MAGNETISM, ELECTRONIC PROPERTIES AND STRUCTURE AT HIGH DENSITY STATE OF MAGNETIC SOLIDS

A.G. GAVRILIUK*, G.N. STEPANOV*, I.A. TROJAN*, V.A. SIDOROV*,
I.S. LYUBUTIN**, B. PALOSZ***, S. STEL'MAKH***, and M. WINZENICK****

* Institute for High Pressure Physics RAS, Troitsk, Moscow reg., 142092, Russia,

E-mail: alexg@ns.hppi.troitsk.ru

** Institute of Crystalography RAS, Leninsky pr. 59, Moscow 117333, Russia

*** High Pressure Research Center, Polish Academy of Sciences, 01-142 Warsaw,

Sokolowska 29.

**** Univesitaet-G-H Paderborn, Fachbereich 6 Physik, Warburgerstr. 100, 33098 Paderborn,

Germany

ABSTRACT

We develop and apply high pressure techniques to study the modification of electronic structure, magnetic properties, and local crystal structure, in magnetic materials under high pressure. The variation in inter-atomic distances and atomic volumes in the high pressure regime provides new information for elucidating electronic processes in solids, and for understanding the connection between electronic structure and magnetic properties of matter.

For our samples we used rare-earth orthoferrites, which are of the perovskite-type structure, and Heusler alloys, with the cubic structure. Those materials are known to be magnetic insulators and magnetic metals, respectively.

We studied the following properties induced by high pressure: electronic structure, spin cross-over effects, magnetic to non-magnetic transitions, and insulator-to-metal transitions. The problems of delocalization, distribution of spin, and charge density distribution in dielectric materials, semiconductors, and metals, as well as chemical bonding can be investigated in this way.

1. INTRODUCTION

We used Mossbauer Sn-119 spectroscopy, optical absorption spectroscopy, magnetic, differential thermal analysis (DTA), and compressibility measurements at high pressure as experimental techniques. High pressure Mossbauer spectroscopy investigations were carried out at room temperatures in the rare-earth orthoferrites NdFeO3 and LuFeO3 at pressures up to 30GPa, and in Heusler alloys Co2MnSn and Ni2MnSn at pressures up to 12 GPa. We measured the pressure dependency of the Neel and Curie temperatures as well as the compressibility at pressures up to 9 GPa. We carried out high pressure optical absorption measurements in LuFeO3 at pressures up to 62 GPa, and found a sharp decrese in the absorption edge in pressure range 30-45 GPa related to the magnetic to non-magnetic transition. For the first time in these materials, the high pressure behaviour of hyperfine magnetic fields at the nuclei of diamagnetic atoms and covalency effects has been explained. These experimental results may be used to improve existing theories, and predict properties of newly synthesized materials.

2. EXPERIMENTAL

2.1. High pressure Mossbauer and optical absorption methods in a DAC

High pressure Mossbauer, optical, and XRD measurements have been carried out in diamond anvil cells (DAC) [1]. The typical view of a DAC is shown in Fig. 1.

In the Mossbauer experiments, the diameter of the anvils was ~600 m. We used the ruby fluorescence technique to determin the pressure exerted on the sample. The rhenium gasket was pressed between the diamond anvils. The initial thickness of the rhenium foil was 200 m; the final indented thickness was ~50 m. A ~200-m diameter hole was produced by a spark cutter in the center of the gasket. The hole was filled with the powdered sample (Fig. 1(b)). Ruby chips (~10 m across) were then placed on the surface of the sample. The maximum difference in the pressure measured at various parts of the sample was 0.1 GPa.

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(b)

(a)

Fig. 1. (a) A typical view of a high pressure diamond anvil cell. C, P: cylinder-piston; S: Mossbauer source or focusing lens; DA: diamond anvils; H: hemisphere; G, L: grooves and lugs; Scr: screw. (b) Enlarged view of the central part of the DAC. Ab: sample (absorber); R: ruby chips.

In the optical absorption experiments, the diameter of the anvils was ~300 m. The initial thickness of the rhenium foil was 200 m, the final indented thickness was ~50 m. A ~100-m diameter hole was produced by a spark cutter in the center of the gasket. A small thin plate was prepared from the initial powder sample by compressing it between the anvils. We then fragmented the plate, and placed one fragment along with the ruby chips (~10 m across) on the surface of the anvil. Silicon liquid was used as the pressure medium.

2.2. High pressure magnetic susceptibility, resistive strain gauge compressibility, and DTA methods in a "toroid" high pressure camera

We measured V-P relationship of Heusler alloys using a resistive strain gauge. Magnetic transitions were detected by measurements of the initial ac-susceptibility. These methods are described in detail in [2] and [3], respectively.

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Fig. 2. The liquid-filled ampoule for studies in a toroid-type apparatus at high hydrostatic pressures and high temperatures. 1: gasket (catlinite), 2: teflon sleeve, 3: lid, 4: thermal insulation, 5: heater, 6: sample, 7: thermocouples, 8: mica, 9: resistive strain gauge; 10: manganin pressure gauge, 11: electrical leads.

To detect the magnetic transitions in rare-earth orthoferrites, we used the DTA method [4], since the temperature dependence of heat capacity has a singularity (-anomaly) at the point of the second order phase transition. This means that the T(T) curve will also have a singularity at the transition point, if the reference sample and the sample of interest both obtain heat from the heater and lose it into surrounding medium.

Resistive strain gauge compressibility, initial ac-susceptibility, and DTA measurements at high pressure were carried out with the use of a toroid-type apparatus [5], capable of creating hydrostatic pressures (up to 9 GPa) at high temperatures (up to 900 K) inside a fluid filled capsule [6]. The designs of the central part of the high pressure cell for the initial ac-susceptibility, and resistive strain gauge methods are shown in Fig.2(b) and 2(c), respectively.

The design of the central part of the DTA high pressure cell is shown in Fig.2(d). Two chromel-alumel thermocouples, 0.05 mm in diameter, are placed inside the resistive heater. One of them has the junction in direct contact with the sample. The other's junction is separated by a mica sheet from the sample. The sample, the thermocouple junction, and the mica spacer are placed in the additional copper thermal shield. In this assembly the surrounding medium (fluid) serves as a reference sample in the DTA measurements. The pressure inside the cell was measured continuously by a manganin gauge, the pressure coefficient of manganin wire being calibrated against the phase transitions in Bi at 2.55 and 7.7 GPa. The gauge was placed in the cold zone of the capsule (Fig.2(a)), and its temperature was nearly constant on heating the sample to 700 K. The thermocouples and pressure gauge were measured with digital voltmeters having 0.01 V resolution and stored and treated by a computer.

The small size of the sample (few mm3) and the low level of expected DTA signal creates a considerable challenge for obtaining reliable data. The typical value of the anomalous part of the T(T) dependence is 1-2 V; that corresponds to 0.05 K at a heating rate of 2-4 K/sec and a sample size of 2x2x0.8 mm3.

3. RESULTS AND DISCUSSION

3.1. High pressure investigations in rare-earth orthoferrites

In rare-earth orthoferrites, the hyperfine magnetic field H was investigated at room temperature using high pressure Mossbauer absorption spectroscopy at nuclei 119Sn. The diamagnetic ions Sn4+ were implanted into the lattice of the antiferromagnetic orthoferrites LuFeO3 and NdFeO3. The samples Nd0.9Ca0.1Fe0.9Sn0.1O3 and Lu0.9Ca0.1Fe0.9Sn0.1O3 were prepared by means of the ceramic method [8]. The orthoferrites have of a perovskite-type crystal structure [7], the schematic view of which is shown in Fig. 3.

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(a) (b)

Fig. 3. (a) The schematic view of the perovskite structure and (b) picture of the change in structure at high pressure.

When diamagnetic ions (D) are implanted into the lattice of magnetic compounds, at the nuclei of these ions arises a hyperfine magnetic field [13,19]. The results of experiments at high pressure may be used to verify the models of the hyperfine interactions, since the theory is far from complete [14,15,16,17,18]. In the absence of a magnetic environment, the electronic shells of the D-ions are compensated. Therefore, magnetic fields at the nuclei of diamagnetic ions HD in a magnetic matrix, appear as a result of interaction between D and the paramagnetic ions M. If the cations D and paramagnetic ions M are separated by anions A, there is no direct mixing of the electronic states of the D- and M-ions. This leads to the so-called "supertransferred hyperfine field" (STHF).

The mechanisms of magnetic ordering in rare-earth orthoferrites are based on the superexchange interaction and may be described within the framework of ligand field theories, whereas week ferromagnetism is due to the Dzyaloshinsky interaction.

Both phenomena, the superexchange and the STHF, depend on the electron transfer from one metal ion to another via a ligand, and so one can expect that the behaviour of H and TN are related. The purpose of the present investigation is to study the dependence of the superexchange and STHF on the interionic distances.

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Fig. 4. The pressure dependence of HSn or
R1-xCaxFe1-xSnxO3.(R=Lu,Nd; x=0.1,0.05).

The high pressure Mossbauer spectroscopy (HP MS) experiments have been carried out in diamond anvils with a rhenium gasket. The hole in the gasket was filled with the powder sample. The 119Sn Mossbauer spectra can be split into three subspectra: two magnetic sextets and one nonmagnetic singlet. The pressure dependence of the maximal supertransferred hyperfine magnetic field at the 119Sn nuclei is shown in Fig. 4.

The measurements of the Neel temperature (TN) have been carried out in the "toroid" hydrostatic high pressure camera. An ethanol-methanol mixture was used as a pressure medium. The samples used were single crystals. The pressure dependence of TN as the pressure increases, and as the pressure decreases are shown in Fig.5. The pressure coefficients for TN in orthoferrites LuFeO3 and NdFeO3 are found to be equal to 8.45 K/Gpa and 11.2 K/Gpa, respectively.

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Fig. 5. The pressure dependence of TN as the pressure increases (solid circles and squares) and decreases (open circles and squares) in LuFeO3 and NdFeO3.

The changes of both TN and HSn(0 K) under pressure have an influence on the measured value of HSn(296 K). We have proposed a procedure for extrapolating the zero temperature pressure dependence of the magnetic field from the room temperature pressure dependence of the magnetic field and Neel temperature:

dd7_78.gif (440 bytes) (1)

where H0sthf is the magnetic field at zero temperature, TN is the Neel temperature,
=-0.000656 K-1, and =0.4998. It is assumed that the shape of the function from Eq.1 is independent of pressure (Fig. 6). This extrapolation procedure has been checked experimentally and gas given good results. Using this procedure, the pressure dependence of the field for NdFeO3 has been derived at zero temperature, dH/dP~1.5kOe/GPa.

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Fig. 6. Extrapolation of the magnetic field from the room temperature to zero temperature. The shape of the function of Eq.1 is assumed to be independent on pressure.

Using the linear combination of atomic orbitals (LCAO) model for the chain M-A-D, the TN and H(0K) values may be expressed in terms of the overlap and covalence parameters [14,15,16,17,18]. Let us consider the simplified model of the origin of H(0 K). In the ground state, the configuration of the outer electron shells in the Fe-O-Sn chain is

Fe3+ O2- Sn4+

3d5 2p2 5so

In an exited state the 2p electron of O2- may transfer partly to the empty 3d shell of Fe3+. As a consequence, an uncompensated spin density appears on the ligand O2-. These unpaired 2p-electrons may transfer to the 5s-shell of Sn4+. The unpaired 5s-electrons of Sn and the overlap of the unpaired 2p-electrons of O2- with the inner ns-electrons of Sn4+ cause the appearance of the Fermi contact hyperfine magnetic field H at the tin nuclei.

The analysis of different contributions into H(0 K) on the basis of the experimental data shows that the largest contribution is caused by the covalent transfer of electrons from 2p-shell of O2- to 5s-shell of Sn4+ [18,19]:

dd7_710.gif (327 bytes) (2)

where a5s is the covalency parameter of transmission pz5s (pz is the 2p-function of O2-, 5s is the 5s-function of Sn4+). To estimate the covalence parameters of the electron transfer from the 2p-shell of O2- to 5s-shell of Sn4+, one can use the available data for the pair In3+-O2- because In3+ and Sn4+ are isoelectronic ions. The parameter of the electron transfer from 2p-orbital of O2- into the 5s- orbital of In3+ increases by 30% if the interionic distance decreases by 10% [20]. The decrease of interionic distances under pressure is determined by the bulk modulus B of the oxygen octahedron:

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To determine the bulk modulus B, and to check possible crystallographic transitions, high pressure x-ray investigations up to 41 GPa have been carried out at a synchrotron facility (HASY LAB Hamburg). The V-P relationship for LuFeO3 as the result of this investigation is shown in Fig. 7. The bulk modulus is equal to ~220 GPa.

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Fig. 7. The V-P relationship for LuFeO3 from high pressure x-ray synchrotron measurements.

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Fig. 8. Pressure dependence of absorption edge in [Lu1-xCax](Fe1-xSnx)O3 (x=0.1) at room temperature.

From Eq.3, the pressure dependence of the hyperfine magnetic field at zero temperature for LuFeO3 is equal to dH/dP~1.6 kOe/GPa. This result seems to indicate that in the chains M-A-D, the LCAO model can describe the dependence of HDsthf(0 K) on the bond angle [19] as well as the interionic distances.

Measurements of the optical absorption edge have been carried out in a diamond anvil cell with a rhenium gasket. Silicon liquid was used as a pressure medium. The samples have been prepared from powder,having the thickness of approximately several m. The average diameter of the working surface of the anvil was ~300m. The pressure dependence of the absorption edge is shown in Fig. 8.

As seen in from optical absorption edge measurements, there is an electronic transition in LuFeO3 at a particular pressure range which causes the edge value to change from ~2.25 eV to ~1.25 eV. It is interesting to note that the value of the pressure at this transition is similar to that found in LaFeO3 at the magnetic to non-magnetic transition [21].

3.2. High pressure investigations in Heusler alloys.

Heusler alloys are ternary intermetallic compounds of stoichiometric composition X2YZ [22,23]. The structure is cubic with the space group L21, with X ions at the cube corners and Y and Z ions occupying alternate body centers of successive cubes (Fig. 9). Alloys with Y=Mn and Z=Sn are magnetically ordered. In such alloys, the large magnetic moment of ~4B is localized on the Mn ion. In Co2MnSn there is also a magnetic moment of ~0.7B on the Co ion [24].

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The purpose of our investigation was to study the dependence of the hyperfine magnetic field HSn on the interatomic distance in the Heusler alloys Ni2MnSn and Co2MnSn, and to compare the experimental observations with the empirical model developed by Delyagin et al. [24]. The hyperfine magnetic field HSn was measured at room temperature using high pressure Mossbauer absorption spectroscopy at the 119Sn nuclei. For details of the investigations of the hyperfine interactions at Sn nuclei in Heusler alloys under pressure, and for a general introduction, see Ref. [25]. The pressure was generated in diamond-anvil cell.

Fig. 9. The crystal structure of the Heusler alloy.

Experimental data were fitted to the linear function H(P)/H(0)=1+kHP. The values of kH(0) and H(0) for Co2MnSn are -0.0200.001 Gpa-1 and 10.50.3 T, respectively, and are -0.0950.015 Gpa-1 and 4.60.12 T for Ni2MnSn. To demonstrate the absence of the irreversible "order-disorder" transition, some experimental points were measured decresing pressure. It is well known that if there is atomic disordering in the Mn-Sn sublattice, the value of H changes dramatically. The observed dependence in the pressure decreasing regime, demonstrates an absence of the irreversible "order-disorder" transition.

Nikolayev et al [27] have measured at room temperature the pressure dependence of the hyperfine fields at 119Sn nuclei in the tin sites of Co2MnSn up to 1.4 GPa (closed circles in Fig.10(a)). Their measured coefficient was (1/H(0))(dH(P)/dP)= -0.022 Gpa-1. The experimental results have been discussed in terms of alternative models of appearance of magnetic fields at nuclei of nonmagnetic atoms in ferromagnetic matrices [23,24].

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(a)

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(b)

Fig. 10. The pressure dependence of HSn in (a) Co2MnSn and (b) Ni2MnSn. Open circles: experimental points measured as the pressure increases; closed squares: experimental points measured as the pressure decreases; closed circles: experimental data from Ref. [27]; solid lines: fits to the experimental data for Co2MnSn and extrapolated curve for Ni2MnSn; dotted line: the theoretical curve using Delyagin's model.

In general, the changes of both TC and H(0K) under pressure, influence the measured value of H(296K). However, the high value of the Curie temperature in Co2MnSn (TC ~ 800K), allows any contribution to H(296K) due to the TC change to be neglcted. Moreover, the position of Co2MnSn near the maximum on Castellitz's empirical curve [28] is an additional reason to ignore the influence of the TC variation on the H(296K) value since this variation must be small. We can also assume that the value of H(296K) is close to that of H(0K).

The situation is different for Ni2MnSn. In our study, the pressure dependence of TC was measured for Ni2MnSn. Experimental procedure was based on the measurements of the initial ac-susceptibility. The data were fitted to the linear function TC(P)=TC(0)+KTcP, with KTc=7.44 K/GP, and TC(0)=341.3 K (see Fig. 11).

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Fig. 11. The pressure dependence of TC for Ni2MnSn: open circles are our experimental points, solid triangles are experimental data from Ref. [28], and solid line is a linear fit of the experimental data.

The V-P relationships for Ni2MnSn and Co2MnSn were measured by means of a resistive strain gauge. The V-P relationships for Ni2MnSn and Co2MnSn are shown in Fig. 12. These experimental data were fitted to one of the standard forms of the state equation of solids:
(V-V0)/V0=-aP+bP2, where parameters a=8.6410-3 Gpa-1 and b=1.1310-4 Gpa-2 were found for Ni2MnSn, and a=7.5210-3 Gpa-1 and b=1.1610-4 Gpa-2 for Co2MnSn.

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Fig. 12. The V-P relationships for Ni2MnSn and Co2MnSn (experi-mental points and fits).

To calculate the extrapolated pressure dependence of H(0K), one can use the pressure dependencies of TC and of H(295K), as in Eq.1. Using the experimental values of HSn at atmospheric pressure: 45 kOe (present work and Ref. [23]), 87 kOe [23], 97 kOe [23,29] for T=295K, T=77K and T=0K, respectively, one can construct the H(T) curve (solid line in Fig. 13). After the fitting procedure, the parameters of this curve have been calculated. Then, let us suppose that a function h(t), where h=H(T)/H(0) and t=T/TN, is pressure independent. The function h(t) for Ni2MnSn is shown in the insert of Fig. 13. With this assumption, and taking into account the dependence H(P) at room temperature and the dependence TC(P), one can draw the curves H(T) for different pressures. These curves have been constructed with that procedure (an example for P=10 GPa is shown in Fig. 13 by the dotted line), and the extrapolated pressure dependence of H(0K) have been computed. In Fig. 10(b) the fitted experimental curve H(P) for room temperature (solid line) and the extrapolated pressure dependence HSnhpf(P) for T=0K (dotted line) are shown.

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Fig.13. Fits and experimental data of the temperature dependence of the magnetic field. Experimental data - filled square: from NMR data of Shinohara [29]; filled triangle: from Mossbauer data of Leiper et al [23]; open circles: our experimental data. Computation - solid line is a fit to the experimental data using Eq.1; dotted line is the extrapolation using H(P) for room temperature and TC(P). Insert : the dependence h=H(T)/H(0) on t=T/TC that has been assumed to be independent of pressure.

Using Delyagin's empirical model [24], one can write the dependence of H on the distance from the Sn atom to the first nearest neighbour (1nn) (r1) and on the distance from Sn atom to the second nearest neighbour (2nn) (r2):

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To calculate HSn(P), we assume: (i) M1, M2 and CB are approximately constant at all pressures, (ii) the largest contribution to the HSn decrease is due to the p(r) variation. We suppose that M1=5.23 for Ni2MnSn, M2=24 and C=20 [24]. To fit H(0K), here we assume that there is the localized magnetic moment of ~0.65 B on the Ni atom. Thus we can write [24]:

dd7_721.gif (515 bytes) for Ni2MnSn (5)

dd7_722.gif (542 bytes) for Co2MnSn (6)

where M1(r1) and M2(r2) are the sum of the magnetic moments of atoms in 1nn and 2nn, respectively (in B units), p(r) is the empirical Delyagin function for the radial dependence of the partial contributions to the hyperfine field (in T/B units), C is the contribution to Hhpf proportional to the average atomic moment of the material . The function p(r) has been constructed as a best fit to the experimental data for b.c.c. and f.c.c. magnetic compounds [24].

The obtained results seem to verify the Delyagin model and allow the pressure dependence of HSn up to 10.8 GPa [24] to be described.

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