Mössbauer spectra of copper ferrites CuGa x Al2x Fe2 − 3x O4 (x = 0.3, 0.4, 0.5) have been measured at temperatures in the range from 90 to 295 K. The degree of disturbance of long-range magnetic order has been determined as a function of the temperature. The Curie temperature has been found for the composition with x = 0.5, and the dependence of the Curie temperature on the concentration of the nonmagnetic impurity has been constructed.

The behavior of the spontaneous magnetization σ s ( T ) and coercive force H c ( T ) of dilute nickel ferrites NiGa x Al x Fe 2 − 2 x O 4 ( x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8) has been studied. Above the transition temperature T t , the coercive force H c is found to reveal an anomalous behavior for compositions with x ≥ 0.4, namely, the temperature dependence of the coercive force H c ( T ) exhibits a maximum in the range from T t to T C . For the reduced temperature θ 2 = 0.8 T C , at which ferrites with the substitution x ≥ 0.4 reside in the spin glass state, the coercive force H c is observed to increase sharply with x . The assumption is made that the clusters prevailing in the spin glass state are no larger than 3 nm in size.

Mössbauer spectra of the CuGa x Al2x Fe2 − 3x O4 system (x = 0.1, 0.2, 0.3, 0.4, 0.5) have been studied at a temperature of 295 K. The results obtained are compared with Mössbauer data for the CuGa x Al x Fe2−2x O4 system. It is established that the hyperfine magnetic fields H B (for octahedral sites) and H A (for tetrahedral sites) for ferrites in both systems with ferrimagnetic ordering decrease linearly depending on the total number of nonmagnetic ions in octahedral and tetrahedral sites of the two systems.

The investigation of magnetic properties of diluted ferrite NiGa0.8Al0.8Fe0.4O4 is carried out. Anomalous behaviour of spontaneous magnetization σs(T), coercive force Hc(T) and volume magnetostriction ω(T) is revealed for the sample with frustrated magnetic structure. It is found that volume magnetostriction ω reaches very large (that is to say “giant”) values (ω ~ 5•10–4) (for T = 84 K). It is revealed that the derivative dω/dH is equal ≈ 43 • 10–9 Oe–1 (for T = 84 K). The assumption is made that the size of clusters is about 3 nm in the spin glass phase (for Tt < T < TC).

An abrupt rise of the permittivity ɛ of nickel ferrite-chromite (NiFeCrO 4 ) is discovered near the compensation temperature T c . The hysteresis character of this increase is demonstrated. Correlation between the magnetic and electrical properties of the ferrite under study is revealed. The observed anomalous behavior of permittivity is thought to be due to a direct overlap of the t 2 g orbitals of the Cr 3+ ions in octahedral nodes.

The behavior of the magnetoresistance of nickel ferrite-chromite NiFeCrO 4 is studied at temperatures in the vicinity of the compensation temperature. A model of magnetic ordering in this compound is proposed. It is established that, at T = 0 K, the sublattice B is responsible for magnetic ordering.

The x-ray structural properties of samples in the CuGa x Al x Fe 2−2 x O 4 ( x = 0−0.7) and CuGa x Al 2 x Fe 2−3 x O 4 ( x = 0−0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.

The behavior of the spontaneous magnetization σ s ( T ) of samples in the Cu x Ni 1- x Fe 0.6 Cr 1.4 O 4 system ( x = 0, 0.1, 0.2, 0.3, 0.4) with variations in temperature is studied. The conclusion is drawn that the degree of frustration of magnetic interactions in the A sublattice increases with increasing x , with the result that the dependence σ s ( T ) transforms from an N -type curve to an anomalous dependence of a new type.

The investigation of magnetic properties of diluted ferrite NiGa0.7Al0.7Fe0.6O4 is carried out. The anomalous behaviour of spontaneous magnetization σs(T), coercive force Hc(T) and derivative of spontaneous magnetization (dσs/dT)(T) is revealed for the sample with frustrated magnetic structure. It is found that the Mossbauer spectrum is doublet at the temperature T = 295 K of the investigated ferrite. This is proved that the magnetic structure represents clusters formed by short-range magnetic order at the temperature below the Curie temperature TC. © MISM2005. All rights reserved

The temperature dependence of the susceptibility of the paraprocess χpara (T) is investigated for samples in the CuGaxAlxFe2−2xO4 (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7), CuGaxAl2xFe2−3xO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5), and GaxFe1− xNiCrO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) systems. It is found that long-range magnetic order arises in spinel ferrites at the temperature Ttran of the transition from a cluster spin-glass state to a frustrated magnetic structure with a maximum in the temperature dependence of the susceptibility of the paraprocess.

The magnetic properties of the NiCr 2 O 4 chromite are investigated and compared with similar properties of the NiFe 1.1 Cr 0.9 O 4 nickel ferrite-chromite material. It is found for the first time that the tetrahedral ( A ) sublattice of the NiCr 2 O 4 chromite is responsible for the total magnetic moment. Moreover, it is revealed that the NiCr 2 O 4 chromite exhibits a giant magnetostriction of the paraprocess (λ para ∼ 200×10 −6 ) and an anomalously large volume magnetostriction (ω ∼ 500×10 −6 ) at a temperature of 4.2 K in magnetic fields up to ∼50 kOe. The inference is made that the observed paraprocess is caused by an increase in the degree of noncollinearity of the magnetic moments induced in the octahedral ( B ) sublattice of the NiCr 2 O 4 chromite in an external magnetic field.

The magnetostriction of the NiFeCrO 4 ferrite is investigated for the first time. It is found that the frustration of magnetic coupling occurs only in the B sublattice of the NiFeCrO 4 ferrite, whereas the A sublattice has a usual magnetic structure. The inference is made that the frustration of magnetic coupling in the B sublattice is caused not only by the negative direct BB exchange interaction Cr B 3+ -Cr B 3+ but also by the positive indirect AB exchange interaction Fe A 3+ -O 2− -Cr B 3+ . Reasoning from the experimental data and an analysis of the exchange interactions in the NiFeCrO 4 ferrite sample, it is demonstrated for the first time that the magnetic moments of Fe A 3+ ions in this ferrite deviate from collinearity. It is established that, at low temperatures, the B sublattice of the NiFeCrO 4 ferrite is responsible for the total magnetic moment n 0exp .

Magnetic properties of samples of the system GaxFe1-x[NiCr]O-4 (X = 0.0,0.2, 0.4, 0.6, 0.8) are investigated. The assumption is made that two magnetic phase transitions take place for the ferrites with x greater than or equal to 0.2: at T-C the transition from the paramagnetic state to a spin-glass state: and at T-1 a second transition from a spin-glass state to a frustrated magnetic phase. This assumption is in good agreement with theoretical models. It is established that long-range magnetic ordering occurs at the temperature T-1.It is suggested that a frustrated magnetic structure in one sublattice is necessary for the occurrence of anomalous dependences of the spontaneous magnetization on temperature sigma(s) (T) of N, P or L type. It is established that if the frustrated magnetic structure is present in both sublattices of the ferrite, an anomalous sigma(s) (T) curve of another type is formed.Our conclusion is that the reason for the frustration of the magnetic connections in the samples of the system GaxFe1-x[NiCr]O-4 with x less than or equal to 0.2 is the presence of strong indirect interlattice interaction Fe-A(3+)-O2--Cr-B(3+) of positive sign.

Summary form only given. For the first time the volume /spl omega/ and anisotropic /spl lambda//sub t/ magnetostrictions of NiFeCrO/sub 4/ is studied. The anomalous behaviour of /spl omega/(T) is revealed below the compensation temperature T/sub c/=325 K. We found, that /spl omega/ grows sharply with reduction of the temperature and reaches the large value (/spl omega//spl ap/200/spl middot/10/sup -6/) at the temperature T=94 K (H=12, 1 kOe). It is found the value /spl omega//spl ap/0 at T>T/sub c/.

For the first time, it is shown that, for ferrite NiFeCrO4 (curve sigma(s)(T) of N type), the frustrated magnetic structure takes place only in the octahedral sublattice. It was established that the replacement of nonmagnetic Ga3+ ions for Fe3+ ions results in an essential change of its magnetic structure: the compensation temperature T-C disappears, in a significant interval of temperatures the dependence sigma(s)(T) carries almost a linear character, and near the Curie temperature T-C there is no maximum on a curve (dsigma(s)/dT)(T). An assumption was made that such behaviour of magnetization sigma(S) for ferrites of the system GaxFe1-x[NiCr]O-4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) demonstrates the fact that in the tetrahedral sublattice the magnetic structure also becomes frustrated.

The magnetostriction of a number of copper-containing ferrite spinels was measured. In all the ferrites studied, below room temperature, |λ‖| was found to decrease with temperature. It is conjectured that this decrease in |λ‖| originates from an increase in the degree of covalency among the Cu2+-O2− ions, which, in turn, reduces their spin-orbit coupling.

A study is reported on the behavior of the isotherms of the magnetization σ(H) and of the longitudinal λ‖(H), transverse λ⊥(H), volume ω(H), and anisotropic λt(H) magnetostrictions measured at T=80 K in the Cu0.4Fe0.6[Ni0.6Cr1.4]O4 and Zn0.4Fe0.6[Ni0.6Cr1.4]O4 ferrite-chromites having a frustrated magnetic structure. It has been established that these ferrite-chromites do not undergo technical magnetization and that the growth of the magnetization with the field is accounted for by two paraprocesses of different natures.

The behaviour of the spontaneous magnetization a,, coercive force H-c, magnetostriction lambda, magnetoresistance DeltaR/R and electroresistance R of the ferrite spinel CuGa0.4Al0.8Fe0.8O4 with frustrated magnetic structure has been investigated.It was found that for a given sample the anomalous behaviour of the magnetostriction and magnetoresistance that occurs differs from that of usual Ferrimagnetic structures, in which the long-range magnetic order penetrates the whole volume of a sample. For example, both the parallel (lambda (\ \)) and the perpendicular (lambda (perpendicular to)) magnetostriction throughout the interval of temperatures investigated in all fields have only positive sign.It has been also established that there is anomalous behaviour of the magnetoresistance: both the parallel ((DeltaR/R)(\ \)) and the perpendicular ((DeltaR/R)(perpendicular to)) magnetoresistance, which are negative throughout the interval of temperatures investigated, decrease monotonically with increasing temperature.The sample investigated shows two magnetic phase transitions when the temperature decreases. The first transition, at the Curie temperature T-c = 460 +/-:5 K, occurs from the paramagnetic state to a phase which consists of spontaneously magnetized areas formed due to short-range order(clusters). The second transition, from this phase to a state with frustrated magnetic structure, takes place at a lower temperature, T-t = 410 +/- 5 K.