- Email: [email protected]
Spin state of manganese in monolayer films of Mn arachidate
Volume 31, number 3
.,
;
~ k?:=rINSTATE OF hVINGAN&
..
M.’
POMEMTZ
15 March
CHEMICdLPHYSICSLE?TERS
1975
IN MONOLA-kR FILMS’OF Mn AkACHIDATE
and R.A. POLLAK
IBM 77lomas L Watson Research Center, Yorktown Heights, New York 10.598, USA
.’ Received 9 December 1974’ :
UringX-raypliotoemission spectroscopy, we km dethin@from aracBidateis S = 512.
tie hln 3s multiplet sp!itting that the.spin
Stateof Mn in I’nDnOl~Y~IS of Nn
strate of cleaved pyrolytic graphite which was pulled
Monolay& films produced by the Lkpuir-Blod-
gett method have.becn studied by many techniques,
pioperties of bulk crystals oi some of these salts have
through tie surface at a rate of about 2 cm/min. There was very good adhesion of the MnAr, to the surface. Graphite has the important property pf having a very low photoemission background’in the region of the M~I3s sp:ctrum which is thus observed with no obscurini background. A Hewlett-Packard 5950A photoelectron JTectrometeJwith monochromatic Al KaI 2 X-rays WV used to obtain the Mn 3s spectrum and i gaustian least-squares fit of the data wis made to de-
been studied over the years, but to OUTknowledp
termine relative pe& positions.
anil have proven both interesti’lng and useful 11). There .is also considera5le interest in the properties of mag netic systems’of dirnensidns lower than three .[2]. We have begun a study of the possibgity of producing two” dimemiqpal magnetic systems using the LangmuirBIodg&t technique by dspcsiting films of the transi-tion nietals .salts of carboxylic scids. The magnetic the
‘. .ma&etic properties of mpnclayers have not beeiexakined; Many basic questions,about such magnetic-
layers are thus unanswered. ?%s letter contains an experiment& result on the most basic question: What is the spin state of the transition metal atom Jvhen’it is :ti-ihe ftil form? we have used X-ray ph&emissiqh spectros’copy (X&5) to determine ths spin state of manganese in ,” manganese arachidate (abr, MnAr2) films. This method :’ is ideally suited to this problem because one can obse&e (iti, fact .one is limited to) the top-most monolayers of a sample. Siegbahn et al. have previously demon-
stratedthe app!icability of X’Sto the detectionof
.3070wei&t ,con&ntiation with the remainder being arachidic acid. Finally; pr&-&ary electron spin rtsonances rrte’asurementsgive a broad (= 300’G wi,de)
: idtie
a$ ,bromine in mu1tiliyer cr-iodostearic and IY~.&mostkaric acid filrrk [3]. ‘. ‘>e MnAr2 filrrk were p&duced’by spieadtig.
m.d so&what
$H,ha’d been adjuited to’7.2 by addition of,‘&OH.* .: to
faqilitate formation of Mn&2 ti’jxeference
arach&c acid: .The iihns. vjere ‘deposited-.& a su$
,: ‘. .’ ., .’ +jj:
;- :,.‘.
-..
‘.
. ..~
.’ ..- ;
: ,.,
.:
. ..
anisotropic l¢ered
at g = 2.0, salts. We deduce the,spiri state of manganese in MnAra from the multiplet splitting [4] ,h, the M.n 3s core level X-my phoi‘okmissio$,$ecJrum. The photoemitted -F/in 3s eIectron has a different biri&g .snergy if, the re-
-. which is t);pical of concen&ated:&2f
: arachidic acid, CHs(CHi) &OOFJ, bn the surface of .. l.O?M Ivin!J~;‘P20 made With d’isti&d water. ‘Tne ” brder to
The &L-n wx intro-
duced,&ectly into the spectrometer after pulling the last monztlayer. ‘The fh had a total’of seven monolayers. Additional experiments to characterize the MnAr, films wel-e performed.on multilayers of about 100 layers pulled on quartz. These include X-ray structural analysis ivhich shows that the matkrial we pull has layer structure with spacing of 33.8. This corresponds to two langths of Mn(Ar)2 chains, as expected from deposition in which the ionic ends of the fatty acid chains are in proximity (sokalled Y-layers [I] ). Chemical analysis of thk films indicates that the MI-&~ is in
’
_.~’ ,. -; ._-.
... .
_.
-’
:‘..
‘.,,
”
-
Volume 3 I, number 3
:
15 !&&I
CHElrIICAL PHYSICS LETTERS
PrOpOrtiOIlti t0 (2s t 1) as predicted
1975
by vzn Vlcck’s
theorem [7]. S is the total spin of the pattiaLly fiied d shell. AE = 6.62 ? 0.01 eV ir, MnF, [S] ivith S = 512, A/Z = 5.6 +-0.2 eV in MnF3 [9] with S = 2,
AE= 4.6 + 0.1 eV in Mn02 [IO] witiS= 312, and Al3 < 0.2 eVin various iron-group complexes [9l in the low~spi~ state. We observe a multiplet splitting of 6.4 f 0.1 eV in the Mn 3s spectrum of hfntil (fig. lb} and &&usby comparing this valile with the above values of LE conclude that the manganese atom in MnAr2 is in the high spin state with S = S/2. This result has implications about the structure of the m=nolayer
films of MnAr2. Et was already noticed
studiesby Langmuir and Schaefer [I I] that the presenceof someions in the subpiraseproduced
in early
remarkable rigidity in the fatty acid monolayer spread on this subphase. It was later observed that there were qualitative differences between the properties of monolayers spread on subphases containing Al or transition metal ions (which we denote by class I)., compared with those containing alkalis. or aikaline earths (class II). For class I, in certain regions of pH, there was a remarkable increase in area [ 121, increase in wettability of deposited films 1131, increased compressibility [12] and increased impenetrabiIity [14]. For class II subphases no such changes were observed. 107 97 87 77 To explain these properties it was suggested [ 121 that B!NDING ENERGY iev) there was a kind of polymcdc structure in the c&s 1 Fig. 1. Mn 3s X-ray photoelectron spectra of (a) hlnF2, from monolayers. It will be noted ;hzt the elements in class 1 ref. [6], shifted in energy to line up with (b) our measuretend to form coordination complexes. hfodels involving ment of spectrum of Mn&. The binding energy of hlnXr~ the linking of coordinated metal atoms have been proMn 3s has been calibrated against the C 1s Iin? of the gaphite posed for Al soaps [IS]. Such web-Iike structures presubstrate, whose binding energy is 284.7 eV. For the pursumably also occur for the transition-metal soap films pose of this paper the important point is the neu identity of the splittings [4] in MnFz and SInA+, and not the absolute and can intuitively axpiain the Fz2perties of class I bindiq energies. monolayers. For class Ii fiis it is assumed that simple metal-fatty acid molecular u&s are formed with little maining Mn 3s electron spin is exchange coupled bonding Setween the molecular units. parallel (7S) or antiparallel (5S) to the unpaired spins There is thus a question of the effect of the bondof thk outer Mn 3d electrons. T-NOstrong peaks aping in the transition-metal soaps on their magnetic pear in the binding eneqg spectrum corresponding to properties. It is well known [ 161 that in atoms conthese two situations (I&_ 1b). Severalyeher peaks obtaining five d electrons (e.g., Mnz’) the total spin served at higher binding energies result from ee corJJate can be either S = 5/Z (high) or S = I[2 QOW-) derelation between electrons in,the 3s and 3d shells pending ofi the strength of the crystal field. In “weak” and can be described in tern-k of configkation intercrystal fields, orbital degeneracy is nearly preserved action [S,6]. and Hund’s tie applies, giving S = 5/2. “Strong” McFeely et al. have shown that for a series of manfields split the orbital degeneracy sufficiently that ganese compounds with different spin configurations lower states are fried with pairs of electrons, leaving the multiplet splitting &E = @S) - EC/S) is directly :.’ i ndt 3 = l/2. Strong ligand fie!ds occur in strongly co.’ .;, ;
.
1
.
603
Vohime 31. nu’inber 3 ;’ valent
comp!=xes
CHEMICALPHYSICS
[ 161. TYIUS,our experiment
tiat
the Iigand fields are not “strong” in MnAr2
monokyers. We,wish to thank Mr. FFank Dzcol for creative contributions to the film pulling technique, and Mr. Frank Cardone foi performing the chemical analyses by the electron
niicfopiobe
method.,
‘. ‘.
Ref&ences
[ i] G-L. Gaines, Insoluble monolayers faces
(\%ley,
New
York,
at liquid,gas
inter-
1956).
121 LJ. de Jongh and A.P. Miedema, Advan. Phys..23 (1974) [3]
k-Sie@ihn;k. Nordtig ,A. Fahbrnar~ R. Nordberg K-. Hamrio, J. %dman, G. Jbhansson, T. IE)&+k, S.;. Ka&son, I. Lindgren and i. Lindberg, Electron‘sp~c-
_.” ._..
.,
15 Makh 1975
.’ tioscop~ for cliemiti.analysis (Almqvist kd Wiksalls, Stoc~olm, 1967) pp. 139-141.
answers
the question of the relative strength of the ligand fieldsjn Mn& soap flms. Tnz result that,S = 512. shows
LE-I-TEXS
[4]
C-S. Fndky,
kz Ekstron
spectro~opy,
cd_ D.A.
Shirley
(North-Holland, Amsterdam, 1972). [S] P.S. Bagus; A.]. Freeman and F. &s&i, Phys. Rev. Letters 30 (1973) 850. [6] S.P. Kow+lcz~:c, L. Ley, R.A. Pollak, F.R. McFeely and D.A. Shirley, Fhys:Rw. B7 (1973) 4039. [7] J.H. van Fleck, Phys. Rev. 45 (1934) 405. [S] F.R. McFeely, S.P. Kowalnyk, L. Ley and D.A. Shirley, Solid &te Commun. 15 (1974) 10.51. :9] J.C. C&&r, G.K. Schweitzei and T.A. Carlson, J. Chem. Fhys. 51 (197%) 973. [lo] C.S. Fadley anmiD.A. Shirley, Phys. Rev. A2 (1970) 1109. [ll] I. LugniuirandV.J. Schaefer, J. Ani. Chem. Sot. 59 (1937) 2400. T. Sa&i and F.. Matuura, Bull. Chcm. Soc:Japan 24 (1,051) 274. A. Inaba, Bull. Cbem. Soc.Japan 2.5 (1952) 174. R. Matuux, BLU. CT-em. Sot Jnp~l24 (1951) 2.78. C.G. McGee, J. Am. Chem. Sot. 71 (1949) 278;
V.R. GTayand A.E. Alexarider,J. phys. Colloid. Chem. 53 (1949) 9. 1161C.J. Ballhauser., Intioduction to ligand field theory (BIcGraw-Hi& New York, 1962).
.,
;
~ k?:=rINSTATE OF hVINGAN&
..
M.’
POMEMTZ
15 March
CHEMICdLPHYSICSLE?TERS
1975
IN MONOLA-kR FILMS’OF Mn AkACHIDATE
and R.A. POLLAK
IBM 77lomas L Watson Research Center, Yorktown Heights, New York 10.598, USA
.’ Received 9 December 1974’ :
UringX-raypliotoemission spectroscopy, we km dethin@from aracBidateis S = 512.
tie hln 3s multiplet sp!itting that the.spin
Stateof Mn in I’nDnOl~Y~IS of Nn
strate of cleaved pyrolytic graphite which was pulled
Monolay& films produced by the Lkpuir-Blod-
gett method have.becn studied by many techniques,
pioperties of bulk crystals oi some of these salts have
through tie surface at a rate of about 2 cm/min. There was very good adhesion of the MnAr, to the surface. Graphite has the important property pf having a very low photoemission background’in the region of the M~I3s sp:ctrum which is thus observed with no obscurini background. A Hewlett-Packard 5950A photoelectron JTectrometeJwith monochromatic Al KaI 2 X-rays WV used to obtain the Mn 3s spectrum and i gaustian least-squares fit of the data wis made to de-
been studied over the years, but to OUTknowledp
termine relative pe& positions.
anil have proven both interesti’lng and useful 11). There .is also considera5le interest in the properties of mag netic systems’of dirnensidns lower than three .[2]. We have begun a study of the possibgity of producing two” dimemiqpal magnetic systems using the LangmuirBIodg&t technique by dspcsiting films of the transi-tion nietals .salts of carboxylic scids. The magnetic the
‘. .ma&etic properties of mpnclayers have not beeiexakined; Many basic questions,about such magnetic-
layers are thus unanswered. ?%s letter contains an experiment& result on the most basic question: What is the spin state of the transition metal atom Jvhen’it is :ti-ihe ftil form? we have used X-ray ph&emissiqh spectros’copy (X&5) to determine ths spin state of manganese in ,” manganese arachidate (abr, MnAr2) films. This method :’ is ideally suited to this problem because one can obse&e (iti, fact .one is limited to) the top-most monolayers of a sample. Siegbahn et al. have previously demon-
stratedthe app!icability of X’Sto the detectionof
.3070wei&t ,con&ntiation with the remainder being arachidic acid. Finally; pr&-&ary electron spin rtsonances rrte’asurementsgive a broad (= 300’G wi,de)
: idtie
a$ ,bromine in mu1tiliyer cr-iodostearic and IY~.&mostkaric acid filrrk [3]. ‘. ‘>e MnAr2 filrrk were p&duced’by spieadtig.
m.d so&what
$H,ha’d been adjuited to’7.2 by addition of,‘&OH.* .: to
faqilitate formation of Mn&2 ti’jxeference
arach&c acid: .The iihns. vjere ‘deposited-.& a su$
,: ‘. .’ ., .’ +jj:
;- :,.‘.
-..
‘.
. ..~
.’ ..- ;
: ,.,
.:
. ..
anisotropic l¢ered
at g = 2.0, salts. We deduce the,spiri state of manganese in MnAra from the multiplet splitting [4] ,h, the M.n 3s core level X-my phoi‘okmissio$,$ecJrum. The photoemitted -F/in 3s eIectron has a different biri&g .snergy if, the re-
-. which is t);pical of concen&ated:&2f
: arachidic acid, CHs(CHi) &OOFJ, bn the surface of .. l.O?M Ivin!J~;‘P20 made With d’isti&d water. ‘Tne ” brder to
The &L-n wx intro-
duced,&ectly into the spectrometer after pulling the last monztlayer. ‘The fh had a total’of seven monolayers. Additional experiments to characterize the MnAr, films wel-e performed.on multilayers of about 100 layers pulled on quartz. These include X-ray structural analysis ivhich shows that the matkrial we pull has layer structure with spacing of 33.8. This corresponds to two langths of Mn(Ar)2 chains, as expected from deposition in which the ionic ends of the fatty acid chains are in proximity (sokalled Y-layers [I] ). Chemical analysis of thk films indicates that the MI-&~ is in
’
_.~’ ,. -; ._-.
... .
_.
-’
:‘..
‘.,,
”
-
Volume 3 I, number 3
:
15 !&&I
CHElrIICAL PHYSICS LETTERS
PrOpOrtiOIlti t0 (2s t 1) as predicted
1975
by vzn Vlcck’s
theorem [7]. S is the total spin of the pattiaLly fiied d shell. AE = 6.62 ? 0.01 eV ir, MnF, [S] ivith S = 512, A/Z = 5.6 +-0.2 eV in MnF3 [9] with S = 2,
AE= 4.6 + 0.1 eV in Mn02 [IO] witiS= 312, and Al3 < 0.2 eVin various iron-group complexes [9l in the low~spi~ state. We observe a multiplet splitting of 6.4 f 0.1 eV in the Mn 3s spectrum of hfntil (fig. lb} and &&usby comparing this valile with the above values of LE conclude that the manganese atom in MnAr2 is in the high spin state with S = S/2. This result has implications about the structure of the m=nolayer
films of MnAr2. Et was already noticed
studiesby Langmuir and Schaefer [I I] that the presenceof someions in the subpiraseproduced
in early
remarkable rigidity in the fatty acid monolayer spread on this subphase. It was later observed that there were qualitative differences between the properties of monolayers spread on subphases containing Al or transition metal ions (which we denote by class I)., compared with those containing alkalis. or aikaline earths (class II). For class I, in certain regions of pH, there was a remarkable increase in area [ 121, increase in wettability of deposited films 1131, increased compressibility [12] and increased impenetrabiIity [14]. For class II subphases no such changes were observed. 107 97 87 77 To explain these properties it was suggested [ 121 that B!NDING ENERGY iev) there was a kind of polymcdc structure in the c&s 1 Fig. 1. Mn 3s X-ray photoelectron spectra of (a) hlnF2, from monolayers. It will be noted ;hzt the elements in class 1 ref. [6], shifted in energy to line up with (b) our measuretend to form coordination complexes. hfodels involving ment of spectrum of Mn&. The binding energy of hlnXr~ the linking of coordinated metal atoms have been proMn 3s has been calibrated against the C 1s Iin? of the gaphite posed for Al soaps [IS]. Such web-Iike structures presubstrate, whose binding energy is 284.7 eV. For the pursumably also occur for the transition-metal soap films pose of this paper the important point is the neu identity of the splittings [4] in MnFz and SInA+, and not the absolute and can intuitively axpiain the Fz2perties of class I bindiq energies. monolayers. For class Ii fiis it is assumed that simple metal-fatty acid molecular u&s are formed with little maining Mn 3s electron spin is exchange coupled bonding Setween the molecular units. parallel (7S) or antiparallel (5S) to the unpaired spins There is thus a question of the effect of the bondof thk outer Mn 3d electrons. T-NOstrong peaks aping in the transition-metal soaps on their magnetic pear in the binding eneqg spectrum corresponding to properties. It is well known [ 161 that in atoms conthese two situations (I&_ 1b). Severalyeher peaks obtaining five d electrons (e.g., Mnz’) the total spin served at higher binding energies result from ee corJJate can be either S = 5/Z (high) or S = I[2 QOW-) derelation between electrons in,the 3s and 3d shells pending ofi the strength of the crystal field. In “weak” and can be described in tern-k of configkation intercrystal fields, orbital degeneracy is nearly preserved action [S,6]. and Hund’s tie applies, giving S = 5/2. “Strong” McFeely et al. have shown that for a series of manfields split the orbital degeneracy sufficiently that ganese compounds with different spin configurations lower states are fried with pairs of electrons, leaving the multiplet splitting &E = @S) - EC/S) is directly :.’ i ndt 3 = l/2. Strong ligand fie!ds occur in strongly co.’ .;, ;
.
1
.
603
Vohime 31. nu’inber 3 ;’ valent
comp!=xes
CHEMICALPHYSICS
[ 161. TYIUS,our experiment
tiat
the Iigand fields are not “strong” in MnAr2
monokyers. We,wish to thank Mr. FFank Dzcol for creative contributions to the film pulling technique, and Mr. Frank Cardone foi performing the chemical analyses by the electron
niicfopiobe
method.,
‘. ‘.
Ref&ences
[ i] G-L. Gaines, Insoluble monolayers faces
(\%ley,
New
York,
at liquid,gas
inter-
1956).
121 LJ. de Jongh and A.P. Miedema, Advan. Phys..23 (1974) [3]
k-Sie@ihn;k. Nordtig ,A. Fahbrnar~ R. Nordberg K-. Hamrio, J. %dman, G. Jbhansson, T. IE)&+k, S.;. Ka&son, I. Lindgren and i. Lindberg, Electron‘sp~c-
_.” ._..
.,
15 Makh 1975
.’ tioscop~ for cliemiti.analysis (Almqvist kd Wiksalls, Stoc~olm, 1967) pp. 139-141.
answers
the question of the relative strength of the ligand fieldsjn Mn& soap flms. Tnz result that,S = 512. shows
LE-I-TEXS
[4]
C-S. Fndky,
kz Ekstron
spectro~opy,
cd_ D.A.
Shirley
(North-Holland, Amsterdam, 1972). [S] P.S. Bagus; A.]. Freeman and F. &s&i, Phys. Rev. Letters 30 (1973) 850. [6] S.P. Kow+lcz~:c, L. Ley, R.A. Pollak, F.R. McFeely and D.A. Shirley, Fhys:Rw. B7 (1973) 4039. [7] J.H. van Fleck, Phys. Rev. 45 (1934) 405. [S] F.R. McFeely, S.P. Kowalnyk, L. Ley and D.A. Shirley, Solid &te Commun. 15 (1974) 10.51. :9] J.C. C&&r, G.K. Schweitzei and T.A. Carlson, J. Chem. Fhys. 51 (197%) 973. [lo] C.S. Fadley anmiD.A. Shirley, Phys. Rev. A2 (1970) 1109. [ll] I. LugniuirandV.J. Schaefer, J. Ani. Chem. Sot. 59 (1937) 2400. T. Sa&i and F.. Matuura, Bull. Chcm. Soc:Japan 24 (1,051) 274. A. Inaba, Bull. Cbem. Soc.Japan 2.5 (1952) 174. R. Matuux, BLU. CT-em. Sot Jnp~l24 (1951) 2.78. C.G. McGee, J. Am. Chem. Sot. 71 (1949) 278;
V.R. GTayand A.E. Alexarider,J. phys. Colloid. Chem. 53 (1949) 9. 1161C.J. Ballhauser., Intioduction to ligand field theory (BIcGraw-Hi& New York, 1962).