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The pulsed fields in studies of spinels
Physica B 155 (1989) 400-402 North-Holland. Amsterdam
THE PULSED
FIELDS IN STUDIES
OF SPINELS
J. JUSZCZYK
The pulsed field technique is discussed for studies high-field susceptibility as well as magnetocrystalline
of magnetization. magnetic phase transitions induced by strong anisotropy for three chosen chromium spine1 systems.
1. Introduction A few years ago we described a pulsed field apparatus developed in our Institute [ 11. It consists of a 100 kJ capacitor bank (2 mF, maximum charging voltage 10 kV) and could produce magnetic fields up to 50 T with a pulse duration longer than 10 ms. At present the 56 kJ capacitor bank (7 mF. maximum charging voltage 4 kV) can produce magnetic fields up to 50T with a pulse duration less than 10 ms and up to 30T with a pulse duration longer than 10 ms. The latter is used in compounds in which we observe dynamic processes during their magnetization because of relaxation effects. This pulsed technique is used by us mainly in the studies of the magnetic properties of the spine1 compounds. In this way we can discuss the magnetization curves, the magnetic phase transitions induced by strong fields. the high-field susceptibility and the magnetocrystalline anisotropy. Recently. Duda has started investigations of transport phenomena at this installation [2]. In this paper 1 present briefly some of the results of a discussion concerning the magnetic properties of three chosen chromium spine1 systems investigated (among others) in pulsed fields.
2. Magnetic spinels
properties
of the chosen
chromium
The object of our interest are the noncollinear chromium spinels, which need strong fields in the magnetization process. Usually we start the mag0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
fields.
netic investigations from the magnetization measurements in high stationary fields at 7‘= 4.2 K and 77 K in order to calibrate and check our results. Therefore in a short time we obtain precise magnetization curves of every sample in a wide temperature range. From these curves we calculate the magnetic moment as a function of temperature. Next, with the use of the pulsed fields we investigate the magnetic phase transitions induced by strong fields [3,4]. The phase transition points are easily seen by the time derivative signal of magnetization (dM/dt) versus the magnetic field. From these curves obtained for the spin (cases simple spiral structures 0.0 G X G 0.025 an d for Zn, I Cu,Cr,Se, Zn I~~rGa2r~~Cr,Se, for 0.0 G x s 0.5), it follows [5] that there are three transition fields separating four structures, namely simple spiral, cone, fan, and parallel alignment. Instead, similar studies of the conical structure, which appears in Zn ,_~,Cu,Cr,Se, for 0.05 G x s 0.7. indicate that the observed transitions concern changes from a through the alignment in the cone structure, meridian plane, an oscillation in this plane. to a parallel alignment. All critical fields between these structures are of the second kind. The values of these fields have been checked with the use of Nagamiya’s theory of magnetization in a screw spin system [5]. In further studies the higher derivatives of d’*M/df” are registrated in order to determine the anisotropy field [3,4, 6-81. From its value the anisotropy constant K, and the anisotropy energy E,a of axial symmetry were calculated. It B.V.
S. Juszczyk
40 1
I Pulsed fields in studies of spineis
the cases of in the out that Cu,Cr,Se, and Zn,_,Ga,,,,Cr?Se, sysZnlmx tems at T = 2.9 K, K, is negative and of the order of 10’ J/m’ and E, changes its sign, indicating that the direction in which E, reaches its minimal value has changed [6]. From a comparison of the exchange and anisotropy energies. it follows that the latter is two orders of magnitude less than the exchange energy. Therefore, the anisotropy energy may be considered as a perturbation (the transition from a simple spiral to a conical structure in Zn,_* Cu,Cr,Se, for 0.0~ x ~0.7 will be caused by the exchange energy and the anisotropy energy will stabilize only the c-axis in simple spiral and conical structures and fix the rotation plane of the magnetic moment vectors of the CT”+ ions (n = 2,3 and turned
4)). On the other hand in Co,, &u,,~,,CrlS,_,Se, spinels K, is positive and of the order of 10’ Jim’ at T = 300 K and the anisotropy energy is a few orders of magnitude less than the exchange one. The pulsed fields were indispensable in the magnetization process of Zn, ,GazI ,,Cr,Se, spinels [9]. because even in fields up to 4.5 T the compounds with x = 0.4 and 0.5 (polycrystals, because a single crystal sample with x = 0.4 reaches saturation) did not exhibit saturation. With the help of pulsed fields the differential high-field susceptibility has been measured in these spine1 systems [l&12]. Using Slater’s ideas on non-integer occupation numbers, numbers of 3d holes (as the carriers of magnetism in the investigated spinels) per magnetic atom (Cr, Co) with spins up and down have been obtained [13]. From the condition of self-consistency for the magnetic moment, the number of occupied states at the Fermi level with a given spin projection N(E,) has been calculated. With the use of the N(E,) values, the Pauli spin paramagnetic susceptibility for all these spine1 systems has been calculated and compared with other terms of the total high-field susceptibility (e.g. the van Vleck paramagnetic susceptibility, the Larmor and Landau diamagnetic susceptibility and the spin wave susceptibility). It turned out that the Pauli susceptibility is generally small and of the same order in the series with Cr as a magnetic atom
for the and instead is higher co I) &%.A5 Cr?S, ,.Se, series - here the magnetic atoms are Co and Cr. Therefore one can conclude that the electronic properties of the compounds under study are governed by the narrow 3d band of chromium and cobalt. In the end it is worth mentioning that the pulsed magnet and the pick-up coil system may be used to obtain, by the dynamic method in a weak mutual field, the critical temperatures of the investigated compounds [l]. This method is so sensitive that with its help one can separate precisely two magnetic or crystalline phases (one can check the purity of the sample too) from each other. The above-mentioned examples show the great power of the pulsed field technique in studies of these spine1 compounds. The results of the pulsed measurements allow us to choose the direction of our next investigations (with the use of other techniques) in order to fully explain the magnetic properties of the compounds under study.
only,
Acknowledgement This work was supported Polish Project CPBP 01.04.
in part
through
the
References [l] S. Juszczyk and H. Duda, J. Magn. Magn. Mat. 44 (1984) 133. [2] H. Duda, private communication. [3] S. Juszczyk, _I. Krok, I. Okonska-Kozlowska, H. Broda, J. Warczewski and P. Byszewski, Phase Transitions 2 (1981) 67. [4] S. Juszczyk. I. Okonska-Koziowska and P. Byszewski, Phase Transitions 4 ( lY84) 29 1. [S] T. Nagamiya, in: Solid State Physics. Vol. 18, F. Seitz and D. Turnbull, eds. (Academic, New York. lY64), p. 306. [6] S. Juszczyk. J. Magn. Magn. Mat. 74 (19X8) 330, 75 (198X) 280. 285. 171 J. Krok. S. Juszczyk, J. Warczewski, T. Mydlarz. W. Szamraj, A. Bombik, P. Byszewski and J. Spaiek, Phase Transitions 4 ( 1983) 1.
402
S. Juszczyk
I Pulsed fields in studies of spine/s
[g] M. Gogoiowicz, S. Juszczyk, J. Warczewski and T. Mydlarz, Phys. Rev. B 35 (1987) 7121. (91 S. Juszczyk, J. Krok, I. Okouska-KozJowska, T. Mydlarz and A. Gilewski, J. Magn. Magn. Mat. 46 (1984) 105.
[IO] S. Juszczyk, H. Duda, T. Mydlarz and J. Krok, Magn. Magn. Mat. 50 (1985) 209. [ll] S. Juszczyk, J. Map. Magn. Mat. 59 (1986) 69. [12] S. Juszczyk, J. Magn. Magn. Mat. 61 (1986) 295. [13] S. Juszczyk,
J. Magn.
Magn.
Mat. 73 (1988)
18.
J
THE PULSED
FIELDS IN STUDIES
OF SPINELS
J. JUSZCZYK
The pulsed field technique is discussed for studies high-field susceptibility as well as magnetocrystalline
of magnetization. magnetic phase transitions induced by strong anisotropy for three chosen chromium spine1 systems.
1. Introduction A few years ago we described a pulsed field apparatus developed in our Institute [ 11. It consists of a 100 kJ capacitor bank (2 mF, maximum charging voltage 10 kV) and could produce magnetic fields up to 50 T with a pulse duration longer than 10 ms. At present the 56 kJ capacitor bank (7 mF. maximum charging voltage 4 kV) can produce magnetic fields up to 50T with a pulse duration less than 10 ms and up to 30T with a pulse duration longer than 10 ms. The latter is used in compounds in which we observe dynamic processes during their magnetization because of relaxation effects. This pulsed technique is used by us mainly in the studies of the magnetic properties of the spine1 compounds. In this way we can discuss the magnetization curves, the magnetic phase transitions induced by strong fields. the high-field susceptibility and the magnetocrystalline anisotropy. Recently. Duda has started investigations of transport phenomena at this installation [2]. In this paper 1 present briefly some of the results of a discussion concerning the magnetic properties of three chosen chromium spine1 systems investigated (among others) in pulsed fields.
2. Magnetic spinels
properties
of the chosen
chromium
The object of our interest are the noncollinear chromium spinels, which need strong fields in the magnetization process. Usually we start the mag0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
fields.
netic investigations from the magnetization measurements in high stationary fields at 7‘= 4.2 K and 77 K in order to calibrate and check our results. Therefore in a short time we obtain precise magnetization curves of every sample in a wide temperature range. From these curves we calculate the magnetic moment as a function of temperature. Next, with the use of the pulsed fields we investigate the magnetic phase transitions induced by strong fields [3,4]. The phase transition points are easily seen by the time derivative signal of magnetization (dM/dt) versus the magnetic field. From these curves obtained for the spin (cases simple spiral structures 0.0 G X G 0.025 an d for Zn, I Cu,Cr,Se, Zn I~~rGa2r~~Cr,Se, for 0.0 G x s 0.5), it follows [5] that there are three transition fields separating four structures, namely simple spiral, cone, fan, and parallel alignment. Instead, similar studies of the conical structure, which appears in Zn ,_~,Cu,Cr,Se, for 0.05 G x s 0.7. indicate that the observed transitions concern changes from a through the alignment in the cone structure, meridian plane, an oscillation in this plane. to a parallel alignment. All critical fields between these structures are of the second kind. The values of these fields have been checked with the use of Nagamiya’s theory of magnetization in a screw spin system [5]. In further studies the higher derivatives of d’*M/df” are registrated in order to determine the anisotropy field [3,4, 6-81. From its value the anisotropy constant K, and the anisotropy energy E,a of axial symmetry were calculated. It B.V.
S. Juszczyk
40 1
I Pulsed fields in studies of spineis
the cases of in the out that Cu,Cr,Se, and Zn,_,Ga,,,,Cr?Se, sysZnlmx tems at T = 2.9 K, K, is negative and of the order of 10’ J/m’ and E, changes its sign, indicating that the direction in which E, reaches its minimal value has changed [6]. From a comparison of the exchange and anisotropy energies. it follows that the latter is two orders of magnitude less than the exchange energy. Therefore, the anisotropy energy may be considered as a perturbation (the transition from a simple spiral to a conical structure in Zn,_* Cu,Cr,Se, for 0.0~ x ~0.7 will be caused by the exchange energy and the anisotropy energy will stabilize only the c-axis in simple spiral and conical structures and fix the rotation plane of the magnetic moment vectors of the CT”+ ions (n = 2,3 and turned
4)). On the other hand in Co,, &u,,~,,CrlS,_,Se, spinels K, is positive and of the order of 10’ Jim’ at T = 300 K and the anisotropy energy is a few orders of magnitude less than the exchange one. The pulsed fields were indispensable in the magnetization process of Zn, ,GazI ,,Cr,Se, spinels [9]. because even in fields up to 4.5 T the compounds with x = 0.4 and 0.5 (polycrystals, because a single crystal sample with x = 0.4 reaches saturation) did not exhibit saturation. With the help of pulsed fields the differential high-field susceptibility has been measured in these spine1 systems [l&12]. Using Slater’s ideas on non-integer occupation numbers, numbers of 3d holes (as the carriers of magnetism in the investigated spinels) per magnetic atom (Cr, Co) with spins up and down have been obtained [13]. From the condition of self-consistency for the magnetic moment, the number of occupied states at the Fermi level with a given spin projection N(E,) has been calculated. With the use of the N(E,) values, the Pauli spin paramagnetic susceptibility for all these spine1 systems has been calculated and compared with other terms of the total high-field susceptibility (e.g. the van Vleck paramagnetic susceptibility, the Larmor and Landau diamagnetic susceptibility and the spin wave susceptibility). It turned out that the Pauli susceptibility is generally small and of the same order in the series with Cr as a magnetic atom
for the and instead is higher co I) &%.A5 Cr?S, ,.Se, series - here the magnetic atoms are Co and Cr. Therefore one can conclude that the electronic properties of the compounds under study are governed by the narrow 3d band of chromium and cobalt. In the end it is worth mentioning that the pulsed magnet and the pick-up coil system may be used to obtain, by the dynamic method in a weak mutual field, the critical temperatures of the investigated compounds [l]. This method is so sensitive that with its help one can separate precisely two magnetic or crystalline phases (one can check the purity of the sample too) from each other. The above-mentioned examples show the great power of the pulsed field technique in studies of these spine1 compounds. The results of the pulsed measurements allow us to choose the direction of our next investigations (with the use of other techniques) in order to fully explain the magnetic properties of the compounds under study.
only,
Acknowledgement This work was supported Polish Project CPBP 01.04.
in part
through
the
References [l] S. Juszczyk and H. Duda, J. Magn. Magn. Mat. 44 (1984) 133. [2] H. Duda, private communication. [3] S. Juszczyk, _I. Krok, I. Okonska-Kozlowska, H. Broda, J. Warczewski and P. Byszewski, Phase Transitions 2 (1981) 67. [4] S. Juszczyk. I. Okonska-Koziowska and P. Byszewski, Phase Transitions 4 ( lY84) 29 1. [S] T. Nagamiya, in: Solid State Physics. Vol. 18, F. Seitz and D. Turnbull, eds. (Academic, New York. lY64), p. 306. [6] S. Juszczyk. J. Magn. Magn. Mat. 74 (19X8) 330, 75 (198X) 280. 285. 171 J. Krok. S. Juszczyk, J. Warczewski, T. Mydlarz. W. Szamraj, A. Bombik, P. Byszewski and J. Spaiek, Phase Transitions 4 ( 1983) 1.
402
S. Juszczyk
I Pulsed fields in studies of spine/s
[g] M. Gogoiowicz, S. Juszczyk, J. Warczewski and T. Mydlarz, Phys. Rev. B 35 (1987) 7121. (91 S. Juszczyk, J. Krok, I. Okouska-KozJowska, T. Mydlarz and A. Gilewski, J. Magn. Magn. Mat. 46 (1984) 105.
[IO] S. Juszczyk, H. Duda, T. Mydlarz and J. Krok, Magn. Magn. Mat. 50 (1985) 209. [ll] S. Juszczyk, J. Map. Magn. Mat. 59 (1986) 69. [12] S. Juszczyk, J. Magn. Magn. Mat. 61 (1986) 295. [13] S. Juszczyk,
J. Magn.
Magn.
Mat. 73 (1988)
18.
J