Magnetoelectric mutual-control in collinear antiferromagnetic NdCrTiO5

Xiang Li,1,2,a) Meifeng Liu,1,2,a),b) Yu Wang,1 Liman Tian,1 Rui Shi,1 Lun Yang,1 Qiyun Pan,1

Juanjuan Han,1 Bo Xie,1 Nian Zhao,1 Xiuzhang Wang,1 Shaozhen Li,3 Lin Lin,2 Zhibo Yan,2

and Jun-Ming Liu1,2

1Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China2Laboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures,Nanjing University, Nanjing 210093, China3School of Mathematics and Physics, Hubei Polytechnic University, Huangshi 435003, China

(Received 5 July 2018; accepted 3 September 2018; published online 19 September 2018)

Strong magnetoelectric (ME) coupling has been one of the dreaming goals in magnetoelectric and

multiferroic materials. In particular, the electro-control of magnetic ordering and magnetization is of

high interest. In this work, we synthesize NdCrTiO5 and perform a set of characterization studies on

the multiferroic properties and the linear ME effect. It is revealed that NdCrTiO5 exhibits a magnetic

phase transition at TN � 20 K, below which a remarkable ME response is observed. On one hand, it

is non-ferroelectric at zero magnetic field and a magnetic field as low as 1.0 T is sufficient to induce

remarkable pyroelectric current below TN, demonstrating the magnetism-induced ferroelectricity.

On the other hand, the remarkable magnetic control of electric polarization and electro-control of

magnetization are recorded. At 10 K, a magnetic field of 1.0 T can lead to a change in polarization

as large as 20%. Moreover, magnetization M can be significantly modulated by an electric field,

with the estimated inverse ME coefficient as large as �1.84 ps/m. The temporal evolution of electri-

cal polarization and magnetization indicates the stable ME mutual control, suggesting potential

applications of NdCrTiO5 as a promising multiferroic. Published by AIP Publishing.https://doi.org/10.1063/1.5047077

The magnetoelectric (ME) effect that denotes the controls

of either magnetization by an electric field or polarization by a

magnetic field in a material has attracted widespread interest

owing to its potential technological applications in high-density

data storage or ME switching devices.1,2 Since the theoretical

prediction of the ME effect in 1959 and experimental observa-

tion of the linear ME effect in Cr2O3, huge effort has been paid

to the design and synthesis of effective ME or multiferroic

materials over the past century.3–6 Subsequently, the linear ME

effect has been observed in many antiferromagnetic (AFM)

systems such as MnTiO3, Co4Nb2O9, and Ni0.4Mn0.6TiO3, and

it is believed that this effect originates from properly broken

inversion symmetry.7–9

Besides, giant ME coupling can be obtained in single-

phase multiferroics in which electric polarization is generated

in a properly spin-ordered phase. Along this line, type-II mul-

tiferroics in which the electric polarization originates from the

spatial inverse symmetry breaking induced by asymmetric

and symmetric striction mechanisms could be one category of

promising candidates.10,11 For the past decade, the long-

sought control of electric properties by a magnetic field has

been achieved in the so-called type-II multiferroics, for exam-

ple, RMnO3, RMn2O5 (R¼ rare earths), LiCu2O2, CuFeO2,

Ba2CoGe2O7, CaMn7O12, and LiFe(WO4)2.12–16 Recent

research on type-II multiferroics with strong ME coupling

demonstrated that it is beneficial to discuss the linear ME

effect from the viewpoint of multiferroicity.17,18 It can help

promote our understanding of additional ME coupling modes

and further find the application-driven ME operations such as

magneto-control of polarization or even electro-control of

magnetization that has been observed in Cr2O3, noting that

any electro-control of magnetization is highly concerned due

to its advantages over magneto-control of electric polariza-

tion.19 Moreover, the large energy barrier between different

ferroic states in most multiferroics produces hysteresis and a

large coercive field, which brings about deleterious effects

such as low precision or asymmetrical oscillations in ME

devices. For such reasons, we discuss one class of collinear

antiferromagnetic oxide NdCrTiO5 that has been concerned

due to the uncertain mechanism wobbling between the linear

ME effect and multiferroicity. It is our motivation to realize

the ME mutual control in this oxide compound and achieve

the stable ME response without significant hysteresis.

As one of the first known ME materials possessing two

distinct magnetic sublattices, NdCrTiO5 was preliminarily

investigated to identify the lattice and magnetic structures in

1970s.20,21 It is now known that NdCrTiO5 crystallizes in the

orthorhombic structure with the Pbam space group, as shown

in Fig. 1(a). There exist cross-site occupations between Cr3þ

and Ti4þ ions. The 4h sites on the bases of oxygen square pyr-

amids are occupied by Cr3þ ions with a probability of 0.95

and Ti4þ ions with a probability of 0.05. The 4f sites in the

center of oxygen octahedra are filled with Cr3þ and Ti4þ ions

in a probability of 0.05 and 0.95, respectively. The 4g sites

are occupied by Nd3þ ions alone. The crystal structure shown

in Fig. 1(a), for simplicity, has ignored the 5% cross-site

occupation of Cr3þ and Ti4þ ions. The octahedra centered at

4f sites form the infinite chains with shared edges along the c-

axis. In the ab-plane, the pairing square pyramids connect

each other with their base sharing oxygen edges, and the octa-

hedra and pyramids are thus linked with corner-sharing

a)X. Li and M. Liu contributed equally to this work.b)Electronic address: lmfeng1107@hbnu.edu.cn

0003-6951/2018/113(12)/122903/5/$30.00 Published by AIP Publishing.113, 122903-1

APPLIED PHYSICS LETTERS 113, 122903 (2018)

oxygen atoms of either apex or bases. The Nd3þ ions locate

on the alternative layers in the octahedral and pyramidal

network.

Such a specific lattice structure allows multifold

exchange interactions, and thus, the magnetic structure of

NdCrTiO5 seems to be a bit complex. As shown in Fig. 1(b),

the magnetic structure consists of two sublattices. One is the

Cr3þ spin sublattice where the spins are collinearly aligned

along the c-axis and antiferromagnetically ordered in the abplane below 13 K, forming the G-type antiferromagnetic

order. The other is the Nd3þ sublattice where the spins order

in the ab plane with the spin-rotation away from the b-axis

by 12�.20

Nevertheless, several major issues with this compound

in terms of linear ME and multiferroic responses remain

unsolved. First, it was argued that the ordering of Nd3þ spins

is driven by the neighboring Cr3þ spins via the Cr3þ-Nd3þ

exchange coupling rather than the independent Nd3þ spin

exchange. This issue remains open yet. It is also uncertain

whether the antiferromagnetic order is the consequence of

Cr3þ spin exchanges or the coupling of these two magnetic

sublattices.22 Second, it was further confirmed that the emer-

gence of the antiferromagnetic order and even ferroelectric

polarization (driven by magnetic field) is around N�eel point

TN¼ 18–21 K rather than 13 K deduced from the neutron dif-

fraction data by Buisson.20 Since no further magnetic phase

transition has been confirmed between these two tempera-

tures, they may imply the same phase transition, i.e., the

ordering of Cr3þ spins. Nevertheless, debatable opinions on

the origin of electric polarization were raised.23–25 Third,

magnetic substitution and doping in NdCrTiO5 brought no

enhanced performance in terms of electric polarization and

ME effect.22,25–27 In fact, no detailed data on the ME

response, especially on the electro-control of magnetism,

have been available so far. These issues thus raise substantial

interest in revisiting the ME and multiferroic properties in

NdCrTiO5 below TN.

Herein, we experimentally demonstrate the non-

hysteretic magneto-control of polarization and robust electro-

control of magnetization in NdCrTiO5. Detailed investigation

of the temperature dependences of magnetization M, specific

heat CP, and electric polarization P induced by a magnetic

field below TN will be reported in detail. Furthermore, we

probe the temporal evolution of P and M in response to the

applied magnetic field and electric field. The stable response

indicates a fascinating ME operation in NdCrTiO5.

The single-phase polycrystalline NdCrTiO5 was prepared

with a conventional solid-state reaction method. The highly

purified powder of oxides Nd2O3, Cr2O3, and TiO2 in stoi-

chiometric ratio was mixed and ground, followed by reaction

in an alumina crucible at 1200 �C for 24 h. The resultant pow-

der was fully re-ground and pelletized under 5000 psi pressure

to a disk of 20 mm in diameter and 1 mm in height. The pellet

was sintered at 1350 �C for 24 h, followed by the natural cool-

ing down to room temperature in air.

The crystal structure of NdCrTiO5 was characterized by

X-ray diffraction (XRD) in the h-2h mode using a Bruker D8

Advanced diffractometer (Cu-Ka radiation) at room tempera-

ture, as shown in Fig. 1(c). All the peaks can be properly

indexed by the standard Bragg reflections without identifiable

impurity phases. For a quantitative evaluation of the phase

purity and lattice distortion, the Rietveld refinement was

adopted to fit the measured XRD data. The high quality of

Rietveld fitting is guaranteed by the obtained Rwp¼ 13.60%,

Rp¼ 9.88%, and v2¼ 1.076. The refined lattice parameters

are a¼ 7.332 A, b¼ 8.508 A, and c¼ 5.662 A, in agreement

with earlier results.20

Subsequently, we look at the magnetic behaviors of the

as-prepared samples. The dc magnetic susceptibilities v as a

function of temperature T are depicted in Fig. 2(a), under the

measuring (cooling) field H¼ 1000 Oe in zero-field cooled

(ZFC) and field cooling (FC) modes using the Quantum

Design Superconducting Quantum Interference Device mag-

netometer (SQUID). The two measured curves are almost

overlapped, indicating strong antiferromagnetic interactions

in NdCrTiO5. The evaluated dv=dT � T curve drawn in Fig.

2(b) shows a sharp peak near TN � 20 K, corresponding to

the tiny kink of the v � T curve around TN, which indicates

the long-range antiferromagnetic ordering of Cr3þ spins. The

broad peak in the v � T curve at around T0 � 10 K might be

ascribed to the ordering of Nd3þ spins.

FIG. 1. (a) A schematic drawing of the lattice structure of NdCrTiO5 with

5% cross-site occupying ions ignored. (b) The orientation of Nd3þ and Cr3þ

spins in NdCrTiO5 denoted by green and red arrows, respectively. (c)

Measured h-2h XRD spectrum of the NdCrTiO5 polycrystalline sample and

the refined results using the Rietveld method.

122903-2 Li et al. Appl. Phys. Lett. 113, 122903 (2018)

A further study was carried out to disclose the possible

phase transitions by measuring the specific heat CP (normal-

ized by T) with the thermal relaxation method using the

Quantum Design Physical Properties Measurement Systems

(PPMS), as drawn in Fig. 2(b). A clear anomaly at TN is

seen, confirming the paramagnetic to antiferromagnetic tran-

sition. Also, a broad shoulder around T0 � 10 K is observed

in accord with our assumption of the magnetic ordering of

Nd3þ moments. There might be two possible origins, one for

the independent Nd3þ spin interactions and the other for the

induction by Cr3þ-Nd3þ exchange coupling, which cannot

be distinguished in this work. Moreover, the ZFC v � Tcurves under different cooling/measuring magnetic fields are

shown in Fig. S1 in the supplementary material. No distinct

change can be identified between these curves, which further

confirms the strong antiferromagnetic interactions of mag-

netic ions in NdCrTiO5.

Before checking possible ME coupling, we first investi-

gate whether spontaneous ferroelectric polarization exists in

NdCrTiO5 or not. We employ the ultra-high sensitive pyro-

electric current method to detect the relatively weak ferroelec-

tricity. For the electrical measurements, a disk-like sample of

3.0 mm in diameter and 0.2 mm in thickness was deposited

with Au electrodes on each side, in order to form a parallel

plate capacitor geometrical structure. First, the sample was

poled with an electric poling field of 10 kV/cm offered by a

source-meter, as well as different applied external magnetic

fields H (H¼ 0 if no magnetic field was applied). By this

scheme, the sample was cooled down from a given tempera-

ture (up to room temperature) to 2 K. Then, the electric field

was removed, and the sample was electrically short-circuited

for sufficient time at 2 K, followed by a slow heating until the

temperature became higher than TN, during which the electric

current released from the capacitor was recorded using a

Keithley 6514A electrometer. The heating rate could be

2–4 K/min under different measuring cycles.

We present in Fig. 3(a) the measured pyroelectric cur-

rent IpyroðTÞ curves under E¼ 10 kV/cm and different mag-

netic fields l0H¼ 0–9 T. It is noted that the measured

IpyroðTÞ data at l0H¼ 0 are almost zero, which indicates no

ferroelectric polarization if the magnetic field is absent. It

suggests that NdCrTiO5 is not an intrinsic ferroelectric.

Upon increasing magnetic field H, the current peak takes

sharp around �19 K and broadens, which gradually exhibits

a slight downshift of the peak temperature. Given that the

IpyroðTÞ signals are purely from the pyroelectric effect (we

discussed it in the supplementary material as shown in Fig.

S2), one has the polarization P(T) data evaluated by integrat-

ing the current plotted in Fig. 3(b). It is seen that the ferro-

electric polarization becomes larger with increasing H and

reaches �13 lC/m2 at l0H¼ 9 T. It is worth noting that the

onset of electric polarization is just around the temperature

TN where the paramagnetic to antiferromagnetic transition

occurs, indicating that the ferroelectricity in NdCrTiO5 does

have the magnetic origin ascribed to the induction of the

magnetic field. Moreover, to check whether this magneti-

cally induced ferroelectricity comes from the linear ME

effect, we plotted the H-induced polarization P(T¼ 10 K) in

the inset of Fig. 3(b). As evidently seen, P is proportional to

the applied H, suggesting that the linear ME effect plays a

major role in NdCrTiO5. The linear ME effect with a rough

coefficient of �2.01 ps/m defined by dP/dH is on the same

order of magnitude as that reported earlier.23

For the characterization of the electro-response to the

magnetic field, the sample was first poled under E¼ 10 kV/

cm and l0H¼ 5 T, similar to the earlier, and cooled down to

FIG. 2. (a) The dc magnetic susceptibilities v in the ZFC and FC modes with

a measuring field of 1000 Oe. (b) The measured derivative dv=dT (left) and

the T-normalized specific heat CP=T (right) as the functions of temperature T.

FIG. 3. (a) Measured pyroelectric current IpyroðTÞ curves at E¼ 10 kV/cm

under different applied magnetic fields with heating rate v¼ 4 K/min. (b)

The polarization P(T) curves evaluated from the pyroelectric current data,

with the inset clarifying the polarization PðT ¼ 10 KÞ as a function of the

applied magnetic field.

122903-3 Li et al. Appl. Phys. Lett. 113, 122903 (2018)

10 K followed by the short-circuit process. Then, the magnetic

field H linearly changes between Hmin and Hmax, during which

the electric current was measured using the electrometer. We

observed a repeatable variation of I induced by a modulated

external magnetic field varying between l0H¼ 4.5 T and

5.5 T at T¼ 10 K, as depicted in Fig. 4(a). The variation of

l0DH¼ 1 T was chosen to minimize the magnetic hystere-

sis.28 It is seen from Fig. 4(a) that the collected current

changes rapidly between I1 � 0.33 pA and I2 � 0.54 pA

without any delay as H changes linearly. The deduced DP¼ PðHÞ � Pð5 TÞ shown in Fig. 4(b) exhibits that in the pres-

ence of a modulated magnetic field, the electric polarization Poscillates linearly with the variation of �1.6 lC/m2, up to

�20% relative to P(5 T) at 10 K. It is noted that no phase shift

occurs between the external magnetic field and the deduced

electric polarization. In other words, with the increasing

(decreasing) magnetic field, the electric polarization increases

(decreases). It is obviously revealed that P is almost linearly

modulated by applied H without hysteresis. The temporal

evolution of P dependent on H indicates a stable ME control.

Furthermore, we investigated the inverse ME effect, that

is, the magneto-response to the electric field. Prior to this

characterization, the sample was poled under the electric field

E¼ 10 kV/cm and magnetic field l0H¼ 4 T and cooled down

to 10 K. After the short-circuit process, both E and H were

removed. Subsequently, E¼ 0 and 10 kV/cm were applied

periodically, during which the magnetization was measured

using the SQUID VSM (measuring field �2000 Oe). Figure

4(c) shows our experimental demonstration of the E-induced

magnetization’s variety without altering H at 10 K. The mea-

sured M remains to be �1:740� 10�3 lB/f.u. while E¼ 0

and changes rapidly to �1:726� 10�3 lB/f.u. while

E¼ 10 kV/cm, with the magnitude of DM being �1:4� 10�5

lB/f.u. The inverse line ME effect with the coefficient of

�1.84 ps/m was defined as DM=DE, which has never been

reported earlier. The temporal evolution of M induced by Eindicates a stable magnetic response to the electric field.

As for the magnetic field control of polarization, the

applied magnetic field could influence the spin orders and

thus modulate the inherent coupled electric orders. To the

contrary, the applied electric field could force the polariza-

tion induced by ME cooling to re-arrange in its direction.

Noting that the polarization originates from the magnetic

order, the variation of polarization would lead to the re-

ordering of antiferromagnetic regions, which accounts for

the electric-field control of magnetism.

In summary, we revisited the classical linear ME effect

in NdCrTiO5 by measuring electric polarization P induced

by a magnetic field H and magnetization M induced by an

electric field E below TN. We experimentally demonstrated

the electric polarization responding to H and the magnetiza-

tion responding to E at 10 K. The observed ME effect shows

an obvious and stable ME mutual control by magnetic and

electric fields. The obtained coefficients of ME and inverse

ME effects are 2.01 ps/m and �1.84 ps/m, respectively. Our

experimental results could provide an appropriate contribu-

tion to a comprehensive understanding of the electro-control

of magnetism on linear ME materials or multiferroics.

See supplementary material for more details of the mag-

netization, pyroelectric measurements, and magnetic control

of electric polarizations.

This work was supported by the National Key Research

Projects of China (Grant No. 2016YFA0300101), the

National Natural Science Foundation of China (Grant Nos.

11704109, 51431006, 51332006, and 11804088), and the

Research Project of Hubei Provincial Department of

Education (Grant Nos. Q20172501 and B2018146).

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