Philosophical Magazine Letters
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
This article was downloaded by: [Gamzatov, A. G.]
On: 26 April 2011
Access details: Access Details: [subscription number 936892050]
Publisher Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-
41 Mortimer Street, London W1T 3JH, UK
Philosophical Magazine Letters
Publication details, including instructions for authors and subscription information:
http://www.informaworld.com/smpp/title~content=t713695410
Specific heat and low-field magnetocaloric effect in A-site ordered
PrBaMn2O6 manganite
A. M. Alieva; A. G. Gamzatova; A. B. Batdalova; V. S. Kalitkab; A. R. Kaulb
a
Laboratory of Low Temperature Physics, Amirkhanov Institute of Physics of Dagestan Scientific
Center of RAS, Makhachkala 367003, Russia b Material Science Department, Moscow State University,
119899 Moscow, Russia
First published on: 22 February 2011
To cite this Article Aliev, A. M. , Gamzatov, A. G. , Batdalov, A. B. , Kalitka, V. S. and Kaul, A. R.(2011) 'Specific heat and
low-field magnetocaloric effect in A-site ordered PrBaMn2O6 manganite', Philosophical Magazine Letters, 91: 5, 354 —
360, First published on: 22 February 2011 (iFirst)
To link to this Article: DOI: 10.1080/09500839.2011.560581
URL: http://dx.doi.org/10.1080/09500839.2011.560581
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial or
systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or
distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses
should be independently verified with primary sources. The publisher shall not be liable for any loss,
actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly
or indirectly in connection with or arising out of the use of this material.Philosophical Magazine Letters
Vol. 91, No. 5, May 2011, 354–360
Specific heat and low-field magnetocaloric effect in A-site ordered
PrBaMn2O6 manganite
A.M. Alieva, A.G. Gamzatova*, A.B. Batdalova, V.S. Kalitkab and A.R. Kaulb
a
Laboratory of Low Temperature Physics, Amirkhanov Institute of Physics of Dagestan
Scientific Center of RAS, Makhachkala 367003, Russia; bMaterial Science Department,
Moscow State University, 119899 Moscow, Russia
(Received 16 April 2010; final version received 2 February 2011)
The specific heat and magnetocaloric effect (MCE) of A-site ordered
PrBaMn2O6 manganite have been studied. The anomalies caused by the
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
ferromagnetic (FM) and antiferromagnetic (AFM) phase transitions are
revealed in the specific heat curve. Direct and inverse MCE are observed at
the Curie and Néel points correspondingly. A value of the inverse MCE in
the heating run is smaller than in the cooling regime. We attribute this
effect to the competition between FM and AFM interactions. A significant
advantage of PrBaMn2O6 as a magnetocaloric material is an MCE
spanning a broad range of temperature with a maximum at room
temperature.
Keywords: manganite; magnetocaloric effect; specific heat
1. Introduction
Recenly, half-doped manganite perovskites with A-site order have attracted interest
owing to their novel physical properties [1–4]. RBaMn2O6 displays remarkable
features: a charge/orbital order (CO) transition at relatively high temperatures, a new
stacking variation of the CE-type CO with fourfold periodicity along the c-axis, the
presence of a structural transition possibly accompanied by dx2y2 orbital order and
an electronic phase segregation. The physical properties of A-site-ordered
PrBaMn2O6 manganites are strongly dependent on an ordering of Pr and Ba
cations [5]. However, the nature of the occurrence of the cation–ordered state in
Ba-substituted manganites is not entirely understood. Such ordering does not occur
for Ca- or Sr-substituted manganites.
In PrBaMn2O6 two phases are observed – a ferromagnetic (FM) phase with
TC ¼ 310–320 K and an A-type antiferromagnetic (AFM) phase with
TN ¼ 200–270 K. Such a spread of critical temperatures results from an important
dependence of the physical properties of these materials on the synthesis
conditions [6,7].
*Corresponding author. Email: gamzatov_adler@mail.ru
ISSN 0950–0839 print/ISSN 1362–3036 online
ß 2011 Taylor & Francis
DOI: 10.1080/09500839.2011.560581
http://www.informaworld.comPhilosophical Magazine Letters 355
Figure 1. Powder XRD pattern of PrBaMn2O2 manganite.
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
2. Results and discussion
A ceramic sample of PrBaMn2O6 was prepared by the chemical homogenization
method. Ash-free paper filters were saturated with a stoichiometric mixture of nitrate
solutions of Pr, Ba and Mn and dried at 100 C. These were then combusted and
annealed. The obtained powder was compressed into pellets and annealed at 1100 C
in an Ar flow for 20 h. As a result, Pr0.5Ba0.5MnO3 manganite was obtained without
A-cation ordering. The pellets were annealed in a closed ampoule at 1100 C and
P(O2) ¼ 10–19 atm for 20 h. The partial pressure of oxygen P(O2) was obtained by a
Fe/FeO getter, held at a temperature of 800 C. Thereby A-site-ordered PrBaMn2O5
manganite was generated. In order to fill the oxygen vacancies, annealing were
carried out at 500 C in an O2 flow for 5 h. Finally, PrBaMn2O5.97(1) manganite was
obtained. The oxygen stoichiometric index was determined by iodometric titration.
From the powder X-ray diffraction data for PrBaMn2O5 sample (after annealing
in a reductive atmosphere), it can be seen in Figure 1 that the diffraction peaks of the
cubic perovskite subcell are split by tetragonal distortion. Also near 2 ¼ 11 a
(001/2) peak can be found, which proves the formation of a superlattice. All peaks in
this pattern belong to the manganite phase. The cell parameters for the oxidized
phase of PrBaMn2O6 were refined from the powder X-ray diffraction data (program
Jana2006) using a lattice-constrained full profile refinement. The X-ray diffraction
analysis confirms that the sample is of single phase. The unit-cell parameters are
a ¼ b ¼ 3.9007(1), c ¼ 7.7486(4) (P4mm group).
The magnetocaloric effect (MCE) measurements were carried out by a modu-
lation technique [8]. The essence of the method is as follows. A low-frequency
modulated magnetic field H ¼ H0cos!t (H0 is the field amplitude and ! is the
frequency) induces temperature oscillations in the sample given by T ¼ T0cos(!t þ ’),
where ’ is the phase shift of the temperature oscillations with respect to the magnetic
field oscillations. An alternate signal from a differential thermocouple glued
to the sample is detected with high accuracy by an SR830 lock-in amplifier.356 A.M. Aliev et al.
Figure 2. Temperature dependences of the magnetization at heating. Inset shows dM/dT(T ).
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
In our experiments a modulated magnetic field with an amplitude up to 2000 Oe and
a frequency of 0.3 Hz was generated by an electromagnet and a power supply unit
with external control. A controlling AC voltage was supplied to the power supply
from the SR830. This technique allows the detection of change in the temperature
with an accuracy of 103 K.
The samples were 3 3 0.3 mm thin plates to which one of the junctions of a
differential chromel–constantan thermocouple with a wire diameter of 0.025 mm
were glued. To improve the thermal contact and reduce inertia, the junction was
flattened to a thickness of 3–5 mm. The magnetic field was directed along the sample
plane. The specific heat was measured by AC calorimetry.
The temperature dependence of the magnetization of PrBaMn2O6 on heating is
shown in Figure 2. On the basis of the Maxwell relation (see inset in Figure 2),
we can expect direct and inverse MCE at FM and AFM magnetostructural phase
transitions with nearly equal values.
The temperature dependence of the specific heat (Cp) of PrBaMn2O6 in a wide
temperature interval is shown in Figure 3. A high-temperature anomaly of the
specific heat with a maximum at T ¼ 308 K is caused by an FM phase transition.
On further decreasing the temperature, a second anomaly in the specific heat, with a
maxima at 214 K on cooling and at 243.6 K on heating, is found. This second
anomaly is caused by an AFM phase transition and is characterized by a wide
hysteresis (DT ¼ 30 K). The hysteresis indicates the first-order nature of the
transition. The wide extent of the hysteresis points to a significant change in the
structure at the AFM transition.
Figure 4a shows the temperature dependences of MCE in PrBaMn2O6 measured
by the modulation technique. The MCE maxima on all curves are near T ¼ 308 K
and amount to DT ¼ 0.051 K at a field change DH ¼ 750 Oe. To extrapolate the
low-field MCE values to high fields, we have measured the field dependence of the
MCE at 308 K (Figure 4a, inset). The field dependence of MCE in a ferromagnetPhilosophical Magazine Letters 357
Figure 3. Temperature dependences of the specific heat of PrBaMn2O6 on cooling and heating
runs.
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
near TC can be expressed as DS ¼ aH2=3 , where n ¼ 2/3 and a is a constant [9]. From
this it follows that DT H2=3 in the vicinity of TC. Note that in the latter expression
any field dependence of the specific heat is not taken into account. Fitting our
experimental data gives n ¼ 0.90, and results in DT ¼ 0.67 K at a magnetic field
change of 11 kOe. This is the mean value of the MCE in manganites [10]. Though the
MCE in PrBaMn2O6 around the FM transition does not reach large values,
the transition width is more than 60 K even in low fields. It is important to note that
the effect is spread over a broad interval around room temperature, which offers a
significant advantage to PrBaMn2O6 as a magnetocaloric material.
More interesting behavior of the MCE is observed around the AFM transition
(Figures 4a and b). First, an inverse MCE is observed, which additionally confirms
the AFM nature of the transition in this temperature interval. As for the specific
heat, the temperature dependence of the MCE is also characterized by hysteresis.
The MCE value obtained in a heating run is smaller than that obtained in the cooling
run. Competition between AFM and FM ordering is suggested to be the reason
for this. The total value of the MCE can be presented as the sum DTtot ¼
DTFM þ DTAFM, where DTFM and DTAFM are magnetocaloric effects due to FM and
AFM processes correspondingly. Near the Néel point, DTtot ¼ DTAFM in the cooling
regime because DTFM ¼ 0, far from the Curie point in low fields. The total MCE
value on heating, DTtot, is the sum of DTAFM with negative sign and non-zero DTFM
with positive sign because the Curie point is close to the Néel point such that the
result is DTtot cooling 4 DTtot heating. With an increasing magnetic field, DTFM will
increase steadily, whereas DTAFM will increase until H 5 Hcr (Hcr is the critical
magnetic field which induces the AFM!FM transition). This leads to different
behavior of the MCE around the AFM transition on heating and cooling. The same
effect must be observed in a region near the FM transition but in a narrow interval of
temperature, slightly above the AFM transition, since AFM ordering will
rapidly decrease above TN. Such a picture is actually observed slightly above TN,358 A.M. Aliev et al.
(a)
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
(b)
Figure 4. (a) Temperature dependences of the MCE in PrBaMn2O6 at various magnetic field
changes. Inset shows field dependence of the MCE at T ¼ 308 K. (b) Temperature dependences
of the MCE in PrBaMn2O6 around the AFM transition on heating and cooling.
and the MCE curves quickly merge on approaching TC. Such behavior of the MCE
has been found by Khovaylo et al. [10] for Ni–Mn–Sn Heusler alloys, where a
difference between heating and cooling values of MCE is explained by the proximity
of FM and AFM transitions.
The MCE at DH ¼ 11 kOe obtained during heating by the classic direct
measurement method are shown in Figure 5. The peak value of MCE
(DT ¼ 0.528 K at DH ¼ 11 kOe) at the FM transition is less than that obtained by
extrapolation of the modulation technique results (DT ¼ 0.67 K at DH ¼ 11 kOe).
A greater discrepancy between the data is observed at the FM–AFM transition.
At low fields the inverse MCE is observed. With an increasing magnetic field, the
compensation temperature (the compensation temperature is the temperature atPhilosophical Magazine Letters 359
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011
Figure 5. Temperature dependence of the MCE of PrBaMn2O6 at DH ¼ 11 kOe (classic direct
method).
which the contributions of direct and inverse MCE are equal and so the total MCE is
zero) shifts to low temperatures and an AFM-FM crossover takes place. So around
the Cp,heating anomaly, where the inverse MCE takes place at low fields, we observe a
direct MCE (DT ¼ 0.13 K) at high magnetic field. Our direct measurements show
that the MCE around the I-order magnetostructural transition has an ordinary
nature and does not achieve giant values. Early reported giant MCE values around
the first-order charge/orbital ordering transition in manganites and other
materials [11–14] can be attributed to inadequate use of the Maxwell relation.
Acknowledgements
This study was supported by the Russian Foundation for Basic Research (Project
No. 09-08-96533), the Program of the Physical Sciences Division of the Russian Academy
of Sciences ‘Highly correlated electrons in solids and structures’.
References
[1] S.V. Trukhanov, I.O. Troyanchuk, M. Hervieu, H. Szymczak and K. Bärner, Phys. Rev. B
66 (2002) p.184424.
[2] T. Ohno, H. Kubo, Y. Kawasaki, Y. Kishimoto, T. Nakajima and Y. Ueda, Physica B
359–361 (2005) p.1291.
[3] R. Vidya, P. Ravindran, P. Vajeeston, A. Kjekshus and H. Fjellvåg, Phys. Rev. B 69 (2004)
p.092405.
[4] Y. Ueda and T. Nakajima, J. Phys. Condens. Matter 16 (2004) p.S573.
[5] T. Nakajima, H. Kageyama, H. Yoshizawa, K. Ohoyama and Y. Ueda, J. Phys. Soc. Jpn.
72 (2003) p.3237.360 A.M. Aliev et al.
[6] S.V. Trukhanov, L.S. Lobanovski, M.V. Bushinsky, V.V. Fedotova, I.O. Troyanchuk,
A.V. Trukhanov, V.A. Ryzhov, H. Szymczak, R. Szymczak and M. Baran, J. Phys.
Condens. Matter 17 (2005) p.6495.
[7] T. Nakajima, H. Yoshizawa and Y. Ueda, J. Phys. Soc. Jpn. 73 (2004) p.2283.
[8] A.M. Aliev, A.B. Batdalov and V.S. Kalitka, JETP Lett. 90 (2009) p.663.
[9] P. Sarkar, P. Mandal and P. Choudhury, Appl. Phys. Lett. 92 (2008) p.182506.
[10] V.V. Khovaylo, K.P. Skokov, O. Gutfleisch, H. Miki, T. Takagi, T. Kanomata,
V.V. Koledov, V.G. Shavrov, G. Wang, E. Palacios, J. Bartolomé and R. Burriel, Phys.
Rev. B 81 (2010) p.214406.
[11] P. Sande, L.E. Hueso, D.R. Miguéns, J. Rivas, F. Rivadulla and M.A. López-Quintela,
Appl. Phys. Lett. 79 (2001) p.2040.
[12] A.L. Lima Sharma, P.A. Sharma, S.K. McCall, S.-B. Kim and S.-W. Cheong, Appl. Phys.
Lett. 95 (2009) p.092506.
[13] B. Hernando, J.L. Sánchez Llamazares, J.D. Santos, V.M. Prida, D. Baldomir,
D. Serantes, R. Varga and J. González, Appl. Phys. Lett. 92 (2008) p.132507.
[14] Ajaya K. Nayak, K.G. Suresh and A.K. Nigam, J. Phys. D Appl. Phys. 42 (2009)
p.035009.
Downloaded By: [Gamzatov, A. G.] At: 13:35 26 April 2011You can also read
