Ci a ion: Manchón-Go dón, A.F.;
Pe ejón, A.; Gil-González, E.;
Kowalczyk, M.; Sánchez-Jiménez,
P.E.; Pé ez-Maqueda, L.A. Low
Tempe a u e Magne ic T ansi ion o
BiFeO3Ce amics Sin e ed by Elec ic
Field-Assis ed Me hods: Flash and
Spa k Plasma Sin e ing. Ma e ials
2023,16, 189. h ps://doi.o g/
10.3390/ma16010189
Academic Edi o : Nes o
Washing on Solís Pina go e
Recei ed: 5 Decembe 2022
Re ised: 15 Decembe 2022
Accep ed: 20 Decembe 2022
Published: 25 Decembe 2022
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ma e ials
A icle
Low Tempe a u e Magne ic T ansi ion o BiFeO3Ce amics
Sin e ed by Elec ic Field-Assis ed Me hods: Flash and Spa k
Plasma Sin e ing
Alejand o Fe nando Manchón-Go dón1,* , An onio Pe ejón1,2,*, E a Gil-González 1,3 , Maciej Kowalczyk 4,
Ped o E. Sánchez-Jiménez 1,3 and Luis A. Pé ez-Maqueda 1
1Ins i u o de Ciencia de Ma e iales de Se illa, CSIC-Uni e sidad de Se illa, C. Amé ico Vespucio 49,
41092 Se illa, Spain
2Depa amen de Química Ino gánica, Facul ad de Química, Uni e sidad de Se illa, 41012 Se illa, Spain
3
Depa amen de Ingenie ía Química, Uni e sidad de Se illa, Escuela Poli écnica Supe io , 41011 Se illa, Spain
4Facul y o Ma e ials Science and Enginee ing, Wa saw Uni e si y o Technology, 141 Wołoska s .,
02-507 Wa saw, Poland
*Co espondence: [email p o ec ed] (A.F.M.-G.); [email p o ec ed] (A.P.)
Abs ac :
Low empe a u e magne ic p ope ies o BiFeO
3
powde s sin e ed by lash and spa k
plasma sin e ing we e s udied. An anomaly obse ed in he magne ic measu emen s a 250 K p o es
he clea exis ence o a phase ansi ion. This ans o ma ion, which becomes less well-de ined as he
g ain sizes a e educed o nanome e scale, was desc ibed wi h ega d o a magne o-elas ic coupling.
Fu he mo e, he samples exhibi ed enhanced e omagne ic p ope ies as compa ed wi h hose o
a pelle p epa ed by he con en ional solid-s a e echnique, wi h bo h a highe coe ci i y ield and
emnan magne iza ion, eaching a maximum alue o 1.17 kOe and 8.5 10
−3
emu/g, espec i ely,
o he specimen sin e ed by lash sin e ing, which possesses he smalles g ains. The specimens
also show mo e signi ican exchange bias, om 22 o 177 Oe o he specimen p epa ed by he solid-
s a e me hod and lash sin e ing echnique, espec i ely. The obse ed inc ease in his pa ame e is
explained in e ms o a s onge exchange in e ac ion be ween e omagne ic and an i e omagne ic
g ains in he case o he pelle sin e ed by lash sin e ing.
Keywo ds:
lash sin e ing; spa k plasma sin e ing; bismu h e i e; magne ic p ope ies; mechanosyn hesis
1. In oduc ion
Mul i e oic ce amic bismu h i on oxide (BiFeO
3
) has ecei ed conside able a en ion
in he esea ch communi y due o i s unique p ope ies o co-exis ence o e oelec ic-
i y and e omagne ism [
1
,
2
]. BiFeO
3
, in i s bulk o m, is a e oelec ic ce amic wi h a
heo e ical sa u a ed pola iza ion o 90
µ
C/cm
2
and a ela i ely high Cu ie empe a u e
TC∼
1100 K [
3
]. A he same ime, BiFeO
3
exhibi s an an i e omagne ic beha io ela ed
o he exchange in e ac ion be ween Fe
+3
ions up o he Néel empe a u e a app oxi-
ma ely
TN∼
643 K [
3
]. Howe e , bulk BiFeO
3
su e s om high leakage cu en [
4
–
9
] and,
gene ally, p esen s a non-homogeneous magne ic s uc u e and a quad a ic e omagne-
oelec ic beha io , esul ing in poo e oelec ic beha io and cancelling mac oscopic
magne iza ion [10,11].
To sol e he p oblem o he poo magne iza ion o BiFeO
3
, di e en app oaches ha e
been add essed, such as he composi ional subs i u ion [
12
–
14
]. Mo eo e , he p epa a ion
o nanoscaled BiFeO
3
samples has been e ealed as an e ec i e me hod o he enhancemen
o i s magne ic p ope ies [
15
–
18
]. Ne e heless, epo ed BiFeO
3
nanos uc u es we e
p epa ed by he con en ional die-p essing me hod, yielding po ous ma e ials wi h poo
elec ical p ope ies, which una oidably es ic s hei applica ions [
1
]. In his sense,
Field-Assis ed Sin e ing Techniques (FAST) ha e been shown o be use ul echniques o
Ma e ials 2023,16, 189. h ps://doi.o g/10.3390/ma16010189 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2023,16, 189 2 o 11
sin e nanos uc u ed ce amics, as hey yield dense ma e ials while minimizing g ain
g ow h [
19
,
20
]. Among FAST, Spa k Plasma Sin e ing (SPS) [
21
] and Flash Sin e ing
(FS) [
22
] echniques can be highligh ed because i has been shown ha nanos uc u ed
BiFeO
3
can be p epa ed by bo h me hods [
15
,
23
–
25
]. Howe e , he magne ic beha io o
his compound p epa ed by FS has no been epo ed.
The analysis o he phase ansi ion beha io in BiFeO
3
is commonly ocused on a high-
empe a u e egime and he na u e o phase ansi ions below 300 K emain unclea . In his
sense, magne ic measu emen s on single c ys als, powde s o nanos uc u ed BiFeO
3
ha e
exposed di e en magne ic ansi ions wi hin his empe a u e ange [
26
–
28
]. Mo eo e ,
di e en expe imen al echniques, such as calo ime y, dielec ic o mechanical measu e-
men s, as well as Raman spec oscopy ha e epo ed possible phase ansi ions close o 25,
38, 55, 140, 150, 178, 200 and 230-260 K [
29
–
31
], assigned o di e en phenomena, such as
magne ic bu glassy ansi ions (38–50 K), o magne oelas ic ansi ion a ound 200–220 K.
The e o e, i is o he mos in e es o pe o m mo e s udies on he low- empe a u e egime
in o de o cla i y he na u e o he obse ed ansi ions. The p esen wo k is ocused on
s udying he in luence o he sin e ing p ocess on he low- empe a u e magne ic beha io
o bulk BiFeO
3
sin e ed by wo di e en FAST me hodologies: lash sin e ing and spa k
plasma sin e ing. The ob ained esul s we e compa ed wi h hose o a BiFeO
3
specimen
p epa ed by solid-s a e eac ion.
2. Ma e ials and Me hods
BiFeO
3
nanopowde s we e p epa ed by milling Fe
2
O
3
(Sigma Ald ich, Da ms ad ,
Ge many; <5
µ
m,
≥
99% pu i y) and Bi
2
O
3
(Sigma Ald ich, Da ms ad , Ge many;
≥
99.9%
pu i y) comme cial powde oxides using a high-ene gy plane a y F i sch Pul e ise e 7
(F i sch GmbH, Ida -Obe s ein, Ge many). A de ailed in o ma ion on he p ocedu e can be
ound elsewhe e [
32
]. The mechano-syn hesized powde s we e subsequen ly sin e ed by
wo echniques: lash-sin e ing and spa k plasma sin e ing. The lash-sin e ing expe imen s
we e ca ied ou using he s anda d p ocedu e [
24
]. The sample was lashed a 100 V cm
−1
and 20 mA mm
−2
o 15 seconds, wi h he lash e en occu ing a 773 K. On he o he
hand, he SPS expe imen was ca ied ou in a comme cial SPS Model 515S (SPS D Sin e
Inc., Japan) unde acuum using a p essu e o 75 MPa a 898 K o 10 min. In e e ence [
10
],
mo e de ailed in o ma ion abou he sin e ing p ocess by SPS can be ound. Fo compa ison
pu poses, a bulk BiFeO
3
pelle was p epa ed by con en ional solid-s a e eac ion using he
same comme cial powde s mixed in an aga e mo a o
∼
10 min and uniaxially p essed o
p epa e a cylind ical pelle . The specimen was i ed a 1123 K o 0.5 h using a hea ing a e
o 10 K min−1in an alumina boa placed on powde o he same composi ion.
The s uc u e o he ob ained pelle s was s udied by X- ay di ac ion, XRD, a oom
empe a u e, using Cu-K
α
adia ion in a Rigaku MiniFlex di ac ome e (Tokyo, Japan).
Phase ansi ion empe a u es we e analyzed by di e en ial scanning calo ime y using a
simul aneous TG/DSC (Q650 SDT; TA Ins umen s, New Cas le, DE 19720, USA) unde a
ni ogen low and 10 K min
−1
hea ing a e. Mic os uc u al cha ac e iza ion was ca ied
ou by scanning elec on mic oscopy in a Hi achi S-4800 mic oscope (Tokyo, Japan).
Magne ic cha ac e iza ion o he pelle s was ca ied ou using he s anda d ib a -
ing sample magne ome e op ion o a Physical P ope ies Measu emen Sys em, PPMS,
(Quan um Design, San Diego, CA, USA) applying an ex e nal magne ic ield o 100 Oe, a
hea ing/cooling a e o
±
1 K/min in ze o- ield cooling (FC), ield hea ing (FH) and ield
cooling modes.
The in si u e olu ion o he c ys allog aphic s uc u e o he samples wi h empe a u e
om oom empe a u e o 180 K (on cooling and hea ing) and wi h a hea ing a e o
10 K/min was measu ed in a B uke D8C di ac ome e (B uke , Bille ica, MA, USA)
wi h Cu-K
α
adia ion. Each pa e n was collec ed a he selec ed empe a u e (measu ed
ime less han 5 min). Phase iden i ica ion and Le Bail e inemen s we e pe o med by
DIFFRAC.EVA ( e sion 6, B uke , Bille ica, MA, USA) and DIFFRAC.TOPAS (Ve sion 6;
B uke , Bille ica, MA, USA) so wa e, espec i ely.
Ma e ials 2023,16, 189 3 o 11
3. Resul s and Discussion
Figu e 1shows XRD pa e ns, aken a oom empe a u e, o he h ee s udied sin e ed
specimens. All he di ac ion peaks o BiFeO
3
specimens sin e ed by non-con en ional
me hods can be indexed as a hombohed al pe o ski e s uc u e wi h an R3c space g oup,
which indica es he e en ion o pu e BiFeO
3
a e he sin e ing p ocess. By con as ,
he con en ionally sin e ed sample pa ially decomposed in o seconda y Bi
25
FeO
40
and
Bi
2
Fe
4
O
9
phases. In ac , hese seconda y phases we e o en obse ed in his compound, as
BiFeO3is me as able and decomposes a ela i ely low empe a u es [33,34].
Ma e ials 2023, 16, x FOR PEER REVIEW 3 o 12
USA) wi h Cu-K𝛼 adia ion. Each pa e n was collec ed a he selec ed empe a u e
(measu ed ime less han 5 min). Phase iden i ica ion and Le Bail e inemen s we e pe -
o med by DIFFRAC.EVA ( e sion 6, B uke , Bille ica, Massachuse s, USA) and DIF-
FRAC.TOPAS (Ve sion 6; B uke , Bille ica, Massachuse s, USA) so wa e, espec i ely.
3. Resul s and Discussion
Figu e 1 shows XRD pa e ns, aken a oom empe a u e, o he h ee s udied sin-
e ed specimens. All he di ac ion peaks o BiFeO3 specimens sin e ed by non-con en-
ional me hods can be indexed as a hombohed al pe o ski e s uc u e wi h an R3c space
g oup, which indica es he e en ion o pu e BiFeO3 a e he sin e ing p ocess. By con-
as , he con en ionally sin e ed sample pa ially decomposed in o seconda y Bi25FeO40
and Bi2Fe4O9 phases. In ac , hese seconda y phases we e o en obse ed in his com-
pound, as BiFeO3 is me as able and decomposes a ela i ely low empe a u es [33,34].
20 30 40 50 60 70
Bi25FeO40
Bi2Fe4O9
In ensi y (a.u.)
2 (deg ees)
SSR
SPS
FS
Figu e 1. XRD pa e ns, aken a oom empe a u e, o he BiFeO3 specimens p epa ed by he
solid-s a e eac ion me hod (SSR) and by mechanosyn hesis and subsequen ly densi ied by SPS
and FS.
The mul i e oic cha ac e and homogenei y o he ob ained pelle s we e s udied by
DSC in a non-iso he mal egime, since phase ansi ion empe a u es a y wi h he exis -
ence o impu i ies [34]. Figu e 2 shows he DSC scans aken a 10 K min-1 on hea ing. All
samples exhibi ed a weak ansi ion a a ound 643 K. Conside ing he da a epo ed in he
li e a u e, his peak co esponds o he an i e omagne ic–pa amagne ic ansi ion o he
samples, i.e. he Néel empe a u e. A much mo e in ense endo he mic peak appea ed
a ound 1093 K. I is associa ed wi h he e oelec ic-pa aelec ic ansi ion and de e -
mines he Cu ie empe a u e. The empe a u e a which bo h ansi ions we e obse ed
a e in e y good ag eemen wi h hose epo ed in he li e a u e o high-quali y BiFeO3
[4,32,35]. Addi ionally, o he sample p epa ed by SSR, a hi d endo he mic peak can be
Figu e 1.
XRD pa e ns, aken a oom empe a u e, o he BiFeO
3
specimens p epa ed by he solid-s a e
eac ion me hod (SSR) and by mechanosyn hesis and subsequen ly densi ied by SPS and FS.
The mul i e oic cha ac e and homogenei y o he ob ained pelle s we e s udied
by DSC in a non-iso he mal egime, since phase ansi ion empe a u es a y wi h he
exis ence o impu i ies [
34
]. Figu e 2shows he DSC scans aken a 10 K min
−1
on hea ing.
All samples exhibi ed a weak ansi ion a a ound 643 K. Conside ing he da a epo ed in
he li e a u e, his peak co esponds o he an i e omagne ic–pa amagne ic ansi ion o
he samples, i.e. he Néel empe a u e. A much mo e in ense endo he mic peak appea ed
a ound 1093 K. I is associa ed wi h he e oelec ic-pa aelec ic ansi ion and de e mines
he Cu ie empe a u e. The empe a u e a which bo h ansi ions we e obse ed a e in
e y good ag eemen wi h hose epo ed in he li e a u e o high-quali y BiFeO
3
[
4
,
32
,
35
].
Addi ionally, o he sample p epa ed by SSR, a hi d endo he mic peak can be clea ly
obse ed a app oxima ely 1057 K. This peak has been ela ed o he exis ence o impu i ies,
in ag eemen wi h he XRD da a.
SEM mic og aphs o he sin e ed pelle s a e p esen ed in Figu e 3. In he case o he
pelle sin e ed con en ionally, he mic og aph shows la ge g ains, ypically o 2.5–10
µ
m. By
con as , he pelle sin e ed by SPS exhibi s a mic os uc u e wi h a g ain size o 100
±
20 nm.
Finally, he mic os uc u e o he pelle sin e ed by FS co esponds o a well-sin e ed ma e ial
wi h smalle g ains o an a e age size o 40 ±12 nm.
Ma e ials 2023,16, 189 4 o 11
Ma e ials 2023, 16, x FOR PEER REVIEW 4 o 12
clea ly obse ed a app oxima ely 1057 K. This peak has been ela ed o he exis ence o
impu i ies, in ag eemen wi h he XRD da a.
600 700 800 900 1000 1100
Endo
641 K
642 K
647 K
Hea low (a.u.)
T (K)
SSR
SPS
FS
TC
TN
1095 K
1099 K
1093 K
Figu e 2. Di e en ial scanning calo ime y (DSC) cu es o BiFeO3 p epa ed by SSR and by mech-
anosyn hesis and subsequen ly densi ied by SPS o FS.
SEM mic og aphs o he sin e ed pelle s a e p esen ed in Figu e 3. In he case o he
pelle sin e ed con en ionally, he mic og aph shows la ge g ains, ypically o 2.5–10 μm.
By con as , he pelle sin e ed by SPS exhibi s a mic os uc u e wi h a g ain size o
100 ± 20 nm. Finally, he mic os uc u e o he pelle sin e ed by FS co esponds o a well-
sin e ed ma e ial wi h smalle g ains o an a e age size o 40 ± 12 nm.
Figu e 2.
Di e en ial scanning calo ime y (DSC) cu es o BiFeO
3
p epa ed by SSR and by mechanosyn-
hesis and subsequen ly densi ied by SPS o FS.
The magne ic beha io o he BiFeO
3
samples was analyzed unde an ex e nally
applied ield o 100 Oe h ough ze o- ield cooling (ZFC) and ield cooling (FC) cu es. The
ield hea ing (FH) cu e is also shown (see Figu e 4). Fo he SSR sample, a pla eau-like
shape can be obse ed in he whole empe a u e ange, al hough he e a e some anomalies.
The i e e sibili y o magne iza ion was e idenced a empe a u es below 100 K, i.e., he
ZFC and FC cu es spli below his empe a u e. Such spli ing phenomena is commonly
a ibu ed o e omagne ic and an i e omagne ic in e aces [
26
], and i has been obse ed
in o he BiFeO
3
- ela ed compounds [
15
,
27
,
36
,
37
]. As can be seen, his phenomenon is
mo e ema kable o he BiFeO
3
nanoce amic samples sin e ed by SPS and FS as compa ed
o he sample p epa ed by SSR. This ac is ela ed o he inc ease in he e omagne ic-
an i e omagne ic in e aces due o he dec ease in he g ain size o he specimens p epa ed
by FAST me hodologies [
26
]. These in e aces a e also he eason o he exis ence o he
exchange bias (EB) e ec ( his e ec will be discussed below). Fu he mo e, bo h ZFC
and FC cu es depic a signi ican inc ease a empe a u es below
∼
20 K due o he weak
e omagne ism o BiFeO
3
a hese empe a u es [
38
,
39
]. Al hough his magne iza ion
enhancemen can be obse ed a low empe a u es o he h ee s udied specimens, i is
highe in he case o he sample p epa ed by he con en ional me hod, p obably due o he
magne ic con ibu ion o he pa asi ic phases [40].
In e es ingly, in addi ion o he ea u es discussed abo e, which a e common o all
specimens, he magne iza ion cu es o he sample densi ied by SPS p esen an anomaly
o abou 250 K, which sugges s he occu ence o a phase ans o ma ion. The he mal
hys e esis be ween FC and FH cu es, ypically obse ed in i s -o de ype ansi ions [
41
],
migh indica e he magne oelas ic na u e o his ansi ion. In ac , he empe a u e ange
assigned o magne oelas ic ansi ion in p e ious wo ks (al hough no by magne ic mea-
su emen s) [
29
–
31
] is in good ag eemen wi h he anomaly obse ed a app oxima ely
250 K o he sample sin e ed by SPS. The magne iza ion cu es o he specimen densi ied
by FS a e qui e simila o hose o he sample densi ied by SPS. Ne e heless, he possible
magne oelas ic ansi ion is weake . This could be ela ed wi h he dec ease in he g ain size.
Ma e ials 2023,16, 189 5 o 11
In ac , i has been epo ed ha below a c i ical g ain size he magne oelas ic ansi ion can
be supp essed in di e en sys ems [42–44].
Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 12
Figu e 3. Scanning elec on mic oscopy mic og aphs o pelle s p epa ed by (a) SSR, and mechano-
syn hesis ollowed by sin e ing using (b) SPS and (c) FS.
The magne ic beha io o he BiFeO3 samples was analyzed unde an ex e nally ap-
plied ield o 100 Oe h ough ze o- ield cooling (ZFC) and ield cooling (FC) cu es. The
ield hea ing (FH) cu e is also shown (see Figu e 4). Fo he SSR sample, a pla eau-like
shape can be obse ed in he whole empe a u e ange, al hough he e a e some anoma-
lies. The i e e sibili y o magne iza ion was e idenced a empe a u es below 100 K, i.e.,
he ZFC and FC cu es spli below his empe a u e. Such spli ing phenomena is com-
monly a ibu ed o e omagne ic and an i e omagne ic in e aces [26], and i has been
obse ed in o he BiFeO3- ela ed compounds [15,27,36,37]. As can be seen, his phenome-
non is mo e ema kable o he BiFeO3 nanoce amic samples sin e ed by SPS and FS as
compa ed o he sample p epa ed by SSR. This ac is ela ed o he inc ease in he e o-
magne ic-an i e omagne ic in e aces due o he dec ease in he g ain size o he speci-
mens p epa ed by FAST me hodologies [26]. These in e aces a e also he eason o he
exis ence o he exchange bias (EB) e ec ( his e ec will be discussed below). Fu he -
mo e, bo h ZFC and FC cu es depic a signi ican inc ease a empe a u es below ~20 K
due o he weak e omagne ism o BiFeO3 a hese empe a u es [38,39]. Al hough his
magne iza ion enhancemen can be obse ed a low empe a u es o he h ee s udied
Figu e 3.
Scanning elec on mic oscopy mic og aphs o pelle s p epa ed by (
a
) SSR, and mechanosyn-
hesis ollowed by sin e ing using (b) SPS and (c) FS.
Fo he pu pose o explo ing he na u e o he magne ic ansi ion ound a a ound 250 K
and o ob ain a be e unde s anding o he phase e olu ion o each sample, empe a u e-
dependen X- ay di ac ion pa e ns we e egis e ed. Figu e S1 (Supplemen a y Ma e ials)
depic s he XRD pa e ns o he sample sin e ed by SPS on cooling and hea ing, and egis e ed
in si u be ween 180–300 K. In he en i e s udied empe a u e ange, no modi ica ion o he
c ys al s uc u e o BiFeO
3
occu ed; only he expec ed shi o he peaks o lowe angles as
he empe a u e was lowe ed om 300 o 180 K can be obse ed.
XRD pa e ns we e analyzed by Le Bail e inemen (goodness o i , GOF
≤
1.6).
Figu e 5depic s he e olu ion o cell olume o he BiFeO
3
phase wi h empe a u e, whe e
a signi ican de ia ion om he end is de ec ed o he sample sin e ed by SPS. This
de ia ion, which is no dis inguished in he case o he SSR sample and is less de ined in
he case o he FS-ed sample, is accompanied by ce ain he mal hys e esis be ween bo h
hea ing and cooling. These ac s suppo ha he ans o ma ion co esponds o a i s -

Ma e ials 2023,16, 189 6 o 11
o de phase ansi ion in e ed om he beha io o he magne iza ion cu es (see Figu e 4).
In ac , he e is a modi ica ion o he olume o he cell wi hou a change o he c ys al
s uc u e. The ob ained esul s allow us o de e mine he magne oelas ic na u e o he
obse ed ansi ion, which has been p e iously a ibu ed o he exis ence o impu i ies [
45
]
o , mo e ecen ly, desc ibed as a magne ic bu glassy ansi ion [37].
Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 12
specimens, i is highe in he case o he sample p epa ed by he con en ional me hod,
p obably due o he magne ic con ibu ion o he pa asi ic phases [40].
6
9
12
6
9
12
0100 200 300 400
6
9
12 FS
SPS
FC
FH
 (10-4 emu/g)
ZFC
SSR
(a)
(c)
(b)
 (10-4 emu/g)
 (10-4 emu/g)
T (K)
Figu e 4. Tempe a u e dependence o he magne iza ion o he specimens p epa ed by (a) SSR,
and mechanosyn hesis and sin e ed by (b) SPS and (c) FS, depic ing ZFC, FH, FC cu es, wi h an
ex e nal applied magne ic ield o 100 Oe.
In e es ingly, in addi ion o he ea u es discussed abo e, which a e common o all
specimens, he magne iza ion cu es o he sample densi ied by SPS p esen an anomaly
o abou 250 K, which sugges s he occu ence o a phase ans o ma ion. The he mal
hys e esis be ween FC and FH cu es, ypically obse ed in i s -o de ype ansi ions
[41], migh indica e he magne oelas ic na u e o his ansi ion. In ac , he empe a u e
ange assigned o magne oelas ic ansi ion in p e ious wo ks (al hough no by magne ic
measu emen s) [29–31] is in good ag eemen wi h he anomaly obse ed a app oxima ely
250 K o he sample sin e ed by SPS. The magne iza ion cu es o he specimen densi ied
by FS a e qui e simila o hose o he sample densi ied by SPS. Ne e heless, he possible
Figu e 4.
Tempe a u e dependence o he magne iza ion o he specimens p epa ed by (
a
) SSR, and
mechanosyn hesis and sin e ed by (
b
) SPS and (
c
) FS, depic ing ZFC, FH, FC cu es, wi h an ex e nal
applied magne ic ield o 100 Oe.
Once he magne ic beha io a low empe a u e o he s udied specimens has been
analyzed, Figu e 6shows he magne ic hys e esis loops a 300 K. As expec ed, he specimens
show an almos linea ield dependence o magne iza ion due o he G- ype an i e omagne ic
beha io o BiFeO
3
, especially in he case o he sample p epa ed by SSR, implying ha he
magne iza ion (o emnan magne iza ion) is p ac ically ze o. This beha io o he hys e esis
loops ag ees wi h he expec ed an i e omagne ic na u e o he s udied compound. The
Ma e ials 2023,16, 189 7 o 11
educ ion in g ain size (see mic og aphs in Figu e 3) leads o he appea ance o some hys e esis
in he case o he samples sin e ed by FAST, wi h a majo e ec in he case o he specimen
sin e ed by FS. The imp o emen o magne ic p ope ies in nanos uc u ed BiFeO
3
is cu en ly
unde discussion, and h ee p incipal ac o s a e unde conside a ion: a pa ial compensa ion
o an i e omagne ic subla ices a he su ace, an inc ease in he spin can ing angle o Fe-O-Fe
bonds in oduced by s ain and an annihila ion o he spi al spin s uc u e [
36
,
46
,
47
]. E en
hough he e a e no impo an disc epancies in he maximum magne iza ion a he ange o
magne ic ields s udied, di e ences in emnan magne iza ion
σ
and coe ci i y
HC
can be
highligh ed. Indeed,
σ
eached he highes alue a 8.5 10
−3
emu/g o he specimen sin e ed
by FS, which possessed he smalles g ains.
Ma e ials 2023, 16, x FOR PEER REVIEW 7 o 12
magne oelas ic ansi ion is weake . This could be ela ed wi h he dec ease in he g ain
size. In ac , i has been epo ed ha below a c i ical g ain size he magne oelas ic ansi-
ion can be supp essed in di e en sys ems [42–44].
Fo he pu pose o explo ing he na u e o he magne ic ansi ion ound a a ound
250 K and o ob ain a be e unde s anding o he phase e olu ion o each sample, em-
pe a u e-dependen X- ay di ac ion pa e ns we e egis e ed. Figu e S1 (Supplemen a y
Ma e ials) depic s he XRD pa e ns o he sample sin e ed by SPS on cooling and hea ing,
and egis e ed in si u be ween 180-300 K. In he en i e s udied empe a u e ange, no mod-
i ica ion o he c ys al s uc u e o BiFeO3 occu ed; only he expec ed shi o he peaks o
lowe angles as he empe a u e was lowe ed om 300 o 180 K can be obse ed.
XRD pa e ns we e analyzed by Le Bail e inemen (goodness o i , GOF ≤ 1.6). Fig-
u e 5 depic s he e olu ion o cell olume o he BiFeO3 phase wi h empe a u e, whe e a
signi ican de ia ion om he end is de ec ed o he sample sin e ed by SPS. This de i-
a ion, which is no dis inguished in he case o he SSR sample and is less de ined in he
case o he FS-ed sample, is accompanied by ce ain he mal hys e esis be ween bo h hea -
ing and cooling. These ac s suppo ha he ans o ma ion co esponds o a i s -o de
phase ansi ion in e ed om he beha io o he magne iza ion cu es (see Figu e 4). In
ac , he e is a modi ica ion o he olume o he cell wi hou a change o he c ys al s uc-
u e. The ob ained esul s allow us o de e mine he magne oelas ic na u e o he obse ed
ansi ion, which has been p e iously a ibu ed o he exis ence o impu i ies [45] o ,
mo e ecen ly, desc ibed as a magne ic bu glassy ansi ion [37].
140 160 180 200 220 240 260 280 300
370.5
371.0
371.5
372.0
372.5
373.0
373.5
374.0
hea ing
V (angs om)3
T (K)
SPS
SSR
FS
cooling
Figu e 5. Cell olume o BiFeO3 as a unc ion o empe a u e o all he s udied samples.
Once he magne ic beha io a low empe a u e o he s udied specimens has been
analyzed, Figu e 6 shows he magne ic hys e esis loops a 300 K. As expec ed, he speci-
mens show an almos linea ield dependence o magne iza ion due o he G- ype an i e -
omagne ic beha io o BiFeO3, especially in he case o he sample p epa ed by SSR, im-
plying ha he magne iza ion (o emnan magne iza ion) is p ac ically ze o. This beha -
io o he hys e esis loops ag ees wi h he expec ed an i e omagne ic na u e o he s ud-
ied compound. The educ ion in g ain size (see mic og aphs in Figu e 3) leads o he ap-
pea ance o some hys e esis in he case o he samples sin e ed by FAST, wi h a majo
Figu e 5. Cell olume o BiFeO3as a unc ion o empe a u e o all he s udied samples.
The exchange aniso opy exis ing a he in e ace be ween e omagne ic and an i e -
omagne ic g ains can o igina e exchange bias (EB) phenomena. In he case o BiFeO
3
, his
e ec can appea a he in e ace as a esul o he in e ac ion be ween he e omagne ic
g ains wi h a size smalle han 62 nm, and an i e omagne ic g ains. The alue o EB,
HEB
, can be de e mined as
HEB
=
(HC++HC−)/
2, whe e
HC+
and
HC−
a e he posi i e
and nega i e ields when magne iza ion is ze o, espec i ely [
48
]. The ob ained alues
a e collec ed in Table 1. A ela i ely low EB e ec is obse ed o he BiFeO
3
sample
p epa ed by SSR, whe eas i inc eases o he specimens sin e ed by FAST echniques, i.e.,
wi h he dec ease in pa icle size. Thus, EB is
∼−
110 Oe o he specimen sin e ed by
SPS and
∼−
177 Oe o he FS specimen. In his la e sample, he exchange in e ac ion
be ween e omagne ic and an i e omagne ic g ains is esponsible o he la ge EB e ec .
Once
HEB
is known,
HC
can be co ec ly de e mined, he alues o which ha e been also
collec ed in Table 1. I can be obse ed ha he inc ease in coe ci i y is caused by a d op o
he pa icle size. This educ ion p o okes he gene a ion o a single magne ic domain o
he g ains. In his way, he mechanism ha gene a e he magne ic beha io changes om
domain wall mo ion o magne iza ion o a ion [
36
]. Mo eo e , a mo e impo an ole o
su ace aniso opy e ec s could be expec ed wi h he dec ease in he pa icle size [49].
Fo compa ison pu poses, Table 1also includes da a om he li e a u e o BiFeO
3
samples
sin e ed by SPS (da a o samples sin e ed by FS ha e no been ound) [
15
,
23
,
50
]. I can be
clea ly obse ed ha he p esen ed pa ame e s a e compa able o hose ob ained in his wo k
o he SPS sample. I is wo h no ing ha he BiFeO
3
specimen p epa ed by FS displays
Ma e ials 2023,16, 189 8 o 11
an enhanced e omagne ic cha ac e compa ed o hose p epa ed by SPS. Gene ally, i is
assumed ha he inc ease in he magne ic pa ame e s is due o he supp essed magne ic
spin s uc u e when he g ain size is below
∼
62 nm [
54
]. On he o he hand, he magne ism
o BiFeO
3
can be ailo ed by s uc u al modi ica ions by he addi ion o di e en ypes o
subs i uen s, as can be seen in Table 1.
Ma e ials 2023, 16, x FOR PEER REVIEW 8 o 12
e ec in he case o he specimen sin e ed by FS. The imp o emen o magne ic p ope ies
in nanos uc u ed BiFeO3 is cu en ly unde discussion, and h ee p incipal ac o s a e
unde conside a ion: a pa ial compensa ion o an i e omagne ic subla ices a he su -
ace, an inc ease in he spin can ing angle o Fe-O-Fe bonds in oduced by s ain and an
annihila ion o he spi al spin s uc u e [36,46,47]. E en hough he e a e no impo an
disc epancies in he maximum magne iza ion a he ange o magne ic ields s udied, di -
e ences in emnan magne iza ion 𝜎𝑟 and coe ci i y 𝐻𝐶 can be highligh ed. Indeed, 𝜎𝑟
eached he highes alue a 8.5 10-3 emu/g o he specimen sin e ed by FS, which pos-
sessed he smalles g ains.
-30 -20 -10 0 10 20 30
-0.2
-0.1
0.0
0.1
0.2
-1000 0 1000
-10
-5
0
5
10
 (emu/g)
H (kOe)
SSR
SPS
FS
(a)
(b)
 (10-3 emu/g)
H (Oe)
Figu e 6. (a) Magne ic hys e esis loops o he s udied specimens aken a 300 K. (b) Low ield e-
gion o he hys e esis loops.
The exchange aniso opy exis ing a he in e ace be ween e omagne ic and an i e -
omagne ic g ains can o igina e exchange bias (EB) phenomena. In he case o BiFeO3, his
e ec can appea a he in e ace as a esul o he in e ac ion be ween he e omagne ic
g ains wi h a size smalle han 62 nm, and an i e omagne ic g ains. The alue o EB, 𝐻𝐸𝐵,
can be de e mined as 𝐻𝐸𝐵 = (𝐻𝐶+ + 𝐻𝐶−)/2, whe e 𝐻𝐶+ and 𝐻𝐶− a e he posi i e and
nega i e ields when magne iza ion is ze o, espec i ely [48]. The ob ained alues a e col-
lec ed in Table 1. A ela i ely low EB e ec is obse ed o he BiFeO3 sample p epa ed by
SSR, whe eas i inc eases o he specimens sin e ed by FAST echniques, i.e., wi h he
dec ease in pa icle size. Thus, EB is ~−110 Oe o he specimen sin e ed by SPS and~−177
Figu e 6.
(
a
) Magne ic hys e esis loops o he s udied specimens aken a 300 K. (
b
) Low ield egion
o he hys e esis loops.
Table 1.
Magne ic p ope ies o BFO-based bulk ce amics sin e ed by elec ic ield assis ed-me hods.
Composi ion Technique d σ
(10−3emu/g) HEB (Oe)HC(Oe) Re e ence
BiFeO3
Solid-S a e Reac ion da a 1.3 −22 177 This wo k
Mechanosyn hesis + SPS ∼100 nm 3.5 −11 451
Mechanosyn hesis + FS ∼30 nm 8.5 −177 1173
BiFeO3Sol-gel + SPS ∼110 nm 11 500 (5K) [15]
BiFeO3Sol-gel + SPS 1–3 µm 0.6 50 [50]
1–3 µm 2.4 120
BiFeO3High-ene gy ball milling + SPS <200 nm 5.7 600 [23]
BiTi0.05Fe0.95O3Sol-gel + SPS <100 nm 10 500 [51]
Bi
0.85
La
0.15
FeO
3
High-ene gy ball c yo milling + SPS
24 nm 5.2 630 [52]
Ma e ials 2023,16, 189 9 o 11
Table 1. Con .
Composi ion Technique d σ
(10−3emu/g) HEB (Oe)HC(Oe) Re e ence
Bi
0.95
Nd
0.05
FeO
3Sol gel + SPS <1 µm 10 685 [53]
Bi
0.90
Nd
0.10
FeO
3101 6721
Bi
0.85
Nd
0.15
FeO
3181 9497
Bi
0.95
Sm
0.05
FeO
321 1954
Bi
0.90
Sm
0.10
FeO
3133 9627
Bi
0.85
Sm
0.15
FeO
3279 15117
4. Conclusions
Pelle s o dense and phase-pu e BiFeO
3
ob ained by mechanosyn hesis and sin e ed
by lash sin e ing, FS and spa k plasma sin e ing, SPS, we e cha ac e ized by magne iza ion
measu emen s. The esul s we e compa ed wi h hose ob ained o a sample p epa ed by a
con en ional solid-s a e eac ion. I is wo h emphasizing ha he magne ic beha io o a
BiFeO3specimen sin e ed by FS has no been p e iously desc ibed.
Low- empe a u e magne ic beha io indica es he co-exis ence o supe pa amag-
ne ic elaxa ion phenomena, which imply he spli ing o magne iza ion cu es a low
empe a u es in oduced by s ong in e pa icle in e ac ions (<100 K). In e es ingly, ze o
ield-cooled, ield-hea ed and ield-cooled magne iza ion cu es e ealed a phase ansi ion
a a ound 250 K in specimens densi ied by ield-assis ed sin e ing echniques, which is pa -
icula ly ema kable in he sample p epa ed by spa k plasma sin e ing. The magne oelas ic
na u e o his ansi ion, he mal hys e esis be ween bo h hea ing and cooling p ocesses
and modi ica ion o he olume wi hou c ys al s uc u e a ia ion, a e suppo ed by in
si u XRD measu emen s.
Supplemen a y Ma e ials:
The ollowing suppo ing in o ma ion can be downloaded a : h ps:
//www.mdpi.com/a icle/10.3390/ma16010189/s1, Figu e S1: XRD pa e ns as a unc ion o em-
pe a u e om 300 o 180 K o he sample p epa ed by SPS on cooling (uppe panel) and hea ing
(lowe panel).
Au ho Con ibu ions:
Concep ualiza ion, A.F.M.-G. and A.P.; me hodology, A.F.M.-G., A.P., E.G.-G.
and M.K.; o mal analysis, A.F.M.-G., A.P., P.E.S.-J. and L.A.P.-M.; in es iga ion, A.F.M.-G., A.P. and
E.G.-G.; esou ces, L.A.P.-M.; da a cu a ion, A.F.M.-G., A.P., E.G.-G. and M.K.; w i ing—o iginal
d a p epa a ion, A.F.M.-G. and A.P.; w i ing— e iew and edi ing, A.F.M.-G., A.P., E.G.-G., M.K.,
P.E.S.-J. and L.A.P.-M.; isualiza ion, A.F.M.-G.; supe ision, A.P., P.E.S.-J. and L.A.P.-M.; p ojec
adminis a ion, P.E.S.-J. and L.A.P.-M.; unding acquisi ion, P.E.S.-J. and L.A.P.-M. All au ho s ha e
ead and ag eed o he published e sion o he manusc ip .
Funding:
This wo k has been unded by he g an CTQ2017-83602-C2-1-R (MCIN/AEI/10.13039/
501100011033 and ERDF A way o making Eu ope by he Eu opean Union), p ojec s P18-FR-1087
(Jun a de Andalucía-Conseje ía de Conocimien o, In es igación y Uni e sidad-Fondo Eu opeo de
Desa ollo Regional P og ama Ope a i o FEDER Andalucía 2014–2020) and INTRAMURAL-CSIC
g an numbe 201960E092. A.F. Manchón-Go dón also acknowledge G an FJC2021-047783-I unded
by MCIN/AEI/10.13039/501100011033 and by “Eu opean Union Nex Gene a ionEU/PRTR”.
Ins i u ional Re iew Boa d S a emen : No applicable.
Da a A ailabili y S a emen : Da a will be a ailable on eques .
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Re e ences
1.
Wu, J.; Fan, Z.; Xiao, D.; Zhu, J.; Wang, J. Mul i e oic bismu h e i e-based ma e ials o mul i unc ional applica ions: Ce amic
bulks, hin ilms and nanos uc u es. P og. Ma e . Sci. 2016,84, 335–402.
2.
Zhang, F.; Zeng, X.; Bi, D.; Guo, K.; Yao, Y.; Lu, S. Dielec ic, e oelec ic, and magne ic p ope ies o Sm-doped BiFeO
3
ce amics
p epa ed by a modi ied solid-s a e- eac ion me hod. Ma e ials 2018,11, 2208. [C ossRe ] [PubMed]