Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ
Resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF)—weak ferromagnetic (WF) transition in the La₂CuO₄ single crystal. The magnetic field (dc or pulsed) was applied normally to the CuO₂ layers. The transition manifested itself in a drastic decrease of the re...
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irk-123456789-1197242017-06-09T03:05:08Z Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ Belevtsev, B.I. Dalakova, N.V. Savitsky, V.N. Bondarenko, A.V. Panfilov, A.S. Braude, I.S. Низкотемпеpатуpный магнетизм Resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF)—weak ferromagnetic (WF) transition in the La₂CuO₄ single crystal. The magnetic field (dc or pulsed) was applied normally to the CuO₂ layers. The transition manifested itself in a drastic decrease of the resistance in critical fields of 5–7 T. The study is the first to display the effect of the AF–WF transition on the conductivity of the La₂CuO₄ single crystal in the direction parallel to the CuO₂ layers. The results provide support for the three-dimensional nature of the hopping conduction of this layered oxide. 2004 Article Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ / B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, I.S. Braude // Физика низких температур. — 2004. — Т. 30, № 5. — С. 551-557. — Бібліогр.: 38 назв. — англ. 0132-6414 PACS: 74.72.Dn, 75.30.Kz, 75.50.Ee http://dspace.nbuv.gov.ua/handle/123456789/119724 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
| spellingShingle |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Belevtsev, B.I. Dalakova, N.V. Savitsky, V.N. Bondarenko, A.V. Panfilov, A.S. Braude, I.S. Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ Физика низких температур |
| description |
Resistive measurements were made to study the magnetic field-induced antiferromagnetic
(AF)—weak ferromagnetic (WF) transition in the La₂CuO₄ single crystal. The magnetic field (dc
or pulsed) was applied normally to the CuO₂ layers. The transition manifested itself in a drastic
decrease of the resistance in critical fields of 5–7 T. The study is the first to display the effect of
the AF–WF transition on the conductivity of the La₂CuO₄ single crystal in the direction parallel
to the CuO₂ layers. The results provide support for the three-dimensional nature of the hopping
conduction of this layered oxide. |
| format |
Article |
| author |
Belevtsev, B.I. Dalakova, N.V. Savitsky, V.N. Bondarenko, A.V. Panfilov, A.S. Braude, I.S. |
| author_facet |
Belevtsev, B.I. Dalakova, N.V. Savitsky, V.N. Bondarenko, A.V. Panfilov, A.S. Braude, I.S. |
| author_sort |
Belevtsev, B.I. |
| title |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ |
| title_short |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ |
| title_full |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ |
| title_fullStr |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ |
| title_full_unstemmed |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ |
| title_sort |
magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal la₂cuo₄+δ |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2004 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119724 |
| citation_txt |
Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ / B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, I.S. Braude // Физика низких температур. — 2004. — Т. 30, № 5. — С. 551-557. — Бібліогр.: 38 назв. — англ. |
| series |
Физика низких температур |
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| fulltext |
Fizika Nizkikh Temperatur, 2004, v. 30, No. 5, p. 551–557
Magnetoresistive study of the antiferromagnetic–weak
ferromagnetic transition in single-crystal La2CuO4��
B.I. Belevtsev1, N.V. Dalakova1, V.N. Savitsky1,
A.V. Bondarenko2, A.S. Panfilov1, and I.S. Braude1
1B. Verkin Institute for Low Temperature Physics and Engineering
of the National Academy of Sciences, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: belevtsev@ilt.kharkov.ua
2V.N. Karazin Kharkov National University, pl. Svobodi 4, Kharkov 61077, Ukraine
Received December 4, 2003
Resistive measurements were made to study the magnetic field-induced antiferromagnetic
(AF)—weak ferromagnetic (WF) transition in the La2CuO4 single crystal. The magnetic field (dc
or pulsed) was applied normally to the CuO2 layers. The transition manifested itself in a drastic
decrease of the resistance in critical fields of 5–7 T. The study is the first to display the effect of
the AF–WF transition on the conductivity of the La2CuO4 single crystal in the direction parallel
to the CuO2 layers. The results provide support for the three-dimensional nature of the hopping
conduction of this layered oxide.
PACS: 74.72.Dn, 75.30.Kz, 75.50.Ee
1. Introduction
Transport and magnetic properties of cuprate
La CuO2 4� � have attracted considerable attention in
the area of superconducting research. This is a parent
compound for one of the family of high-temperature
superconductors, and study of its properties is consid-
ered to be important for elucidation of still unclear na-
ture of superconductivity in cuprates. Stoichiometric
La2CuO4 (� � 0) is an antiferromagnetic (AF) insula-
tor with a Néel temperature TN of about 320 K, but
doping it with bivalent metals (such as Sr) or with ex-
cess oxygen (� � 0) leads to destruction of the
long-range AF order and a decrease in TN [1–3]. A
fairly high doping results in a transition to a metallic
state.
The perovskite crystal lattice of La2CuO4 is ortho-
rhombic (below about 530 K) consisting of CuO2 lay-
ers separated by La2O2 layers (the latter consisting of
two buckled La–O layers) [2,3]. In the Bmab space
group the CuO2 layers are perpendicular to the c axis
and parallel to the ab plane [3]. The CuO6 octahedra
are tilted in a staggered way; the tilting is uniform in a
given cb plane. The AF state is strongly connected with
crystal lattice features [4]. The magnetic state is deter-
mined by d9 2Cu � ions with spin S � 0 5. . In the CuO2
planes, the magnetic structure is characterized by a
simple two-dimensional (2D) AF array with nearest
neighbors having antiparallel moments [4]. Due to the
above-mentioned tilting of the CuO6 octahedra, the
spins are canted 0.17� in the cb plane away from the b
axis [4,5]. As a result, a weak ferromagnetic (FM) mo-
ment perpendicular to the CuO2 plane appears in each
layer. Below TN , the directions of the FM moments are
opposite in neighboring CuO2 planes, so that the sys-
tem as a whole is a three-dimensional (3D) AF [5].
Application of high enough magnetic field along
the c axis causes a magnetic transition into a weak-fer-
romagnetic (WF) state, in which all canted moments
are aligned along the field direction [5]. The transi-
tion is accompanied by a jump-like change in the resis-
tivity [5]. The critical field Hc of the transition is
temperature dependent. It goes to zero for T approach-
ing TN , but increases with decreasing temperature and
amounts up to 5–6 T below 100 K. Hole doping of
La CuO2 4, leading to lower TN , causes smaller Hc
values as well (down to about 3 T at low temperature
for samples with TN about 100 K) [6]. Some magnetic
transitions have also been found for field applied par-
allel to the ab plane [7]. In this case, for field parallel
© B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, and I.S. Braude, 2004
to b axis, a spin-flop transition was found at field H1
about 10 T, and a transition to the FM state at a field
H2 of about 20 T. These transitions manifest them-
selves as weak knees (no jumps) in the MR curves [7].
It is believed that no magnetic transition should take
place when field is applied parallel to the a axis, which
is perpendicular to the staggered moments [7,8].
Doping with excess oxygen introduces charge carri-
ers (holes) in the CuO2 planes. At small enough
�( .� 0 01), La2CuO4� � remains insulating, althongh
TN is lowered considerably [9,10]. The excess oxygen
atoms reside at interstitial sites between La–O planes
[11]. Each such excess atom is surrounded by a tetra-
hedron of apical oxygen atoms. For layered cuprates,
in which the CuO2 planes are the main conducting
units, a quasi-2D behavior is expected for the in-plane
transport. This has actually been found in many
cuprates [12] but not in La2CuO4� �. In this com-
pound, the Mott’s variable-range hopping (VRH),
with temperature dependence of the resistance de-
scribed by the expression
R R
T
T
�
�
�
�
0
0
1 4
exp ,
/
(1)
is found [13,14] at low T for both the in-plane (cur-
rent J parallel to the CuO2 planes) and out-of-plane
( | | )J c transport. The fractional exponent in Eq. (1)
equal to 1/4 corresponds to 3D system (for 2D sys-
tems, it should be equal to 1/3) [15]. At the same
time, the hopping conduction in La2CuO4� � samples
with fairly high crystal perfection shows a consider-
able anisotropy, so that the values of R0 and T0 in
Eq. (1) are different for the in-plane and out-of-plane
transport. The in-plane conductivity � ab is found to
be considerably higher than the out-of-plane conduc-
tivity � c. The ratio � �ab c/ is strongly temperature
dependent. It is minimal (about 10) in the liquid-he-
lium temperature range, but increases dramatically
with temperature and saturates above 200 K to maxi-
mal values of the order of 100 [16–18].
The 3D character of VRH in La2CuO4� � testifies
that a hole transfer between CuO2 is likely not only at
J c| | , but at J a b| | , as well. In considering this question
it is important to know the exact nature of the holes in
La CuO2 4� �. Although about 17 years has passed
since the discovery of superconductivity in doped
La CuO2 4� �, the nature of the holes in it still cannot be
considered completely clear. This in turn makes it hard to
gain insight into the nature of the cuprate’s superconduc-
tivity. In the undoped state, the CuO2 planes present a
lattice of d9 2Cu � (S � 0 5. ) and p6 2O � (S � 0) ions.
Doping with excess oxygen causes (to ensure neutrality)
the appearance of additional holes in the planes. This can
be achieved in two ways: 1) some of the d9 2Cu � ions
change into the d8 3Cu �( )S � 0 state, or 2) some of
the in-plane oxygen ions p6 2O � change into the p5 1O �
( .S � 0 5) state. In either case, the holes induce strong lo-
cal perturbations of the AF order. In the known literature
[19–25], both kinds of holes have been taken into account
in theoretical models of fundamental properties of the
cuprates. There is much speculation, however, that holes
in La2CuO4� � have a strong oxygen character [19–24],
and this view has strong experimental support [20,22,26].
At the same time, due to the overlapping of the d and p
orbitals and hybridization of the d and p bands, the d
orbitals exert a significant influence on the hole motion.
According to Ref. 21, owing to the special the char-
acter of the excess oxygen as interstitial atoms [11]
with weak oxygen—oxygen bonding, the holes can be
delocalized from the CuO2 planes onto the apical O
atoms, i.e., into the La O2 2� � region between adjacent
CuO2 planes. This assures the 3D nature of VRH in
La2CuO4� �. In this way the La2CuO4� � differs dras-
tically from the Sr-doped system, where the holes re-
main quasi-two-dimensional. In fact, the ratio � �ab c/
in lightly doped La Sr CuO1 4�x x crystals of good qual-
ity can be as high as several thousand [27].
In this communication we report the results of a
study of the AF–WF transition by magnetoresistance
(MR) measurements in a La2CuO4� � single crystal. In
the known previous studies [5,28–31] the MR investi-
gations of AF–WF transition in La2CuO4� � were done
for the case when both the magnetic field and transport
current are perpendicular to the CuO2 planes (i.e.,
H c| | and J c| | ). Under these conditions a rather sharp
decrease in the resistance has been found as the critical
field Hc was approached from below. The amplitude of
the relative change in resistance (�R/Rn , where Rn is
the resistance in the AF state) due to the AF–WF tran-
sition depends on temperature. It is maximal in the
range 20–30 K, where it can amount to 0.30–0.50 in
fairly perfect crystals [5,7,29–31].
It is known that the enhancement of spin order usu-
ally leads to a decrease in resistivity of metallic sys-
tems. For example, a considerable decrease in resisti-
vity can occur at transitions from the paramagnetic
to the FM or AF state in some metals, alloys or even
in some FM perovskite oxides, like mixed-valence
manganites [32–34]. This is usually attributed to a de-
crease in the scattering rate of quasi-free charge car-
riers on disordered local spins as a result of the
above-mentioned magnetic transitions. The situa-
tion is rather different in the case of insulating
La CuO2 4� �. Here the transition to the 3D AF state
produces hardly any noticeable change in the hopping
conductivity at TN (apparently for the reason that 2D
AF correlations in the CuO2 planes persist up to
temperatures far above TN [1–3]). But transition to
552 Fizika Nizkikh Temperatur, 2004, v. 30, No. 5
B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, and I.S. Braude
the 3D WF state increases the conductivity enor-
mously. Since VRH in La2CuO4� � has a pronounced
3D character, it can be expected that the AF–WF
transition would manifest itself in resistivity in field
H c| | not only for the transport current perpendicular
to the CuO2 planes, as was found in Refs. 5, 29–31,
but for the in-plane hole transport as well. In this
study this effect has been actually revealed, as
described below.
2. Experimental
A single-crystal La2CuO4� � sample with dimen-
sions of 1.3�0.3�0.39 mm is investigated. This sample
was studied previously in Ref. 18, where it was indi-
cated as sample No. 1 with TN = 188 K. After that
study, the sample was annealed additionally in an
oxygen atmosphere (700 �C, 5.5 days) in the hope
that oxygen content (that is, �) would be increased. It
turned out, however, that the thermal treatment
caused only a slight decrease in TN (down to 182 K)
and in the resistivity. The TN value was determined
from magnetic susceptibility measurements.
The crystallographic orientation of the sample was
determined from an x-ray diffraction study. This re-
veals that the sample has a quantity of twins, which
inevitably appear in La2CuO4� � crystals when cooled
through tetragonal-to-orthorhombic structural transi-
tion at T � 530 K [3]. As a result, a peculiar domain
structure is developed. The orientation of the c axis is
the same in each domain, but the orientations of the a
and b axes are switched (or reversed) in a fixed way
between two possible orientations upon crossing the
domain (twin) boundaries. In this connection, al-
though we will speak conventionally in the following
about a or b directions of transport current in the sam-
ple studied, they should be taken, first of all, as the
two in-plane current directions in the twinned crystal,
which are perpendicular to each other. In a heavily
twinned crystal no significant anisotropy in the
in-plane conductivity can be expected even assuming
that some intrinsic conductivity anisotropy within the
CuO2 planes is present. We have found, however, a
pronounced anisotropy in the conductivity (and a
rather significant one in the MR) for these two
in-plane directions. This matter will be touched upon
in the next Section of the paper. In contrast, we can
speak about the c directions in the sample studied
without any reservation or possible misunderstanding.
In this study, the dc resistance in the directions
parallel to CuO2 planes was measured by the Mont-
gomery method [35], which is appropriate for systems
with a pronounced anisotropy of the conductivity.
Contacts between the measuring wires and the sample
were made using a conducting silver paste. The mea-
surements were done in field H c| | in a helium
cryostat with a superconducting solenoid. Although
the maximum field in the cryostat (about 6 T) has ap-
peared to be quite sufficient in most cases to reveal
manifestations of the AF–WF transition in MR of the
sample studied, a somewhat higher field is needed to
study the transition more thoroughly, especially for
the study of hysteretic phenomena in the R H( ) curves
in the vicinity of the critical field Hc [5,29–31]. This
hysteretic behavior is considered as an indication of a
first-order transition. For this reason, a part of the dc
resistance measurements in this study were done in
pulsed magnetic field with amplitude up to 15 T. The
nearly sinusoidal pulse has a duration about 33 ms,
during which the field is swept from zero to a maxi-
mum amplitude and back to zero. For these measure-
ments the field H c| | and transport currents, J c| | and
J a| | , were used. The rate of variations in magnetic
field was up to 103 T/s. Other essential details of the
pulse measuring technique employed can be found in
Ref. 36.
3. Results and discussion
The temperature dependences of the resistivity �a
measured along the a axis ( | | )J a is shown in Fig. 1 for
different magnitudes of the measuring current. It is
seen that the �( )T behavior does not depend essen-
tially on current in the whole measuring temperature
Magnetoresistive study of antiferromagnetic–weak ferromagnetic transition in single crystal
Fizika Nizkikh Temperatur, 2004, v. 30, No. 5 553
300 100 50 20 10 5
T, K
10
4
103
10
2
101
100
10–1
10 mA
1 mA
100 A�
10 A�
1 A�0.01 A�
0.1 A�
0.2 0.3 0.4 0.5 0.6 0.7 0.8
T , K
–1/4 –1/4
�
,
�
cm
a
.
Fig. 1. The temperature dependences of resistivity �a of
single-crystal La CuO2 4� � measured at different values of
transport current. In all cases the current was directed
parallel to the crystallographic axis a. The dependences
are presented as lg �a versus T�1 4/ .
range, 4 2 300. K K� �T , for current magnitude less
than about 1 �A; that is, Ohm’s law holds in this case.
For better consideration, one of these ohmic �( )T
curves (at J � 1 �A) is presented separately in Fig. 2.
It can be seen that Mott’s law [Eq. (1)] is obeyed
fairly well in the range 20 K � T � 200 K. In the range
T � 20 K, a steeper [as compared to Eq. (1)] increase
in R with decreasing temperature is found. This devia-
tion from Mott’s law at low temperature is rather typ-
ical for La2CuO4� � and was observed earlier in
Refs. 14, 31. In Ref. 14, a possible reason for this be-
havior is suggested: the presence of superconducting
inclusions in the insulating sample due to phase sepa-
ration in La2CuO4� �.
Magnetic structure of La2CuO4� �, according to
neutron diffraction data [2,4], is anisotropic for all
three orthorhombic axes. The same can be expected,
therefore, for transport and magnetic properties. In
the presence of twins, however, the measured trans-
port and magnetic properties usually show quite defi-
nite anisotropy solely for directions parallel and
perpendicular to the CuO2 planes. Recently, in un-
twinned La2CuO4� � crystals, a clear in-plane aniso-
tropy of the magnetic susceptibility � was found [37].
A similar phenomenon may be expected in the trans-
port properties of La2CuO4� � samples without twins.
In a sample with multiple twins, no considerable
in-plane anisotropy could be expected. The measured
ratio � �b a/ in the sample studied (see inset in Fig. 2)
reveals, however, a rather distinct anisotropy. The ra-
tio is close to unity at T � 11–12 K, but it increases
with temperature and approaches value of about 3 at
room temperature. A similar behavior was found in
the previously studied sample with somewhat higher
TN � 188 K [18]. The a–b anisotropic conductivity
behavior in a twinned sample (in the case that the con-
ductivities � a and �b are inherently different) can be
observed only when, first, the existing twins are few
in number (so that the measured resistivity is not
properly averaged between the two possible crystal
orientations), and, second, a given current direction is
really parallel to the a (or b) axis in most of the crys-
tal. The results of this study therefore give evidence
that the intrinsic conductivity anisotropy in the CuO2
planes of La CuO2 4� � is quite credible.
We found that the MR behavior of the sample stud-
ied depends significantly on the magnitude of the mea-
suring current, especially at low temperature. The up-
per panel of Fig. 3 presents the MR curves recorded at
T � 5 K for the case J a| | . It can be seen that for low
currents (that is in the Ohmic regime) the MR is posi-
tive, but for high enough currents (J � 1 �A) the MR
becomes negative and strongly increases above
H � 5 T. Positive MR was observed only at low tem-
perature (T � 20 K) for both the in-plane current
554 Fizika Nizkikh Temperatur, 2004, v. 30, No. 5
B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, and I.S. Braude
300 100 50 20 10 5
T, K
104
103
10
2
101
10
0
10
–1
�
,
�
cm
a
.
0.2 0.3 0.4 0.5 0.6 0.7
T , K
–1/4 –1/4
0 100 200 300
3
2
1
�
�/
b
a
T, K
Fig. 2. The temperature dependence of resistivity �a of sin-
gle-crystal La2CuO4� � measured for transport current equal
to 1 �A. The current was directed parallel to the crystallo-
graphic axis a. The inset shows temperature behavior of the
ratio of the resistivities �b and �a for measuring currents
directed along the crystallographic axes b and a.
J ||a
J ||a
0.1 A�
1 A�
1 A�
10 A�
10 A�
100 A�
100 A�
1 mA
T = 20 K
T = 5 K
�
R
(H
)/
R
(0
)
�
R
(H
)/
R
(0
)
0 1 2 3 4 5 6
0
–0.02
–0.04
–0.06
–0.08
–0.10
–0.12
0.04
0.02
0
–0.02
–0.04
–0.06
H, T
Fig. 3. Magnetoresistance curves at T � 5 and 20 K mea-
sured for single-crystal La2CuO4� � in the out-of-plane dc
magnetic field (H c| | ) for different amplitudes of measur-
ing current directed along the crystallographic axis a.
directions used, J a| | and J b| | . At fairly high temper-
ature, T � 20 K, only negative MR is observed, which
increases profoundly above H � 5 T, as well (lower
panel of Fig. 3). We have attributed this rather sharp
increase to an influence of the AF–WF transition, as
will be discussed in more detail below. As to the posi-
tive MR at low temperature (T � 20 K), this could be
attributed to the presence of superconducting inclu-
sions due to phase separation, as was mentioned
above. For example, in Ref. 38, positive MR attrib-
uted to superconducting inclusions has been found in
the low-temperature range (T � 10 K) in even more re-
sistive La CuO2 4� � with higher TN .
For all of the temperature range in which the MR
was measured in this study (4.2 K � �T 90 K), the
MR magnitude is strongly dependent on the measu-
ring current (as illustrated by Fig. 3). For this reason,
to compare MR curves with an evident effect of
AF–WF transition at different temperatures we have
used only data for rather high currents, that is for the
non-Ohmic conduction regime. Some examples of the
MR curves at H c| | for the cases J a| | and J b| | and
current J � 100 �A are shown in Figs. 4 and 5 for cer-
tain selected temperatures. It is obvious from the
curves that a rather sharp decrease in resistance occurs
when H exceeds some critical magnitude (in the range
5–6 T). All main features of this resistive transition
are quite identical to those found in MR behavior of
La2CuO4� � at the AF–WF transition for the case
H c| | , and the out-of-plane current direction (J c| | ),
when mainly interplane hopping is affected by the
transition [5,28–30]. The results obtained show that
the AF–WF transition influences hopping conduction
in the directions parallel to CuO2 planes as well. This
effect, although being anticipated (as is indicated
above), have never been seen previously in
La CuO2 4� �, to our knowledge.
Magnetoresistive study of antiferromagnetic–weak ferromagnetic transition in single crystal
Fizika Nizkikh Temperatur, 2004, v. 30, No. 5 555
0
–2
–4
–6
–8
–10
T = 20 K�
R
(H
)/
R
(0
) ,
%
0
–2
–4
–6
–8
T = 50 K
�
R
(H
)/
R
(0
) ,
%
0 1 2 3 4 5 6
0
–2
–4
–6
H, T
�
R
(H
)/
R
(0
),
%
T = 8 K
Fig. 4. Magnetoresistance curves at various fixed tempera-
tures measured for single-crystal La CuO2 4� � in the
out-of-plane dc magnetic field (H c| | ) for measuring cur-
rent (100 �A) directed along the crystallographic axis a.
0
–10
–20
–30
0
–10
–20
–30
0
–10
–20
–30
T = 50 K
T = 10 K
T = 5 K
H, T
0 1 2 3 4 5 6
�
R
(H
)/
R
(0
)
�
R
(H
)/
R
(0
) ,
%
�
R
(H
)/
R
(0
) ,
%
Fig. 5. Magnetoresistance curves at various fixed tempera-
tures measured for single-crystal La CuO2 4� � in the
out-of-plane dc magnetic field (H c| | ) for measuring cur-
rent (100 �A) directed along the crystallographic axis b.
The following features of the resistive transition
can be pointed out. First, the transition is sharper and
the relative changes in resistance, �R/Rn , are larger
for the b direction of the transport current than those
for a direction (compare Figs. 4 and 5). Second, the
MR curves are hysteretic in the field range of the tran-
sition, as expected. The hysteresis becomes more pro-
nounced for decreasing temperature. The latter feature
of the MR curves is quite consistent with that found
previously at the AF–WF transition for J c| |
[5,29–31]. Third, a considerable negative MR in the
low-field range below the magnetic transition can be
observed (Figs. 4 and 5). This contribution to the to-
tal MR is not hysteretic and, maybe, has little if any
relationship to the magnetic transition. For a given
current (for example, for J � 100 �A, as in Figs. 4 and
5) the contribution of this type of MR increases with
decreasing temperature and is more pronounced for
the a direction of the measuring current. It is found as
well that the negative MR at low field increases with
current magnitude (Fig. 3) and, therefore, with an ap-
plied voltage, so it is much more pronounced in the
non-Ohmic regime of hopping conductivity (compare
Fig. 1 and 3). In previous studies, negative MR in the
AF La CuO2 4� � for the case of both the current and
field parallel to CuO2 was found and discussed to a
certain degree [14,18]. The nature of the negative MR
in rather low field H c| | and J a b| | , revealed in this
work is not clear and is worthy of additional study.
It is evident from Figs. 4 and 5 that the maximal dc
field about 6 T, used for these measurements, is not
high enough to accomplish the magnetic transition in
full measure. To overcome this disadvantage, measure-
ments were done in pulsed magnetic field with magni-
tude up to 15 T. The MR curves were recorded at tem-
peratures T � 4.2 K, 20.4 K, and 77 K for both the
in-plane and out-of-plane directions of the transport
current. Examples of MR curves for pulsed field at T �
= 4.2 and T � 77 K are shown in Figs. 6 and 7.
The pulsed MR measurements enabled us to see the
magnetic transitions in full measure. The MR curves
for low temperature region were found to be quite simi-
lar for both methods (compare Figs. 4 and 6). It is also
seen that the resistive transition for the out-of-plane
556 Fizika Nizkikh Temperatur, 2004, v. 30, No. 5
B.I. Belevtsev, N.V. Dalakova, V.N. Savitsky, A.V. Bondarenko, A.S. Panfilov, and I.S. Braude
0
–1
–2
–3
–4
0
–5
–10
–15
–20
�
R
(H
)/
R
(0
) ,
%
0 2 4 6 8
H, T
�
R
(H
)/
R
(0
) ,
%
J || a
J || c
Fig. 6. Magnetoresistance curves registered for single-crys-
tal La2CuO4� � in the out-of-plane pulsed magnetic field
( | | )H c at T � 4.2 K for the in-plane and out-of-plane cur-
rent directions ( | |J a and J c| | ) with current magnitudes
6 �A and 7.4 �A, respectively.
0 2 4 6 8 10
H, T
0
–10
–20
–30
–40
–50
J || a
J || c
0
–10
–20
–30
–40
�
R
(H
)/
R
(0
) ,
%
�
R
(H
)/
R
(0
) ,
%
Fig. 7. Magnetoresistance curves registered for single-crys-
tal La CuO2 4� � in the out-of-plane pulsed magnetic field
( | | )H c at T � 77 K for the in-plane and out-of-plane cur-
rent directions (J a| | and J c| | ) with current magnitudes
5.93 mA and 178 �A, respectively.
current direction is sharper, and the relative changes in
resistance, �R/Rn, are generally larger than those for
in-plane direction. The MR curves in pulsed magnetic
field at T � 77 K are less hysteretic than those at
T � 4 2. K, as expected (Fig. 7). The maximum values
of �R/Rn � 50 % found in this study for pulsed mag-
netic field agree well with those found in previous stud-
ies in dc magnetic field [7].
In conclusion, we have found that the AF–WF
transition in La2CuO4� � clearly manifests itself in
the in-plane hopping conductivity. This supports the
3D nature of hopping conduction in this compound.
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