NiMn,NiMnP+ ve CrFe alaşım ince filmlerinde elektron spin rezonans (ESR) ve direnç ölçümleri
Başlık çevirisi mevcut değil.
- Tez No: 75020
- Danışmanlar: PROF. DR. YILDIRHAN ÖNER
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Fizik Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 243
Özet
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Özet (Çeviri)
SUMMARY Electron Spin Resonance (ESR) and Resistivity Measurements on NiMn, NiMnPt and CrFe Alloys Thin Films Electrical and magnetic properties of Nİ76Mn24, Nİ74Mn24Pt2, Nİ62Mn38, Nİ77Mn23, Cri^Fe^ and Nii-^M^sPtx alloys film with various thicknesses have been investigated as a function of temperature from 4K up to 300K. Electron Spin Resonance (ESR) methode has been used to study the flush evaporated amorphous Nİ76Mn24 Nİ74Mn24Pt2, Nİ62Mn38 and e-beam grown polycristalline Nİ77M1123 films. Transport properties of binary Cri_xFe;r and ternary Niı-^M^sPta amorphous alloys have been studied by applying dc and ac resistivity measure ment techniques. A theoretical model has been developed to analyse the experi mental data. It has been found that while the surface anisotropy almost remains unchanged the the effective bulk ansotropy is increased with decreasing film thick ness. The Pt impurity has higly induced the exchange anisotropy and increased the canting temperature. The bulk forms of above alloys exhibit unusual physical behavior such as dis placed magnetic histeresis loop, onset of strong irreversibilities in magnetization, an asymmetric magnet or esistance, a minimum in the resistivity curves, a triple- peaked structure in the low-field susceptibility and shifths in the electron spin resonance curves toward the negative fields upon cooling the sample in an ex ternal field, etc. These pecular behavior are hardly understood in bulk form of these alloys. All of these behavior have been attributed to a new kind magnetic anisotropy, namely, exchange unisotropy. it is sometime called as uni-directional anisotropy since the direction of the effective magnetic field due to this unisotropy is always induced along the local magnetization vector during the cooling process Moreover this anisotropoy field is independent of the crystalline axes. It is claimed that these unisotropies arise from the competition between the antiferromagnetic interactions of nearest Mn-Mn neighbors and the ferromagnetic interactions of Ni-Mn and Ni-Mn pairs. Indeed, the_co existence of such competition leads to the frustration phenomenon and cause the Dzialoshinskii-Moria(D-M) type unidirec tional anisotropy field observed in many other magnetic system. It has also been shown that these properties are changed if one use thin films instead of bulk sample of the same alloy. Electron Spin Resonance (ESR) has XVllproven to be most sensitive and precissive technique to get signal from very small magnetic systems. The order of accuracy of magnetic parameters deduced by thise methode is not affected from the size of the sample under investigation as generally occured in many other techniqes. Therefore in order to derive the magnetic parameters Electron Spin Resonance (ESR) measurements have been carried out on amorphous Nİ76Mn24, Nİ74Mn24Pt2, Nİ62Mn38 and policrystalline Nİ77Mn23 films of varios thicknesses in the temperature range 4K-300K. The ESR spectra from all samples have been recorded at room temperature for the different angle between the applied dc magnetic field vector and film normal. But at lower temperature the angular dependent spectra have been recorded only for Nİ77Mn23 films. Low-temperatur ESR studies of Nİ76Mn24, Nİ74Mn24Pt2 and Nİ62Mn38 films have been done only for parallel geometry (where the dc applied magnetic field lies in the surface of the films) and perpendicular geometry (where the magnetic field perpendicular to the surface). ESR experiments were carried out by an x-band ESR spectrometer employing 100kHz field modulation to get the field derivative ESR spectra as a function of dc magnetic field. A continuous helium gas flow cryostat was used to coole the samples down to 4 K and to controle the temperature. The thin film samples were cut 2x1 mm in dimensions and put in the cavity by using a goniometer to rotate the sample arond the vertical axis which is perpendicular to dc magnetic field of the electromagnet. The spectra were stored into a personal computer in order to analyse them through a convenient theoretical model as will be descibed below. At room temperatures, most of the ESR spectra exhibited multi-peak absorbtion lines which are regularly ordered from the left to the right when the angle between the external field and film normal is smaller than a critical values which is varied from sample to sample. The amplitudes of these lines increase from the lower field to the higher field side. The separation of these peaks from each other increases as the angle decreases. The resonance field values decrease with increasing angle. The character of angular variation of the field for resonace depends on pysical structure of the sample. The number of the resonance peaks regularly increases with film thickness of the same alloy. The relative intensity of the sresonance peak at highest field also increases as one go from thickest to thinest film of the same alloy. All of these are found to be some expected behavior of Spin Wave Resonance (SWR) spectra from ferromagnetic thin films. The angular dependence (shape anisotropy) comes from dimagnetizating field due to magnetic north and south poles at opposite surface of the ferromagnetic films. For policrystalline or amorphous samples this demagnetzing field is expected to be symmetric with respect to the film normal. However, for Nİ77İVIn23 film assymetric behaviour has been observed and this behavior was attributed to an oblique (geometric) anisotropy arrised from the film growth procedure. The aspects of the ESR spectra for almost all of these samples seemed to be dramatically temperature dependent.At low temperature the resonance curves shift to lower fields. The lines become broader at lower temperatures. For some samples these peaks are overlapp and give a very broad single line due to this broadening effect. Also the spectra for all samples showed very strong magnetic hysteresis effects at lower temperatures. That is the the futures of the absorption lines depend on the sweep direction of the external dc field. When the spectrum is taken with the increasing field direction the lower field portion of any absorption xvmline is totally disappeared while it is reappeared again when the field is swept from higher to lower values. Some spectra have been taken after cooling the sample in an external magnetic filed (FC case). It has been seen that when the measurement field is paralel to the cooling field (n-FC case) the resonance curve is shifted to lower field with respect to that of zero field cooling case. On the other hand the resonance curves were shifted to higher field when the measurement field is applied in opposite direction of the cooling field (r-FC case). The absuloute values of these shifts in field for both cases are aqual to each other. All of these behavior have been attributed to the uni-directional exchange anisotropy induced at lower temperatures. Resistivity measurements on flush evaporated amorphous Cr82.4Fei7.6,Cr74Fe26 and Nİ62Mn38 Nİ62Mn2sPtıo films were carried out by using four point ac and dc techniques between 1.5-300 K. For high precission measurements Keithly-220 model curent source and Keithly-196 model multimeter, Scientific Instrument-830 model lock-in amplifier, Hewlet Packard ac amplifier, an inductive voltage divider and isolation transformer have been used. A conventional and and a home made helium flow-cryostat were used to cool the sample. Temperature was controled in 0.1 K accuracy by using Lake- Shore 321model temperature controler employing Si-diyot and calibrated carbon glass resistor as temperature sensors. A computer program have been written and all the measurements have been controlled by a computer to store the data for theoretical analyse. The resistivity of both of amorphous C^^Fe^^,and Cr74Fe26 films first increase slowly with decreasing temperature down to 43 K, passing through a local maximum exhibit a sharp decrease in a very narrow temperature range and then start to increase monoton- ically with decreasing temperature. At the same time some small oscilations in the resistivity curves have been observed below this transition temperature. This resistivity versus temperature curve showed hysteresis effect with temperature. This minima was also seen in resistivity curves of Nİ62Mn38 and Nİ62Mn2sPtıo films. But the histeresis effect is observed below the temperature correspond ing the resistivity minima in this case. The shape of these histeresis loope is changed after in a few cycle in the temperature below this temperature. The overal magnitude of the resistivities of all the samples above are much greater than expected. The magnetic parameters of the samples have been deduced from the analy sis of the data by a theoretical model described below. The overall behavior of high temperature ESR spectra were seen to be well explained by using a clas sical spin wave resonace (SWR) theory based on the classical Landau-Liftshitz equation of motion for magnetization with Bloch-type damping term. The ESR peaks in higher fields attributed to the surface modes while the remaining ones at lower fields were assumed to correspond to bulk SWR modes. As wll known the distances between bulk modes are a rough measure of the exchange interaction amoung the neigboring spins while the distance of the surface mode to uniform Kittel's mode is related to the surface anisotropy parameters. In order to explain the low temperature properties of the ESR spectra the magnetic systems have been modeled by adding various anisotropy terms to the magnetic free energy as follows xixET=EZ+Ed+Eb+Eexc+Eg (1) By chosing the z axis of referance coordinate system to be paralel to the film normal these term can be axplained as follows: In Eq.l the first term is usual Zeeman energy of a magnetic moment M in an effective magnetic field and given as Ez= -M0H [sin(0) sin(0H) sin( and a dc field H the resonance relation has been derived from the classical energy density func tion by using the Landau-Lifshitz dynamic equation of motion for magnetization as - =7(V.£T+Mx^V2M+M*h)+M^]/ (2) dt Ml T2 where 7 is the gyromagnetic ratio, V.Er=-ipdET/d0+6(l/sm(6))dET/dtp is the torque due to the energy density E in spherical coordinates. The second term arises from the exchange interaction between the spins and characterized by the parameter D (=2A/M0,where A is called as exchange stiffness). The last term was included to account for the any relaxation of dynamic magnetization with characteristic time T2. This differantial eaquation has been solved by trying fol lowing spin wave functions for nth mode m£(z,t)=m° expi(wt ± knz) m.ğ (z,t)=m£ exp i(wt ± knz) and following dispersion relation has been obtained for spin waves with wave vector kn and characteristic frequency w“ for 11th spin wave mode along z direc tion in a thin film as 0 \2 7 1 d2ET\ ( 1 82ET\2 1 Mosin2(0) dip2 J ”\Mosm(0)d6dtp) +72T22 (3) On the other hand, the power absorption from the rf field of frequency u in a unit volume of a magnetic sample has been obtained by using the well known expression xxi = -uxihl (4) where hi is the amplitude of the magnetic field component of rf field, X2 is the imaginary part of the high frequency susceptibility x(x = Xi ~ Xi) which is de fined as x= TT I vL" (5) For ip = 7r/2 the contribution to the susceptibility from each mode (for instance nth mode) was obtained to be X2=i-rr: - -^2 7- - (6) L(^)2 - (*)*] - 4,V74r| In addition to the inhomogeneous magnetization near the surface regions, we assumed an uniaxial surface anisotropy energy Es = -ksl2cosz(6) and used it in famous Rado-Weertman general exchange boundary condition to deduce an expression MW^M (7) for bulk modes corresponding real values of wave vector and xxntanh(fcnL)=- ki + PiPi (8) for the surface modes to complex values of wave vector. Here L is the film thickness and P? are the surface spin pinning parameters which were determided by the expression T 2ksF (9) The solution of Eq.9 for k was substituted in Eq.8 to obtain the contribution from nt mode to the microwave absotrption in SWR spectra. The magneto-resistivity data were analized by using Baxter's expression öp WL /fa/ 2^ [UD-Hi;)} -H J) ?2^ftV n \.^ (-^{t_-t+)+Vl-Vt+i) (10) to account for the combined effects of Zeeman splitting and spin-orbit scattering together with magnetic spin-flip scattering. The terms in Eq.ll are explained as follows t=- 3Bw 4(Bso-Ba) t±=t+^(l±0^) B±=:B0+-(BSO-BS)(1 ± y/l^f)+2Bs, B2=B;+-Bs+-Bso, B0=Bj+2Bs, 7 = 3g>BB 8eD(Bso-Bs) where D is the electronic diffusivity. The characteristic fields are related to char acteristic electron scattering times through relations of the type xxmBx = h/4:eDTx where x=i, so, and s refer to the inelastic, spin-orbit, and magnetic spin-flip scat tering times respectively. Here fa(x) is the Kawabata function for 3D disordered system. Several computer programs have been written to redrive the theroretical data for comparison to the corresponding experimental ones in order to analyse both SWR and resistivity data and to deduce physical parameters, namely, effective magneto-crystalline bulk ( Ki)and surface anisotropies, line-width (I/T2), spec troscopic siplitting factor, uni-directional (H^, exchange) bulk and surface (Q and kf, k|. ks) anisotropies, exchange stiffness parameter (D), electronic scat tering such as inelastic, spin-orbit, spin flip, the characteristic temperature for resistivity minima, canting temperatutres for above samples. Quite satisfactory agreements have been obtained between the experimental data and theoretical SWR data at higher temperatures. However, we were not able to analyze the ESR spectra due to very broad lines, which are associated with the frustration of spins at lower temperatures. The frustration manifests itself at higher tem peratures for Nİ74Mn24Pt2 than for Nİ76Mn24 and becomes more severe with Pt impurities. The induced exchange bulk anisotropy, (H^), which is created during cool ing of the sample was found to rotate freely towards the applied field, especially for amorphous samples. It shoul be remembered that this anisotropy field is quite rigid at low temperature for bulk samples of the same alloys. But the absolute value has been significantly increased in thin film cases. The tem perature behaviour of this anisotropy is in agreement with the exponential law (exp(- a/T) for almost all samples. The temperature behaviour of this exchange bulk anisotropy reflects itself on film surfaces as to be surface anisotropy which has been attributed to the D-M interactions. The uni-directional anisotropy in duced at both surface seemed to be practically the same. It has also be seen that there is a remarkable contribution to this surface anisotropy from inhomogeneous magnetisation. Actually, it is almost impossible to make exact determination of this anisotropy due to inavitable field treatment effects during experimental processes. Magneto crystalline and the shape anisotropy component to the effective bulk anisotropy energy exhibitted strong temperature dependence. The mein contri bution is from demagnetising field. The qualitative behavior is smilar for both crystalline and amorphous samples. However the Pt dopant foun to highly ef fected the temperature dependence, that is, this anisotropy changed its sign from negative to positive below 150 K when the sufficient amount of Pt has been doped. The positivity of this anisotropy is a sign for strong easy axis (hard plane) magneto-crystalline anisotropy. Magnetocrystalline anisotropy was also found to overdominete the other components at low temperatures with Pt doped samples. Morover, for the film with nominal thicknesses so called oblique anisotropy has xxivbeen induced depending on the film preparation condition for e-beam growth sam ples. The order of this anisotropy is comparable, even stronger, to any magneto- crystalline anisotropy. Prom the temperature behaviour of the bulk anisotropy constants, we have concluded that magnetic structures of almost all samples con sist of mainly 180° domains lying in the film surface, in accordance with the TEM study on policrystalline couterparts given in the literature. The Pt impurities also change drastically the domain structure. Pure magnetic anisotropy component can not be determined unless the magnetisation data from any independent mea surement technique is provided.. The surface anisotropy field for almost all samples showed a uniaxial character with hard axis (the easy plane) oriented along the film normal. The symmetric boundary condition has been observed for all samples but Pt doped ones. A weak linear temperature dependence down to 100 K have been observed for surface anisotropy energy. The absolute values of this anisotropy starts to rapidly increase below this temperature. It has also been seen that the surface anisotropy weakly depend on the film thickess so that its values can be taken to be the same for each sample of the same alloys. Therefore the magnetic structure of thinner films (below 300 A) is primarily controlled by only the surface anisotropy. It is also found that the Pt impurities strongly modifies the surface structure, exhibiting a very complex symmetry in the surface field. However, the resonance parameters for the surfaces of both samples can be described quite well by asymmetric surface condition. The origine of this anisotropy was foud to be as D-M interactions. The values of exchange interaction parameter D was obtained in a quite good accuracy as a function of temperature. The monotonically increasing D-T curve passes through a broad maxima at about 150 K and a minima at about 100 K and then rapidly inreases with decreasing temperature. No corelation was observed between film thickness and the stifness parameter. But the magnitude of this parameter at a certain temperature is highly affected by physical structure and composition of the alloys. We have also obtained a very close connection between the stiffness constants and the surface anisotropy constants. Finally, magneto resistivity data from CrFe and NiMnPt amorphous alloys have been sucsesfully analyzed by using Anderson's Weak Localization modelat relatively higher temperature range. Below 50 K for CrFe samples Stoner anhans- ment factor has been considered bsides WL model. The parameters, inelastic n and spin-flip scattering rate ts,have been determided as a function of tempera ture. Also Pt concetration dependence of rso was obtained for NiMnPt samples. Taking into account the local spin fluctuation effects besides the strong spin-orbit effects, we have seen that the electrical transport mechanism for the CrFe, NiM nPt systems can be explained only by the localization.. At low temperatures, local spin fluctuations found to play a predominant role on the temperature be haviour of the resistivity in agreement with prediction of the model of Bergmann and Beckmann. In conclusions; all films exhibit a very strong spin disorder at a microscopic scale at low temperatures and this frustration of the spins which menifest it self in the broadening of SWR lines, rapid change of magnetic parameters like surface anisotropy, bulk energy, effective exchange interaction parameter and uni directional anisotropy energy. This common effect revealed that magnetic state of the system is controled by D-M interaction. As for the resistivity data, the xxvspin-orbit scattering and spin-flip scattering due to the local spin fluctuations must be strong, yielding antilocalization. Therefore, the elastic and all the in elastic scattering processes connot be treated independently. Our conclusion is in disagreement with the work of Altounian who have analyzed their data for the Fea;Nii_-cZr2 system by consedering the resistivity due to the local spin fluctua tions to be simply additive. xxvi
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