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Spin flopping transition in a linear chain-like organic free radical, tanol, below 0.5 K
Volume 70. number
1
CHEMICAL
SPIN FLOPPING TRANSlTION TANOL, BELOW 0.5 K Masafuml
KUMANO
and Yusaku
IN A LINEAR CHAIN-LIKE
3i October
ORGANIC
15 February 1980
FREE RADICAL,
LKECAMI
Ciwrnrcal Research Institute of Non-Aqueous Katahrm 2-I -1, Setrdal980. Japan Recewed
PHYSICS LETTERS
Sohrtrotx, Tohoku Utzwerstty.
1979
A spm flopping transltlon m a lmear cham-hhe orgaruc free radical, TANOL, has been observed below 0 5 K The maxlmum value of the amsotropy energy was estimated to be 230 erg/cm3. The origm of the magnetx amsotropy IS discussed The temperature dependence of the floppmg field IS also reported
TANOL (4-hydroxy-2,2,6,6-tetramethylpiperrdinoxy) IS one of the model compounds havmg an anrlferromagnetic hear cham [ 1 ] _Characterlstlc behavior of the linear chain appears tn the heat capacity [2] and the magnetic susceptibtity [3,4] as a broad maxunum at around 4 K due to the Helsenberg exchange mteractlon in one dimension (I e short-range ordermg) wth J/k = 4.16 K. The magnetic lmear cham IS expected to
c-axis [5,6] _The mterdoes not vanish m the actual materials, causes an antlferromagnetic long-range ordermg at 0 49 K and J./J IS es& mated to be l/i80 [7,8]. The temperature dependence of the sublattlce magnetlzatton measured with the NMR method efiblted a T’ dependence with lowermg of the temperature to 0 K [S]. To obtain a more detalled profile on the magnetic amsotropy, magnetic measurements were performed m an external magnetic be along the crystallograpluc cham exchange
InteractIon
J’. which generally
field.
The smgle crystal of TANOL IS set W&I its b-axis parallel to the applied magnetic field and cooled down below 0.1 K using a 3He/4He dllutlon refrigerator *. Cerium magnesium nitrate (CMN) mounted on the rmxing chamber was used as a low-temperature therrnometer, which was calibrated by a germanium reslstdnce thermometer at tigher temperatures. To maintain the thermometer free from the magnetic flux, it takes * A SHE ddutlon refrigerator Tohoku
114
Umversity,
system m the Cryogemc was used for the measurements
Center,
about 45 cm between the thermometer and the sample. To mmunize the temperature difference between the sample and the thermometer, the sample IS duectly immersed m the superfluid phase of 4He, as shown II-I fig. 1. (Though the mixing chamber could be used as a sample cell, a trace amount of 3He causes a vertltally dlstrlbuted temperature gradient [9] _) Magnetic susceptlblhty measurements were performed with a semi-automatic mutual inductance bndge of the Hartshorn type operated at 80 Hz [lo]. To ehmmate the magnetic dipole coupling between the superconducting solenoid and the sample cod, the pnmary wmdlng of the sample co11 was cyhndrlcally split into two parts so as to satisfy the relation r1 12: = r2rzz, where I-, and t-* are the diameters and nl and n2 the number of turns of the Inner and outer cyhnder, respectively, and then the two wmdmgs were connected III series but Inverse UI polarity. With this procedure, the net altematmg pnmary wmdmgs,
magnetic
field produced
by the
exists only inside the umer cyhnder and no net dipole field was produced outslde the co11 Fig. 2 shows a typical recorder trace of x’ as a function of the apphed external magnetic field. In the lowest temperature region, a sharp peak appeared at about 2.0 kOe, which corresponds to the spin flopping transltion. When the external magnetic field was apphed m the CICplane (perpendicular to the b-axis), spm floppmg was also observed at about 2 2 kOe usmg the NMR method [I l] . So the easy axis does not coincide with each of the three crystallographc axes, but 1s at
I
Volume 70, number
1
CHEMICAL
PHYSICS
LEITERS
15 February lk
2k
1980
3k Q?
I-
Fig 1 Sample cell for magnetic measurements In a magnetic field. Pure 4He was introduced through the capillary tube (B) into the lower portlon of the cell(F). made of quartz tube SuperfluId 4He unmerses the sample (H) and the bundle of copper wlze (C) attached to the top copper plate (A). which 1s further brought mto thermal equllibrlum with the mlxmg chamber Indnun-sealed flange (D) IS used for introducmg the sample mto the celi (E) is a Kovar seal (G) IS the SWIPI.Z COII and (I) IS the sample holder made of TFE.
an angle of some degrees with the ac plane. To estimate the amsotropuz energy m TANOL, orthorhomblc amsotropy IS assumed Further, the floppmg field,Hf, when Ho 1s parallel to the easy axis, IS assumed to be the same order of magmtude as that when Ho is parallel to the b+xus, because the difference m the flopping field u1 the cases of Ho parallel to the b-axis and of Ho apphed in the &z plane is small. Then the flopping field IS expressed as Hf = (2KA )‘/2,
(1)
where K is an anisotropy constant and A is the molecular field constant. In the conventional antlferromagnet,A is approximately equal to (Q - x,,)-1, in the hmlt of 0 K, which IS estimated from the susceptlbility of the powdered sample, xp as follows: x1 - XII= Xl =+ X,.
(2)
Fig. 2 Typical recorder traces of x’. The weak peak at 1 kOe corresponds to the critical field of solder used for the onnectlons of the co11leads
The magnetic susceptlbihty of the powdered sample of TANOL is reported to be about 0 01 cgs emu/m01 at 0.2 K [ 121. So the minunum value of the molecular field constant IS obtained by neglecting the parallel susceptibility at 0.2 K to be 1 .OS X 10m4 cm3/cgs emu and the maximum K value is determined from eq. (I) to be 230 erg/cm3, which corresponds to 6.03 X 10S20 erg/spin. The magnetic amsotropy is mainly contriiuted from both the magnetic dipole-dipole interaction and the anisotropic exchange Interaction. The lattice sums of the magnetic
dlpole4pole
interaction
do not
vanish when the lattice lacks cubic symmetry. In the case of TANOL, 6 X lo-20 erg/spin is estimated for the &pole-dipole interaction, where the magnetic moment due to TANOL’s spin is assumed to be 037
Bohr magneton from NMR measurements [II]. Magnetlc anisotropic exchange mteraction, caused by the spin-orbit interactlon, is approximately expressed as (g - 2 .0023)2J. EPR measurements
(3) showed
the observed
g-vaIue to be
Volume 70, number
gll -g1 =67X
1
10-3,
CHEMICAL PHYSICS LETTERS
g,=2002
Then, the anrsotroprc exchange energy IS about 8 6 X IO-20 erg/spur At present, It cannot be decrded whrch of the two types of interactron dommates the magnetrc anisotropy The temperature dependence of the spur flopping transitron is shown m fig.2. On ratsmg the temperature, Hf decreases from 2 0 kOe to 0 kOe at about 0.5 K Thts behavror cannot be observed m conventional threedrmensronal spm systems, whrch have a posrtrve temperature coefficrent. Recently, de Jonge et al. found the same characterrstrc behavror m the S = 5/2 spur system [13]. For analysrs of the magnetrc phase dragram, tt IS necessary to know the zero-field spin arrangement. Unfortunately, for TANOL the easy axrs has not yet been found, and detarled study of thus pomt IS under way. The authors wash to acknowledge the assrstance of Dr. S. Sakatsume of the Cryogemc Center, Tohoku University, for the operation of the dllutton refrrgerator.
116
15 February
1980
References [I J Yu S Kanmov and E.G. Rosantzev. [2] [3] [4j
(5 ] [6] [7] 181 [9]
Soviet Phys. Sobd State 8 (I 967) 2225 H. Lematre, P Rey. A Rassat, A de Combaneu and J C. Mtchel, Mol Phys. 14 (1968) 201. Yu S Kanmov, Sonet Phys JETP 30 (1970) 1062 J Yamauchr, T FuJtto, E. Ando, H Ntshrguchr and Y Degucht, J. Phys Sot Japan 25 (1968) 1558 M Kumano and Y. Ikegami, Chem Phys. Letters 54 (1978) 109. J P. Boucher, F Femeu and M Nechtschem, Phys Rev. B9 (1974) 3871. J P Boucher, N. Nechtschem and M. Samt Paul, Phys. Letters 42A (1973) 397 M Kumano, Y Ikegamt, T. Sato and S. Satto, Chem Phys Letters 52 (1977) 497 M. Kumano, Y Ikegamt, T Sato and S Saito, Rev SCI tnstr. 50 (1979) 24
[JO] M. Kumano
and Y Ikegaml, Rev SCI Instr. 50 (1979) 921 i I 1 J M Kumano, Thess, Tohoku Unwerstty (1976). [ 12 1 hl Kumano, Y Ikegami. T Sato and S. Satto. Chem Phys Letters41 (1976) 354 [ 131 W J 111de Jonge, J P_A M. HlJmans. F Boersman. J-C. Schouten and K. Kopmga, Phys Rev. B17 (1978) 2922.
1
CHEMICAL
SPIN FLOPPING TRANSlTION TANOL, BELOW 0.5 K Masafuml
KUMANO
and Yusaku
IN A LINEAR CHAIN-LIKE
3i October
ORGANIC
15 February 1980
FREE RADICAL,
LKECAMI
Ciwrnrcal Research Institute of Non-Aqueous Katahrm 2-I -1, Setrdal980. Japan Recewed
PHYSICS LETTERS
Sohrtrotx, Tohoku Utzwerstty.
1979
A spm flopping transltlon m a lmear cham-hhe orgaruc free radical, TANOL, has been observed below 0 5 K The maxlmum value of the amsotropy energy was estimated to be 230 erg/cm3. The origm of the magnetx amsotropy IS discussed The temperature dependence of the floppmg field IS also reported
TANOL (4-hydroxy-2,2,6,6-tetramethylpiperrdinoxy) IS one of the model compounds havmg an anrlferromagnetic hear cham [ 1 ] _Characterlstlc behavior of the linear chain appears tn the heat capacity [2] and the magnetic susceptibtity [3,4] as a broad maxunum at around 4 K due to the Helsenberg exchange mteractlon in one dimension (I e short-range ordermg) wth J/k = 4.16 K. The magnetic lmear cham IS expected to
c-axis [5,6] _The mterdoes not vanish m the actual materials, causes an antlferromagnetic long-range ordermg at 0 49 K and J./J IS es& mated to be l/i80 [7,8]. The temperature dependence of the sublattlce magnetlzatton measured with the NMR method efiblted a T’ dependence with lowermg of the temperature to 0 K [S]. To obtain a more detalled profile on the magnetic amsotropy, magnetic measurements were performed m an external magnetic be along the crystallograpluc cham exchange
InteractIon
J’. which generally
field.
The smgle crystal of TANOL IS set W&I its b-axis parallel to the applied magnetic field and cooled down below 0.1 K using a 3He/4He dllutlon refrigerator *. Cerium magnesium nitrate (CMN) mounted on the rmxing chamber was used as a low-temperature therrnometer, which was calibrated by a germanium reslstdnce thermometer at tigher temperatures. To maintain the thermometer free from the magnetic flux, it takes * A SHE ddutlon refrigerator Tohoku
114
Umversity,
system m the Cryogemc was used for the measurements
Center,
about 45 cm between the thermometer and the sample. To mmunize the temperature difference between the sample and the thermometer, the sample IS duectly immersed m the superfluid phase of 4He, as shown II-I fig. 1. (Though the mixing chamber could be used as a sample cell, a trace amount of 3He causes a vertltally dlstrlbuted temperature gradient [9] _) Magnetic susceptlblhty measurements were performed with a semi-automatic mutual inductance bndge of the Hartshorn type operated at 80 Hz [lo]. To ehmmate the magnetic dipole coupling between the superconducting solenoid and the sample cod, the pnmary wmdlng of the sample co11 was cyhndrlcally split into two parts so as to satisfy the relation r1 12: = r2rzz, where I-, and t-* are the diameters and nl and n2 the number of turns of the Inner and outer cyhnder, respectively, and then the two wmdmgs were connected III series but Inverse UI polarity. With this procedure, the net altematmg pnmary wmdmgs,
magnetic
field produced
by the
exists only inside the umer cyhnder and no net dipole field was produced outslde the co11 Fig. 2 shows a typical recorder trace of x’ as a function of the apphed external magnetic field. In the lowest temperature region, a sharp peak appeared at about 2.0 kOe, which corresponds to the spin flopping transltion. When the external magnetic field was apphed m the CICplane (perpendicular to the b-axis), spm floppmg was also observed at about 2 2 kOe usmg the NMR method [I l] . So the easy axis does not coincide with each of the three crystallographc axes, but 1s at
I
Volume 70, number
1
CHEMICAL
PHYSICS
LEITERS
15 February lk
2k
1980
3k Q?
I-
Fig 1 Sample cell for magnetic measurements In a magnetic field. Pure 4He was introduced through the capillary tube (B) into the lower portlon of the cell(F). made of quartz tube SuperfluId 4He unmerses the sample (H) and the bundle of copper wlze (C) attached to the top copper plate (A). which 1s further brought mto thermal equllibrlum with the mlxmg chamber Indnun-sealed flange (D) IS used for introducmg the sample mto the celi (E) is a Kovar seal (G) IS the SWIPI.Z COII and (I) IS the sample holder made of TFE.
an angle of some degrees with the ac plane. To estimate the amsotropuz energy m TANOL, orthorhomblc amsotropy IS assumed Further, the floppmg field,Hf, when Ho 1s parallel to the easy axis, IS assumed to be the same order of magmtude as that when Ho is parallel to the b+xus, because the difference m the flopping field u1 the cases of Ho parallel to the b-axis and of Ho apphed in the &z plane is small. Then the flopping field IS expressed as Hf = (2KA )‘/2,
(1)
where K is an anisotropy constant and A is the molecular field constant. In the conventional antlferromagnet,A is approximately equal to (Q - x,,)-1, in the hmlt of 0 K, which IS estimated from the susceptlbility of the powdered sample, xp as follows: x1 - XII= Xl =+ X,.
(2)
Fig. 2 Typical recorder traces of x’. The weak peak at 1 kOe corresponds to the critical field of solder used for the onnectlons of the co11leads
The magnetic susceptlbihty of the powdered sample of TANOL is reported to be about 0 01 cgs emu/m01 at 0.2 K [ 121. So the minunum value of the molecular field constant IS obtained by neglecting the parallel susceptibility at 0.2 K to be 1 .OS X 10m4 cm3/cgs emu and the maximum K value is determined from eq. (I) to be 230 erg/cm3, which corresponds to 6.03 X 10S20 erg/spin. The magnetic amsotropy is mainly contriiuted from both the magnetic dipole-dipole interaction and the anisotropic exchange Interaction. The lattice sums of the magnetic
dlpole4pole
interaction
do not
vanish when the lattice lacks cubic symmetry. In the case of TANOL, 6 X lo-20 erg/spin is estimated for the &pole-dipole interaction, where the magnetic moment due to TANOL’s spin is assumed to be 037
Bohr magneton from NMR measurements [II]. Magnetlc anisotropic exchange mteraction, caused by the spin-orbit interactlon, is approximately expressed as (g - 2 .0023)2J. EPR measurements
(3) showed
the observed
g-vaIue to be
Volume 70, number
gll -g1 =67X
1
10-3,
CHEMICAL PHYSICS LETTERS
g,=2002
Then, the anrsotroprc exchange energy IS about 8 6 X IO-20 erg/spur At present, It cannot be decrded whrch of the two types of interactron dommates the magnetrc anisotropy The temperature dependence of the spur flopping transitron is shown m fig.2. On ratsmg the temperature, Hf decreases from 2 0 kOe to 0 kOe at about 0.5 K Thts behavror cannot be observed m conventional threedrmensronal spm systems, whrch have a posrtrve temperature coefficrent. Recently, de Jonge et al. found the same characterrstrc behavror m the S = 5/2 spur system [13]. For analysrs of the magnetrc phase dragram, tt IS necessary to know the zero-field spin arrangement. Unfortunately, for TANOL the easy axrs has not yet been found, and detarled study of thus pomt IS under way. The authors wash to acknowledge the assrstance of Dr. S. Sakatsume of the Cryogemc Center, Tohoku University, for the operation of the dllutton refrrgerator.
116
15 February
1980
References [I J Yu S Kanmov and E.G. Rosantzev. [2] [3] [4j
(5 ] [6] [7] 181 [9]
Soviet Phys. Sobd State 8 (I 967) 2225 H. Lematre, P Rey. A Rassat, A de Combaneu and J C. Mtchel, Mol Phys. 14 (1968) 201. Yu S Kanmov, Sonet Phys JETP 30 (1970) 1062 J Yamauchr, T FuJtto, E. Ando, H Ntshrguchr and Y Degucht, J. Phys Sot Japan 25 (1968) 1558 M Kumano and Y. Ikegami, Chem Phys. Letters 54 (1978) 109. J P. Boucher, F Femeu and M Nechtschem, Phys Rev. B9 (1974) 3871. J P Boucher, N. Nechtschem and M. Samt Paul, Phys. Letters 42A (1973) 397 M Kumano, Y Ikegamt, T. Sato and S. Satto, Chem Phys Letters 52 (1977) 497 M. Kumano, Y Ikegamt, T Sato and S Saito, Rev SCI tnstr. 50 (1979) 24
[JO] M. Kumano
and Y Ikegaml, Rev SCI Instr. 50 (1979) 921 i I 1 J M Kumano, Thess, Tohoku Unwerstty (1976). [ 12 1 hl Kumano, Y Ikegami. T Sato and S. Satto. Chem Phys Letters41 (1976) 354 [ 131 W J 111de Jonge, J P_A M. HlJmans. F Boersman. J-C. Schouten and K. Kopmga, Phys Rev. B17 (1978) 2922.