoreflection were
detected at λ > 1 µm connected with the interaction of
light with free charge carriers. The magnetorefractive effect in magnetotransmission and magnetoreflection modes
3.2 Visible Range
The giant MRE (∼2–4 %) in the wide temperature interval from 10 K to 300 K was for all studied manganite films
J Supercond Nov Magn
Fig. 2 On the left—spectra of the magnetoreflection (a) in the out-ofplane magnetic field of 11 kOe. and magnetotransmission (b) in the
in-plane magnetic field of 2.8 kOe of the La0.7 Ca0.3 MnO3 films at the
temperatures of the maximum of effects. On the right—the tempera-
ture dependences of the magnetoreflection in the La0.7 Ca0.3 MnO3 film
at wavelengths 0.4 µm (solid line) and 12 µm (symbols) in the out-ofplane and in-plane magnetic field H = 11 and 3 kOe respectively
detected in the visible spectral range (Fig. 2). However, no
strict correlation between MRE and colossal ρ/ρ for all
samples was observed. For example, there is change of sign
of magnetoreflection at 0.6 µm which is absent in the magnetotransmission. The spectra of the MRE on reflection and
transmission modes for films are similar to the spectra of
the optical density in the region of interband electron transitions. Temperature dependences of R/R in the region
of fundamental absorption for films are complicated substantially. The contribution of free charge carrier to MRE
is very small in the visible range. Meanwhile, there is a
clearly pronounced feature in MT and magnetoreflection
effects near TC for all samples. The estimations of MRE
in the framework of developed theory have given a contribution related with MR about only 0.3 % with maximum near the Curie point that is less than experimental
date.
First, the clearly expressed anomaly near the TC is manifested in R(T , H )/R dependence for all films. Secondly,
there is effect of magnetoreflection in the paramagnetic region whereas in the IR range effects of ρ/ρ, R/R, and
t/t are approached to zero. Thirdly, it was discovered a
weak temperature dependence of R/R in the ferromagnetic region and magnetic fields of 11 kOe, which is in contradiction with the data for the IR-range, where the magnetoreflection approaches to zero values at T < (TC –20 K)
regardless of presence or absence of resonance-like contribution to magnetoreflection. The observed spectral peculiarities take place in the interval between the two absorption bands at 1.5 and 4.5 eV from the spectrum of
optical density of manganites, where optical absorption is
small. So from our point of view, according to the experimental date, the nature of the magnetorefractive effect
in manganite films in visible range may be explained by
the alteration of the density of electronic states and shift-
ing of resonance-like absorption bands under the magnetic
field.
4 Application
Although the obtained magnitude of the MRE is one order
less in comparison with that in the IR-region, but it greatly
exceeds a level of linear magnetooptical effects (Faraday,
Kerr effects) in this region at the same conditions. Taking in
account our results, a “twin” modulator of light on the base
of early developed infrared modulator [5] was proposed.
The magnetooptical elements of the modulator (MOE) were
made from epitaxial lanthanum manganite films grown by
different methods. The principal working scheme of this
modulator is as follows: the unpolarized light falls on the
working surface of the MOE at different angles from 5
to 75 degrees. The light is modulated in the MOE under the
magnetic field applied. Part of the modulated light passes
through the magnetooptical material, the remaining part of
modulated light reflects from the surface of MOE. The value
of passed and reflected light depends on the thickness of the
MOE and the angle of light incident. The Curie temperature
of the magnetooptical materials (manganite films) determines the working temperature of modulator. The depth of
modulation of light was proportional to the value of applied
magnetic field, thickness of the MOE and the magnitude of
the magnetorefractive effect. The operation speed of an optical modulator employing the magnetotransmission effect
in lanthanum manganites is determined by rate of magnetization reversal processes (up to 10 GHz [6]). Furthermore,
the thermostabilization system of this IR-modulator may be
simplified by using MOE on the base of an artificially created thin multilayer structure made from manganites films
with various TC [7].
J Supercond Nov Magn
5 Summary
Finally, the huge magnetorefractive effect in the wide spectral range was detected for manganites. By the analysis of
the obtained data for magnetotransport, optical and magnetooptical properties of doped thin films of manganites, we
concluded that magnetorefractive effect has a different nature depending on the spectral range of investigation. At
visible range, the effect is related with the alteration of
the electronic structure of films in the region of interband
transitions described by different mechanisms and shifting
of the resonance-like absorption bands under the magnetic
field applied; at infrared—one is determined by the interaction of light with free charge carriers and resonance-like
contribution connected with shifting of position of minimum of reflectance. Though there is a small contribution of
free charge carriers to the magnetorefractive effect (e.g. the
small feature in magnetoreflection dependence on Fig. 2)
in the visible region (the area of fundamental absorption
of light for manganites) the effect is much less in comparison with that in the infrared. The obtained magnitude
of the magnetorefractive effect at visible is less than one
in the infrared, but it exceeds a level of linear magnetooptical effects in this region and can be proposed for using
in various optoelectronic devices, e.g., twin-modulator of
light.
Acknowledgements This work was supported by the RFBR Grant
10-02-00038, the program of DPS RAS “Physics of new materials and
structures,” and the Grant of the President of Russia MK-1048.2012.2.
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