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Pressure dependence of magnetic properties of Fe4N and Mn4N
MI
Journal of Magnetism and Magnet ic Materials 104-107 (1992) i 935-1936 North-Holland
Pressure dependence of magnetic properties of Fe4N and Mn4N J.G.M. Armitage ", R.G. Graham '~, J.S. Lord 4, P.C. Riedi 4, S.F. Matar b and G. Demazeau b a ZF. Allen Laboratories, Departmalt of Physics & Astronomy, Unicersi~. of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, Scotland, UK b Laboratoire de Chimie du Solide du CNRS, Unit'ersit# Bordeato:.l, 351 course de la Lib#ration, 33405, Talence, France
The value of the magnetization of ferromagnetic Fe4N is found to decrease under pressure while that of ferrimagnetic
Mn4N increases. The magnitude of the effective field at the 55Mn nucleus of the Mn(l) site of Mn~N also increases under pressure. It is deduced that the magnitude of the moment at the Mn(ll) site decreases under pressure, while that of the Mn(l) site is independent of pressure, in agreement with the model of covalent magnetism for Mn4N. There has been renewed interest in the transition metal nitrides recently because Fe4 N has been investigated as a possible material for recording (e.g. ref. [1]). The nitrides are however of more general interest because they contain two inequivalent magnetic sites, the formula may be written M(I) M3(II) N, where M is Fe or Mn, which have very different bonding. The atom at the M(I) site has hardly any interaction with the N and a large moment 3.0g a (3.85ga) for Fe(Mn) while there is covalent bond between M(II) and nitrogen leading to moments of 2.0tz a ( - 0 . 9 0 g o ) for Fe(Mn) [2-5]. Since the properties of Fe and Mn have been drastically changed by the introduction of N, which leads to ,-, 30% increase in volume, it is particularly interesting to look at the magnetic properties of the nitrides as a function of pressure. The pressure dependence of the magnetization of F e r N and Mn4N was dcduccd from forced magnctostriction measurements at 4.2 K and the pressure dependence of the effective field at the SSMn nucleus of the Mn(I) site measured at room temperature, 77 and 4.2 K. We interpret the results to show that the magnitude of the magnetic moment at the Mn(II) site decreases under pressure, as does the N6el temperature. while the moment at the Mn(I) site is indcpcnden, of pressure in agreement with the theoretical picture of covalent magnetism in these materials [2]. A series of powder samples of composition close to Mn4N was examined using a swept-frequency N M R spin echo spectrometer [6]. The Mn(I) site has cubic symmetry and a narrow 55Mn resonance line is observed near i34 MHz at 4.2 K (fig. i) in agreement with earlier work [7]. Additional lines were always observed below 134 MHz, indicating vacant N sites, but in the present paper we will only discuss the sample, whose spectrum is shown in fig. 1, which had the lowest intensity satellite lines. The Mn(II) site has noncubic symmetry and a very broad (20-60 MHz) spectrum which will not be considered here. The pressure measurements were performed in a
liquid filled B e - C u pressure ceil. The pressure was applied at room temperature and locked into the cell before it was placed in liquid nitrogen or helium. Thc pressure in the cell was measured using the resistance of a calibrated semiconductor pressure transducer. There was no evidence of nonhydrostatic broadening of the 55Mn N M R line under pressure. A check on the semiconductor pressure transducer was obtained from the 59Co N M R of a small quantity of fcc cobalt powder in the same coil as the Mn4N. The pressure dependence of the 55Mn NMR of Mn4N is shown in fig. 2 and the results summarized in table 1. Forced volume magnetostriction measurements were carried out at 4.2 K on Fc4N and Mn4N in thc form of a thin disc of pressed powder. The change of length of the disc in fields up to 12 T was ~neasurcd using a capacitance cell that has bccn dcscribcd prcviously [8,9]. Thc forced volume magnctostriction may bc uscd to derive the pressure dependence of the magnetization per unit mass (or) via the thermodynamic Maxwell relation ( 0 o r r / 0 P ) = - ( l / p ) (a In V / O H ) r where p is
110
120150 135 U (MHz) Fig. 1. The 55Mn NMR spectrum of Mn4N at 4.2 K and a pressure of 3.5 kbar. The main line is due to the Mn(l) site and the satellite line due to Mn next to nitrogen defects.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
115
J.G.M. Armitage et al. / Pressure dcT)emh'nce of properties of kk"4 N, Mn 4N
1936 1'o
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2 4 6 8 10 Pressure (kbar)
12
Fig. 2. The reduced 55Mn NMR frequency of the Mn(l) site of Mn4 N as a function of pressure at three temperatures.
the density. The volutnc of Fe4N at 4.2 K was found to bc a lincar function of field with (0 In I//OH) r = 54.4 x 10- (' T - i leading to (0 In (ro/aP) = - 3 . 6 Mbar The V - H plot for Mn4N was nonlinear but it was estimated that (i:) In V/OH). r = - 1 . 0 x 10 -(' T - t , (a In cro/OP) = + 1.0 M b a r - I The 55Mn N M R frcqucncy z, of the Mn(l) sitc of Mn4N was found to be a linear function of prcssurc (fig. 2) but thc value of (0 In v/OP) r was a function of temperature. The N6cl temperature of Mn4N is 755 K and by room temperature the bulk magnetization has dccrcascd to 0.69, whereas the Mn(1) moment has dccrcascd to 0.88, of thc value at 0 K [5]. Thc prcssurc dcpcndcncc of the Mn moments at room tcmpcraturc, and of the NMR frequency, will thcrcforc d c p c n d upon the valJc of (i)In Tx/i)P). Using thc thermodynamic rclation ((3 in u / O P ) r = (0 in v / a P ) o - ( 0 In u / In TX0 In TN/aP), scc c.g. rcf. [10], wc find that InTN/OP = 5.2 M b a r - i . Thc cffcct of pressure on Table ! The SSMn NMR frequency of the Mn(1) site of Mn4N ;is a function of tumperature and pressure T (K)
z, (MHz)
(0 In ~,/OP) (Mbar - t)
290 77 4.2
I 17,12 132.66 133.82
I. 12 + 11.1)2 0.25 + 0.02 0.17 + 0.02
ferromagnetic Fe4N is to decrease the magnetization at 0 K, as expected for a band fcrromagnct, but is to increase the nctt magnetization of fcrrimagnctic Mn4N and thc magnitude of the hypcrfine field at the nuclcus of the Mn(1) site. Since the Mn(1) site is rather isolated in the crystal it is plausible that the large moment on this site is i n d e p e n d e n t of pressure, while the magnitude of the antiparallcl moment on the M n ( l l ) site decreases under pressure, in qualitative agreement with the electron distribution found using the augmented spherical-wave m e t h o d [2]. It would bc very interesting to repeat this calculation as a function of lattice spacing. Thc effective field at the SSMn nucleus of the Mn(l) site of M n 4 N is negative, i.e. antiparallel to the Mn(I) moment and nett magnetization, as is usual for a contact interaction. Assuming that all the pressure dependence of the magnetization and Mn(I) effective field at 4.2 K is due to the decrease of the Mn(II) moment with pressure, the contribution to the effective field from the parent Mn and the transferred field from neighbouring Mn are estimated to be - 17.8 and +5.1 T, respectively. These values are plausible but the band calculations [2] are not yet accurate enough to provide an estimate of the contributions to the cffcctivc ficlds of Mn4N.
References
[1] G. Demazeau, D. Andriamandroso, M. Pouchard, B. Tanguy and P. Hagenmuller, C.R. Hebd. S6an, Acad. Sci 11 297 11983) 843. [2] S. Matar, P. Mohn, G. Demazeau and B. Siberchicot, J. de Phys. 49 11988) 1761. [3] B.C. Frazer, Phys. Rev. 112 (1958) 751. [4] W.J. Takei, R.R. Heikes and G. Shirane, Phys. Rev. 125 11962) 1893. [5] D. Fruchart, D. Givord, P. Convert, P. L'H~ritier and J.P. S6nateur, J. Phys. F. 9 (1979) 2431. [6] T. Dumelow and P.C. Riedi, Hyperfine Interactions 35 (1987) 1061. [7] M. Matsuura, J. Phys. Soc. Jpn. 2i 11966) 886. [8] J.G.M. Armitage, T. Dumelow, P.C. Riedi and J.S. Abell, J. Phys.: Condens. Matter ! 11989) 3987. [9] S.F. Matar, G. Demazeau, P. Hagenmuller, J.G.M. Armitage and P.C. Riedi, Eur. J. Solid State lnorg. Chem. ,'o (1989) 517. [I0] P.C. Riedi, Phys. Rev. B 24 11981) 1593.
Journal of Magnetism and Magnet ic Materials 104-107 (1992) i 935-1936 North-Holland
Pressure dependence of magnetic properties of Fe4N and Mn4N J.G.M. Armitage ", R.G. Graham '~, J.S. Lord 4, P.C. Riedi 4, S.F. Matar b and G. Demazeau b a ZF. Allen Laboratories, Departmalt of Physics & Astronomy, Unicersi~. of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, Scotland, UK b Laboratoire de Chimie du Solide du CNRS, Unit'ersit# Bordeato:.l, 351 course de la Lib#ration, 33405, Talence, France
The value of the magnetization of ferromagnetic Fe4N is found to decrease under pressure while that of ferrimagnetic
Mn4N increases. The magnitude of the effective field at the 55Mn nucleus of the Mn(l) site of Mn~N also increases under pressure. It is deduced that the magnitude of the moment at the Mn(ll) site decreases under pressure, while that of the Mn(l) site is independent of pressure, in agreement with the model of covalent magnetism for Mn4N. There has been renewed interest in the transition metal nitrides recently because Fe4 N has been investigated as a possible material for recording (e.g. ref. [1]). The nitrides are however of more general interest because they contain two inequivalent magnetic sites, the formula may be written M(I) M3(II) N, where M is Fe or Mn, which have very different bonding. The atom at the M(I) site has hardly any interaction with the N and a large moment 3.0g a (3.85ga) for Fe(Mn) while there is covalent bond between M(II) and nitrogen leading to moments of 2.0tz a ( - 0 . 9 0 g o ) for Fe(Mn) [2-5]. Since the properties of Fe and Mn have been drastically changed by the introduction of N, which leads to ,-, 30% increase in volume, it is particularly interesting to look at the magnetic properties of the nitrides as a function of pressure. The pressure dependence of the magnetization of F e r N and Mn4N was dcduccd from forced magnctostriction measurements at 4.2 K and the pressure dependence of the effective field at the SSMn nucleus of the Mn(I) site measured at room temperature, 77 and 4.2 K. We interpret the results to show that the magnitude of the magnetic moment at the Mn(II) site decreases under pressure, as does the N6el temperature. while the moment at the Mn(I) site is indcpcnden, of pressure in agreement with the theoretical picture of covalent magnetism in these materials [2]. A series of powder samples of composition close to Mn4N was examined using a swept-frequency N M R spin echo spectrometer [6]. The Mn(I) site has cubic symmetry and a narrow 55Mn resonance line is observed near i34 MHz at 4.2 K (fig. i) in agreement with earlier work [7]. Additional lines were always observed below 134 MHz, indicating vacant N sites, but in the present paper we will only discuss the sample, whose spectrum is shown in fig. 1, which had the lowest intensity satellite lines. The Mn(II) site has noncubic symmetry and a very broad (20-60 MHz) spectrum which will not be considered here. The pressure measurements were performed in a
liquid filled B e - C u pressure ceil. The pressure was applied at room temperature and locked into the cell before it was placed in liquid nitrogen or helium. Thc pressure in the cell was measured using the resistance of a calibrated semiconductor pressure transducer. There was no evidence of nonhydrostatic broadening of the 55Mn N M R line under pressure. A check on the semiconductor pressure transducer was obtained from the 59Co N M R of a small quantity of fcc cobalt powder in the same coil as the Mn4N. The pressure dependence of the 55Mn NMR of Mn4N is shown in fig. 2 and the results summarized in table 1. Forced volume magnetostriction measurements were carried out at 4.2 K on Fc4N and Mn4N in thc form of a thin disc of pressed powder. The change of length of the disc in fields up to 12 T was ~neasurcd using a capacitance cell that has bccn dcscribcd prcviously [8,9]. Thc forced volume magnctostriction may bc uscd to derive the pressure dependence of the magnetization per unit mass (or) via the thermodynamic Maxwell relation ( 0 o r r / 0 P ) = - ( l / p ) (a In V / O H ) r where p is
110
120150 135 U (MHz) Fig. 1. The 55Mn NMR spectrum of Mn4N at 4.2 K and a pressure of 3.5 kbar. The main line is due to the Mn(l) site and the satellite line due to Mn next to nitrogen defects.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
115
J.G.M. Armitage et al. / Pressure dcT)emh'nce of properties of kk"4 N, Mn 4N
1936 1'o
? 90i,,/
C7) 0
o10
£
./
//-, /
/
l
T-,r--v-",
0
I-
,
~
~
i
'
~
2 4 6 8 10 Pressure (kbar)
12
Fig. 2. The reduced 55Mn NMR frequency of the Mn(l) site of Mn4 N as a function of pressure at three temperatures.
the density. The volutnc of Fe4N at 4.2 K was found to bc a lincar function of field with (0 In I//OH) r = 54.4 x 10- (' T - i leading to (0 In (ro/aP) = - 3 . 6 Mbar The V - H plot for Mn4N was nonlinear but it was estimated that (i:) In V/OH). r = - 1 . 0 x 10 -(' T - t , (a In cro/OP) = + 1.0 M b a r - I The 55Mn N M R frcqucncy z, of the Mn(l) sitc of Mn4N was found to be a linear function of prcssurc (fig. 2) but thc value of (0 In v/OP) r was a function of temperature. The N6cl temperature of Mn4N is 755 K and by room temperature the bulk magnetization has dccrcascd to 0.69, whereas the Mn(1) moment has dccrcascd to 0.88, of thc value at 0 K [5]. Thc prcssurc dcpcndcncc of the Mn moments at room tcmpcraturc, and of the NMR frequency, will thcrcforc d c p c n d upon the valJc of (i)In Tx/i)P). Using thc thermodynamic rclation ((3 in u / O P ) r = (0 in v / a P ) o - ( 0 In u / In TX0 In TN/aP), scc c.g. rcf. [10], wc find that InTN/OP = 5.2 M b a r - i . Thc cffcct of pressure on Table ! The SSMn NMR frequency of the Mn(1) site of Mn4N ;is a function of tumperature and pressure T (K)
z, (MHz)
(0 In ~,/OP) (Mbar - t)
290 77 4.2
I 17,12 132.66 133.82
I. 12 + 11.1)2 0.25 + 0.02 0.17 + 0.02
ferromagnetic Fe4N is to decrease the magnetization at 0 K, as expected for a band fcrromagnct, but is to increase the nctt magnetization of fcrrimagnctic Mn4N and thc magnitude of the hypcrfine field at the nuclcus of the Mn(1) site. Since the Mn(1) site is rather isolated in the crystal it is plausible that the large moment on this site is i n d e p e n d e n t of pressure, while the magnitude of the antiparallcl moment on the M n ( l l ) site decreases under pressure, in qualitative agreement with the electron distribution found using the augmented spherical-wave m e t h o d [2]. It would bc very interesting to repeat this calculation as a function of lattice spacing. Thc effective field at the SSMn nucleus of the Mn(l) site of M n 4 N is negative, i.e. antiparallel to the Mn(I) moment and nett magnetization, as is usual for a contact interaction. Assuming that all the pressure dependence of the magnetization and Mn(I) effective field at 4.2 K is due to the decrease of the Mn(II) moment with pressure, the contribution to the effective field from the parent Mn and the transferred field from neighbouring Mn are estimated to be - 17.8 and +5.1 T, respectively. These values are plausible but the band calculations [2] are not yet accurate enough to provide an estimate of the contributions to the cffcctivc ficlds of Mn4N.
References
[1] G. Demazeau, D. Andriamandroso, M. Pouchard, B. Tanguy and P. Hagenmuller, C.R. Hebd. S6an, Acad. Sci 11 297 11983) 843. [2] S. Matar, P. Mohn, G. Demazeau and B. Siberchicot, J. de Phys. 49 11988) 1761. [3] B.C. Frazer, Phys. Rev. 112 (1958) 751. [4] W.J. Takei, R.R. Heikes and G. Shirane, Phys. Rev. 125 11962) 1893. [5] D. Fruchart, D. Givord, P. Convert, P. L'H~ritier and J.P. S6nateur, J. Phys. F. 9 (1979) 2431. [6] T. Dumelow and P.C. Riedi, Hyperfine Interactions 35 (1987) 1061. [7] M. Matsuura, J. Phys. Soc. Jpn. 2i 11966) 886. [8] J.G.M. Armitage, T. Dumelow, P.C. Riedi and J.S. Abell, J. Phys.: Condens. Matter ! 11989) 3987. [9] S.F. Matar, G. Demazeau, P. Hagenmuller, J.G.M. Armitage and P.C. Riedi, Eur. J. Solid State lnorg. Chem. ,'o (1989) 517. [I0] P.C. Riedi, Phys. Rev. B 24 11981) 1593.