Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂
The effect of linearly polarized light illumination on the metamagnetic phase transition in the antiferromagnetic garnet Ca₃Mn₂Ge₃O₁₂ is studied.
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2002
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| Cite this: | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ / V.A. Bedarev, V.I. Gapon, S.L. Gnatchenko, M. Baran, R. Szymczak, J.M. Desvignes, H.Le Gall // Физика низких температур. — 2002. — Т. 28, № 1. — С. 51-60. — Бібліогр.: 19 назв. — англ. |
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| author | Bedarev, V.A. Gapon, V.I. Gnatchenko, S.L. Baran, M. Szymczak, R. Desvignes, J.M. Gall, H. Le |
| author_facet | Bedarev, V.A. Gapon, V.I. Gnatchenko, S.L. Baran, M. Szymczak, R. Desvignes, J.M. Gall, H. Le |
| citation_txt | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ / V.A. Bedarev, V.I. Gapon, S.L. Gnatchenko, M. Baran, R. Szymczak, J.M. Desvignes, H.Le Gall // Физика низких температур. — 2002. — Т. 28, № 1. — С. 51-60. — Бібліогр.: 19 назв. — англ. |
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| description | The effect of linearly polarized light illumination on the metamagnetic phase transition in the antiferromagnetic garnet Ca₃Mn₂Ge₃O₁₂ is studied.
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Fizika Nizkikh Temperatur, 2002, v. 28, No. 1, p. 51–60Bedarev V. A., Gapon V. I., Gnatchenko S. L., Baran M., Szymczak R., Desvignes J. M., and Gall H. LeEffect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca3Mn2Ge3O12Bedarev V. A., Gapon V. I., Gnatchenko S. L., Baran M., Szymczak R., Desvignes J. M., and Gall H. LeEffect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca3Mn2Ge3O12
Effect of light illumination on
antiferromagnet–metamagnet phase transitions in the
garnet Ca3Mn2Ge3O12
V. A. Bedarev, V. I. Gapon, and S. L. Gnatchenko
B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences
of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail:bedarev@ilt.kharkov.ua
M. Baran and R. Szymczak
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
J. M. Desvignes
Laboratory Charles Fabry de l’Institut d’Optique, bat.503, 91403 Orsay, France
H. Le Gall
Laboratory of Magnetism of Bretagne, 6 Avenue Le Gorgeu, 29285 Brest, France
Received June 13, 2001, revised September 13, 2001.
The effect of linearly polarized light illumination on the metamagnetic phase transition in
the antiferromagnetic garnet Ca3Mn2Ge3O12 is studied. The crystal is exposed to light
propagating both along the tetragonal axis [001] and along the [100] direction. In both cases,
a change of the field Ht of the metamagnetic phase transition is observed under illumination,
and this change depends on the orientation of plane of polarization of the light with respect
to the crystal axes. In the first case, k || H || [001], the value of Ht decreases on exposure to
light with the polarization E || [110] and increases on exposure to light with the polarization
E || [11
__
0]. In the second case, k || H || [100], the value of Ht decreases irrespective of the
orientation of the plane of polarization of the light with respect to the crystal axes. However,
the magnitudes of the change of Ht are different for light with the polarization E || [011] and
with the polarization E || [01
__
1]. The change of the field of the metamagnetic phase transition
in the second case was much larger than in the first case. A phenomenological theory of the
photomagnetic effects observed in the antiferromagnetic garnet Ca3Mn2Ge3O12 is developed.
It is shown that the effect of light illumination on the metamagnetic phase transition is
related to the photoinduced magnetic moment in this antiferromagnet. The magnetic moment
induced by linearly polarized light in the garnet Ca3Mn2Ge3O12 is detected experimentally by
means of a SQUID magnetometer.
PACS: 75.30.Kz, 78.20.Ls
Effect of light illumination on antiferromagnet–metamagnet phase transitions
Introduction
The study of possibility of controlling the mag-
netic state of crystals by using light illumination is
attracting interest in terms of both a better insight
into the nature of the effect and its application.
Light-induced phase transitions have been observed
in some magnetically ordered crystals [1]. For in-
stance, the linearly polarized light illumination of
yttrium iron garnets results in a spin-reorientation
phase transition as a result of the photoinduced
change of crystalline magnetic anisotropy [2,3]. The
© V. A. Bedarev, V. I. Gapon, S. L. Gnatchenko, M. Baran, R. Szymczak, J. M. Desvignes, and H. Le Gall, 2002
changes in magnetic and electronic states in re-
sponse to light irradiation was recently observed in
manganites with colossal magnetoresistance [4–7].
In these compounds the light induces a phase tran-
sition from an insulating antiferromagnetic (AFM)
state to a conducting ferromagnetic one. At the
moment, the mechanism of the light-induced phase
transition in manganites remains unclear. The tran-
sition is thought to be a associated with the pho-
toinduced melting of the charge-ordered state
caused by optical transitions with charge transfer
between Mn3+ and Mn4+ ions.
The light-induced modification of the magnetic
state related to the redistribution of the Mn3+ and
Mn4+ ions in the crystal lattice due to the photoin-
duced charge transfer between the ions was recently
observed in another manganese oxide, namely in the
AFM garnet Ca3Mn2Ge3O12 [8]. Linearly polarized
light alters the field of the metamagnetic (MM)
phase transition. The effect of light on the phase
transition in the garnet Ca3Mn2Ge3O12 is related to
the light-induced magnetic moment that arises in
the AFM state.
The mechanisms of the photoinduced phase tran-
sitions in the above manganese oxides have some
common features; in particular, the optical transi-
tions with charge transfer between the Mn3+ and
Mn4+ ions are of great importance. A comprehensive
elucidation of the mechanisms of the photoinduced
phase transitions in manganese oxides is of obvious
interest for the physics of photoinduced phenomena
in magnets and is also necessary for application of
these compounds. The photoinduced effects obser-
ved in manganese oxides could be used in devices
for the storage and processing of information.
The paper reports experimental data on the effect
of illumination on the first-order MM phase transi-
tions induced by magnetic field in the AFM garnet
Ca3Mn2Ge3O12 . The experiments were carried out
for two directions of propagation of the inducing
light and two directions of magnetic field with
respect to the crystallographic axes: along the tet-
ragonal axis [001] (k || H || [001]) and normal to this
axis, i.e., along the crystallographic direction [100]
(k || H || [100]). It was found that the effect of light
on the MM phase transition in the above two cases
differs substantially. A phenomenological theory
has been developed to describe the observed pho-
toinduced effects.
Experimental technique
The effect of light irradiation on the first-order
MM phase transitions in calcium–manganese–ger-
manium garnet (CaMnGeG) was investigated by
means of magnetooptical and magnetometric tech-
niques. The field dependences of the angle of rota-
tion of the plane of polarization of the light and the
field dependences of the magnetization were meas-
ured. Visual observations of the two-phase domain
structure formed during the MM phase transition
were also performed. The samples under study were
plates of several tens of microns in thickness. The
single crystal plates were cut perpendicular to a
direction of type [100]. The elastic stresses gene-
rated in the plate surface layers due to mechanical
polishing were removed by annealing at a tempera-
ture of about 1000 °C as well as by chemical polish-
ing in orthophosphoric acid. It is known that the
CaMnGeG crystals display the Jahn–Teller phase
transition from the cubic to a tetragonal phase at
T ≈ 500 K [9]. This transition results in the forma-
tion of crystal twins in the low-symmetry phase
[9–11]. A special thermal treatment was used to
obtain single-domain samples [10]. As a result of
the treatment, single-domain plates with the
tetragonal axis oriented perpendicular or parallel to
the plate surface were obtained.
In the magnetooptical and visual experiments the
sample was placed on a holder in an optical helium
cryostat and was kept in vacuum. Temperature was
measured by means of a resistance thermometer
with an accuracy of about 0.1 K. A superconducting
magnet produced a magnetic field that was perpen-
dicular to the plate surface and parallel to the
direction of light propagation.
The optical scheme of the experimental setup for
visual observation of the two-phase domain struc-
ture is shown in Fig. 1,a. The light from filament
lamp (1) passes through polarizer (5), the sample
(7), and analyzer (9). The sample image is pro-
jected by the objective (8) onto the photocathode of
a TV camera (10) and is displayed on a monitor
(11) and saved by a videorecorder (12). To ensure
that the magnetic state of the crystal under study
remains unchanged, the light flux density is de-
creased to 0.01 W/cm2 by means of the filters. To
investigate the effect of light illumination on the
MM phase transition, the sample is exposed to the
light of a helium–neon laser (15) with a wave-
length λ = 633 nm and a light flux density of about
0.1 W/cm2. The optical system is also equipped
with rotary mirrors (13) and (14) and with a field
diaphragm (3) having a certain shape which allows
local irradiation of a chosen area of the sample. The
diaphragm image on the sample surface is formed by
the lens (4).
When the field dependences of the angle of
rotation of the plane of polarization were measured,
V. A. Bedarev et al.
52 Fizika Nizkikh Temperatur, 2002, v. 28, No. 1
it should have been taken into account that several
magnetooptical effects (the Faraday effect, Cotton–
Mouton effect, and the linear magnetooptical effect
(LMOE) [12]) arise in a magnetic field in the
antiferromagnet CaMnGeG. Therefore, the incident
linearly polarized light became elliptically polarized
even in the case when it passed through the crystal
along the tetragonal axis. In our experiments we
measured the angle of rotation Φ of the ellipse axis
of the transmitted light with respect to the plane of
polarization of the incident light. The angle Φ
depended on the Faraday rotation and linear bire-
fringence. When the measuring beam of light passed
through the crystal along the [100] direction, i.e.,
perpendicular to the tetragonal axis, the plane of
polarization of the incident light was chosen paral-
lel to the crystallographic direction [010] or [001].
In this case, the contribution of crystalline birefrin-
gence to the modification of the polarization of the
transmitted light was minimal. As a rule, the light
ellipticity was slight (about 1°) in our experiments.
That made possible to use a modulation technique
with light modulation in the plane of polarization
and synchronous detection of the signal to measure
the angle Φ (Fig. 1,b). The measurement of field
dependences Φ(H) and the light illumination of the
sample were carried out with the use of helium–
neon laser of wavelength λ = 633 nm. The light flux
density used for illumination of the sample was
about 0.1 W/cm2. To measure the field depen-
dences Φ(H), the flux density of the laser beam was
attenuated down to 0.01 W/cm2.
The field dependences of the magnetization and
photoinduced magnetic moment were measured by
means of a commercial SQUID magnetometer
(Quantum Design MPMS-5).
Experimental results
Before studying the effect of light illumination
on the MM phase transitions, we examined these
transitions in an unexposed crystal. The data from
the magnetooptical and visual studies were used to
construct the H–T magnetic phase diagrams of
CaMnGeG for two orientations of external mag-
netic field: H || [001] and H || [100] (Fig. 2). In
both cases the transition from the AFM to the MM
state is a first-order phase transition at low tem-
peratures. For H || [100] the first-order AFM–MM
Fig. 1. The optical system of the experimental setup for
visual observation of the domain structure: 1 — filament
lamp; 2, 4, 8 — lenses; 3 — field diaphragm; 5 — po-
larizer; 6 — solenoid; 7 — sample; 13, 14 — mirrors; 15
— helium–neon laser (a). The optical system for meas-
uring the angle of rotation of the plane of polarization
of the ligth: 1 — helium–neon laser; 2 — light attenu-
ator; 3 — polarizer; 4 — solenoid; 5 — sample; 6 —
modulator; 7 — analyzer; 8 — photomultiplier; 9 — re-
corder; 10 — lock-in amplifier (b).
a
b
Fig. 2. Magnetic phase diagrams of the garnet
Ca3Mn2Ge3O12 for H || [001] (a) and H || [100] (b). The
solid and dotted lines correspond to the first- and se-
cond-order phase transitions, respectively.
a
b
Effect of light illumination on antiferromagnet–metamagnet phase transitions
Fizika Nizkikh Temperatur, 2002, v. 28, No. 1 53
phase transition is observed in the whole tempera-
ture range T < TN ≈ 13.5 K where TN is the Neel
temperature. In the case H || [001], the H–T phase
diagram exhibits a critical point, Hcr ≈ 18 kOe,
Tcr ≈ 11.5 K, at which the line of first-order phase
transitions passes into a line of second-order ones.
Because the effect of light illumination on the
first-order MM phase transition was studied, in the
case H || [001] the experiments were performed at
T < Tcr .
In the first experiment we studied the effect of
light irradiation on the MM phase transition in-
duced by a magnetic field H || [001]. In this case the
directions of the inducing light and the measuring
light beam were parallel to the [001] crystal axis.
The effect of light on the MM phase transition was
first studied visually. For this purpose a single
domain AFM state was prepared by applying a
magnetic field [12]. The process of monodomainiza-
tion was monitored visually through the LMOE.
After the process was completed and the magnetic
field was switched off, the upper half of the sample
was exposed to laser light with the polarization
E || [110] whereas the lower one to light with the
polarization E || [110]. Thereupon we observed visu-
ally the magnetic-field-induced phase transition
from the AFM to the MM state. The two-phase
domain structure formed during the transition in
the exposed sample is shown in Fig. 3. The AFM–
MM interphase wall is indicated by the dashed line,
and the boundary between the crystal parts exposed
to the light with the different polarizations is
shown by the solid heavy line. As is evident from
Fig. 3,a, the transition from the AFM to the MM
state in the upper part of the sample occurs before
that in the lower part. To be sure that the difference
in the transition fields between the upper and lower
parts of the sample is caused by the light illumina-
tion and not accidental factors (internal mechanical
stresses, temperature gradient, etc.), in the second
stage of the experiments the upper part of the
sample was exposed to light with the polarization
E || [110] and the lower one to light with the
polarization E || [110]. In this case we observed the
inverse effect, namely, the magnetic-field-induced
phase transition from the AFM to the MM state in
the lower part of the sample occurs before that in
the upper part (Fig. 3,b). Thus the visual observa-
tion permits us to conclude that exposure of the
crystal to light with the polarization E || [110]
stimulates the MM phase transition in the garnet
Ca3Mn2Ge3O12 , while exposure to light with the
polarization E || [110] inhibits the transition.
To determine the value of the photoinduced
change of the phase transition field, ∆Ht , we mea-
sured the field dependences of the angle rotation,
Φ(H), shown in Fig. 4. The dependences were
measured in the same area of the sample (∼ 100 µm
in diameter) exposed first to light with the polari-
zation E || [110] and then to light with the polariza-
tion E || [110]. In both cases the intensity and the
duration of the exposure were the same. The jump
in the Φ(H) curve (Fig. 4) corresponds to a first-
order MM phase transition. The transition is accom-
Fig. 4. The field dependences of the rotation angle
measured in the case k || H || [001] in a sample area of
about 100 µm in diameter exposed beforehand to line-
arly polarized light with the polarization E || [110] or
E || [110]. The sample temperature T = 7 K.
Fig. 3. The two-phase domain structure formed in the
Ca3Mn2Ge3O12 plate during the MM phase transition
after exposure of the crystal to linearly polarized light.
The AFM–MM interphase wall is denoted by a dashed
line. The sample temperature T = 7 K; the applied mag-
netic field H || [001]. Part 1 (above the solid heavy line)
exposed to light with the polarization E || [110] and part
2 (below the solid heavy line) to light with the polari-
zation E || [110] (a); part 1 exposed to light with the
polarization E || [110] and part 2 to light with the po-
larization E || [110] (b).
a b
V. A. Bedarev et al.
54 Fizika Nizkikh Temperatur, 2002, v. 28, No. 1
panied by relatively small hysteresis. The difference
in transition fields, 2∆Ht = Ht2 − Ht1 , between the
cases of exposure of the crystal to light with polari-
zation E || [110] and E || [110] is about of 180 Oe at
the temperature T = 7 K, i.e., ∆Ht ≈ 90 Oe. The
values Ht1 and Ht2 were determined from the mid-
point of the section of sharp change of the rotation
angle on the curve Φ(H) measured with an increas-
ing (or decreasing) magnetic field for E || [110] and
E || [110] (Ht = (H′ + H′′)/2 (see Figs. 5 and 6)).
The MM phase transition field in the unexposed
sample, Ht , was equal to ∼ 31.2 kOe. This is close
to the value of (Ht1 + Ht2)/2.
In the second experiment we investigated the
effect of light illumination on the MM phase tran-
sition induced by a magnetic field H || [100]. In this
case the studies were performed by means of magne-
tooptical and magnetometric techniques. The field
dependences of the angle of rotation of the plane of
polarization and the field the dependences of the
magnetization were measured. The direction of pro-
pagation of the inducing light was parallel to the
crystal axis [100]. In the magnetooptical experi-
ments, the direction of propagation of the measu-
ring light beam was also parallel to the [100] axis.
At k || H || [100] we also observed a photoin-
duced change of the MM phase transition field. The
value ∆Ht = Ht1 − Ht0 (Ht = (H′ + H′′)/2) also de-
pended on the polarization of the inducing light.
However, exposure to light with any linear polari-
zation always resulted in a decrease of the MM
transition field. The field dependences of the rota-
tion angle, Φ(H), measured at the temperature T =
= 11 K, are shown in Fig. 5. The jump in the Φ(H)
curves corresponds to the MM phase transition. As
it is evident from Fig. 5, after exposing the crystal
to light with the polarization E || [011] and
E || [011], the phase transition occurs at a lower
field that in the unexposed crystal. The photoin-
duced decrease in the transition field for E || [011]
was larger than that for E || [011]. It is noted that
the different magnitudes of the rotation angle Φ in
the MM state on the curves shown in Fig. 5 are
related to the appearance of photoinduced linear
birefringence in the illuminated crystal [13].
Figure 6 shown the field dependences of the
magnetization measured at the temperature T = 11 K
in the unexposed sample and in the sample exposed
to light with the polarization E || [011]. It is also
seen in Fig. 6 that a decrease of the MM transition
field is observed after illumination of the sample.
In the case E || [011] (larger magnitude of ∆Ht),
the values of the photoinduced changes in the tran-
sition field, ∆Ht , at different temperatures were
determined from the field dependences, Φ(H), mea-
sured in the temperature range 7–13 K. The depen-
dence ∆Ht(T) is shown in Fig. 7. As is seen in
Fig. 7, the value of ∆Ht increases with decreasing
temperature, peaks at T = 10.5–11 K, and then
decreases with further reduction in temperature. In
the case under consideration the maximum value of
∆Ht is about of 1.2 kOe. This is much larger than in
Fig. 5. The field dependences of the rotation angle
measured in the case k || H || [100] in a sample area
about 100 µm in diameter exposed beforehand to line-
arly polarized light with the polarization E || [011] or
E || [011] as well as in the unexposed sample. The tem-
perature of the sample T = 11 K.
Fig. 6. The field dependences of the magnetization
measured in a field H || [100] at a temperature T = 11 K.
m — unexposed sample; l — the sample was prelimi-
narily exposed to linearly polarized light with polariza-
tion E || [011]. The direction of propagation of the in-
ducting light k || [100].
Effect of light illumination on antiferromagnet–metamagnet phase transitions
Fizika Nizkikh Temperatur, 2002, v. 28, No. 1 55
the previous case where the inducing light propa-
gated along the tetragonal axis. The sign of the
∆Ht remains unchanged in the whole temperature
range studied.
Discussion
The experimental investigations demonstrated that
the exposure of the garnet Ca3Mn2Ge3O12 to line-
arly polarized light propagating along [001] the
direction resulted in an increase or a decrease in the
MM phase transition field, depending on the polari-
zation of the inducing light. On exposure of the
crystal to light propagating along [100] the direc-
tion, the MM phase transition field decreases with-
out regard to the of the polarization inducing light.
The reason why the light irradiation affects the MM
phase transition in the CaMnGeG may be magnetic
moment stimulation by the light. Depending on the
direction of the photoinduced magnetic moment,
the AFM phase in the magnetic field is more or less
stable in the exposed crystals than in the unexposed
crystal, and that results in the variation in the MM
phase transition field.
Let us consider a phenomenological model for
the appearance of a magnetic moment in this garnet
under illumination. It is known that on exposure of
the crystal to light, its internal energy per unit
volume varies as follows [14]:
∆F = − (1/16π) [d(ωεik)/dω] Ei Ek , (1)
where ω is the light frequency, E is the electric field
strength of the light wave, and εik is the dielectric
tensor of the crystal. The change in the internal
energy under illumination can generate a light-in-
duced magnetic moment phm in the crystal. The
value and the direction of the photoinduced mag-
netic moment are dependent on the polarization of
the inducing light as well as on the crystalline and
magnetic symmetry of the crystal [15,16]. Using
(1), we can derive an expression for phm. To do
this, we first expand the ∆F series in H, restricting
to the second-order terms, and then we differentiate
the resulting expression with respect to H. As a
result we obtain:
phml = Clik Ei Ek + Blmik Hm Ei Ek , (2)
where Clik and Blmik are the first and the second
derivatives of the expression (1/16π)[d(ωεik)/dω].
Using (2), we can derive an expression for the
magnetic moment phm induced by linear by pola-
rized light propagating along the [001] direction.
For this purpose, we rewrite Eq. (2) in the follow-
ing form:
phml = phml
(1) + phml
(2) . (3)
In this expression phml
(1) = Clik Ei Ek , where Clik is
an axial c-tensor symmetric in i and k, and
phml
(2) = Blmik Ei Ek Hm = Ph∆χlmHm , where Blmik
is a polar i-tensor symmetric on two pairs of indices
i, k and l, m. Let us obtain first phml
(1) . Taking into
account that CaMnGeG belongs to the point mag-
netic group 4′/m, the tensor matrix Clik can be
written in the form:
Clµ = ±
0
0
C15
0
0
−C15
0
0
0
C14
−C15
0
C15
C14
0
0
0
C14
. (4)
The reduction of indices was used in (4). In this
expression, «+» and «−» correspond to the two
time-inverted antiferromagnetic states, AFM+ and
AFM–, and C15 = Cxxz = Cyyz = Cxzx and C14 =
= Cyzx = Cxzy = Cxyz . If the inducting light propa-
gates along the [001] direction, the components
ph ml
(1) can be written as follows:
ph mx
(1) = 0
ph my
(1) = 0
ph mz
(1) = ± (CzxxExEx − CzyyEyEy + 2CzxyExEy) .
(5)
To determine the second term ph ml
(2) in Eq. (3),
we calculate ph∆χlm taking into consideration the
fact that the Laue crystal class of the garnet under
study is C4 :
Fig. 7. The temperature dependence of the change in the
MM phase transition field caused by light irradiation in
the case k || H || [100] and E || [011].
V. A. Bedarev et al.
56 Fizika Nizkikh Temperatur, 2002, v. 28, No. 1
χlm =
B11 B12 B13 0 0 B16
B12 B11 B13 0 0 −B16
B31 B31 B33 0 0 0
0 0 0 B44 B45 0
0 0 0 −B45 B44 0
B61 −B61 B63 0 0 B66
E1 E1
E2 E2
0
0
0
2E1 E2
=
=
B11E1
2 + B12E2
2 + 2B16E1E2 B61E1
2 − B61E2
2 + 2B16E1E2 0
B12E1
2 + B11E2
2 − 2B16E1E2 0
B31E1
2 + B31E2
2
. (6)
Then, using (6), we can derive an expression for the magnetic moment phml
(2) :
phml
(2) =
B11E1
2 + B12E2
2 + 2B16E1E2 B61E1
2 − B61E2
2 + 2B16E1E2 0
B12E1
2 + B11E2
2 − 2B16E1E2 0
B31E1
2 + B31E2
2
Hx
Hy
Hz
, (7)
where
phmx
(2) = (Bxxxx Ex
2 + Bxxyy Ey
2 + 2Bxxxy ExEy)Hx + (Bxyxx Ex
2 − Bxyxx Ey
2 + 2Bxxxy ExEy)Hy
phmy
(2) = (Bxxyy Ex
2 + Bxxxx Ey
2 − 2Bxxxy ExEy)Hy + (Bxyxx Ex
2 − Bxyxx Ey
2 + 2Bxxxy ExEy)Hx
phmz
(2) = Bzzxx (Ex
2 + Ey
2)Hz .
(8)
It should be noted that the matrix of the tensor Blmik is determined only by the crystal symmetry of CaMnGeG.
Therefore, the direction of the moment phml
(2) is independent of the magnetic state of the crystal. If we know
the expressions for phml
(1) and phml
(2), we can determine the magnetic moment phm induced by light
propagating along the [001] direction. Upon introducing the azimuthal angle ϕ measured from the axis
x || [100], the components of this magnetic moment, phmx ,
phmy ,
phmz , can be given as follows:
phmx = (Bxxxx cos
2 ϕ + Bxxyy sin
2 ϕ + Bxxxy sin 2ϕ)|E|2Hx + Hy(Bxyxx + Bxxxy)|E|2(sin 2ϕ + cos 2ϕ) ;
phmy = (Bxxyy cos
2 ϕ + Bxxxx sin
2 ϕ − Bxxxy sin 2ϕ)|E|2Hy + Hx|E|2(Bxyxx cos 2ϕ + Bxxxy sin 2ϕ) ;
phmz = Bzzxx|E|2Hz ± |E|2(Czxx cos 2ϕ + Czxy sin 2ϕ) .
(9)
The signs «±» correspond to the antiferromagnetic states AFM+ and AFM–. At k || H || [001], field
components Hx and Hy are equal zero. Therefore, phmx = 0, phmy = 0 and phmz = Bzzxx|E|2Hz ±
± |E|2(Czxx cos 2ϕ + Czxy sin 2ϕ) in this case.
If the inducing light propagates along the [100] direction, the induction of a magnetic moment is also
allowed by the symmetry. Repeating the line of calculation made for the previous case, we can derive
expressions for the photoinduced magnetic moment components:
phmx = (HxBxxxx + HyBxyxx)|E|2 cos2 ϕ + HyBxxzz|E|2 sin2 ϕ + (HzBxzxz ± 2Cxxz)|E|2 sin 2ϕ ;
phmy = [(HxBxyxx + HyByyxx)|E|2 cos2 ϕ + HyByyzz|E|2 sin2ϕ] + (HzByzxz ± 2Cyxz)|E|2 sin 2ϕ ;
phmz = Hy Bzzxx|E|2 + (Hy Byzxz + Hx Bxzxz)|E|2 sin 2ϕ ± Czxx|E|2 cos2 ϕ .
(10)
Effect of light illumination on antiferromagnet–metamagnet phase transitions
Fizika Nizkikh Temperatur, 2002, v. 28, No. 1 57
Here, the angle ϕ is measured from the axis
y || [010], and the signs «±», as in the previous case,
correspond to the two antiferromagnetic states
AFM+ and AFM–. At k || H || [100], field compo-
nents Hy and Hz are equal zero. Therefore, phmx =
= HxBxxxx|E|2 cos2 ϕ ± 2Cxxz|E|2 sin 2ϕ, phmy =
= HxBxyxx|E|2 cos2 ϕ ± 2Cyxz|E|2 sin 2ϕ and phmz =
= HxBxzxz|E|2 sin 2ϕ ± Czxx|E|2 cos2 ϕ, i.e. in this
case all components of photoinduced magnetic mo-
ments are not equal zero.
Experimental verification of the fact that a mag-
netic moment is really induced by linearly polarized
light in Ca3Mn2Ge3O12 came from the magnetic
measurements. The photoinduced magnetic moment
was measured by means of a SQUID magnetometer.
The uniformly magnetized crystal was irradiated
along the [001] direction by helium–neon laser
light with a wavelength λ = 633 nm and a light flux
density of about 0.1 W/cm2. The plane of polariza-
tion the inducing light was close to the (110) plane
of crystal. The magnetization was measured along
the axis z || [001]. The sample was in a magnetic
field Hz = 6 kOe at a temperature T = 5 K. The
change of the magnetization of crystal under illumi-
nation, i.e., the photoinduced magnetic moment,
was measured. Figure 8 presents the kinetics of the
induced magnetic moment measured for the two
antiferromagnetic states of the sample, AFM+ and
AFM–. It follows from (9) that the value of phmz
changes when the antiferromagnetic state is modi-
fied from AFM+ to AFM–. Indeed, the same expo-
sure of the crystal induced magnetic moments of
opposite directions in the AFM+ and AFM– states
(Fig. 8). The absolute magnitude of the photoin-
duced magnetic moment upon saturation was 0.12
and 0.13 G in the AFM+ and AFM– states, respec-
tively. Thus the absolute magnitudes of the pho-
toinduced magnetic moments in the antiferromag-
netic states AFM+ and AFM– are almost identical.
One can suggest that the polarization-independent
magnetic moment phmz
(2) is appreciably less than
phmz
(1) or is equal to zero. It should be noted that in
the case k || [001] the photoinduced magnetic mo-
ment was not found in the MM state.
In the case k || [100], the magnetic measurements
also revealed a photoinduced magnetic moment. In
AFM state the value of ph m was lower than in the
previous case. As in the above case, the photoin-
duced magnetic moments in the antiferromagnetic
states AFM+ and AFM– are almost the same in
value and opposite in direction. Thus, in the case
considered, the value of the magnetic moment
phmx
(2) is also much lower than phmx
(1) or is also
equal to zero. The absolute magnitude of the mo-
ment was approximately 0.05 G at T = 5 K and
H = 6 kOe. In the case under consideration, the
magnetic measurements also revealed a photoinduced
magnetic moment in the MM state. Its value was
approximately 0.25 G at T = 5 K and H = 35 kOe.
Using the values of phm measured with the
SQUID magnetometer, one can estimate the change
in the MM phase transition field, ∆Ht , caused by
the light irradiation in CaMnGeG. The energy of a
magnet in a magnetic field can be given in the form
of expansion in powers of H [17]:
E = E0 − mi
0Hi − χij HI Hj + ... , (11)
where E0 is the energy of the magnet in the absence
of magnetic field; m0 is the spontaneous magnetic
moment; χij is the magnetic susceptibility. The
photo induction of a magnetic moment results in a
change of the energy of the magnet by a quan-
tity phmi Hi . Taking this addition into considera-
tion and restricting ourselves to the second-order
term in the expansion in H, we can rewrite (11) as
follows:
E = E0 − mi
0Hi − χij Hi Hj − phmi (H)Hi . (12)
Fig. 8. The time dependences of the photoinduced mag-
netic moment in the garnet Ca3Mn2Ge3O12 exposed to
linearly polarized light for two time-inverted antiferro-
magnetic states AFM+ and AFM–. The direction of
propagation and the polarization of the inducing light
are k || [001] and E || [110], respectively. The sample
temperature T = 5 K, and the applied magnetic field is
equal to 6 kOe and is oriented along the [001] axis.
a
b
V. A. Bedarev et al.
58 Fizika Nizkikh Temperatur, 2002, v. 28, No. 1
where phmi (H) = phmi
(1) + ph∆χij Hi . Then, the en-
ergies of the AFM and MM states in both cases
considered above can be written as follows:
AE = AE0 − Aχii Hi
2 − phmi
A(H)Hi ;
ME = ME0 − mi
0Hi − Mχii Hi
2 − phmi
M(H)Hi .
(13)
In (13) the notations A and M refer to the AFM
and MM phases, respectively. Equating the energies
of the AFM and MM states in the point of MM
phase transition for unexposed and exposed crystal
and solving the obtained set of equations, we find
the following expression for the photoinduced
change in the MM transition field:
∆Ht = −
phmi
M(Ht) − phmi
A(Ht)
2(Mχii − Aχii) + mi
0/Ht
(14)
where Ht is the field of the MM transition in the
unexposed crystal.
Using expression (14), we can estimate the value
of ∆Ht in both cases considered k || H || [001] and
k || H || [100]. In the first case no photoinduced
magnetic moment was found in the MM state by
means of the SQUID magnetometer. By substitut-
ing into Eq. (14) phmz
M ≈ 0, phmz
A ≈ ± 0.12 G as
well as (Mχzz − Aχzz) = −3.6⋅10−4 and mz
0 ≈ 47 G, we
obtained ∆Ht ≈ ± 150 Oe. The estimated value ∆Ht
is somewhat (less than two times) higher than the
experimental magnitude ∆Ht ≈ ± 90 Oe. However,
taking into account the errors in the determination
of the parameters substituted in (14), the agree-
ment between the experimental and the calculated
values of the ∆Ht is considered satisfactory. It
should be mentioned that Mχzz ,
Aχzz , and mz
0
were determined from the field dependence M(H)
(mz
0 was determined by extrapolating the linear
field dependence of the magnetization in the MM
state to H = 0).
In the case k || H || [100], one has (Mχxx − Aχxx) =
= −9⋅10−4 and mx
0 ≈ 23 G at the temperature T = 11 K
(see Fig. 6), as well as phmx
M ≈ 0.25 G, phmx
A ≈
≈ ± 0.05 G. By substituting these parameters into
Eq. (14), we obtain two values for the change of
the transition field ∆Ht ≈ −1.2 kOe and ∆Ht ≈
≈ −0.8 kOe. These could correspond either to the
two antiferromagnetic states AFM+ and AFM– or to
the exposure of crystal by the light with the two
polarizations E || [011] and E || [011]. The estimated
values are in a good agreement with the experimen-
tal values ∆Ht ≈ −1.2 kOe and ∆Ht ≈ −0.7 kOe that
were obtained for illumination of the crystal by
light with the polarizations E || [011] and E || [011]
at the temperature T = 11 K. As can be seen from
Fig. 7, the magnitude of ∆Ht decreases at higher
and lower temperatures. Apparently, the decrease
∆Ht as the temperature increases from 11 K to TN is
related to the decrease of the photoinduced mag-
netic moment near the Neel temperature. Some
decrease of ∆Ht at temperatures T < 10.5 K can be
explained by a decreasing absolute value of (χM − χA)
while the photoinduced magnetic moment reaches
saturation. For instance, one obtains a calculated
value ∆Ht ≈ −1 kOe at the temperature T = 9 K.
Conclusion
It follows from a comparison of the experimental
results obtained and the results of a theoretical
consideration that the change of the MM transition
field induced by linearly polarized light in the
garnet Ca3Mn2Ge3O12 is due to the induction of a
magnetic moment under illumination. The appear-
ance of the photoinduced magnetic moment can be
explained by the redistribution of the Mn4+ ions
between the magnetic sublattices in the crystal. The
garnet Ca3Mn2Ge3O12 contains Mn4+ ions in a low
concentration [13]. In the ground state these ions
are uniformly distributed between the sublattices.
The illumination of the crystal by linearly polarized
light leads to a nonuniform distribution of the
Mn4+ ions between the sublattices as a result of
optical transitions with charge transfer [13,16]. As
a result of the redistribution, the magnetic sublat-
tices become nonequivalent and a photoinduced
magnetic moment appears.
This research was supported in part by the
INTAS grant N 97-366.
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|
| id | nasplib_isofts_kiev_ua-123456789-129271 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-07T17:41:47Z |
| publishDate | 2002 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Bedarev, V.A. Gapon, V.I. Gnatchenko, S.L. Baran, M. Szymczak, R. Desvignes, J.M. Gall, H. Le 2018-01-18T15:48:22Z 2018-01-18T15:48:22Z 2002 Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ / V.A. Bedarev, V.I. Gapon, S.L. Gnatchenko, M. Baran, R. Szymczak, J.M. Desvignes, H.Le Gall // Физика низких температур. — 2002. — Т. 28, № 1. — С. 51-60. — Бібліогр.: 19 назв. — англ. 0132-6414 PACS: 75.30.Kz, 78.20.Ls https://nasplib.isofts.kiev.ua/handle/123456789/129271 The effect of linearly polarized light illumination on the metamagnetic phase transition in the antiferromagnetic garnet Ca₃Mn₂Ge₃O₁₂ is studied. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Низкотемпеpатуpный магнетизм Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ Article published earlier |
| spellingShingle | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ Bedarev, V.A. Gapon, V.I. Gnatchenko, S.L. Baran, M. Szymczak, R. Desvignes, J.M. Gall, H. Le Низкотемпеpатуpный магнетизм |
| title | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ |
| title_full | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ |
| title_fullStr | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ |
| title_full_unstemmed | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ |
| title_short | Effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet Ca₃Mn₂Ge₃O₁₂ |
| title_sort | effect of light illumination on antiferromagnet-metamagnet phase transitions in the garnet ca₃mn₂ge₃o₁₂ |
| topic | Низкотемпеpатуpный магнетизм |
| topic_facet | Низкотемпеpатуpный магнетизм |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/129271 |
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